Meso-scale modelling of stress effect on chloride diffusion in concrete using three-phase composite sphere model

At mesoscopic scale of modelling, concrete can be conceived as a three-phase composite material which is composed of coarse aggregate, mortar matrix and interfacial transition zone (ITZ) between the coarse aggregate and the mortar. This paper proposes a numerical model of stress effect on chloride diffusion in concrete from the meso-scale perspective. The concrete is idealised by using the three-phase composite sphere model. Based on the theory of elasticity, the constitutive relationships between the porosity of mortar, porosity of ITZ and the stress of concrete are established. Subsequently, the relationship among apparent chloride diffusion coefficient of concrete, chloride diffusion coefficients of mortar and ITZ, and volume fractions of three phases is established, and an apparent chloride diffusion coefficient model with respect to the elastic deformation stage of concrete is obtained. Finally, to extend the applicability of the model to a wide range of concrete stress levels under tension and compression, the model is modified to cater for the inelastic deformation stage of concrete. The modified model is applied to evaluate the apparent chloride diffusion coefficient of experimental specimens in the literature, and good agreement between experimental data and computed results is achieved. For concrete under tensile stress, the maximum prediction error of the modified model in the elastic stage and inelastic stage of concrete is 8.1% and -11.6%, respectively. For concrete under compressive stress, the maximum prediction error of the modified model in the elastic stage and inelastic stage of concrete is -8.2% and -9.3%, respectively. Hence, the modified model is of high accuracy and can serve as a useful tool for durability design and service life prediction of concrete structures, especially those susceptible to chloride attack.