FIELD MONITORING OF SHRINKAGE CRACKING
POTENTIAL IN A HIGH-PERFORMANCE CONCRETE
BRIDGE DECK
By
TIM WALKOWICH
A Thesis submitted to the
Graduate School – New Brunswick
Rutgers, The State University of New Jersey
in partial fulfillment of the requirements
for the degree of
Master of Science
Graduate Program in Civil and Environmental Engineering
Written under the direction of
Dr. Hani Nassif
and approved by
New Brunswick, New Jersey
January 2011 ABSTRACT OF THE THESIS
Field Monitoring of Shrinkage Cracking Potential in a High-
Performance Concrete Bridge Deck
Thesis Director:
Dr. Hani H. Nassif
Over the past decade many state engineers throughout New Jersey have reported cracking on High Performance Concrete (HPC) bridge decks at early ages. The presence of cracking early in the life of a high performance deck offsets the benefits gained in using the material as the potential for corrosion begins at the onset of cracking. While many factors apply to bridge deck cracking, the shrinkage of the concrete’s mass is a primary concern. Because of shear studs and boundary conditions, among other causes that act in restraining the deck itself, it is important to understand the mechanics of concrete under restraint.
The AASHTO Passive Ring Test (PP 34-06) is seeing an increase in use in studies analyzing restrained shrinkage. The test simulates a concrete member of infinite length and allows researchers to study the effects of various parameters on restrained shrinkage. This thesis presents the results of a study that analyzed the ring test’s ability to simulate restrained shrinkage on HPC bridge decks. The investigation incorporated an instrumented, simply supported
ii composite bridge deck with laboratory samples taken on the day of the pour as well as a finite element analysis. The results suggest the AASHTO Passive Ring Test simulates the restrained shrinkage of simply supported HPC decks reasonably well. Fewer than 1% of all cracking present on the ring specimens saw complete penetration through the sample with 80-90% of all cracking considered to be micro cracking. While the presence of several cracks along the bridge deck itself showed no correlation with the shrinkage ring specimens, finite element analysis suggests these cracks are a result of adjacent live load. Also, the findings of this study highlight the importance of following design in the field as well as the effect of live load on staged construction of HPC bridge decks.
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ACKNOWLEDGMENTS
I would like to thank Dr. Hani H. Nassif for all the opportunities I have been fortunate enough to take as well as his support throughout my time at Rutgers. The knowledge and experience I have gained through my relationship with him has been and I am sure will continue to be invaluable.
I would also like to thank Dr. Husam S. Najm and Dr. Kaan Ozbay for being on my committee and providing their insight.
I would like to thank my father and mother, Anthony and Mary Ellen Walkowich for helping me to grow into the person I am today. Without their guidance I would not have achieved half of what I have at this point in my life. Special thanks to my sisters, Heather and
Jessica, without whom I would not have the laughter and memories that keep me going when times are rough.
I am forever in debt to Carl Fleurimond, Dan Su, Etkin Kara, Ufuk Ates and Gunup
Kwan. Their friendship and guidance were critical in my success at Rutgers and I wish them all nothing but the best in their future efforts.
Without the help and participation of both the NJ Turnpike Authority and the SHAW
Group, Inc., this thesis would not be possible. Thank you in particular to Adel, Scott, and Paul for allowing me on site whenever my research required.
Thank you to Mike, Chris, Alex, Parth, John, Peng and everyone and anyone that ever helped in the lab in any way big or small. The assistance you provided made my experience at
Rutgers possible.
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Last, I would like to thank my closest friends Artie, Eric, Chris, Bryan, Matt and Jake.
Through the good times and the bad you have all stuck with me and I cannot put into words what that has meant to me.
