Abstract: The digital image processing was used to capture the microstructure,and the finite-element model including the aggregate,sand mastic and air voids was established.The simulations were performed to investigate crack evolution under monotonic loading using a bilinear cohesive zone model.Results show that the inhomogeneous distributions of the aggregates and the air voids have some influence on the stress distribution.When the damage variable (D) increases to 1,the crack initiation will occur,and there are great agreements between the crack initiating location of simulation and that of experiment.The growth rate of the crack quantities decreases with the crack propogation occuring.The bilinear cohesive zone model can simulate the damage evolution,crack initiation and propagation well.