Effect of recycled aggregate and supplementary cementitious material on the chloride threshold for steel bar corrosion in concrete

Under marine environment, the passive film on the surface of steel bar embedded in reinforced concrete structures will be broken once a threshold value of chloride, i.e., chloride threshold (Cth), is reached at the rebar-concrete interface by diffusing through the concrete cover. The value of Cth is essential for determining service life of concrete structures. In this investigation, the effect of recycled aggregate (RA, i.e., recycled coarse aggregate) and supplementary cementitious material (SCM), including ground granulated blast furnace slag, red mud and glass powder, on the value of Cth was investigated. A chloride threshold test method for accurate determination of Cth was proposed by the alternation of accelerated chloride migration test and artificial-natural diffusion test, which can simulate the marine environment and accelerate the diffusion of chloride. The corrosion activity of steel bar was monitored using two electrochemical methods, including linear polarization resistance method and macro-cell corrosion method, to detect the initiation of corrosion. Immediately after the corrosion initiation, corresponding specimen was taken out to measure the chloride content of concrete adjacent to the surface of steel bar. A calculating method of chloride threshold Cth was presented considering the actual dis-tribution of coarse aggregate in concrete adjacent to the surface of steel bar. The results of the chloride threshold tests show that RA and SCM have a significant influence on the protective properties of passive films, and it is worth noting that the positive effect of RA on the quality of passive film has been observed. For concrete with the same substitution level of SCM, the chloride threshold Cth increases linearly with RA content. Subsequently, the suggested values of Cth for natural aggregate concrete (NAC) with different substitution level of SCM are given, and the suggested values of Cth for corresponding recycled aggregate concrete can be given by an amplified factor kRA multiplied by Cth for NAC with +/- 10% relative error.