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TABLE OF CONTENTS
ABSTRACT OF THE THESIS ii
ACKNOWLEDGMENTS vi
CHAPTER I – INTRODUCTION 1
1.1. PROBLEM STATEMENT 1 1.2. RESEARCH OBJECTIVES AND SCOPE 2 1.3. THESIS ORGANIZATION 2
CHAPTER II – LITERATURE REVIEW 4
2.1. INTRODUCTION 4 2.2. TYPES OF SHRINKAGE 4 2.2.1. PLASTIC SHRINKAGE 5 2.2.2. THERMAL SHRINKAGE 5 2.2.3. AUTOGENOUS SHRINKAGE 6 2.2.4. DRYING SHRINKAGE 6 2.3. SHRINKAGE FACTORS 7 2.4. RESTRAINED SHRINKAGE RING TEST 8 2.4.1. RING TEST BACKGROUND 8 2.4.2. RING TEST SETUP 11 2.5. PREVIOUS WORK 12
CHAPTER III – EXPERIMENTAL SETUP 35
3.1. INTRODUCTION 35 3.2. MATERIAL PROPERTIES OF MIX 36 3.3. MIXING AND FRESH SAMPLING OF CONCRETE 37 3.3.1. SLUMP TEST 38 3.3.2. AIR CONTENT 39 3.3.3. SAMPLING OF SPECIMENS AND CONSOLIDATION 40 3.3.4. CURING 41 3.4. INSTRUMENT DETAILS AND FIELD IMPLEMENTATION 42 3.4.1. EMBEDDED VIBRATING WIRE STRAIN GAUGES 42 3.4.2. PORTABLE DATA LOGGER 45 3.4.3. ACCELEROMETERS 46 3.4.4. LASER DOPPLER VIBROMETER 46 3.4.5. STRUCTURAL TESTING SYSTEM 47 3.5. LABORATORY TESTING PROCEDURES 49 3.5.1. COMPRESSIVE STRENGTH OF CONCRETE SPECIMENS 49 3.5.2. SPLITTING TENSILE STRENGTH OF CONCRETE SPECIMENS 50 3.5.3. MODULUS OF ELASTICITY 51 3.5.4. FREE SHRINKAGE TEST 52 3.5.5. RESTRAINED SHRINKAGE TEST 53 3.5.5.1. ENVIRONMENTAL CHAMBER 54
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CHAPTER IV – TEST RESULTS 56
4.1. INTRODUCTION 56 4.2. MECHANICAL PROPERTIES 56 4.2.1. COMPRESSIVE STRENGTH 56 4.2.2. SPLITTING TENSILE STRENGTH 58 4.2.3. ELASTIC MODULUS 59 4.2.4. FREE SHRINKAGE 60 4.3. LABORATORY TEST RESULTS 61 4.3.1. SHRINKAGE RINGS 61 4.4. FIELD TEST RESULTS 68 4.4.1. FIELD STRAINS 68 4.4.2. BRIDGE DECK CRACKING 76
CHAPTER V – FINITE ELEMENT MODELING 82
5.1. INTRODUCTION 82 5.1.1. MODEL ELEMENT TYPES 82 5.1.1.1. BEAM ELEMENT 83 5.1.1.2. SHELL ELEMENT 83 5.1.1.3. STEEL REINFORCEMENT 84 5.1.1.4. SHEAR STUDS 84 5.1.1.5. BOUNDARY CONDITIONS 84 5.1.1.6. CONSTRAINT AND RELEASE ELEMENTS 85 5.1.2. MATERIAL PROPERTIES 85 5.2. FINITE ELEMENT ANALYSIS RESULTS 87 5.2.1. BRIDGE DECK ANALYSIS 87 5.2.2. FINITE ELEMENT CONCLUSION 102
CHAPTER VI – SUMMARY AND CONCLUSIONS 103
6.1. SUMMARY AND CONCLUSIONS 103 6.2. FUTURE SCOPE OF WORK 104
REFERENCES 105
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LIST OF TABLES
Table 2.5-1. Phase I and II Mix Proportions (Whiting, et. al. 2000) 17 Table 2.5-2. Time to First Crack for FDD Mixes (Whiting, et. al. 2000) 20 Table 2.5-3. Time to First Crack for TDO Mixes (Whiting, et. al. 2000) 20 Table 2.5-4. Time to First Crack for Phase II Mixes (Whiting, et. al. 2000) 21 Table 2.5-5. North Eastern Ohio Field Survey Results (Delatte, et. al.) 23 Table 2.5-6. Absorptive Light Weight Aggregate Mix Proportions (Delatte, et. al.) 23 Table 2.5-7. Cracking Dates for Ring Specimens (Delatte, et. al.) 24 Table 2.5-8. Time to Cracking of Ring, RE and RBE Test Samples (Weiss, et. al.) 26 Table 3.2-1. NJ Turnpike Mix Design 37 Table 3.2-2. NJ Turnpike Admixture Content 37 Table 3.4-1. VWSG Orientations and Locations 44 Table 3.5-1. Summary of Laboratory Tests Performed 49 Table 4.2-1. Compressive Strength of Concrete Mix over Time 57 Table 4.2-2. Compressive Strength of Concrete Mix over Time 58 Table 4.2-3. Compressive Strength of Concrete Mix over Time 60 Table 4.2-4. Free Shrinkage of Concrete Mix over Time 61 