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Carbonation in Concrete and Effect on Steel Corrosion

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Carbonation in Concrete and Effect on Steel Corrosion Report Prepared by: Alberto A. Sagues Eric I. Moreno' Walter Morris Carmen Andradet (Section 2) 'Permanent Affiliation: CINVESTAV-Merida, Mexico +lnstituto Eduardo Torroja, Madrid, Spain CARBONATION IN CONCRETE AND EFFECT ON STEEL CORROSION Final Report, State Job No. 99700-3530-119 WPI 0510685 Prof. Alberto A. Sagiies. Ph.D., P.E. Principal Investigator June, 1997 Department of Civil and Environmental Engineering College of Engineering University of South Florida Tampa Florida 33620 Technical Report Documentat~onPage I 1 Re~ortNo 1 2 Government Accesrlan No 1 3 Reclo8enis Catalos No 1 I I 4 Title and Subtitle 1 5 Reoort Date June, 1997 CARBONATION IN CONCRETE AND EFFECT ON STEEL CORROSION Alberto A. Sagues, Eric. I. Moreno, Walter Morris and Carmen Andrade 9 Pefiormlng Oiganizatlon Name and Address 10 Work Unlt Na VRAIS) Department of Civil and Environmental Engineering University of South Florida Tampa, FL 33620 8-8383 Final Report 12 s~~~~~~~~Agency ~~~~~~d hddress August 1993 - February 1997 Florida Department of Transportation 605 Suwannee Street 14 Spanrar#ngAgency Code Tallahassee, FL 32399-0450 15 Supplementary Notes l-----l Prepared in cooperation with the U.S. Department of Transportation and the Federal Highway Administration 16 Abstract An investigation was conducted to establish a prognosis for carbonation-induced corrosion of steel in concrete in present and future Florida highway structures. A survey of 18 existing bridges built between 1939 and 1981 (ages 14 years < t < 56 years) revealed carbonation depthsx, :50 mm with a median of. 10 mm. Carbonation coefficients Kc=x, t-'I2ranged from 0 to 14 mm/y"2, with a median value of 1.4 mm/y"2. The highest values of Kc were observed on the decks of inland bridges. The projected time to corrosion initiation for the combination worst 10-percentile K, values and lowest 10-percentile reinforcement cover was 266 years. Only a very small fraction of the present inventory of Florida D.O.T. bridges is expected to exhibit carbonation-induced corrosion over a 75-year service life. Laboratory tests were conducted to determine the influence of mix design on the carbonation resistance of concretes to be used in new Florida D.O.T. construction. The results indicate that the time for initiation of carbonation-induced corrosion may be shortened by -360% when the fly ash cement replacement is increased from 20% to 50%. The initiation time may be shortened by ;:35 when cement replacement is increased from 20% fly ash to 20% fly ash plus 8% silica fume. 17 KeyWardr 78 D8sfrib~tanStatement Reinforcing Steel, Corrosion. Carbonation, Concrete, No restrictions. This document is available to the public Fly Ash, Silica Fume, Bridges through the National Technical information Service. Springfield. VA 22161 19. Secuilty Clarr$f(offh\r repod) 20 Securty Carsif (of fhs page) 21 NO of pages 22 pr,ce Unclassified Unclassified 251 =arm DOT F 1700.7 (8-72) Reproduction of completed page authorized Technical Report Documentation Page CONVERSION FACTORS, US CUSTOMARY TO METRIC UNITS Multiply to obtain inch mm foot meter square inches square mm cubic yard cubic meter poundlcubic yard kglcubic meter gallonlcubic yard literlcubic meter standard cubic feeffhour mllminute ounces gram pound kilogram pound (Ib) newtons kip (1000 lb) kilo newton (kN) poundlin2 MPa kiplin2 MPa ft-kip kN-m in-kip kN-m ACKNOWLEDGMENT This investigation was supported by the State of Florida Department of Transportation, and this report is prepared in cooperation with the State of Florida Department of Transportation and the U.S. Department of Transportation. The opinions, findings, and conclusions expressed here are those of the authors and not necessarily those of the Florida Department of Transportation or the U.S. Department of Transportation. The authors are indebted to Dr. S. Kranc for valuable discussions. The extensive technical support and many helpful discussions provided by Rodney G. Powers, and the assistance of the Corrosion Section of the FDOT Materials Office are gratefully acknowledged. One of the authors (E.Moreno) acknowledges the scholarship provided by the National Council for Science and Technology (CONACYT-Mexico). TABLE OF CONTENTS Cover Page Conversion Factor Acknowledgment Executive Summary Section 1. INTRODUCTION Section 2. STATE OF THE ART KNOWLEDGE Section 3. FIELD INVESTIGATION 3.1 Procedure 3.2 Results 3.3 Discussion 3.4 Conclusions References Section 4. LABORATORY INVESTIGATION 4.1 Procedure 4.2 Results 4.3 Discussion 4.4 Conclusions References Statement of Benefits APPENDICES EXECUTIVE SUMMARY Corrosion of reinforcing steel in concrete often occurs in the substructure of Florida marine bridges as a result