Technical Report Documentation Page Report No. 2. Government Accession No. 3. Recipient’s Catalog No. FHWA/OH-2003/006 4. Title and subtitle 5. Report Date Environmental/Durability Evaluation of FRP Composite March 2003 Strengthened Bridges
7. Author (s) 6. Performing Dr. Nabil F. Grace Organization Code 9. Performing Organization Name and Address 8. Performing Organization Lawrence Technological University, Civil Engineering Department Report No. 21000 West 10 Mile Rd, Southfield, MI 48075-1058, USA 10. Work Unit No. (TRAIS) 12. Sponsoring Agency Name and Address Ohio Department of Transportation 11.Contract/Grant No. 1980 W Broad Street, Columbus, OH 43223 State Job No. 14718 (0) 15. Supplementary Notes 13. Type of Report and Period Covered Final Report 14. Sponsoring Agency Code
16. Abstract
The experimental evaluation of the durability of RC beams externally strengthened with CFRP plates/fabrics is presented. Experimental investigation consisted of testing a total of seventy eight RC beams under various environmental conditions such as 100% humidity, dry-heat, salt-water and alkaline solutions, freeze-thaw and thermal expansion cycles, and repeated load effects. Accelerated aging procedures were used to correlate the strength and stiffness degradation as per ASTM standards. All the test beams were subjected to ultimate load test after appropriate environmental conditioning. A durability based design approach for analysis and design of externally strengthened beams is also presented. This report consists of 6 chapters as given below: Chapter 1- Introduction Chapter 2- Literature Review Chapter 3- Experimental Program Chapter 4- Results and Discussion Chapter 5- Durability based design Chapter 6- Conclusions and Recommendations
17. Keywords Carbon Fiber Reinforced Polymer 18. Distribution Statement Plates, Carbon Fiber Reinforced Fabrics, No restrictions. This document is available to the Reinforced Concrete, Durability, External public through the National Technical Strengthening. Information Service, Springfield, Virginia 22161 19. Security 20. Security Classification 21. No. of pages 22. Price Classification (page) unclassified 319 (Report) Unclassified LAWRENCE TECHNOLOGICAL UNIVERSITY
ENVIRONMENTAL/DURABILITY EVALUATION OF FRP COMPOSITE
STRENGTHENED BRIDGES
Prepared in Cooperation with the Ohio Department of Transportation and the U. S. Department of Transportation, Federal Highway Administration
Contract No. 14718 (0)
by
Nabil F. Grace, Ph.D., PE Project Director
Civil Engineering Department Lawrence Technological University Southfield, MI 48075-1058, USA
March, 2003 Ohio Department of Transportation- Technical Advisory Group
Monique R. Evans
Brad Fagrel
Vikram Dalal
Valerie Frank
Karen Pannell
Lawrence Technological University- Research Team
Nabil F. Grace, Project Director
Emad N. Ibrahin, Research Assistant
Nate Blackburn, Research Assistant
Shamsher B. Singh, Postdoctoral Research Scholar
ODOT Contract No. 14718 (0)
Project Director: Nabil F. Grace, LTU
Keywords:
Carbon Fiber Reinforced Polymer (CFRP) CFRP Plate CFRP Fabrics Composite Materials Concrete Dry-Heat Durability Epoxy Environmental Conditioning External Strengthening System Freeze-Thaw Humidity Repeated Loads Alkaline Solution Salt solution
Civil Engineering Department Lawrence Technological University Southfield, MI 48075-1058 Tel: 1-248-204-2556; Fax: 1-248-204-2568
The contents of this report reflect the views of the authors who are responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Ohio Department of Transportation or the Federal Highway Administration. This report does not constitute a standard, specification or regulation.
TABLE OF CONTENTS LIST OF FIGURES ...... viii LIST OF TABLES ...... xviii ABSTRACT...... xix ACKNOWLEDGMENTS ...... xxi CHAPTER 1 INTRODUCTION...... 1 1.1 STATEMENT OF PROBLEM ...... 1
1.2 BACKGROUND TO THE FRP SYSTEMS...... 2 1.2.1 Historical Development of Externally Bonded FRP System...... 4 1.2.2 Commercially Available Externally Bonded FRP Systems...... 6
1.3 PROJECT OBJECTIVES ...... 21 1.4 EXPERIMENTAL INVESTIGATION...... 22
CHAPTER 2 LITERATURE REVIEW...... 24 2.1 GENERAL...... 24
2.2 EFFECT OF HUMIDITY...... 24 2.3 EFFECT OF DRY-HEAT...... 26
2.4 EFFECT OF ALKALINE ENVIRONMENT ...... 28
2.5 EFFECT OF REPEATED LOAD...... 31 2.6 EFFECT OF FREEZE/THAW ...... 32
2.7 EFFECT OF SALT-WATER SOLUTION...... 35 2.8 MATERIALS DESIGN VALUES ...... 36 2.8.1 Aerospace-based Determination...... 36 2.8.2 Naval and Marine-Based Determination...... 37 2.8.3 Determination in Infrastructure Applications ...... 37
CHAPTER 3 EXPERIMENTAL PROGRAM ...... 39 3.1 OUTLINE OF THE EXPERIMENTAL PROGRAM ...... 39
3.2 CONSTRUCTION DETAILS OF TEST BEAMS...... 40 3.2.1 Reinforcement Details...... 40 3.2.2 Application of CFRP fabrics and CFRP plates...... 41
3.3 ENVIRONMENTAL CONDITIONING AND TEST PROCEDURES ...... 43 3.3.1 100% Humidity Exposure...... 45 3.3.2 Dry-Heat Exposure ...... 45 3.3.3 Alkaline Solution Exposure ...... 46 3.3.4 Salt-Water Exposure ...... 47 3.3.5 Freeze/Thaw Exposure...... 48 3.3.6 Thermal Expansion Cycles ...... 50 3.3.7 Repeated Load Effects ...... 51
3.4 INSTRUMENTATION AND TESTING PROCEDURE ...... 51 3.4.1 Instrumentation...... 51 3.4.2 Testing Equipment ...... 52 3.4.3 Testing Procedure ...... 55
CHAPTER 4 RESULTS AND DISCUSSION ...... 83 4.1 INTRODUCTION ...... 83 4.2 BEAMS EXPOSED TO 100% HUMIDITY ...... 84 4.2.1 Beams strengthened with CFRP plates ...... 85 4.2.2 Beams strengthened with CFRP fabrics ...... 88
4.3 BEAMS EXPOSED TO DRY-HEAT CONDITION AT 140OF (60OC)...... 90 4.3.1 Beams strengthened with CFRP plates ...... 90 4.3.2 Beams Strengthened with CFRP Fabrics...... 92
4.4 BEAMS EXPOSED TO ALKALINE SOLUTION AT 73OF (23OC)...... 94 4.4.1 Beams strengthened with CFRP plates ...... 95 4.4.2 Beams strengthened with CFRP fabrics ...... 97
4.5 BEAMS EXPOSED TO SALT-WATER AT 73OF (23OC)...... 98 4.5.1 Beams Strengthened with CFRP Plates ...... 98 4.5.2 Beams strengthened with CFRP fabrics ...... 100 4.6 BEAMS EXPOSED TO FREEZE/THAW CYCLES ...... 102 4.6.1 Beams strengthened with CFRP plates ...... 102 4.6.2 Beams Strengthened with CFRP Fabrics...... 103 4.7 BEAMS EXPOSED TO THERMAL EXPANSION CYCLES ...... 105 4.7.1 Beams Strengthened with CFRP Plates ...... 105
vi 4.7.2 Beams Strengthened with CFRP Fabrics...... 107 4.8 WEIGHT CHANGE DUE TO EXPOSURE TO VARIOUS ENVIRONMENTAL CONDITIONS..108
4.9 BEAMS EXPOSED TO REPEATED LOADS...... 108 4.9.1 Beams Strengthened with CFRP Plates ...... 109 4.9.2 Beams Strengthened with CFRP Fabrics...... 111
4.10 COMPARISON OF ULTIMATE LOAD RESULTS ...... 113 4.10.1 Beams strengthened with CFRP plates ...... 113 4.10.2 Beams Strengthened with CFRP Fabrics...... 118
CHAPTER 5 DURABILITY BASED DESIGN...... 257 5.1 DESIGN EXAMPLE...... 270
CHAPTER 6 CONCLUSIONS AND RECOMMENDATIONS...... 283 6.1 CONCLUSIONS ...... 283
6.2 RECOMMENDATIONS ...... 284
REFERENCES ...... 286 APPENDIX-A ...... 299 A.1 Notations ...... 299
vii LIST OF FIGURES
Figure 3.1 Details of the experimental program...... 57
Figure 3.2 Construction details of test beams ...... 58
Figure 3.3 Metal forms and reinforcement arrangement...... 59
Figure 3.4 Preparation of structural epoxy for beam strengthening...... 60
Figure 3.5 Application of structural epoxy for CFRP plates/fabrics installation ...... 61
Figure 3.6 Installation of first layer of CFRP fabric ...... 62
Figure 3.7 Application of structural epoxy on top of first layer of CFRP fabric...... 63
Figure 3.8 Installation of second layer of CFRP fabric ...... 64
Figure 3.9 Use of hand-rollers to bond CFRP fabric ...... 65
Figure 3.10 Weight evaluation of test beams...... 66
Figure 3.11 Schematic of tanks used for humidity, salt-water, and alkaline solution exposures...... 67
Figure 3.12 Side view of tanks used for humidity, salt-water, and alkaline solution exposures...... 68
Figure 3.13 Arrangements for humidity, salt-water, alkaline solution, and dry-heat exposure...... 69
Figure 3.14 Bottom row of four beams exposed to hot water for 10,000 hours ...... 70
Figure 3.15 Bottom row of four beams exposed to 10,000 hours of dry-heat...... 71
Figure 3.16 Beams exposed to salt-water for 10,000 hours...... 72
Figure 3.17 Beams exposed to freeze/thaw test inside environmental chamber ...... 73
Figure 3.18 Water-thawing for beams exposed to freeze/thaw test...... 74
Figure 3.19 Freeze/thaw cycle for CFRP externally strengthened beams according to ASTM C666-B ...... 75
Figure 3.20 Arrangement for thermal expansion test...... 76
Figure 3.21 Thermal expansion test cycles...... 77
viii Figure 3.22 Range of repeated load test Applied @ 3.25 Hz...... 78
Figure 3.23 Strain gages instrumentation along beam-span...... 79
Figure 3.24 Deflection instrumentation along beam span...... 80
Figure 3.25 Four-point ultimate and repeated load test set-up...... 81
Figure 3.26 Loading frame for ultimate and repeated load tests...... 82
Figure 4.1 Close-up of ultimate load test of beam strengthened with CFRP plate after 10,000 hours exposure to 100% humidity condition ...... 125
Figure 4.2 Deflection at midspan of beams strengthened with CFRP plates and exposed to 100% humidity condition @100°F ...... 126
Figure 4.3 Deflected shape at 12 kip load for beams strengthened with CFRP plates exposed to 100% humidity condition @100°F ...... 127
Figure 4.4 Strain in CFRP plate at midspan of beams strengthened with CFRP plates and exposed to 100% humidity condition @100°F ...... 128
Figure 4.5 Strain in CFRP plate of beams strengthened with CFRP plates and exposed to 100% humidity @100°F at 12 kip load...... 129
Figure 4.6 Deflected shape at failure load of beams strengthened with CFRP plates and exposed to 100% humidity @100°F ...... 130
Figure 4.7 Load versus deflection relationships for beams strengthened with CFRP plates and exposed to 3000 hours of 100% humidity @100°F ...... 131
Figure 4.8 Load versus deflection relationships for beams strengthened with CFRP plates and exposed to10,000 hours of 100% humidity @100°F...... 132
Figure 4.9 Load versus strain relationships for beams strengthened with CFRP plates and exposed to 3000 hours of 100% humidity @100°F...... 133
Figure 4.10 Load versus strain relationships for beams strengthened with CFRP plates and exposed to 10,000 hours of 100% humidity @100°F ...... 134
Figure 4.11 Deflection at midspan of beams strengthened with CFRP fabrics and exposed to 100% humidity condition @ 100°F ...... 135
Figure 4.12 Deflected shape at 12 kip load for beams strengthened with CFRP fabrics and exposed to 100% humidity @100°F ...... 136
Figure 4.13 Strain at midspan of CFRP fabric for beams strengthened with CFRP fabrics and exposed to100% humidity @100°F ...... 137
ix Figure 4.14 Strain in CFRP fabric of beams strengthened with CFRP fabrics and exposed to 100% humidity @100°F at 12 kips load ...... 138
Figure 4.15 Deflected shape at failure load for beams strengthened with CFRP fabrics and exposed to 100% humidity @100°F ...... 139
Figure 4.16 Load versus deflection relationships for beams strengthened with CFRP fabrics and exposed to 3000 hours 100% humidity @100°F ...... 140
Figure 4.17 Load versus deflection relationships for beams strengthened with CFRP fabrics and exposed to 10,000 hours of 100% humidity @100°F...... 141
Figure 4.18 Load versus strain relationships for beams strengthened with CFRP fabrics and exposed to 3000 hours of 100% humidity @100°F...... 142
Figure 4.19 Load versus strain relationships for beams strengthened with CFRP fabrics and exposed to 10,000 hours of 100% humidity @100°F ...... 143
Figure 4.20 Deflection at midspan of for beams strengthened with CFRP plates and exposed to dry heat condition @140°F ...... 144
Figure 4.21 Deflected shape at 12 kips load for beams strengthened with CFRP plates and exposed to dry heat condition @ 140°F ...... 145
Figure 4.22 Strain at midspan of CFRP plate for beams strengthened with CFRP plates and exposed to dry-heat condition @ 140°F...... 146
Figure 4.23 Strain in CFRP plate at 12 kip load for beams strengthened with CFRP plates and exposed to dry heat condition @ 140°F ...... 147
Figure 4.24 Deflected shape at failure load for beam strengthened with CFRP plates and exposed to dry heat condition @ 140°F ...... 148
Figure 4.25 Load versus deflection relationships for beams strengthened with CFRP plates and exposed to dry heat condition for 3000 hours @ 140°F ...... 149
Figure 4.26 Load versus deflection relationships for beams strengthened with CFRP plates and exposed to dry heat condition for 10,000 hours @ 140°F...... 150
Figure 4.27 Load versus strain relationships for beams strengthened with CFRP plates and exposed to 3000 hours of dry heat condition @ 140°F...... 151
Figure 4.28 Load versus strain relationships for beams strengthened with CFRP plates and exposed to 10,000 hours of dry-heat condition @ 140°F...... 152
Figure 4.29 Close-up of ultimate load test of beam strengthened with CFRP fabric after 10,000 hours of exposure to to dry-heat...... 153
x Figure 4.30 Midspan deflection of beams strengthened with CFRP fabrics and exposed to dry heat condition @140°F...... 154
Figure 4.31 Deflected shape at failure load for beams strengthened with CFRP fabrics exposed to dry heat condition at 140°F...... 155
Figure 4.32 Strain in CFRP fabric at midspan of beams strengthened with CFRP fabrics and exposed to dry heat condition @ 140°F ...... 156
Figure 4.33 Strain in CFRP fabric at 12 kip load for beams strengthened with CFRP fabrics and exposed to dry heat condit ion @ 140°F ...... 157
Figure 4.34 Deflected shape at failure load for beams strengthened with CFRP fabrics and exposed to dry heat condition @ 140°F ...... 158
Figure 4.35 Load versus deflection relationships for beams strengthened with CFRP fabrics exposed to dry heat condition for 3000 hours at 140°F ...... 159
Figure 4.36 Load versus deflection relationships for beams strengthened with CFRP fabrics exposed to dry heat condition for 10,000 hours at 140°F ...... 160
Figure 4.37 Load versus strain relationships for beams strengthened with CFRP fabrics and exposed to 3000 hours of dry heat condition @ 140°F...... 161
Figure 4.38 Load versus strain relationships for beams strengthened with CFRP fabrics and exposed to 10,000 hours of dry heat condition @ 140°F ...... 162
Figure 4.39 Deflection at midspan of CFRP plate for beams strengthened with CFRP plates and exposed to alkaline solution @ 73°F ...... 163
Figure 4.40 Deflected shape at 12 kip load for beams strengthened with CFRP plates and exposed to alkaline solution @ 73°F ...... 164
Figure 4.41 Strain in CFRP plates at midspan of beams strengthened with CFRP plates and exposed to alkaline solution @ 73°F ...... 165
Figure 4.42 Strain in CFRP plate at 12 kips load for beams strengthened with CFRP plates and exposed to alkaline solution @ 73°F ...... 166
Figure 4.43 Deflected shape at failure load for beams strengthened with CFRP plates and exposed to alkaline solution @ 73°F ...... 167
Figure 4.44 Load versus deflection relationships for beams strengthened with CFRP plates and exposed to alkaline solution @ 73°F for 3000 hours ...... 168
Figure 4.45 Load versus deflection relationships for beams strengthened with CFRP plates and exposed to alkaline solution @ 73°F for 10,000 hours...... 169
xi Figure 4.46 Load versus strain relationships for beams strengthened with CFRP plates and exposed to alkaline solution @ 73°F for 3000 hours...... 170
Figure 4.47 Load versus strain relationships for beams strengthened with CFRP plates and exposed to alkaline solution @ 73°F for 10,000 hours ...... 171
Figure 4.48 Close-up of ultimate load test of beam strengthened with CFRP fabric after 10,000 hours exposure to alkaline solution ...... 172
Figure 4.49 Midspan deflection of beams strengthened with CFRP fabrics and exposed to alkaline solution @ 73°F ...... 173
Figure 4.50 Deflected shape at 12 kips load for beams strengthened with CFRP fabrics and exposed to alkaline solution @ 73°F ...... 174
Figure 4.51 Strain in CFRP fabric at midspan of beams strengthened with CFRP fabrics and exposed to alkaline solution @ 73°F ...... 175
Figure 4.52 Strain in CFRP fabrics at 12 kips for beams strengthened with CFRP fabrics and exposed to alkaline solution @ 73°F ...... 176
Figure 4.53 Deflected shape at failure load of beams strengthened with CFRP fabrics and exposed to alkaline solution @ 73°F ...... 177
Figure 4.54 Load versus deflection relationships for beams strengthened with CFRP fabrics and exposed to alkaline solution @ 73°F for 3000 hours ...... 178
Figure 4.55 Load versus deflection relationships for beams strengthened with CFRP fabrics and exposed to alkaline solution @ 73°F for 10,000 hours...... 179
Figure 4.56 Load versus strain relationships for beams strengthened with CFRP fabrics and exposed to alkaline solution @ 73°F for 3000 hours...... 180
Figure 4.57 Load versus strain relationships for beams strengthened with CFRP fabrics and exposed to alkaline solution @ 73°F for 10,000 hours ...... 181
Figure 4.58 Beams strengthened with CFRP plates and fabrics after exposure to salt-water for 10,000 hours ...... 182
Figure 4.59 Deflection at midspan of beams strengthened with CFRP plates and exposed to salt solution @ 73°F ...... 183
Figure 4.60 Deflected shape at 12 kips load for beams strengthened with CFRP plates and exposed to salt solution @ 73°F...... 184
Figure 4.61 Strain in CFRP plates at midspan of beams strengthened with CFRP plates and exposed to salt solution @ 73°F...... 185
xii Figure 4.62 Strain in CFRP plate for beams at 12 kip strengthened with CFRP plates and exposed to salt solution @ 73°F...... 186
Figure 4.63 Deflected shape at failure load for beams strengthened with CFRP plates and exposed to salt solution @ 73°F...... 187
Figure 4.64 Load versus deflection relationships for beams strengthened with CFRP plates and exposed to salt solution @ 73°F for 3000 hours ...... 188
Figure 4.65 Load versus deflection relationships for beams strengthened with CFRP plates and exposed to salt solution @ 73°F for 10,000 hours ...... 189
Figure 4.66 Load versus strain relationships for beams strengthened with CFRP plates and exposed to salt solution @ 73°F for 3000 hours ...... 190
Figure 4.67 Load versus strain relationships for beams strengthened with CFRP plates and exposed to salt solution @ 73°F for 10,000 hours ...... 191
Figure 4.68 Deflection at midspan at 12 kips load of beams strengthened with CFRP fabrics after exposure to salt solution @ 73°F...... 192
Figure 4.69 Deflected shape at 12 kip load for beams strengthened with CFRP fabrics and exposed to salt solution @ 73°F...... 193
Figure 4.70 Strain in CFRP fabric at midspan of beams strengthened with CFRP fabrics and exposed to salt solution @ 73°F...... 194
Figure 4.71 Strain in CFRP fabrics at 12 kips load for beams strengthened with CFRP fabrics and exposed to salt solution @ 73°F...... 195
Figure 4.72 Deflected shape at failure load for beams strengthened with CFRP fabrics and exposed to salt solution @ 73°F...... 196
Figure 4.73 Load versus deflection relationships for beams strengthened with CFRP fabrics and exposed to salt solution @ 73°F for 3000 hours ...... 197
Figure 4.74 Load versus deflection relationships for beams strengthened with CFRP fabrics and exposed to salt solution @ 73°F for 10,000 hours ...... 198
Figure 4.75 Load versus strain relationships for beams strengthened with CFRP fabrics and exposed to salt solution @ 73°F for 3000 hours ...... 199
Figure 4.76 Load versus strain relationships for beams strengthened with CFRP fabrics and exposed to salt solution @ 73°F for 10,000 hours ...... 200
Figure 4.77 Midspan deflection of beams strengthened with CFRP plates and exposed to freeze/thaw condition...... 201
xiii Figure 4.78 Deflected shape at 12 kips load for beams strengthened with CFRP plate and exposed to freeze/thaw condition ...... 202
Figure 4.79 Strain in CFRP plate at midspan of beams strengthened with CFRP plates and exposed to freeze/thaw condition ...... 203
Figure 4.80 Strain in CFRP plate at 12 kips load for beams strengthened with CFRP plates and exposed to freeze/thaw condition ...... 204
Figure 4.81 Deflected shape at failure load for beams strengthened with CFRP plates and exposed to freeze/thaw condition ...... 205
Figure 4.82 Load versus deflection relationships for beams strengthened with CFRP plates and exposed to 350 cycle of freeze/thaw condition ...... 206
Figure 4.83 Load versus deflection relationships for beams strengthened with CFRP plates and exposed to 700 cycle of freeze/thaw condit ion ...... 207
Figure 4.84 Load versus strain relationships for beams strengthened with CFRP plates and exposed to 350 cycles of freeze/thaw condition...... 208
Figure 4.85 Load versus strain relationships for beams strengthened with CFRP plates and exposed to 700 cycles of freeze/ thaw condition...... 209
Figure 4.86 Close-up of ultimate load test of beam strengthened with CFRP fabric after exposure to 350 freeze/thaw cycles...... 210
Figure 4.87 Deflection at midspan at 12 kip load of beams strengthened with CFRP fabrics and exposed to freeze/thaw condition ...... 211
Figure 4.88 Deflected shape at 12 kips load for beams strengthened with CFRP fabrics and exposed to freeze/thaw condition ...... 212
Figure 4.89 Strain in CFRP fabric at midspan of beams strengthened with CFRP fabrics and exposed to freeze/thaw condition ...... 213
Figure 4.90 Strain in CFRP fabric at 12 kip load for beams strengthened with CFRP fabrics and exposed to freeze/thaw condition ...... 214
Figure 4.91 Deflected shape at failure load for beams strengthened with CFRP fabrics and exposed to freeze/thaw condition ...... 215
Figure 4.92 Load versus deflection relationships for beams strengthened with CFRP fabrics and exposed to 350 cycle of freeze/thaw condition ...... 216
Figure 4.93 Load versus deflection relationships for beams strengthened with CFRP fabrics and exposed to 350 cycle of freeze/thaw condition ...... 217
xiv Figure 4.94 Load versus strain relationships for beams strengthened with CFRP fabrics and exposed to 350 cycles of freeze/thaw condition...... 218
Figure 4.95 Load versus strain relationships for beams strengthened with CFRP fabrics and exposed to 700 cycles of freeze/thaw condition...... 219
Figure 4.96 Close-up of ultimate load test of beam strengthened with CFRP plate after exposure to 35 thermal expansion cycles ...... 220
Figure 4.97 Midspan deflection at at 12 kips load of beams strengthened with CFRP plates and exposed to 35 thermal expansion test cycles...... 221
Figure 4.98 Deflected shape at 12 kip load for beams strengthened with CFRP plates and exposed to 35 thermal expansion test cycles ...... 222
Figure 4.99 Strain in CFRP plates at midspan of beams strengthened with CFRP plates and exposed to 35 thermal expansion test cycles ...... 223
Figure 4.100 Strain in CFRP plates at 12 kips load for beams strengthened with CFRP plates and exposed to 35 thermal expansion test cycles...... 224
Figure 4.101 Deflected shape at failure load for beams strengthened with CFRP plates and exposed to 35 thermal expansion test cycles ...... 225
Figure 4.102 Load versus deflection relationships for beams strengthened with CFRP plates and exposed to 35 thermal expansion test cycles...... 226
Figure 4.103 Load versus strain relationships for beams strengthened with CFRP plates and exposed to 35 thermal expansion test cycles ...... 227
Figure 4.104 Midspan deflection at 12 kips load of beams strengthened with CFRP fabrics and exposed to 35 thermal expansion test cycles...... 228
Figure 4.105 Deflected shape at 12 kips load for beams strengthened with CFRP fabrics and exposed to 35 thermal expansion test cycles ...... 229
Figure 4.106 Strain in CFRP fabrics at midspan of beams strengthened with CFRP fabrics and exposed to 35 thermal expansion test cycles ...... 230
Figure 4.107 Strain in CFRP fabrics at 12 kip load for beams strengthened with CFRP fabrics and exposed to 35 thermal expansion test cycles...... 231
Figure 4.108 Deflected shape at failure load for beams strengthened with CFRP fabrics exposed to 35 thermal expansion test cycles ...... 232
Figure 4.109 Load versus deflection relationships for beams strengthened with CFRP fabrics exposed to 35 thermal expansion test cycles ...... 233
xv Figure 4.110 Load versus strain relationship for beams strengthened with CFRP fabrics and exposed to 35 thermal expansion test cycles ...... 234
Figure 4.111. Weight of beams before and after 10,000 hours of exposure to various environmental conditions ...... 235
Figure 4.112 Repeated load tests for strengthened beams with CFRP plates and fabrics...... 236
Figure 4.113 Load versus deflection at midspan of beams strengthened with CFRP plate and subjected to repeated load test cycles ...... 237
Figure 4.114 Deflected shape at 12 kips static load for beams strengthened with CFRP plate and subjected to repeated load test cycles...... 238
Figure 4.115 Deflection at midspan of beams strengthened with CFRP plate and subjected to repeated load test cycles...... 239
Figure 4.116 Load versus strain relationship at midspan of CFRP plate for beams subjected to repeated load test cycles...... 240
Figure 4.117 Strain in CFRP plates at 12 kips static load and subje cted to repeated load test cycles ...... 241
Figure 4.118 Strain in CFRP at midspan of beams strengthened with CFRP plates during repeated load tests...... 242
Figure 4.119 Average ultimate loads after the conclusion of repeated load tests of beams strengthened with CFRP plate...... 243
Figure 4.120 Load versus deflection at midspan of beams strengthened with CFRP fabrics and subjected to repeated load test cycles...... 244
Figure 4.121 Deflected shape at 12 kips static load for beams strengthened with CFRP fabric during repeated load tests...... 245
Figure 4.122 Deflection at midspan of beams strengthened with CFRP fabrics during repeated load tests...... 246
Figure 4.123 Load versus strain at midspan of CFRP fabrics during repeated load tests...... 247
Figure 4.124 Strain in CFRP fabrics at 12 kips static load during repeated load tests...... 248
Figure 4.125 CFRP strain at midspan of beams strengthened with CFRP fabric during repeated load tests...... 249
Figure 4.126 Average ultimate loads after the conclusion of repeated load tests of beams strengthened CFRP fabrics...... 250
Figure 4.127 Average ultimate failure loads for beams strengthened with CFRP plates...... 251
xvi Figure 4.128 Ultimate failure loads for beams strengthened with CFRP plates after exposure to thermal cycles...... 252
Figure 4.129 Average midspan deflections for beams strengthened with CFRP plates ...... 253
Figure 4.130 Average ultimate failure loads for beams strengthened with CFRP fabrics ...... 254
Figure 4.131 Average ultimate failure loads for beams strengthened with CFRP fabrics exposed to thermal cycles...... 255
Figure 4.132 Midspan deflections for beams strengthened with CFRP fabrics...... 256
Figure 5.1 Stress, strain, and force diagrams across depth of beam cross-section ...... 281
Figure 5.2 Cross-sectional details of CFRP strengthened beam...... 282
xvii LIST OF TABLES
Table 1.1 Typical properties of fibers (Feldman 1989, Kim 1995) ...... 11
Table 1.2 Typical properties of prefabricated FRP strips and comparison with steel (ACI 440.2R-02, 2002) ...... 14
Table 1.3 Typical densities of FRP materials lb/ft3 (g/cm3) (ACI 440.2R-02, 2002)...... 14
Table 1.4 Typical coefficients of thermal expansion for FRP materials (ACI 440.2R-02, 2002) ...... 14
Table 1.5 Sustained plus cyclic service load stress limits in FRP reinforcement (ACI 440.2R-02, 2002) ...... 20
Table 3.1 Mechanical properties of CFRP strengthening materials...... 42
Table 4.1 Ultimate load (kips) for beams strengthened with CFRP plates and exposed to various environmental conditions...... 114
Table 4.2 Ultimate load for beams strengthened with CFRP plates and exposed to various thermal conditioning ...... 116
Table 4.3 Midspan deflection (in.) at failure for beams strengthened with CFRP plates and exposed to different environmental conditions...... 117
Table 4.4 Ultimate load (kips) for beams strengthened with CFRP fabrics and exposed to various environmental conditions...... 120
Table 4.5 Ultimate load (kips) for beams strengthened with CFRP fabrics and exposed to various thermal conditioning...... 121
Table 4.6 Midspan deflection (in.) at failure for beams strengthened with CFRP fabrics and exposed to different environmental conditions...... 122
Table 4.7 CFRP strength reduction factors (y) for different environmental conditionings...... 123
Table 5.1 Allowable service load stresses ...... 271
xviii ABSTRACT
Currently, most of the infrastructures such as bridges and buildings are in the need of continuous maintenance and repair because of the stiffness and strength degradation caused by aggressive environmental condition. The salt-water solution, alkali solution, and humidity cause severe degradation in the strength and stiffness of reinforced concrete (RC) beams by the corrosion of steel reinforcements. It is known that advanced fibrous composite materials such as carbon fiber reinforced polymers (CFRP) can eliminate the problem of corrosion and substantially increase the strength and stiffness of the beams internally reinforced with CFRP bars. However, in the case of RC beams externally strengthened with CFRP plates and/or fabrics and exposed to aggressive environmental conditions, corrosion of steel is not the only factor affecting the strength of externally reinforced RC beams, but the bond between the CFRP plates and surface of RC beam plays an important role for limiting the strength of such beams. Thus, it is essential to evaluate the overall response of the RC beams externally strengthened with CFRP plates/fabrics and exposed to different environmental conditions. This report presents the results obtained from experimental evaluation of the durability of RC beams externally strengthened with CFRP plates/fabrics. The experimental investigation consisted of testing a total of seventy eight RC beams under various independent environmental conditions such as 100% humidity, dry-heat, salt-water and alkaline solutions, freeze-thaw and thermal expansion cycles, and repeated load effects. Accelerated aging procedures were used to correlate the strength and stiffness degradation as per ASTM standards. All the test beams were subjected to ultimate load test after appropriate environmental conditioning. From the experimental results, it was confirmed that CFRP has the excellent durability, however, the degradation of the structural epoxy bonding the beam and CFRP plates/fabrics reduced the stiffness and strength of the beam. The most severe reduction in the strength (about 33% reduction) was observed in the case of beam strengthened with CFRP plate and exposed to 10,000 hours of 100% humidity at 38oC. Based on the test results, an analysis and design approach for design of RC beams externally strengthened with CFRP plates/fabrics is presented along with a design example. In addition, appropriate recommendations are made for the future research to evaluate the durability of CFRP strengthened system under combined environmental conditions.
xx ACKNOWLEDGMENTS
This research was funded by the Ohio Department of Transportation and the Federal Highway Administration under contract No. 14718. The authors would like to thank the following ODOT personnel for their help and suggestions: Brad Fagrel, Monique R. Evans, Vikram Dalal, Valerie Frank, and Karen Panell of ODOT office of Structural Engineering and office of Research and Development.
CHAPTER 1 INTRODUCTION
1.1 Statement of Problem
The nation has a great need to simultaneously repair existing structures and attempt to increase both their performance levels and life spans. Bridges are considered to be one of the major infrastructure elements in need of continuous maintenance and repair. A large number of techniques currently exist for strengthening highway bridges. Strengthening (improving the load carrying capacity) or stiffening (reducing the vertical deflection at the service load) of concrete members is usually accomplished by construction of external reinforced concrete or shotcrete jackets, by post-tensioning, or by epoxy bonding of steel plates to the tension face of structural members. For the large number of structures located in close vicinity of a marine environment, and for structures subjected to the effects of road salt usage, steel corrosion represents a considerable problem when used externally or when embedded in concrete. The idea of finding new materials to replace the steel has become the concern of many researchers. For that purpose, fiber reinforced polymers (FRP) have become of considerable interest in civil engineering as non-metallic reinforcement. A range of different fiber reinforcement materials, such as aramid, glass, and carbon fiber reinforced polymers (AFRP, GFRP, and CFRP), have been used for over 25 years in aerospace and manufacturing applications where low weight, high tensile strength and noncorrosive structural properties are required. The proven materials in these fields exhibit low creep and elongation, and are thinner, lighter and stronger than steel. FRP materials were introduced to the construction industry in the late seventies. The development of FRP stiffening and strengthening techniques started in Germany, Japan, and Switzerland. The short-term flexural and shear strength of reinforced concrete beams strengthened with aramid, glass, or carbon FRP epoxy bonded laminates has been examined by several researchers and institutions. Highway bridge construction or rehabilitation projects represent one of the major domains where FRP materials can be used. The use of FRP in the form of plates or fabrics, in lieu of steel plates, in strengthening or stiffening concrete structural members has gained momentum in the construction repair industry. The use of fiber reinforcements in the highway bridge industry can provide flexibility for structural upgrades and a significant savings relative to conventional strengthening methods because of the low maintenance and life cycle costs. Even though several demonstrations of bridge rehabilitation projects have shown the viability of such schemes, a number of critical questions related to long-term durability as well as damage and failure mechanisms still remain unaddressed. As a result, only a few structural engineers, contractors, and building and bridge owners are willing to accept the risk without addressing the durability concern. Conventional strengthening methods continue to be implemented when rehabilitating reinforced concrete structures. However all concerned parties would be willing to accept strengthening technology if the durability concern is adequately addressed. Although short-term behavior of the concrete structures strengthened with CFRP is well documented, long-term issues still remain. Various environmental effects on the composite material systems for strengthening highway bridges, related to long-term performance, need to be properly predicted. Strength reduction factors related to various environmental conditions have to be precisely determined to allow the design of safely strengthened concrete structures. In this experimental investigation, the long-term durability of reinforced concrete bridge beams strengthened with CFRP plates and fabrics is examined under various independent environmental conditions.
1.2 Background to the FRP Systems
The issue of upgrading the existing civil engineering infrastructure has been one of great importance for over a decade. Deterioration of bridge decks, beams, girders and columns, buildings, parking structures and others may be attributed to ageing, environmentally induced degradation, poor initial design and/or construction, lack of maintenance, and to accidental events such as earthquakes. The infrastructure’s increasing decay is frequently combined with the need for upgrading so that structures
2 can meet more stringent design requirements such as increased traffic volumes in bridges exceeding the initial design loads, and hence, the aspect of civil engineering infrastructure renewal has received considerable attention over the past few years throughout the world. Although detailed information and background materials can be found in ACI 440.2R-02 (2002) and FIB Bulletin (2001), the essential background material for fiber reinforced composites is reproduced in the following sections for the sake of completeness. The strengthening or retrofitting of existing concrete structures to resist higher design loads, correct deterioration-related damage, or increased ductility has traditionally been accomplished using conventional materials and construction techniques. Externally bonded steel plates, steel or concrete jackets, and external post-tensioning are just some of the many traditional techniques available. Recently, composite materials made of fibers in a polymeric resin, also known as fiber-reinforced polymers (FRP), have emerged as an alternative to traditional materials and techniques. An FRP system is defined as all the fibers and resins used to create the composite laminate, all applicable resins used to bond it to the concrete substrate, and all applied coatings used to protect the constituent materials (ACI 440.2R-02, 2002). Coatings are used exclusively for aesthetic reasons and are not considered part of an FRP system. FRP materials are lightweight, noncorrosive, and exhibit high tensile strength. Additionally, these materials are readily available in several forms ranging from factory- made laminates to dry fiber sheets that can be wrapped to conform to the geometry of a structure before adding the polymer resin. The relatively thin profile of cured FRP systems is often desirable in applications where aesthetics or access is a concern. The growing interest in FRP systems for strengthening and retrofitting can be attributed to many factors such as immunity to corrosion, low weight (about ¼ of steel, FIB Bulletin, 2001), resulting in easier application in confined space, elimination of the need for scaffolding and reduction in labor costs, very high tensile strength (both static and long-term, for certain type of FRP materials), stiffness which may be tailored to the design requirements, large deformation capacity, and practically unlimited availability in FRP sizes and FRP geometry, and dimensions. Although the fibers and resins used in FRP systems are relatively expensive compared to traditional strengthening materials like
3 concrete and steel, labor and equipment costs to install FRP systems are often lower. FRP systems can also be used in areas with limited access where traditional techniques would be difficult to implement, for example, a slab shielded by pipe and conduit. FRP composites suffer from certain disadvantages too, which are not to be neglected by engineers. For example, contrary to steel, which behaves in an elastoplastic manner, FRP composites in general are linear elastic to failure (although the latter occurs at large strains) without any significant yielding or plastic deformation, leading to reduced ductility. Additionally, the cost of materials on a weight basis is several times higher than that for steel, but when cost comparisons are made on strength basis, they become less unfavorable. Moreover, some FRP materials, e.g. carbon and aramid, have incompatible thermal expansion coefficients with concrete. Finally, their exposure to high temperatures (e.g. in case of fire) may cause premature degradation and collapse (some epoxy resins start softening at about 45-70oC). Hence, FRP materials should not be thought of as a blind replacement of steel (or other materials) in structural intervention applications. Instead, the advantages offered by them should be evaluated against potential drawbacks, and final decisions regarding their use should be based on consideration of several factors, including not only mechanical performance aspects, but also constructibility and long-term durability.
