The changes in the sonic surface wave velocity of concrete under stress were investigated in this paper. Surface wave velocities at sonic frequency range were measured on a prismatic concrete specimen undergoing several cycles of uniaxial compression. The loading was applied (or removed) gradually in predefined small steps (stress-controlled). The surface wave velocity was measured at every load step during both loading and unloading phases. Acoustic Emission (AE) test was conducted simultaneously to monitor the microcracking activities at different levels of loading. It was found that the sonic surface wave velocity is highly stress dependent and the velocity-stress relationship follows a particular trend. The observed trend could be explained by a combination of acoustoelasticity and microcracking theories, each valid over a certain range of applied stresses. Having measured the velocities while unloading, when the material suffers no further damage, the effect of stress and damage could be differentiated. The slope of the velocity-stress curves over the elastic region was calculated for different load cycles. This quantity was normalized to yield a dimensionless nonlinear parameter. This parameter generally increases with the level of induced damage in concrete. 1. Introduction To ensure the integrity of an existing structure, one needs to have a reliable estimation of in-situ material properties, the remaining strength and relevant damages in the load-carrying components. Nondestructive testing (NDT) techniques which can provide a reliable assessment of one or more of these parameters, without harming the structure itself, are invaluable to inspectors and engineers. Among the applicable NDT methods, acoustic techniques have long been used for inspection of concrete structures, both in defect detection and material characterization applications. When used for defect detection, acoustic techniques are especially powerful tools for locating the defects, which introduce an impedance discontinuity within the concrete structure (e.g., voids, cracks, and flaws). In applications concerning material characterization, their advantage lies in their ability to give a direct estimation of mechanical material properties through measuring the acoustic wave velocities. The results of acoustic tests are usually analyzed and interpreted based on the theory of elastic wave propagations in linearly elastic homogenous solids. According to this theory, the velocity of acoustic waves propagating in linear elastic materials is a function of elastic material properties
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