Fatigue of Partially Prestressed Concrete

Abstract

The fatigue behavior of partially prestressed concrete beams is investigated in a series of tests of bonded post‐tensioned T‐beams. The beams are simply supported with the same overall dimensions, and designed for the same flexural strength. The main parameter in the study is the degree of prestress (from 0.2–1.0). Eight beams are tested in fatigue: Five beams are subjected to a constant load cycle, while the remaining three beams are subjected to cumulative fatigue loading. All beams are initially cracked before the application of repeated loading. Rapid changes in cracking, deflections, and nonprestressed steel stress occur in the early stages of fatigue loading, and are followed by a stable period until just prior to failure. Cumulative fatigue loading always results in reduced life of the member. Fatigue failure is caused by successive fracturing of the non‐prestressed reinforcement; and no failure of pretensioned reinforcement is observed. Crack spacing approximately coincides with shear reinforcement spacing which is constant in the tests, indicating that the latter spacing may have some effect on spacing of cracks. Based on the test results, it is concluded that at present there is no justification for the use of complicated expressions to predict maximum crack width, and that the simple expression proposed by the CEB‐FIP Code gives satisfactory predictions. It appears that a degree of prestress between 0.4 and 0.6 is optimum for satisfactory cracking and fatigue response.