MOISTURE MOVEMENT IN CONCRETE DURING DRYING

The prediction of moisture distribution within concrete slabs has considerable practical importance as it affects the time at which coverings can be safely applied. Moisture in concrete affects creep, shrinkage, strength development, durability and must be present in sufficient quantities for full hydration to occur and for workability on site. However, an excess of moisture in concrete at the point of covering particularly can lead to a number of problems such as delamination and blistering of vinyl, buckling of timber floors and rising of tiles. Moisture migrates in concrete slabs as a diffusion-type process during drying and is lost to the ambient air via evaporation from exposed surfaces. Drying will continue until equilibrium is reached between the humidity in the concrete and in the ambient air. The measured relative humidity (rh) profiles indicate that non-linear profiles exist early on as drying was more rapid near the surface, particularly during accelerated drying. Over time, however, the moisture movement became more uniform through the slab. This thesis presents the results from a series of rh measurements taken at the surface and through the depth in a number of concrete slabs with varying thicknesses (100, 150 and 200mm) and w/c ratios (0.4, 0.5 and 0.6) and allowed to dry in two different ambient drying environments; naturally in a laboratory and artificially in a control room where drying was accelerated using a heater and dehumidifier. These properties were chosen as they represent typical ranges of slab thicknesses and w/c ratios used in practice for most ground bearing slab applications. From these results, a commercial finite element model (FEM) (DIANA) has been set up to predict the changing rh over time using calibrated material properties, namely the diffusion coefficients and evaporation rates. The FEM demonstrated that it could be used to predict the rh over time