Modeling of Moisture Diffusivity of Concrete at Low Temperatures

Abstract

Moisture diffusivity of saturated concrete varies with decreasing temperature. The variation depends on the amount of ice formed in pores and on the extent of the damage induced by the formation of ice. Ice crystals in the pores decrease the moisture diffusivity of concrete because of the low moisture diffusivity of ice, and the damage attributable to excessive ice formation in the pores increases the diffusivity. An analytical model is developed to characterize the effect of the two opposing processes on the diffusivity of the concrete under low temperatures. Three pore configurations are defined as (1) pores with no ice, (2) pores with ice and no damage, and (3) pores with ice and damage. The effective diffusivity of concrete is a combination of the diffusivities of the three types of porous media, each containing one configuration of pores. In the current study, the three types of porous media are combined by the composite model developed by Hashin and Shtrikman. The volume fraction of each configuration of pores is evaluated by a pore-size distribution based on the adsorption isotherm and a function of freezing temperature in terms of pore sizes. The diffusivities of the three types of porous media are evaluated by considering the ice-formation and damage-development mechanisms. The Druker-Prager plasticity model was used, and pores are assumed to be spherically shaped for evaluating the internal damage. The results of model prediction show that the moisture diffusivity in concrete decreases when the solution is frozen in some of the pores. If the temperature reduces further and causes damage attributable to excessive ice formation, then the diffusivity decreases at a lower rate than that at the same temperature without damage.