Numerical simulation of external sulphate attack in concrete considering coupled chemo-diffusion-mechanical effect

Abstract

This paper proposes an extended numerical method based on Fick’s second law and second-order reaction kinetics for predicting the erosion process of concrete exposed to sodium sulphate solution. A three-phase concrete model was introduced for calculating the diffusivity of concrete to consider the effect of the interfacial transition zone (ITZ), and the diffusivities of diffusible phases were obtained based on the general effective medium equation. In addition, the effects of pore-filling and chemo–mechanical damage on the diffusivity of concrete were considered in the model, using the volume increase approach. Moreover, the expansive products, mainly gypsum and ettringite, were assumed to precipitate unevenly in the ITZ and cement paste owing to the different diffusivities and sulphate ion concentrations of these two phases. The distributions of the sulphate ions, gypsum, and ettringite in concrete, the expansion of concrete specimens, and the damage depth were determined through numerical calculations, and the simulation reliability was validated based on the comparisons with the available experimental data. Furthermore, the effects of the water–cement ratio and external solution concentration on the sulphate diffusion and specimen expansion were determined. The simulation results showed that the ITZ influenced the calculated results and should therefore be considered in the numerical model. The diffusivity of concrete decreased owing to the pore-filling effect but increased as the chemical–mechanical damage increased.