Permeability and relative permeability of mortar undergoing damage: a hierarchical capillary bundle approach

The purpose of this work is to achieve a better understanding of the relationship between mechanical damage, pore size distribution and transport properties of cementitious materials. In the literature, analyses are usually restricted to intrinsic permeability of the material and the evolution of the apparent permeability with respect to the pressure gradient and to the nature of the fluid considered are left aside. A new model capable to provide the apparent permeability of a porous material to gas, directly from the pore size distribution and from the properties of the gas is discussed. Comparisons with experimental data on mortar specimens show that the model can reproduce the intrinsic permeability and its evolution when the material is subjected to mechanical damage, provided the pore size distributions are available. Extension to the transport of different phases (e.g. water and water vapor) is discussed, with a view towards the simulation of nuclear accident in containment vessels. It is shown that small pores that are not affected by damage according to the pore size distribution are of great importance in the evaluation of the relative permeability to liquid and vapor as a function of the saturation. A tentative model is discussed and compared with the existing – standard – approach relying on Van Genuchten relationships.