Experimental and analytical investigation on chloride ions transport in concrete considering the effect of dry-exposure ratio under diurnal tidal environment

Abstract

For reinforced concrete structures in tidal zone, the dry-wet ratio (the ratio of drying time to wetting time in one cycle) of member varies with the elevation. Chloride ions transport in concrete is affected by coupling action of diffusion and convection under dry-wet cycle environment. The erosion of concrete corresponding elevation of the most unfavorable dry-wet ratio is the most intense, in which invade the most chloride ions. In existing studies, the most unfavorable dry-wet ratio determined by different scholars is not consistent, and there is a lack of quantitative indexes to describe the impact of convection action on chloride concentration distribution. In this research, a series of natural exposure experiments were carried out in submarine environment and dry-wet cycle environment using the indoor tidal automatic circulation device independently invented by the laboratory. A new index, dry-exposure ratio (the ratio of drying time to exposure time in a cycle), was proposed to quantify the dry-wet cycle environment. The mechanism of convection and the influence of different dry-exposure ratios on chloride ions transport in concrete were discussed, and the most unfavorable dry-exposure ratio was determined. Under the condition of diurnal tide (dry-wet cycle period is 24 h), while wetting time is 10 min, shallow part of concrete is the most seriously corroded by chloride ions due to the coupling action of convection and diffusion, while wetting time is 45 min, deep part of concrete is the most seriously corroded by chloride ions diffusion. The variations of peak chloride concentration, depth of convection zone and apparent chloride diffusion coefficient with the dry-exposure ratio and exposure duration were analyzed. The results show that the peak chloride concentration and depth of convection zone can be used as the indexes of quantitative convection. By introducing the depth of convection zone and modifying the peak chloride concentration and apparent chloride diffusion coefficient, a time-dependent model of chloride ions transport in concrete considering the effect of dry-exposure ratio was established. The proposed empirical model is expected to be useful in realistically predicting chloride ions transport in concrete under dry-wet cycle environment.