Table 4.3-1. Crack Width Distribution Over All Samples 67 Table 4.4-1. Crack Map Details 77 Table 5.2-1. Strain Validation 88 Table 5.2-2. Concrete Properties for New Concrete Sections 89 Table 5.2-3. FE Model Strains Resulting from Case 1 Loading 91 Table 5.2-4. FE Model Strains Resulting from Case 2 Loading 93 Table 5.2-5. FE Model Strains Resulting from Case 3 Loading 95 Table 5.2-6. FE Model Strains from Case 4 Loading 96 Table 5.2-7. FE Model Strains from Case 4 Loading 97 Table 5.2-8. FE Model Strains from Case 6 Loading 99 Table 5.2-9. FE Model Strains from Case 7 Loading 100 Table 5.2-9. FE Model Strains from Case 7 Loading 101
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LIST OF FIGURES
Figure 2.4.1. AASHTO Ring Test Geometry 11 Figure 2.5.1. Shah, Ouyang, et. al. Element Mesh 14 Figure 2.5.2. Tensile Strength Prediction (A) and Experimental (B) (Shah, Ouyang, et. al.) 16 Figure 2.5.3. Shrinkage of HPC mixes with varying w/c and silica fume content (Whiting, et. al) 18 Figure 2.5.4. Shrinkage of HPC mixes with varying geometry and w/c (Whiting, et. al) 19 Figure 2.5.5. RE and RBE Test Method Setup (Weiss, et. al.) 25 Figure 2.5.6. Typical Cracking Observed on NJ Bridge Decks (Saadeghvaziri and Hadidi) 27 Figure 2.5.7. Bridge Survey Distribution throughout NJ (Saadeghvaziri and Hadidi) 27 Figure 2.5.8. Bridge Survey Form (Saadeghvaziri and Hadidi) 29 Figure 2.5.9. End Condition Effect on Deck Cracking (Saadeghvaziri and Hadidi) 30 Figure 3.1.1. Site Plan and Cross-Section 35 Figure 3.3.1. Measurement of Slump 39 Figure 3.3.2. Type – B Pressure Meter 40 Figure 3.3.3. Free Shrinkage and Cylinder Sample Molds 41 Figure 3.3.4. Restrained Shrinkage Ring Mold 41 Figure 3.3.5. Sample Covered with Wet Burlap 42 Figure 3.3.6. Sample Sealed in Polyethylene Sheet 42 Figure 3.4.1. Geokon, Inc. VWSG for Embedment 42 Figure 3.4.2. VWSG Installation 43 Figure 3.4.3. VWSG Locations 44 Figure 3.4.4. Portable CR1000 Data Logger 45 Figure 3.4.5. Kistler Single Axis Accelerometer 46 Figure 3.4.6. Laser Doppler Vibrometer (Polytec, Inc.) 47 Figure 3.4.6. Installed Strain Transducer 48 Figure 3.4.7. STS Transducer, Receiver Box and Collection Unit 48 Figure 3.5.1. Forney One Million Pound Machine 50 Figure 3.5.2. Tinius Olsen Compression Machine 50 Figure 3.5.3. Concrete Sample under Loading 50 Figure 3.5.4. Concrete Sample with Compressometer 51 Figure 3.5.5. Free Shrinkage Prisms 52 Figure 3.5.6. Length Comparator with Reference Bar (right) and Concrete Prism (left) 52 Figure 3.5.1. Environmental Chamber 54 Figure 4.2.1. Compressive Strength of Concrete Mix over Time 57 Figure 4.2.2. Splitting Tensile Strength of Concrete Mix over Time 58 Figure 4.2.3. Elastic Modulus of Concrete Mix over Time 59 Figure 4.2.4. Free Shrinkage of Concrete Mix over Time 60 Figure 4.3.1. Final Crack Mapping of Ring Specimen 1 (Side Profile) 62 Figure 4.3.2. Final Crack Mapping of Ring Specimen 1 (Top/Bottom Profile) 63 Figure 4.3.3. Final Crack Mapping of Ring Specimen 2 (Side Profile) 65 Figure 4.3.4. Final Crack Mapping of Ring Specimen 2 (Top/Bottom Profile) 66 Figure 4.4.1. Longitudinal Mid-span Strains at Top of Deck During Early Age of Concrete 68 Figure 4.4.2. Transverse Mid-span Strains at Top of Deck During Early Age of Concrete 69 Figure 4.4.3. Longitudinal Mid-span Strains at Bottom of Deck During Early Age of Concrete 69 Figure 4.4.4. Transverse Mid-span Strains at Bottom of Deck During Early Age of Concrete 70 Figure 4.4.5. Longitudinal Quarter-span Strains at Top of Deck During Early Age of Concrete 71 Figure 4.4.6. Transverse Quarter-span Strains at Top of Deck During Early Age of Concrete 71 Figure 4.4.7. Longitudinal Strains Along Top of Mid-span During Long-Term Period 72
