Crack propagation in plain concrete

The propogation of existing cracks in plain concrete under static loading is influenced by the properties of the cement paste matrix and the aggregates. Initial cracks are assumed to exist in the matrix and at the matrix-aggregate interface. Crack propogation through the matrix occurs in two stages: slow growth and rapid propagation. A hypothesis for the fracture of concrete is presented. In the energy concept of fracture mechanics of homogeneous material the fracture toughness or material resistance to crack propogation is expressed as the stress intensity factor for the region near the crack tip at the onset of rapid crack propagation. A pseudo-fracture toughness is obtained for concrete when it is analyzed as a homogeneous material. The pseudo-fracture toughness is equal to the summation of the fracture toughness of the cement paste and an arresting action that is developed by the aggregates. An experimental investigation was undertaken to provide an evaluation of the hypothesis. Five mortar and five concrete mixes were used. The variables included the water-cement ratio; the sand-cement ratio and the gravel-cement ratio. Five notched flexural specimens for each mix design were tested at 28 days. A continous record of applied load vs. tensile surface deformation was obtained during each test. The load-deformation records indicated that the behavior of the notched specimens under loading included four stages; the "linear" stage, "microcracking" stage, "slow growth" stage and "fracture" stage. Analysis of the test data indicated that the pseudo-fracture toughness varied directly with the coarse aggregate content. The range of water-cement ratio and fine aggregate content investigated had negligible effect on the pseudo-fracture toughness. Suggestions are included for further studies of crack propagation in plain concrete. These studies would include investigations of the variables that affect the fracture toughness of cement paste and the crack arresting action of aggregates. It is anticipated that these would greatly extend the scope of the findings of the present study of concrete fracture.Illinois Highway Division 64/08; Comm Department Bureau of Public Roads 64/0