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 of it. Chloride ions transport in concrete is affected by coupling action of diffusion and convection under dry-wet cycling environment. The erosion of concrete at the elevation corresponding to the most unfavorable dry-wet ratio is the most intense, in which invade the most chloride ions. In existing researches, the most unfavorable dry-wet ratio determined by different scholars is not consistent, and there is no index to quantify the influence of convection action on chloride concentration distribution. In this research, a series of chloride natural diffusion experiments under submarine environment and dry-wet cycling environment were carried out with the indoor tidal automatic circulation device independently invented by the laboratory. A new index as dry-exposure ratio (the ratio of drying time to exposure time in one cycle) was proposed to quantify the dry-wet cycling 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 ratios were determined. Under the condition of diurnal tide (dry-wet cycling 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 variation 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 to quantify the convection action. By introducing the depth of convection zone and modifying 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 cycling environment.

Keywords: chloride ions transport, dry-wet cycling, depth of convection zone, peak chloride concentration