Cover-zone protective qualities under corrosive environments

Abstract

The protective qualities of the concrete cover-zone and the condition of the embedded steel (active/passive) surface were studied using reinforced concrete slabs subjected to intermittent wetting/drying. The cementitious binders used within the experimental programme comprised plain Portland cement (CEM I) and Portland cement partially replaced with fly-ash (FA) and ground granulated blast-furnace slag (GGBS) with water/binder (w/b) ratios of 0.4 and 0.6. The concrete surface was exposed to a 0.55 Molar (3.2%) chloride solution with the intermittent regime extending over a period of ~400 days. Pairs of stainless steel electrodes were positioned within the cover-zone to monitor changes in electrical resistance/resistivity as chlorides migrated through the concrete, whereas macro-cell current and half-cell potential measurements were used to study corrosion initiation on the steel surface. In addition, at the end of the experimental programme, chloride profiling and gravimetric mass-loss measurements were obtained. For the analysis of electrical measurements, a normalisation method was employed to study chloride transport within the cover-zone. It was found that ongoing hydration (and resulting pore structure refinement) was the main factor for the increase in the bulk electrical resistance/resistivity of concrete and that chloride ingress was the main factor for the reduction in resistance/resistivity. The resistance/resistivity varied depending on the depth of the chloride penetration. It was shown that both binder type and w/b had a significant influence on the performance of the cover-zone and that the electrical properties of concrete could be developed as a candidate indicator of concrete performance.