Multiscale quantification of thermal expansion of concrete and thermal stresses of concrete structures

Abstract

The mechanical behavior of concrete structures subjected to temperature changes is significantly influenced by hygro-thermal processes occurring at the nanoscopic material scale. In order to quantify this relation exemplarily, a multiscale structural analysis of a simply supported concrete beam subjected to sudden cooling at its top surface is carried out. The overall analysis is organized in three steps. The first step refers to upscaling of thermoelastic properties of concrete by means of a multiscale model. It uses measured “hygrothermic coefficients” as input. They quantify the change of the internal relative humidity resulting from a temperature change. The multiscale model links temperature-induced changes of effective pore pressures in nanoscopic gel and capillary pores to the macroscopic thermal expansion behavior of the cement paste and the concrete. In the present contribution, this upscaling approach is validated by comparing model-predicted thermal expansion coefficients of cement paste with measured counterparts. The second step of the overall analysis consists of a macroscopic thermoelastic Finite Element analysis of the aforementioned concrete beam. These simulations are based on the homogenized elastic stiffness and the homogenized thermal expansion coefficient of concrete obtained in the first step. The simulations deliver distributions of the temperature and of the macroscopic stresses inside the analyzed concrete beam. In the third step, the obtained macroscopic stresses of concrete and the corresponding temperature changes are downscaled to average stress states of the cement paste matrix and of the aggregate inclusions, respectively. This way, it is shown that the significant mismatch of the thermal expansion coefficients of cement paste and aggregates results in microscopic tensile stresses of cement paste, which are significantly larger than the macroscopic tensile stresses experienced by concrete.