Construction Of Image-based Meso-scale Concrete Models And Investigation Of Dynamic End Friction Effects

Concrete is the most widely used material in engineering construction.It is paramount to oobtain an in-depth understanding of its mechanical properties for better design and safety evaluation of cooncrete structures.The frequent occurrences of engineering fracture accidents have made people realize that research on the fracture of concrete at the meso-scale is very important.Due to experimental limitations,numerical modelling has become the common method in meso-scale studies of concrete fracture,to understand the failure mechanisms of concrete and to develop new concrete materials with higher properties.Firstly,using the pixel replacing method,meso-scale concrete models based on CT images are built to simulate the end friction effects of dynamic compressive strength under strain rates ranging 10-5?2s-1.Monte Carlo simulations are conducted for 20 image-based models.Through statistical analyses and curve fitting of the simulation results,a quantitative relation of dynamic compressive strength with end friction coefficient and strain rate is obtained.It is also found that the cracks in concrete tend to initiate in weak areas such as voids and interfacial transitional zones.Besides,with the increasing of strain rate,the cracks move closer to the center of the specimen.Secondly,a new method,called background point array method,is developed,by taking pixel points as the background and determining material properties according to the number of pixels in the elements.The method is implemented in a Matlab program and used to build meso-scale concrete models from CT images of rectangular and circular specimens with the shape and distribution of aggregates well reproduced.Finally,the above meso-scale concrete models built by the background point array method are used to model complicated crack propagation by inserting nonlinear cohesive element.Direct tensile tests of 20 CT image-based models and one Brazilian disk splitting test are simulated.It is found that the failure mode of direct tensile tests is generally a macroscopic crack passing the pores and being perpendicular to the tensile force;and the crack of the Brazilian disk is generally located in the center of the specimen along the direction of force.