Mechanical Properties Investigation Of Heated Hardened Cement Paste

To date, the studies on mechanical properties degradation of concrete exposed to elevated temperatures are mainly focused on concretes with different mix proportions. Since concrete is a multiphase composite material, the disadvantage of this method is that the contributions of different phases in concrete to the apparent material degradation of concrete couldn’t be distinguished. Cement paste, the cohesion phase in concrete, will undergo thermal dehydration and decomposition, resulting in the variation of the microstructure and formation of cracking of cement paste. As an extrinsic material property, strength is governed by the defects of the material. The high temperature-induced damage will lead to the strength degradation of cement paste. Therefore, the investigation of the mechanical properties degradation of heated cement paste is essential to the understanding of the mechanical properties degradation of heated concrete.In this thesis, the mechanical properties degradation of heated cement paste is investigated both experimentally and theoretically. In the experiments, the compressive strength and splitting tensile strength of different cement pastes exposed to different target temperatures are measured. The cracking of the specimens and the phenomena of the experiments are observed. In the theoretical analysis, by analyzing the thermal decomposition-induced volume fraction variations of different components in cement paste and by applying the experimental data on surface cracking of specimens, the effect of thermal damage on the stress field in cement paste is analyzed by using finite element method and the contributions of the two types of damage are compared. The study of this thesis can provide research basis for the understanding of the mechanical properties degradation of concrete under high temperature conditions.The experimental results show that, with the increase in the target temperature, the mass loss of specimens with three water to binder ratios of 0.5,0.26,0.26 (with silica fume) is increasing, the maximum mass loss occurs in the temperature range of 20℃～ 100℃. The crack width, depth and density of the three types of specimens is constantly increasing with the increase in temperature. The silica fume can incraese the degree of development of cracking width, but reduce the depth and density development. The compressive strength of of hardened cement paste with a water to binder ratio of 0.5 and 0.26 specimens declines with temperature increasing and the compressive strength of 0.26 specimen is higher than 0.5, but the compressive strength of 0.26 (silica fume) specimen is first increasing and then decreasing. In the temperature range of 100℃-450℃, its compressive strength is higher than the strength under room temperature. The splitting tensile strength of all the three types of specimens are first increasing and then decreasing, the maximum splitting tensile strengths all happen at 100℃, the splitting tensile strength of 0.26 specimen is higher than 0.5, the silica fume can reduce the splitting tensile strength. The simulation results show that, for the degradation of tensile strength, the cracks evolution dominates in the temperature range of 100℃-500℃, and in the temperature range of 500℃～700℃, the pore coarsening effect dominates.