Determination of the Isothermal Moisture Transport Properties of Porous Building Materials

Abstract

A multiscale network approach is proposed as a practical method for estimating the isothermal permeability of porous building materials covering hygroscopic and overhygroscopic ranges. The multiscale approach is based on the concept of examining the porous space at different levels of magnification, and it allows modeling of combined liquid water and water vapor transfer over a wide saturation range. The proposed method simply requires the knowledge of common moisture properties, such as the capillary pressure curve, the capillary absorption coefficient and water vapor permeability, which can be determined from standard experiments. Hysteresis between drainage and wetting due to air entrapment can be adequately modeled by considering only those pores that become filled by water. The calculated network permeability is validated comparing experimental and simulation results of isothermal capillary absorption and drying processes in ceramic brick and calcium silicate. It is shown that transient moisture processes of capillary absorption from a dry sample and the isothermal drying from an initially capillary saturated sample can be accurately modeled by a single nonlinear permeability and capillary pressure curve. The capillary pressure curve to be used is the main wetting curve, which describes the moisture content relationship from the dry state to the capillary moisture content, correctly taking into account air entrapment phenomena. The description of the absorption or drying moisture processes by a single or a double exponential diffusivity relation was found to be less accurate.