A study of initiation and active reinforcement corrosion in conventional reinforced concrete

Reinforced concrete structures in marine environments usually are considered problematic as reinforcement corrosion may occur relatively quickly, usually attributed to the ingress of aggressive chlorides. However, practical cases exist where reinforced concrete structure have been exposed for many years in seawater conditions but with little or no evidence of reinforcement corrosion. This is despite extremely high chloride contents in the concrete. Herein results are reported of a long-term (10+ year) study in which small samples of specially made reinforced concretes, of various water-cement and aggregate-cement ratios and of various densities with identical reinforcement bars and with identical compaction were exposed to a high humidity temperature controlled environment. All specimens were made with seawater. Removal of the reinforcing bars revealed a strong diffrerence in concrete colour between the outer layers and the core of the specimens. XRD and EDS analyses and pH measurements show that the outer layers are depleted of Ca(OH)2, the critical component in maintaining sufficient alkalinity for prevention of active corrosion. They did not have an elevated CaCO2 content as would be the case if carbonation was involved. The samples were wet throughout the total period of exposure. It is well-known that water-based leaching of Ca(OH)2 is very slow but it is also known to increased in the presence of chlorides. The results are interpreted as showing that the role of chlorides is not that of directly attacking the steel reinforcement but rather one of increasing the rate of leaching of alkalis from the concrete that then eventually allows reinforcement corrosion to become active. This cast a new light on the role of chlorides in corrosion of reinforcement.