Redistribution of moisture and ions in cement based materials

Abstract: There were two principal objectives for this thesis. The first was to develop a methodology and evaluation model of moisture redistribution in order to make the future relative humidity in a screeded concrete slab predictable. The second objective was to develop a method to determine the critical humidity for ion transport in concrete. Residual moisture in screeded concrete slabs may be redistributed to the top screed surface under a semi-permeable flooring, thus elevating the relative humidity, RH, and possibly exceeding the critical humidity level. Passing the critical humidity level may result in material damage to the flooring and adhesive. In order to avoid such damage there is a need of a methodology to estimate the maximum humidity obtained underneath flooring. The redistribution of residual moisture may increase the concentration of alkali at the contact area between the adhesive and concrete. This alkali increase may initiate and sustain adhesive deterioration, which is a common moisture related damage. Several screeded concrete slabs with PVC flooring were prepared to reproduce and monitor moisture distribution and the subsequent redistribution. The moisture distribution before flooring and after a certain period 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 levelling flooring compound. Repeated absorption and desorption scanning curves starting from the desorption isotherm were also investigated. The measurements performed made it possible to develop both a qualitative and quantitative model of moisture redistribution and to quantify the humidity achieved under the flooring. 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 homogeneous slabs. Results from previous research suggested that early drying of the concrete slab can mitigate the increase in relative humidity that occurs when an impermeable floor covering is installed. A study was conducted to verify these results. The results of this new study of the effects of early drying could not demonstrate any significant difference between an early and late dehydration. The redistribution of ions under different moisture conditions was investigated with a newly developed method. The method is divided into a preconditioning of test specimens and also a method to determine the redistribution of potassium ions. This preconditioning provides a well defined moisture condition of the specimen, which is important for determining the critical humidity for the transport of ions. The redistribution is detected by examining the ion content distribution before and after the preconditioning of ion species already present in the concrete. Previous research in this area has been conducted through adding chloride ions to the specimens. However, redistribution of chloride ions is impeded in that they bind to the inner pore surfaces, which poses a risk that the critical moisture level may be affected/misjudged.