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Figure 4.4.8. Transverse Strains Along Top of Mid-span During Long-Term Period 73 Figure 4.4.9. Longitudinal Strains Along Bottom of Mid-span During Long-Term Period 73 Figure 4.4.10. Transverse Strains Along Bottom of Mid-span During Long-Term Period 74 Figure 4.4.11. Transverse Strains Along Top of Quarter-span During Long-Term Period 75 Figure 4.4.12. Transverse Strains Along Top of Quarter-span During Long-Term Period 75 Figure 4.4.13. Crack Mapping of Bridge Deck with Sensor Locations 77 Figure 4.4.14. Crack Microscope Used in Deck Crack Mapping 78 Figure 4.5.15. VWSG Temperatures During Initial 36 Hour Period 80 Figure 5.1.1. Four Node Shell Element Detailed with Integration Points 83 Figure 5.1.2. Typical stress-strain curves of structural steel (Salmon and Johnson, 1997) 86 Figure 5.2.1. ABAQUS FE Model Layout 89 Figure 5.2.2. Loading Cases Considered in FE Analysis 90 Figure 5.2.3. FE Diagram Index 91 Figure 5.2.4. Deck Strains Due to Case 1 Live Load 92 Figure 5.2.5. Deck Strains Due to Case 2 Live Load 93 Figure 5.2.6. Comparison of Stress Distribution to Crack Orientation 94 Figure 5.2.7. Deck Strains Due to Case 3 Live Load 95 Figure 5.2.8. Deck Strains Due to Case 4 Live Load 96 Figure 5.2.9. Deck Strains Due to Case 5 Live Load 98 Figure 5.2.10. Deck Strains Due to Case 6 Live Load 99 Figure 5.2.11. Deck Strains Due to Case 7 Live Load 100 Figure 5.2.11. Deck Strains Due to Case 7 Live Load 101
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CHAPTER I
INTRODUCTION
1.1 PROBLEM STATEMENT
Possibly the greatest threat to the integrity of concrete is the unavoidable cracking that occurs. Excessive cracking results in weakening from freeze-thaw cycles, corrosion of reinforcing steel because of chloride infiltration and increased maintenance costs. The ability of water and chlorides to penetrate through cracks reduces the service life of the structure and can affect the ability to function as designed. Cracking can occur for several reasons including concrete shrinkage during curing, temperature changes, lack of satisfactory support, magnification of applied loads and restraint conditions.
Proposed in 1998 as a provisional standard the AASHTO Passive Ring Test simulates shrinkage cracking experienced by concrete under partially restrained conditions. The test consists of casting a concrete ring around an inner steel ring with foil strain gauges installed along the middle of the ring’s inner circumference. As shrinkage occurs compressive stresses form in the steel while tensile stresses generate in the concrete. If the strains produced in the concrete exceed that of the splitting tensile strain, cracking will occur. Due to the many variables responsible for deck cracking the tests design is for comparison of mixes and selection of a mix that will perform well
2 under restraint. As of 2006 AASHTO has been balloting for the acceptance under full standards.
