Mesoscale modelling of size effect on the evolution of fracture process zone in concrete

A comprehensive mesoscopic investigation has been conducted to examine the evolution of the fracture process zone (FPZ), using notched plain concrete beams subjected to three-point bending as a generic representation. The concrete beams are modelled as random heterogeneous materials containing three components, namely coarse aggregates, mortar and the interface transition zone (ITZ). To better represent the fracture process in concrete, a coupled cohesive-contact interface approach is employed to model the crack initiation, crack propagation and the friction mechanisms during the fracture process. Thus, the FPZ is naturally captured in the simulation as the zone composed by microcracks along the ITZ or through the mortar matrix in the mesoscale model. The macroscopic response of load-deformation curves as well as the shapes and sizes of FPZ calculated from numerical results are validated against experimental observations and good agreement is achieved. Subsequent modelling results demonstrate that the FPZ tend to exhibit a main crack and multiple secondary microcracks. During the growth of the main crack, new microcracks initiate while some microcracks formed earlier stop growing and even close. The evolution path of the FPZ is strongly irregular due to the random spatial distribution of the aggregate particles with weak ITZs. The influence of the size effect on the FPZ is also investigated from the numerical simulation. Results show that the width of the FPZ is insensitive to the beam size but the length of the FPZ is strongly dependent on the beam size, and such characteristics of the FPZ are deemed to be intrinsic reasons for the overall size effect phenomenon in concrete structures.