1.2.1 Historical Development of Externally Bonded FRP System
The initial development of externally bonded FRP systems for the retrofit of concrete structures occurred in the 1980s in both Europe and Japan. In Europe, FRP systems were developed as alternates to steel plate bonding. Bonding steel plates to the tension zones of concrete members with epoxy resins were shown to be viable techniques for increasing their flexural strengths (Fleming and King, 1967). This technique has been used to strengthen many bridges and buildings around the world. Because steel plates can corrode, leading to a deterioration of the bond between the steel and concrete, and that are difficult to install, and requiring the use of heavy equipment, researchers looked to FRP materials as an alternative to steel. Experimental work using FRP materials for retrofitting concrete structures was reported as early as 1978 in Germany (Wolf and Miessler, 1989). Research in Switzerland led to the first applications of externally
4 bonded FRP systems to reinforced concrete bridges for flexural strengthening (Meier 1987, Rostassy, 1987). FRP systems were first applied to reinforced concrete columns for providing additional confinement in Japan in the 1980s (Fardis and Khalili, 1981, Katsumata et al., 1987). A sudden increase in the use of FRPs in Japan was observed after the 1995 Hyogoken Nanbu earthquake (Nanni, 1995). The United States has had a long and continuous interest in fiber-based reinforcement for concrete structures since the 1930s. Actual development and research into the use of these materials for retrofitting concrete structures, however, started in the 1980s through the initiatives of the National Science Foundation (NSF) and the Federal Highway Administration (FHWA). The research activities led to the construction of many field projects encompassing a wide variety of environmental conditions. Previous research and field applications for FRP rehabilitation and strengthening are described in ACI 440R-96 and conference proceedings (Japan Concrete Institute, 1997, Neale, 2000, Dolan et al., 1999, Shehata et al., 1999, Saadatmanesh and Ehsani, 1998, Benmokrane and Rahman, 1998, Neale and Labossiere, 1997, Hassan and Rizkalla, 2002). The developments of codes and standards for externally bonded FRP systems are ongoing in Europe, Japan, Canada, and the United States. Within the last 10 years, the Japan Society of Civil Engineers (JSCE) and the Japan Concrete Institute (JCI) and the Railway Technical Research Institute (RTRI) published several documents related to the use of FRP materials in concrete structures. In Europe, Task Group 9.3 of the International Federation for Structural Concrete (FIB) recently published a bulletin on design guidelines for externally bonded FRP reinforcement for reinforced concrete structures (FIB Bulletin, 2001). The Canada Standards Association and ISIS have been active in developing guidelines for FRP systems. Section 16, “Fiber Reinforced Concrete”, of the Canadian Highway Bridge Design Code was completed in 2000 (CSA S806-02) and the Canadian Standards Association (CSA) recently approved the code “Design and Construction of Building Components with Fiber Reinforced Polymers” (CSA S806-02). In the United States, criteria for evaluating FRP systems are becoming available to the construction industry (CALTRANS 1996, Hawkins et al., 1998).
5 1.2.2 Commercially Available Externally Bonded FRP Systems
FRP systems come in variety of forms, including wet layup systems and precured systems. FRP system forms can be categorized based on how they are delivered to the site and installed. The FRP system and its form should be selected based on the acceptable transfer of structural loads and the ease and simplicity of application.
Common FRP system forms (ACI 440.2R-02, 2002) suitable for the strengthening of structural members are listed as follows:
· Wet layup systems- Wet layup FRP systems consist of dry unidirectional or multidirectional fiber sheets or fabrics impregnated with a saturating resin on-site. The saturating resin, along with the compatible primer and putty, is used to bond the FRP sheets to the concrete surface. Wet layup systems are saturated in-place and cured in- place and, in this sense, are analogous to cast-in-place concrete. Three common types of wet layup systems are listed as follows:
1. Dry unidirectional fiber sheets where the fibers run predominantly in one planar
direction.
2. Dry multidirectional fiber sheets or fabrics where the fibers are oriented in at least
two planar directions, and
3. Dry fiber tows that are wound or otherwise mechanically applied to the concrete
surface. The dry fiber tows are impregnated with resin on-site during winding
operation.
· Prepreg systems- Prepreg FRP systems consist of uncured unidirectional or multidirectional fiber sheets or fabrics that are preimpregnated with a saturating resin in the manufacturer’s facility. Prepreg systems are bonded to the concrete surface with or
6 without an additional resin application, depending upon specific system requirements.
Prepreg systems are saturated off-site and, like wet layup systems, cured in place.
Prepreg systems usually require additional heating for curing. Prepreg system manufacturers should be consulted for storage and shelf-life recommendations and curing procedures. Three common types of prepreg FRP systems are listed as follows:
1. Preimpregnated unidirectional fiber sheets where the fibers run predominantly in one
planar direction,
2. Preimpregnated multidirectional fiber sheets or fabrics, where fibers are oriented in at
least two planar directions,
3. Preimpregnated fiber tows that are wound or otherwise mechanically applied to the
concrete surface.
· Precured systems- Precured FRP systems consist of a wide variety of composite shapes manufactured off-site. Typically, an adhesive along with the primer and putty is used to bond the precured shapes to the concrete surface. The system manufacturer should be consulted for recommended installation procedures. Precured systems are analogous to precast concrete. Three common types of procured systems are listed as follows:
1. Precured unidirectional laminate sheets, typically delivered to the site in the form of
large flat stock or as thin ribbon strips coiled on a roll.
2. Precured multidirectional grids, typically delivered to the site coiled on a roll,
3. Precured shells, typically delivered to the site in the form of shell segments cut
longitudinally so they can be opened and fitted around columns or other members,
7 multiple shell layers are bonded to the concrete and each other to provide seismic
confinement.
· Other FRP forms-Other FRP forms are not covered in this document. These include cured FRP rigid rod and flexible strand or cable (Saadatmanesh and Tannous 1999, Dolan
1999, Fukuyama, 1999, ACI 440R-96, and ACI 440.1R-01).
Constituent Materials and Properties
The constituent materials used in commercially available FRP repair systems, including all resins, primers, putties, saturants, adhesives, and fibers, have been developed for strengthening of structural concrete members based on materials and structural testing.
Resins- A wide range of polymeric resins, including primers, putty fillers, saturants, and adhesives are used with FRP systems. Commonly used resin types including epoxies, vinylesters, and polysters have been formulated for use in wide range of environmental conditions. FRP system manufacturers use resins that have the following characteristics.
· Compatibility with and adhesion to the concrete substrate,
· Compatibility with and adhesion to the FRP composite system
· Resistance to environmental effects, including but not limited to moisture, salt water,
temperature extremes, and chemicals normally associated with exposed concrete
· Filling ability
· Workability
· Pot life (time interval after preparation during which a liquid or plastic mixture is to
be used) consistent with the application
· Compatibility with and adhesion to the reinforcing fiber, and
8 · Development of appropriate mechanical properties for the FRP composite
Primers- The primer is used to penetrate the surface of concrete, providing an improved adhesive bond for the saturating resin or adhesive.
Putty fillers- The putty is used to fill small surface voids in the substrate, such as bug holes, and to provide a smooth surface to which the FRP system can bond. Filled surface voids also prevent bubbles from forming during curing of the saturating resin.
Saturating resin- The saturating resin is used to impregnate the reinforcing fibers, fix them in place, and to provide a shear load path to effectively transfer load between fibers.
The saturating resin also serves as the adhesive for wet layup systems, providing a shear load path between the previously primed concrete substrate and the FRP system.
Adhesives- Adhesives are used to bond precured FRP laminate systems to the concrete substrate. The adhesive provides a shear load path between the concrete substrate and the
FRP reinforcing laminate. Adhesives are also used to bond together multiple layers of precured FRP laminates.
Protective coatings- The protective coating is used to protect the bonded FRP reinforcement from potentially damaging environmental effects. Coatings are typically applied to the exterior surface of the cured FRP system after the adhesive or saturating resin has cured.
Fibers- A great majority of materials are stronger and stiffer in the fibrous form than as a bulk material. A high fiber aspect ratio (length to diameter ratio) permits very effective
9 transfer of load via matrix materials to the fibers, thus enabling full advantage of the properties of fibers to be taken. Therefore, fibers are very effective and attractive reinforcement materials. Fibers can be manufactured in continuous or discontinuous
(chopped) form. Continuous glass, aramid, and carbon fibers are common reinforcements used with FRP systems for strengthening of civil engineering structures.
Such fibers have a diameter in the order of 5-20 mm, and can be manufactured as unidirectional or bi-directional reinforcement. The fibers used for strengthening all FRPs exhibit a linear elastic behavior up to failure and do not have a pronounced yield plateau as for steel. The fibers give the FRP system its strength and stiffness.
Glass fibers for continuous fiber reinforcement are classified into three types: E- glass fibers, S-glass fibers, and alkali resistant AR-glass fibers. E-glass fibers, which contain high amounts of boric acid and aluminate, are disadvantageous in having low alkali resistance. S-glass fibers are stronger and stiffer than E-glass fiber, but still not resistant to alkali. To prevent glass fiber from being eroded by cement alkali, a considerable amount of zircon is added to produce alkali resistance AR-glass fibers; such fibers have mechanical properties similar to E-glass. An important aspect of glass fiber is their low cost. Aramid fibers were first introduced in 1971, and today are produced by several manufactures under various brand names. The structure of aramid fiber is anisotropic and gives higher strength and modulus in the fiber longitudinal direction. The diameter of aramid fiber is approximately 12 mm. Aramid fibers respond elastically in tension but they exhibit non-linear and ductile behavior under compression; they also exhibit good toughness, damage tolerance, and fatigue characteristics.
10
Table 1.1 Typical properties of fibers (Feldman 1989, Kim 1995) Elastic modulus Tensile strength Ultimate tensile Material (GPa) (MPa) strain (%)
Carbon
High strength 215-235 3500-4800 1.4-2.0
Ultra high strength 215-235 3500-6000 1.5-2.3
High modulus 350-500 2500-3100 0.5-0.9
Ultra high modulus 500-700 2100-2400 0.2-0.4
Glass
E 70 1900-3000 3.0-4.5
S 85-90 3500-4800 4.5-5.5
Aramid
Low modulus 70-80 3500-4100 4.3-5.0
High modulus 115-130 3500-4000 2.5-3.5
11 Carbon fibers are normally either based on pitch or PAN, as raw materials. Pitch fibers are fabricated by using refined petroleum or coal pitch that is passed through a thin nozzle and stabilized by heating. PAN fibers are made of polyacrylonitrile that is carbonized through burning. The diameter of pitch-type fibers measures approximately 9-18 mm and that of the PAN type measures 5-8 mm. The structure of this carbon fiber varies according to the orientation of the crystals; the higher the orientation degree and rigidity because of growing crystals. The pitch base carbon fibers offer general purpose and high strength/elasticity materials. The PAN-type carbon fibers yield high strength materials and high elasticity materials. Typical properties of various types of fiber materials are provided in Table 1.1. Note that values in this table are only indicative of the static strength of unexposed fibers. Design values of the FRP composite systems should account both for the presence of resin and for reduction due to long-term loading, and environmental exposure etc.
Rule of Mixtures
Depending on the type of fiber, FRP materials are referred to as AFRP (aramid fiber reinforced polymer), CFRP (carbon fiber reinforced polymer), and GFRP (glass fiber reinforced polymer). Typically, the volume fraction of fibers in FRPs equals about 50-70% for strips and about 25-35% for sheets. Hence, fibers are the principal stress bearing constituents, while the resin transfers stresses among fibers and protects them. Basic mechanical properties of FRP materials may be estimated, if the properties of the constituent materials (fibers and matrix) and their volume fraction are known. This may be accomplished by applying the “rule of mixtures” simplification as follows:
Ef = Efib Vfib + Em Vm (1.1)
ff = ffib Vfib + fm Vm (1.2)
where Ef is the Young’s modulus of FRP in fiber direction; Efib is Young’s modulus of fibers ; Em is Young’s modulus of matrix; Vfib is volume fraction of fibers;
Vm is volume fraction of matrix; ff is tensile strength of FRP in fiber direction; ffib tensile strength of fibers, and fm tensile strength of matrix. Note that in the above equations Vfib
12 + Vm =1. Typical values for the volume fraction of fibers in prefabricated strips are in the order of 0.5-0.65. As a rule of mixture approximates the micro-mechanical behavior of fiber composites, a more detailed prediction of the stress-strain behavior should be obtained through tensile testing. Hence, the material properties should be given for the combined FRP directly, so to reflect the fiber and matrix characteristics as well as the micro-structural aspects such as fiber diameter, distribution, and parallelism of fiber, local defects, volume fractions, and fiber-matrix interfacial properties. Typical FRP commercial products in the form of prefabricated strips have the properties presented in Table 1.2, where the properties of mild steel are given for comparison.
Tensile Behavior- When loaded in direct tension, FRP materials do not exhibit any plastic behavior (yielding) before rupture. The tensile behavior of FRP materials consisting of one type of fiber material is characterized by a linearly elastic stress-strain relationship until failure, which is sudden and can be catastrophic. The tensile strength and stiffness of an FRP material is dependent on several factors. Because the fibers in an FRP material are the main load-carrying constituent, the type of fiber, the orientation of fibers, and quantity of fibers primarily govern the tensile properties of the FRP material. Due to the primary role of the fibers and methods of application, the properties of an FRP repair system are sometimes reported based on the net fiber area. In other instances, the reported properties are based on gross-laminate area. It should be noted that in case of prefabricated strips, the material properties based on the total cross-sectional area can be used in calculations and are usually supplied by the manufacturer (see Table 1.2). However, in case of in-situ resin impregnated systems, a calculation based on the FRP properties for the total system (fibers and matrix) and the actual thickness is not appropriate, because the final FRP thickness and with that the fiber volume fraction is uncertain and may vary.
Thus, it is very important that in calculations the appropriate material properties for the applied system are used. It must be also noted that for a comparison of FRP materials it may not be sufficient only to compare values for strength and/or stress-strain relations. It is also important to know the composition of the FRP material to which the
13
Table 1.2 Typical properties of prefabricated FRP strips and comparison with steel (ACI 440.2R-02, 2002) Material Elastic Tensile strength, ff Ultimate tensile,efu modulus, (MPa) strain (%) Ef (GPa) Prefabricated strips Low modulus CFRP strips 170 2800 1.6 High modulus CFRP strips 300 1300 0.5 Mild steel 200 400 25* * Yield strain =0.2%
Table 1.3 Typical densities of FRP materials lb/ft3 (g/cm3) (ACI 440.2R-02, 2002) Steel GFRP CFRP AFRP 490 75 to 130 90-100 75-90 (7.9) (1.2 to 2.1) (1.5-1.6) (1.2-1.5)
Table 1.4 Typical coefficients of thermal expansion for FRP materials (ACI 440.2R- 02, 2002) Coefficient of thermal expansion, 10-6/oF (´ 10-6/oC) Direction GFRP CFRP AFRP
Longitudinal, aL 3.3 to 5.6 -0.6 to 0 -3.3 to –1.1 (6 to 10) (-1 to 0) (-6 to –2) 10.4 to 12.6 12 to 27 33 to 44 Transverse, a T (19 to 23) (22 to 50) (60 to 80)
14 given property belongs. In case of uncertainty about the thickness (like with in-situ resin impregnated systems), it may be more convenient to base calculations on the fiber properties and fiber cross-sectional area than on properties for the total system. The latter approach is still possible, however, the material properties and thickness (cross-sectional area) as specified by the manufacturer should then be used and not the actual thickness that is realized in practice (FIB Bulletin, 2001).
As per ACI 440 committee recommendations (ACI 440.2R-02, 2002), the tensile properties of a particular FRP system, however, should be obtained from the FRP system manufacturer. Manufacturers should report an ultimate tensile strength defined by this guide as the mean tensile strength of a sample of test specimens minus three times the
* * standard deviation ( ffu = ffu-3s) and, similarly, report an ultimate rupture strain (efu = efu-3s). These statically based ultimate tensile properties provide a 99.87% probability that the indicated values are exceeded (Mutsuyoshi et al., 1990). Young’s modulus should be calculated as the chord modulus between 0.003 and 0.006 strain, in accordance with ASTM D 3039. A minimum number of 20 replicate test specimens should be used to determine the ultimate tensile properties. The manufacturer should provide a description of the method used to obtain the reported tensile properties, including the number of tests, mean values, and standard deviations.
Compressive Behavior- Externally bonded FRP systems should not be used as compression reinforcement due to insufficient testing validating its use in this type of application (ACI 440.2R-02, 2002). Although it is not recommended to rely on externally bonded FRP systems to resist compressive stresses, the compressive behavior of FRP materials is discussed below in the following paragraphs.
15 Coupon tests on FRP laminates used for repair on concrete have shown that the compressive strength is lower than the tensile strength (Wu, 1990). The mode of failure for FRP laminates subjected to longitudinal compression can include transverse tensile failure, fiber microbuckling, or shear failure. The mode of failure depends on the type of fiber, the fiber-volume fraction, and the type of resin. Compressive strength of 55, 78, and 20% of the tensile strength has been reported for GFRP, CFRP, and AFRP, respectively (Wu, 1990). In general, compressive strengths are higher for materials with higher tensile strengths, except in the case of AFRP where fibers exhibit nonlinear behavior in compression at a relatively low level of stress.
The compressive modulus of elasticity is usually smaller than the tensile modulus of elasticity of FRP materials. Tests reports on samples containing a 55 to 60% volume fraction of continuous E-glass fibers in a matrix of vinylester or isophthalic polyster resin have reported a compressive modulus of elasticity of 5,000 to 7,000 ksi (34,000 to 48,000 MPa) (Wu, 1990). According to reports, the compressive modulus of elasticity is approximately 80% for GFRP, 85% for CFRP, and 100% for AFRP of the tensile modulus of elasticity for the same product (Ehsani, 1993).
Physical Properties
A brief description of physical properties of FRP materials is given below:
Density- FRP materials have densities ranging from 75 to 130 lb/ft3 (1.2 to 2.1 g/cm3)
(ACI 440.2R-02, 2002), which is four to six times lower than that of steel (Table 1.3).
Coefficient of thermal expansion for FRP materials- The coefficients of thermal expansion of unidirectional FRP materials differ in the longitudinal and transverse directions, depending upon the types of fiber, resin, and volume fraction of fiber. Table
16 1.4 lists the longitudinal and transverse coefficients of thermal expansion for typical unidirectional FRP materials. Note that a negative coefficient of thermal expansion indicates that the material contracts with increased temperature and expands with decreased temperature. Concrete has a coefficient of thermal expansion that varies from
4 ´ 10-6 to 6 ´ 10-6/oF (7 ´ 10-6 to 11 ´ 10-6/oC) and is usually assumed to be isotropic
(Mindess and Young, 1981). Steel has an isotropic coefficient of thermal expansion of
6.5 ´ 10-6/oF (11.7 ´ 10-6/oC).
Effects of high temperatures- Beyond the glass transition temperature (Tg), the elastic modulus of a polymer is significantly reduced due to changes in its molecular structure.
The glass transition temperatures is defined as the midpoint of the temperature range over which an amphoras material (such as glass or high polymer) changes from (or to) a brittle, vitreous state to (or from) a plastic state. The value of Tg depends on the type of resin but is normally in the region of 140 to 180oF (60 to 82oC). In an FRP composite material, the fibers, which exhibit better thermal properties than the resin, can continue to support some load in the longitudinal direction until the temperature threshold of the fibers is reached. This can occur at temperatures near 1800oF (1000oC) for glass fibers and 350 oF (175 oC) for aramid fibers. Carbon fibers are capable of resisting temperatures in excess of 500oF (275 oC). Due to a reduction in force transfer between the fibers through bond to the resin, the tensile properties of the overall composites are reduced. Test results have indicated that temperatures of 480oF (250 oC), much higher than the resin Tg, much higher than the resin Tg, will reduce the tensile strength of GFRP and CFRP materials in excess of 20% (Kumhara et al., 1993). Other properties affected
17 by the shear transfer through the resin, such as bending strength, are reduced significantly at lower temperatures (Wang and Evans, 1995).
For bond-critical applications of FRP systems, the properties of the polymer at the fiber-concrete interface are essential in maintaining the bond between FRP and concrete.
At a temperature close to its Tg, the mechanical properties of the polymer are significantly reduced, and the polymer looses its ability to transfer stresses from the concrete to the fibers.
Time Dependent Behavior
The following sections present the time dependent behavior of FRP materials.
Creep-rupture- FRP materials subjected to a constant load over time can suddenly fail after a time referred to as the endurance time. This type of failure is known as creep- rupture. As the ratio of the sustained tensile stress to the short-term strength of the FRP laminate increases, endurance time decreases. The endurance time also decreases under adverse environmental conditions, such as high temperature, ultraviolet-radiation exposure, high alkalinity, wet and dry cycles, or freezing-and-thawing cycles.
In general, carbon fibers are the least susceptible to creep-rupture; aramid fibers are moderately susceptible, and glass fibers are most susceptible. Creep-rupture tests conducted at room temperature on 0.25 in. (6 mm) diameter FRP bars reinforced with glass, aramid, and carbon fibers indicated that a linear relationship exist between creep- rupture strength and the logarithm of time for all load levels (ACI 440.2R-02, 2002). The ratios of stress level at creep-rupture after 500,000 h (about 50 years) to the initial ultimate strength of the GFRP, AFRP, and CFRP bars were extrapolated to be 0.3, 0.47,
18 and 0.91, respectively (Yamaguchi et al., 1997). Similar values have been determined elsewhere (Malavar, 1998). In Table 1.5, sustained plus cyclic service load stress limit in
FRP reinforcement are presented. As long as the sustained stress in the FRP is below the creep rupture stress limits, the strength of the FRP is available for nonsustained loads.
Fatigue- A substantial amount of data for fatigue behavior and life prediction of stand- alone FRP materials have been generated in the last 30 years (National Research Council,
1991). During most of this period, aerospace materials were the primary subjects of investigation. Despite the differences in quality and consistency between aerospace and commercial grade FRP materials , some general observations on the fatigue behavior of
FRP materials can be made. Unless specifically stated otherwise, the following cases being reviewed are based on an unidirectional material with approximately 60% fiber volume fraction and subjected to tension-tension sinusoidal loading at:
· A frequency low enough to not cause self-heating
· Ambient laboratory environments
· A stress ratio (ratio of minimum applied stress to maximum applied stress) of 0.1;
and
· A direction parallel to the principal fiber alignment
Test conditions that raise the temperature and moisture content of FRP materials generally degrade the ambient environmental fatigue behavior. Of all types of FRP composites for infrastructure applications, CFRP is the least prone to fatigue failure. An endurance limit of 60-70% of the initial static ultimate strength of CFRP is typical. On a plot of stress versus the logarithm of the number of cycles at failure (S-N curve), the
19 downward slope of CFRP is usually about 5% of the initial static ultimate strength per decade of logarithmic life. At one million cycles, the fatigue strength is generally between 60 and 70% of the initial static ultimate strength and is relatively unaffected by the moisture and temperature exposures of concrete structures unless the resin or fiber/resin interface is substantially degradred by the environment.
For GFRP, a cyclic tensile fatigue effect of approximately 10% loss in the initial static strength per decade of logarithmic lifetime is observed (Mandell, 1982). This fatigue effect is thought to be due to fiber-fiber interactions and not dependent on the stress corrosion mechanism described for individual fibers. Usually, no clear fatigue limit can be defined. Environmental factors can play an important role in the fatigue behavior of glass fibers due to their susceptibility to moisture, alkaline, and acidic solutions.
Neglecting the rather poor durability of all aramid fibers in compression, the tension-tension fatigue behavior of an impregnated aramid fiber strand is excellent. Strength degradation per decade of logarithmic lifetime is approximately 5 to 6% (Roylance and Roylance, 1981). While no distinct endurance limit is known for AFRP, two million cycle endurance limits of commercial AFRP tendons for concrete
Table 1.5 Sustained plus cyclic service load stress limits in FRP reinforcement (ACI 440.2R-02, 2002) Fiber type Stress type GFRP AFRP CFRP
Sustained plus 0.22 ffu 0.30 ffu 0.55 ffu cyclic stress limit
20 applications have been reported in the range of 54 to 73% of the ultimate strength
(Odagiri et al., 1997). Based on these findings, Odagiri suggested that the maximum strsss be set to 0.54 to 0.73 times the tensile strength.
1.3 Project Objectives
To study the long-term durability issues related to strengthening with CFRP fabrics and plates, an experimental investigation was carried out with the following objectives:
1. To examine the response of beams strengthened with CFRP plates and fabrics before and after exposure to thermal environmental conditions such as freeze/thaw cycles, thermal expansion, and dry heat. 2. To investigate the behavior of the strengthened beams with CFRP plates and fabrics after exposure to humidity, salt-water, and alkali environmental conditions. 3. To study the behavior of concrete beams strengthened with CFRP plates and fabrics and subjected to repeated load testing. 4. To determine the strength reduction factors related to various environmental exposures. 5. To verify the allowable maximum strain values in CFRP fabrics and plates after exposure to various conditions. 6. To identify the different possible failure modes of reinforced concrete beams strengthened with CFRP fabrics or plates, and to relate them with various types of exposures.
It should be noted that the CFRP was chosen over the other FRP materials because of the fact that other FRP materials are significantly susceptible to aggressive environmental conditions. Moreover, the CFRP is the least susceptible to creep-rupture and has higher fatigue resistance characteristics in comparison to GFRP and AFRP. In addition, the CFRP has higher strength and stiffness and hence is a potential FRP material for civil engineering infrastructure under harsh environmental condition.
21 1.4 Experimental Investigation
For the purpose of this experimental investigation, 78 reinforced concrete beams of rectangular cross-section were constructed and strengthened with CFRP plates and fabrics. Beams were constructed in a 1:4 scale to simulate the beams commonly used for highway bridges. These strengthened beams were exposed to various environmental conditions. Accelerated ageing procedures following ASTM standards were used correlating strength and stiffness degradation. The test procedures were carefully selected to accomplish the objectives of the research investigation. In addition to three stainless steel tanks, an environmental chamber and a dry-heat chamber were constructed for the investigation. The two chambers and the three tanks were constructed following the ASTM specifications for the required testing procedures. Environmental exposures included the following:
1. salt-water 2. alkaline solution 3. repeated loading at 15%, 25%, and 40% of the strengthened beam’s ultimate load carrying capacity 4. dry heat conditioning at 140±3oF (60±2oC) 5. 100% humidity at 100±3oF (38±2oC) 6. freeze/thaw cycles between 0 and 40oF (-18oC and 4oC) 7. thermal expansion cycles between 80oF and 120oF (27oC and 49oC)
Beams exposed to salt-water, alkaline solution, hot water (100% humidity), and dry heat were tested under ultimate loading after 1000, 3000 and 10,000 hours of exposure. Beams tested for freeze/thaw were exposed to 350 and 700 cycles. Beams tested for thermal expansion condition were exposed to 35 cycles. All beams were tested under ultimate loading after full exposure. Beams subjected to 2,000,000 cycles of repeated loading under a load range of 15%, 25%, and 40% of the beam’s ultimate load carrying capacity were also tested to failure following the repeated load tests. The weight of the beams was recorded before and after exposure to different environmental conditions. The obtained results confirmed those formerly reported by earlier researchers.
22 After the completion of the experimental investigation, several crucial conclusions were established. In general, failure modes were carefully monitored and recorded by pictures, and they confirmed that any limitations in the carbon/epoxy system were due to the bonding agent (epoxy) matrix. In particular, moisture absorption by the epoxy matrix was the dominant characteristic in determining the strengthening durability of tested beams.
For both strengthening systems (plates and fabrics) and all environments considered in this investigation, the reduction in the load carrying capacity of the strengthened beams was less than 15% after exposure. However, beams strengthened with CFRP plates and exposed to 100% humidity (at 38o±2oC) for 10,000 hours experienced an average of 33% reduction in the load carrying capacity.
23 CHAPTER 2 LITERATURE REVIEW
2.1 General
The use of fiber-reinforced polymers (FRP) in civil infrastructure applications is increasing remarkably. FRP composites can be used as rebars (as an alternative to the traditional steel), seismic retrofit of concrete columns, wall panels and profiles, and in new bridge deck systems. External reinforcement for concrete structures by FRP materials represents another important and viable application where these materials can be employed. A considerable need to investigate the effects of various environmental exposures still exists. Many researchers have studied short-term durability earlier, as stated here, while issues concerning long-term durability necessitate more perfection and exactness that will be provided hereafter. Common environmental exposure conditions to be considered include dry heat, freeze/thaw cycles, alkaline solution, salt-water, repeated loading, and humidity. In this investigation, these types of exposures were considered using ASTM accelerated aging test procedures.
This chapter reviews some of the notable publications related to the effects of environmental conditions on FRP in general, and more specifically on carbon fiber reinforced polymer (CFRP) strengthened reinforced concrete (RC) structures. The chapter is divided mainly into several distinct sections. Sections 2.2 to 2.7 are concerned with various environmental effects studied in this investigation. The last section (section 2.8) covers some of the available materials design values.
2.2 Effect of Humidity
The resistance to corrosion is one of the major advantages of CFRP materials used in external strengthening of reinforced concrete structural members. That property promotes the use of CFRP in underground applications, waterways, water barriers, and other structures usually exposed to water. The effect of water on CFRP is due to the degradation of the bond between CFRP materials and the concrete; the carbon fibers are not affected by water. Water absorption first affects the resin, causing changes in the polymer structure; however, degradation can be retarded if complete curing of the resins is allowed. Hunston et al. (2000) recommended that the stress level in the composite, under sustained factored loads, should be less than 40% of the guaranteed design strength for CFRP.
Leung et al. (2001) investigated the flexural capacity of strengthened concrete beams exposed to different environmental conditions. They used 80 plain concrete beams strengthened with mild steel or carbon fiber reinforced polymer plates. Specimens were kept under water at 81oF (27oC) for 28 and 60 days. In this investigation, it was proved that strengthening with CFRP provides greater enhancement than strengthening with steel plates. Exposure to water for long periods caused the reduction of the load-carrying capacity as well as the midspan deflection.
David and Neuner (2001) tested two systems of carbon wet-lay-up and E-glass. They concluded that the exposure of 10,000 hours to 100% humidity at a high temperature caused a 36% loss in the tensile strength for an E-glass system, while the degradation for the carbon wet-lay-up system was much smaller. It was indicated that this loss could be due to the interfacial characteristics that involve the presence of silanes. They further concluded that the durability of the systems used in their investigation was highly dependent on the durability of the epoxy resin.
Karbhari and Engineer (1996) tested un-reinforced concrete specimens strengthened externally by carbon fiber reinforced polymers after immersion in water. Two different epoxy systems were used, both cured under ambient conditions. It was revealed that depending on the compatibility between the fiber and the resin and the characteristics of the resin system, degradation in strengthening efficiency, even over a short time period, can be significant.
Zheng and Morgan (1993) investigated the synergistic thermal-moisture damage mechanisms of epoxies and their carbon fiber composites. Reverse thermal effects were investigated after measuring the weight change of the epoxy resins and their carbon fiber
25 composites when immersed in distilled water at temperatures ranging between 33oF (0.5oC) and 176oF (80oC). It was determined that a critical temperature regime exists, above which the resin has the ability to absorb greater amounts of water.
Anstice and Beaumont (1982) studied the effects of the hygrothermal aging on mechanisms of fracture of glass and carbon fiber reinforced epoxies. In this investigation, fracture mechanisms of debonding, fiber-fracture, and pullout after exposure to various levels of humidity at 212oF (100oC), were considered. They also presented a life- prediction method and mechanism of damage anticipation based on fracture maps for different composites of glass and carbon fibers considered in that study.
Karbhari et al. (1997) presented a modified peel test method for assessing changes in bonding between composites and concrete. They exposed both glass and carbon fibers with two different epoxies to fresh water and synthetic seawater, at a temperature of 4oF (-15.5oC), and freeze-thaw conditions. Two different modes, mode I and mode II of critical energy release rates, were considered in this investigation. It was concluded that the exposure to aqueous solutions had a significantly deleterious effect, indicating that most of the degradation was at the level of the epoxy layer between the composite and the concrete. Carbon fiber systems experienced lower affects than those of the glass fiber system for both modes I and II.
Woo and Piggot (1988) investigated the diffusion of water in carbon and E-glass reinforced epoxy composites fabricated by a pultrusion process over a range of temperatures. They indicated that a linear relationship between weight gain and temperature had been established. This demonstrated that both types of fibers acted as barriers to diffusion. Preferential absorption of water at the fiber-matrix interface was illustrated. The effect was considerably larger in glass fiber composites than in CFRP specimens, irrespective of the type of resin used.
2.3 Effect of Dry-Heat
FRP composites are exposed to thermal effects both during their fabrication and their usable life when strengthening concrete structures. The response of the components
26 of composite systems, mainly the resins and the adhesives, should be considered and studied separately. FRP composites subjected to high temperatures experience an initial post-cure in the first stage followed by degradation in thermal effects increasing with time. The coefficient of thermal expansion of the thermosetting resins (polyester, vinylesters, and epoxies) is about 3 to 10 times that of plain concrete, i.e., the coefficient of thermosetting resins ranges between 40 to 90 x 10-6/oC (Agarwal and Broutman, 1990), while that of thermal expansion of plain concrete is about 10 x 10-6/oC. It was noted that the variation of thermal expansion coefficient of plain concrete depends on the used aggregate (Tia et al. 1991, Beer and Johnston, 1992). The differential coefficient of thermal expansions of concrete and resin may lead to the breaking of the bond between the FRP and concrete at a higher temperature. Juska et al. (2000) analyzed the available data related to the effects of various types of thermal exposures, and presented a gap analysis. They concluded that the most important exposures to be considered are the elevated high temperatures and the freeze/thaw conditions. They also presented results of the analysis concerning the importance of data in relation to both elevated temperatures and freeze/thaw conditions. It was concluded that for the elevated temperatures, all the data in connection with prolonged exposures and thermal cycling is considered to be of great importance. Finally, they suggested that FRP composites should not be used at temperatures above their glass transition temperature (Tg), and they recommended that selected materials should have a o o Tg at least 86 F (30 C) above the maximum use temperature. Ferrier and Hamelin (1999) tested concrete slabs joined by two symmetrically arranged carbon/epoxy composite strips bonded to the slabs. Three different types of epoxy polymers with glass transition temperatures of 104oF (40oC), 122oF (50oC), and 140oF (60oC) were used. Elevated temperatures caused a degradation of static tension and tensile creep responses. The polymer with the highest Tg experienced the minimum decrease from the mechanical response. They recommended using a polymer with Tg o o above 122 F (50 C) for civil engineering applications. Almusallam et al., (2001) investigated the durability and long-term behavior of reinforced concrete beams strengthened with FRP composites. GFRP was used to
27 strengthen ninety-nine beams of 150x150x1200 mm (6x6x48 inch), constructed and exposed to hot-dry and wet-dry conditions for twelve months. It was found that the exposure to high temperatures caused no significant degradation in the flexural strength of the tested beams. In the report prepared by the “Fédération Internationale du Béton (FIB)”, 2001, an investigation conducted by Pantuso et al. (2000) was presented. In this investigation, the behavior of strengthened specimens with two types of unidirectional CFRP strips (E=175 GPa and E=300 GPa) was studied at three different temperatures: -148oF (-100oC), -22oF (-30oC), and 104oF (40oC). It was concluded that the test specimens reinforced with high elastic modulus strips showed a greater reduction in the ultimate bond force compared to the specimens reinforced with low elastic modulus strips. The Aerospace Corporation presented the report (ATR-99 (7524)-2, 1999) where it was indicated that elevated temperatures cause reduction in tensile strength of FRP composites by about 20% at 140oF (60oC). Similarly, Kshirsagar et al. (1998) concluded that the dry-heat conditioning might cause more brittleness in FRP composites. Steckel et al. (1998) tested twelve composite overwrap systems against environmental durability. The systems included carbon/epoxy materials. Systems were exposed to dry heat at 140oF (60oC) for 1,000 and 3,000 hours. No loss of property was observed regarding stiffness, strength, or failure strain. Mosallam and Dutta (2001) studied the behavior of epoxy adhesives for repair and rehabilitation of concrete structures under extreme temperature environments. Four different types of adhesives were considered in this study, at five cure temperatures, namely: 120oF (49oC), 73oF (23oC), 120oF (49oC), 25oF (-4oC), and -30oF (-34oC). It was concluded that the strength of an adhesive bond is entirely dependent on cure time and temperature, with temperature playing the more significant role. The increase in both time and temperature was found to be a major factor that improves the epoxy bond strength.
2.4 Effect of Alkaline Environment
FRP composites can be exposed to various sources of alkaline environments. Alkaline chemicals, concrete pore solution, and soil represent the main sources of
28 alkaline attacks to the FRP composites. Resins in FRP composites can protect the fibers from the alkaline attacks, but not for long periods of time, especially if the FRP fibers were not completely cured. Alkaline solution can accelerate the degradation of some types of resins and also the bond between the concrete and the FRP materials. Benmokrane et al. (2000) prepared a gap analysis for the availability and importance of data concerning the effects of alkaline environments on FRP composite materials used in civil infrastructure applications. They found that data related to alkaline exposures under wet conditions and in absence of stress is still sparse and questionable, while for other types of alkaline exposures data is widely available and also validated. It was indicated that exposure to alkaline environments causes degradation of both stiffness and strength values of various FRP composites. Sen (2001) studied the durability of advanced composites in a marine environment. He tested reinforced concrete beams strengthened with carbon fibers, glass fibers, and aramid reinforced polymers. Specimens were pre-cracked prior to their exposure to wet/dry cycles in an alkaline environment. Specimens were tested to ultimate load after 36 months of exposure in the case of carbon fibers and aramid, while the duration of exposure was of 20 months for glass fibers. Beams strengthened with carbon fiber polymers maintained the same level in ultimate capacity when compared to the unexposed beams. The epoxy resin was ineffective to protect the glass fiber in the case of concrete’s alkaline environment. The bond between aramid and concrete was only affected when the system was exposed to tidal/thermal cycles. That was explained by the difference of the thermal expansion coefficients of the aramid and the concrete. According to the report prepared by the “Fédération Internationale du Béton (FIB)”, (2001), carbon fiber is considered resistant to alkali and acidic environments, whereas glass fiber can degrade in the same environments. It was recommended to apply a well-selected resin matrix to isolate and protect the fiber and to postpone the deterioration. GangaRao et al. (2001) studied the aging of the bond between FRP and concrete cubes. In their experimental investigation, they used carbon and glass fabric strips bonded to four-inch concrete cubes by epoxy resin. Specimens were subjected to an alkaline solution of a pH varying from 3 to 13 for periods of up to 3 months. They concluded that
29 the average variations in bond strength of specimens, for both glass and carbon, under accelerated aging were found to be less than 10% of the unaged. Chin et al. (1998) studied the environmental effects on composite matrix resins used in construction. Specimens were exposed to an alkaline solution combined with a high temperature of 194oF (90oC). After ten weeks, specimens were severely degraded and changes in glass transition temperatures and tensile strengths were observed. Subsequent to the immersion in an alkaline solution, examination of the polymers showed no ionic penetration into the bulk of the polymer. GangaRao and Vijay (1997) have described the accelerated ageing response of glass fiber reinforced plastic structural composite (GFRP) bars and plates, concrete beams reinforced with GFRP bars, and concrete beams wrapped with CFRP fabrics. The accelerated ageing parameters considered included sustained stress level, pH, temperature, and humidity variations. In addition, accelerated testing procedures correlating strength and stiffness degradation were presented in terms of master curves for durability of structural composites. Chin et al. (1997) conducted research to study chemical and mechanical changes in polymer matrix composites following exposure to various environmental effects. They had an ultimate goal to identify factors that contributed to matrix resin degradation under environmental conditions. Epoxy vinylesters and isophthalic polyester resins were selected to be studied in this investigation. Samples were immersed in an alkaline solution having a pH value of 13.5 for 1300 hours. No significant changes in tensile strength were observed. Chin et al. evaluated and presented all changes that resulted from exposure to the alkaline environment in strength, viscoelastic response, and thermal properties. Rahman et al. (1998) investigated the mechanism of deterioration of FRP reinforcement for concrete. Specimens were prepared following a procedure similar to that used in the actual manufacture of FRP grid. Both carbon and glass fibers were used to prepare the specimens. Specimens were exposed to alkaline solutions under a tensile load of 50% or 30% of their ultimate strengths for 45, 122, and 370 days. Glass fiber specimens failed prior to the end of the 45 days. Carbon fiber specimens showed signs of physical damage in the form of etching, cracking, and breaking of the resin.
