ABSTRACT
The heterogeneous nature of cementitious composites introduces an internal length scale which leads to size dependent fracture behaviour. In this chapter a review is given of size and shape dependent fracture behaviour of plain concrete and mortar under compressive and tensile loadings. The approach is rather fundamental, in the sense that physical mechanisms underlying softening are studied. Uniaxial compression tests on prisms with varying height indicate that fracturing in compression is a localised phenomenon, and may be described with a compressive fracture energy concept. Recent results show, however, that boundary condition effects may have a significant effect on the fracture response in compression. In tension the softening can be explained from crack interface bridging near aggregates. It is shown that the post-peak carrying capacity depends on the size of the aggregates in the concrete mix. A crack interface bridge consists of two overlapping cracks with an intact ligament in between. The (ftexural) failure of this ligament is a very stable process and may explain the long tail of the softening diagram. Moreover, this type of crack interface bridging has been observed in a variety of materials and scales over nine orders of magnitude. Flexural crack interface bridging seems to be a universal fracture mechanism in brittle heterogeneous materials, and may be responsible for the observed size effects in structures.
