Computational modeling of cyclic wetting and drying in building envelopes containing salts

Prediction of water and salt movement and salt crystallization in porous materials can be done effectively by means of mathematical and computational modeling. In this way, the time development of water and salt concentration fields can be obtained which is crucial for a proper assessment of possible future damage of materials. In this paper, computational simulation of water and salt movement and salt crystallization in envelope parts of historical buildings is presented. The diffusion-advection model is used for basic description of coupled water and salt transport, taking into account both water movement due to the moisture gradient and diffusion and dispersion effects within the liquid phase due to the concentration gradient. Salt crystallization in the porous body is modeled using an equilibrium model. The effect of salt bonding on the pore walls is taken into account as well. All water and salt transport and storage parameters are assumed to be hysteretic. The mathematical model is solved using the standard Galerkin finite element approach, and a computational implementation is performed. The computer code in C++ is then used in a series of computational simulations for characteristic types of historical building envelopes provided by new surface layers at their renovation. Climatic data corresponding to the test reference year for Prague are used as boundary conditions, so that cyclic wetting and drying occurs in the envelope. The results of computer simulations of moisture and salt concentration fields in historical building envelopes are utilized in the subsequent service life analysis. On the basis of changes of moisture and salt concentration, particularly the number of wetting-drying cycles and frost cycles in a combination with moisture and salt conditions, the durability of new surface layers is assessed.