Corrosion rate of macrocell corrosion in reinforced concrete and service life modeling of carbonated concrete containing supplementary cementitious materials

Abstract

The corrosion propagation stage is essential in the design of service life of a reinforced concrete (RC) structure. However, this is challenged by the measurement of the corrosion rate in chloride-induced macrocell corrosion and the limited knowledge of the corrosion propagation of carbonated concrete. Different sources of error have been reported to interrupt the evaluation of the corrosion rate in chloride-induced macrocell corrosion. The corrosion propagation of carbonated concrete becomes more and more important due to the replacement of ordinary Portland cement (OPC) by supplementary cementitious materials (SCMs). The objective of this thesis is herein to investigate the corrosion rate measurement in chloride-induced macrocell corrosion and develop a service life model for carbonated SCM concretes with the inclusion of the corrosion propagation stage. The applicability of the Stern-Geary equation to macrocell corrosion has been often ignored. To address this research gap, this study investigates the corrosion rate measurement by using the polarization resistance method on the macrocell corrosion in a simulated pore solution. A modified Stern-Geary equation for macrocell corrosion is proposed to consider the effects of the macrocell current. The proposed equation is validated for both active and passive steel rebars. A significant overestimation is found for passive steel rebars while a substantial measurement error is found for active steel rebars due to the macrocell current. The corrosion rate measurement of reinforcing steel under chloride-induced macrocell corrosion in RC is investigated. The corrosion rates of RC columns with different cover thicknesses and chloride concentrations were evaluated using different techniques and cathode over anode ratios. The results show that the corrosion rate of highly corroding steel can be underestimated by an order of magnitude by the linear polarization resistance method due to a failure to compensate for diffusion resistance. The galvanostatic polarization technique can eliminate the influence of diffusion resistance and therefore reliably measure polarization resistance. A B value of 52 mV is recommended under macrocell corrosion. The macrocell corrosion of carbonated concrete is studied by replacing OPC with silica fume (SF), pulverized fuel ash (PFA) and ground granulated blastfurnace slag (GGBS). The corrosion rates and corrosion potentials in the microcell corrosion and the macrocell corrosion of carbonated SCM concretes were obtained. The electrical resistivities of the non-carbonated concrete and carbonated concrete in microcell corrosion are also presented. Results indicate that it is necessary to consider the macrocell corrosion in carbonated concretes. The replacement of OPC by SCMs can enhance the contribution of the macrocell corrosion in carbonated concrete. A novel Carbonation-induced Early Corrosion Propagation based Service-life Assessment Model (CECP-SAM) is proposed and compared with field data in Hong Kong. The effects of exposure condition, concrete constituents, water/binder ratio, cover thickness, rebar diameter, semi-carbonation zone and macrocell corrosion on the service life are presented. Results indicate that partial replacement of OPC by PFA and GGBS can significantly decrease the service life. Practical equations are proposed to facilitate performance-based service life design. Prescriptive limits are also provided for service lives of 50 and 100 years.