30 2.5 Effect of Repeated Load
Fatigue due to repeated load is defined as the phenomenon causing the failure of a material or component after the application of a repeated load cycles, even though the level of that load is not high enough to cause failure on the first cycle (Lesko et al. 2001). Fatigue life is usually measured as the number of cycles to failure for a given applied load level. Lesko et al. indicated that the degradation and failure of bridges are usually associated with cyclic dynamic loading. Gheorghiu et al. (2001) examined reinforced concrete beams externally strengthened with unidirectional CFRP strips. Beams were exposed to either wet/dry cycles or to continuous immersion in water, then loaded in fatigue and tested quasi- statically to failure under four-point bending. The ultimate loads were not significantly different as a result of the environmental exposure. It was shown that the delamination of the interface was more progressive for the exposed specimens than for the control beams. For the design purposes of RC structures reinforced externally with FRP materials, the “Fédération Internationale du Béton (FIB)” (2001), studied the effects of repeated loads on the CFRP materials. It was found and confirmed that the CFRP materials exhibit a superior fatigue performance to that of steel. Furthermore, it was recommended that the criteria for the fatigue design of the CFRP strengthened beams should be limited to the stress limits of the steel rebars, and to neglect the high properties of the FRP as if the beams were not strengthened. Ferrier et al. (1999) studied the fatigue behavior of CFRP composite reinforcement for concrete structures. For the purpose of this investigation program, specimens composed of two concrete blocks connected by a carbon fabric and epoxy polymer were fabricated. Specimens were subjected to cyclic shear stresses until failure. The investigation illustrated that the average shear stress for a double lap joint is 0.80 MPa for a lifetime of 106 cycles under a loading of 1 s-1. It was concluded that the average shear stress for carbon epoxy composite plate is 550 MPa for the same lifetime. Mandell et al. (1981) examined the fatigue performance of glass fiber dominated composites. One of the tested composites (0/90o crossply glass epoxy) experienced 10% loss in strength when subjected to 106 cycles of fatigue cycling. Woven fabric (style 181
31 polyester E-glass) lost 17% of its strength after exposure to 104 cycles of fatigue, but leveled out at 4% strength loss for the same number of cycles of fatigue. Static fatigue (creep) of E-glass strands has been seen to have 3.6% strength reduction after being exposed to fatigue conditions for 106 cycles. Another investigation done by Mandell et al. (1985) showed that for a 30-strand 25- mm diameter E-glass fiber bundle with and without epoxy matrix, fatigue caused 10% strength reduction per million cycles. Ten percent has been reported for the standard 9 mm diameter E-glass fiber impregnated strand after being exposed to 106 cycles of repeated loading. Single 25 mm diameter E-glass fiber in dynamic fatigue had 3-5% strength reduction for the same number of cycles that was reported in static fatigue tests. Conners et al. (1979) determined that soaking graphite epoxy for 13 weeks at 180oF (82oC) in distilled water had very little effect on the fatigue performance. Denton studied the tensile cyclic and static fatigue on SMC-R50% fiber volume, taking into consideration thermal effects. It was noted that the relationship between the tensile modulus reduction and the fatigue stress was linear at a temperature of 73oF (23oC).
2.6 Effect of Freeze/Thaw
Due to the large difference in thermal expansion coefficients of CFRP and concrete (by a factor of 10 for CFRP), changes in temperature can generate significant shear stresses between the concrete and the CFRP fabrics and plates. These stresses may cause bond deterioration. It was reported by different researchers that matrix hardening, matrix microcracking, and fiber matrix bond degradation could occur due to the exposure to temperatures below zero. Presence of salt when combined with freeze/thaw conditions can accelerate the degradation of composite systems due to the formation and expansion of salt deposits in addition to the moisture induced swelling and drying effects (Juska et al., 2000). In this paper, Juska et al. presented a gap analysis and indicated that data related to thermal gradients of FRP composites in the case of freeze and freeze/thaw conditions was still questionable and not available. They presented different test methodologies related to the material operational limit of laminating resin, debonded laminates, torque for composite/concrete bond strength, and material operational limits of
32 adhesion. It was concluded that the freeze/thaw conditions could potentially result in debonding of laminates, either from concrete, or from other FRP composite elements, particularly in the case of gaps presence at the adhesive bond line.
Bisby and Green (2002) conducted an investigation using 39 under reinforced concrete beams strengthened externally by bonding FRP sheets to the tension side. Three types of CFRP sheets, from three different manufacturers, in addition to one type of GFRP, were tested during that investigation. Beams were subjected to freeze-thaw conditioning of to 200 or 300 cycles. The results obtained showed that the exposure to freeze-thaw cycles did not decrease the bond between the FRP sheets and the concrete.
Lopez et al. (1999), tested 36 concrete beams strengthened with glued-on CFRP laminates in bending after exposure to freeze-thaw cycles. Exposure to freeze-thaw cycles was carried out using ASTM C666 procedure B. Different sets of beams were exposed to different numbers of freeze-thaw cycles (0, 100, 200, and 300 cycles). Some of the beams were pre-cracked while the others were kept uncracked. It was noticed that both the moment capacity and the ultimate deflection decreased with the increase of the number of cycles. Pre-cracked beams suffered from a larger decrease than that of the uncracked beams.
Yagi et al. (1997) investigated the deterioration of bond strength between CFRP sheets and the concrete, and in the tensile strength of CFRP materials due to the exposure to freeze-thaw cycles. RC beams were wrapped with CFRP sheets and exposed to freeze/thaw cycles for periods of up to 3 years prior to ultimate load tests. The results of this investigation showed that there was no loss in strength due to the exposure to freeze- thaw cycles. Some deterioration occurred to the outermost layer of the epoxy resin. This deterioration did not reach the interior of the resin underneath the CFRP sheets. The results confirmed that the strengthened beams didn’t encounter any significant weakening because of the exposure to freeze/thaw.
The report prepared by “Fédération Internationale du Béton (FIB)” (2001) discussed the effects of freeze/thaw cycles on the durability of FRP externally bonded
33 reinforcement. Previous literature presented in this report confirms that freeze/thaw cycles has no effect on FRP materials, however, it increases the concrete crack width, causing deterioration to the bond between concrete and FRP. Problems resulted from freeze/thaw cycles were greater in cases where the quality of concrete is poor.
Toutanji and Rey (1997) investigated the durability characteristics of concrete columns wrapped with FRP tow sheets. Specimens consisted of concrete columns wrapped with carbon and glass FRP composite sheets, and were subjected to dry-wet and freeze-thaw conditions. Specimens wrapped with GFRP sheets and exposed to freeze/thaw cycles demonstrated 28% reduction in their strength, while specimens wrapped with CFRP sheets exhibited a maximum strength reduction of 19%.
Steckel et al. (1998) examined the performance of twelve composite overwrap systems against environmental durability. Systems included carbon/epoxy materials exposed to freeze/thaw cycles between 0oF (-18oC) and 100oF (38oC). No loss of property was observed with regard to stiffness, strength, or failure strain, although one carbon system showed a 23% reduction in short-term shear stress.
Weiss (1982) studied low temperature properties of carbon fiber reinforced epoxide resins. The strength, stiffness and thermal properties were monitored over the range of 68oF to -320oF (20oC to -195oC). After lowering the temperature from 68oF (20oC) (room temperature) to -320oF (-195oC), Young’s modulus rose by 7%, while Poisson’s ratio decreased 6%. It was also determined that the fracture strain is 10% larger at -320oF (-195oC) than at room temperature.
Dutta (1992) examined the longitudinal tensile strength of the unidirectional fiber- reinforced polymer composites. In his research investigation, he considered the effects of cold thermal cycling on a number of composite materials (glass fiber and glass-epoxy). Dutta stated that high residual stresses occurred in composites at low temperature because of the large differences in coefficients of thermal expansion of the constituent materials. He detected degradation in the strength related to low temperatures. Hawkins et al. (1998) tested freeze/thaw resistance of composites designed for seismic retrofit of bridge
34 columns. Composite systems were subjected to 20 cycles of 24 hours each at 100% humidity, between 0oF (-18oC) and 100oF (38oC). No degradation of composites was observed.
Green et al. (1998) investigated four different products to evaluate the effects of freeze/thaw cycles on the bond between FRP sheets and the concrete. Beams were subjected to 50, 150, or 300 freeze/thaw cycles. Each cycle was divided into 16 hours of freezing at 0oF (-18oC), then 8 hours of thawing at 59oF (15oC). The results showed that no bond deterioration was experienced due to freeze/thaw.
2.7 Effect of Salt-Water Solution
Many of the civil engineering infrastructures are in close proximity to the sea and subjected to salt effects in addition to high humidity. Many researchers studied the effect of salt water on the FRP composites to determine the level of degradation related to that particular condition. Some of these results are presented hereafter.
Sonawala and Spontak (1996) tested two systems of glass fiber composites immersed in brine over a period of 270 days. The tested vinylester system showed more relative resistance than the polyester system. A significant deterioration in tensile strength occurred in the polyester system. Karbhari and Engineer (1996) examined unreinforced concrete specimens strengthened externally by both carbon and glass fiber reinforced epoxy. Specimens were subjected to immersion in seawater at ambient conditions. Two different epoxy systems were used. It was shown that the compatibility between the fiber and resin, and the properties of the resin are the major factors affecting degradation in strengthening efficiency.
Searle and Summerscales (1999) investigated the structural integrity of composite laminates in a marine environment. They also discussed aspects related to damage initiation, moisture absorption, blistering, and property degradation of E-glass and carbon reinforced composites. They provided some recommendations concerning the gel coats to be used to protect the composite systems, and the selection of the most suitable resin and gel coat for each particular condition.
35 2.8 Materials Design Values
Fiber-reinforced polymer composites used in civil infrastructures are significantly different in material form and process than the autoclave-cured prepreg-based components used in aerospace and defense sectors. A well-established materials database is required for routine civil engineering design. Moreover, generic material properties useful for the purpose of material selection and conceptual or even preliminary ‘back-of- the-envelope’ design are not in a form for final design and stress analysis until they are reduced to design allowables and/or reported in a format that makes it easy to use directly in the design. For most materials, such as steel, aluminum, titanium, or alloys, it is easy to obtain the design allowables because the number of viable alternatives for use is relatively small. However, the myriad combinations and configurations possible with FRP composites make the determination of design allowables for FRP a relatively extensive and expensive task. There are a number of common methods used for the determination of design allowables or even safety factors, primarily based on the application area, some of which are described below.
2.8.1 Aerospace-based Determination
The use of composite materials in the aerospace industry is predicted through the establishment of design allowables through extensive programs of specification and qualification. The two most commonly used basis levels are A-basis and B-basis allowables (Kharbari and Seible, 1997). The A-basis allowable is the level of mechanical properties above which at least 99 percent of the population of values is expected to fall with a confidence level of 95 percent. In comparison, the B-basis allowable is the level of mechanical property above which at least 90 percent of the population of values is expected to fall with a confidence level of 95 percent. The basis values are based on a minimum of 30 specimens (MIL-HDBK-17, 1997) from at least five batches of material per environmental condition and direction.
In the aerospace world, safety factors used can approach one in some cases, (such as fairings). Due to that reason, the need to ensure a very high level of specification, qualification, and testing exists. The method implemented to select and test new materials
36 is very rigorous, but does not allow for ease in replacement of constituent materials. Even minor modifications to qualified material systems can only be made after extensive testing, thus, this method, when applied to an emerging applications area, can stifle growth and/or field adoption of better forms of materials in a timely fashion.
2.8.2 Naval and Marine-Based Determination
The FRP composite materials have been extensively used in naval and marine applications and for the most part their use is predicated through the adoption of a safety factor of 4-6 (Puccini, 1993, Gibbs and Cox Inc., 1960) based on the combined effects of stress rupture and fatigue. Note that hulls face a greater degree of loading than a number of thin-skin composite elements in other application areas; hulls in general are not subjected to sustained loading but rather to impact and slamming loads of short and intermittent duration (Puccini 1993).
In almost all cases, durability of marine structures for long term use is dependent on the use of an appropriate gel coat, without which there is a realistic fear of premature degradation through surface blistering. This can lead to further deterioration and delamination of the FRP composites, especially those fabricated using orthophthalic polyester. With the use of an empirical safety factor of 4-6, as noted earlier, and the use of gel coats and regular maintenance of surfaces, along with careful attention to detailing and processing, marine vessels fabricated from FRP composite materials are noted to have long service lives.
2.8.3 Determination in Infrastructure Applications
Many FRP structures are designed using the basis of allowables being not greater than ten percent of the ultimate strength of the FRP material (Karbhari, 2000). This level is based on the use of brittle resin systems and E-glass reinforcements. However, for use in diverse systems, guaranteed values of material properties are used. These guaranteed values of performance lie within a specified percentile of ultimate performance level. As per the standards provided by the “Fédération Internationale de la Précontrainte” (1999), characteristic value for design (guaranteed material property) is taken as the mean minus
37 1.65 times the standard deviation (i.e. ninety-fifth percentile). As per the Japan Society of Civil Engineers (JSCE), guaranteed value is defined as the mean minus three times the standard deviation of a test population.
Byars et al. (2001) compared different design codes and guidelines in Japan, UK, Norway, USA, and Canada. They concluded that:
· The JSCE design guidelines (1997) use a single factor that incorporates several uncertainty aspects including environmental durability. Stress limits for sustained stress are used in the JSCE design codes. · The UK design guidelines (published by the British Institute of Structural Engineers, 1999) deal with the environmental degradation of FRP by using one factor that takes account of the influence of environment, sustained stress and a few other uncertainties. · The Norwegian standard NS3473 (Dejke, 2001) and the ACI design guidelines (1998 and 2000) have an exclusive factor for environmental deterioration. · In the Canadian design guidelines (1996), liberal stress limits/design strengths are adopted, completed by restrictions in the use of certain types of FRP in some applications. There are also restrictions on the selection of resin types for FRP reinforcement.
More design issues related to the environmental effects were presented in the technical report on the “Design and use of externally bonded fiber reinforced polymer reinforcement (FRP EBR) for reinforced concrete structures,” (2001) prepared by a working party of the task group 9.3 FRP reinforcement concrete structures. This report stressed the fact that the presence of moisture represents a particularly harsh parameter for all structural materials, especially for the resin matrix used to bond the FRP materials to the concrete. The report clearly stated that carbon fiber is relatively inert to clear water, but stated that the only effects of water are those of the moisture on the resin matrix.
38 CHAPTER 3 EXPERIMENTAL PROGRAM
3.1 Outline of the Experimental Program
For the purpose of examining the effects of different environmental conditions on CFRP strengthened reinforced concrete (RC) beams, seventy-eight test beams were fabricated. Figure 3.1 shows the various types of environmental exposures considered, number of tested beams for each type of exposure, exposure duration, and the ASTM specifications used. For each type of environmental exposure, half of the test beams were strengthened with CFRP plates and the other half with CFRP fabrics. Environmental exposures included 100% humidity at 38±2oC, dry-heat at 60±2oC, alkaline solution at pH of 9.5, salt-water (substitute ocean water). Cyclic freeze/thaw tests, and cyclic thermal expansion test were conducted. Beams exposed to freeze/thaw condition were subjected to 350 and 700 cycles of freeze/thaw, for a temperature ranging between 40 and 0oF (4oC to –17.8oC). Beams tested for thermal expansion conditioning were exposed to 35 cycles, for a temperature varying between 80±2oF (26.7±1.5oC) and 120±2oF (48.9±1.5oC). Repeated loading at 15%, 25%, and 40% of the beam ultimate carrying capacity was also considered in the investigation. Two types of CFRP strengthening systems were considered in this investigation. These types were CFRP plates and CFRP fabric systems. Currently, both systems are used commercially to increase the load carrying capacity or to reduce the deflection of beams. As shown in Figure 3.1, two unstrengthened beams (beams without CFRP strengthening) and four baseline beams (two beams strengthened with CFRP plates and the other two strengthened with CFRP fabrics), were tested under ultimate load without exposure to any environmental conditions. The purpose of these tests were to determine the strengthening effect due to the use of CFRP fabrics and CFRP plates prior to the exposure to any environmental condition. In addition to these six beams, four sets of twelve beams were each subjected to one of the following conditions: 100% humidity (hot water), dry-heat, alkaline solution, and salt-water conditionings. For each set of twelve beams exposed to these environmental conditions, four beams were exposed for 1000 hours, four other beams for 3000 hours, and the remaining four beams for 10,000 hours durations. It should be noted that each twelve beam set consisted of six beams strengthened with CFRP plates and six beams strengthened with CFRP fabric sheets. Twelve beams were tested for evaluating the repeated load effects. Three sets, each composed of 4 beams, were subjected to 2 million cycles of repeated load applied at 3.25 Hz, to study the fatigue response of the strengthened beams. The first, second, and the third sets of beams were subjected to a load range corresponding to 15%, 25%, and 40% of the ultimate load carrying capacity of the strengthened beam, respectively. In addition, eight test beams were exposed to freeze/thaw cycle effects. Four beams were exposed to 350 freeze/thaw cycles of four hours duration, while the other 4 beams were subjected to similar 700 freeze/thaw cycles. To investigate the effect of the thermal expansion on the CFRP strengthened beams, an additional set of four beams was subjected to 35 thermal cycles, each of five hours duration. During the exposure to the different environmental conditions, all beams were closely observed to monitor the discoloring and any change in the external appearance. All the beams were instrumented and tested under static ultimate loading after the completion of desired duration of environmental exposure. The experimental data such as loads, deflections, and CFRP strains were recorded and analyzed.
3.2 Construction Details of Test Beams
3.2.1 Reinforcement Details
Seventy-eight beams of rectangular cross-section [6 x 10 in. (152 x 254 mm)] (Figure 3.2.b) were supplied to the Structural Testing Center of Lawrence Technological University by United Precast Inc., Ohio. The beams were 9 ft (2.74 m) long (Figure 3.2.a). Concrete mix design providing a characteristic 28-day strength of 4.5 ksi (31 MPa) was used. Average actual strength for concrete cylinders was found to be exceeding this value by 10 to 20%. The flexural reinforcement consisted of two #5 bars (15.9 mm diameter) at the bottom, and two #3 bars (9.5 mm diameter) bars at the top of
40 the beam. Shear reinforcement was provided in the form of two-legged rectangular stirrups with standard hooks. Stirrups were made of #3 bars (9.5 mm diameter) bars. The stirrups were 4 in. (100 mm) wide, and 8 in. (200 mm) in depth. The center-to-center spacing of the stirrups was 4 in. (100 mm). Beams were cast using metal forms as shown in Figure 3.3. All steel bars used had a characteristic strength of 60 ksi (414 MPa)
3.2.2 Application of CFRP fabrics and CFRP plates
Beams strengthened with CFRP fabrics had two layers of fabrics installed, while only one plate was installed for the beams strengthened with CFRP plates. This configuration allowed maintaining the nominal load carrying capacity constant for both types of strengthened beams. Beams strengthened with CFRP plates were directly supplied to the Structural Testing Center of Lawrence Technological University in ready-to-test state. United Precast Inc., Ohio, fabricated these beams while Fiber Reinforced SystemsTM (FRSTM)1 completed the installation of the CFRP plates. CFRP fabrics were applied to the beams at the Structural Testing Center. Installation procedures for both the CFRP fabrics and CFRP plates are outlined below.
CFRP Fabrics Installation Procedure The Fiber Reinforced Systems (FRSTM) CFRP fabrics were bonded to the test beams as per the instructions provided by the manufacturer. All irregularities found on the concrete surface of the beam were removed using a hand grinder and a masonry- grinding wheel. Surface was sand blasted to allow ensuring proper bonding for the CFRP fabrics. Structural epoxy supplied by the same company (FRSTM) was used to fill voids and low spots on the surfaces of the beams and was allowed to cure for 24 hours. FRSTM structural epoxy has a tensile strength of 8.8 ksi (60.7 MPa), adhesion greater than 290 psi (2 MPa), average flexural strength of 14.5 ksi (100 MPa), and a flexural modulus of 310 ksi (2140 MPa). The preparation of the structural epoxy is shown Figure 3.4, while its application on the bottom surface of the beam is shown in Figure 3.5. Material
1 FRSTM: FIBER REINFORCED SYSTEMSTM, 4636 Shuster Road, Columbus, Ohio, 43214.
41
Table 3.1 Mechanical properties of CFRP strengthening materials. Property CFRP Plates CFRP Fabrics
Width, in. (mm) 3.0 (76.2) 6.0 (152.4)
Thickness, in. (mm) 0.047 (1.2) 0.007 (0.2)
Average Modulus of Elasticity, msi (GPa) 20 (138) 33 (227 MPa)
Average Ultimate Strain, percent 1.5 1.8
Average Ultimate Tensile Strength, ksi (MPa) 300 (2070) 400 (2758)
42 properties of CFRP fabrics and CFRP plates, as provided by the manufacturer are presented in Table 3.1. Two layers of fabric of 0.007 in. (0.2 mm) thick, 6 in. (152 mm) width, and 88 in. (2235 mm) length were used for strengthening of each beam. The first layer of the CFRP fabric was bonded on the prepared surface of the concrete as shown in Figure 3.6. Subsequently, the structural epoxy was applied on the top of the first layer of the CFRP fabric (Figure 3.7). Figure 3.8 shows the placement of the second layer of the CFRP fabric bonded with an epoxy coat to the first layer. Hand rollers, fabricated from steel cover by cotton cloth, were used to properly bond the sheets together as shown in Figure 3.9 and to eliminate any trapped air between the two layers.
CFRP Plates Installation Procedure
The beams strengthened with CFRP plates were prepared in the same way as the beams strengthened with CFRP fabrics. The CFRP strips were of 3 in. x 0.047 in. (76 mm x 1.2 mm) cross-section. A batch of FRSTM structural epoxy (prepared as per manufacturer’s instructions) was used to fill the voids and low spots on the surface of the beams to be strengthened with CFRP plates. The concrete surfaces treated with epoxy were cured for 24 hours prior to the installation of the CFRP Plates. The CFRP plates were cut to an 88 in. (2235 mm) length with a die grinder and an abrasive cut-off wheel. Wiping the CFRP plate surface with an acetone-soaked cloth ensured removal of all carbon dust. Using a trowel, a thin (1/16 in. or 1.6 mm) coat of epoxy was applied to the surface of the concrete beams in the area where the plates were to be bonded, as well as to the bonding surface of the CFRP plates. The plates were then applied to the epoxy coated beam surface and rolled out with a roller to ensure proper bonding.
3.3 Environmental Conditioning and Test Procedures
All the beams were weighed prior to the start of exposure to any of the environmental conditions. They were also weighed after the completion of the exposure duration before the testing to the ultimate failure load. Two Ohaus Corporation2 weighing indicators, models CD-31 and CD-33, were used. Each indicator had a capacity of up to
2 Ohaus Corporation, 29 Hanover road, Florham Park, NJ 07932
43 199,999 pounds, and a tolerance of 0.05%. Figure 3.10 shows one of the beams while weighing it on the scales. Three stainless steel tanks were designed and constructed to allow the exposure of the beams to 100% humidity, alkaline solution, and salt-water conditions. Each tank was designed to accommodate 12 beams arranged in three rows of four beams each. Each row consisted of four beams, two beams strengthened with CFRP plates, and two beams strengthened with CFRP fabrics. Beams were spaced apart using three 4 in. x 4 in. (100 mm x 100 mm) treated lumber blocks to ensure an adequate soaking (Figures 3.11 and 3.12). Beams were arranged in such way that the top row of beams is removed after 1000 hours, while the middle row is removed after 3000 hours. The bottom row of beams was removed after 10,000 hours. Each of the three tanks was 4 ft wide, 10 ft long, 4 ft high, (1.22 m x 3.05 m x 1.22 m). Figure 3.13 illustrates the tanks placed in the durability testing facility. Each tank was equipped with the following: - Heating blanket placed underneath the bottom surface of the tank to maintain a constant temperature as specified by the ASTM standards. The heating blankets were supplied by BenchMark Thermal3, in form of copper elements covered by silicon rubber with a high-strength woven fiberglass core as standard sheath material. - Two submersible pumps positioned at opposing corners of the tank to allow for adequate water circulation and to ensure that the water temperature will remain constant at all points in the tank. These pumps, model PES-WG, were manufactured by Little Giant Pump Company4. The lifting capacity of each pump was 20 gallons/minute (75.5 liter/min).
- Two thermocouples to continuously monitor and maintain fluids temperature inside the tank at the required temperature. The thermocouples acted as a switch turning the heating system on and off due to change in the temperature of the tank.
Appropriate ASTM standards were closely followed as discussed hereafter. Details of different environmental conditionings are outlined in the following sections.
3 BENCHMARK THERMAL, P.O. Box 1799, Grass Valley, CA 95949 4 Little Giant Pump Company, P. O. Box 12010, Oklahoma City, OK 73157
44 3.3.1 100% Humidity Exposure
In order to examine the effect of water on the reinforced concrete beams strengthened with CFRP plates and fabrics, twelve beams were exposed to 100% humidity conditioning as per the procedures of ASTM D2247 (1997). The twelve beams consisted of six beams strengthened with CFRP plates and six beams strengthened with CFRP fabrics. The aqueous solution tank was prepared by cleaning of all extraneous material and then filled with regular tap water of approximately 1100 gallons (4100 liters). The water was then promptly warmed to a temperature of 38o±2oC (100o ± 3oF) using the heating blanket as described earlier. Beams were weighed, physically inspected, and photographed prior to their placement into the prepared tank. Beams were then carefully placed into the tank and excess water was allowed to overflow the sides of the tank (Figure 3.14). Beams were exposed to humidity for durations of 1000, 3000, and 10,000 hours. After the completion of the duration of exposure to humidity conditioning for each set of four beams (placed together in one row), they were weighed, instrumented, and tested to failure under four-point flexural ultimate loads.
3.3.2 Dry-Heat Exposure To examine the effect of dry-heat conditioning on the beams strengthened with CFRP plates and fabrics, twelve beams were subjected to different durations of dry-heat exposure. The dry-heat conditioning followed the procedure of ASTM D3045 (1992). Specially designed and manufactured dry-heat chamber was used to meet the ASTM D3045 (1992) requirements. The dry-heat chamber was fabricated by Russells Technical Products5 using insulated stainless steel panels. The dry-heat chamber was 5 ft wide x 11 ft long x 7 ft high (3.4 m x 1.5 m x 2.1 m), made of stainless steel insulated walls. The heating system was accomplished by electric resistance heater elements. The elements responded rapidly to instrument demand with minimum residual heating effects minimizing overshoot. The heaters were staged as required by the above-mentioned ASTM, for ease regulation of heat input during varying electric load and temperature conditions. The heaters were baffled from the test space to prevent direct radiation to the
5 Russells Technical Products, 1145 S. Washington Ave., Holland, MI 49423
45 beams on test. The heaters were easily accessible for maintenance or removal. Blowers were used to provide air circulation within the conditioned area. The blowers were designed to assure proper air distribution and prevent air stratification throughout the chamber. Prior to exposure to the dry-heat conditioning, twelve beams (six beams strengthened with plates and six beams strengthened with fabrics) were weighed and the surface condition of each beam was documented. Thermocouples were attached to the center-most beams to ensure that the high temperature of the beams is maintained accurate and constant. The chamber was then heated to 140oF (60oC) and maintained at this temperature for the entire period of exposure. The beams were kept under steady heat for duration of 1000, 3000, and 10,000 hours, and were spaced apart using three treated lumber blocks of 4 in. x 4 in. (100 mm x 100 mm) to ensure an adequate heat circulation and exposure. Beams were stacked in three rows, each composed of two beams strengthened with CFRP plates and two beams strengthened with CFRP fabrics. The top row of beams was removed from the chamber after 1000 hours of exposure, the middle after 3000, and the bottom row after 10,000 hours of exposure. Beams were visually inspected on a biweekly basis for the entire duration of exposures. Figure 3.15 shows the bottom row of beams inside the dry-heat chamber after the completion of 10,000 hours of exposure. Each set of beams was cooled to the ambient temperature and then weighed. Each beam was then instrumented and loaded to failure under four-point flexural loading.
3.3.3 Alkaline Solution Exposure
In order to examine the effect of alkalis on the RC beams externally strengthened with CFRP plates and fabrics, twelve beams were immersed in 100% alkaline solution. The alkaline solution was prepared as per section 6.1 of the ASTM D 1141 (1998). The alkaline conditioning followed the procedures of ASTM C581 (1994). A stainless steel tank was used as to accommodate the beams during their exposure to the alkaline solution as described in section 3.3. The tank was cleaned of all extraneous materials and then filled with the alkaline solution. Each 10.0 liters of the solution contained 0.20 g of reagent-grade Ca(OH)2 powder and 0.25 g of reagent grade
46 of CaCO3 powder. The volume of alkaline solution used was 850 gallons (3217 liters). The solution had a pH of 9.5. The solution was then promptly warmed to a temperature of 73o±3oF (23o±2oC) using the heating blanket positioned below the bottom surface of the tank. Beams were examined for appearance, weight, description of physical features, and photographs were taken. Beams were carefully placed into the tank, in the same manner used for humidity conditioning, and any excess solution was allowed to overflow the sides of the tanks. Beams were stacked in rows of four horizontally and three vertically, and were spaced apart to allow for adequate soaking, using treated lumber blocks of 4 in. x 4 in. x 45 in. (100 mm x 100 mm x 1140 mm). Once the beams were in place, the two pumps were positioned at opposing corners of the tank for adequate circulation of the alkaline solution. Beams were exposed to the alkaline solution for durations of 1000, 3000, and 10,000 hours. The pH concentration of the solution was checked on a weekly basis to ensure that the proper level of pH was maintained. When the pH concentration level dropped to 8.5, the alkalinity of the solution was restored by adding 0.002 g of
Ca(OH)2 per 10.0 liters of solution. After the completion of the duration of the exposure for each row of beams, the four beams were removed, weighed, and then tested to failure under four-point flexural loading.
3.3.4 Salt-Water Exposure
Salt-water solution was prepared based on the concept of substitute ocean water (ASTM D1141, 1998). Salt-water solution was prepared by mixing synthetic sea salt, manufactured by Aquarium Systems6, with tap water. As per the instructions given by the manufacturer in the data sheet, a salt volume was mixed to the water until obtaining a specific gravity of 1.022 to ensure that the solution had the same composition of the ocean water. The solution contained Chlorine, Sodium, Magnesium, Sulphur, Calcium, Potassium, Bromine, Carbon, Nitrogen, Strontium, Oxygen, Boron, Silicon, Fluorine, and Argon. The salinity concentration of the substitute ocean water was 1500 ppm. The tank was first cleaned of all extraneous materials and then filled with ocean water solution. The water was then promptly warmed to a temperature of 73o±3oF (23o±2oC). Beams were then examined for their weight and physical features. All beams
6 Aquarium Systems, 8141 Tyler Blvd., Mentor, Ohio 44060
47 were carefully placed into the tanks and excess salt-water was allowed to overflow the sides of the tanks. The beams were stacked in rows of four horizontally and three vertically, and spaced apart as to allow for an adequate soaking using treated lumber blocks of 4 in. x 4 in. x 45 in. (100 mm x 100 mm x 1140 mm). Once the beams were in place, the pumps were positioned at opposing corners. The tank was equipped with aerators to allow for adequate circulation, and oxygenation of the solution. These aerators were fabricated by Maxima/Optima7 and consisted of two air pumps placed outside the tank to deliver air through houses and slotted pipes fixed at the bottom of the tank. These air pumps could supply a constant airflow of 5600 cc/minute through two outlets. Submersible pumps were installed to ensure that the water temperature would remain constant at all points in the tank. Thermocouples were installed to ensure that the tank remained at a constant temperature. Beams were exposed to the substitute ocean water for periods of 1000, 3000, and 10,000 hours. They were visually examined on a weekly basis, and any change in their external appearance was documented through photographs. Figure 3.16 shows the discoloration (to a yellow color) of the water and the concrete surface due to salt-water solution. The specific gravity of the water was checked and maintained at a level of 1.0022 to 1.0030 in order to ensure a constant level of salinity. Finally, all the beams were weighed and tested to failure under four-point flexural loading, each after the completion of its duration of exposure to the salt-water.
3.3.5 Freeze/Thaw Exposure
In order to determine the performance of concrete beams strengthened with CFRP plates and CFRP fabrics after exposure to repeated freeze and thaw cycles, eight beams were exposed to freeze/thaw cycles. A set of four beams, two strengthened with CFRP plates and two strengthened with CFRP fabrics, was subjected to 350 cycles, while another identical set of four beams was subjected to 700 cycles of freeze/thaw cycles. According to the ASTM C666-1997, Procedure B, the nominal freeze/thaw cycle includes reducing the temperature of the beams surface from 40 to 0oF (4oC to –17.8oC), and then raising it from zero to 40oF (–17.8oC to 4oC). Air was used to freeze the beams (Figure 3.17), while water was used to thaw them (Figure 3.18). Each cycle of
7 Maxima/Optima, P. O. Box 9107, 50 Hampden Road, Mansfield, MA 02048
48 freeze/thaw took four hours as shown in Figure 3.19. The temperature of the chamber was maintained at 0±3oF (–17.8±1.5oC) at the end of the cooling period for 30 minutes, and at 40±3oF (4±1.5oC) at the end of the thawing period for 50 minutes. It took 90 minutes to reduce the temperature at the center of any beam from 37oF to 3oF (2.7oC to - 16oC), while it took 70 minutes to raise the temperature from 3 to 37oF (-16oC to 2.7oC). The freeze/thaw chamber was fabricated by Russells Technical Products8 using insulated stainless steel panels. The internal dimensions of the chamber were 20 ft long x 12 ft wide x 9 ft high (6.1 m x 3.6 m x 2.7 m). The chamber allows having a temperature range from -90oF (–68oC) to 185oF (85oC). The cooling system consisted of two 60 HP Carlyle semi-hermetic compressors, Ozone friendly refrigerants 507 and 23, and a remote air-cooled condenser with 100 linear feet of interconnect. The air system had the ability to provide up to 8,480 cfm of conditioned air. A fluid reservoir was located outside the chamber to accommodate the water used to thaw the beams. This reservoir was manufactured from 304 series stainless steel, and it was piped to the machine section with copper tubing with an insulation sleeve. The reservoir had a capacity of 72 cubic feet, and it was connected to a pump able to transfer approximately 24 GPM of water. All four beams were placed inside the freeze/thaw chamber within a tank 4 ft wide x 1.5 ft high x 10 ft high (1.2 m x 0.45 m x 3.0 m) for exposure to freeze/thaw condition. This internal storage reservoir was manufactured from one-inch thick co- polymer polypropylene material. The reservoir was equipped with one high water level control to prevent overfilling of the reservoir. A low water level control was installed to prevent the system operating without adequate water in the system. Before the beginning of each of the two tests, beams were immersed in water at 73.4 ± 3oF (22.8±1.5oC) for 48 hours to ensure proper curing, as per the instructions stated in the ASTM C666-97 Procedure B. They were then examined for their weight and external appearance. Thermocouples were installed on the concrete surface and also on the CFRP plates and fabrics at the midspan and the ends of each beam. Four more thermocouples were installed inside the concrete at the center of the beams after drilling
8 Russells Technical Products, 1145 S. Washington Ave., Holland, MI 49423
49 special holes for that purpose. All the thermocouples were connected to the computer system to monitor the temperature of the CFRP fabrics and CFRP plates, and the concrete. Freeze and thaw cycles were started for the beams placed in the freeze/thaw tank for 350 and 700 cycles, respectively. After the exposure to freeze/thaw cycles was completed, each set of beams was brought to the Structural Testing Center for proper instrumentation and testing as described in section 3.4 entitled “Instrumentation and Testing Procedure”.
3.3.6 Thermal Expansion Cycles
In order to evaluate the effect of thermal expansion characteristics of the FRP material on the strength of reinforced concrete beams strengthened with FRP materials, four test beams (two beams strengthened with CFRP plates and two beams strengthened with CFRP fabrics) were placed in a tank inside a 20 ft x 12 ft x 9 ft (6.1 m x 3.6 m x 2.7 m) heating chamber as shown in Figure 3.20. The heating chamber was the same chamber used for the freeze/thaw conditioning. The chamber was specified for the maximum temperature of 168oF (75.5oC) and the maximum humidity of 100%. Strain gages were attached along the length of the CFRP plates or fabrics and then connected to the test control software (TCS), and thermocouples were attached to the surface of the beams (Figure 3.21). The beam surfaces were then heated to 80±2oF (26.7±1.5oC). The temperature of the beams was then raised to 120±2oF (48.9±1.5oC) and immediately cooled back down to 80±2oF. The raise of the temperature to 120±2oF (48.9±1.5oC) and cooling down to 80±2oF formed one complete cycle. There were 35 cycles over the duration of this test; each consisted of a 2.5-hours heating phase and a 2.5-hours cooling phase (Figure 3.21). During the complete duration of the thermal expansion exposure, surface temperature of the tank and air temperature were maintained using BS1 software (specifically developed for the Russell Technologies Panel Heating Chamber). The TCS software package was used to gather the data, which were analyzed and displayed, along with physical monitoring. Beams were inspected regularly for any discolorations or changing in CFRP fabrics and plates, and the corresponding data were recorded. Finally, the beams were loaded to failure.
