Instantaneous chloride diffusion coefficient and its time dependency of concrete exposed to a marine tidal environment

Based on the field exposure test in a marine tidal environment, the free chloride concentrations of nine concrete mixtures at different exposed times were obtained. Then the apparent chloride diffusion coefficients in concrete were calculated by Fick's second law, according to the calculated values and the time dependent characteristic, the method for analyzing the instantaneous chloride diffusion coefficient of concrete was studied. Furthermore, effects of water to binder (W/B) ratio, admixture and exposure time on the age reduction factor of apparent diffusion coefficient and instantaneous diffusion coefficient have been analyzed respectively. The suggested time dependent model of instantaneous chloride diffusion coefficient was verified by the measured chloride concentrations. Results show that the convection zone has been appeared in test concrete at 120 d exposure time, and the addition of admixtures can reduce the free chloride concentrations effectively. Besides, the instantaneous chloride diffusion coefficients are all less than the corresponding apparent chloride diffusion coefficients, the difference between them decreases with W/B ratio, but increases with admixture, except silica fume. In addition, at 360 d exposure time, the age reduction factor of instantaneous diffusion coefficients tend to be stable generally. Based on the time dependent model of instantaneous chloride diffusion coefficient in concrete at 360 d exposure time, the derived instantaneous chloride diffusion coefficients can be well coincident with the fitted calculated by the measured chloride concentrations. In addition, the relative error of chloride concentration between the values predicted by the time dependent model of instantaneous diffusion coefficients and the measured values is less than that predicted by the time dependent model of apparent diffusion coefficients.