Using mixes with low cracking potential is a great way to increase the service life of a bridge and reduce maintenance requirements. The ability to determine the performance of a mix prior to its use in the field would improve the integrity of partially restrained bridge decks. The goal of this thesis is to determine the validity of the laboratory ring tests method to simulate actual bridge deck restrained shrinkage.
1.2 RESEARCH OBJECTIVES AND SCOPE
The goal of this research is to test the ring tests prediction of restrained shrinkage of high performance concrete bridge decks. Collection of field samples took place during the deck pouring. All samples underwent a fourteen-day wet-cure. Determination of basic properties was possible through testing of the samples. Properties necessary for analysis include compressive and splitting tensile strength, modulus of elasticity, and free and restrained shrinkage.
1.3 THESIS ORGANIZATION
This thesis is structured into five chapters as follows:
Chapter I covers the introduction, research objectives and scope as well as the organization of the thesis.
Chapter II covers a general background as well as a literature review of the various types of shrinkage, the factors that affect the shrinkage of a concrete structure, the
3 performance of HPC as it pertains to free shrinkage and restrained shrinkage as well as similar work performed by others.
Chapter III covers the experimental setup including all mechanical and field testing, field implementation and the material properties of the concrete mix analyzed.
Chapter IV covers the finite element modeling employed in this study.
Chapter V covers all test results.
Chapter VI covers the summary and conclusions of the thesis as well as goals for future research.
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CHAPTER II
LITERATURE REVIEW
2.1 INTRODUCTION
Concrete will always experience some change in volume. If located in a region with no restraint, this change will not result in any cracking. The concrete mass will merely change in volume. Foundations, subgrades, reinforcement and connecting members all act as partial restraint in field conditions. To examine the effects of volume change it is important to understand the reasons for this change to occur.
Because most concrete elements are significantly longer in one dimension than the other two, volume change is described linearly when referring to concrete. Typically, volume reduction occurs because of changes in moisture and temperature. This change is termed shrinkage. The four types of shrinkage of concern to engineers are plastic, thermal, autogenous and drying shrinkage.
2.2 TYPES OF SHRINKAGE
Shrinkage begins immediately after the pouring if fresh concrete. Volume change can continue for years after curing. Cement properties, type and gradation of aggregates, rate of drying and other reasons affect the type and rate of shrinkage. The following sections detail the types of shrinkage.
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2.2.1 Plastic Shrinkage
Plastic shrinkage of concrete occurs at the surface of fresh concrete. Shortly after pouring concrete, excess water makes way to the surface. Evaporation of surface water faster than the rate of bleeding results in plastic shrinkage, and therefore a reduction in volume of the fresh concrete. HPC displays a low rate of bleeding in comparison to normal concrete. This low rate of bleeding requires caution to avoid plastic shrinkage during pouring.
Protection from evaporation removes plastic shrinkage as a concern. Fogging, windbreaks, shading, plastic sheet covers, or wet burlap help in protecting against plastic shrinkage.
2.2.2 Thermal Shrinkage
Thermal shrinkage is a result of the generation of heat of hydration. Heat is a by- product of the chemical process by which the cement paste hydrates. This can cause an expansion of the concrete during initial curing. As curing continues and this temperature begins to drop the concrete experiences a decrease in volume and undergoes thermal contraction. This can result in cracking if the concrete cannot dissipate heat effectively.
In slender structures such as bridge decks, heat dissipation occurs rather rapidly.
Therefore any thermal shrinkage is negligible.
2.2.3 Autogenous Shrinkage
Autogenous shrinkage is a change in volume because of cement hydration.
Curing requires water for cement hydration to occur. Without an external water supply
6 the cement will use pore water found through the concretes voids. Continued consumption of pore water results in self-desiccation.
Concrete with a low water to cement ratio, such as HPC, is susceptible to autogenous shrinkage. Curing with an external water supply for a minimum of seven days will eliminate autogenous shrinkage. Specialized shrinkage reducing admixtures are an acceptable method as well.
2.2.4 Drying Shrinkage
Drying shrinkage is the result of volumetric changes in hardened concrete because of evaporation. This occurs gradually and can continue for years after the pouring.
