Moisture movement in concrete during drying

<SPAN style="LINE-HEIGHT: 115%; FONT-FAMILY: 'Times New Roman','serif'; FONT-SIZE: 12pt">This thesis presents the results from a series of relative humidity (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. 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.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

<SPAN style="LINE-HEIGHT: 115%; FONT-FAMILY: 'Times New Roman','serif'; FONT-SIZE: 12pt; mso-fareast-font-family: Calibri; mso-fareast-theme-font: minor-latin; mso-ansi-language: EN-GB; mso-fareast-language: EN-US; mso-bidi-language: AR-SA">When the surface reached 75% rh, the point specified by the British Standard to be achieved before any covering should be applied, an impervious floor covering (vinyl) was applied and the rh continued to be monitored through the depth as the residual moisture redistributed. For the concrete to reach a surface rh of 75%, it may take several months or even years, particularly for very thick slabs. It is common on many construction sites to speed up the process by accelerating drying by using dehumidifiers and heaters. However, it has been shown here that this only leads to rapid drying near the surface with large residues of moisture remaining deep in the concrete. As a consequence, when an impervious covering is applied, this residue of moisture will slowly re-distribute due to the humidity gradient between the top and bottom of the slab and slowly builds-up under the covering. To model this, the FEM was again set up to suit the new initial and boundary conditions with the impervious covering now applied and the rh through the depth. Again, the results compared well with those measured. Along with this, a vapour pressure is created and this combines with the moisture build-up to cause the problems above. This thesis presents measurement of this pressure as well as a number of pull-off tests that physically assess what forces are required to lift sealed vinyls from concrete surfaces.