50 3.3.7 Repeated Load Effects
To examine the effect of repeated loads on the externally CFRP strengthened beams, twelve beams were subjected to constant amplitude cyclic loading. Three sets, each composed of four beams, were subjected to 2 million cycles of repeated load. Each set of beams had 2 beams strengthened with CFRP plates and 2 beams strengthened with CFRP fabrics. The tests were accomplished for 100,000, 1,000,000 and 2,000,000 cycles of repeated loading. The repeated loading was applied with a servo-hydraulic actuator to a loading beam resting on top of the test beam. The actuator controller was programmed to oscillate the load sinusoidally, at a frequency of 3.25 Hz between load limits specified for each of the three sets of beams.
The repeated load range was 15%, 25%, and 40% of the ultimate load of the strengthened beams, for the three sets of tested beams. As shown in Figure 3.22, the predicted ultimate load was 30,000 lbs (133.5 kN).
3.4 Instrumentation and Testing Procedure
Instrumentation and testing procedure for all beams were kept the same for all types of exposures. They are explained in the following sections.
3.4.1 Instrumentation
A variety of instruments were used in measuring and recording data during the testing of beams under different loading conditions. These instruments included strain gages (Figure 3.23), string pots (Figure 3.24), load cells, and computer equipment.
Strain Gages
Micro-measurement CEA-series electrical resistance strain gages provided strain measurements on the CFRP plates and fabrics. The strain gages had a resistance of 350 ohms at 24oC, a gage factor of 2.125 ± 0.5% at 24oC, a temperature range of -75oC to 205oC, gage length of 0.75 inch (19.1 mm), and fatigue life of 10 million cycles at 1500 microstrain. The CFRP surface was prepared for the application of strain gages by
51 cleaning and leveling the desired locations and applying a thin coat of epoxy. When dry, the epoxy was chemically neutralized. The strain gages were then bonded to the surface using Micro-Measurements 200 Catalyst-B and M-Bond 200 adhesive. Next, the strain gages were wired into the MEGADAC data acquisition unit. All the lead wires near the gages were affixed to the bridge and coated with polyurethane for protection. The use of lead wires reduced the risk of accidentally tearing the gage off the surface of the CFRP plates or fabrics. Beams strengthened with CFRP fabrics had five electrical resistance strain gages placed along the centerline of the beam, whereas beams strengthened with CFRP plates had six strain gages. Five out of six strain gages were placed along the length of the beam on one side of the longitudinal axis, while the sixth strain gage was placed at the midspan of the beam on the other side of the longitudinal axis. String Pots During the ultimate loading of the beams, deflections were measured using the devices commercially known as “string pots.” These were retractable strings connected to a radial potentiometer. The degree to which the string is pulled out is proportional to the resistance of the potentiometer, which can be measured by the data acquisition system. These devices offered usable ranges of roughly 50 inches (1.27 m) and provided very stable measurements during the testing of the beams. Figure 3.24 shows the position of the string pots on one of the beams during loading.
3.4.2 Testing Equipment
In addition to the instruments, equipment was needed to conduct the testing of seventy-eight beams. In this section, the testing equipment is described.
Dynamic Excitation System for Repeated Load Test
An MTS loading system, in conjunction with an MTS hydraulic pump, provided the fatigue, static, and ultimate load capabilities. This arrangement consisted of: 1- a 22 kips (98 kN) actuator for fatigue testing, MTS Model 244.22 equipped with a position sensor, velocity feedback transducer, and a load cell.
52 2- an 82 kips (365 kN) actuator for ultimate loading, MTS Model 234.35 equipped with a position sensor, velocity feedback, and a load cell. 3- a digital display console, MTS Model 458.20 Micro Console unit. 4- an MTS 30 gpm (1.892 l/s) hydraulic pump.
MTS Actuator
The MTS actuators function under precision servovalve control in a closed loop system. Precision control over-force generation and piston displacement was one of the many features of the actuators. The load cells monitored applied load, while the piston’s displacements were monitored by the internally mounted linear variable differential transducer (LVDT).
The actuator operated by supplying high-pressure hydraulic fluid to one side of the piston and returning the fluid to the other side of the piston. A servovalve, which is attached to the manifold of the actuator, controlled the flow rate and direction of the hydraulic fluid. The servovalve maintained its control by comparing the desired input signal from the Micro Console unit to the actual output signal from the transducer. A voltage difference between the two signals (DC error) caused the servovalve to react with the appropriate hydraulic fluid flow rate and direction, thus providing the desired operation from the actuator. A swivel head, which was included in the actuator’s components, allowed the instruments to be leveled and aligned properly prior to testing. Loads up to 82 kip (365 kN) could be applied by the actuator.
MTS Digital Display Unit
The 458.20 Micro Console unit provided a digital display and hydraulic pressure control. Furthermore, the console also accommodated the 458.12 DC controller (load control), 458.14 AC (position control) and 458.90 Function Generator modules. The function generator module controlled the dynamic load waveforms and waveform frequency used throughout testing. The function generator maintained its control by commanding the servo control loop. Dynamic loads of cyclic sine, square or triangle waveforms, at frequencies ranging from 0.01 to 1100 Hz, were options supported by the
53 function generator. The function generator in conjunction with the DC controller produced the desired dynamic excitation used for the testing of beams. The DC and AC controller modules included:
1- transducer conditioning. 2- command conditioning. 3- servo loop control.
The difference between the DC and AC controllers was the electrical nature of the command signal, namely direct (DC) or alternating (AC) current. Excitation required by the actuator’s load cell and LVDT was provided by the transducer conditioning circuit. Set point and span control were adjusted with the command conditioning circuit. The set point controlled a mean reference signal while the span option controlled the amplitude of the signal. Servo loop control allowed for adjustment of the program’s DC error, thus enabling the desired tracking of the feedback with command signals.
MTS Hydraulic Pump
An MTS model 510.30A, 30 gpm (1.892 l/s), 3,000 psi (20.7 MPa) hydraulic pump provided the hydraulic power needed to operate the MTS loading system. The reservoir of this model has a capacity of 80.5 gallons (305 liters). The hydraulic fluid temperature was maintained by a water-regulating valve which automatically controlled cooling water flow. The pump was designed to provide continuous operation at the rated pressure with a 100% duty cycle. The pump was operated by the 458.20 Micro Console unit as described in section 3.4.2.3.
Data Acquisition System
An OPTIM Electronics MEGADAC 3415AC data acquisition system was used to monitor the test sensors throughout all phases of the experimental investigation. This model possessed numerous capabilities for accurate dynamic capture of analog data from a wide range of active and passive transducers. This instrument allows having a maximum sampling rate of 25,000 samples per second. It provides up to one Gigabyte of internal RAM for data storage. The data acquisition was able to maintain all the necessary channels of input with each test sensor having its own channel.
54 A MEGADAC input module model AD-1 808FB-1 was used to transfer the data from the transducers to the system. This module provides signal conditioning, three selectable voltage excitation levels, anti-alias filtering, and have two selectable gain settings per channel. The module had eight input channels, and was connected to a standard terminal block model STB 808FB/120. This terminal block consists of an eight- channel standard junction box. These channels were connected to different strain gages, string pots, and load cells.
Computer Equipment
A Pentium III was used in conjunction with TCS data acquisition software for data management and transmission. The computer was interfaced with the MEGADAC data acquisition unit through an input module model AD-1 808FB-1, allowing the computer software to provide means for controlling each of the test sensors when measurements were taken. Since each type of test sensor had its own designated channel on the data acquisition unit, a transducer table was set up in the software for each set of sensors. The table included such information as the type of test sensor, card and post gain, and the engineering units and calibration value of the sensor. Once the sensors were defined in the transducer’s library, experiment tables were set up to control the data measurements. The experiment tables provided control over the sequence of channels to be sampled, the scanning rate for the channels to be utilized and the starting and stopping criteria for data sampling. After experiment tables were defined in the software, experimental tests could be controlled with the computer. When conducting a test, the data was stored in computer memory. The TCS software provided numerous data reduction and analysis options.
3.4.3 Testing Procedure
The experimental procedure for test beams was aimed at collecting data concerning the static and ultimate load characteristics after exposure to various environmental conditions. A schematic of the test set up is presented in Figure 3.25. Beams were placed under the actuator mounted on a steel frame (Figure 3.26). The
55 center-to-center distance between supports was 100 in. (2540 mm). A four-point loading system was used to apply the static load at the center of the beams. All the beams were loaded and unloaded until failure. The loading and unloading sequences followed two steps. First, the load was applied up to a load of 12 kips (53.4 kN) and unloaded to zero load, and subsequently beams were loaded up to 24 kips (106.8 kN) and unloaded to zero load again. Finally, the beams were loaded to failure. Throughout the testing, the rate of loading was 0.004 in./second, while the unloading rate was 0.01 in./second. During different phases of the test, deflections and strains were measured and recorded at uniform intervals.
56
Experimental Program.
Base Line Freeze/Thaw Alkaline Solution @ Water @ 100oF (4 Beams) 350 cycles 73oF (12 Beams) + (4 Beams) (12 Beams) 1000 hrs, Un-strengthened + 1000 hrs, 3000 hrs, (2 Beams) 700 cycles 3000 hrs, 10,000 hrs (4 Beams) 10,000 hrs ASTM D2247 5 hrs duration ASTM C581 between 0o & 40oF ASTM C666-B
Repeated load Dry Heat @ 140oF Salt Water @ Thermal Expansion o (12 Beams) (12 Beams) 73 F (4 beams) 40 % (1,250 -12,500 lbs) 1000 hrs, (12 Beams) 35 cycle s 25 % (750 - 7,500 lbs) 3000 hrs, 1000 hrs, 5 hrs duration 15 % (350 – 3,500 lbs) 10,000 hrs 3000 hrs, between o o ASTM D3405 10,000 hrs 80 & 120 F ASTM D1141 ASTM C531
Figure 3.1 Details of the experimental program
57 Two-legged # 3 stirrups @ 4"
2 # 3 top reinforcement
2 # 5 bottom reinforcement
10"
CFRP 88" plate 108"
(a) Longitudinal Section
2 # 3 1"
1"
# 3 @ 4" 10"
2 # 5
6"
(b) Cross-Section
Figure 3.2 Construction details of test beams
58
Figure 3.3 Metal forms and reinforcement arrangement
59
Figure 3.4 Preparation of structural epoxy for beam strengthening
60
Figure 3.5 Application of structural epoxy for CFRP plates/fabrics installation
61
Figure 3.6 Installation of first layer of CFRP fabric
62
Figure 3.7 Application of structural epoxy on top of first layer of CFRP fabric
63
Figure 3.8 Installation of second layer of CFRP fabric
64
Figure 3.9 Use of hand-rollers to bond CFRP fabric
65
Weighing scales
Weight indicators
Figure 3.10 Weight evaluation of test beams
66
Water circulation pump Water circulation pump
10" Top row of 4 beams exposed to 1,000 hours of conditioning 48"
10" Middle row of 4 beams exposed to 3,000 hours of conditioning
CFRP Fabric or Plate 10" Bottom row of 4 beams exposed to 10,000 hours of conditioning
Heating blanket 108"
120"
Figure 3.11 Schematic of tanks used for humidity, salt-water, and alkaline solution exposures
67
Beams Strengthened Beams Strengthened with CFRP Plates with CFRP fabrics
10"
2" Spacer 48"
10"
2" Spacer
10"
2" Spacer
Heating blanket 6" 6" 4" 6" 4" 6" 4" 6" 6"
48"
Figure 3.12 Side view of tanks used for humidity, salt-water, and alkaline solution exposures
68
Dry- heat Freeze/ thaw
Salt Hot water water
Alkaline solution
Figure 3.13 Arrangements for humidity, salt-water, alkaline solution, and dry-heat exposure
69
Water circulation pump
Figure 3.14 Bottom row of four beams exposed to hot water for 10,000 hours
70
CFRP CFRP plates fabrics
Figure 3.15 Bottom row of four beams exposed to 10,000 hours of dry-heat
71
Water circulation pump
Figure 3.16 Beams exposed to salt-water for 10,000 hours
72
Beams with CFRP plates
Beams with CFRP fabrics
Figure 3.17 Beams exposed to freeze/thaw test inside environmental chamber
73
Beams with CFRP plates
Beams with CFRP fabrics
Figure 3.18 Water-thawing for beams exposed to freeze/thaw test
74
100 Cycle Time 4 hours
80 100% Humidity at 43° F 50 Minute Soak 60
40
F) 20 o
0 0° F
Temperature ( 30 Minute Soak -20
-40
-60 Air Temperature Water Temperature Surface Temperature of Beam Internal Temperature of Beam
1:00:00 2:00:00 3:00:00 4:00:00 5:00:00 6:00:00 7:00:00 8:00:00 9:00:00 10:00:00
Time (hours)
Figure 3.19 Freeze/thaw cycle for CFRP externally strengthened beams according to ASTM C666-B
75
Beams with CFRP fabrics
Beams with CFRP plates Thermocouples
Figure 3.19 Arrangement for thermal
Figure 3.20 Arrangement for thermal expansion test
76 180 Cycle Time 5 hours
160
120° F 140 60 minutes
120
F) o 100
80 80° F
Temperature ( 60 minutes
60
40
20 Air Temperature Surface Temperature of Beam Internal Temperature of Beam
1:00:00 2:00:00 3:00:00 4:00:00 5:00:00 6:00:00 7:00:00 8:00:00 9:00:00 10:00:00
Time (hours)
Figure 3.21 Thermal expansion test cycle s
77
-4000 0
Load (lbs) -8000
-12000 Repeated load range #3: 40% of beam ultimate strength (1200-12000 lbs)
0
-4000
-8000 Load (lbs) Static set-point -12000 Repeated load range #2: 25% of beam ultimate strength (750-7500 lbs)
0
-4000
Load (lbs) -8000
-12000 Repeated load range #1: 15% of beam ultimate strength (450-4500 lbs)
0 0.25 0.5 0.75 1 1.25 1.5 1.75 2
Number of Cycles (millions)
Figure 3.22 Range of repeated load test Applied @ 3.25 Hz
78
(a) Strain gage installed on CFRP fabric. (b) Strain gage installed on CFRP plate.
CFRP
3" 3" 8" 16" 25" 25 " 16" 8 " 5" 98" 5 " 108"
(c) Bottom view of CFRP plate/fabric
Figure 3.23 Strain gages instrumentation along beam-span
79
Linear variable differential transducers
Figure 3.24 Deflection instrumentation along beam span
80
Load
Loading beam
32" 10"
CFRP Strengthening 100"
108"
Figure 3.25 Four-point ultimate and repeated load test set-up
81
Figure 3.26 Loading frame for ultimate and repeated load tests
82 CHAPTER 4 RESULTS AND DISCUSSION
4.1 Introduction
The experimental investigation was carried out to study the long-term durability issues related to strengthening with carbon fiber reinforcement polymers (CFRP). In this chapter, the deflection and strain responses of strengthened beams with and without exposure to different environmental conditionings, such as 100% humidity, salt-water, alkali-solution, freeze/thaw cycles, thermal expansion cycles, repeated load cycles, and dry-heat conditioning, are discussed in detail. The deflections, CFRP strains, and ultimate load carrying capacity of beams exposed to different environmental conditions are compared to that of unstrengthened beams and baseline beams (strengthened beams without environmental exposure). In addition, strength reduction factors associated with various environmental conditions are proposed and evaluated as a ratio of the load carrying capacity of CFRP strengthened beams exposed to a specific environmental condition and to that of corresponding baseline beam.
The failure modes and weight changes associated with various types of environmental exposures were examined for all test beams. This investigation confirmed the outstanding durability characteristics of CFRP fabrics and plates, and the effective role of structural epoxy for development of a stronger and more durable bond between CFRP plates/fabrics and beams. It was concluded that humidity is the major factor that reduced the bond strength between CFRP plates and beam significantly. It was also observed that the reduction in the load carrying capacities of beams strengthened with CFRP plates/fabrics after exposure to different environmental conditioning was less than 15%, except for the beams strengthened with CFRP plates and exposed to 100% humidity (at 38o ± 2oC) for 10,000 hours. These beams experienced an average loss of 33% of their load carrying capacity. The average load carrying capacity of the unstrengthened beams was 19.3 kips (86 kN), while the average load carrying capacities of the CFRP plates and CFRP fabrics strengthened baseline beams were 30.7 and 30.0 kips (137 and 134 kN), respectively. The deflections associated with ultimate loads of the beams strengthened with CFRP plates and CFRP fabrics were 0.63 and 0.9 in. (16 and 22.9 mm), respectively. The maximum strains developed in the CFRP plates and CFRP fabrics of the baseline beams at the above-mentioned loads and deflections, were 3,988 and 6,150 millionths, respectively.
For each type of environmental exposure and strengthening system, a set of graphs has been prepared in order to view and analyze the results. Each set of graphs include the following:
- Load versus midspan deflection up to 12 kips of static load. - Deflected shape at midspan at 12 kips of static load. - Load versus midspan CFRP strain up to 12 kips of static load. - CFRP strain along length of beams at 12 kips of static load. - Deflected shape at midspan at failure load. - CFRP strain along length of beams at failure load. - Load versus midspan deflection during the loading and unloading up to the failure load. - Load versus midspan CFRP strain during the loading and unloading up to the failure load.
The 12 kips of static load represents the service load of the test beams. The responses of the strengthened beams exposed to different environmental conditions are presented in the following sections.
4.2 Beams exposed to 100% humidity
In this section, responses of the beams strengthened with CFRP plates and CFRP fabrics exposed to 100% humidity at 100oF (38oC) are discussed. In the beam designations as given in the figures, P stands for the beam strengthened with CFRP plates, while F stands for the beam strengthened with CFRP fabrics. The other notations such as W1k, W3k, and W10k refer to the beams exposed to 1000, 3000, and 10,000
84 hours of 100% humidity conditioning at 100oF (38oC), respectively. The numeral 1 or 2 associated with the designation of beams represents the designation number.
4.2.1 Beams strengthened with CFRP plates
Figure 4.1 shows a close-up view of the ultimate load test of the beam strengthened with CFRP plates after exposure to 10,000 hours to 100% humidity. Figures 4.2a, 4.2b, and 4.2c show the load-deflection responses up to a load of 12 kips (53.4 kN) for the beams strengthened with CFRP plates and exposed to 100% humidity conditioning for 1000, 3000, and 10,000 hours, respectively. It is shown that the responses of the beams exposed to humidity condition and those of the baseline beams are identical. Thus, it is observed that there is no significant effect of humidity on the load-deflection response of strengthened beams up to 12 kips (53.4 kN) of static load.
To examine the pre-failure responses of the strengthened beams exposed to humidity conditioning in comparison to those of baseline beams, deflection profiles for baseline beams and beams exposed to 1000, 3000, and 10,000 hours of humidity conditioning are shown in Figure 4.3 for a load of 12 kips (53.4 kN). It is observed that the beams exposed to 1000 and 3000 hours of 100% humidity conditioning have similar deflection profiles with almost the same deflection values as those of baseline beams, (along the length of the beams). However, as shown in Figure 4.3c, deflections of the beams exposed to 10,000 hours of humidity conditioning are larger than those of the baseline beams all along the length of the beams, except at the midspan section where both beams have the same deflection.
Figures 4.4a, 4.4b, and 4.4c show the load versus CFRP strain relationships up to 12 kips (53.4 kN) of static load for beams strengthened with CFRP plates and exposed to humidity condition for 1000, 3000 and 10,000 hours at 100oF (38oC), respectively. It is shown that CFRP strains for the beams strengthened with CFRP plates and exposed to humidity condition are scattered in a banded form. For a specific load, strains for plate baseline beams (Figure 4.4a) lie between those of the beams exposed to 1000 hours of 100% humidity at 100oF (38oC). It is observed that strains in the CFRP plates of the beams exposed to 3000 and 10,000 hours of humidity are less than those for the baseline
85 beams after 9 kips (40 kN) of load. It is worth noting that the scattering of strain data is reduced for the longer durations of exposure (Figures 4.4b and 4.4c).
The CFRP plates strain profiles corresponding to the load of 12 kips (53.4 kN) are shown in Figures 4.5a, 4.5b, and 4.5c for the beams exposed to 1000, 3000 and 10,000 hours of humidity, respectively. It is observed that the developed strains in the CFRP plates of the beams exposed to humidity condition are lower than those of the baseline beams. The deflections (Figure 4.3) and strains (Figure 4.5) profiles of the beams strengthened with CFRP plates exhibit a decrease in the stiffness of the beam due to 100% humidity conditioning at 100oF (38oC).
The deflection profiles of baseline beams and beams exposed to humidity conditioning at their ultimate failure load are shown in Figure 4.6. As it was expected, deflections of the beams exposed to 1000 hours of exposure to humidity conditioning are larger than those of the corresponding baseline beams. However, deflections of the beams exposed to 3000 hours of humidity are almost the same as those of corresponding baseline beams (Figure 4.6b). Similarly, for 10,000 hours of exposure, deflections are slightly larger in the end quarter spans and slightly lower at the midspan in comparison to those of the baseline beams. Based on these observations, it is noted that unlike the beams exposed to 1000 hours of humidity conditioning, deflections of the beams exposed to 3000 and 10,000 hours of humidity are not significantly larger than those of the corresponding baseline beams. This is attributed to the fact that the load carrying capacity of beams exposed to 1000 hours is almost the same as that of baseline beams. However, the ultimate load carrying capacities of CFRP plates strengthened beams, exposed to 3000 and 10,000 hours of humidity, are reduced slightly, and reduced significantly in comparison to the corresponding baseline beams.
The loads versus deflection relationships up to the ultimate failure loads of beams strengthened with CFRP plates and exposed to 3000 and 10,000 hours of 100% humidity are presented in Figures 4.7 and 4.8. It is shown that the load-deflection responses for the
86 baseline beams and beams exposed to 3000 hours of humidity conditioning are identical up to the ultimate failure load. As shown in Fig. 4.7, baseline beams and beams exposed to humidity conditioning show a sudden drop in the load followed by a large deflection at a constant load before the complete collapse of the beams. It may be noted that the failure of the beams was initiated by the onset of delamination of the CFRP plates at a load close to the observed ultimate failure load of the beam. A similar response is observed for the beams exposed to 10,000 hours of exposure to humidity conditioning, except that the load carrying capacity of the beams exposed to 10,000 hours of humidity conditioning is significantly reduced (37% reduction) in comparison to that of the baseline beams.
The average ultimate failure loads of the beams strengthened with CFRP plates and exposed to 1000, 3000, and 10,000 hours of 100% humidity conditioning were 33.3, 25.3, and 20.9 kips (148, 113, and 93 kN), respectively. It is worth noting that 1000 hours of exposure resulted in about an 8% increase in the load carrying capacity of the beams, while 3000 and 10,000 hours of exposure resulted in about 24% and 37% reductions in load carrying capacity with respect to that of baseline beams. The 8% increase in the ultimate load carrying capacity of beams after 1000 hours of exposure to humidity may be due to efficient curing of the structural-epoxy facilitating bond between CFRP plates and surface of the beams.
Figures 4.9 and 4.10 show the load versus CFRP strain relationships of the beams strengthened with CFRP plates and exposed to 3000 and 10,000 hours of humidity conditioning. It is observed that for a specific load, CFRP strain in the beam designated as plate baseline1 is larger than that of other beams. It is also observed that the developed strains in the CFRP plates of the beams exposed to humidity conditioning are lower in comparison to those of baseline beams for loads greater than 12 kips (53.4 kN). The decrease in the strains of CFRP plates is aggravated due to the increase in the duration of exposure to humidity. This may be attributed to the differential change in the stiffness of the CFRP plates and structural epoxy. This differential change in the stiffness of the CFRP plates and structural epoxy also causes the early failure of the beams exposed to humidity condition due to the onset of delamination of CFRP plates from the concrete surface. It is worth mentioning that all the beams strengthened with CFRP plates failed
87 due to the onset of delamination between the CFRP plates and concrete surface. The onset of delamination was primarily dependent on the bond between the concrete surface and the CFRP plates through structural epoxy. The decrease in the shearing strength of structural epoxy due to continuous exposure to 100% humidity conditioning at 100oF (38o±2oC) significantly reduces the load corresponding to the onset of delamination. This fact confirms the results obtained by earlier researchers (Leung et al., 2001), Zheng and Morgan, 1993).
Although there was no deterioration or change in appearance of the CFRP plates after exposure to humidity for different durations, it can be concluded that the decrease in the differential stiffness of CFRP plates and structural epoxy caused an early failure of the beams.
The average values of deflections corresponding to the ultimate failure loads were 0.71, 0.62, and 0.53 inch (18.0, 16.0, and 13.5 mm) for 1000, 3000 and 10,000 hours of humidity exposures, respectively. Similarly, average values of strains in the CFRP plates at the ultimate failure loads were 3,701, 3,641, and 1,834 millionths for 1000, 3000 and 10,000 hours of humidity exposure, respectively.
4.2.2 Beams strengthened with CFRP fabrics
Figures 4.11a, 4.11b, and 4.11c show the load-deflection responses up to a load of 12 kips (53.4 kN) for the beams strengthened with CFRP fabrics and exposed to 100% humidity conditioning for 1000, 3000, and 10,000 hours, respectively. It is shown that there is no significant effect of 100% humidity at 100oF (38oC) on the load-deflection response up to 9 kips (40 kN) load. However, it is observed that for loads between 9 to 12 kips (40 to 53.4 kN), deflections of the beams exposed to humidity conditioning are smaller than those of corresponding baseline beams.
Figures 4.12a, 4.12b, and 4.12c show the deflection profiles along the length of the beam strengthened with CFRP fabrics for a load of 12 kips (53.4 kN). It is observed that deflections of the beams strengthened with CFRP fabrics and exposed to 3000 hours of humidity conditioning are smaller than those of baseline beams. However, after 10,000
88 hours of exposure to humidity (Figure 4.12c), deflections of the beams are closer to those of baseline beams in the end quarter spans. In the central portion of the strengthened beams exposed to 10,000 hours of humidity, deflections are less than those of baseline beams. This response of CFRP fabric strengthened beams indicates that short-term exposure to 100% humidity (hot water) may increase the stiffness of the beams. However, long-term exposure (beyond 10,000 hours) to humidity may decrease the stiffness of the beam, which may cause early onset of delamination of fabrics from the concrete surface.
Figures 4.13a, 4.13b, and 4.13c show the load versus CFRP fabrics strain responses up to 12 kips (53.4 kN) of load for beams exposed to 1000, 3000, and 10,000 hours of humidity at 100oF (38oC), respectively. It is observed that for a specific load, the strains in the CFRP fabrics of the beams exposed to humidity conditioning are lower than those of the baseline beam. The strain profiles of the CFRP fabrics along the length of the beams for a load of 12 kips (53.4 kN) are shown in Figure 4.14. It is shown that the strain profiles are almost symmetrical about the midspan of the beam. It is also shown that at a specific section of the beams, strain is smaller for the beams exposed to 100% humidity at 100oF (38oC) condition than that of the baseline beam.
The deflection profiles corresponding to the failure loads of the beams strengthened with CFRP fabrics and exposed to 1000, 3000, and 10,000 hours of humidity are shown in Figures 4.15a, 4.15b, and 4.15c, respectively. In general, it is observed that the deflections are smaller for the beams exposed to humidity conditioning in comparison to those of baseline beams. It is also worth mentioning that the effect of humidity on the beams strengthened with CFRP fabrics is not as significant as on the beams strengthened with CFRP plates.
Figures 4.16 and 4.17 show the load versus deflection relationships for beams strengthened with CFRP fabrics and exposed to 3000 and 10,000 hours of 100% humidity at 100oF (38oC), respectively. It is shown that the fabric baseline beams have a higher load carrying capacity in comparison to the beams exposed to 3000 and 10,000 hours of humidity conditioning. However, the reduction in the load carrying capacity of beams is
89 larger for the longer duration of exposure to humidity. It is worth noting that like the case of beams strengthened with CFRP plates, beams strengthened with CFRP fabrics undergo substantial deformation after a sudden drop of the load after the ultimate failure. The sudden drop in the load is associated with the onset of delamination of the fabrics from the concrete surface.
The load versus CFRP fabric strain relationships up to ultimate failure loads of the beam exposed to 3000 and 10,000 hours of 100% humidity at 100oF (38oC) are shown in Figures 4.18 and 4.19, respectively. It is shown that load-strain data are staggered and the load versus strain response for the beam exposed to humidity conditioning lies between those of baseline beams. However, the developed CFRP strain corresponding to the ultimate failure load of the beam exposed to humidity conditioning is significantly lower than that of the baseline beam. The reduction in the strain corresponding to the ultimate failure load of the beam is greater for the longer duration of exposure.
4.3 Beams exposed to dry-heat condition at 140oF (60oC)
To examine the effect of dry-heat conditioning on the CFRP strengthened beams, responses of the CFRP plates/fabrics strengthened beams exposed to dry-heat conditioning for 1000, 3000, and 10,000 hours at 140oF (60oC) are presented along with those of corresponding baseline beams in the following sections.
4.3.1 Beams strengthened with CFRP plates
Figures 4.20a, 4.20b, and 4.20c show the load versus deflection relationships for beams strengthened with CFRP plates and exposed to dry-heat conditioning at 140oF (60oC) for
1000, 3000, and 10,000 hours, respectively. It is observed that there is no significant effect of dry-heat conditioning on the load-deflection responses up to a load of 12 kips
(53.4 kN). The deflection profiles along the length of beams are presented in Figures
4.21a, 4.21b, and 4.21c for the beams exposed to dry-heat conditioning for 1000, 3000,
90 and 10,000 hours, respectively. It is observed that the deflection profiles do not change significantly at least for 10,000 hours of exposure to dry-heat conditioning.
Figures 4.22a, 4.22b, and 4.22c show the load versus CFRP plates strain responses for the beams exposed to 1000, 3000 and 10,000 hours of dry-heat conditioning, respectively. It is observed from Figure 4.22a that for a specific load, the strain in the CFRP plates of the beam exposed to dry-heat conditioning is larger in comparison to that of plate baseline beams, irrespective of the duration of exposure. However, the effect of dry-heat conditioning is most significant for 10,000 hours of exposure in comparison to that for 1000 and 3000 hours of exposure (see Figure 4.22c). The strain profiles along the length of the baseline beams and beams exposed to dry-heat conditioning are presented in Figures 4.23a, 4.23b and 4.23c for 1000, 3000, and 10,000 hours of exposure. It is observed that there is no appreciable difference in the strain profiles of the baseline beams and those of beams exposed to 1000 and 3000 hours of exposure to dry-heat conditioning. However, one of the beams exposed to 10,000 hours of dry-heat conditioning exhibited substantial midspan strain in the CFRP plates in comparison to that of other beams.
The deflection profiles at failure loads along the length of the beams strengthened with CFRP plates and exposed to dry-heat conditioning for 1000, 3000 and 10,000 hours are shown in Figures 4.24a, 4.24b and 4.24c, respectively. It is shown that the deflection profiles are symmetrical about the midspan of the beams. The symmetrical deflection profiles represent the uniformity of the dry-heat conditioning along the length of the beam. It is also observed that there is no significant effect of dry-heat conditioning on the deflection profiles of CFRP plates strengthened beams, except that the overall stiffness of the beams exposed to 10,000 hours of dry-heat conditioning is reduced to a value lower than that of baseline beams.
The load versus deflection relationships for beams strengthened with CFRP plates and exposed to 3000 and 10,000 hours of dry-heat conditioning are presented in Figures 4.25 and 4.26, respectively. It is shown that the ultimate failure loads of baseline beams
91 are higher than those of beams exposed to dry-heat conditioning. However, the duration of exposure to dry-heat conditioning has a significant effect on the ultimate load carrying capacity. It is observed that there is no significant effect of dry-heat conditioning on the load-deflection response up to ultimate failure loads. The average failure loads of the beams exposed to 3000 and 10,000 hours of dry-heat condition were 26.8 and 27.6 kips (119 and 123 kN), respectively. Figures 4.27 and 4.28 show the load versus strain relationships (up to the failure loads) for beams strengthened with CFRP plates and exposed to 3000 and 10,000 hours of dry-heat conditioning, respectively. It is shown that there is no significant effect of dry-heat conditioning on the load versus strain relationships of the beams strengthened with CFRP plates. The sudden drop in load (immediately after the ultimate failure load) signifies the failure of the beams due to debonding of the CFRP plates from the concrete surface, which led to the complete loss of the stiffness of the beams, as indicated by substantial deformation at the constant load.
4.3.2 Beams Strengthened with CFRP Fabrics
Figure 4.29 shows a close-up view of the ultimate load test for the beam strengthened with CFRP fabrics and exposed to dry-heat condition for 10,000 hours. The load versus midspan deflection relationships up to a load of 12 kips (53.4 kN) for the beams strengthened with CFRP fabrics and exposed to 1000, 3000, and 10,000 hours of dry-heat conditioning are presented in Figures 4.30a, 4.30b, and 4.30c, respectively. It is shown that there is no significant effect of dry-heat conditioning on the load-deflection responses up to a load of 12 kips (53.4 kN).
Figures 4.31a, 4.31b, and 4.31c show the deflection profiles along the length of the beams strengthened with CFRP fabrics and exposed to dry-heat conditioning for 1000, 3000 and 10,000 hours, respectively. It is observed that there is no significant effect of dry-heat conditioning on the strain profiles of beams strengthened with CFRP fabrics. It is also observed that as in the case of beams strengthened with CFRP plates, deflection profiles are symmetrical about the midspan of the beams with the maximum deflection at the center of the beam. However, the midspan deflection of the beams strengthened with
92 CFRP fabrics and exposed to dry-heat conditioning are higher in comparison to the corresponding CFRP plates strengthened beams (Figure 4.24).
The load versus strain relationships up to a load of 12 kips (53.4 kN) for the beams strengthened with CFRP fabrics and subjected to 1000, 3000 and 10,000 hours of dry- heat conditioning are presented in Figures 4.32a, 4.32b and 4.32c, respectively. In general, strains in CFRP fabrics of the beams exposed to dry-heat conditioning are smaller than those of corresponding baseline beams. However, as shown in Figure 4.32, some residual strains are present in the CFRP fabrics after unloading the beams exposed to dry-heat conditioning. This residual strain is not observed in the CFRP fabrics of baseline beams.
Figures 4.33a, 4.33b, and 4.33c show the strain profiles along the length of the beams strengthened with CFRP fabrics and exposed to dry-heat conditioning for 1000, 3000, and 10,000 hours, respectively. It is observed that there is no significant effect of 1000 hours of dry-heat conditioning on the CFRP fabrics strain profiles at 12 kips (53.4 kN) of load. However, CFRP fabric strains are lower in the case of beams exposed to 3000 and 10,000 hours of dry-heat conditioning in comparison to those of baseline beams.
The deflection profiles (at failure loads) along the length of beams strengthened with CFRP fabrics and exposed to dry-heat conditioning at 140oF (60oC) for 1000, 3000, and 10,000 hours are presented in Figures 4.34a, 4.34b, and 4.34c, respectively. It is shown that the deflections of the beams exposed up to 3000 hours of dry-heat conditioning (Figure 4.34b) are slightly lower than those of baseline beams. However, as seen from Figure 4.34c, dry-heat conditioning for 10,000 hours significantly changes the deflection profiles of the beam, which indicates the substantial loss in the stiffness of CFRP fabric strengthened beams.
The load versus deflection relationships up to the ultimate failure loads for beams strengthened with CFRP fabrics and exposed 3000 and 10,000 hours of dry-heat conditioning are presented in Figures 4.35 and 4.36, respectively. It is worth noting that
93 the ultimate load carrying capacity of beams strengthened with CFRP fabrics and exposed to dry-heat conditioning is larger than that of corresponding baseline beams. The average ultimate failure load of the baseline beams was 30.0 kips (134 kN), while the ultimate failure loads of beams exposed to 1000, 3000, and 10,000 hours of dry-heat conditioning were 30.5, 30.8, and 30.6 kips (136, 137, and 137 kN), respectively. As in the case of beams strengthened with CFRP plates, beams strengthened with CFRP fabrics also exhibited a sudden drop in the load (immediately after the ultimate failure load) and substantial deformation at the constant load (lower than ultimate failure load). This load- deflection response after the ultimate failure load signifies complete loss of stiffness before collapse of the beams.
Figures 4.37 and 4.38 show the load versus strain relationships for beams strengthened with CFRP fabrics and exposed to 3000 and 10,000 hours of dry-heat conditioning at 140oF (60oC), respectively. It is observed that the fabric baseline beam has residual strain after unloading of the beam before ultimate failure load, while the beams strengthened with CFRP fabrics and exposed to dry-heat conditioning do not show significant residual deformation. It is also observed that for a specific load below 9 kips
(40 kN), strain in the CFRP fabrics of the baseline beams is lower than that of beams exposed to dry-heat conditioning. For loads of 9 to 21 kips (40 to 94 kN), strains for fabric baseline and dry-heat conditioned beams are almost the same. However, beyond 24 kips (107 kN) of load, the strain in the CFRP fabrics of the baseline beam for a specific load is larger than that of the beams exposed to dry-heat conditioning.
4.4 Beams exposed to alkaline solution at 73oF (23oC)
Responses, such as deflection, strain, and failure modes of the beams strengthened with CFRP plates and CFRP fabrics and exposed to alkaline solution at 73oF (23oC), are presented in the following sections.
94 4.4.1 Beams strengthened with CFRP plates
The load versus deflection relationships up to a load of 12 kips (53.4 kN) are shown in Figures 4.39a, 4.39b, and 4.39c for the beams strengthened with CFRP plates and exposed to alkaline solution for 1000, 3000 and 10,000 hours at 73oF (23oC), respectively. It is shown that there is no significant effect of alkaline solution on load- deflection response of CFRP plate strengthened beams, irrespective of the duration of exposure. The corresponding deflection profiles of the beams strengthened with CFRP plates and exposed to alkaline solution for 1000, 3000, and 10,000 hours are presented in Figures 4.40a, 4.40b, and 4.40c, respectively. It is observed that there is no significant effect of alkaline solution on the deflection profiles of the beam strengthened with CFRP plates. The deflection profiles are symmetrical about the midspan with the maximum deflection being at the center of the beam.
