Corrosion of Reinforcement Steel Embedded in High Water-cement Ratio Concrete Contaminated with Chloride

摘要

The aim of this investigation was to examine the role of internal chloride on the corrosion behaviour of steel in ordinary Portland cement (OPC), and sulphate resistant Portland cement (SRPC) mortar mixes of higher water-cement (w/c) ration. For this purpose, the steel electrodes embedded in mortar specimens were subjected to short-term electrochemical monitoring. On completion of electrochemical monitoring, pore solution was extracted from the specimens for analysis. For the purpose of comparison, pore solution was also extracted from separated OPC and SRPC cylindrical mortar specimens hydrated in an environment isolated from the atmosphere. Results from micoanalysis using scanning electron microscopy (SEM) are also reported. The results demonstrated that mortars prepared with a cement of high tricalcium aluminate (C_3A) content, even when the w/c ratio of the mix was maintained at 0.70, still passivated the steel embedded in it even in the presence of chloride as high as 1% by weigh of cement. Under a similar condition, the steel embedded in mortar prepared with a cement of low C_3A content was unable to passivate the steel electrode. The special protection mechanism by the lime-rich layer as proposed by Page for low w/c ratio concretes is also valid for the steel embedded in high w/c ratio mortar. The type of exposure condition (e.g. exposure to atmosphere) during the hydration period strongly influences the hydroxide ion concentration in the pore solution, while this has no effect on the free-chloride concentration in the pore solution. This study reconfirms the significance of the C_3A phase of cement in respect to chloride-binding processes.