Experimental Study On Failure Criterion And Constitutive Relation Of Early-age C20 Concrete Under Biaxial Stress State

The failure criterion and constitutive relation of the early-age concrete is the basis of the nonlinear analysis on the concrete structure in the construction. However, due to the lagging theoretical research, the impreciese theory can not match the advanced algorithms in the nonlinear analysis, even there are not the ture failure criterion and constitutive relation, which will have a serious effect on the application and development of the advanced technology such as the early dismantling formwork construction. As a result, the research on the failure criterion and constitutive relation have important practical value and theoretical significance.In order to give theoretical basis for the non-linear analysis of the early-age C20 concrete structure in the most-unfavorable diathesis-based early dismantling formwork construction technology, the test of the concrete strength and deformation was carried out under the biaxial stress state. On an experimental results, firstly the failure criterion and the initial yield criterion of early-age C20 concrete materials are established, and then the subsequent yield criterion can be derived based on the non-uniform isotropic strengthening criterion, finally elastic-plastic constitutive relation is established according to the three criteria. The main research results are as follows:1. The cube compressive strength, prism compressive strength, axial tensile strength and static elastic modulus of compression can be set out in the early-age C20 concrete strength test. According to the test data, the best mathematical regression equation of the strength indicators in the test age range can be made, which will provide the theoretical basis for the mechanical parameters of the calculation.2. In the experimental basis, the difference of uniaxial compression stress-strain curve between early-age C20 concrete materials and conventional concrete materials is analyzed, and the more precise uniaxial compression stress-strain standard curve equation can be established through mathematical method, so as to provide the theoretical basis for non-linear analysis of early-age C20 concrete structures or structural members under the uniaxial state.3. According to the distribution characteristics of strength test data in the octahedral stress space, using the method of the linear fitting and mathematical derivation, the failure criterion, the initial yield criterion and the subsequent yield criterion of early-age C20 concrete materials in the octahedral stress space are established, and then the biaxial strength envelope curve of early-age C20 concrete materials in the form of principal stress is achieved on the basis of failure criterion, so as to provide the theoretical foundation for the strength analysis of the early-age C20 concrete materials under uniaxial and biaxial stress state.4. Considering the elastic-plastic constitutive theory and based on the early-age C20 concrete failure envelope, the initial yield envelope and the reference surface, octahedral stress space can be divided. Based on the location relationship in stess space according to the divided region, the two intensity envelope and the reference surface, the elastic-plastic constitutive model of the divided region is established, which the general elastic-plastic constitutive relation for early-age C20 concrete materials is made. All of this can supply the theoretical basis for non-linear analysis of early-age C20 concrete structures or structural members under the biaxial state.