Load effects on fracture of concrete

Failure of concrete under any type of loading is associated with the development of visible cracks and very large inelastic deformations. These cracks are the precipitation of internal microcracks under high strains. The concrete system consists of microcracks even before the application of load. These microcracks propagate under different types of loading to form failure planes with a resulting loss in structural strength leading to failure. This paper discusses the formation of microcracks in concrete under short-term and sustained loadings. Concrete specimens were subjected to constant and incremental sustained loading and the deformations observed are related to the intensity of microcracking. The results show that at equal strains, the degree of internal microcracking is considerably reduced if the sustained load is applied incrementally over a period of time rather than applied as a step. It is concluded that the development of combined bond and mortar cracks is essential to cause failure under sustained loading. The longer the time to failure the higher the intensity of internal cracking at failure.