The load versus strain relationships for beams strengthened with CFRP plates and exposed to alkaline solution at 73oF for 1000, 3000 and 10,000 hours are presented in Figures 4.41a, 4.41b, and 4.41c, respectively. It is shown that for a specific load, midspan strain in CFRP plates of beams exposed to alkaline solution is lower in comparison to that of baseline beams. Figures 4.42a, 4.42b, and 4.42c show the strain profiles along the length of the beams strengthened with CFRP plates and exposed to alkaline solution for 1000, 3000, and 10,000 hours, respectively. It is observed that alkaline solution reduces the developed strain level in CFRP plates of the beam.
The deflection profiles along the length of the beam are presented in Figures 4.43a, 4.43b, and 4.43c for 1000, 3000 and 10,000 hours of exposure to alkaline solution at 73oF (23oC), respectively. It is observed that the effect of alkaline solution after 1000 hours of exposure on deflection profile is not clear, as indicated by deflections of one beam being lower while that of another being higher in comparison to those of baseline beams. It is also observed that after 3000 hours of exposure, beams exposed to alkaline solution and baseline beams exhibit almost identical deflection all along the length of the beam. However, the deflections of beams exposed to 10,000 hours of alkaline solution are larger than those of baseline beams. Thus, on average, it could be concluded that short-term exposure of alkaline solution will not significantly change the stiffness of the beam
95 strengthened with CFRP plates. However, the long-term exposure may cause significant reduction in the stiffness of the CFRP plate strengthened beams.
The load versus deflection relationships for beams exposed to 3000 and 10,000 hours of alkaline solution at 73oF (23oC) are shown in Figures 4.44 and 4.45, respectively. It is observed that the ultimate failure load of baseline beams is slightly lower than that of the beams exposed to alkaline solution. The average ultimate failure loads for baseline beams, and beams exposed to alkaline solution for 1000, 3000, and 10,000 hours were 30.7, 30.7, 33.5, and 32.1 kips (137, 137, 149, and 143 kN), respectively. The increase in the load carrying capacity of the beams exposed to alkaline solution is due to the improved bond between the CFRP plates and concrete surface through the structural epoxy. Thus, it can be concluded that the exposure to alkaline solution of the beams strengthened with CFRP plates is beneficial in increasing the bond strength of the CFRP plates with the concrete surface. It should be noted that all strengthened beams with and without exposure to environmental conditions failed due to debonding and/or onset of delamination of the CFRP plates from the concrete surface. Thus, the debonding and/or onset of delamination of the CFRP plate are the actual modes of failure and govern the load carrying capacity of strengthened beams. The debonding of CFRP plates depends on the inherent bond strength of CFRP plates and RC beams with epoxy binder. Thus, the increased strength is definitely due to improved bond strength of CFRP plates and RC beams through epoxy binder. This in turn increases the load carrying capacity of the beam by delaying the onset of delamination of the CFRP plates from the concrete surface.
Figures 4.46 and 4.47 show the load versus strain relationships for the beams exposed to 3000 and 10,000 hours of alkaline solution and for baseline beams. It is shown that strains in the beams exposed to 3000 hours of alkaline solution are lower than those of corresponding baseline beams. However, after 10,000 hours of exposure to alkaline solution, it is observed that there is redistribution of strain in the CFRP plates (Figure 4.47). Unlike 3000 hours of exposure, strain in the beams exposed to 10,000 hours of exposure to alkaline solution lies in between those of baseline beams for most of the load range. Thus, it should be noted that the improved strength and stiffness of
96 structural epoxy under alkaline environment increased the load carrying capacity of the strengthened beams and developed strain in the CFRP plates. The increase in the developed strain in the CFRP plates under alkaline environment is indicative of the improvement in the structural efficiency of CFRP plates bonded to the concrete surface of beams.
4.4.2 Beams strengthened with CFRP fabrics
Figure 4.48 shows a close-up view of the ultimate load test for the beam strengthened with CFRP fabrics and exposed to alkaline solution for 10,000 hours. Figure 4.49 shows the load versus midspan deflection responses of beams strengthened with CFRP fabrics and exposed to alkaline solution at 73oF (23oC) along with those for baseline beams. It is shown that there is no appreciable effect of the alkaline solution on the load-deflection response of CFRP fabrics strengthened beams at least up to 12 kips (53.4 kN) of load. The deflection profiles corresponding to the load of 12 kips (53.4 kN) are shown in Figure 4.50 for the beam strengthened with CFRP fabrics and exposed to alkaline solution at 73oF (23oC). It is shown that deflections of the baseline beams are larger than those of beams exposed to alkaline solution. This indicates that alkaline solution is beneficial with regard to increasing the stiffness of the beams strengthened with CFRP fabrics. Figure 4.51 shows the load versus midspan CFRP fabrics strain relationships for the beams strengthened with CFRP fabrics and exposed to alkaline solution at 73oF (23oC). It is shown that for a specific load, strains are larger for the beams exposed to alkaline solution in comparison to those of baseline beams, however, the difference in strains for the baseline beams and beams exposed to alkaline solution is reduced in the case of 10,000 hours of exposure. The strain profiles along the length of the beams strengthened with CFRP fabrics and exposed to alkaline solution for 1000, 3000, and 10,000 hours at 73oF (23oC) are presented in Figures 4.52a, 4.52b and 4.52c, respectively. It is observed that the strains in the CFRP fabrics of the baseline beams are larger than those of the beams exposed to alkaline solution at 73oF (23oC). The deflection profiles along the length of beams strengthened with CFRP fabrics and exposed to alkaline solution at 73oF (23oC) for 1000, 3000 and 10,000 hours are
97 presented in Figures 4.53a, 4.53b, and 4.53c, respectively. It is shown that unlike the beams strengthened with CFRP plates, deflection of beams (at failure load) strengthened with CFRP fabrics and exposed to alkaline solution is lower than that of baseline beams. This is attributed to the lower load carrying capacity of the beams strengthened with CFRP fabrics and exposed to alkaline solution. The load versus deflection relationships for beams strengthened with CFRP fabrics and exposed to alkaline solution for 3000 and 10,000 hours at 73oF (23oC) are shown in Figures 4.54 and 4.55, respectively. It is observed that there is no significant difference between the load-deflection responses of the baseline beams and those exposed to alkaline solution, except that the failure loads of the beams exposed to the alkaline solution are lower than those of baseline beams.
However, after the ultimate failure of the beam, all beams experience substantial loss in their load carrying capacity. The corresponding load versus strain relationships for the beams strengthened with CFRP fabrics and exposed to alkaline solution for 3000 and 10,000 hours at 73oF (23oC) are presented in Figures 4.56 and 4.57, respectively. It is observed that on average, for a particular value of load, strains in the beam exposed to alkaline solution are smaller than those of baseline beams.
4.5 Beams Exposed to Salt-Water at 73oF (23oC)
Figure 4.58 shows the discoloration of the beams strengthened with CFRP plates and fabrics after 10,000 hours of exposure to salt water. It is noticed that the beams turned to a yellow color because of the presence of the salt in the water. In this section, the deflection and strain responses of the beams strengthened with CFRP plates and CFRP fabrics and exposed to salt-water solution at 73oF (23oC) are presented.
4.5.1 Beams Strengthened with CFRP Plates
Figures 4.59a, 4.59b, and 4.59c show the load versus deflection relationships for the beams strengthened with CFRP plates and exposed to salt-water solution at 73oF (23oC) for 1000, 3000 and 10,000 hours, respectively. It is shown that there is no significant effect of salt-water solution on the load-deflection response of the beams up to 12 kips
98 (53.4 kN) load. Similarly, it is observed from Figure 4.60 that there is no significant effect of salt-water solution on the deflection profiles of beams strengthened with CFRP plates, irrespective of the duration of exposure.
The load versus strain relationships [up to 12 kips load (53.4 kN)] for beams strengthened with CFRP plates and exposed to salt-water solution for 1000, 3000 and 10,000 hours at 73oF (23oC) are presented in Figures 4.61a, 4.61b and 4.61c, respectively. It is observed that on average, strain in the CFRP plates of the beam exposed to salt-water solution is smaller than that of corresponding baseline beams. The strain profiles for the load of 12 kips (53.4 kN) along the lengths of the beams strengthened with CFRP plates and exposed to salt-water solution for 1000, 3000 and 10,000 hours at 73oF (23oC) are presented in Figures 4.62a, 4.62b and 4.62c, respectively. It is observed that there is no significant difference in the strain profiles of beams exposed to salt-water solution and those of baseline beams. It is also observed that strains in the beams exposed to salt-water solution for 3000 and 10,000 hours are lower than those of baseline beams. However, it must be noted that the effect of salt-water solution after 3000 hours of exposure is most appreciable and redistribution of strain after 10,000 hours of exposure is noticed. It is indicated from Figure 4.62c that the difference in the strain profiles of the beams exposed to 10,000 hours of salt-water solution and that of baseline beams is less in comparison to the corresponding difference of strain profiles for 3000 hours of exposure (Figure 4.62b).
The deflection profiles (at failure load) of the beams strengthened with CFRP plates and exposed to salt-water for 1000, 3000 and 10,000 hours at 73oF (23oC) are shown in Figures 4.63a, 4.63b, and 4.63c, respectively. It is shown that the deflections of beams strengthened with CFRP plates and exposed to salt-water solution are larger than that of corresponding baseline beams.
The load versus deflection relationships for beams strengthened with CFRP plates and exposed to salt-water solution for 3000 and 10,000 hours at 73oF (23oC) are shown in Figures 4.64 and 4.65, respectively. It is shown that on average, the strength of the beams exposed to salt-water solution for 3000 hours is higher in comparison to that of baseline beams. However, after 10,000 hours of exposure, one of the beams exposed to salt-water
99 solution exhibited significantly lower strength, while the other showed significantly higher strength in comparison to those of baseline beams. Thus, it is observed that the exposure to salt-water solution may be beneficial as far as the increase in the load carrying capacity of the beam is concerned. However, long-term exposure may cause early failure of the strengthened beams. The corresponding load versus strain relationships are shown in Figures 4.66 and 4.67 for beams strengthened with CFRP plates and exposed to salt-water solution for 3000 and 10,000 hours at 73oF (23oC), respectively.
4.5.2 Beams strengthened with CFRP fabrics
The Load versus deflection relationships for beams strengthened with CFRP fabrics and exposed to salt-water solution at 73oF (23oC) for 1000, 3000 and 10,000 hours are presented in Figures 4.68a, 4.68b and 4.68c, respectively. It is shown that there is no significant effect of salt-water solution on the load-deflection relationships up to 12 kips (53.4 kN). Figures 4.69a, 4.69b, and 4.69c show the deflection profiles at a load of 12 kips (53.4 kN) for beams strengthened with CFRP fabrics and exposed to salt-water solution for 1000, 3000 and 10,000 hours at 73oF (23oC), respectively. It is shown that deflections of the beams exposed to salt-water solution are smaller than those of baseline beams. It is observed from Figure 4.69 that the salt-water solution may increase the stiffness of the beam strengthened with CFRP fabrics. Figures 4.70a, 4.70b, and 4.70c show the load versus strain relationships [up to a load of 12 kips (53.4 kN)] for the beams strengthened with CFRP fabrics and exposed to salt-water solution for 1000, 3000 and 10,000 hours at 73oF (23oC), respectively. It is shown that the strain in the CFRP fabrics of the beams exposed to salt-water solution is lower in comparison to that of baseline beams. The strain profiles at the load of 12 kips (53.4 kN) along the length of beams strengthened with CFRP fabrics and exposed to 1000, 3000 and 10,000 hours of salt-water solution at 73oF are shown in Figures 4.71a, 4.71b, and 4.71c, respectively. It is shown that strains in the CFRP fabrics of baseline beams are larger than those of the beams exposed to salt-water solution. It should be noted that the responses of beams strengthened with CFRP fabrics are significantly
100 influenced with exposure to salt-water solution as observed in the case of beams strengthened with CFRP plates.
The deflection profiles at failure loads of the beams strengthened with CFRP fabrics and exposed to salt-water solution at 73oF (23oC) for 1000, 3000 and 10,000 hours are shown in Figures 4.72a, 4.72b and 4.72c, respectively. It is shown that all along the length of the beams, deflections in the beams exposed to salt-water solution are smaller in comparison to those of baseline beams. The effect of salt-water solution on the deflection of the beam is greater for the longer durations such as 3000 and 10,000 hours of exposure to salt-water solution. It should be noted that the difference in the deflection profiles of the baseline beams and beams exposed to salt-water solution is due to the combined effect of salt-water solution and reduced failure load of the beams exposed to salt-water solution.
The load-deflection responses for the beams strengthened with CFRP fabrics and exposed to salt-water solution for 3000 and 10,000 hours at 73oF (23oC) are shown in Figures 4.73 and 4.74, respectively. It is observed that the average deflection corresponding to the failure load of beams exposed to the salt-water solution is lower than that of baseline beams. Average ultimate failure loads of the beams remained almost the same after 1000, 3000, and 10,000 hours of exposure to salt-water solution. The average failure loads of the baseline beams, and beams exposed to 1000, 3000, and 10,000 hours of exposure were 30.0, 28.4, 28.4 and 28.4 kips (134, 126, 126 and 126 kN), respectively.
Figures 4.75 and 4.76 show the load versus strain relationships for beams exposed to salt-water solution at 73oF (23oC) for 3000 and 10,000 hours, respectively. It is observed that for a specific load, average strain in the CFRP fabrics of the beams exposed to 10,000 hours of salt-water solution is lower than that of baseline beams. It is also observed that the CFRP fabric strains for 10,000 hours exposure to salt-water solution are larger than corresponding strains for 3000 hours of salt-water solution.
101 4.6 Beams Exposed to Freeze/Thaw Cycles
In this section, the deflection and strain responses of beams strengthened with CFRP plates and CFRP fabrics and exposed to 350 and 700 freeze/thaw test cycles are presented and discussed. Two sets of beams were tested. Each set consisted of 2 beams strengthened with CFRP plates and 2 beams strengthened with CFRP fabrics. The first set of beams was exposed to 350 freeze/thaw test cycles, while the second set of beams was exposed to 700 freeze/thaw test cycles as per the procedure outlined in Chapter III (Experimental Program).
4.6.1 Beams strengthened with CFRP plates
Figures 4.77a and 4.77b show the load versus deflection relationships [up to a load of 12 kips (53.4 kN)] for the beams strengthened with CFRP plates and exposed to freeze/thaw conditioning for 350 and 700 cycles, respectively. It is shown that on average, there is no significant effect of freeze/thaw cycles on the load-deflection responses of the beams strengthened with CFRP Plates. The deflection profiles of beams exposed to 350 and 700 freeze/thaw test cycles along with those of baseline beams are shown in Figures 4.78a and 4.78b, respectively. It is shown that there is no significant effect of 350 freeze/thaw cycles on the deflection profiles, whereas 700 freeze/thaw cycles show appreciable effect on the deflection profile, indicating the loss of overall stiffness of the beam. The load versus CFRP plates strain (at midspan) relationships, up to a load of 12 kips (53.4 kN), are shown in Figures 4.79a and 4.79b for beams exposed to 350 and 700 freeze/thaw test cycles, respectively. It is observed that the developed CFRP plate strains corresponding to 350 and 700 freeze/thaw cycles are larger and identical, respectively, in comparison to those of baseline beams. Figures 4.80a and 4.80b show the strain profiles for the beams strengthened with CFRP plates exposed to 350 and 700 freeze/thaw test cycles, respectively. It is shown that the strains for the beams exposed to 350 and 700 cycles of freeze/thaw conditioning at 12 kips (53.4 kN) of load are lower than those of corresponding baseline beams.
102 The deflection profiles corresponding to the ultimate failure load of the beams strengthened with CFRP plates and exposed to 350 and 700 freeze/thaw test cycles are shown in Figures 4.81a and 4.81b, respectively. The deflections for the beams exposed to 350 freeze/thaw cycles are almost identical to those of baseline beams. However, deflections of the beams exposed to 700 freeze/thaw cycles are larger than those of corresponding baseline beams. The load-deflection responses of beams strengthened with CFRP plates and exposed to 350 and 700 freeze/thaw cycles are shown in Figures 4.82 and 4.83, respectively. The average ultimate failure loads of the beams exposed to 350 and 700 freeze/thaw cycles were 29.7 and 27.8 kips (132 and 124 kN), respectively. The deflections corresponding to the ultimate failure loads were larger for the beams exposed to freeze/thaw test cycles in comparison to those of baseline beams, especially for 700 freeze/thaw cycles.
Figures 4.84 and 4.85 show the load-strain responses of the beams strengthened with CFRP plates and exposed to 350 and 700 freeze/thaw test cycles, respectively. It is shown that for a specific value of the load below 24 kips (107 kN), a higher strain is observed for the beams exposed to 350 cycles of freeze/thaw conditioning. However, beyond the 24 kips (107 kN) of load, strain in the baseline beams increases and reaches those of the beams exposed to freeze/thaw test cycles.
4.6.2 Beams Strengthened with CFRP Fabrics
Figure 4.86 shows a close-up of the ultimate failure load test of the beam strengthened with CFRP fabrics and exposed to 350 cycles of freeze/thaw. Figures 4.87a and 4.87b show the load versus midspan deflection responses [up to a load of 12 kips (53.4 kN)] of the beams exposed to 350 and 700 freeze/thaw test cycles, respectively. It is shown that there is no significant effect of freeze/thaw cycles on the load-deflection response of the beam up to the applied load of 12 kips (53.4 kN). The deflection profiles corresponding to 12 kips (53.4 kN) load are shown in Figures 4.88a and 4.88b for beams strengthened with CFRP fabrics and exposed to 350 and 700 freeze/thaw cycles, respectively. It is shown that the deflection at a particular section of the beam exposed to
103 350 freeze/thaw cycles is lower than that of baseline beams. However, after exposure to 700 freeze/thaw cycles, the deflection of the beam becomes almost identical to that of baseline beams. Thus, it is noted that the greater number of freeze/thaw cycles causes redistribution of stress and restores the stiffness of the freeze/thaw conditioned beam to that of baseline beams.
Figures 4.89a and 4.89b show the load-strain responses [up to a load of 12 kips (53.4 kN)] of the beams exposed to 350 and 700 freeze/thaw test cycles, respectively. It is shown that for a specific load, strains are smaller in beams exposed to freeze/thaw cycles in comparison to those of corresponding baseline beams. However, after 700 cycles of freeze/thaw conditioning, the responses for the baseline beams and beams exposed to freeze/thaw cycles are closer than those observed in the case of beams exposed to 350 freeze/thaw test cycles. The strain profiles for a load of 12 kips (53.4 kN) are shown in Figures 4.90a and 4.90b for beams strengthened with CFRP fabrics and exposed to 350 and 700 freeze/thaw cycles, respectively. It is shown that the strains for the beams exposed to freeze/thaw test cycles are lower than those of baseline beams all along the length of beams. However, similar to deflection profiles, the strain profiles of the baseline beams and beams exposed to 700 freeze/thaw test cycles are closer than those observed for 350 freeze/thaw cycles. The deflection profiles corresponding to ultimate failure loads of the beams exposed to 350 and 700 freeze/thaw test cycles are shown in Figures 4.91a and 4.91b, respectively. It is shown that deflections for the beams strengthened with CFRP fabrics and exposed to 350 freeze/thaw cycles are lower than corresponding deflections of baseline beams. However, after 700 cycles of freeze/thaw conditioning, one of the beams shows greater deflections (except at the midspan), while the other shows lower deflections (as in the case of 350 freeze/thaw cycles) in comparison to those of baseline beams. The difference in the deflection profiles of the beams exposed to freeze/thaw conditioning and those of baseline beams is attributed to the reduction in the ultimate load carrying capacity of the beams exposed to freeze/thaw cycles. It should be noted that the difference in the deflection profiles of two beams exposed to 700 freeze/thaw cycles is due to the significant difference in their ultimate failure loads.
104 The load versus deflection responses of the beams strengthened with CFRP fabrics and exposed to 350 and 700 freeze/thaw test cycles are presented in Figures 4.92 and 4.93, respectively. Average failure loads of the baseline beams and beams exposed to 350 and 700 freeze/thaw cycles were 30.0, 28.1, and 26.1 kips (134, 125, and 116 kN), respectively. As shown in Figure 4.92, deflection corresponding to the ultimate failure load of the beam exposed to 350 freeze/thaw cycles is lower than that of the baseline beams. However, one of the two beams exposed to 700 freeze/thaw cycles exhibited lower deflection while the other showed larger deflection than that of baseline beams. This difference in responses of the two beams exposed to 700 freeze/thaw cycles is due to significant difference in their failure loads. The corresponding load- strain responses for the beams strengthened with CFRP fabrics and exposed to 350 and 700 freeze/thaw cycles are presented in Figures 4.94 and 4.95, respectively. It is shown that for a specific load, CFRP fabric strains in the beams exposed to freeze/thaw cycles are lower than those in the baseline beams. However, the difference in the load-strain responses of the beams exposed to 700 freeze/thaw cycles and that of baseline beams is reduced significantly due to further redistribution in the strain of the CFRP fabrics.
4.7 Beams Exposed to Thermal Expansion Cycles
To examine the change in the response of the beams under cyclic thermal loading, four beams consisting of two beams strengthened with CFRP plates and two beams strengthened with CFRP fabrics were exposed to 35 thermal expansion test cycles as per the procedure outlined in Section 3.3.6 of Chapter III. The test results of the beams strengthened with CFRP plates and CFRP fabrics are presented and discussed in the following sections.
4.7.1 Beams Strengthened with CFRP Plates
Figure 4.96 shows a close-up of the ultimate failure load test of the beam strengthened with CFRP plates and exposed to 35 thermal expansion cycles. Figure 4.97 shows the load versus deflection relationships up to 12 kips (53.4 kN) of load for the beams strengthened with CFRP plates and exposed to 35 thermal expansion test cycles. It is
105 observed that the deflections of beams exposed to thermal expansion test cycles are larger than those of baseline beams. The deflection profiles for the load of 12 kips (53.4 kN) are shown in Figure 4.98 for the beams strengthened with CFRP plates and exposed to 35 thermal expansion test cycles. It is observed that the deflections in the beams exposed to 35 thermal expansion test cycles are larger than those of baseline beams all along the length of beams. These deflection profiles indicate that the stiffness of beams strengthened with CFRP plates is reduced due to exposure to thermal expansion test cycles.
The load versus strains responses up to a load of 12 kips (53.4 kN) are shown in Figure 4.99 for beams exposed to 35 thermal expansion test cycles and for the baseline beams. It is shown that for a specific load, average strains in beams exposed to thermal expansion test cycles are larger than those of corresponding baseline beams. The strain profiles for the load of 12 kips (53.4 kN) are shown in Figure 4.100. It is observed that the strains in the beam exposed to 35 thermal expansion test cycles are larger than those of baseline beams all along the length of the beam.
The deflection profiles corresponding to the failure loads of the baseline beams and beams exposed to 35 thermal expansion test cycles are shown in Figure 4.101. It may be noted that the deflections of the beams exposed to thermal expansion test cycles are larger than those of baseline beams all along the length of the beams, while the ultimate failure loads of the beams exposed to thermal expansion is obviously due to the decrease in the beam stiffness as mentioned earlier.
Figure 4.102 shows the load-deflection responses of the baseline beams and beams exposed to 35 thermal expansion test cycles. It is observed that the average ultimate failure load [26 kips (116 kN)] of beams exposed to the thermal expansion test cycles is lower than that of baseline beams by about 15%. It is also observed that for a specific load, the deflection of the beam exposed to thermal expansion test cycles is larger than that of the baseline beam. The corresponding load-strain responses of the baseline beams and beams exposed to thermal expansion test cycles are shown in Figure 4.103. It is observed that for a specific load, strain in the beam exposed to thermal expansion test
106 cycles is larger than that of baseline beam. However, the maximum developed strain is observed in the case of baseline beams because of the early failure of the beams exposed to thermal expansion at a load lower than the ultimate failure load of baseline beam.
4.7.2 Beams Strengthened with CFRP Fabrics
Figure 4.104 shows the load-deflection responses up to 12 kips (53.4 kN) of load for beams strengthened with CFRP fabrics and exposed to 35 thermal expansion test cycles. It is observed that there is no significant effect of thermal expansion test cycles on the load-deflection responses of the beams strengthened with CFRP fabrics. Similarly, it is also observed from Figure 4.105 that the deflection profiles of the beam strengthened with CFRP fabrics remain almost unchanged after exposure to thermal expansion test cycles.
Figure 4.106 shows the load versus midspan CFRP fabrics strain relationship up to 12 kips (53.4 kN) of load for the beams exposed to 35 thermal expansion test cycles. It is observed that up to 9 kips (40 kN) of load, there is no significant difference in the load- strain responses of baseline beams and beams exposed to thermal expansion test cycles. However, beyond 9 kips (40 kN) of load, a slightly lower strain is observed in beams exposed to thermal expansion in comparison to that of baseline beams. The strain profiles corresponding to the load of 12 kips (53.4 kN) are shown in Figure 4.107. It is observed that the strains in the beams exposed to thermal expansion test cycles are significantly lower than those of baseline beams.
The deflection profiles corresponding to the ultimate failure loads of the baseline beams and beams exposed to 35 thermal expansion test cycles are shown in Figure 4.108. It is observed that the deflections of beams exposed to thermal expansion test cycles are slightly higher than those of baseline beam except at the midspan, where deflections of the baseline beam and beams exposed to thermal expansion are almost identical.
The load versus deflection relationships for the beams exposed to 35 thermal expansion test cycles and those of baseline beams are presented in Figure 4.109. It is observed that the ultimate failure loads of the beams exposed to thermal expansion cycles
107 are greater than those of the baseline beams. However, the deflection associated with ultimate failure load is slightly greater in the case of the baseline beams. The corresponding load-strain responses are shown in Figure 4.110. It is shown that for a specific load, the strains in the CFRP fabrics of the beams exposed to thermal expansion test cycles are lower than that of baseline beams.
4.8 Weight Change Due to Exposure to Various Environmental Conditions
All the beams were weighed before their exposure to various environmental conditions. After the end of the exposure to 10,000 hours for the beams exposed to 100% humidity, alkaline-solution, salt-water, and dry-heat, the beams were left for 14 days prior to weighing them to recognize the difference in weight. The results obtained for the weight change confirmed those found in the studied literature (Woo and Piggot (1988)). Figure 4.111 presents the weights of the beams before and after exposure to different environmental conditions. The beams strengthened with CFRP plates and exposed to dry- heat lost on average 2.5% of their weight, while those strengthened with CFRP fabrics lost an average of 4.5% of their original weight.
The beams soaked in salt-water, alkaline solution, or 100% humidity in hot water and strengthened with CFRP plates gained on average 2% of their weight original weight, while this percentage was only 1% for those beams strengthened with CFRP fabrics. For beams exposed to freeze/thaw cycles, and for both types of strengthening with plates or with fabrics, the weight of the beams experienced only minor changes.
4.9 Beams Exposed to Repeated Loads
In order to examine the effects of repeated loads on the load-deflection response, and ultimate failure loads of the beams strengthened with CFRP plates/fabrics, beams strengthened with CFRP plates and CFRP fabrics were subjected to constant amplitude repeated load cycles. Three constant amplitude load ranges equal to 15%, 25%, and 40% of the ultimate failure loads of the strengthened beams were considered (Figure 4.112). A set of four beams consisting of two beams strengthened with CFRP plates and two beams strengthened with CFRP fabrics were subjected to each load range for a total of 2 million
108 cycles. The load-deflection responses (up to a load of 12 kips) were predicted by conducting static load tests at the beginning of the repeated load cycles (0 cycle) and after 0.1, 1, and 2million cycles. The load-deflection responses (up to a load of 12 kips) were predicted by conducting static load tests at the beginning of the repeated load cycles (0 cycle) and after completion of 0.1, 1, and 2 million cycles of repeated load test. However, the ultimate load test was conducted only after execution of 2 million cycles of repeated load of constant amplitude.
The mean values of the repeated loads were 2,475, 4,125, and 6,600 lbs, i.e., 8.3%, 13.8%, and 22% of the ultimate strength of the beams, and correspond to the load ranges of 15%, 25%, and 40% of the ultimate strength, respectively. It should be noted that the CFRP plates and CFRP fabrics strengthened beams were designed to have the same ultimate strength. The responses of beams strengthened with CFRP plates and CFRP fabrics are discussed in the following sections. In the discussion of test results, the repeated load ranges of 15%, 25%, and 40% are referred as load ranges 1, 2, and 3, respectively. It should be also noted that the load ratio, R (defined as the ratio of minimum value of the load cycle to the maximum value of the load) was kept constant (0.1). In figures, alphanumeric characters such as P-R15-1, and F-R15-1 designate beams. In the beam designations, P refers to the beam strengthened with CFRP plates; F refers to the beam strengthened with CFRP fabrics; R15 refers to the repeated load range of 15% of the ultimate strength of the CFRP strengthened beam, and the last numeral represents the beam designation number.
4.9.1 Beams Strengthened with CFRP Plates
Figures 4.113a, 4.113b, and 4.113c show the load versus midspan deflection relationships for beams strengthened with CFRP plates subjected to 0, 0.1, 1, and 2 million test cycles of the repeated loads of load ranges 1, 2, and 3, respectively. It is shown that for load range 1 (Figure 4.113a), the beam behaves as a linear elastic structure, irrespective of the number of repeated load cycles. However, for the load range 2, the beam strengthened with CFRP plates sustained initial deformation after 0.1 million cycles of repeated load and this deformation remains unchanged for the next repeated
109 load cycles. However, it is observed that for the load range 3 (Figure 4.113c), initial sustained deformation caused due to repeated load test cycles is negligible.
The deflection profiles corresponding to the load of 12 kips (53.4 kN) for the beam strengthened with CFRP plates and subjected to repeated load ranges 1, 2, and 3 are shown in Figures 4.114a, 4.114b, and 4.114c, respectively. It is shown that the deflection profiles are similar to the maximum deflection at the center of the beam for all load ranges, irrespective of number of test cycles. In general, it is observed that repeated load test cycles reduce the stiffness of the beam. It is also observed that for smaller load ranges such as 1 and 2, increasing the repeated load test cycles beyond 0.1 million has no appreciable effect on the deflection profiles.
Unlike the cases of load ranges 1 and 2, the deflection profiles (Figure 4.114c) for load range 3 show that the deflections along the length of beam remain unchanged for 0 to 1 million repeated load test cycles. Thus, it could be concluded that there is no effect of increase in the number of cycles beyond 0.1 million cycles for small load ranges (1 and 2), whereas for the highest load range (i.e., load range 3), the repeated load cycles will not appreciably affect the deflection profile of the beam. The variation of cyclic deflection with number of cycles of repeated load ranges 1, 2, and 3 are shown in Figures 4.115a, 4.115b, and 4.115c, respectively. It is shown that the variation in the amplitude of the deflection is consistent with the amplitude of load range.
Figures 4.116a, 4.116b, and 4.116c show the load versus strain (at midspan of the CFRP plates) relationships up to a load of 12 kips (53.4 kN) for load ranges 1, 2, and 3, respectively for 0, 0.1, 1, and 2 million test cycles. It is shown that there is no significant effect of the repeated load cycles on the load-strain responses for load ranges 1 and 2. However, for load range 3, it is observed that for a specific load, strain in the beam subjected to repeated load cycles is larger than without repeated load cycles. Also, no significant change in the load-strain response is observed after 1 million repeated load test cycles.
110 The CFRP plate strain profiles corresponding to the load of 12 kips (53.4 kN) are shown in Figures 4.117a, 4.117b, and 4.117c for load ranges 1, 2, and 3, respectively for 0, 0.1, 1, and 2 million cycles. It is observed that for load range 1, the maximum strain in the CFRP plates occurred at the midspan of the plates. However, for load ranges 2 and 3, a horizontal strain plateau is formed in the middle portion of the length of the plates. It must be noted that for load range 1, strains corresponding to the 0.1, 1 and 2 million repeated load test cycles are larger than that of 0 cycle. However, at the midspan of the plates, strains for 0 and 0.1 million cycles are almost the same and less than those for 1 and 2 million cycles. From Figure 4.117b, it is observed that there is no significant effect of repeated load cycles on the strain profiles for the load range 2. However, strain values for 2 million cycles of repeated load range 3 are significantly larger than that of the other load cycles. The variation of cyclic strain of the CFRP plates of the beam strengthened with CFRP plates with number of repeated load test cycles is shown in Figures 4.118a, 4.118b, and 4.118c for load ranges 1, 2, and 3, respectively. It is observed that like deflection, cyclic strain is consistent with constant amplitude load cycles.
Figure 4.119 shows the comparison of the ultimate load carrying capacity of unstrengthened beams and baseline beams (strengthened beams without repeated load effects), and beams (P-R15-1, P-R25-1, and P-R40-1) subjected to repeated load test cycles. It is shown that the baseline beam has 59% higher strength than that of the unstrengthened beam. As shown in Figure 4.119, the repeated load test cycles of load ranges 1, 2, and 3 have no significant effect on the ultimate load carrying capacity of the beams strengthened with CFRP plates.
4.9.2 Beams Strengthened with CFRP Fabrics
Figures 4.120a, 4.120b, and 4.120c show the load versus deflection relationships for the beams strengthened with CFRP fabrics and subjected to repeated load test cycles of load ranges 1, 2, and 3, respectively. It is shown that there is no significant effect of the repeated load test cycles on the load-deflection responses of the beams strengthened with CFRP fabrics, irrespective of the magnitude of the repeated load range. Similarly, it is observed from Figure 4.121 that deflection profiles of the beams strengthened with CFRP
111 fabrics remain almost unchanged, irrespective of the magnitude of load range and number of repeated load test cycles.
The variation of the cyclic deflection of the beam strengthened with CFRP fabrics with the number of repeated load test cycles is shown in Figures 4.122a, 4.122b, and 4.122c for load ranges 1, 2, and 3, respectively. As in the case of the beam strengthened with plates, cyclic deflection of the beam strengthened with CFRP fabrics is also of constant amplitude throughout the loading cycles. As expected, higher the load range causes higher amplitude of cyclic deflection.
The load versus midspan CFRP fabric strain responses for the beam strengthened with CFRP fabrics are shown in Figures 4.123a, 4.123b, and 4.123c for load ranges 1, 2, and 3, respectively. It is observed (Figure 4.123a) that after the application of the load of 12 kips (53.4 kN) at the beginning (at 0 cycle) of the repeated loading cycles and subsequent unloading develops a residual strain in the fabrics. Also, the load strain relationship is observed to be piecewise linear. However, no residual deformation is observed after loading and unloading of the beam subjected to repeated load cycles of load range 1. Figures 4.123b and 4.123c indicate that number of repeated load cycles has significant effect on the load-strain responses of beams strengthened with CFRP fabrics. It is observed that for a specific load, strain is higher (Figures 4.123a and 4.123b) for the beam subjected to a greater number of repeated load cycles. It is worth noting that unlike load range 2, there is no residual strain in the CFRP fabrics after the static loading and unloading cycle of the beam subjected to the load range 3.
The CFRP fabrics strain profile along the length of the beam is shown in Figures 4.124a, 4.124b, and 4.124c, respectively for load ranges 1, 2, and 3, respectively. It is observed that unlike the beams strengthened with CFRP plates (Figure 4.117), the strain profiles of the beam strengthened with CFRP fabrics indicate that the effect of repeated load test cycles on strain profiles decreases as the magnitude of load range increases. The variation of cyclic CFRP fabric strain with the number of repeated load test cycles is shown in Figures 4.125a, 4.125b, and 4.125c for load ranges 1, 2, and 3, respectively. It is shown that during load range 1 (Fig. 4.127a), the amplitude of the cyclic strain remains
112 constant for the entire number of cycles, however, the mean value of the cyclic strain is slightly increased after 1 million cycles. Similar observations are made for cyclic strains of load ranges 2 and 3. However, the variation in the amplitude of cyclic strain is significant for load range 3.
Figure 4.126 shows the comparison of the ultimate failure loads of the unstrengthened beam, baseline beam, and beams strengthened with CFRP fabrics and subjected to repeated load ranges 1, 2, and 3. It is observed that the effect of repeated load on the ultimate strength of the beam strengthened with CFRP fabrics is not significant. The maximum decrease in the load carrying capacity of the beam strengthened with CFRP fabrics is observed to be 2.7% for the load ranges 1 and 2. However, the repeated load of the load range 3 causes about a 0.7% increase in the strength of the beam strengthened with CFRP fabrics.Thus, like in the case of beams strengthened with CFRP plates, the repeated load cycles have no significant effect on the load carrying capacity of the beams strengthened with fabrics.
4.10 Comparison of Ultimate Load Results
In order to compare the ultimate load carrying capacity and corresponding deflection of the beams exposed to environmental conditions with that of baseline beams, results at failure loads of the beams strengthened with CFRP plates and CFRP fabrics are presented in the following sections.
4.10.1 Beams strengthened with CFRP plates
Figure 4.127 and Table 4.1 show the ultimate failure loads of the unstrengthened beams, baseline beams, and beams exposed to dry-heat, humidity, salt-water, and alkaline solution for 1000, 3000, and 10,000 hours. It is observed that the strengthening of beams using CFRP plates increased the load carrying capacity of beams by about 59%. It is also observed that the long-term humidity conditioning of beams strengthened with CFRP plates significantly reduces the load carrying capacity of these beams. However, there is no significant effect of dry-heat conditioning on the ultimate load capacity of the beam.
113
Table 4.1 Ultimate load (kips) for beams strengthened with CFRP plates and exposed to various environmental conditions. Environmental Load at 1000 Hrs Load at 3000 Hrs Load at 10,000 Hrs Exposure Dry-heat 28.50 26.78 27.64
Humidity 33.25 25.30 20.87
Salt-Water 32.50 33.69 33.10
Alkali Solution 30.70 33.47 32.09
Baseline* 30.70* ------
Control Beams* 19.30* ------
* No exposure to environmental conditions.
114 The percentage reduction in the strength of the beams after 3000 and 10,000 hours of humidity conditioning are 18% and 32%, respectively. However, it is observed that beams strengthened with CFRP plates and exposed to salt-water and alkaline solution exhibit improved load carrying capacity, irrespective of number of hours of exposure. The maximum load carrying capacity of CFRP plate strengthened beams is observed for 3000 hours of exposure. Thus, it could be concluded that the salt-water and alkaline solution conditionings of the strengthened beams for 3000 hours are beneficial with regard to the improved load carrying capacity of the strengthened beams. However, long- term exposure may decrease the strength. The possible cause for the improvement of the load carrying capacity of these beams is the better curing resulting from the immersion in salt-water or alkaline solution. It should be noted that the baseline beams were not cured by immersion.
