DOI: 10.14457/tu.the.2019.766
title: Electrochemical properties of reinforcing steel corroded by carbonation before and after being repaired

description: A study of electrochemical properties of reinforcing steel in concrete has earned interest from many researchers due to their useful information for predicting, understanding, and characterizing the severity of reinforced concrete structures due to corrosion. Carbonation causes the corrosion of reinforcing steel in concrete. In this study, effects of carbonation on electrochemical properties of reinforcing steel in concrete and repair materials are discussed. In addition, the effects of moisture on corrosion behavior are also presented in this research work. The low-carbon reinforcing steel bars were embedded in concretes with water/binder ratios of 0.4 and 0.6. A coal fly ash was used to replace cement at 0% and 30% by weight of binder. In addition, the same type of steel was embedded in six brands of repair materials. Three brands were patch-repair materials and the other three brands were grout-repair materials. The 0.6 water/binder concretes were coated by three brands of surface coating materials after corrosion initiation. All samples were exposed to accelerated carbonation environment (4% CO2, 50 ± 5% relative humidity (RH), and 40 0C temperature) and laboratory environment (0.04% CO2, 75 ± 5% RH, and 28 0C). The carbonation depth, Four-point Wenner probe, Linear polarization resistance (LPR), Electrochemical impedance spectroscopy (EIS), and Potentiodynamic polarization resistance (PPR) were monitored in this study. The results showed that the corrosion rates of steel increased with an increase of carbonation depth, even the carbonation depth has not reached the steel surface. The remaining uncarbonated depth to initiate the corrosion of reinforcing steel depends on concrete mix proportion. Patch materials resulted in the carbonation depth comparable to 0.4OPC. The Grout materials had high resistance to carbonation, so no carbonation depth was observed. The corrosion rate of steel increased when the moisture content increased. For the steel in repair materials, an increase of corrosion rate was observed in samples that was susceptible to carbonation, especially for patch-repair materials. The void contents of repair materials were lower than that of the concrete, and Grout materials had the lowest void content. The PSB and GSK showed the highest electrical resistivity that was similar to that of 0.4FA. The other repair materials showed similar electrical resistivity to 0.4OPC. The electrical resistivity of repair materials depended more on their compositions than their void content. The electrical resistivity measured by EIS was a little higher than that measured by four-probe due to different measured depth of sample. The corrosion rate of surface coating sample was lower than that of the uncoated sample. However, corrosion rate still increased after water supplying due to an increase of moisture. The electrochemical properties of reinforcing steel corroded by carbonation can be understood in this study. Guideline to determine corrosion initiation of reinforcing steel based on remaining uncarbonated depth is proposed. Also, corrosion rate of reinforcing steel before and after carbonation in different moisture conditions are proposed. The results from this study can be used to propose a guideline in order to evaluate the service life of reinforced concrete structures under carbonation. The selection of repair materials for carbonation-induced corrosion can be recommended.