Research On Stochastic Cohesive Laws And Crack Propagation Of Concrete

Concrete is a heterogeneous mixture of coarse aggregate,fine aggregate and cement slurry.Initial imperfections such as micro-cracks and micro-holes randomly produced during molding process.All the factors above make the mechanical properties of concrete become extremely complex.Actually,due to the influence of raw materials,construction environment and test conditions,the mechanical parameters of concrete,such as tensile strength,elastic modulusdisperse widely.The tensile strength of different parts of the material is inconsistent,which leads to the irregularity of crack development in concrete,in other words,the crack propagation is random and discrete.To conclude,the properties of concrete determine the complexity and dispersion of its mechanical properties.Therefore,the stochastic and nonlinear are the important features of concrete.It is important for scientific research to studythe fracture properties of concreteconsidering the stochastic damage mechanics.Coupling stochastic damage mechanics with fracture mechanics,the expression of type I crack tip were deduced.The method of stochastic cohesive laws of concrete was proposed.Then the stochastic cohesive laws obtained were applied to the fracture of normal concrete and dam concrete.The following results have been achieved:1)Based on the parallel springstochastic damage model,damage theory was used to the concrete fracture criterion,the boundary equations of stochastic damage zone and damage area of type Icrack tip were deduced.The method is used to calculate the damage of the concrete sample for fracture toughness test.The discrete range of completely undamaged zone,damaged areas and stochastic crack propagation are obtained.Then the results from calculation were with the available experimental data,indicating a good agreement.The results obtained were verified by comparing with the existing experiment data.2)Based on the principle of dissipative energy equivalence,the energy dissipated using the parallel spring stochastic damage model and the fracture model is equal,which assume the micro-spring failure strain were a lognormal stochastic variable.Then the procedure was presented.The stochastic laws obtained were combined with the existing experiment data.Through axial tensile test,the parameters of stochastic damage model were obtained.The influence of model parameters on the stochastic cohesive laws of was studied.The shape and variance of stochastic cohesive laws were affected by model parameters.3)Based on the stochasticcohesive law calculated by this method,the crack propagation of concrete three-point bending beam with different coarse aggregate content was simulated.Through numerical method,the P-CMOD mean curve and its upper and lower limit curves were obtained with maximum aggregate particle size of 20 mm.The fracture tests with different coarse aggregates contents were carried out,and the calculated P-CMOD curves were compared with the experimental results to verify the effectiveness of the proposed method.The range of the characteristic length of concrete with 5~20mm aggregate size was studied with different coarse aggregatecontent.The results show that when the coarse aggregate content is in the range of 15% to 60%,comparison with the test results,the characteristic height of the concrete takes 5 to 7 times the maximum aggregate size.