Moisture re-distribution in concrete under impermeable coverings

It is normally considered safe to apply an impermeable floor covering to concrete surfaces when the surface relative humidity reaches 75% as determined by a surface hygrometer. However, over time, defects can appear on the covering such as blistering of vinyl and rising of tiles from the surface. One cause is the on-going diffusion of the residual moisture deep within the slab to the surface. The covering traps this residual moisture, thus preventing evaporation to the ambient air and gradually generates a vapour pressure underneath the covering, which can result in damage. Here, experimental results on the longterm influence of an impermeable covering on the moisture condition in concrete in normal and forced drying conditions are presented. They show primarily that slabs in a forced drying environment result in a significant residue of moisture deep in the slab. As a result, a greater vapour pressure is generated under the covering compared with those drying at normal conditions. In order to predict this, a commercial finite element package (DIANA) has been used to model the influence of an impermeable covering on the subsequent re-distribution of the moisture through the depth of the concrete over time.