Thermal stresses and associated damage in concrete at elevated temperatures

Abstract

This thesis is devoted to study the thermal stress field and associated fracture of cement-based composites, such as mortor 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 mortor. The evolution of micro-/meso-structure with temperatures and the temperature-dependent stress-strain relations where observed and determined respectively in a real-time mode. The dehydration-induced cracks in HCP and three types of thermal cracks in mortor during heating were observed successfully. It was found that both the decomposition of hydration products and 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 the 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 MPFA2D (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-dehyrated 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. The results showed that the crack formation, propagation and coalescence were closely connected with heterogeneity. The crack patterns were dependent on the thermal mismatch as well as the temperature gradient. The effects of temperature dependence of mechanical properties of phase materials on the thermal stress and the associated cracking were analyzed and discussed. Finally, two numerical case studies were carried out to study the thermal response of a cement-based composite subjected to elevated temperature using T-MFPA2D. In the first case, a series of numerical simulations were carried out to study the effects of the severe thermal mismatch between the concrete and the fiber reinforced polymer/plastic (FRP) on the critical temperature increments of cement-based composites reinforced with single bar and multiple bars at low elevated temperatures. The second case was devoted to chanical behavior of a cement-based composite under uniaxial compressive load and thermal load. It was found that a composite material had higher rate of reduction in mechanical properties then the related single-phase material. The transition from brittleness to ductility of a composite material subjected to high-elevated temperatures was simulated.