Damage Characteristics Of Concrete Under Multiaxial Loading State Study

The structural safety, durability and evaluation of damage in concrete under various loading patterns are of primary interest in every country. A simple and efficient method for the early and regular engineering measurement of damage, as well as for quality assurance during and after the construction of new structures and of reconstruction processes, is urgently required. Generally, it can be stated that the process of progressive cracking is the primary cause of the behavior of concrete observed under compression and subsequent failure. The density of fine cracking caused by the loading history is therefore a measure of material integrity, and is defined in the present work as damage.Firstly, the damage mechanism under various loading patterns, the theory of continuum damage mechanics and multiform damage constitutive models of concrete are investigated. Based on the damage theory by Najar, the damage model has been developed using the Simpson integral method.Secondly, the damage model of concrete under uniaxial compression has been applied to triaxial compressive state, to analysis damage evolvement of concrete under variety triaxial loading pattern, which is the most important part of the dissertation. In this paper, loading patterns include uniaxial compression, uniaxial tension, biaxial proportional compression, triaxial proportional compression and triaxial equi-compression. The damage evolution rule of concrete under triaxial and uniaxial compression shows the similar tendency. With the increase in the strain of the loading history, the incremental speed of damage becomes less and less, and finally the damage approaches a stable extreme value. Experienced the same axial strain loading history, the damage value of concrete is lower than that under uniaxial compression, and the damage of concrete in axial direction is reduced gradually with the increase in lateral pressures, which means that the lateral pressures can reduce the damage effectively. In addition, under triaxial equi-compression loading history, the damage of concrete is found to be in linear dependence relationship with the increase of axial strain, which means that kind of loading pattern could develop the compressive feature of concrete adequately.Finally, the relationship between the damage and the peak strain under the condition of triaxial proportional compression is investigated. It is concluded that the damage of concrete subjected triaxial proportional compression can be determined by the peak strain of the loading histories conveniently and accurately.