Investigation of volumetric effect of coarse aggregate on corroding steel reinforcement at the interfacial transition zone of concrete

Abstract

Corrosion is an electrochemical process which requires electrolyte for the occurrence of corrosion reaction. Therefore it is necessary to calculate the effective corrosion rate with reference to the saturated area of concrete only when it comes to corrosion of steel reinforcement embedded in concrete. Theoretically and numerically the saturated area depends on the capillary zone porosity, gel zone porosity and their respective degree of saturation in the aggregate free volume of concrete which is a heterogeneous material in nature. This makes it important to deeply understand the effect of aggregate on corrosion in concrete. Investigation was carried out to find the effect of coarse aggregate volume on the corrosion rate and potential of reinforcement steel in concrete. The initiative for this research came from the observation that the interfacial transition zone around the steel bar in concrete is surrounded primarily by paste only and hence the coarse aggregate volume should not influence the corrosion rate principally. Also there are chances that some fine aggregate could be present in the vicinity of steel bar surface being finer than coarse aggregate which may affect the corrosion rate. The previous research data in this field was found to be limited and has a difference of opinion. Therefore, these factors have been investigated in this research. Prismatic concrete and mortar specimens were cast and their corrosion potential values were compared with each other to find the effect of the volume of coarse aggregate on corrosion. The experiment results showed that the effect of the volume of coarse aggregate on corrosion potential is not significant since the area around the steel bar especially in the case of deformed steel is mostly surrounded by paste only. The corrosion potential values obtained in the case of mortar and concrete specimens showed same averaged magnitude. This fact was further strengthened by measuring the corrosion mass loss and resulting corrosion rate in the two cases. Furthermore, in the light of experiment results obtained in this research, the numerical FEM model DuCOM developed by our research group at the University of Tokyo, Japan incorporating the effect of aggregate on corrosion rate and potential of corroding steel in concrete undertaken in the past has been successfully enhanced and verified in this research.