of chloride ions from seawater. The chloride ions penetrate through the concrete cover and cause breakdown of the otherwise protective passive layer on the steel surface. A slower process that also causes steel passivity breakdown is carbonation of the concrete due to chemical reaction with atmospheric carbon dioxide. A low pH, reacted layer of concrete forms at the surface and penetrates inward to a depth x proportional to the square root of the exposure time t (x = Kc t'" , where Kc is called the carbonation coefficient). When xis equal to the rebar cover depth, steel depassivation takes place and corrosion begins. This process does not require chloride ions and may eventually affect the entire FDOT bridge inventory. Progress in preventing chloride-induced corrosion has now significantly extended the design service life of new structures, and slower forms of deterioration such as carbonation-induced corrosion merit serious consideration when attempting to achieve a 75-year service life goal. Moreover, thousands of existing FDOT bridges both over seawater and inland are reaching service lives at which carbonation-induced corrosion can be of concern. The present investigation was conducted to establish a prognosis for carbonation- induced corrosion in present and future FDOT structures. Specific objectives included (a) determining the state of the art knowledge of the carbonation-corrosion process; (b) conduct a survey of bridges in the FDOT inventory to determine the extent of concrete carbonation, establishing quantitative service life predictions and compare with the effects of chloride-induced corrosion; (c) conduct a laboratory investigation to determine the influence of mix design parameters on the carbonation resistance of concretes to be used in new construction. The results from the knowledge base developed under objective (a) were used to select structures in the State with the most propitious conditions for carbonation. To address objective (b) a total of 18 structures was identified for examination, incorporating bridges built between 1939 and 1981 in Dade, Duval, Hillsborough and Monroe Counties. Sixteen of the bridges showed evidence of significant concrete carbonation, but only one (built in 1962) showed corrosion damage attributable to carbonation. Concrete carbonation depths observed in the bridges were as high as 50 mm, with an average value of =I0mm. The carbonation coefficients measured from bridge- extracted cores ranged from 0 to 14 mmly'", with a median value of 1.4 mmly'". Cores extracted from bridge decks presented higher values of x, and Kc compared with the substructure. This difference was most pronounced on bridges built over water, where the average values of x, were 12.3 and 2.1 mm for the deck and the substructure respectively. The most severe conditions for carbonation-induced corrosion were obtained on the decks of bridges built overland (highest 10 percentile Kc > 4.35 mmly1'2). Measurements of concrete covers in the bridges examined showed that in 10% of the cases the cover was < 3.8 cm. The estimated time for corrosion initiation assuming the combination of worst 10-percentiles for carbonation coefficient and cover indicated above (4.35 mmlyl'z and 3.8 cm) was = 70 years. Time to carbonation-induced corrosion projections for the median conditions were much in excess of 100 years. As expected, evaluation of the propensity for chloride- induced corrosion in substructure of the marine bridges included in the survey resulted in much lower projections of time to corrosion. The results of the field survey suggest that only a small fraction (on the order of 1%) of the present FDOT bridge inventory is expected to show significant carbonation-induced corrosion during a 75-year service life. Objective (c) stemmed from FDOT concrete formulations for new construction that are relying increasingly on pozzolanic additions (primarily class F fly ash) for increased performance. Because the pozzolanic reaction consumes part of the calcium hydroxide reserve of concrete, its resistance to carbonation may be compromised. Laboratory experiments were designed to determine the carbonation resistance of concretes with 20% fly ash cement replacement (typical of high performance Section 346 concretes) and an extreme case of 50% replacement. Variations with additional pozzolan (8% silica fume), and chloride contamination were also examined. Test specimens were exposed to accelerated carbonation test atmospheres containing 0.5% CO, (16 times more than normal atmospheric content) and the carbonation coefficients were determined after 100 days of exposure. Comparison between carbonation coefficients obtained in the the accelerated test exposure and initial atmospheric conditions confirmed the theoretical expectations that Kc - %COP. The results from the accelerated tests could then be
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