Modelling heat and moisture transfer in deformable porous building materials

A fully coupled numerical model to simulate slow transient phenomena involving heat, capillary water and moist air transfer in deformable porous building materials is presented. It makes use of the modified effective stress concept together with the capillary pressure - saturation relationship. Inner structure of the material is described using a log-normal distribution of the pore size, which is very convenient from the numerical point of view. Phase changes (condensation and evaporation), heat transfer through conduction, convection and latent heat transfer are taken into account. The governing equations in terms of temperature, capillary pressure, air pressure and displacement vector are coupled non- linear differential equations. They are discretised by the finite element method in space and by finite differences in the time domain. Two numerical examples are solved to present the capabilities of the model for analysing the hygrothermic behaviour of porous building materials and structures in a wide range of moisture content, starting from hygroscopic one up to almost fully saturated with water. The first example, dealing with a drying process of a flat slab, is solved for three different building materials, i.e. clay brick, cellular concrete and standard concrete, resulting in a qualitatively different hygrothermic behaviour. The second example concerns a condensation process on the inner surface of a corner of a brick wall during winter after a rapid increase of the relative humidity of the inner air. It shows that the present model is a useful tool for analysis of the transient behaviour of complex building structures subjected to boundary conditions varying in time.