Mechanisms underlying the relationship between electrical resistivity and corrosion rate of steel in mortars

The linear relationship between electrical resistivity of concrete and corrosion rate of steel in logarithmic coordinates (referred as R-C relationship) has been widely reported. However, R-C relationships are not universal but highly dependent on binder chemistry and corrosion environment, and there has been no consensus on the reasons behind the discrepancy of R-C relationships reported in the literature. In this work, the R-C relationships of alkali-activated slag (AAS) mortars are investigated and compared with that of reference ordinary Portland cement (OPC) mortars, aiming at uncovering the fundamental compositional parameters that cause differences in R-C relationships of various cementitious systems. The results show that the hydroxide ion concentration (pH value) of pore solution has a significant and decisive effect on the R-C relationship. With the decrease of pH, the material resistivity increases because of decreased pore solution conductivity while corrosion rate of steel increases due to the drop of [Cl ⁻]/[OH ⁻], leading to a shift of R-C relationship lines. A new mechanism is proposed to explain the differences in the R-C relationship caused by binder chemistry and corrosion type (chloride- versus carbonation-induced corrosion), which is further verified using the published experimental results in the literature.