Influence of Load Duration on Shear Strength of Reinforced Concrete Members

The significance of sustained loading on the compressive and tensile strength of concrete has been experimentally verified since the 1950s and is currently acknowledged in most design codes implicitly or explicitly. However; its influence on other potentially sensitive phenomena, as for instance the case of members subjected to shear; has not yet been clearly established. Currently; there is still scanty experimental data available on long-term shear tests and they present inconclusive results to assess whether high levels of sustained load have a detrimental effect on the shear strength. To advance on the knowledge of the phenomenon, this manuscript presents the results of an experimental program on 16 reinforced concrete full-scale beams without shear reinforcement tested under varying loading rates (associated to different times to failure, from some seconds to some months). The program consists of two series: one dedicated to slender beams and another to squat members (with different potential significance of the arching action in their shear response). The results show no sign cant reduction on the shear strength for low loading rates (long times of application of load) in any of the two tests series investigated. Yet, some enhancement is observed in the shear strength for high loading rates (failures in some seconds) compared to typical test durations. The experimental results are analyzed and discussed with reference to refined measurements performed during the tests in terms of crack shape and development. These observations, together with the evaluation of the contributions of the potential shear transfer actions in the specimens, lead to a series of practical design considerations.