Chloride threshold value for reinforcement corrosion in concrete with additions of silica fume or fly ash

The objective of this study is to evaluate the influences of silica fume and pulverised fly ash on the chloride threshold value for the corrosion of steel reinforcement in concrete. The chloride ions were introduced into concrete specimens made with ordinary Portland cement alone and a mixture of ordinary Portland cement and the partial replacements of cement with silica fume and pulverised fly ash at three levels by an immersing–drying cycle procedure. Half-cell potential and linear polarisation methods were applied to detect the initiation of pitting corrosion for the steel reinforcement. The threshold values were expressed as free chloride or total chloride by mass of binder, as well as [Cl⁻]/[OH⁻] and [Cl⁻]/[H⁺]. It has been found that the changes of the chloride threshold value with the additions of pulverised fly ash and silica fume intensively depend on the representations. Especially, the critical [Cl⁻]/[H⁺], regardless of the type and content of mineral admixtures, approximately keeps constant at 0·02. Accordingly, [Cl⁻]/[H⁺] is suggested as a more suitable form to represent the chloride threshold value.