Determination of nonlinear elastic constants and stress monitoring in concrete by coda waves analysis

For non destructive stress evaluation in prestressed concrete, we describe in this paper a promising ultrasonic method. In "ordinary" isotropic homogeneous materials such as steel, ultrasonic determination of third order elastic constants provides means for stress evaluation since half a century. Acoustoelasticity is aimed at measuring ultrasonic velocity changes at progressively increasing applied stress. In case of concrete, example of a class of materials that exhibit strong multiple scattering as well as significant elastic nonlinear response, accurate velocity monitoring is challenging. In this paper, we purpose a novel method originally from geophysics. This treatment is aimed at detecting small changes in velocity or attenuation in the earth crust, by analyzing coda waves. So called "coda wave interferometry", this technique is transposed to concrete. We show that intense scattering can be applied to robustly determine velocity changes. CWI is implemented at progressively increasing applied stress, and thereby, we extract nonlinear elastic coefficients. We show that the nonlinear behavior of concrete is around two order magnitude greater than ordinary materials such as steel or aluminum. In addition, we show how this method could be transposed in case of in situ measurements in concrete structures and outline future possible applications.