Composite materials such as concrete and ceramics with coarse material microstructures exhibit size-dependent fracture properties when the size of damage zone or fracture process zone (FPZ) around a crack tip is comparable to the structural or specimen size. These materials do not comply with the linear elastic fracture mechanics (LEFM), and therefore, are referred to as quasi-brittle materials. This size-dependence of fracture behaviour has been attributed to the interactions between the crack tip, fracture process zone and specimen boundaries [1,2]. It was pointed out recently that when a crack tip is close to a free boundary or a bimaterial interface, the stress/strain fields around the crack tip and associated damage zone will interact with the boundary, and the fracture behaviour of the structure is changed [1-5]. Therefore, the size effect in fracture of quasibrittle materials is in fact, due to the influence of specimen boundaries and/or bimaterial interfaces. This means that the size effect is a boundary effect in reality.