In this paper the dependence of the failure mode upon the material microstructure is investigated through uniaxial compression tests of hydrated cement paste and mortar samples with different fillers. Since both splitting and shear failure mechanisms can be viewed as the propagation of microcracks, the results are analysed using the criteria of macrocrack growth taking into account the non-singular terms in calculating the near-crack tip stresses. The study shows that the micro-plastic behaviour of the material controls the macroscopic fracture pattern and the axial stress-strain relationship in uniaxial compression. High micro plasticity results in a long process zone which enables the high ambient compression to direct the growing macrocracks and thus produce splitting. In microscopically brittle materials the compression is not sufficient to prevent interaction with a lateral boundary from turning the macrocrack into a shear fracture.
Rock splitting in uniaxial compression tests is usually attributed to uniform loading conditions and is associated with high rock brittleness. On the other hand, shear (oblique) fracture is supposed to be indicative of the presence of plasticity in rock behaviour (eg, Paul, 1968). Thus, understanding the mechanism responsible for the transition of one type of fracture to the other is important for rock characterization and, since similar loading conditions can occur in high pillars, for predicting failure in rock structures. Rock behaviour and failure mechanism depend on the rock microstructure. The influence of rock microstructure on internal fracture processes is complicated by the great variations in the micro- fabric from sample to sample, which makes refined tests non-reproducible. In this situation it is attractive to base the initial stage of investigation at least on physical models of rocks made from materials with controllable microstructure. Obviously, cementitious materials, eg cements and mortars with different fillings would be the first choice for such a study. The difference in the failure patterns of different types of materials under the same loading and boundary conditions cannot be simply explained in terms of confining stresses at the end of the specimens since the loading conditions were the same for all tested samples. Other researchers have observed that the formation of an inclined failure surface is a fundamental form of failure and not only a consequence of end restraint (eg, Santiago et al., 1973). Therefore, the explanation for the different phenomena goes beyond the confining pressure and should be sought in the microstructure of the material itself. Experiments (Santiago et al., 1973) show that the fracture mode of the cementitious materials depends on the composition: pure cement samples fail by splitting whereas conretes exhibit shear failure mode. It was suggested that the failure mode is determined by the grain size. In the present paper the dependence of the failure mode and the type of axial stress-strain relationship on the material microstructure is further investigated both experimentally and theoretically. The theoretical model is based on the analysis of the direction of macrocrack propagation which takes into account the non-singular terms of the near crack-tip stress concentration.
