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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 long-term 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.
Holmes, N. and West, R., Moisture re-distribution in concrete under impermeable coverings, in Role of Concrete. International Congress on Celebrating Concrete: People and Practice, University of Dundee, Scotland, 2003