Triaxial Pressure Under Concrete Damage Model And Damage Characteristics

As it is well known, microscopic flaws are present in concrete even before loading, mainly along mortar-aggregate interfaces. Under compression, they increase in length in a stable manner until continuous crack patterns are formed at higher stresses due to mortar cracking that bridges the interfacial cracks. 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. The wider utilization of concrete under triaxial compressive stress state, for example, concrete column with spiral hoop, has necessitated a better understanding of its damage characteristics and behavior under triaxial compressive loading patterns(monotonic and cyclic). The purpose of the study is to investigate the damage behaviour of concrete under multiaxial stress state based on hypothesis that the damage is orthogonally aeolotropic in principal stress space. A concrete damage model for triaxail stress state concrete is proposed based on damage theory of Najar in the paper, and damage characteristics of concrete are also investigated.Firstly, research status of concrete damage theory is reviewed, and several damage models for concrete under uniaxial stress state are compared. In view of energy loss, based on the damage theory of Najar combined with typical concrete uniaxial compression stress-strain relations proposed by Saenz and the uniaxial tensile stress-strain relationship recommended by the actual Concrete Structure Design Codes (GB50010-2002), uniaxial compression and tensile damage model for concrete are established. Using the Gaussian integral calculation, the uniaxial concrete damage model is verified by calculation examples and simulation results of other models.Secondly, according to the Four-parameter failure criterion for concrete proposed by Ottosen, the ultimate stress of concrete under multiaxial stress state is determined, and then the enhanced coefficient of concrete strength due to multiaxial stress state is also determined. Combined with the typical constitutive relation presented by Saenz, an equivalent uniaxial constitutive relation is established. Then a composite damage variable D is defined based on the damage theory of Najar, and a damage model for concrete under multiaxial loading patterns is proposed. Gaussian integral method is used to calculate the value of damage, and the validity of the model has been verified preliminarily. The results show that the model has many advantages such as simple form, high precision and convenient for engineering application. Some concrete damage development rules about concrete under constant-unequal lateral pressure, constant-equal lateral pressure, and proportional loading are investigated from the computational results and experimental data. The damage of concrete under constant-unequal lateral pressure, constant-equal lateral pressure, proportional compression loading is developed faster than that under uniaxial compression loading. When concrete is applied proportional compression loading, the ratio of lateral pressure to vertical pressure has different influence on damage develop. The damage of concrete developed slower with the ratio increase if the ratio is unchanged. Otherwise, the damage of concrete is developed earlier than that with the unchanged ratio. Under certain pressure range, the damage of concrete under constant-equal lateral pressure is developed slower than that under constant unequal lateral pressure and proportional compression loading.Finally, the damage of concrete under triaxial repetitive proportional compression loading is studied. The concept of equivalent envelope is proposed. Then, concrete damage model is established with the idea of studying triaxial monotonous proportional compression stress state. Evolution laws of the model are the same as the experimental results conducted by other scholars, which proves that the damage model for concrete is valid.