Cohesive zone and crack-resistance (R)-curve of cementitious materials and their fibre-reinforced composites

This paper reviews the fracture mechanics descriptions of the failure behaviours of cementitious materials, e.g. concretes, and their fibre composites. Analogies are drawn from examples on ceramics and ceramic composites. The presented materials are mainly based on the author's past work and experience at the University of Sydney. The concept of the cohesive zone is identified for these materials and is found to relate to aggregate- and fibre-bridging for concretes and fibre cements, respectively. Methods to obtain the constitutive stress-displacement relation of the cohesive zone are given and discussed. By assuming the cohesive zone as a fictitious crack, and using the stress intensity factor (K)-superposition method, it is shown that the load-deflection and crack-resistance (R)-curves for any given specimen geometry and size can be numerically obtained which are in good agreement with experimental data. Except for very large size specimens in which the crack-bridging zone is much smaller than the crack length and other dimensions, the R-curve is not a material property. It is concluded that the most important and fundamental property is the stress-displacement relation of the cohesive zone for the cementitious material concerned.