Numerical modelling on moisture and sulfate ion transport in unsaturated concrete slab under dry-wet cycles

The spatio-temporal evolution of sulfate ion in concrete is important for the durability assessment of the structures under external sulfate attack (ESA) and dry-wet cycles. In this paper, the transport behavior of moisture and sulfate ion in unsaturated concrete under dry-wet cycles is numerically simulated. Firstly, the time-varying equation of sulfate ion concentration in concrete is firstly established. Secondly, the equations of sulfate ion diffusion and consumption, moisture transport under dry-wet cycles, and heat transfer are given. Thirdly, considering the behaviors of water evaporation and ion consumption, the Neumann boundary conditions of moisture content and ion concentration at the dry stage are adopted. Then, the numerical solutions are obtained by using the Crank-Nicolson scheme of FDM. On the basis of model validation, a numerical simulation is performed to analyze the distribution of moisture and sulfate ion in cement mortar slab under dry-wet cycles The results show that, the moisture content and ion concentration at the surface layer of slab undergo a cyclic change of first increasing and then decreasing, and finally trend to stabilize. At the dry stage, the peaks of moisture content and sulfate ion concentration form at the sulfate layer, dividing the slab into convection and diffusion zones. The moisture content and ion concentration in their convection zones gradually decrease, while decrease in their diffusion zones. Additionally, the depths of ion peak have always smaller that of moisture peak, which confirms the ion transport lags behind water penetration. This work can provide a basis for the analysis on ESA-induced mechanical response of concrete structures under sulfate environment with dry-wet cycles.