On the influence of the relative humidity on the thermal response of a concrete plate: A mesoscale phase-field analysis

This study contains an investigation of the thermomechanical response and the cracking behavior of a concrete plate, subjected to diurnal temperature changes, with a particular focus on the influence of the relative humidity (RH). The latter contributes to the heterogeneity of the thermal expansion of the concrete constituents and, thereby, has an influence on the mesoscopic cracking behavior. In the present work, a mesoscale thermomechanical phase-field fracture model is established, considering the mesostructure of concrete, consisting of mortar, aggregates, and interfacial transition zones (ITZs). The ITZs are regularized with an auxiliary interfacial phase-field. The mesoscale results are compared with the ones from macroscopic phase-field analyses. It is found that, while the relative humidity exhibits an insignificant impact on the macrostresses, it has a significant influence on the mesostress fluctuations and the fracture damage. Astonishingly, mesocracking even occurs in macroscopically-compressed regions of the plate. This is primarily due to the large RH-dependent expansive thermal eigenstrains in the mortar, which result in tensile stresses in the aggregates and the ITZs, and in an increase of risk of mesocracking. These cracks start in the ITZs. Their propagation and orientation are governed by the local principal stresses. This agrees with the results of microelastic analyses. Therefore, both the mesoscale phase-field simulations and the microelastic models can be employed to predict the initiation of cracking, providing insight into the design and the durability assessment of thermally-loaded concrete structures.