Redistribution of moisture and ions in cement based materials

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

Uttorkningstider och kritiska fukttillstånd: kemisk fuktbindning och alkalitransport

Två helt olika alternativa möjligheter att förkorta torktiderna för byggfukt i betonggolv har undersökts. Den ena möjligheten var att försöka utnyttja de resultat som erhållits i ett antal tidigare projekt där det visat sig att fuktbelastningen på ett mer eller mindre tätt ytskikt under vissa förhållanden varit mycket lägre efter omfördelning av kvarvarande byggfukt än vad man kunde förvänta sig. Hypotesen har varit att tidig uttorkning skapade en mängd kvarvarande ohydratiserat cement som under omfördelningen, när dessa delar blir fuktiga igen, skulle fortsätta att binda vatten kemiskt och därigenom sänka fuktnivån. Erforderlig torktid skulle därmed bli kortare. Detta har studerats genom att jämföra två fall, med tidig respektive sen uttorkning, till samma nivå. Särskild omsorg har ägnats åt att genomföra experimenten utan att något som helst läckage uppstått. Ett sådant hade annars kunnat ge liknande resultat. RF-profilerna efter omfördelning har sedan uppmätts. Det visade sig att det finns en liten effekt av den typ som hypotesen säger men att denna är så liten att den inte är signifikant. Det har alltså inte gått att förklara de tidigare, positiva resultaten. Andra förklaringar måste sökas. Den andra möjligheten var att klargöra var de verkliga kritiska fukttillstånden ligger för vandring av alkali mot ett tätt ytskikt och förhoppningsvis kunna visa att de är högre än vad som motsvarar dagens kritiska fukttillstånd. Det skulle i så fall kunna innebära att man inte skulle behöva torka betong lika långt som idag, varvid erforderliga torktider skulle bli kortare. Dessa kritiska fukttillstånd har undersökts på ett helt nytt sätt som aldrig tidigare prövats. Provkroppar har torkats ensidigt till olika fuktnivåer och förseglats. Efter omfördelning av kvarvarande fukt har andra sidan öppnats för uttorkning. Om den initiala fuktnivån då hade varit över den kritiska för alkalivandring skulle detta visa sig genom att alkali ansamlats vid, eller på något djup under, den torkande ytan. Alkaliprofiler har mätts upp med ett antal olika metoder. Tyvärr har dessa inte gett så klara resultat som fordras. Alkalivandring har därför hittills inte kunnat tydligt påvisas. Dessa försök fortsätter med andra metoder

Moisture Redistribution in Screeded Concrete Slabs

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