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In May 1993, the Federal Highway Administration (FHWA) began a 5-year research project, Corrosion Resistant Reinforcing for Concrete Components. The objective of the study was to develop cost-effective new breeds of organic, inorganic, ceramic, and metallic coatings, as well as metallic alloys that could be utilized on or as reinforcement for embedment in portland cement concrete and ensure a corrosion-free design life of 75 to 100 years when exposed to adverse environments. The 1993–1998 research program involved testing more than 52 different organic, inorganic, ceramic, and metallic coatings on steel bars, as well as solid metallic bars. Specifically, these included epoxy-coated, other polymer-coated, ceramic-coated, galvanized-clad, epoxy-coated galvanized-clad, stainless steel-clad, nickel-clad, copper-clad, corrosion resistance alloy-clad, inorganic silicate-clad, solid corrosion-resistance alloy steel, solid aluminum-bronze, solid stainless steels, and solid titanium reinforcing bars. Consequently, 12 different bar types were selected for long-term durability tests in concrete exposed to the very aggressive Southern Exposure (SE) testing, which involved alternating wetting with 15 weight percent NaCl solution and drying cycles for 96 weeks. About 150 test slabs were fabricated for the selected 12 different bar types.
After the conclusion of the 96-week SE testing in 1998, 31 post-SE test slabs that were not autopsied were then exposed to a long-term natural weathering at an outdoor test yard in Northbrook, IL, from September 1998 to December 2002. Periodic macrocell corrosion current between two mats and short-circuit potential of top mat bars (while they were connected to the bottom mat bars) data were collected during the exposure test program. When the test program ended after about 7 years, autopsy and subsequent laboratory analysis was performed with the test slabs, and the results are reported here. The tests include chlorides in the concrete, condition evaluation at bar/concrete interface, and visual examination of extracted bars.
Macrocell current density was a good indicator of corrosion performance of the various reinforcements. The black bars produced the highest mean macrocell current density (least corrosion resistant) among various combinations of test variables regardless of slab configuration. The stainless steel bars exhibited negligible mean macrocell current density. Whenever an epoxy-coated reinforcing bar (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, and the coating looked new with a glossy texture. However, when ECRs slabs with a high macrocell current density were autopsied, they revealed coating deterioration due to corrosion and exhibited numerous hairline cracks and/or blisters in conjunction with reduction in adhesion, coating disbondment (permanent loss of adhesion), and underlying steel corrosion. Generally, the number of final coating defects on the autopsied ECRs increased from their initial values determined before embedment in concrete. There was no consistent trend found between the level of macrocell current density and the extent of coating adhesion loss. The earlier FHWA studies investigated the adhesion of the coatings using accelerated solution immersion tests and cathodic disbonding tests. Based on the review of the test results, the adhesion, as tested by solution immersion and cathodic disbonding tests, appeared to be a poor indicator of long-term performance of the coated bars in chloride contaminated concrete after 96‑week SE. These findings led researchers to conclude that there is no direct relationship between loss of adhesion and the effectiveness of ECR to mitigate corrosion.
In general, bent ECRs in the top mat coupled with black bars in the bottom mat performed the worst among all ECR cases. For straight top-mat ECRs, the mean macrocell current density was influenced by the size of initial coating damage and type of bar in the bottom mat. Their performance varied from 7 to 40 percent of the highest black bar case as measured by macrocell current density. However, 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, regardless of the initial coating defect size. Both mat ECR systems behaved comparable to stainless steel bars. According to impedance modulus, alternating current (AC) resistance, macrocell current density data, and autopsy results, the excellent performance of test slabs containing ECRs in both mats should be attributed to the facts that electrical resistance was very high between the two mats, and the ECRs in the bottom mat suppressed the corrosion activity by minimizing the area for the cathodic reaction (oxygen reduction).
The 2-year saltwater ponding with alternate wetting, heating, and drying, followed by 5-year outdoor weathering, confirmed that use of ECRs in the top mat only (uncoated bottom mat) reduced the corrosion susceptibility to at least 50 percent of the black bar case, even when the coating has damage. Hence, ECR used in the top mat alone would not provide optimum corrosion protection. If ECRs are used in both mats in uncracked concrete, corrosion resistance increases dramatically, even when the rebar coatings have defects. Such improved corrosion resistance can be attributed to a (1) reduction in cathodic area; (2) higher electrical resistance; and (3) reduced cathodic reaction.
Table of Contents | Chapter 1: Introduction and Project History
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