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Corrosion of epoxy-coated rebar in marine bridges - Part 2: Corrosion in cracked concrete

Lau, K. / Sagues, A.A. / Powers, R.G.

The extent of corrosion of epoxy-coated rebar (ECR) in marine bridges was found to be generally correlated with concrete chloride diffusivity, D_App, with significant corrosion observed for bridges with D_App values reaching up to ~10^-7 cm²/s but not for sound concrete locations in bridges with D_App approaching the order of 10^-9 cm²/s. However, significant ECR corrosion was observed in several preexisting cracked concrete locations of a low D_App bridge after only ~15 years of service. Well manifested, enhanced chloride and moisture penetration took place through the preexisting cracks of this bridge. The corrosion products in that case were solid and grew underneath the epoxy coating and had a composition consistent with that of chloride-substituted Akaganeite (beta-FeO[OH]). Laboratory experiments showed that oxygen presence was not necessary for the formation of similar corrosion products, suggesting that corrosion damage could develop even with thin, moisture-filled cracks that would restrict oxygen flow to the corroding region. Other surveyed low D_App bridges built with ECR showed also preferential chloride penetration at cracks, but no corrosion. An initial model formulation is introduced that may serve as a starting point for quantitative corrosion forecasting of possible damage in those other bridges. Exploratory projections indicate that, as expected, relatively isolated preexisting cracking should only create topical concrete damage with limited maintenance requirements. However, if the preexisting crack orientation with respect to the rebar were adverse and chloride transport were greatly enhanced (as it could be expected in relatively wide cracks), projected corrosion damage was more substantial.