The influence of binder type, cracking and cover on corrosion rates of steel in chloride-contaminated concrete

Cracking of reinforced concrete can result in substantial reduction in service life owing to rapid initiation of steel corrosion. Supplementary cementitious materials (SCMs) can alter not only the pore solution chemistry but also the environment surrounding the steel and lead to significant reductions in corrosion rate. The impact of binder type on corrosion rate was assessed using seven concrete mixtures comprising ordinary Portland cement (PC) and blends of PC with ground granulated blast-furnace slag, fly ash, condensed silica fume and a ternary blend. Corrosion rates were measured in prismatic specimens with crack widths of 0.2 mm or 0.7 mm. For 20 mm cover, all the SCMs resulted in at least a 50% reduction in corrosion rate compared with the PC control. Increase in crack width from 0.2 mm to 0.7 mm increased corrosion rate in all cases, but had far less impact than that of the SCMs. Increase in cover depth from 20 mm to 40 mm had substantial benefits for PC specimens, reducing corrosion rates by more than half; the same benefits were not observed in specimens using SCMs. This was ascribed to corrosion rates of SCM concretes being controlled primarily by resistivity of the system, while rates in PC specimens were controlled mainly by oxygen availability and thus cover depth.