The ultimate load carrying capacities of CFRP plate strengthened beams exposed to freeze/thaw and thermal expansion test cycles, are presented in Figure 4.128 and Table 4.2. It is observed that the 35 thermal expansion test cycles reduced the strength of the beams by about 15%. It is also observed that in general, freeze/thaw test cycles decrease the load carrying capacity of the beams. The decreases in the load carrying capacity of beams strengthened with CFRP plates and exposed to 350 and 700 freeze/thaw test cycles were about 3.3% and 9.5%, respectively.
The deflections corresponding to the ultimate failure loads of the beams strengthened with CFRP plates and exposed to dry-heat, humidity, salt-water, and alkaline solutions are presented in Figure 4.129 and Table 4.3. It is shown that the deflection corresponding to the ultimate failure loads of the beams strengthened with CFRP plates is the highest for 10,000 hours of exposure to alkaline solution, while it is the lowest for 10,000 hours of humidity conditioning. It is worth noting that the increase in the duration of exposure of humidity conditioning decreases the deflection, while the corresponding increase in the duration of alkali conditioning increases the deflection associated with ultimate failure loads.
115
Table 4.2 Ultimate load for beams strengthened with CFRP plates and exposed to various thermal conditioning Test Load
Baseline Beams* 30.7* Freeze/thaw 350 cycles 26.85 Freeze/thaw 700 cycles 27.82
Thermal Expansion 35 cycles 25.94
* No exposure to environmental conditions
116
Table 4.3 Midspan deflection (in.) at failure for beams strengthened with CFRP plates and exposed to different environmental conditions
Environmental Deflection Deflection Deflection Exposure at 1000 Hrs at 3000 Hrs at 10,000 Hrs
Dry-heat 0.58 0.67 0.65
Humidity 0.71 0.62 0.53
Salt-Water 0.67 0.75 0.68
Alkali Solution 0.63 0.75 0.81
Baseline* 0.63 ------
* No exposure to environmental conditions.
117 4.10.2 Beams Strengthened with CFRP Fabrics
Figure 4.130 and Table 4.4 show the ultimate failure loads of the unstrengthened beams, baseline beams, and beams exposed to dry-heat, humidity, salt-water, and alkaline solution, while Figure 4.131 and Table 4.5 show the ultimate failure loads of the CFRP fabrics strengthened beams exposed to freeze/thaw and thermal expansion test cycles. It is shown that the short-term exposure (up to 3000 hours) to dry-heat conditioning increases the load carrying capacity of the CFRP fabric strengthened beams in comparison to that of baseline beams. It is observed that humidity and salt-water solution decrease the load carrying capacity of the beams strengthened with CFRP fabrics, however, duration of exposure to humidity and salt-water solution have no significant effect, as observed in the case of beams strengthened with CFRP plates (Figure 4.127).
It is worth noting that the reduction in the ultimate load carrying capacity of beams strengthened with CFRP fabrics and exposed to salt-water solution is the same (about 5.3%), irrespective of duration of exposure. From Figure 4.131, it is observed that thermal expansion test cycles have no significant effect on the load carrying capacity of the beam, while 700 freeze/thaw test cycles reduced the load carrying capacity of the beam by about 13%.
The deflections corresponding to the ultimate failure loads of beams strengthened with CFRP fabrics and exposed to 1000, 3000, and 10,000 hours of dry-heat, humidity, salt-water, and alkali-solutions are presented in Figure 4.132 and Table 4.6. It is observed that the maximum deflection occurs for 10,000 hours of exposure to dry-heat conditioning, while the lowest deflection occurs in the case of salt-water conditioning. Thus, it is observed that a maximum of about a 17% difference in the deflection due to environmental conditioning (salt-water) results with respect to the baseline beam.
Based on the experimental results for the beams strengthened with CFRP plates and CFRP fabrics, and exposed to various environmental conditions, strength reduction factors were predicted corresponding to the CFRP strengthened beams exposed to a specific environmental condition. The strength reduction factors (Table 4.7) are based on the ratio of measured load carrying capacity of strengthened beams after being exposed to
118 a specific independent environmental condition to that of baseline beams not exposed to the environmental conditioning. It should be noted that these strength reduction factors do not take into account the aged concrete and associated concrete defects, deterioration or contamination. However, the effect of concrete deterioration caused due to exposure to a specific aggressive prevalent environmental condition will be taken care of by the proposed strength reduction factor. These strength reduction factors are valid for real world situation, i.e., service bridges under a specific prevalent environmental condition. The long-term strength reduction factors associated with different environmental conditions for the CFRP strengthened beams are proposed in Table 4.7 for easy reference. These strength-reduction factors are useful for durability-based analysis and design of CFRP strengthened beams. The reduction factors established during this investigation were compared to those presented by Seible et al. (1997). It should be noted that the properties of the CFRP materials and bonding agents used in both research investigations were different. In both investigations, the immersion in water had a severe effect on the CFRP strengthened beams. The strength reduction factor for that effect was 0.80 for Seible et al., while in this investigation it was found to be 0.70 for the beams strengthened with CFRP plates and exposed to hot water for 417 days. Results related to the immersion in seawater were also consistent. Seible et al. showed that the exposure to seawater decreased the ultimate load carrying capacity of the beams by 75% to 85%. In this report, the reduction factor related to the exposure to a salt-water solution for 417 days was 0.95 for the beams strengthened with CFRP plates, and 0.90 for those strengthened with CFRP fabrics. Other types of environmental conditions were not considered in Seible et al. research.
119
Table 4.4 Ultimate load (kips) for beams strengthened with CFRP fabrics and exposed to various environmental conditions. Environmental Load at 1000 Hrs Load at 3000 Hrs Load at 10,000 Hrs Exposure Dry-heat 30.50 30.78 30.64 Humidity 27.90 28.28 27.10 Salt-Water 28.40 28.39 28.39 Alkali Solution 29.90 28.31 29.10 Baseline* 29.95* ------Control Beams* 19.30* ------
* No exposure to environmental conditions.
120
Table 4.5 Ultimate load (kips) for beams strengthened with CFRP fabrics and exposed to various thermal conditioning. Test Fabrics
Baseline Beams* 29.95* Freeze/thaw 350 cycles 28.12 Freeze/thaw 700 cycles 26.12
Thermal Expansion 35 cycles 30.29
* No exposure to environmental conditions.
121
Table 4.6 Midspan deflection (in.) at failure for beams strengthened with CFRP fabrics and exposed to different environmental conditions.
Environmental Deflection Deflection Deflection Exposure at 1000 Hrs at 3000 Hrs at 10,000 Hrs
Dry-heat 0.86 0.90 1.01
Humidity 0.78 0.76 0.80
Salt-Water 0.85 0.75 0.84
Alkali Solution 0.85 0.77 0.87
Baseline* 0.90* ------
* No exposure to environmental conditions.
122
Table 4.7 CFRP strength reduction factors (y) for different environmental conditionings. Environmental Condition CFRP Plates CFRP Fabrics
100 % Humidity 0.70 0.90
Dry-heat conditioning 0.90 1.00
Alkaline conditioning 1.00 0.90
Freeze/thaw conditioning 0.90 0.85
Salt-water solution 0.95 0.90
123 Repeatability
The ultimate failure loads presented in Tables 4.1, 4.2, 4.4, and 4.5 represent the average values for two identical beams (strengthened with either CFRP plates or fabrics), exposed to the same environmental condition. Table 4.8 presents the ultimate failure load for each beam tested during this investigation, in order to allow the reader to evaluate the minimum and maximum values encountered. It is noticed that in most cases the maximum range of variation was within ±10% of the average value of the ultimate failure load. The major exception encountered was in the case if the beams strengthened with CFRP plates and exposed to 100% humidity for 3000 hours where one of the beams had an ultimate failure load of 29.79 kips while the other beam had a failure load of 20.81 kips.
124
Figure 4.1 Close-up of ultimate load test of beam strengthened with CFRP plate after 10,000 hours exposure to 100% humidity condition
125
12 12 12
9 9 9
6 6 6 Load (kip) Load (kip) Load (kip)
3 3 3 P-W1k-1 P-W3k-1 P-W10k-1 P-W1k-2 P-W3k-2 P-W10k-2 Plate Baseline 1 Plate Baseline 1 Plate Baseline 1 Plate Baseline 2 Plate Baseline 2 Plate Baseline 2
0 0 0 0.0 0.5 1.0 0.0 0.5 1.0 0.0 0.5 1.0
Deflection (inch) Deflection (inch) Deflection (inch) (a) Exposure of 1000 hours (b) Exposure of 3000 hours (c) Exposure of 10,000 hours
Figure 4.2 Deflection at midspan of beams strengthened with CFRP plates and exposed to 100% humidity condition @100°F
126
0.0 (a) Exposure of 1000 hours
P-W1k-1 P-W1k-2 Plate Baseline 1 Plate Baseline 2 0.5 Deflection (in.) 0 1 2 3 4 5 6 7 8 Distance along length of beam (feet)
(in.) 0.0 (b) Exposure of 3000 hours
P-W3k-1 P-W3k-2 Plate Baseline 1 Plate Baseline 2 Deflection 0.5 0 1 2 3 4 5 6 7 8 Distance along length of beam (feet)
0.0 (c) Exposure of 10,000 hours
P-W10k-1 P-W10k-2 Plate Baseline 1 Plate Baseline 2 0.5 Deflection (in.) 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
Figure 4.3 Deflected shape at 12 kip load for beams strengthened with CFRP plates exposed to 100% humidity condition @100°F
127
12 12 12
9 9 9
6 6 6 Load (kip) Load (kip)
Load (kip)
3 3 3 P-W1k-1 P-W3k-1 P-W10k-1 P-W1k-2 P-W3k-2 P-W10k-2 Plate Baseline 1 Plate Baseline 1 Plate Baseline 1 Plate Baseline 2 Plate Baseline 2 Plate Baseline 2 0 0 0 0 1000 2000 3000 0 1000 2000 3000 0 1000 2000 3000
MicroStrain (in./in.) MicroStrain (in./in.) MicroStrain (in./in.)
(a) Exposure of 1000 hours (b) Exposure of 3000 hours (c) Exposure of 10,000 hours
Figure 4.4 Strain in CFRP plate at midspan of beams strengthened with CFRP plates and exposed to 100% humidity condition @100°F
128 3000
2000 P-W1k-1 P-W1k-2 Plate Baseline 1 Plate Baseline 2
1000 (a) Exposure of 1000 hours 0 MicroStrain (in./in.) 0 1 2 3 4 5 6 7 8 Distance along length of beam (feet)
3000 P-W3k-1 P-W3k-2 Plate Baseline 1 Plate Baseline 2 2000
1000 (b) Exposure of 3000 hours
MicroStrain (in./in.) 0 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
3000 P-W10k-1 P-W10k-2 Plate Baseline 1 Plate Baseline 2 2000
1000 (c) Exposure of 10,000 hours
MicroStrain (in./in.) 0 0 1 2 3 4 5 6 7 8
Distance along length of Beam (feet)
Figure 4.5 Strain in CFRP plate of beams strengthened with CFRP plates and exposed to 100% humidity @100°F at 12 kip load
129 0.0
(a) Exposure of 1000 hours 0.5
P-W1k-2 Deflection (in.) P-W1k-1 Plate Baseline 1 Plate Baseline 2 1.0 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
0.0
(b) Exposure of 3000 hours 0.5
P-W3k-1 Plate Baseline 1 Plate Baseline 2 Deflection (in.) 1.0 0 1 2 3 4 5 6 7 8
Distance along beam length (feet)
0.0 (c) Exposure of 10,000 hours n (in.) 0.5
P-W10k-1 P-W10k-2 Plate Baseline 1
Deflectio Plate Baseline 2 1.0 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet) Figure 4.6 Deflected shape at failure load of beams strengthened with CFRP plates and exposed to 100% humidity @100°F
130 36
33
P-W3k-1 30 Plate Baseline 1
27 Plate Baseline 2
24
21
Load (kip) 18
15
12
9
6
3
0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Midspan Deflection (in.) Figure 4.7 Load versus deflection relationships for beams strengthened with CFRP plates and exposed to 3000 hours of 100% humidity @100°F
131 36
33 P-W10k-1
30 P-W10k-2 Plate Baseline 1 27 Plate Baseline 2
24
21
18 Load (kip) 15
12
9
6
3
0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Midspan Deflection (in.)
Figure 4.8 Load versus deflection relationships for beams strengthened with CFRP plates and exposed to10,000 hours of 100% humidity @100°F
132 36
33
30
27
24
21
18 Load (kip)
15
12
P-W3k-1 9 Plate Baseline 1
6 Plate Baseline 2
3
0 0 1000 2000 3000 4000 5000
Midspan MicroStrain (in./in.)
Figure 4.9 Load versus strain relationships for beams strengthened with CFRP plates and exposed to 3000 hours of 100% humidity @100°F
133 36
33
30
27
24
21
18
Load (kip)
15
P-W10k-1 12
P-W10k-2
9 Plate Baseline 1
Plate Baseline 2
6
3
0 0 1000 2000 3000 4000 5000
Midspan MicroStrain (in./in.)
Figure 4.10 Load versus strain relationships for beams strengthened with CFRP plates and exposed to 10,000 hours of 100% humidity @100°F
134
12 12 12
9 9 9
6 6 6 Load (kip) Load (kip) Load (kip)
3 3 3 F-W10k-1 F-W1k-1 F-W3k-1
F-W1k-2 F-W3k-2 F-W10k-2 Fabric Baseline 1 Fabric Baseline 1 Fabric Baseline 1
0 0 0 0.0 0.5 1.0 0.0 0.5 1.0 0.0 0.5 1.0
Deflection (inch) Deflection (inch) Deflection (inch)
(a) Exposure of 1000 hours (b) Exposure of 3000 hours (c) Exposure of 10,000 hours
Figure 4.11 Deflection at midspan of beams strengthened with CFRP fabrics and exposed to 100% humidity condition @ 100°F
135 0.0 (a) Exposure of 1000 hours
F-W1k-1 F-W1k-2 Fabric Baseline 1 Deflection (in.) 0.5 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
0.0
(b) Exposure of 3000 hours
F-W3k-1 F-W3k-2 Fabric Baseline 1 Deflection (in.) 0.5 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
0.0 (c) Exposure of 10,000 hours
F-W10k-1 F-W10k-2 Fabric Baseline 1 Deflection (in.) 0.5 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
Figure 4.12 Deflected shape at 12 kip load for beams strengthened with CFRP fabrics and exposed to 100% humidity @100°F
136
12 12 12
9 9 9
p) 6 6 6 Load (ki Load (kip) Load (kip)
3 3 3 F-W1k-1 F-W3k-1 F-W10k-1
F-W1k-2 F-W3k-2 F-W10k-2
Fabric Baseline 1 Fabric Baseline 1 Fabric Baseline 1
0 0 0 0 1000 2000 3000 0 1000 2000 3000 0 1000 2000 3000
MicroStrain (in./in.) MicroStrain (in./in.) MicroStrain (in./in.)
(a) Exposure of 1000 hours (b) Exposure of 3000 hours (c) Exposure of 10000 hours
Figure 4.13 Strain at midspan of CFRP fabric for beams strengthened with CFRP fabrics and exposed to100% humidity @100°F
137 3000 F-W1k-1 F-W1k-2 Fabric Baseline 1 2000
1000 (a) Exposure of 1000 hours MicroStrain (in./in.) 0 0 1 2 3 4 5 6 7 8
Distance along length of Beam (feet)
3000 F-W3k-1 F-W3k-2 Fabric Baseline 1
2000
1000 (b) Exposure of 3000 hours MicroStrain (in./in.) 0 0 1 2 3 4 5 6 7 8
Distance along length of Beam (feet)
3000 F-W10k-1 F-W10k-2 Fabric Baseline 1 n./in.) (i 2000
1000 MicroStrain (c) Exposure of 10,000 hours
0 0 1 2 3 4 5 6 7 8
Distance along length of Beam (feet)
Figure 4.14 Strain in CFRP fabric of beams strengthened with CFRP fabrics and exposed to 100% humidity @100°F at 12 kips load
138 0.0
(a) Exposure of 1000 hours 0.5
Deflection (inch) 1.0 F-W1k-1 F-W1k-2 Fabric Baseline 1 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
0.0
0.5 (b) Exposure of 3000 hours
Deflection (inch) 1.0 F-W3k-1 F-W3k-2 Fabric Baseline 1
0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
0.0
(c) Exposure of 10,000 hours 0.5
Deflection (inch) 1.0 F-W10k-1 F-W10k-2 Fabric Baseline 1
0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
Figure 4.15 Deflected shape at failure load for beams strengthened with CFRP fabrics and exposed to 100% humidity @100°F
139
36
33 F-W3k-1
30 F-W3k-2 Fabric Baseline 1
27 Fabric Baseline 2
24
21
18
Load (kip) 15
12
9
6
3
0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Midspan Deflection (in.)
Figure 4.16 Load versus deflection relationships for beams strengthened with CFRP fabrics and exposed to 3000 hours 100% humidity @100°F
140
36
33 F-W10k-1
F-W10k-2 30 Fabric Baseline 1 27 Fabric Baseline 2
24
21
18
Load (kip) 15
12
9
6
3
0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Midspan Deflection (in.) Figure 4.17 Load versus deflection relationships for beams strengthened with CFRP fabrics and exposed to 10,000 hours of 100% humidity @100°F
141
36
33
30
27
24
21
(kip) 18 Load 15
F-W3k-1 12
Fabric Baseline 1 9
Fabric Baseline 2 6
3
0 0 1000 2000 3000 4000 5000 6000 7000 8000
Midspan MicroStrain (in./in.)
Figure 4.18 Load versus strain relationships for beams strengthened with CFRP fabrics and exposed to 3000 hours of 100% humidity @100°F
142
36
33
30
27
24
21
18 Load (kip) 15
12 F-W10k-1 F-W10k-2 9 Fabric Baseline 1
6 Fabric Baseline 2
3
0 0 1000 2000 3000 4000 5000 6000 7000 8000
Midspan MicroStrain (in./in.)
Figure 4.19 Load versus strain relationships for beams strengthened with CFRP fabrics and exposed to 10,000 hours of 100% humidity @100°F
143
12 12 12
9 9 9
6 6 6 Load (kips) Load (kips) Load (kips)
3 3 3 P-H3k-1 P-H10k-1 Beam P -H1k-1 Beam P -H1k-2 P-H3k-2 P-H10k-2 Plate Baseline 1 Plate Baseline 1 Plate Baseline 1 Plate Baseline 2 Plate Baseline 2 Plate Baseline 2
0 0 0 0.0 0.5 1.0 0.0 0.0 0.5 1.0 0.5 1.0 Deflection (in.) Deflection (in.) Deflection (in.)
(a) Exposure of 1000 hours (b) Exposure of 3000 hours
Figure 4.20 Deflection at midspan of for beams strengthened with CFRP plates and exposed to dry heat condition @140°F
144
0.0 (a) Exposure of 1000 hours
Beam P-H1k-1 Beam P-H1k-2 Plate Baseline 1 Plate Baseline 2 Deflection (in.) 0.5 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
0.0 (b) Exposure of 3000 hours
Beam P-H3k-1 Beam P -H3k-2 Plate Baseline 1 Plate Baseline 2 Deflection (in.) 0.5 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
0.0 (c) Exposure of 10,000 hours
Beam P -H10k-1 Beam P-H10k-2 Plate Baseline 1 Plate Baseline 2 Deflection (in.) 0.5 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
Figure 4.21 Deflected shape at 12 kips load for beams strengthened with CFRP plates and exposed to dry heat condition @ 140°F
145
12 12 12
9 9 9
6 6 6 Load (kip) Load (kip) Load (kip)
P-H10k-1 3 P-H1k-1 3 P-H3k-1 3 P-H10k-2 P-H1k-2 P-H3k-2 Plate Baseline 1 Plate Baseline 1 Plate Baseline 1
Plate Baseline 2 Plate Baseline 2 Plate Baseline 2
0 0 0 0 1000 2000 3000 0 1000 2000 3000 0 1000 2000 3000
MicroStrain (in./in.) MicroStrain (in./in.) MicroStrain (in./in.)
(a) Exposure of 1000 hours (b) Exposure of 3000 hours (c) Exposure of 10,000 hours
Figure 4.22 Strain at midspan of CFRP plate for beams strengthened with CFRP plates and exposed to dry-heat condition @ 140°F
146
3000
Beam P -H1k-1 Beam P -H1k-2 Plate Baseline 1 Plate Baseline 2 2000
1000
MicroStrain (in./in.) (a) Exposure of 1000 hours
0 0 1 2 3 4 5 6 7 8
Distance along length of Beam (feet) 3000
Beam P -H3k-1 Beam P -H3k-2 Plate Baseline 1 Plate Baseline 2 2000
1000
MicroStrain (in./in.) (b) Exposure of 3000 hours 0 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
3000 Beam P -H10k-1 Beam P -H10k-2 Plate Baseline 1 Plate Baseline 2
2000
1000
MicroStrain (in./in.) (c) Exposure of 10,000 hours
0 0 1 2 3 4 5 6 7 8
Distance along length of Beam (feet)
Figure 4.23 Strain in CFRP plate at 12 kip load for beams strengthened with CFRP plates and exposed to dry heat condition @ 140°F
147 0.0
(c) Exposure of 10,000 hours
0.5
Deflection (in.) P-H10k-1 P-H10k-2 Plate Baseline 1 Plate Baseline 2 1.0 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
0.0
(a) Exposure of 1000 hours
0.5
Deflection (in.) P-H1k-1 P-H1k-2 Plate Baseline 1 Plate Baseline 2
1.0 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
0.0
(b) Exposure of 3000 hours 0.5
Deflection (in.) P-H3k-1 P-H3k-2 Plate Baseline 1 Plate Baseline 2
1.0 0 1 2 3 4 5 6 7 8
Distance along beam length (feet)
Figure 4.24 Deflected shape at failure load for beam strengthened with CFRP plates and exposed to dry heat condition @ 140°F 148 36
33 P-H3k-1
P-H3k-2 30 Plate Baseline 1 27
Plate Baseline 2 24
21
18 Load (kip)
15
12
9
6
3
0 3.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0
Midspan Deflection (in.) Figure 4.25 Load versus deflection relationships for beams strengthened with CFRP plates and exposed to dry heat condition for 3000 hours @ 140°F
149 36
33 P-H10k-1 30 P-H10k-2
Plate Baseline 1 27 Plate Baseline 2 24
21
18 Load (kip)
15
12
9
6
3
0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Midspan Deflection (in.) Figure 4.26 Load versus deflection relationships for beams strengthened with CFRP plates and exposed to dry heat condition for 10,000 hours@140°F
150
33
30
27
24
21
18
Load (kip) 15
12
9 P-H3k-1
P-H3k-2 6
Plate Baseline 1
3 Plate Baseline 2
0 0 1000 2000 3000 4000 5000
Midspan MicroStrain (in./in.)
Figure 4.27 Load versus strain relationships for beams strengthened with CFRP plates and exposed to 3000 hours of dry heat condition @ 140°F
151 36
33
30
27
24
21
18
Load (kip) 15
12
P-H10k-1 9 P-H10k-2
6 Plate Baseline 1
Plate Baseline 2
3
0 0 1000 2000 3000 4000 5000
Midspan MicroStrain (in./in.)
Figure 4.28 Load versus strain relationships for beams strengthened with CFRP plates and exposed to 10,000 hours of dry-heat condition @ 140°F
152
Figure 4.29 Close-up of ultimate load test of beam strengthened with CFRP fabric after 10,000 hours of exposure to to dry-heat
153
12 12 12
9 9 9
6 6 6
Load (kips) Load (kips) Load (kips)
3 3 3
F-H1k-1 F-H3k-1 F-H10k-1 F-H1k-2 F-H3k-2 F-H10k-2 Fabric Baseline 1 Fabric Baseline 1 Fabric Baseline 1 Fabric Baseline 2 Fabric Baseline 2 Fabric Baseline 2
0 0 0 0.0 0.5 1.0 0.0 0.5 1.0 0.0 0.5 1.0 Deflection (in.) Deflection (in.) Deflection (in.)
(a) Exposure of 1000 hours (b) Exposure of 3000 hours (c) Exposure of 10,000 hours
Figure 4.30 Midspan deflection of beams strengthened with CFRP fabrics and exposed to dry heat condition @140°F
154
0.0 (a) Exposure of 1000 hours
F-H1k-1 F-H1k-2 Fabric Baseline 1 Fabric Baseline 2 Deflection (in.) 0.5 0 1 2 3 4 5 6 7 8 Distance along length of beam (feet) 0.
(b) Exposure of 3000 hours
Deflection (in.) F-H3k-1 F-H3k-2 Fabric Baseline 1 Fabric Baseline 2 0. 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
0.0
(c) Exposure of 10,000 hours
F-H10k-1 F-H10k-2 Fabric Baseline 1 Fabric Baseline 2 Deflection (in.) 0.5 0 1 2 3 4 5 6 7 8 Distance along length of beam (feet)
Figure 4.31 Deflected shape at failure load for beams strengthened with CFRP fabrics exposed to dry heat condition at 140°F
155 12 12 12
9 9 9
6 6 6 Load (kip) Load (kip) Load (kip)
3 3 3 F-H10k-1 F-H1k-1 F-H3k-1
F-H1k-2 F-H3k-2 F-H10k-2
Fabric Baseline 1 Fabric Baseline 1 Fabric Baseline 1
0 0 0 0 1000 2000 3000 0 1000 2000 3000 0 1000 2000 3000
MicroStrain (in./in.) MicroStrain (in./in.) MicroStrain (in./in.)
(a) Exposure of 1000 hours (b) Exposure of 3000 hours (c) Exposure of 10,000 hours
Figure 4.32 Strain in CFRP fabric at midspan of beams strengthened with CFRP fabrics and exposed to dry heat condition @ 140°F
156 3000
F-H1k-1 F-H1k-2 Fabric Baseline 1 2000
1000
MicroStrain (in./in.) (a) Exposure of 1000 hours 0 0 1 2 3 4 5 6 7 8 Distance along length of Beam (feet) 3000
F-H3k-1 F-H3k-2 Fabric Baseline 1 2000
1000
MicroStrain (in./in.) (b) Exposure of 3000 hours 0 0 1 2 3 4 5 6 7 8 Distance along length of Beam (feet) 3000
F-H10k-1 F-H10k-2 Fabric Baseline 1 2000
1000
MicroStrain (in./in.) (c) Exposure of 10,000 hours 0 0 1 2 3 4 5 6 7 8 Distance along length of Beam (feet)
Figure 4.33 Strain in CFRP fabric at 12 kip load for beams strengthened with CFRP fabrics and exposed to dry heat condition @ 140°F
157 0.0
F-H1k-1 F-H1k-2 Fabric Baseline 1
0.5 (a) Exposure of 1000 hours Deflection (in.) 1.0
0 1 2 3 4 5 6 7 8 Distance along length of beam (feet) 0.0 F-H3k-1 F-H3k-2 Fabric Baseline 1
0.5 (b) Exposure of 3000 hours
Deflection (in.) 1.0
0 1 2 3 4 5 6 7 8 Distance along length of beam (feet)
0.0 F-H10k-1 F-H10k-2 Fabric Baseline 1
0.5 (c) Exposure of 10,000 hours Deflection (in.)
1.0
0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
Figure 4.34 Deflected shape at failure load for beams strengthened with CFRP fabrics and exposed to dry heat condition @ 140°F
158 36
33 F-H3k-1
F-H3k-2 30 Fabric Baseline 1
27 Fabric Baseline 2
24
21
18 Load (kips) 15
12
9
6
3
0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Midspan Deflection (in.)
Figure 4.35 Load versus deflection relationships for beams strengthened with CFRP fabrics exposed to dry heat condition for 3000 hours at 140°F
159 36
33 F-H10k-1 30 F-H10k-2 Fabric Baseline 1 27 Fabric Baseline 2
24
21
18
Load (kips) 15
12
9
6
3
0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Midspan Deflection (in.)
Figure 4.36 Load versus deflection relationships for beams strengthened with CFRP fabrics exposed to dry heat condition for 10,000 hours at 140°
160
36
33
30
27
24
21
18 Load (kip)
15
12
F-H3k-1 9
F-H3k-2 6
Fabric Baseline 1 3
0
0 1000 2000 3000 4000 5000 6000 7000
Midspan MicroStrain (in./in.)
Figure 4.37 Load versus strain relationships for beams strengthened with CFRP fabrics and exposed to 3000 hours of dry heat condition @ 140°F
161
36
33
30
27
24
21
18
Load (kip) 15
12 F-H10k-1 9 F-H10k-2
6 Fabric Baseline 1
3
0 0 1000 2000 3000 4000 5000 6000 7000
Midspan MicroStrain (in./in.) Figure 4.38 Load versus strain relationships for beams strengthened with CFRP fabrics and exposed to 10,000 hours of dry heat condition @ 140°F
162 12 12 12
9 9 9
6 6 6 Load (kip) Load (kip)
Load (kip)
3 3 3 P-A1k-1 P-A3k-1 P-A10k-1 P-A1k-2 P-A3k-2 P-A10k-2 Plate Baseline 1 Plate Baseline 1 Plate Baseline 1 Plate Baseline 2 Plate Baseline 2 Plate Baseline 2
0 0 0 0.0 0.5 1.0 0.0 0.5 1.0 0.0 0.5 1.0
Deflection (in.) Deflection (in.) Deflection (in.) (a) Exposure of 1000 hours (b) Exposure of 3000 hours (c) Exposure of 10,000 hours Figure 4.39 Deflection at midspan of CFRP plate for beams strengthened with CFRP plates and exposed to alkaline solution @ 73°F
163
0.0 (a) Exposure of 1000 hours
P-A1k-1 P-A1k-2 Plate Baseline 1 Plate Baseline 2
Deflection (in.) 0.5 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
0.0
(b) Exposure of 3000 hours
P-A3k-1 P-A3k-2 Plate Baseline 1 Plate Baseline 2 Deflection (in.) 0.5 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
0.0 (c) Exposure of 10,000 hours
P-A10k-1 P-A10k-2 Plate Baseline 1 Plate Baseline 2
Deflection (in.) 0.5 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet) Figure 4.40 Deflected shape at 12 kip load for beams strengthened with CFRP plates and exposed to alkaline solution @ 73°F
164 12 12 12
9 9 9
6 6 6 Load (kip) Load (kip)
Load (kip)
P-A1k-1 P-A3k-1 3 3 3 P-A10k-1
P-A1k-2 P-A3k-2 P-A10k-2
Plate Baseline 1 Plate Baseline 1 Plate Baseline 1
Plate Baseline 2 Plate Baseline 2 Plate Baseline 2
0 0 0 0 1000 2000 3000 0 1000 2000 3000 0 1000 2000 3000
MicroStrain (in./in.) MicroStrain (in./in.) MicroStrain (in./in.)
(a) Exposure of 1000 hours (b) Exposure of 3000 hours (c) Exposure of 10,000 hours
Figure 4.41 Strain in CFRP plates at midspan of beams strengthened with CFRP plates and exposed to alkaline solution @ 73°F
165 3000
P-A1k-1 P-A1k-2 Plate Baseline 1 Plate Baseline 2 2000
1000
MicroStrain (in./in.) (a) Exposure of 1000 hours
0 0 1 2 3 4 5 6 7 8
3000 Distance along length of Beam (feet)
2000 P-A3k-2 Plate Baseline 1 P-A3k-1 Plate Baseline 2
1000
MicroStrain (in./in.) (b) Exposure of 3000 hours 0 0 1 2 3 4 5 6 7 8
3000 Distance along length of beam (feet)
P-A10k-1 P-A10k-2 Plate Baseline 1 Plate Baseline 2 2000
1000
MicroStrain (in./in.) (c) Exposure of 10,000 hours 0 0 1 2 3 4 5 6 7 8 Distance along length of Beam (feet)
Figure 4.42 Strain in CFRP plate at 12 kips load for beams strengthened with CFRP plates and exposed to alkaline solution @ 73°F
166 0.0
(a) Exposure of 1000 hours
0.5
Deflection (in.) 1.0 P-A1k-1 P-A1k-2 Plate Baseline 1 Plate Baseline 2
0 1 2 3 4 5 6 7 8 Distance along length of beam (feet) 0.0
(b) Exposure of 3000 hours 0.5
Deflection (in.) P-A3k-1 P-A3k-2 Plate Baseline 1 Plate Baseline 2 1.0 0 1 2 3 4 5 6 7 8
Distance along beam length (feet)
0.0
(c) Exposure of 10,000 hours 0.5
Deflection (in.) P-A10k-1 1.0 P-A10k-2 Plate Baseline 1 Plate Baseline 2
0 1 2 3 4 5 6 7 8 Distance along length of beam (feet)
Figure 4.43 Deflected shape at failure load for beams strengthened with CFRP plates and exposed to alkaline solution @ 73°
167 36
33 P-A3k-1
30 P-A3k-2 Plate Baseline 1 27 Plate Baseline 2
24
21
18
Load (kip) 15
12
9
6
3
0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Midspan Deflection (in.)
Figure 4.44 Load versus deflection relationships for beams strengthened with CFRP plates and exposed to alkaline solution @ 73°F for 3000 hours
168 36
33 P-A10k-1
30 P-A10k-2
Plate Baseline 1 27 Plate Baseline 2 24
21
18
Load (kip)
15
12
9
6
3
0 0.0 0.5 1.0 1.5 2.0 2.5 3.0
Midspan Deflection (in.)
Figure 4.45 Load versus deflection relationships for beams strengthened with CFRP plates and exposed to alkaline solution @ 73°F for 10,000 hours
169
36
33
30
27
24
21
18
Load (kip)
15
12
P-A3k-1
9 P-A3k-2 Plate Baseline 1 Plate Baseline 2 6
3
0 0 1000 2000 3000 4000 5000 Midspan MicroStrain (in./in.)
Figure 4.46 Load versus strain relationships for beams strengthened with CFRP plates and exposed to alkaline solution @ 73°F for 3000 hours
170
36
33
30
27
24
21
18 Load (kip)
15
12
9
P-A10k-1 6 P-A10k-2 Plate Baseline 1
3 Plate Baseline 2
0
0 1000 2000 3000 4000 5000
Midspan MicroStrain (in./in.)
Figure 4.47 Load versus strain relationships for beams strengthened with CFRP plates and exposed to alkaline solution @ 73°F for 10,000 hours
171
Figure 4.48 Close-up of ultimate load test of beam strengthened with CFRP fabric after 10,000 hours exposure to alkaline solution
172 12 12 12
9 9 9
6 6 6 Load (kip) Load (kip) Load (kip)
3 3 3 F-A1k-1 F-A3k-1 F-A10k-1 F-A10k-2 F-A1k-2 F-A3k-2 Fabric Baseline 1 Fabric Baseline 1 Fabric Baseline 1 Fabric Baseline 2 Fabric Baseline 2 Fabric Baseline 2
0 0 0 0.0 0.5 1.0 0.0 0.5 1.0 0.0 0.5 1.0 Deflection (inch) Deflection (inch) Deflection (inch) (a) Exposure of 1000 hours (b) Exposure of 3000 hours (c) Exposure of 10,000 hours
Figure 4.49 Midspan deflection of beams strengthened with CFRP fabrics and exposed to alkaline solution @ 73°F
173
0.0 (a) Exposure of 1000 hours
F-A1k-1 F-A1k-2 Fabric Baseline 1 Fabric Baseline 2
Deflection (inch) 0.5 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
0.0 (b) Exposure of 3000 hours
F-A3k-1 F-A3k-2 Fabric Baseline 1 Fabric Baseline 2
Deflection (inch) 0.5 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
0.0 (c) Exposure of 10,000 hours
F-A10k-1 F-A10k-2 Fabric Baseline 1 Fabric Baseline 2
Deflection (inch) 0.5 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
Figure 4.50 Deflected shape at 12 kips load for beams strengthened with CFRP fabrics and exposed to alkaline solution @ 73°F
174 12 12 12
9 9 9
6 6 6 Load (kip) Load (kip) Load (kip)
3 3 3 F-A3k-1 F-A1k-1 F-A10k-1 F-A1k-2 F-A3k-2 F-A10k-2 Fabric Baseline 1 Fabric Baseline 1 Fabric Baseline 1
0 0 0 0 1000 2000 3000 0 1000 2000 3000 0 1000 2000 3000
MicroStrain (in./in.) MicroStrain (in./in.) MicroStrain (in./in.)
(a) Exposure of 1000 hours (b) Exposure of 3000 hours (c) Exposure of 10,000 hours
Figure 4.51 Strain in CFRP fabric at midspan of beams strengthened with CFRP fabrics and exposed to alkaline solution @ 73°F
175 3000
F-A1k-1 F-A1k-2 Fabric Baseline 1 2000
1000 (a) Exposure of 1000 hours MicroStrain (in./in.) 0 0 1 2 3 4 5 6 7 8
Distance along length of Beam (feet)
3000
F-A3k-1 F-A3k-2 Fabric Baseline 1
2000
1000
MicroStrain (in./in.) (b) Exposure of 3000 hours 0 0 1 2 3 4 5 6 7 8
Distance along length of Beam (feet)
3000 F-A10k-1 F-A10k-2 Fabric Baseline 1
2000
1000
MicroStrain (in./in.) (c) Exposure of 10,000 hours
0 0 1 2 3 4 5 6 7 8
Distance along length of Beam (feet)
Figure 4.52 Strain in CFRP fabrics at 12 kips for beams strengthened with CFRP fabrics and exposed to alkaline solution @ 73°F
176 0.0
0.5 (a) Exposure of 1000 hours
Deflection (inch) 1.0 F-A1k-1 F-A1k-2 Plate Baseline 1 Plate Baseline 2
0 1 2 3 4 5 6 7 8
Distance along length of beam (feet) 0.0
0.5 (b) Exposure of 3000 hours
Deflection (inch) 1.0 F-A3K-1 F-A3K-2 Plate Baseline 1 Plate Baseline 2
0 1 2 3 4 5 6 7 8
Distance along length of beam (feet) 0.0
0.5 (c) Exposure of 10,000 hours Deflection (inch) 1.0 F-A10k-1 F-A10k-2 Plate Baseline 1 Plate Baseline 2
0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
Figure 4.53 Deflected shape at failure load of beams strengthened with CFRP fabrics and exposed to alkaline solution @ 73°F
177 36 F-A3k-1
33 F-A3k-2
Fabric Baseline 1 30 Fabric Baseline 2 27
24
21
18
Load (kip) 15
12
9
6
3
0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Midspan Deflection (inch)
Figure 4.54 Load versus deflection relationships for beams strengthened with CFRP fabrics and exposed to alkaline solution @ 73°F for 3000 hours
178 36
F-A10k-1 33 F-A10k-2
30 Fabric Baseline 1
27 Fabric Baseline 2
24
21
18
Load (kip) 15
12
9
6
3
0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Midspan Deflection (inch)
Figure 4.55 Load versus deflection relationships for beams strengthened with CFRP fabrics and exposed to alkaline solution @ 73°F for 10,000 hours
179
36
33
30
27
24
21
)
18
Load (kip 15
12
9 F-A3k-1
6 F-A3k-2
3 Fabric Baseline 1
0 0 1000 2000 3000 4000 5000 6000 7000
Midspan MicroStrain (in./in.)
