ABSTRACT
So far attention has been given to mode I, or tensile, fracture of concrete. This is undoubtedly the most important fracture mode, but shear can have in¸uence as well. Consider, for example, the case of bending, where along the tensile part of the beam shear forces must be active too. For a material such as concrete, and many similar materials such as rock, one could easily argue that tensile fracture will always prevail. The tensile strength of normal strength concrete is no more than 10% of its compressive strength. Because shear can be interpreted as equal-biaxial tension-compression (i.e., σ1 = –σ2), the enormous imbalance between tensile and compressive fracture strength will inevitably lead to premature tensile fracture. Shear, or mode II or III fracture, would only be possible if through some means the imbalance between the tensile and compressive strength is restored, as argued in Van Mier (1997, 2004c). In this chapter we explore fracture of concrete subjected to combined tension and shear. We distinguish two shear modes (see Figure 2.3), namely mode II or in-plane shear and mode III or out-of-plane shear, therefore we discuss both modes. In both sections, Section 7.1 devoted to in-plane shear and tension, which, in fracture mechanics terminology would be referred to as mixed mode I and II fracture, and in Section 7.2 debating torsion, or mode III fracture, the loading combination refers to the externally applied stress. The pure fracture mechanics modes, as well as the mixed-modes refer to the crack-tip loading, which may differ substantially from the external loading on the specimen or structure. This normally may lead to confusion, so, once more: the loading cases mentioned always refer to the external loading on the considered structure.
