Microstructure, microchemistry, and prediction of long-term diffusion behavior of chloride in concrete

Abstract

Microstructural and microchemical features of two types of concrete are investigated employing electron and ion optical techniques. The first type is the concrete cured in seawater or normal water. The second type is concrete cured in normal water and exposed subsequently to seawater. Major constituent phases of concrete and differences in their distribution due to different curing media are identified. Chloride profiles in different concretes are evaluated using the proton induced X-ray emission technique. Diffusion coefficient D was calculated by modeling the diffusion process and comparing with measured profiles. D, thus estimated, is found to be ~1.8×10⁻⁹ m²/s, which is higher than the reported values of ~10⁻¹¹ to 10⁻¹³ m²/s. The faster diffusion of chloride in seawater-cured concrete can be attributed to the availability of water medium in wet concrete, in the initial stages of the hydration of cement. The prediction of the concentration profile of chloride in a layer of 100 mm of 28% fly ash containing concrete over concrete exposed to seawater is carried out. For the worst scenario, analytical estimates of the concentration of chloride as a function of time at a distance of 100 mm in the fly ash containing concrete were made. The concentration profiles of chloride expected after 40 years in the fly ash-containing concrete were also estimated using diffusion coefficient values available in the literature.