Figure 4.56 Load versus strain relationships for beams strengthened with CFRP fabrics and exposed to alkaline solution @ 73°F for 3000 hours
180
36
33
30
27
24
21
18
15 Load (kip)
12
9
6 F-A10k-1
F-A10k-2 3
Fabric Baseline 1
0 0 1000 2000 3000 4000 5000 6000 7000
Midspan MicroStrain (in./in.)
Figure 4.57 Load versus strain relationships for beams strengthened with CFRP fabrics and exposed to alkaline solution @ 73°F for 10,000 hours
181
Figure 4.58 Beams strengthened with CFRP plates and fabrics after exposure to salt-water for 10,000 hours
182 12 12 12
9 9 9
6 6 6 Load (kip) Load (kip) Load (kip)
P-S3k-1 P-S10k-1 3 3 3 P-S1k-1 P-S3k-2 P-S10k-2
Plate Baseline 1 Plate Baseline 1 Plate Baseline 1
Plate Baseline 2 Plate Baseline 2 Plate Baseline 2
0 0 0 0.0 0.5 1.0 0.0 0.5 1.0 0.0 0.5 1.0
Deflection (in.) Deflection (in.) Deflection (in.)
(a) Exposure of 1000 hours (b) Exposure of 3000 hours (c) Exposure of 10,000 hours
Figure 4.59 Deflection at midspan of beams strengthened with CFRP plates and exposed to salt solution @ 73°F
183
0.0 (a) Exposure of 1000 hours
P-S1k-1 Plate Baseline 1 Plate Baseline 2
Deflection (inch) 0.5 0 1 2 3 4 5 6 7 8
Distance along length of Beam (feet)
(b) Exposure of 3000 hours 0.0
P-S3k-1 P-S3k-2 Plate Baseline 1 Plate Baseline 2
0.5 Deflection (inch) 1 7 8 0 2 3 4 5 6
Distance along length of beam (feet)
0.0 (c) Exposure of 10,000 hours
P-S10k-1 P-S10k-2 Plate Baseline 1 Plate Baseline 2
0.5 Deflection (inch) 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
Figure 4.60 Deflected shape at 12 kips load for beams strengthened with CFRP plates and exposed to salt solution @ 73°F
184 12 12 12
9 9 9
6 6 6 Load (kip) Load (kip) Load (kip)
3 3 P-S10k-1 P-S1k-1 3 P-S3k-1
P-S10k-2 P-S3k-2 Plate Baseline 1 Plate Baseline 1 Plate Baseline 1 Plate Baseline 2 Plate Baseline 2 Plate Baseline 2
0 0 0 0 1000 2000 3000 0 1000 2000 3000 0 1000 2000 3000
MicroStrain (in./in.) MicroStrain (in./in.) MicroStrain (in./in.)
(b) Exposure of 3000 hours (c) Exposure of 10,000 hours (a) Exposure of 1000 hours
Figure 4.61 Strain in CFRP plates at midspan of beams strengthened with CFRP plates and exposed to salt solution @ 73°F
185 3000
2000 P-S1k-1 Plate Baseline 1 Plate Baseline 2
1000 (a) Exposure of 1000 hours MicroStrain (in./in.) 0 0 1 2 3 4 5 6 7 8
Distance along length of Beam (feet)
3000
P-S3k-1 P-S3k-2 Plate Baseline 1 Plate Baseline 2 2000
1000
MicroStrain (in./in.) (b) Exposure of 3000 hours 0 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
3000
P-S10k-1 P-S10k-2 Plate Baseline 1 Plate Baseline 2 2000
1000
MicroStrain (in./in.) (c) Exposure of 10,000 hours 0 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
Figure 4.62 Strain in CFRP plate for beams at 12 kip strengthened with CFRP plates and exposed to salt solution @ 73°F
186 0.0
(a) Exposure of 1000 hours 0.5
Deflection (in.) P-S1k-1 Plate Baseline 1 Plate Baseline 2 1.0 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
0.0
(b) Exposure of 3000 hours 0.5
Deflection (in.) P-S3k-1 P-S3k-2 Plate Baseline 1 Plate Baseline 2 1.0 0 1 2 3 4 5 6 7 8 Distance along length of beam (feet) 0.0
(c) Exposure of 10,000 hours
0.5
Deflection (in.) P-S10k-1 P-S10k-2 Plate Baseline 1 Plate Baseline 2 1.0 0 1 2 3 4 5 6 7 8 Distance along length of beam (feet) Figure 4.63 Deflected shape at failure load for beams strengthened with CFRP plates and exposed to salt solution @ 73°F
187 36
33
P-S3k-1 30 P-S3k-2 27 Plate Baseline 1
24 Plate Baseline 2
21
18 Load (kip) 15
12
9
6
3
0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Midspan Deflection (in.)
Figure 4.64 Load versus deflection relationships for beams strengthened with CFRP plates and exposed to salt solution @ 73°F for 3000 hours
188 36
33
P-S10k-1 30 P-S10k-2
27 Plate Baseline 1
Plate Baseline 2 24
21
18
Load (kip) 15
12
9
6
3
0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Midspan Deflection (in.)
Figure 4.65 Load versus deflection relationships for beams strengthened with CFRP plates and exposed to salt solution @ 73°F for 10,000 hours
189
36
33
30
27
24
21
18
15 Load (kip)
12
9 P-S3k-1
P-S3k-2 6 Plate Baseline 1
3 Plate Baseline 2
0 0 1000 2000 3000 4000 5000
Midspan MicroStrain (in./in.)
Figure 4.66 Load versus strain relationships for beams strengthened with CFRP plates and exposed to salt solution @ 73°F for 3000 hours
190
36
33
30
27
24
21
18
Load (kip)
15
12
9
P-S10k-1 6 P-S10k-2
Plate Baseline 1 3 Plate Baseline 2
0 0 1000 2000 3000 4000 5000
Midspan MicroStrain (in./in.)
Figure 4.67 Load versus strain relationships for beams strengthened with CFRP plates and exposed to salt solution @ 73°F for 10,000 hours
191 12 12 12
9 9 9
6 6 6 Load (kip) Load (kip) Load (kip)
3 3 3 F-S1k-1 F-S3k-1 F-S10k-1
F-S3k-2 F-S10k-2 F-S1k-2
Fabric Baseline 1 Fabric Baseline 1 Fabric Baseline 1
0 0 0 0.0 0.5 1.0 0.0 0.5 1.0 0.0 0.5 1.0
Deflection (in.) Deflection (in.) Deflection (in.)
(c) Exposure of 10,000 hours (a) Exposure of 1000 hours (b) Exposure of 3000 hours
Figure 4.68 Deflection at midspan at 12 kips load of beams strengthened with CFRP fabrics after exposure to salt solution @ 73°F
192 0.0
(a) Exposure of 1000 hours
ction (inch) F-S1k-1 F-S1k-2 Fabric Baseline 1 Defle 0.5 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
0.0 (b) Exposure of 3000 hours
F-S3k-1 F-S3k-2 Fabric Baseline 1 Deflection (inch) 0.5 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
0.0 (c) Exposure of 10,000 hours
F-S10k-1 F-S10k-2 Fabric Baseline 1 Deflection (inch) 0.5 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
Figure 4.69 Deflected shape at 12 kip load for beams strengthened with CFRP fabrics and exposed to salt solution @ 73°F
193 12 12 12
9 9 9
6 6 6
Load (kip) Load (kip) Load (kip)
3 3 3 F-S10k-1 F-S1k-1 F-S3k-1
F-S1k-2 F-S3k-2 F-S10k-2 Fabric Baseline 1 Fabric Baseline 1 Fabric Baseline 1
0 0 0 0 1000 2000 3000 0 1000 2000 3000 0 1000 2000 3000 MicroStrain (in./in.) MicroStrain (in./in.) MicroStrain (in./in.)
(a) Exposure of 1000 hours (b) Exposure of 3000 hours (c) Exposure of 10,000 hours
Figure 4.70 Strain in CFRP fabric at midspan of beams strengthened with CFRP fabrics and exposed to salt solution @ 73°F
194 3000
F-S1k-1 F-S1k-2 Fabric Baseline 1 2000
1000 (a) Exposure of 1000 hours
MicroStrain (in./in.) 0 0 1 2 3 4 5 6 7 8
Distance along length of Beam (feet)
3000
F-S3k-1 F-S3k-2 Fabric Baseline 1 2000
1000 (b) Exposure of 3000 hours MicroStrain (in./in.) 0 0 1 2 3 4 5 6 7 8
Distance along length of Beam (feet)
3000 F-S10k-1 F-S10k-2 Fabric Baseline 1 2000
1000 (c) Exposure of 10,000 hours MicroStrain (in./in.) 0 0 1 2 3 4 5 6 7 8
Distance along length of Beam (feet)
Figure 4.71 Strain in CFRP fabrics at 12 kips load for beams strengthened with CFRP fabrics and exposed to salt solution @ 73°F
195 0.0
0.5 (a) Exposure of 1000 hours
Deflection (inch) 1.0 F-S1k-1 F-S1k-1 Fabric Baseline 1 Fabric Baseline 2
0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
0.0
0.5 (b) Exposure of 3000 hours
Deflection (inch) 1.0 F-S3k-1 F-S3k-1 Fabric Baseline 1 Fabric Baseline 2 0 1 2 3 4 5 6 7 8 Distance along length of beam (feet)
0.0
0.5 (c) Exposure of 10,000 hours
Deflection (inch) 1.0 F-S10k-1 F-S10k-1 Fabric Baseline 1 Fabric Baseline 2 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
Figure 4.72 Deflected shape at failure load for beams strengthened with CFRP fabrics and exposed to salt solution @ 73°
196 36
33 F-S3k-1
30 F-S3k-2 Fabric Baseline 1 27 Fabric Baseline 2
24
21
18
Load (kip) 15
12
9
6
3
0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Midspan Deflection (in.)
Figure 4.73 Load versus deflection relationships for beams strengthened with CFRP fabrics and exposed to salt solution @ 73°F for 3000 hours
197 36
33 F-S10k-1
F-S10k-2 30 Fabric Baseline 1
27 Fabric Baseline 2
24
21
18
Load (kip) 15
12
9
6
3
0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Midspan Deflection (in.)
Figure 4.74 Load versus deflection relationships for beams strengthened with CFRP fabrics and exposed to salt solution @ 73°F for 10,000 hours
198
36
33
30
27
24
21
18
Load (kip)
15
12
9 F-S3k-1
F-S3k-2 6 Fabric Baseline 1
Fabric Baseline 2 3
0 0 1000 2000 3000 4000 5000 6000 7000
Midspan MicroStrain (in./in.)
Figure 4.75 Load versus strain relationships for beams strengthened with CFRP fabrics and exposed to salt solution @ 73°F for 3000 hours
199
36
33
30
27
24
21
18 Load (kip)
15
12
9 F-S10k-1
6 F-S10k-2 Fabric Baseline 1
3 Fabric Baseline 2
0 0 1000 2000 3000 4000 5000 6000 7000 Midspan MicroStrain (in./in.)
Figure 4.76 Load versus strain relationships for beams strengthened with CFRP fabrics and exposed to salt solution @ 73°F for 10,000 hours
200 12 12
9 9
6 6 Load (kip) Load (kip)
3 3 P-F 350 Cyc-1 P-F 700 Cyc-1
P-F 350 Cyc-2 P-F 7000 Cyc-2
Plate Baseline 1 Plate Baseline 1 Plate Baseline 2 Plat e Baseline 2
0 0 0.0 0.5 1.0 0.0 0.5 1.0 Deflection (inch) Deflection (inch)
(a) Exposure of 350 Cycle (b) Exposure of 700 Cycle Figure 4.77 Midspan deflection of beams strengthened with CFRP plates and exposed to freeze/thaw condition
201
0.0 (a) Exposure of 350 Cycle
P-F350 Cyc-1 P-F 350 Cyc-2 Plate Baseline 2 Plate Baseline 1 Deflection (inch)
0.5 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
0.0
(b) Exposure of 700 Cycle
P-F 700 Cyc-1 P-F 700 Cyc-2 Plate Baseline 1 Plate Baseline 2 Deflection (inch)
0.5 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
Figure 4.78 Deflected shape at 12 kips load for beams strengthened with CFRP plate and exposed to freeze/thaw condition
202 12 12
9 9
6 6 Load (kip) Load (kip)
3 3 P-F 700 Cyc-1 P-F 350 Cyc-1 P-F 700 Cyc-2 P-F 350 Cyc-2 Plate Baseline 1 Plate Baseline 1 Plate Baseline 2 Plate Baseline 2 0 0 0 1000 2000 3000 0 1000 2000 3000
MicroStrain (in./in.) MicroStrain (in./in.) (a) Exposure of 350 Cycle (b) Exposure of 700 Cycle
Figure 4.79 Strain in CFRP plate at midspan of beams strengthened with CFRP plates and exposed to freeze/thaw condition
203
3000
P-F350 Cyc-1 P-F 350 Cyc-2 Plate Baseline 1 Plate Baseline 2 2000
1000
MicroStrain (in./in.) (a) Exposure of 350 Cycle 0 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
3000
2000 P-F700 Cyc-1 P-F 700 Cyc-2 Plate Baseline 1 Plate Baseline 2
1000
MicroStrain (in./in.) (b) Exposure of 700 Cycle 0 0 1 2 3 4 5 6 7 8
Distance along length of Beam (feet)
Figure 4.80 Strain in CFRP plate at 12 kips load for beams strengthened with CFRP plates and exposed to freeze/thaw condition
204 0.0
(a) Exposure of 350 Cycle 0.5
Deflection (inch)
1.0 P-F 350 Cyc-1 P-F 350 Cyc-2 Plate Baseline 1 Plate Baseline 2
0 1 2 3 4 5 6 7 8
Distance along beam length (feet)
0.0
(b) Exposure of 700 Cycle 0.5
Deflection (inch)
1.0 P-F 700 Cyc-1 P-F 700 Cyc-2 Plate Baseline 1 Plate Baseline 2
0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
Figure 4.81 Deflected shape at failure load for beams strengthened with CFRP plates and exposed to freeze/thaw condition
205 36
33 P-F 350 Cyc-1
30 P-F 350 Cyc-2
Plate Baseline 1 27 Plate Baseline 2 24
21
p) 18
Load (ki 15
12
9
6
3
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0.0
Midspan Deflection (in.)
Figure 4.82 Load versus deflection relationships for beams strengthened with CFRP plates and exposed to 350 cycle of freeze/thaw condition
206 36
33 P-F 700 Cyc-1
30 P-F 700 Cyc-2
Plate Baseline 1 27 Plate Baseline 2
24
21
(kip) 18
Load 15
12
9
6
3
0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Midspan Deflection (in.)
Figure 4.83 Load versus deflection relationships for beams strengthened with CFRP plates and exposed to 700 cycle of freeze/thaw condition
207
36
33
30
27
24
21
18 Load (kip)
15
12
9 P-F 350 Cyc-1
P-F 350 Cyc-2 6 Plate Baseline 1
3 Plate Baseline 2
0
0 1000 2000 3000 4000 5000
Midspan MicroStrain (in./in.)
Figure 4.84 Load versus strain relationships for beams strengthened with CFRP plates and exposed to 350 cycles of freeze/thaw condition
208
36
33
30
27
24
21
18 Load (kip)
15
12
9 P-F 700 Cyc-1
P-F 700 Cyc-2 6
Plate Baseline 1 3
Plate Baseline 2
0 0 1000 2000 3000 4000 5000
Midspan MicroStrain (in./in.) Figure 4.85 Load versus strain relationships for beams strengthened with CFRP plates and exposed to 700 cycles of freeze/ thaw condition
209
Figure 4.86 Close-up of ultimate load test of beam strengthened with CFRP fabric after exposure to 350 freeze/thaw cycles
210 12 12
9 9
6 6 Load (kip) Load (kip)
3 3 F-F 700 Cyc-1 F-F 350 Cyc-1 F-F 700 Cyc-2 F-F 350 Cyc-2 Fabric Baseline 1 Fabric Baseline 1
0 0 0.0 0.5 1.0 0.0 0.5 1.0 Deflection (in.) Deflection (in.)
(a) Exposure of 350 Cycle (b) Exposure of 700 Cycle
Figure 4.87 Deflection at midspan at 12 kip load of beams strengthened with CFRP fabrics and exposed to freeze/thaw conditio n
211
0.0
(a) Exposure of 350 Cycle
Deflection (inch) F-F 350 Cyc-1 F-F 350 Cyc-2 Fabric Baseline 1 0.5 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
0.0
(b) Exposure of 700 Cycle
Deflection (inch) F-F 700 Cyc-1 F-F 700 Cyc-2 Fabric Baseline 1
0.5 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
Figure 4.88 Deflected shape at 12 kips load for beams strengthened with CFRP fabrics and exposed to freeze/thaw condition
212 12 12
9 9
6 6 Load (kip) Load (kip)
3 3 F-F 700 Cyc-1 F-F 350 Cyc-1 F-F 700 Cyc-2 F-F 350 Cyc-2 Fabric Baseline 1 Fabric Baseline 1
0 0 0 1000 2000 3000 0 1000 2000 3000 MicroStrain (in./in.) MicroStrain (in./in.)
(a) Exposure of 350 Cycle (b) Exposure of 700 Cycle
Figure 4.89 Strain in CFRP fabric at midspan of beams strengthened with CFRP fabrics and exposed to freeze/thaw condition
213
3000
F-F 350 Cyc-1 F-F 350 Cyc-2 Fabric Baseline 1
2000
1000 MicroStrain (in./in.) (a) Exposure of 350 Cycle
0 0 1 2 3 4 5 6 7 8
Distance along length of Beam (feet)
3000
F-F 700 Cyc-1 F-F 700 Cyc-2 Fabric Baseline 1
2000
1000 (b) Exposure of 700 Cycle MicroStrain (in./in.)
0 0 1 2 3 4 5 6 7 8
Distance along length of Beam (feet)
Figure 4.90 Strain in CFRP fabric at 12 kip load for beams strengthened with CFRP fabrics and exposed to freeze/thaw condition
214
F-F 350 Cyc-1 F-F 350 Cyc-2 Fabric Baseline 1 0.0
0.5 (a) Exposure of 350 Cycle
Deflection (inch) 1.0
0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
0.0 Fabric Baseline 1 F-F 700 Cyc-1 F-F 700 Cyc-2
(b) Exposure of 700 Cycle 0.5
Deflection (inch) 1.0
0 1 2 3 4 5 6 7 8
Distance along length of beam (feet) Figure 4.91 Deflected shape at failure load for beams strengthened with CFRP fabrics and exposed to freeze/thaw condition
215 36
33 F-F 350Cyc-1
30 F-F 350Cyc-2 Fabric Baseline 1 27 Fabric Baseline 2
24
21
18
Load (kip) 15
12
9
6
3
0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Midspan Deflection (in.)
Figure 4.92 Load versus deflection relationships for beams strengthened with CFRP fabrics and exposed to 350 cycle of freeze/thaw condition
216 36
33 F-F 700Cyc-1
30 F-F 700Cyc-2 Fabric Baseline 1 27 Fabric Baseline 2
24
21
18
Load (kip) 15
12
9
6
3
0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Midspan Deflection (in.)
Figure 4.93 Load versus deflection relationships for beams strengthened with CFRP fabrics and exposed to 350 cycle of freeze/thaw condition
217
36
33
30
27
24
21
18
Load (kip) 15
12
F-F 350Cyc-1 9 F-F 350Cyc-2
6 Fabric Baseline 1
3
0 0 1000 2000 3000 4000 5000 6000 7000
Midspan MicroStrain (in./in.)
Figure 4.94 Load versus strain relationships for beams strengthened with CFRP fabrics and exposed to 350 cycles of freeze/thaw condition
218
36
33
30
27
24
21
18
Load (kip)
15
12
9 F-F 700Cyc-1 F-F 700Cyc-2
6 Fabric Baseline 1
3
0 0 1000 2000 3000 4000 5000 6000 7000
Midspan MicroStrain (in./in.)
Figure 4.95 Load versus strain relationships for beams strengthened with CFRP fabrics and exposed to 700 cycles of freeze/thaw condition
219
Figure 4.96 Close-up of ultimate load test of beam strengthened with CFRP plate after exposure to 35 thermal expansion cyc les
220
12
9
6
Load (kip)
P-TE 35Cyc-1
3 P-TE 35Cyc-2
Plate Baseline 1
Plate Baseline 2
0
0.0 0.5 1.0
Deflection (in.)
Figure 4.97 Midspan deflection at at 12 kips load of beams strengthened with CFRP plates and exposed to 35 thermal expansion test cycles
221
P-TE 35 Cyc-1 P-TE 35 Cyc-2 Plate Baseline 1 Plate Baseline 2
0.0
0.5 Deflection (inch) 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
Figure 4.98 Deflected shape at 12 kip load for beams strengthened with CFRP plates and exposed to 35 thermal expansion test cycles
222 12
9
6
Load kip)
P-TE 35Cyc-1 3 P-TE 35Cyc-2 Plate Baseline 1
Plate Baseline 2
0 0 1000 2000 3000
MicroStrain (in./in.)
Figure 4.99 Strain in CFRP plates at midspan of beams strengthened with CFRP plates and exposed to 35 thermal expansion test cycles
223
3000 P-TE 35 Cyc-1 P-TE 35 Cyc-2 Plate Baseline 1 Plate Baseline 2
2000
1000 MicroStrain (in./in.)
0 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
Figure 4.100 Strain in CFRP plates at 12 kips load for beams strengthened with CFRP plates and exposed to 35 thermal expansion test cycles
224
P-TE 35 Cyc-1 P-TE 35 Cyc-2 Plate Baseline 1 Plate Baseline 2
0.0
0.5
Deflection (inch)
1.0
0 1 2 3 4 5 6 7 8
Distance along beam length (feet)
Figure 4.101 Deflected shape at failure load for beams strengthened with CFRP plates and exposed to 35 thermal expansion test cycles
225 36
33 P-TE 35Cyc-1 30 P-TE 35Cyc-2
Plate Baseline 1 27 Plate Baseline 2 24
21
18
Load (kip) 15
12
9
6
3
0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 Midspan Deflection (inch)
Figure 4.102 Load versus deflection relationships for beams strengthened with CFRP plates and exposed to 35 thermal expansion test cycles
226
36
33
30
27
24
21
18
Load (kip) 15
12
9 P-TE 35Cyc-1
6 P-TE 35Cyc-2
Plate Baseline 1
3 Plate Baseline 2
0 0 1000 2000 3000 4000 5000
Midspan MicroStrai n (in./in.)
Figure 4.103 Load versus strain relationships for beams strengthened with CFRP plates and exposed to 35 thermal expansion test cycles
227
12
9
6
Load (kip)
F-TE 35Cyc-1 3 F-TE 35Cyc-2
Fabric Baseline 1
0 0.0 0.5 1.0 Deflection (inch)
Figure 4.104 Midspan deflection at 12 kips load of beams strengthened with CFRP fabrics and exposed to 35 thermal expansion test cycles
228
F-TE 35 Cyc-1 F-TE 35 Cyc-2 Fabric Baseline 1
0.0
Deflection (inch)
0.5 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
Figure 4.105 Deflected shape at 12 kips load for beams strengthened with CFRP fabrics and exposed to 35 thermal expansion test cycles
229
12
9
6
Load (kip)
F-TE 35Cyc-1
3 F-TE 35Cyc-2
Fabric Baseline 1
0 0 1000 2000 3000
MicroStrain (in./in.)
Figure 4.106 Strain in CFRP fabrics at midspan of beams strengthened with CFRP fabrics and exposed to 35 thermal expansion test cycles
230
3000
F-TE 35 Cyc-1 F-TE 35 Cyc-2 Fabric Baseline 1
2000
1000
MicroStrain (in./in.)
0 0 1 2 3 4 5 6 7 8
Distance along length of Beam (feet)
Figure 4.107 Strain in CFRP fabrics at 12 kip load for beams strengthened with CFRP fabrics and exposed to 35 thermal expansion test cycles
231
F-TE 35 Cyc-1 F-TE 35 Cyc-2 Fabric Baseline 1
0.0
0.5
Deflection (in.)
1.0
0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
Figure 4.108 Deflected shape at failure load for beams strengthened with CFRP fabrics exposed to 35 thermal expansion test cycles
232 36
33
30
27
24
21
18
Load (kip) 15
12 F-TE 35Cyc-1
9 F-TE 35Cyc-2
Fabric Baseline 1 6
3
0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Midspan Deflection (in.)
Figure 4.109 Load versus deflection relationships for beams strengthened with CFRP fabrics exposed to 35 thermal expansion test cycles
233
36
33
30
27
24
21
18
Load (kip) 15
12
F-TE 35Cyc-1 9 F-TE 35Cyc-2
6 Fabric Baseline 1
3
0
0 1000 2000 3000 4000 5000 6000 7000
Midspan MicroStrain (in./in.)
Figure 4.110 Load versus strain relationship for beams strengthened with CFRP fabrics and exposed to 35 thermal expansion test cycles
234 700 700 100% Humidity Salt-Water
600 585 581 578 582 600 589 569 573 566 570 579 581 574 554 558 561 566 500 500
400 400
300 300 Weight (lb) Weight (lb) 200 200
100 100
0 0 Plate 1 Plate 2 Fabric 1 Fabric 2 Plate 1 Plate 2 Fabric 1 Fabric 2
Weight before exposure Weight after 10,000 hours of exposure
700 Dry-Heat 700 Alkaline Solution
600 600 586 587 591 594 598 575 575 585 582 584 579 568 577 566 565 553 500 500
400 400
300 300 Weight (lb) Weight (lb) 200 200
100 100
0 0 Plate 1 Plate 2 Fabric 1 Fabric 2 Plate 1 Plate 2 Fabric 1 Fabric 2
Figure 4.111. Weight of beams before and after 10,000 hours of exposure to various environmental conditions
235 0
-4000
-8000
Load (lbs)
-12000 (c) Repeated load range #3: 40% beam ultimate strength (1200-12000 lbs)
0
-4000 oad (lbs) L -8000 Static set-point (b) Repeated load range #2: 25% beam ultimate strength (750-7500 lbs) -12000
0
-4000
-8000 Load (lbs) -12000 (a) Repeated load range #1: 15% beam ultimate strength (450-4500 lbs)
0 0.25 0.5 0.75 1 1.25 1.5 1.75 2
Number of Cycles (millions) Figure 4.112 Repeated load tests for strengthened beams with CFRP plates and fabrics
236 (a) Repeated load range 1 (b) Repeated load range 2 (c) Repeated load range 3 15% beam ultimate strength 25% beam ultimate strength (450-4500 lbs) 40% beam ultimate strength (750-7500 lbs) (1200-12000 lbs)
12 12 12
9 9 9
6 6 6 d (kips)
Loa Load (kips) Load (kips)
3 3 3 @ 0 Cycles @ 0 Cycles @ 0 Cycles @ 100,000 Cycles @ 100,000 Cycles @ 100,000 Cycles @ 1,000,000 Cycles @ 1,000,000 Cycles @ 1,000,000 Cycles @ 2,000,000 Cycles @ 2,000,000 Cycles @ 2,000,000 Cycles
0 0 0 0 0.5 1 0.0 0.5 1.0 0.0 0.5 1.0 Deflection (in.) Deflection (in.) Deflection (in.)
(b) Beam P-R25-1 (a) Beam P-R15-1 (c) Beam P-R40-1
Figure 4.113 Load versus deflection at midspan of beams strengthened with CFRP plate and subjected to repeated load test cycles
237
0.0 (c) Repeated load range #3: 40% beam ultimate strength (1200-12000 lbs, beam P-R40-1)
Deflection (in.) 0.5
(b) Repeated load range #2: 25% beam ultimate strength (750-7500 lbs, beam P-R25-1)
0.0
Deflection (in.) 0.5
0.0 (a) Repeated load range #1: 15% beam ultimate strength (450-4500 lbs, beam P -R15-1)
Deflection (in.)
0.5 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
@ 0 Cycles @ 100,000 Cycles @ 1,000,000 Cycles @ 2,000,000 Cycles
Figure 4.114 Deflected shape at 12 kips static load for beams strengthened with CFRP plate and subjected to repeated load test cycles
238 -0.1
-0.2
-0.3 (c) Repeated load range #3: 40% beam ultimate strength (1200-12000 lbs, beam P-R40-1)
Deflection (in.) -0.4
0
-0.1
-0.2
-0.3
Deflection (in.) (b) Repeated load range #2: 25% beam ultimate strength (750-7500 lbs, beam P-R25-1) -0.4
0
-0.1
(in.) -0.2
(a) Repeated load range #1: 15% beam ultimate strength (450-4500 lbs, beam P -R15-1)
Deflection -0.3
-0.4 0 0.25 0.5 0.75 1 1.25 1.5 1.75 2
Number of Cycles (millions)
Figure 4.115 Deflection at midspan of beams strengthened with CFRP plate and subjected to repeated load test cycles
239 (a) Repeated load range 1 (b) Repeated load range 2 (c) Repeated load range 3 15% beam ultimate strength 25% beam ultimate strength 40% beam ultimate strength (450-4500 lbs) (750-7500 lbs) (1200-12000 lbs)
12 12 12
9 9 9
6 6 6
Load (kips) Load (kips) Load (kips)
3 3 3 @ 0 Cycles @ 0 Cycles @ 0 Cycles @ 100,000 Cycles @ 100,000 Cycles @ 100,000 Cycles @ 1,000,000 Cycles @ 1,000,000 Cycles @ 1,000,000 Cycles @ 2,000,000 Cycles @ 2,000,000 Cycles @ 2,000,000 Cycles
0 0 0 0 1000 2000 3000 0 1000 2000 3000 0 1000 2000 3000
MicroStrain (in./in.) MicroStrain (in./in.) MicroStrain (in./in.)
(b) Beam P-R25-1 (c) Beam P-R40-1 (a) Beam P-R15-1 Figure 4.116 Load versus strain relationship at midspan of CFRP plate for beams subjected to repeated load test cycles
240 3000
(c) Repeated load range #3: 40% beam ultimate strength (1200-12000 lbs, beam P-R40-1)
2000
1000
MicroStrain (in./in.) 0 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
3000 (b) Repeated load range #2: 25% beam ultimate strength (750-7500 lbs, beam P-R25-1) 2000
1000
MicroStrain (in./in.) 0 5 7 0 1 2 3 4 6 8 Distance along length of beam (feet)
3000
(a) Repeated load range #1: 15% beam ultimate strength (450-4500 lbs, beam P -R15-1) 2000
1000
MicroStrain (in./in.) 0 0 1 2 3 4 5 6 7 8
Distance along length of beam (feet)
@ 0 Cycles @ 100,000 Cycles @ 1,000,000 Cycles @ 2,000,000 Cycles
Figure 4.117 Strain in CFRP plates at 12 kips static load and subjected to repeated load test cycles
241 2500 (c) Repeated load range #3: 40% beam ultimate strength (1200-12000 lbs, beam P-R40-1)
2000
1500
1000 500 MicroStrain (in./in.) 0
2500 (b) Repeated load range #2: 25% beam ultimate strength (750-7500 lbs, beam P-R25-1) 2000
1500 1000
500 MicroStrain (in./in.) 0
2500
2000 (a) Repeated load range #1: 15% beam ultimate strength (450-4500 lbs, beam P -R15-1)
1500 n (in./in.) 1000
500 MicroStrai 0 0 0.25 0.5 0.75 1 1.25 1.5 1.75 2
Number of Cycles (millions) Figure 4.118 Strain in CFRP at midspan of beams strengthened with CFRP plates during repeated load tests
242
33.0 35.0 31.1 29.2 28.4 30.0
25.0
19.3
20.0
Load (kips)
15.0
No Repeated Load Test 000 lbs) 4500 lbs) - 12 10.0 -
Repeated Load Range = 15% (450 7500 lbs) -
Repeated Load Range = 40% (1200 No Repeated Load Test
Repeated Load Range = 25% (750 5.0
0.0 Unstregthened Baseline Beam P-R15-1 Beam P-R25-1 Beam P-R40-1
Figure 4.119 Average ultimate loads after the conclusion of repeated load tests of beams strengthened with CFRP plate
243 (a) Repeated load range 1 (b) Repeated load range 2 (c) Repeated load range 3 15% beam ultimate strength 25% beam ultimate strength 40% beam ultimate strength (450-4500 lbs) (750-7500 lbs) (1200-12000 lbs) 12 12 12
9 9 9
6 6 6
Load (kips) Load (kips) Load (kips)
3 @ 0 Cycles 3 @ 0 Cycles 3 @ 0 Cycles
@ 100,000 Cycles @ 100,000 Cycles @ 100,000 Cycles @ 1,000,000 Cycles @ 1,000,000 Cycles @ 1,000,000 Cy cles @ 2,000,000 Cycles @ 2,000,000 Cycles @ 2,000,000 Cycles
0 0 0 0.0 0.5 1.0 0.0 0.5 1.0 0.0 0.5 1.0
Deflection (in.) Deflection (in.) Deflection (in.)
(c) Beam F-R40-1 (a) Beam F-R15-1 (b) Beam F-R25-1
Figure 4.120 Load versus deflection at midspan of beams strengthened with CFRP fabrics and subjected to repeated load test cycles
244
(c) Repeated load range #3: 40% beam ultimate strength (1200-12000 lbs, beam F-R40-1) 0.0
Deflection (in.) 0.5
(b) Repeated load range #2: 25% beam ultimate strength (750-7500 lbs, beam F-R25-1)
0.0
Deflection (in.) 0.5
(a) Repeated load range #1: 15% beam ultimate strength (450-4500 lbs, beam F-R15-1) 0.0
Deflection (in.) 0.5 7 0 1 2 3 4 5 6 8 Distance along length of beam @ 0 Cycles @ 100,000 Cycles @ 1,000,000 Cycles @ 2,000,000 Cycles
Figure 4.121 Deflected shape at 12 kips static load for beams strengthened with CFRP fabric during repeated load tests
245 (c) Repeated load range #3: 40% beam ultim ate strength (1200-12000 lbs, beam F-R40-1)
-0.1
-0.2
Deflection (in.) -0.3
-0.4
0
-0.1
-0.2
Deflection (in.) -0.3 (b) Repeated load range #2: 25% beam ultimate strength (750-7500 lbs, beam F-R25-1)
-0.4
0
-0.1
-0.2 (a) Repeated load range #1: 15% beam ultimate strength (450-4500 lbs, beam F-R15-1)
Deflection (in.) -0.3
-0.4 1.5 1.75 0 0.25 0.5 0.75 1 1.25 2 Number of Cycles (millions)
Figure 4.122 Deflection at midspan of beams strengthened with CFRP fabrics during repeated load tests
246 (b) Repeated load range 2 (c) Repeated load range 3 (a) Repeated load range 1 25% beam ultimate strength 15% beam ultimate strength 40% beam ultimate strength (750-7500 lbs) (450-4500 lbs) (1200-12000 lbs)
12 12 12
9 9 9
6 6 6
Load (kips) Load (kips) Load (kips)
3 3 @ 0 Cycles 3 @ 0 Cycles @ 0 Cycles
@ 100,000 Cycles @ 100,000 Cycles @ 100,000 Cycles @ 1,000,000 Cycles @ 1,000,000 Cycles @ 1,000,000 Cycles @ 2,000,000 Cycles @ 2,000,000 Cycles @ 2,000,000 Cycles
0 0 0 0 1000 2000 3000 0 1000 2000 3000 0 1000 2000 3000
MicroStrain (in./in.) MicroStrain (in./in.) MicroStrain (in./in.)
(c) Beam F-R40-1 (a) Beam F-R15-1 (b) Beam F-R25-1
Figure 4.123 Load versus strain at midspan of CFRP fabrics during repeated load tests
247 3000 (c) Repeated load range #3: 40% beam ultimate strength (1200-12000 lbs, beam F-R40-1)
2000
1000 MicroStrain (in./in.) 0
(b) Repeated load range #2: 25% beam ultimate strength (750-7500 lbs, beam F-R25-1)
3000
2000
1000
MicroStrain (in./in.) 0
3000 (a) Repeated load range #1: 15% beam ultimate strength (450-4500 lbs, beam F-R15-1)
2000
1000
MicroStrain (in./in.) 0 0 1 2 3 4 5 6 7 8 Distance along length of Beam (feet)
@ 0 Cycles @ 100,000 Cycles @ 1,000,000 Cycles @ 2,000,000 Cycles Figure 4.124 Strain in CFRP fabrics at 12 kips static load during repeated load tests
248 (c) Repeated load range #3: 40% beam ultimate strength (1200-12000 lbs, beam F-R40-1) 2500
2000
1500 1000
500 MicroStrain (in./in.) 0
2500 (b) Repeated load range #2: 25% beam ultimate strength (750-7500 lbs, beam F-R25-1)
2000
1500 1000
500 MicroStrain (in./in.) 0
2500 (a) Repeated load range #1: 15% beam ultimate strength (450-4500 lbs, beam F-R1 5-1) 2000
1500
1000 500 MicroStrain (in./in.) 0 0 0.25 0.5 0.75 1 1.25 1.5 1.75 2
Number of Cycles (millions) Figure 4.125 CFRP strain at midspan of beams strengthened with CFRP fabric during repeated load tests
249
35.0 29.1 29.3 28.3 28.3 30.0
25.0 19.3
20.0
15.0 12000 lbs) - Load Range = 40% Load (kips)
(1200
10.0 Repeated
4500 lbs) 7500 lbs) - - No Repeated Load Test No Repeated Load Test 5.0 Repeated Load Range = 15% (450 Repeated Load Range = 25% (750
0.0 Unstrengthened Baseline Beam F-R15-1 Beam F-R25-1 Beam F-R40-1
Figure 4.126 Average ultimate loads after the conclusion of repeated load tests of beams strengthened CFRP fabrics
250 40
35 33.7 33.5 33.5 33.1 32.5 32.1 30.7 30.7 30 28.5 27.6 26.8 25.3 25
20.9
20 19.3
Load (kip) 15
10
5
0 Un-strengthened Baseline Dry Heat Humidity Salt-Water Alkaline Solution
Beams exposed to 1000 Hrs Beams exposed to 3000 Hrs Beams exposed to 10,000 Hrs
Figure 4.127 Average ultimate failure loads for beams strengthened with CFRP plates
251
35
30.70 30 27.82 26.85 25.95 25
20
Load (kip) 15
10
5
0 Baseline Freeze 700 cycles Freeze 350 cycles Thermal 35 cycles
Figure 4.128 Ultimate failure loads for beams strengthened with CFRP plates after exposure to thermal cycles
252 0.90
0.81 0.80 0.75 0.75 0.71 0.70 0.68 0.67 0.67 0.65 0.63 0.63 0.62 0.60 0.58 0.53
0.50
0.40
Deflection (in.)
