Measurement and modelling of the moisture distribution in early-age concrete in the joint of composite beam and hollow core slab

The building moisture in concrete can damage the surface material if applied too early before the excess moisture has dried out. The composite beam forms an abnormal structure at the joint of the hollow-core slab, in which the thickness of the structural concrete is almost the thickness of the entire slab. For this reason, the joint of the beam and hollow core slab must be taken into account when assessing the adequate drying of concrete structures. The considered composite beams are steel casing beams, which are casted full of concrete. To achieve a composite structure, there are holes in the sides of the casing, through which moisture from the concrete transfers to the surrounding structure. However, the drying of the concrete inside the beam is significantly limited. The level of relative humidity of the concrete inside the beam remains moderately high for a long time, depending on the quality of the concrete and, consequently, the self-desiccation of the concrete. When using concrete with a low water binder ratio, drying occurs at a level of about 85%RH as a result of self-desiccation. In the study, relative humidity in the joint was measured continuously and with instantaneous measurements. The relative humidity in the joint concrete was also monitored after the surface structure layers were installed. In addition, humidity was measured from below the surface material and continued until equilibrium was reached. The structure was simulated as a 3D model and studied using a concrete material model which takes into account the self-desiccation and the thermal dependence of the sorption. A good agreement was found to describe the effect of temperature changes on the relative humidity of concrete that can be observed in continuous measurement. In addition, a suitable prediction of relative humidity below the surface material can be achieved based on modelled relative humidity distribution before the installation of the surface structure.