Long-Term Performance of Epoxy-Coated Reinforcing Steel in Heavy Salt- Contaminated Concrete [MF]

This report describes long-term natural weathering exposure testing of the remaining 31 post-Southern Exposure (SE) test slabs that were not autopsied during the 1993–1998 Federal Highway Administration (FHWA) research project. The samples were exposed from September 1998 to December 2002 at an outdoor test yard in Northbrook, IL. The 1993– 1998 research program involved testing more than 52 different bar materials and, consequently, 12 different bar types were selected for long-term durability tests in concrete exposed to the very aggressive SE test, which involved alternating wetting with 15 weight percent NaCl solution and drying cycles for 96 weeks. Periodic macrocell corrosion current between top and bottom mats and short-circuit potential data were collected during the exposure test rogram. p Upon termination of the test program, autopsy and subsequent laboratory analysis was performed on the test slabs. The test results confirmed that the black bars produced the highest mean macrocell current density (least corrosion resistant) among various combinations of test variables regardless of slab configuration, and that the stainless steel bars exhibited negligible mean macrocell current density. In general, bent epoxy-coated reinforcing bar (ECR) in the top mat, coupled with black bars in the bottom mat, performed the worst among all ECR cases. The straight top-mat ECRs’ macrocell current density varied from 7 to 40 percent of the highest black bar case, depending on the size of initial coating damage and type of bar in the bottom mat. ECR used in the top mat alone reduced the corrosion susceptibility to at least 50 percent of the black bar case, even when it contained coating damage and was connected to the black bar bottom mat. In contrast, if straight ECRs in the top mat were connected to ECRs in the bottom mat, the mean macrocell current density was no greater than 2 percent of the highest black bar case even when rebar coatings had defects, and approach the corrosion resistant level of stainless steel reinforcement. Such improved corrosion resistance can be attributed to (1) reduction in cathodic area; (2) higher electrical resistance; and (3) reduced cathodic reaction. Whenever an ECR slab with negligible macrocell current density was autopsied, the appearance of the extracted ECR and concrete/bar interface was excellent with no sign of corrosion. However, when ECRs specimens with high macrocell current densities were autopsied, they revealed coating deterioration due to corrosion and exhibited numerous hairline cracks and/or blisters in conjunction with reduced adhesion, coating disbondment (permanent adhesion loss), and underlying steel corrosion. No consistent trend was found between the level of macrocell current density and the extent of coating adhesion loss. The present test results and the earlier FHWA studies indicate that adhesion appeared to be a poor indicator of long-term performance of the coated bars in chloride contaminated concrete; it is concluded that there is no direct relationship between loss of adhesion and the effectiveness of ECR to mitigate corrosion

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