FRACTURE PROCESS ZONE PRESENCE AND BEHAVIOR IN MORTAR SPECIMENS

A general consensus exists in relating fracture behavior of cement-based materials to a fracture process zone that develops around and ahead of the main crack tip. Nevertheless, there remains some disagreement as to the nature and dimensions of the process zone. Some authors refer to the process zone as the main crack and its branches, while others define it as a zone of microcracks around the main crack. Density and distribution of increased microcracking around the crack have not yet been directly observed and documented. Paper presents experimental evidence of the process zone occurring as a cloud of unconnected microcracks around and ahead of a Mode I macro-crack. Results show that, as the loading increases, a long and narrow process zone develops ahead of the main crack tip. Its length is at least 15.2 cm (6 in.), and its width is about 2 cm (0.8 in.) during the main crack movement. As the load further increases and the main crack starts moving, the process zone widens and reaches a width of up to 6 cm in its wake. The measured changes in the characteristics of the process zone (i.e., its width and micro-crack density) have been related to the change in specimen toughness. Influence of specimen geometry on the process zone dimensions and specimen toughness has also been assessed. Microcrack data were collected through direct material observation by means of a scanning electron microscope.