Investigation of moisture transport properties of cementitious materials

Abstract

Moisture transport in cementitious materials is directly related to the durability of concrete structures. When the material loses moisture, the drying shrinkage induces cracks which are harmful to the solid body. When liquid moves into the material, it can carry aggressive ions. Hence, moisture transport properties are important to cope with durability issues. The present paper is focused on two ways to enhance our understanding of moisture transport properties of cementitious materials. They are based on the indirect determination of the liquid water permeability and water vapour diffusion coefficient. The first one is known as the "inverse analysis" method. In a moisture transport model, the liquid permeability and diffusion coefficient can be adjusted in order to fit the measured mass loss curves and water content profiles. In the second way, measured apparent diffusion coefficient (also called moisture diffusivity) curves are fitted over a large range of relative humidity (RH) by a general expression which includes both liquid transport and vapour diffusion. Due to different RH ranges of predominance of liquid and vapour transport, the liquid water permeability and vapour diffusion coefficient can be determined separately. Input experimental data on cement pastes are collected from the literature. Discussions on relative permeability and Knudsen diffusion show a significant influence on modelling of moisture transport. A further comparison with measured permeability data is able to provide a better understanding of moisture transport properties.

Pore-scale modelling of relative permeability of cementitious materials using X-ray computed microtomography images

“…It should be mentioned that the simulated intrinsic water permeability of saturated specimens seems to be 2 or 3 times higher than experimental data from literature shown in [18,22,37]. This can be attributed to the fact that (i) the saturation conditions of specimens in simulations and experiments are generally not the same.…”

Determination of water permeability for a moisture transport model with minimized batch effect

“…Theoretically, K l should be independent of the external drying conditions. The previous study [8] argued that the discrepancy between K l determined at different RHs was thought to be affected by the preparation of different specimens of the same mix design. Nonetheless, the batch effect is minimized in this study.…”

“…The model selected here to simulate moisture transport in cementitious materials is the semi-simplified version of the multiphase modelincluding the transport of both liquid water and water vapour -which considers that gas pressure is constant and the liquid phase remains incompressible. The governing equation for the mass balance is written as [6,7,8]…”

“…Thiéry et al [53] suggested x D = 2.74 based on the fitting of experimental data for cement pastes and mortars taken from Papadakis et al [54]. The comparison of f (S, φ) calculated by these two proposed values of x D shows that Thiéry's suggestion provides smaller f (S, φ) values (higher resistance) which may be closer to the real conditions of cementitious materials than the original x D value for granular materials [8].…”

“…In fact, K l in the moisture transport model can be determined by back calculation from the measured mass loss curve at a constant RH. This method was known as "inverse analysis" in the previous studies [78,2,79,7,8]. As discussed in [7,8], the initial moisture state of calculations corresponds to the state after self-desiccation which is about the same RH used in this study for the preparation of specimens for drying tests.…”