Chloride Corrosion Threshold in Concrete
Chloride ions can destroy the passivity of reinforcing steel in concrete and corrosion is likely to procede if moisture and oxygen availability are such as to support corrosion. Chloride may be introduced from different sources such as admixtures containing chloride, chloride bearing aggregates, or deicing salt solution permeating through hardened concrete. In this study, concrete slabs containing either single steel rods or electrically connected steel were subjected to various outdoor and indoor exposure conditions. It was found that corrosion rates of the single steel increased with increasing chloride content. The chloride corrosion threshold limit was between about 0.2% and 0.4% calcium chloride dihydrate by mass of cement (0.014% and 0.022% soluble chloride ion by mass of concrete) depending on the test method. Macrocell corrosion rate with electrically coupled steel was influenced greatly by the differential concentration of chloride ions in anode and cathode regions; large differences in chloride concentration resulted in a high corrosion rate. (Author/TRRL)