Finite Element Modelling of Moisture Movement in Concrete Floors

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 at Trinity College Dublin 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 [1]. When an impermeable floor covering is applied tothe floor surface, this residue of moisture will gradually equilibrate and generate (over a long time) avapour pressure that can result in substantial damage to the covering, resulting in expensive repairwork.This paper presents a finite element model that predicts the changing moisture content, in terms ofthe internal RH, during drying and after the application of the floor covering as the internal RHequilibrates over time. The model accounts for the thickness of the slab, w/c ratio, environmentalconditions, boundary conditions and uses nonlinear diffusion coefficients and evaporation rates toaccurately model the moisture movement in the slab. The results from the model give goodcorrelations with the experimental readings taken at the various depths over time using hand-heldhumidity probes.