Experimental Study On Dynamic Strength And Deformation Of Concrete Under Uniaxial And Blaxial Action

The large concrete structures , such as bridges, safety hull of nuclear power plants,sea oil plateforms et al. are subjected to not only static loads, but also dynamic loadings such as vehicles, nuclear power, wind, wave, water flow and violence earthquake. So the research on dynamic property of concrete is key to reasonable analysis and design of these structures. Merely the fatigue strength in uniaxial tension or compression under constant-amplitude cyclic loading is given in the existing Design Code. The design result by using the fatigue strength in the Code is apt to conservative under variable-amplitude or triaxial cyclic compression loadings, but sometimes apt to dangerous in uniaxial or triaxial tension-compression fatigue loadings. Moreover, there is not the value of Ihe influence of loading rates on the behavior of concrete in the states of complex stresses. The thesis is a part of the project of National Natural Science Foundationæ¡­ailure criterion of concrete under multiaxial variable-amplitude fatigue loading. The following researches were carried out.(1) Fatigue tests of concrete under uniaxial tension and tension-compression with constant amplitude cyclic loading were carried out. The fatigue behavior of strength, stiffness, and deformation of concrete is analysed systematically. The fatigue equation is established. The empirical expressions of the total longitudinal strain and the elastic modulus are given, and the relationships between the increasing rate of the total second-stage strain and the decreasing rate of the elastic modulus with the cyclic number are obtained.(2) The tests on the fatigue behavior of concrete under uniaxial tension with variable-amplitude fatigue loading were carried out.The damage variable is defined. The law of cumulative damage and its evolution equation of concrete under uniaxial tension with constant amplitude is established, and the validity of which is verified by the test resultsof variable amplitude loading.(3) The fatigue behavior of concrete under uniaxial tension with constant amplitude cyclic loading but high temperatures is investigated based on test results. The effects of temperatures on tensile fatigue strength, stiffness, and deformation of concrete are studied. The unified fatigue formular of considering the effect of temperature is derived. Empirical expressions of the total longitudinal strain and the secant modulus are given. In addition, the relationships of the increasing rate of the total second-stage strain and the decreasing rate of the secant modulus with the number of cycles are obtained respectively.(4) The stress level, stress ratio, loading frequency and failure probability, which determine the fatigue life of concrete ,are served as the input of the neural network while the fatigue life as output. The nonlinear relations of them are described wih the structure of neural network. The training of the network is done for experimental data, and the estimation of fatigue life of simulated specimens is proceeded to examine the validity and reliability of the BP network for estimating the fatigue life of concrete. Moreover, influence of the number of trained specimens on the estimation precision of fatigue life is studied based on an actual example.(5) Fatigue tests of concrete specimens under biaxial compression while subjected to constant lateral stresses are conducted. The failure modes of concrete are aquired. The fatigue strength, deformation and stiffness of concrete in such stress states are analyzed. The fatigue equation, strain formula and the relationship between the increasing rate of the second-stage strain and the number of cycles are established.(6) The biaxial compressive tests of concrete under constant lateral stress are carried out. The effects of lateral stress and loading rate on the strength and deformation of concrete under impact loading are discussed systematically. The failure criterions of principal stress, octahedral stress and strain space are established.(7) The biaxial compre...