PROBABILISTIC ASPECTS OF FRACTURE ENERGY OF CONCRETE

A test method to determine fracture energy and strain-softening in direct tension is described. Experimental results on cylinders of equal diameter and varying length are reported. It is found that the tensile strength decreases with increasing volume while the fracture energy remains constant within the observed volume range. By means of numerical simulation, it is shown that in a direct tension test several fracture process zones appear in the initial states of cracking and that final rupture is induced by the development of only one of these fracture zones. This phenomenon has been observed experimentally by other authors. A comparatively large number (44) of identical samples were tested by using the wedge-splitting test. Half the specimens were grooved. The fracture energy of the grooved and ungrooved specimens turned out to be the same within the given range of accuracy. It was observed experimentally and simulated numerically that in grooved specimens the crack is forced to follow a ragged fracture surface which is statistically not the weakest one. In an ungrooved specimen the crack path generally diverts from the centre line and advances through weaker zones. For the formation of these skew cracks, however, more energy is consumed due to aggregate interlock. In addition, the fracture process zone observed in ungrooved specimens is generally wider.