Predicting moisture movement during the drying of concrete floors using finite elements

Predictive methods for determining the point at which it is safe to apply floor coverings to concrete floors can save time and money for the client, contractor and the floor installer. The current standard states that impervious floor coverings should not be applied until the surface of the floor reaches a relative humidity (RH) of 75%, established using a surface hygrometer test. Tests by the authors on drying concrete slabs in natural and forced drying environments show that there is a large variation in the residue of moisture deep in the concrete when the floor covering is applied, particularly so in a forced drying environment. After an impermeable floor covering is applied to the floor surface, this residue of moisture will gradually equilibrate within the slab depth and generate (over a long time) a vapour pressure at the surface that can result in substantial damage to the covering, resulting in expensive repair work. This paper presents a finite element model that predicts the changing moisture content, in terms of the internal RH, during drying and after the application of the floor covering as the internal RH equilibrates over time. The model accounts for the thickness of the slab, w/c ratio, environmental conditions, boundary conditions. It uses non-linear diffusion coefficients and evaporation rates to accurately model the moisture movement in the slab. The results from the model give good correlations with the experimental readings taken at the various depths over time using hand-held humidity probes.