Moisture re-distribution in concrete under impermeable coverings

Abstract

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 cover traps this residual moisture, thus preventing evaporation to the ambient air gradually generates a vapor pressure underneath the cover, which can result in damage. Here, experimental results on the long-term influence of an impermeable covering of the moisture condition in concrete in normal and forced drying conditions are presented in the paper. 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 vapor pressure is generated under the cover 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 cover on the subsequent re-distribution of the moisture through the depth of the concrete over time.