Quantitative relationship between microstructure of steel-concrete interface and chloride-induced corrosion rate of steel in unsaturated cementitious materials

While extensive evidence indicates that the porous microstructure of the steel-concrete interface (SCI) is the key factor contributing to early depassivation and expedited corrosion propagation of steel rebar, there remains a lack of quantitative relationship between the SCI microstructural parameters and corrosion rate of steel, particularly under unsaturated conditions. In this work, the effects of rebar arrangement direction (i.e., horizontal and vertical orientations), binder type (i.e., ordinary Portland cement and alkali-activated slag), presence of aggregate, and chloride content, on both the SCI and chloride-induced corrosion rate of steel were systematically investigated and quantified at different relative humidity levels. The results indicated that in comparison with Portland cement counterparts, the reaction products of alkali-activated slag fill the gap under the horizontally oriented steel rebars, favoring more densified SCI microstructure and better corrosion protection. Quantitative analysis reveals that in the unsaturated state, the corrosion rate of steel decreases more slowly in more porous SCI microstructure. An image-based model is proposed to quantitatively link SCI microstructure and corrosion rate of steel, which is applicable to both Portland cement and alkali-activated slag systems in saturated and unsaturated conditions.