The Influence of Concrete Nonlinear Elastic Behavior under Tensile Stresses on Longitudinal and Shear Ultrasonic Pulse Propagation

Longitudinal and shear ultrasound pulse velocity (UPV) were measured in concrete under tensile stresses. 17 beams made of 7 mixtures were tested under flexural tensile loading until failure while UPVs were recorded. At low stresses, before 30% of the maximum tensile stress, mixtures showed acoustoelastic response, with linear UPV variation as the stress increased. Beyond 30% of the maximum load UPVs started to decay, indicating the degradation of the material due to the onset and propagation of microcracks. Close to failure, mixtures presented an even steeper UPV decay, indicating the degradation of the elastic properties. Measured third-order elastic constants indicate a high acoustoelastic sensitivity of concrete under tensile stresses. The different magnitudes of the third-order parameters of the mixtures demonstrate the potential of acoustoelasticity in ultrasonic investigation of concrete. Results also proved that mechanical damage due to tensile stresses, beyond the acoustoelastic region, induce a decrease in both Lame constants.