Experimental and Numerical Assessment of the Hydrodynamical Properties of Cementitious Materials

Durability of concrete structures is strongly conditioned by the occurrence of several chemical and physical phenomena that can be linked to the displacement of water in the constitutive material. Previous studies have already emphasized that in moderate temperature and pressure conditions, water transfers in weakly permeable cementitious material happen predominantly in the liquid phase, and can be modelled as a nonlinear diffusive process. The main goal of this study is to evaluate the intrinsic parameters involved in this diffusive process. For that purpose, a set of isothermal drying experiments are conducted on samples of different types, compositions and sizes. The classical Mualem expressions are fitted to the experimental sorption isotherms, and a numerical ID model is used to fit the intrinsic permeability of the material. On the one hand, the values of the Mualem parameters we obtain show a significant sensitivity on the nature of the material and on the sample size. On the other hand, the Mualem expression, originally formulated for soils, does not fit perfectly experimental data, especially near saturation. A dual porosity formulation is proposed and allows a better representation of the material behaviour. Numerical simulations conducted with the Mualem tortuosity parameter L set to its usual value of 0.5 show a large under-prediction of the value of the unsaturated permeability for low saturation values. It appears necessary to fit this parameter to values between -2.5 and -3 to obtain a good agreement with observed drying kinetics. This discrepancy must be linked to microcracks development in the sample which leads to larger apparent hydraulic conductivity at low saturation states.