MODELLING OF CHLORIDE DIFFUSION INTO SURFACE-TREATED CONCRETE

Abstract Chloride penetration into concrete is the main cause of the corrosion of steel in concrete structures exposed to chloride-rich environments. As a preventive or remedial method, surface treatments on concrete have been increasingly applied to both new and existing concrete structures to combat this problem. Ion diffusion is regarded as the dominant chloride transport process for structures constantly immersed in water. So far, knowledge of how a surface treatment reduces chloride diffusion is limited and the relationship between chloride diffusion resistance and surface treatment parameters, such as thickness, porosity and diffusion coefficient, has not been quantitatively identified. To gain an insight into the protective mechanism of surface treatments, a theoretical study of chloride diffusion through surface-treated concrete is required. This work proposes a unified physical model for all types of surface treatment and the concept of water-percolated porosity. The influences of surface treatment and substrate properties on chloride diffusion are studied using a finite difference model. Results indicate that chloride diffusion is controlled by both the surface treatment and the substrate. A surface treatment can significantly reduce the chloride concentration at the concrete surface, but this interfacial concentration increases with time. Hence the chloride profile in the concrete substrate does not obey Fick's law with a constant concentration boundary condition. If Fick's law is applied to such a surface-treated concrete, a diffusion coefficient, termed a pseudo diffusion coefficient, of less than the true diffusion coefficient of the substrate material is obtained.