Effects of coating a drying Calcium Sulphate Floor on the relative humidity of the Structure

Cast structures must be allowed to dry sufficiently before they can be covered with impermeable coatings. During a commissioned study on the drying rate of calcium sulphate screed cast floor, it was observed that the measured relative humidity of the structure increased significantly after a vinyl flooring was glued to the surface of the structure. The cause for the phenomenon could not be determined based on the carried-out test arrangements. Therefore, a follow-up study with identical test structures was conducted to isolate the cause of the increase in measured relative humidity after coating. Tests confirmed that the effect was caused by the water contained in the adhesive. The adhesive was initially dosed by applying it with a trowel with specified notching as instructed. In the follow-up study, weighing the applied amount of adhesive revealed that the instructed amount was exceeded significantly. Due to the shape of the material’s equilibrium moisture content curve, even a small increase in moisture content significantly increases the structure’s relative humidity. Therefore, dosing instructions of applied adhesive must be followed precisely, as overdosing can potentially lead to moisture damages in related structures.