We propose a mathematical non-steady-state, time/depth dependent diffusion-migration model based on the modified Fick’s second law of diffusion. It considers the effects of different chloride binding mechanisms for the transport process of chloride ions diffused in a porous concrete on the variation of its chloride profile. Four different chloride-binding isotherms, i.e., no binding, linear binding, Langmuir binding and Freundlich binding, together with the time/depth dependent chloride diffusivities are investigated. Results of this study show that the variations of amounts of available free chloride ions in the solution of saturated concrete resulting from different chloride-binding isotherms have significant effects on the chloride profile. Comparison between the analytical solution of the proposed mathematical model and previous experimental data shows a satisfactory agreement for the chloride concentrations 0.05 M and 0.1 M. The orders of reduction amount of chloride diffusivities from large to small are found to be in the following sequence: (1) no binding isotherm, (2) Freundlich binding isotherm, (3) Linear binding isothern, and (4) Langmuir binding isotherm.

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