Should there be no restraining conditions present the mass will shrink without any increase in stresses or cracking. Most field conditions, however, include some form of partial restraint (shear connectors, foundations, etc.) which result in shrinkage cracking.
Larger masses exhibit lower rates of drying shrinkage because of their lower surface area to volume ratios. Bridge decks on the other hand, whose dimensions yield high surface areas, are more susceptible to volume change.
The most controllable factor in minimizing drying shrinkage is the water to cement ratio of a concrete mix. Using the smallest possible water content will result in the lowest shrinkage. Similarly, aggregate properties play a role in concrete shrinkage.
Harder aggregates such as quartz, granite, feldspar, limestone and dolomite resist the effects of drying shrinkage because of their high resistance to compression (ACI
Committee 224). Various forms of wet curing will hold off drying shrinkage until curing has ended.
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2.3 SHRINKAGE FACTORS
The most important controllable variable in shrinkage is the water content. The water per cubic yard of concrete will contribute more to shrinkage than any other aspect of a mix. Higher the water content yields more evaporation during drying. This means greater overall levels of shrinkage. Keeping the water content as low as possible reduces shrinkage experienced by concrete. The water required in a mix is affected by several factors including, but not limited to, aggregate volume, aggregate size and gradation, cement content and fineness, chemical admixtures, and curing history.
One way to regulate the water needed is to use more aggregate. Aggregates typically display higher resistance to compression than the surrounding cement paste.
Added coarse aggregate resists the shrinking paste better and decreases the overall shrinkage experienced. The more aggregate used (within reasonable limits) the higher the concretes resistance to shrinkage. Similarly, the size and gradation of aggregate affects how much shrinkage will result. Using the largest practical size reduces the total surface area of aggregate that requires coating with cement. This results in lower water and cement demands in the mix effectively reducing shrinkage. Aggregate gradation is equally important in reducing shrinkage. A well graded concrete will have smaller and finer aggregate filling in voids leaving less room for cement. This overall decrease in void space translates to a lower cement and water content requirement of the mix.
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2.4 RESTRAINED SHRINKAGE RING TEST
There are several methods available for testing the restrained shrinkage performance of concrete mixes. These methods include the flat panel test, linear restrained test, and restrained shrinkage ring test. The most popular is the ring test as it is simplistic and economical.
2.4.1 Ring Test Background
The restrained shrinkage ring test consists of casting a concrete ring around a steel ring of smaller diameter. As shrinkage occurs stresses form throughout the system.
Within the steel these stresses are compressive and four foil strain gauges (FSG), installed along the inner circumference of the ring, measure the resulting strain. The concrete experiences tensile stresses that counterbalance the compressive stresses in the steel. Should these tensile stresses exceed the allowable stresses within the concrete cracking will occur. Installing VWSG along the top of the rings allows for quantification of concrete stresses even if cracking does not occur.
The first testing using restrained rings were performed by R.W. Carlson and T.J.
Reading (1988) in the study of shrinkage cracking in concrete building walls. At the time there were no standard methods for testing restrained shrinkage available. In the test concrete was cast around polished steel rings and dried 25, 50 and 75% relative humidity to discover the effects on restrained shrinkage. The rings were smaller than modern
3 methods allowing the aggregate possible for testing to be no more than /8” (9 mm) in size. The steel rings used were coated with paraffin wax to allow slippage of concrete on the ring if cracking occurred. Two sides of the ring were sealed to allow drying only
9 from the outer surface. The investigation found the concrete exposed to lower humidity, and therefore more rigorous drying, withstood higher stresses before cracking. Cracking also occurred more quickly in these samples. The study found that aggregate type plays a large role in crack resistance and the ring test provided useful information and warranted further study as a possible standard test method.
Grzybowski and Shah (1990) modified the previous test in their analysis of shrinkage cracking of fiber reinforced concrete. Incorporating two different types of fibers, steel and polypropylene, the study focused on the resistance to shrinkage cracking as a result of composite reinforcement. The findings stated introducing a small amount of steel fibers (0.25% by volume) reduced average crack widths by as much as 20% and maximum crack widths by 50%. Polypropylene fibers, on the other hand, were determined to be far less effective at reducing crack widths.
