Moisture redistribution in screeded concrete slabs

Abstract: The principal objective for this licentiate thesis is to develop a methodology and evaluation model in order to make the future relative humidity in a screeded concrete slab predictable. Residual moisture in screeded concrete slabs may redistribute to the top screed surface under semi-permeable flooring, thus elevating the relative humidity, RH, and possibly exceed the critical humidity level. Passing the critical humidity level may result in material damages on the flooring and adhesive. In order to avoid such damages there is a need of a methodology to estimate the maximum humidity obtained underneath flooring. Several screeded concrete slabs with PVC flooring, were prepared to reproduce and monitor moisture distribution and the occurring redistribution. The moisture distribution before flooring and after a certain time of redistribution is presented. In addition, sorption isotherms including scanning curves were determined in a sorption balance for materials used in the floor constructions, viz, w/c 0.65 concrete, w/c 0.4 concrete, w/c 0.55 cement mortar, and Floor 4310 Fibre Flow, a self leveling flooring compound. Repeated absorption and desorption scanning curves starting from the desorption isotherm were also investigated. Performed measurements enabled the development of both a qualitative and quantitative model designed to resemble the actual moisture redistribution and quantify the humidity achieved under flooring respectively. The hysteresis phenomenon of the sorption isotherm is considered in the model. The model is well suited for estimations of the future RH underneath flooring in a screeded concrete slab and may also be used on homogenous slabs.