| Ultra-high toughness cementitious composites (UHTCC) is a new type of engineering material which is composed of Portland cement, fiber, water, fine aggregate, and active mineral admixtures such as fly ash, silica fume and so on. The static mechanics and durability of this material are outstanding, due to its typical characteristics of strain hardening and multiple cracking under uniaxial tension. Then it is hoped that UHTCC can be used to improve the performance of concrete structures, which are under dynamic and cyclic loads, e.g. seismic load, burials load, vehicle load, et al. As a result, it is necessary to investigate the dynamic mechanics of this material. For this purpose, this thesis took an experimental study on the performances of impact resistance and flexural fatigue of this material. Moreover, exploratory research on its fatigue crack propagation law and the theoretical basis, fracture property, was carried out based on some experiments. The research is supported by the Key Program of the National Natural Science Foundation of China (No.50438010) and the Research and Application Program of Key Technologies for Major Constructions in the South-North Water Transfer Project Construction in China (JGZXJJ2006-13). The detailed contents of this thesis are as follows(1) The impact resistance property of UHTCC was studied. The results showed that multiple cracks were generated in UHTCC impact specimens, which delayed the development of the failure crack, prevented the fragmentation, and improved the whole ability of impact resistance. Four-point fatigue bending tests were conductedto study its flexural fatigue property. Multiple cracks were produced during the fatigue progress, and the total number of cracks dropped with the decrease of fatigue stress levels, for which, a bi-linear relationship was found. Due to these multiple cracks, a ductile fatigue failure exhibited for UHTCC, and its S-N bi-logarithm relation was found to be bi-linear, which proved that the effect of PVA fibers was less obvious at low stress levels.(2) The damage mode of UHTCC under flexure fatigue was investigated. A ductile damage happened for UHTCC. The plastic tensile strain, εp, was adopted to calculate the damage quantity, D. Then a fatigue damage evolution function of UHTCC was constructed, on the basis of elastic-plasitic isotropic damage model.(3) Three-point flexural fracture tests were carried out on UHTCC specimens with a single notch, and the non-linear fracture mechanics was employed here to study the fracture property of this material. Through analysis, the double J integral, initial cracking value, JIC, and failure value, JIF, could be used to evaluate UHTCC's fracture toughness. That was, macrocacks began to develop if the energy absorbed by the plastic zone was larger than while the failure crack began to develop if that was larger than JIf. The growth of macrocracking coveraging area, AA, here was selected here as the parameter to describe the development of multiple cracks. Based on the experimental results, a simplified JR resistance curve was introduced. From this curve, a good bi-linear relationship existed between J integral and△A before the failure crack initiated, with JIc as the dividing point. Consequently, it could be considered that the energy in need for the same amount of cracking area propagation was equal during the stable developing stage.(4) Based on the Paris law of fatigue crack propagation rate, a theoretical and experimental investigation was undertaken to study the fatigue crack propagation mechanism of UHTCC. During the experiment, it was observed that multiple cracks were formed, with a random distribution of their locations, shapes and lengths. According to some discussion, Paris law was applicable for this material, with the function form as, dA/dN=C(△J)m, in which, the coveraging area of multiple cracks, A, was applied in instead of the length of a single crack, a; meanwhile, the J integral substituted for the stress singularity intensity, K. A fatigue crack propagation threshold was found in the experiment, that is to say, the cracks did not develop while the fatigue range of J integral value,△J, was smaller than the critical value,△Jth. It was found that, the fatigue crack propagation rate slowed down with the increases of PVA fiber fraction. Furthermore, the influence of PVA fiber on the propagation rate was found to become obvious with the increase of J integral.(5) The flexural fatigue tests on UHTCC/concrete composite beam (UC composite beam) were carried out to study the effects of UHTCC layer on concrete layer. The results showed that, the section deformation of the composite beam fitted to the plane section assumption for both static and fatigue loading. In addition, the depth of compression zone at higher stress levels was smaller than that of lower stress levels for the same load cycles. Good bond strength was realized and no relative slip happened under fatigue loading. During the fatigue process, several visible cracks generated on the UHTCC layer, and the number reduced with the decrease of stress levels, while, about1-3cracks were produced on the concrete layer. Due to the multiple-cracking characteristic, ductile deformation was found for UC beams under fatigue load. Three stages of deformation existed, while the deformation ability became weaker with the decrease of stress levels. Through analysis, the fatigue life of a UC composite beam was defined by the complete damage of UHTCC layer. The regression function on UHTCC's flexural fatigue damage was applied to calculate the evolution process of maximum tensile strain, and the calculated results were a little smaller than the experimental results, expecially at lower stress levels, the two results were in good agreement.
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