In 2002, Weiss and Shah studied the role of shrinkage reducing admixtures as well as specimen geometry and their relationship to shrinkage cracking. The study used two different sizes of restraining rings. The tall group consisted of 9 rings with a height of 150 mm while the short group incorporated 9 rings with a height of 30 mm. Each group was further classified according to wall thicknesses of 25, 75, and 150 mm (the study consisted of three samples of each per group). Analysis of the samples showed cracking occurring in the 25 mm short rings at an average of 8 days as opposed to 11 days in the 50 mm ring. No cracking occurred on the 150 mm thick ring. Within the tall group only two of the three 25 mm thick rings showed signs of cracking while all other specimens remained uncracked. These results led the researchers to conclude the early age cracking behavior of concrete is geometrically dependent.
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The work of Grzybowski and Shah was expanded on by Ahmed and Mihasi
(2009) by applying the study of restrained shrinkage to High Performance Fiber
Reinforced Cementitious Composites (HPFRCC). The test samples were prepared by casting a 40 mm thick concrete ring around a 150 mm steel ring. The drying environment consisted of a relative humidity of 60% and a temperature of 68 ⁰ F. A premix mortar specimen was cast for comparison to the fiber reinforced specimen. The study found that while the reinforced sample yielded many cracks (the premix mortar showed only 6 cracks), the crack widths were significantly smaller in the reinforced mortar. The higher number of cracks with smaller crack widths was credited to the strain hardening and toughness properties of the HPFRCC.
2.4.2 Ring Test Setup
The AASHTO Ring Test was developed for testing the effects of concrete mix parameters on shrinkage under restrained conditions. Altering mix ingredients with the ring test provides researchers with the opportunity to understand the effects each component has on the restrained shrinkage of a particular concrete or cementitious composite. The advantage of this test lies in its simplicity of construction and versatility.
The test allows customization to study nearly any variable of a concrete mix design.
The ring setup consists of a concrete ring cast around an inner steel ring. The
1 apparatus requires a steel ring with a wall thickness of 12.7 mm ± 0.4 mm (0.5 in ± /64 in), an outer diameter of 305 mm (12 in) and a height of 152 mm (6 in). The concrete mold should allow for the hardened concrete ring to have an outer diameter of 457 mm
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(18 in). These dimensions will result in the desired 3” concrete wall thickness. The
AASHTO geometric requirements can be seen in Figure 2.4.1.
Figure 2.4.1. AASHTO Ring Test Geometry
The AASHTO ring test uses a smaller steel ring than similar test methods (ASTM
Ring Test). This allows for the use of larger aggregate sizes in testing. The increased concrete wall thickness causes cracking to occur at later ages than in similar test methods.
Occasionally no cracking occurs at all. Installing FSGs along the inner circumference of the steel ring aids in determining the cracking age of a mix. These gauges measure the compressive strain in the steel throughout the test. Abrupt changes in strain are credited to cracking in the concrete. This allows for precise determination of cracking age.
2.5 Previous Work
In an attempt to develop a method to predict the shrinkage cracking of concrete
Shah, Ouyang, et. al (1998), used standard ring tests to assess shrinkage ring cracking.
Up to that time ring testing had mainly been used for evaluation of concrete cracking as
12 opposed to crack prediction. The goal of the study was to produce a model that could be provide a guide for the design of concrete pavements and slabs.
The model itself was founded on the principles of fracture mechanics. The assumption that cracking in concrete is related more to fracture energy than tensile strength was backed by fiber reinforcement. The research team sited introducing fiber reinforcement delays cracking without significantly affecting the allowable tensile stress of the concrete. Fiber reinforcement does however alter the fracture energy of a concrete specimen. This reasoning led the team to focus on fracture mechanics in their study as opposed to the more traditional allowable tensile stress method.
The fracture resistance curve approach (R-curve) was used in the generation of the model. Fracture resistance theory is described as follows: By applying a load to a structure with an initial crack length of a 0, strain energy, U, is created. The rate of strain energy release with any arbitrary crack length, a, is known as the strain energy release rate, G. As load is applied some of the energy is also consumed as the crack tip propagates. This propagation energy is defined as W. The rate of change of W with respect to crack length, a, is known as the fracture resistance, R. A crack propagates unsteadily when the following two conditions are true: