FRACTURE AND MULTIPLE CRACKING OF CEMENTITIOUS COMPOSITES.

Cementitious composites consist essentially of a Portland cement paste, mortar or concrete matrix reinforced with steel (primarily) or mineral fibers. Compared to fiber reinforced polymeric and metallic matrices they form in a way a different class of composites. Their primary characteristics are: (1) that the ultimate tensile strain of the matrix is much lower than the yield or ultimate tensile strain of the fibers, and (2) the bond strength at the fiber matrix interface is relatively small (there is little adhesion in the chemical sense). Attempts have been made to calculate some fracture properties of these composites, using linear elastic fracture mechanics approach, by testing notched beam specimens, notched tension specimens, or cleavage type specimens. The results have been only partly successful because these composites may not show a brittle type of failure depending upon the relative dimensions of the specimen and the length of the critical crack (in the Griffith's sense). It is concluded that for cementitious composites for which no fracture testing standards exist, specific experimental techniques to measure fracture properties and analytical approaches to interpret their values, must be developed.