| This thesis is devoted to study the thermal stress field and associated fracture of cement-based composites, such as mortar and concrete. The following works have been completed.; A series of experiments were conducted at a macro-level to determine the temperature-dependent stress-strain relationship of high strength concrete (HSC), and to measure the coefficient of thermal expansion (CTE) of hardened cement paste (HCP), mortar and concrete at elevated temperatures through specially designed equipment. The effects of admixtures, and unstressed and stressed conditions on the mechanical properties of HSC were studies.; Further to the macroscopic experiments, a set of scanning electron microscope (SEM) experiments were carried out to study the mechanisms of thermal damage of HCP and mortar. The evolution of micro-/meso-structure with temperatures and the temperature-dependent stress-strain relations were observed and determined respectively in a real-time mode. The dehydration-induced cracks in HCP and three types of thermal cracks in mortar during heating were observed successfully. It was found that both the decomposition of hydration products and the dehydration-induced cracks significantly affected the mechanical properties of HCP. As for the mortar specimens, the cracks were induced not only by the decomposition of hydration products, but also by the thermal mismatch between the HCP and the sand.; A methodology of thermal damage analysis was developed to study the thermal degradation of cement-based composite (HCP, mortar and concrete), taking into account of heterogeneity, temperature gradient and temperature-dependent properties of phase materials. Two thermo-elastic mesoscopic damage models at a meso-level were proposed based on the results of SEM experiments conducted in this research. A finite element program T-MFPA2D, that was based on the existing MFPA2D (Material Fracture Process Analysis developed in CRISR, Northeastern University, Shenyang, China) program, was developed to calculate the thermal stresses and analyse the crack development accompanying for thermo-mechanical and thermo-dehydrated damages.; A series of numerical parametric studies were conducted to identify the effects of various factors on the thermal damage of a cement-based composite at elevated temperatures. The factors were heterogeneity of phase materials, thermal mismatch between the phase materials, temperature gradient and temperature-dependence. (Abstract shortened by UMI.)... |