0.30
0.20
0.10
0.00 Baseline Dry-Heat Humidity Salt-Water Alkaline Solution
Beams exposed to 1000 Hrs Beams exposed to 3000 Hrs Beams exposed to 10,000 Hrs
Figure 4.129 Average midspan deflections for beams strengthened with CFRP plates
253 35
30.5 30.8 30.6 30 29.9 29.9 29.1 28.4 28.4 28.4 28.3 27.9 28.3 27.1
25
20 19.3
Load (kip) 15
10
5
0 Un-strengthened Baseline Dry Heat Humidity Salt-Water Alkaline Solution
Beams exposed to 1000 Hrs Beams exposed to 3000 Hrs Beams exposed to 10,000 Hrs
Figure 4.130 Average ultimate failure loads for beams strengthened with CFRP fabrics
254 35
30.3 30 30 28.1
26.1
25
20
Load (kips) 15
10
5
0
Baseline Freeze 700 cycles Freeze 350 cycles Thermal 35 cycles
Figure 4.131 Average ultimate failure loads for beams strengthened with CFRP fabrics exposed to thermal cycles
255 1.20
1.01 1.00
0.90 0.90 0.86 0.87 0.85 0.84 0.85 0.80 0.80 0.78 0.77 0.76 0.75
0.60
Deflection (in.) 0.40
0.20
0.00 Baseline Dry Heat Humidity Salt-Water Alkali Solution
Beams exposed to 1000 Hrs Beams exposed to 3000 Hrs Beams exposed to 10,000 Hrs
Figure 4.132 Midspan deflections for beams strengthened with CFRP fabrics
256 CHAPTER 5 DURABILITY BASED DESIGN
The following steps can be taken for design of reinforced concrete (RC) beams to be externally strengthened with fiber reinforced polymer (FRP) plates or fabric sheets for retrofitting and/or repairing applications. It should be noted that the proposed methodology presented herein is applicable only for external strengthening of RC beams/slabs using CFRP plates/fabrics. This is not applicable to prestressed concrete bridges or to steel beam/girder bridges. Figure 5.1 shows the strain and stress distributions along the depth of cross-section and forces in the steel reinforcements and concrete for a FRP strengthened beam of rectangular cross-section.
I. Compute Design Material Properties:
Design ultimate tensile strength of FRP plate, ffd = CE ffu
Design rupture strain of FRP plate, efd = CE efu where
CE = environmental tensile strength reduction factor ffu = specified tensile strength of FRP efu = specified rupture strain of FRP
The values of CE for carbon fiber reinforced polymer (CFRP)/Epoxy system are recommended to be 0.95 and 0.85 for enclosed and unenclosed exposure conditions (ACI Committee 440.2R-02, 2002), respectively. However, in case the FRP strengthened structural system is exposed to aggressive environmental condition, the nominal FRP contributed strength of the structure should be modified using strength reduction factors based on the long-term exposures. The design equations presented in this chapter are based on the following assumptions: (i) Plane sections before bending remain plane after bending (ii) Ultimate strain of concrete is 0.003 in./in. (iii) Tensile strength of concrete is ignored (iv) FRP materials behave linearly elastically up to failure (v) Perfect bond exists between the concrete and FRP
II. Compute the Existing Substrate Strain (Arduini and Nanni, 1997):
If the unfactored service moment acting on the RC beam before upgrading
(strengthening/repairing) is defined as Mo, then strain at the bottom face of the concrete, ebi, where the composite plate is to be bonded can be obtained by considering linear variation of strain along the depth of the beam.
Mo (h -co ) e bi = (5.1) Ec I tro where co = depth of the neutral axis of unstrengthened beam
Ec = modulus of elasticity of concrete h = total depth of beam cross-section
I tro = moment of inertia of the transformed cracked section of unstrengthened beam
III. Compute the Balanced Plate Ratio, rf,b (Saadatmanesh and Malek, 1998):
Balanced plate ratio gives the maximum cross-sectional area of the plate to assure yielding of the tensile reinforcement and crushing of the concrete simultaneously. The balanced plate ratio should be calculated depending on whether compression steel yields or not.
eu - e y Critical compression depth, dc = d eu + e y where d = effective depth of RC beam, i.e., distance from extreme compression fiber to the
centroid of tensile steel reinforcement
258 eu = ultimate compressive strain of concrete ey = yield strain of steel
(a) Compression steel yields (d¢ £ dc): The balanced plate ratio ( ? f,b ) corresponding to yielding of compression steel at balanced condition can be computed using Eq.5.2. This equation is obtained using equilibrium and compatibility conditions, which takes into account Hognestad’s parabola of idealized stress-strain curve (Park and Paulay, 1975) for concrete.
' a f c k1+(?' - ?)f y ? f,b = (5.2) efe Ef where k1 = neutral axis depth ratio (c/d)
eu = eu + e y
(h -k1 d) efe = e u - ebi £ km e fd (5.3) k1 d c = depth to the neutral axis from the extreme compression fiber d¢ = distance from the extreme compression fiber to the centroid of compression steel
Ef = modulus of elasticity of FRP plate
' fc = compressive strength of concrete fy = yield stress of steel reinforcement a = Mean stress factor (Ann et al., 1991)
e æ e e 2 ö 0.15 e 0.075 æ e 2 ö u ç u o ÷ æ ö æ u ö æ ö ç 0 ÷ =1+ 1 - - -ç ÷ ç - e ÷- ç ÷ for eo £ eu ç 2 ÷ ç ÷ ç o ÷ ç ÷ ç ÷ eo 3eo e 0.004 - eo 2 0.004-eo eu è u ø è ø è ø è ø è ø
259 2 eu eu = - 2 for eo ³ eu eo 3eo
A s r = (5.4) b d
' As r¢ = (5.5) bd
Af ? = (5.6) f b d
As = cross-sectional area of tensile steel
' As = cross-sectional area of compression steel
Af = cross-sectional area of FRP plate b = width of RC beam km = delamination factor (ACI Committee 440.2R-02, 2002, and REPLARK SYSTEM, 2000) ebi = strain in concrete substrate at the bottom of RC beam efe = effective strain in CFRP plates/fabrics eo = concrete strain corresponding to the maximum concrete stress
' 2 fc = Ec
Ec = modulus of elasticity of concrete r = tension reinforcement ratio r¢ = compression reinforcement ratio
? f = CFRP reinforcement ratio
The value of factor km can be evaluated as per Eqs. 5.7 and 5.8 for English and
Metric Systems of units, respectively. This value of km is based on the maximum tensile force expression (FIB bulletin, 2001), which can be transferred from the CFRP
260 plate/fabrics to the concrete by means of bond stresses at the anchorage zone. Here, calibration coefficient (FIB bulletin 14, 2001) has been taken as inversely proportional to the total thickness of bonded CFRP plates/fabrics and the constant of proportionality is based on experimental results. English System of Units
' 1/4 1.0 (fc f ctm) k = £ 0.5 (5.7a) m 30 nt g e 1/2 f fd (Ef )
' f ctm= 0.19 f c ksi (ACI 318) (5.7b)
where n tf is the total thickness of CFRP plates/fabrics (inches); n and tf are the number of layers of CFRP plates/fabrics bonded together and thickness of each layer of
' plates/fabrics, respectively; and Ef , f c , and fctm are expressed in ksi units. fctm is the mean tensile strength of concrete. g is material safety factor for concrete.
Metric System of Units
' 1/4 1.0 (fc fctm) k = £ 0.5 (5.8a) m 1.2 nt g e 1/ 2 f fd (Ef )
' fctm = 0.5 fc MPa (ACI 318) (5.8b)
' where n tf is the total thickness of CFRP plates (mm); and Ef , f c , and f ctm are expressed in MPa units.
(b) Compression steel does not yield (dc < d¢): In case the yielding of compression steel, at balanced condition, does not occur, Eq. 5.9 can be used to compute balanced plate ratio (? ). f,b
261 (k1d - d') ' eu Es ?' + a fc k1 - ? fy k1d rf,b = (5.9) efe Ef
Es = Young’s modulus of elasticity of steel reinforcements.
IV. Compute the Maximum Allowable Plate Ratio, ? : Based on ACI 318, the f,max maximum allowable plate ratio, rf,max can be computed using Eq. 5.10 as given below:
? = 0.75? (5.10) f,max f,b
V. Proportion the FRP Plate: Choose a plate with appropriate dimensions and specified material properties such that plate ratio remains below the maximum allowable plate ratio. Here, specified material properties refer to the property of material provided by manufacturers.
VI. Compute the Balanced Plate Ratio, ? , to Determine Failure Mode : The f,bb balanced plate ratio, ? refers to the condition at which the maximum compressive f,bb stress in the concrete and maximum effective tensile stress in the composite plate reach simultaneously. This can be used to characterize the two primary modes of failure such as crushing of concrete and plate failure. Here, plate failure refers to the failure of the beam by the onset of delamination and/or plate rupture. It must be noted that the occurrence of these two primary beam failure modes are based on the assumption that the premature beam failure due to the onset of delamination or concrete cover delamination may reduce the maximum effective stress in the FRP plate at the ultimate failure of beam. The balanced plate ratio, ? is expressed by Eqs. 5.11a and 5.11b. f,bb
' h ' a f c k 2 + es Es ?' - ? f y ? = d for ? £ ? (5.11a) f,bb f f,max k mefd Ef
262 ' h ' a f c k 2 + es Es ?' - e s Es ? ? = d for ? > ? (5.11b) f,bb f f,max k mefd Ef
eu k 2 = (5.12) eu + k me fd+ ebi
' æ d' ö es = ç1 - ÷ e u £ e y (5.13) è k 2 d ø
æ 1- k 2 ö e s = ç ÷ e u £ e y (5.14) è k 2 ø where k2 = neutral axis depth coefficient
' es = strain in compressive steel reinforcement es = strain in tensile steel reinforcement
VII. Determine the Critical Plate Ratio, ? : This ratio is determined to ensure f,c yielding of compression steel at ultimate beam failure by the rupture of plate or by the concrete crushing.
(a) Rupture of Plate: The critical plate ratio corresponding to the rupture of the plate is defined as, ? and is expressed by Eq. 5.15, as given below: f,cf
' c a f c + (?' - ?) f y ? = d (5.15) f,cf k m efdEf where
ey h + k m efd d' c= (5.16) k m efd+ e y
263
(b) Crushing of Concrete: The critical plate ratio corresponding to the crushing of concrete is defined as ? and is expressed by Eq. 5.17, as given below: f,cc
d' a f ' k + (?' - ?) f c 3 d y ? = (5.17) f,cc E f e fe where
e u k 3 = (5.18) e u - e y
e u (h -k 3 d') e fe = - e bi £ km efd (5.19) k 3 d'
VIII. Determine the Mode of Failure: The following criteria are used to determine the failure modes of strengthened beams to compute their nominal moment capacity. These failure criteria are based on the assumption that premature failure due to the onset of delamination and concrete cover delamination is prevented by taking into account the factor km in design equations.
(a) Plate failure and yielding of tension and compression steel for ? £ ? and ? f f,bb f
³ ? f,cf
(b) Plate failure and yielding of tension steel but no yielding of compression steel for
? £ ? and ? £ ? f f,bb f f,bb
(c) Crushing of concrete and yielding of tension and compression steel for ? ³ f
? and ? ³ ? f,bb f f,cc
264 (d) Crushing of concrete and yielding of tension steel but no yielding of compression
steel for ? ³ ? and ? £ ? f f,bb f f,cc
IX. Nominal Moment Capacity of Strengthened Beams: Based on the above failure modes, the nominal flexural capacity of the strengthened beam, Mn is calculated using one of the following equations:
(a) Plate failure and yielding of tension and compression steels:
' Mn = A s f y (?c - d') +A s f y (d - ? c ) +? km efd Ef A f (h -?c ) (5.20) where
?c =ß1c (5.21)
1 e c - 3 12 eo ß1 = if 0 £ ec £ eo (Ann et al., 1991) (5.22a) ec 1- 3 e0
3 2 2 2 ec - 5.1eo ec - 0.004eo + 0.024ec =1- if eo £ e c £ eu (5.22b) 2 2 ec (3.925 eo -10.2eo ec - 0.9 ec -0.016 eo + 0.048 ec )
(k mefd + ebi ) c e = (5.23) c h -c
ec = concrete strain at the extreme compression fiber at a particular load
ß1 = ratio of the depth of the centroid of resultant concrete compression force to the depth of the neutral axis
?c = depth of the centroid of resultant concrete compression force from the extreme compression fiber ? =FRP strength reduction factor to take into account the long-term aggressive environmental exposure
265 ' k m Afd efd Ef + As f y -As f y c = (5.24) ' a fc b
2 ec ec a = - for 0 £ e c £ eo (5.25a) 2 eo 3 eo
2 2 ec æ ec eo ö æ 0.15 ö æ e c ö æ 0.075 öæ e0 ö =1+ ç1 - - ÷-ç ÷ ç - e ÷- ç ÷ç ÷ for eo £ e ç 2 ÷ ç ÷ ç o ÷ ç ÷ç ÷ c eo è 3eo ec ø è 0.004 -eo ø è 2 ø è 0.004- eo øè ec ø
£ eu (5.25b)
The value of ‘c’ has to be obtained using iterative procedure to ensure equilibrium of forces in steel, FRP plate, and concrete. b) Plate failure and yielding of tensile steel but no yielding of compression steel:
Using Eq. 5.23, obtain the value of concrete strain, e c , at the extreme compression fiber.
For a particular value of e c , the value of a can be obtained using Eq. 5.25a or 5.25b, whichever is applicable.
Recalculate the value of c using Eq. 5.26.
' ' k m Afd e fdEf + As f y -es Es As c = (5.26) ' a f c b where
(k m e fd+ ebi )(c -d') e' = £ e (5.27) s h -c y where
' es = strain in compressive steel reinforcement
If the assumed value of c and that obtained by Eq. 5.26 are the same, the nominal moment capacity can be obtained using Eq. 5.28.
266 ' ' Mn = A s fs (?c - d') +A s f y (d - ? c ) + ? k mA f efd Ef (h - ?c ) (5.28)
' ' f s = Es es (5.29) where
' fs = stress in the compressive steel reinforcement
(c) Crushing of concrete and yielding of tensile and compression steels:
For this mode of failure, nominal moment capacity of strengthened beam can be determined by Eq. 5.30. The location of the centroid of concrete compression force, gc and a can be determined using Eq. 5.21 and Eq. 5.25, respectively for ec being equal to eu.
' Mn = A s f y (? c - d') +A s f y (d - ? c ) + ? A f e fe E f (h - ?c ) (5.30) where
(h -c) eu e = - e £ k e (5.31) fe c bi m fd
' A f Ef efe + (As - A s ) f y c = (5.32) ' a fc b
(d) Crushing of concrete and yielding of tensile steel but no yielding of compression steel:
For this mode of failure, nominal moment capacity of the strengthened beam can be obtained using Eq. 5.33. The value of a and gc can be determined by substituting the value of ec, which equals crushing strain of concrete, eu in Eqs. 5.25 and 5.22, respectively.
' ' Mn = A s fs (? c - d') +A s f y (d - ? c ) +? A f efe Ef (h -?c ) (5.33)
267 The neutral axis depth, c can be expressed by Eq. (5.34) using equation of equilibrium.
' ' A f efe Ef + As f y - A s f s c = (5.34) ' a f c b
(c - d') e u e' = £ e (5.35) s c y
' ' f s = Es es (5.36)
The value of effective plate strain, efe can be computed by Eq. 5.31.
X. Compute Design Moment Capacity: The design moment capacity, Md of an FRP strengthened beam can be determined using strength reduction factor, f, which takes into account the loss of ductility, which may be caused due to externally bonding the FRP plates to the concrete surface. The strength reduction factor, f can be determined as per
Eq. 5.38 (ACI 318 and ACI Committee 440.2R-02, 2002).
Md = f Mn (5.37) f = 0.90 for es ³ 0.005 (5.38a)
(es - e y ) = 0.70 + 0.20 for e y £ es £ 0.005 (5.38b) (0.005 - e y )
= 0.70 for es £ ey (5.38c) where es is the strain in tensile steel reinforcement.
It should be noted that the design moment, Md should be greater than or equal to the required moment capacity, Mu of the strengthened beam. The required moment capacity of the strengthened beam is expressed by Eq. 5.39.
Mu = 1.4 MD + 1.7 ML (AASHTO, 1998) (5.39) where MD is the dead load moment and ML is the live load moment.
268 XI. Allowable Service Stresses: In addition to determining the ultimate strength of the
FRP upgraded RC beam, the stresses in the concrete, steel, and FRP plate must be determined at service load condition (ACI Committee 440.2R-02, 2002). The equations for allowable stresses presented in the ACI Committee 440.2R-02 (2002) have been modified to take into account the compression reinforcement and are presented in Eqs.
5.40-5.43. The service load stresses resulting from all the sustained dead and live loads should be checked against allowable stresses. Table 5.1 lists the allowable service load stresses for the constituent materials. The following expressions can be used to determine the stresses in concrete, steel, and FRP plate under sustained service load condition.
é æ k döù M + e A E ç h - ÷ (d - kd) E ê s bi f f ç ÷ú s ëê è 3 øûú fs,s= æ k d ö æ k d ö æ k d ö A E çd - ÷ d - k d + A' E ç -d'÷(kd - d') +A E çh- ÷ h- kd s s ç ÷( ) s s ç ÷ f f ç ÷ ( ) è 3 ø è 3 ø è 3 ø
(5.40)
æ E ö k d f , = f ç c ÷ (5.41) c s s,s ç ÷ è Es ø (d -k d)
æ Ef ö æ h - k d ö ffe,s = fs,s ç ÷ ç ÷ - e E (5.42) ç E ÷ ç d - k d ÷ bi f è s ø è ø
æ k d- d'ö f ' = f ç ÷ (5.43) s,s s,s ç h-k d ÷ è ø where fc,s = maximum compressive stress in concrete under sustained service loads ffe,s = maximum stress in FRP under service loads fs,s = maximum stress in tensile steel reinforcement under service loads
269 ' fs,s = maximum stress in compression steel under sustained service loads, ksi k = neutral axis depth coefficient at service load condition
It should be noted that as the service load stress for the FRP plate under sustained loads (dead load) plus sustained portion of the live load is less than the allowable value, the FRP will have adequate safety against failure due to creep rupture.
5.1 Design Example
Problem: A deficient simply supported RC beam is to be strengthened to carry a nominal ultimate midspan load of 21 kips using 4-point loading system (see Fig. 3.25) in addition to its self-weight on a long-term basis under aggressive humidity conditions (hot water
40o C). Figure 5.2 shows the cross-sectional details of the beam. The total span of the beam is 108 in., while the center-to-center distance between supports is 100 in. The effective span of central 4-point loading beam is 32 in. The beam is internally reinforced with two Grade 60 steel bars of 3/8 in. and 5/8 in. diameter in compression and tension, respectively. Use of CFRP plate is recommended for external strengthening. Assume that the material being added through the placement in the enclosed space uses wet lay-up process and adhesive bonding of pre-fabricated sections with ambient cure. Design a suitable strengthening system using CFRP plates and epoxy. The material properties of
' CFRP plate and epoxy are given in Table 3.1. Take the strength of concrete fc as equal to
4.5 ksi and yield stress of steel is 60 ksi.
270
Table 5.1 Allowable service load stresses Material Allowable Stress
Tension Steel 0.80 fy
Compression Steel 0.40 fy
CFRP Plate/Fabric 0.55 ffd
271 Solution: The design steps for the CFRP strengthened beam are explained as follows.
Step 1: Compute the Design Material Properties
Design ultimate tensile strength of CFRP plate, ffd = CE ffu
= 0.95 ´ 300
= 285 ksi
Design rupture strain of CFRP plate, efd = 0.95 ´ 0.015
= 0.014
Step 2: Compute the Existing Substrate Strain, e bi
6´ 10 ´150 Dead load of the beam Wd = = 62.5 lb/ft 12 ´12 2 Unfactored service moment before strengthening = Wd l / 8 62.5 ´ 1002 = = 0.54 kip-ft 122 ´ 8´ 1000
' Strength of concrete, f c = 4.5 ksi
Modulus of elasticity of concrete, Ec = 57000 Ö 4500 = 3824 ksi
29,000 Modular ratio of steel reinforcement and concrete, m = = 7.6 3,824 From Fig. 5.2, b = 6 in. d ' = 1.5625 in. d =8.3125 in.
' 2 As = 0.22 in.
2 As = 0.62 in.
272 Let the neutral axis depth of unstrengthened beam be co. Equating the first moment of tensile and compressive areas (based on concrete) about the neutral axis results in
Eq.5.44.
c2 b + (m-1) A' (c –d') = m A (d-c ) (5.44) 2 s o s o
Substituting the parametric values in Eq. 5.44 results in Eq. 5.45, 2 co + 2.06 co - 13.81 = 0 (5.45)
From the solution of Eq. 5.45,
co = 2.83 in.
Transformed moment of inertia of elastic cracked unstrengthened beam section,
3 bco I = + (m-1) A' (c -d')2 + m A (d- c )2 tro 3 s o s o
6 ´ 2.833 = + (7.6 -1) ´ 0.22 ´(2.83-1.5625)2 + 7.6 ´ 0.62´ (8.3125- 2.83)2 3
=189.3 in.4
Mo (h - co ) Existing substrate strain, ebi = EcItro
0.54 ´ 12 ´ (10 - 2.83) = 3824 ´ 189.3
= 6.42 ´ 10-5
Step 3: Compute the Balanced Plate Ratio, ? f,b :
e u = 0.003
273 60 -3 e y = = 2.1´10 29,000
e u - e y Critical compression depth, dc = d eu + e y
= 1.47 in. d¢ = 1.5625 in. > dc Þ compression steel does not yield at the balanced condition.
Thus, balanced plate ratio, ? is given by Eq. 5.9. f,b
Value of km can be obtained using Eq. 5.7, where g equals 1.5 (FIB Bulletin 14, 2001) and n equals 1. f ctm= 0.19 4.5 = 0.403 ksi
1.0 (4.5´0.403)1/4 km = = 0.27 1.5´30´1´0.047´0.014 (20,000)1/2
-3 km e fd = 0.27 ´ 0.014 = 3.78 ´ 10
eu k1 = eu + e y
0.003 = 0.003 + 2.1´10-3
= 0.588
0.22 ?' = = 4.41´ 10-3 6´ 8.3125
0.62 ? = =0.012 6´ 8.3125
(h -k1 d) efe = e u -ebi k1 d
274 (10-0.588 ´ 8.3125) = ´ 0.003-6.42´10-5 = 3.07 ´10-3 £ k e 0.588 ´8.3125 m fd
' 2 fc 2´ 4.5 -3 eo = = = 2.4 ´10 < e u E c 3824
From Eq. 5.25b, a = 0.92
0.588´8.3125 -1.5625 ´ 0.003´.29,000 ´ 4.41´10-3 +0.92´ 4.5´ 0.588 - 0.012´ 60 0.588 ´8.3125 ? f,b = 3.07´10 -3´ 20,000 = 0.03
Step 4: Compute the maximum allowable plate ratio, ? : f,max
? = 0.75 ´ 0.03 f,max = 2.25 ´ 10-2
Step 5: Proportion the CFRP Plate:
Choosing two plates of 3 ´ 0.047 in. dimensions and bonding them side by side along the width of beam cross-section.
Total width of bonded plates, bf = 6 in.
Total thickness of plate, tf = 0.047 in.
6 ´ 0.047 Plate ratio, ? = =5.65 ´10-3 < ? O.K. f 6 ´ 8.3125 f,max
Step 6: Compute the Balanced Plate Ratio, ? to Determine the Failure Modes: f,max
0.003 k 2 = = 0.438 0.003 + 0.27´ 0.014 + 6.42 ´10-5
275 1.5625 ' æ ö -3 e =ç 1- ÷ ´ 0.003 = 1.71 ´ 10 < ey s ç 0.438 ´ 8.3125÷ è ø
? = f,bb
10 0.92 ´ 4.5 ´ 0.438 ´ + (1.71´10-3 ´ 4.41´10-3- 2.1 ´10-3´ 0.012) ´ 29,000 8.3125 0.27 ´ 0.014 ´ 20,000
-2 = 2.2 ´ 10 > ? f
Since the actual plate area is less than the balanced plate area (at ultimate condition), the plate failure by the onset of delamination will govern the design.
Step 7: Compute the Critical Plate Ratio, ? : f,cf
2.1 ´10-3´10 + 0.27 ´ 0.014 ´1.5625 c = 0.27 ´ 0.014 + 2.1´10-3 = 4.57 in.
4.57 0.92 ´ 4.5 ´ + (4.41´ 10-3 - 0.012) ´ 60 ? = 8.3125 = 2.4´10-3 > ? f,cf 0.27 ´ 0.014´ 20,000 f
Since ? is greater than ? , the condition of yielding of compression steel at f,cf f plate failure condition is not satisfied. Thus, nominal moment capacity of strengthened beam will be based on plate failure, yielding of tensile steel, and no yielding of compression steel.
Step 8: Compute Nominal Moment Capacity of Strengthened Beam
-3 k m efd = 3.78 ´ 10
-3 eo = 2.4 ´ 10
276 (k m efd +ebi ) c Strain in concrete, ec = h-c
' (c -1.5625) ec Strain in compression steel, es = c
Assume c = 3.35 in.
-3 ec =1.93´10 < e u O.K.
' -3 es =1.03 ´10 < e y O.K.
From Eq. 5.25a, a = 0.58
From the solution of the equlibrium equation (Eq. 5.46),
' ' ' a f c b c + As fs =Af k m efd Ef + As fs (5.46) where
2 Af = 0.282 in.
2 As = 0.62 in.
' 2 As = 0.22 in.
f s = f y = 60 ksi
45.24 (c -1.5625) f ' = s c
c = 3.3 in. @ assumed value of c; Thus, take c = 3.35 in.
From Eq. (5.22b), b1 = 0.36 gc = b1 c = 0.36 ´ 3.35
= 1.21 in.
Strength reduction factor, ? (Table 4.7) for hot water conditioning (100% humidity )
277 = 0.70
Nominal moment capacity of strengthened beam, Mn is given by Eq. 5.47.
' ' Mn = As fs (d'-?c ) + As f s (d -?c ) + ? Af Ef k m efd (h - ? c ) (5.47)
= 6.57 (1.5625 - 1.21) + 37.2 (8.3125 - 1.21) + 0.70 ´ 21.32 (10 - 1.21)
= 397.7 kip-in.
= 33.1 kip-ft
(33.1-0.75)´2´12 Nominal load carrying carrying capacity of beam = 34 = 22.8 kips > 21 kips O.K.
Required moment capacity of the beam, Mu can be determined using Eq. 5.39.
Mu = 1.4 MD + 1.7 ML
1.4´62.5´1002 21´34 = + = 30.5 kip -ft (Experimetal value) 122´8 ´1000 2
The nominal moment capacity of strengthened beam is about 8.5% higher than the corresponding experimental value. It should be noted that the above required moment capacity is based on ultimate load carrying capacity of beam,i.e., 21 kips.
Step 9: Compute Design Moment Capacity of Strengthened Beam
Strain in tensile steel at ultimate load of strengthened beam,
1.93 ´10-3 (8.3125- 3.35) e = s 3.35
= 0.0029
From Eq. 5.38, f = 0.76
Md = f Mn = 0.76 ´ Mn = 25.8 kip-ft < Mu
278 It should be noted that the design moment capacity of beam is less than the required moment capacity of the beam. This is attributed to the reduced ductility of the beam due to the onset of delamination. Hence, the design load carrying capacity of the beam should be specified based on the above design moment capacity.
(25.8 - 0.75)´ 2 ´12 Design load carrying capacity of beam = 34 = 17.7 kips
Thus, the given beam is capable of carrying the nominal ultimate load of 21 kips. However, its design load carrying capacity should be limited to 17.7 kips.
Step 10: Check for stresses under sustained service load condition
In the present problem, sustained service load is only self-weight of the beam.
62.5 ´1002 ´12 Thus, moment due to sustained service load, Ms = =6.51 kip -in. 12 2 ´ 8 ´ 1000
The stresses in steel, CFRP plate, and concrete under service load condition can be determined using Eqs. 5.40 to 5.43.
20,000 Modular ratio of CFRP plates and concrete, m = = 5.24 f 3824
Modular ratio of steel and concrete, m = 7.6
Let the neutral axis depth of the strengthened beam corresponding to the service load condition is c. Equating the first moment of the compression and tension area (based on concrete) about the neutral axis results in the following quadratic equation (Eq. 5.48): b c2 + (m-1) A' (c -d') = m A (h - c) + m A (d-c) (5.48) 2 s f f s
Substituting the parametric values in Eq. 5.48 results in Eq. 5.49. c2 + 2.55c - 18.74 = 0 (5.49)
279 From the solution of Eq. 5.49, c = 3.1 in.
f s,s = é æ 3.1öù ê6.51+ 6.42´ 10-5´ 0.282 ´ 20,000 ç10 - ÷ú(10 -3.1) ´ 29,000 ç 3 ÷ ëê è øûú æ 3.1ö æ 3.1 ö 0.62 ´ 29,000 ç8.3125 - ÷(8.3125 -3.1) + 0.22 ´ 29,000 ç - 1.5625 ÷(3.1-1.5625 )+ 0.282 ´ ç 3 ÷ ç 3 ÷ è ø è ø æ 3.1ö 20,000 ç10 - ÷ ´ (10-3.1) ç 3 ÷ è ø
= 1.92 ksi < 0.8 fy O.K.
æ 1 ö 3.1 ' f = 1.92 ´ ç ÷ ´ = 0.15 ksi < 0.45 f c,s ç 7.6÷ (8.3125- 3.1) c è ø O.K.
æ 20,000 öæ 10- 3.1 ö -5 ffe,s = 1.92 ´ ç ÷ç ÷ -6.42´10 ´20,000 = 0.45 ksi < 0.55 ffd O.K. ç 29,000 ÷ç8.3125 -3.1÷ è øè ø
3.1-1.5625 f ' =1.92´ = 0.60 ksi < 0.4 f O.K. s,s (8.3125-3.1) y
Since the service load stresses under the sustained self-weight of the beam are under allowable limits, the CFRP plates will have adequate safety against failure due to creep rupture.
280
Compression steel, A´s Stirrup
b ec a f¢c
d´ e¢ F¢ s f¢ gc s s C c
N. A.
d h
es f s Fs ff Ff tf efe ebi Tension Steel, As bf
FRP plate, Af
Cross-section Strain Stress Force
Figure 5.1 Stress, strain, and force diagrams across depth of beam cross-section
281
Two-legged 6 in. 3/8 in. diameter 1 in. stirrups
Two Grade 60 steel bars of 10 in. Two Grade 60 steel bars of 3/8 in. diameter 5/8 in. diameter
1 in. 0.047 in.
3 in. 3 in.
CFRP plate (Typ.)
Figure 5.2 Cross-sectional details of CFRP strengthened beam
282 CHAPTER 6 CONCLUSIONS AND RECOMMENDATIONS
In this experimental investigation, CFRP plates and fabrics were used to externally strengthen reinforced concrete beams. The beams were then exposed to various independent environmental conditionings. These conditionings included, 100% humidity, dry-heat, alkaline solution, salt-water, freeze/thaw cycles, thermal expansion cycles, and repeated load cycles.
6.1 Conclusions
This investigation confirmed the excellent durability of carbon fiber reinforced polymers. The accelerated aging tests ensure that CFRP is long lasting, even when exposed to severe environmental conditions for long durations. For both strengthening systems, with CFRP plates or with the CFRP fabrics, the beams showed approximately the same efficiency and long-term durability in most cases, confirming the satisfactory mechanical properties of CFRP. Based on the results of this study, the following conclusions may be drawn:
1. In general, most of the beams had less than a 15% reduction in their load carrying capacity after exposure to various independent environmental conditions. The beams strengthened with CFRP plates and exposed to 100% humidity for 10,000 hours showed about a 33% reduction in the load carrying capacity and that represented the only case encountered where the strength reduction was significantly high. 2. The strength reduction factors obtained from this research investigation varied between 0.70 and 1.0 for the beams strengthened with CFRP plates. For the beams strengthened with CFRP fabrics, the reduction factors varied between 0.85 and 1.0, for the various independent environmental conditions considered. These strength reduction factors do not take into account the aged concrete and associated defects, deterioration or contamination. However, the proposed strength reduction factors take into account the deterioration of concrete caused due to a specific aggressive environmental condition to which they refer to. 3. Most of the beams showed more ductility after being subjected to different environments. The values of deflections recorded after exposures were greater, in most of the cases, than those of the baseline beams. The beams strengthened with CFRP fabrics were more ductile than those strengthened with CFRP plates exposed to the same environmental conditions. 4. The values of strains recorded confirmed those provided by the CFRP manufacturers in the materials data sheets. It was found that in the majority of the tests, actual strains correlated with the corresponding loads appropriately. 5. The typical failure mode encountered during the tests resulted from the delamination between the CFRP fabrics or plates and the concrete. CFRP plates and fabrics were in good condition after all types of environmental exposure, and no deterioration was noticed for them. 6. The weight of the beams was increased by 1.5% on average for the beams exposed for 10,000 hours to alkaline solution, humidity, and salt-water. The weight of the beams exposed to dry-heat was decreased by 3.5% after 10,000 hours of exposure. For the beams tested for freeze/thaw cycles, or for thermal expansion cycles, minor changes in the beams weights occurred, and the maximum weight change recorded was less than 0.25%.
6.2 Recommendations
Based on the results and observations acquired from the entire investigation, the following recommendations could be made.
· Effects of combined environmental conditions were not considered in this investigation. More research is needed to study combined effects, especially when combining different environments with the repeated loading. The strength reduction factors presented in this investigation are estimated for each environmental condition separately; other reduction factors should be established for combined environmental conditions.
284 · The real need is to find, consider, investigate, and improve new materials to be used as bonding agents. The delamination between the CFRP fabrics or plates and the epoxy establishes the need of further research in order to find a better compatible material to bond the CFRP to the concrete. · This investigation, and all the previous research, confirmed that CFRP materials are efficient to be used for strengthening concrete structures. Even though the CFRP maximum capacities have not yet been used, it is recommended to use the CFRP fabrics and plates broadly to strengthen the beams of highway bridges without significant concern about the long-term durability, providing appropriate strength reduction factors are considered in the design calculations. · The situation where the external use of CFRP is not appropriate is the exposure to long-term hot water condition, which can lead to a significant reduction in the load carrying capacity of strengthened beams. · The presented design methodology is applicable only for external strengthening of RC beams/slabs using CFRP plates/fabrics. This is not applicable to prestressed concrete bridges or to steel beam/girder bridges.
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298 APPENDIX-A
A.1 Notations
2 Af cross-sectional area of FRP plate, in.
2 As cross-sectional area of tension steel reinforcement, in.
' 2 As cross-sectional area of compression reinforcement, in. b width of the beam, in. b f total width of plates, in. c depth of the neutral axis of strengthened beam from the extreme compression fiber, in.
C resultant compressive force in concrete, kips c o depth of the neutral axis of unstrengthened beam from the extreme compression fiber, in.
CE environmental tensile strength reduction factor for FRP materials d, d¢ depth of centroid of tensile and compression steel reinforcements from the extreme compression fiber, respectively, in. dc critical compression depth, in.
Ec modulus of elasticity of concrete, ksi
Ef modulus of elasticity of FRP material, ksi
E fib Young’s modulus of fiber, ksi
Em Young’s modulus of matrix, ksi
Es modulus of elasticity of steel, ksi
' f c characteristic strength of concrete, ksi
f c,s maximum compressive stress in concrete under sustained service loads, ksi ff stress in FRP plate at ultimate condition, ksi
Ff resultant force in FRP plate at ultimate condition, kips ffe effective stress in FRP plate, ksi fft tensile strength of FRP in fiber direction, ksi ffib tensile strength of fibers, ksi fm tensile strength of matrix, ksi ffu specified tensile strength of FRP plate, ksi fs,s maximum stress in tensile steel under sustained service loads, ksi fs stress in tensile steel at ultimate condition, ksi
Fs resultant force in tensile steel at ultimate condition, kips
' f s,s maximum stress in compression steel under sustained service loads, ksi
' f s stress in compressive steel at ultimate condition, ksi
' Fs resultant force in compressive steel at ultimate condition, kips fy yield stress of steel, ksi h overall depth of the beam, in.
I tro moment of inertia of the transformed cracked section of unstrengthened beam, in.4 k neutral axis depth factor at the service load condition k1 ,k2 neutral axis depth factors at balanced and ultimate conditions, respectively.
k 3 neutral axis depth factor corresponding to yielding of compression steel at the crushing of concrete km bond-dependent coefficient for flexure m modular ratio of steel and concrete
300 mf modular ratio of FRP plate and concrete
Md design moment capacity of strengthened beam, kip-ft
MD unfactored maximum bending moment due to dead load, kip-ft
ML unfactored maximum bending moment due to live load, kip-ft
Mn nominal moment capacity of strengthened beam, kip-ft
Mo unfactored service moment acting on RC beam before upgrading, kip-ft
Ms unfactored moment due to sustained portion of service loads, kip-ft
Mu ultimate moment capacity of strengthened beam, kip-ft n number of layers of FRP plates t thickness of FRP plate, in. f
Vfib Volume fraction of fibers
Vm Volume fraction of matrix
Wd Dead load of the beam, lbs a mean stress factor ebi strain level in the concrete substrate at the time of FRP installation
ec concrete strain at the extreme compression fiber at a particular load
e fd design strain for FRP plate efe effective design strain for FRP plate
e fu specified rupture strain for FRP plate eo concrete strain corresponding to the maximum concrete stress es strain in tensile steel
' es strain in compressive steel
301 eu ultimate concrete strain ey yield strain of steel b1 ratio of the depth of the centroid of resultant concrete compression force to the depth of the neutral axis gc depth of the centroid of resultant concrete compression force from the extreme compression fiber, in. f strength reduction factor rf ratio of plate to the beam cross-sectional area r tensile steel reinforcement ratio r¢ compressive steel reinforcement ratio rfb balanced plate ratio rfbb balanced plate ratio at ultimate rf,c critical plate ratio for yielding of compression steel at ultimate condition rf,cf critical plate ratio for yielding of compression steel at the plate rupture rf,cc critical plate ratio for yielding of compression steel at crushing of concrete rf,max maximum allowable plate ratio y FRP strength reduction factor corresponding to aggressive environmental condition
302