PERFORMANCE EVALUATION OF CORROSION PROTECTION SYSTEMS FOR REINFORCED CONCRETE

Abstract

In this study the performance of corrosion protection systems for reinforced concrete is evaluated. Conventional bare and epoxy-coated reinforcement are compared with alternative forms of reinforcement–galvanized steel, MMFX steel containing 9% and 4% chromium (ASTM A1035 Type CS and CM steel), and epoxy-coated MMFX steel containing 4% and 2% chromium (epoxy-coated ASTM A1035 Type CM and CL steel). Furthermore, corrosion performance of reinforced concrete with partial replacement of cement by 20% fly ash, 40% fly ash, 5% silica fume, 10% silica fume, 20% slag cement, and 40% slag cement in bridge decks containing uncoated conventional steel as well as 40% fly ash, 10% silica fume, and 40% slag cement in bridge decks containing conventional epoxy-coated reinforcement are compared with the concrete bridge decks containing only portland cement along with epoxy-coated and uncoated reinforcement. The corrosion performance of systems are evaluated using bench-scale specimens (Southern Exposure, cracked beam, and beam specimens) and rapid macrocell tests. Macrocell corrosion rates, corrosion potential, and total corrosion rates, which are measured by Linear Polarization Resistance test, are used to monitor the corrosion performance of specimens. Critical corrosion loss required to crack concrete cover in specimens containing galvanized bars and conventional steel are investigated and compared with the results of predictive equations introduced in the literature. The critical chloride threshold of conventional reinforcement in concrete containing different supplementary cementitious materials (fly ash, silica fume, and slag cement) are compared. The chloride contents are measured based on the free chloride content (water soluble chloride) of concrete samples at the level of bar. The life-expectancy and cost effectiveness of a bridge deck constructed with each system are estimated for a 75-year design period based on the obtained results. Results show that galvanized steel exhibits better performance than conventional bars against corrosion; galvanized steel requires over twice the corrosion loss and has an expected-life about three times as long as conventional steel. The average critical corrosion loss to crack concrete with 1-in. cover is found to be approximately 25 µm, very close to the value obtained by O’Reilly’s (2011) predictive equation. While MMFX bare bars show higher corrosion resistance than conventional bars, those with 9% chromium exhibit better corrosion performance than MMFX bars containing 4% chromium; however, critical chloride threshold of both MMFX bars are about three times of that for conventional steel. Although use of galvanized steel and uncoated MMFX bars are more cost effective than conventional steel, they are not as cost effective as epoxy-coated bars. Epoxy-coated MMFX bars containing 2% chromium do not show significant better performance against corrosion compared to conventional epoxy-coated bars; however, those with 4% chromium have an appreciably higher corrosion resistance and life-expectancy than conventional ECR. Using supplementary cementitious materials in concrete enhances the corrosion resistance of the systems; with increasing the amount of SCM, the time to initiation increased and the corrosion rates decreased. Chloride ingress rate is significantly lower in concrete containing SCM compared to those without it, with the lowest rate in concrete with silica fume. Most specimens containing 40% fly ash, 20% slag, 40% slag, and 10% silica fume repassivate after initiation, with corrosion re-initiating at a higher chloride threshold. The initial critical chloride thresholds for slag cement and 40% fly ash specimens are similar to that for 100% ordinary portland cement, but the secondary CCCT values are significantly higher. For 10% silica fume specimens, the initial CCCT value is lower, but the secondary CCCT value is similar to the critical chloride threshold of conventional steel in specimens with 100% portland cement. While using epoxy-coated reinforcement and supplementary cementitious materials separately, increases the life-expectancy and cost effectiveness of a corrosion protection system, using them together exponentially increases the effects.