| Concrete is the most widely used material in Civil Engineering. With the development of architecture technology, many high-rise structures, marine structures, long-span bridges, and other special constructions appeared. High performance materials with high strength and good durability are needed for constructing these vital structures. However, normal concrete has moderate strength and poor durability, which cannot meet these requirements. In the past three decades, considerable attention has been paid to the development of high-strength and high-performance concrete (HS-HPC). HS-HPC can meet the requirements of the strength, rigidity and durability of modern engineering structure. So it is now widely used in civil engineering. Concrete is commonly subjected to multiaxial stress in most structures. Hence, the mechanical behaviors of concrete under multiaxial stress states are necessary to be investigated. Moreover, with the development of computing technique, the finite method is being widely used in analysis and design of concrete structures, which also requires a better understanding of realistic mechanical behaviors of concrete under multiaxial stress states. Up to now, very limited studies on the behaviors of HS-HPC under multiaxial stress states can be found in the literatures. Therefore, it is very important to study the behavior of HS-HPC under multi-axial stress states.In this research, the behavior of HS-HPC under multiaxial stress states are evaluated through experimental studies, theoretical analysis and numerical simulations. The main contents and conclusions are as follows:1.Splitting tensile test and compression shear test have been used to obtain the bond strength and shear strength between aggregate and cement paste. The influence of aggregate size and type, the water/cement ratio on the bond strength of the ITZ is also investigated. It is found that aggregate size has a great effect on the bond strength of ITZ. The bond strength of ITZ decreases with increasing aggregate size. When the water/cement ratio is lower, it could get higher bond strength of ITZ. And also, the aggregate type has a great effect on the bond strength of ITZ.2. Biaxial compression tests are carried out in this study for HS-HPC. According to the experimental results, it is confirmed that the biaxial compressive strength is dependent on the biaxial stress ratio and the uniaxial compression strength. The biaxial compressive ultimate strength of HS-HPC is greater than the uniaxial compressive strength. The highest biaxial compressive strength occurred at the stress ratio of 0.5. Under biaxial compression, the specimens failed into a series of plate-type fragments due to confinement of the lateral stress and the formation of the micro-cracks. And the inclination angle of the major cracks decreased with the stress ratio. According to the stress-strain curves, it could be found that the lateral stress has significant effect on the stiffness and ductility of HS-HPC in the direction of the major principal stress under biaxial compressive loading.3. Biaxial compression-tension tests are carried out in this study for HS-HPC. According to the test results, it is indicated that the ultimate strength of HS-HPC under biaxial compression-tension stress states is lower than the uniaxial compressive strength, and the strength decreased with the stress ratio increasing. Compared with normal concrete, it is indicated that the behavior of HS-HPC under biaxial compression-tension shows significant difference. Under biaxial compression-tension, a tensile failure mode was observed, and only one single macro crack appeared in the middle of the specimens for all stress ratios. Under biaxial compression-tension loading, the stress-strain curves were approximately linear in the pre-peak region. And the stiffness and ductility decreased with the stress ratio increasing. The HS-HPC shows more brittle than that under uniaxial compressive loading.4. Biaxial tension tests are carried out in this study for HS-HPC. According to the experimental results, it is demonstrated that the biaxial tensile strength of HS-HPC is approximately equal to its uniaxial tension strength. Meanwhile, the normalized biaxial strength decreases with the uniaxial compressive strength of HS-HPC increasing. Under biaxial tensile loading, a tensile failure was also observed. The inclination angle of the macro cracks to the direction of the major principal stress decreased with the stress ratio increasing. Under biaxial tensile stress state, the stress-strain curves were similar with that under uniaxial tension. And the elastic modulus increased with the stress ratio.5. Triaxial compression tests are carried out in this study for HS-HPC. According to the experimental results, it is manifested that the triaxial strength is also dependent on the triaxial stress ratio and the uniaxial compression strength. The triaxial compressive strength of HS-HPC is greatly improved by the introduction of the minor compressive stress, compared to biaxial compressive strength. For the specimens with fixed stress ratio between major and minor compressive stress, the highest triaxial compressive strength occurred at the stress ratio of 0.5 because of the intermediate principal stress effect. According the stress-strain curve, it is found that under triaxial compression, the increases of stiffness and ductility are significant compared to uniaxial compressive loading.6. Triaxial compression-compression-tension tests are carried out in this study for HS-HPC. The experimental results showed that under triaxial compression-compression-tension stress states, the ultimate strength of HS-HPC is much lower than the uniaxial compressive strength. As the tensile stress increasing, the ultimate strength decreased significantly. In comparison to triaxial compressive stress state, intermediate principal stress effect is not obvious for HS-HPC under triaxial compression-compression-tension. Similar to the failure mode under biaxial compression-tension loading, a tensile failure was observed for HP-HSC under triaxial compression-compression-tension loading. One single macro crack appeared in the middle of the specimens for all stress ratios, which is perpendicular to the direction of the major stress. From the stress-strain curves of HS-HPC in triaxial compression-compression-tension stress states, it is found that the application of tensile stress significantly decreased the stiffness and ductility compared to uniaxial compressive loading.7. By means of regression analysis, the strength criterion of HS-HPC under biaxial stress state is obtained. A four-parameter strength criterion is proposed for HS-HPC by modifying Ottosen criterion. Compared with the existed models, the new model shows simpler and more accurate.8. A multi-axial constitutive model for HS-HPC is developed by modifying the orthogonal anisotropic increment model for ordinary concrete. Through a program complied by matlab, it is found that both the strength and the deformation of high strength and high performance concrete could be well predicted using the predicted model.9. According to the meso analysis method, a discrete element model is established for HS-HPC using particle flow code (PFC 3D). The crack propagation mechanism and basic mechanical response of the concrete under multiaxial loads could be obtained from the simulation. The comparison of numerical and experimental results shows this method could effectively describe the failure behavior of HS-HPC under various stress state. |
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