NDE of two-layered mortar samples using high-frequency Rayleigh waves.

The Spectral Analysis of Surface Waves (SASW) is a popular technique in seismics for imaging the ground subsurface. It uses the dispersive properties of Rayleigh waves in a transversely homogeneous, multilayered medium. The SASW approach is being transposed into the civil engineering domain to characterize subsurface damage in concrete structures. Such a damage consists in a few millimeters thick surface layer with porosity slightly higher than in the sound material. It is induced by contact with moisture or chemicals at the surface of the structure and may facilitate penetration of aggressive agents. In this study, two-layered mortar samples are made to mimic concrete cover damage in real structures. The dispersive behavior of Rayleigh waves arises when the wavelength is comparable to the thickness of the first layer. Given the small thickness of this layer, it requires increasing the frequency up to several hundreds of kHz, which means high attenuation and low signal-to-noise ratio. Rayleigh waves with 0.5 MHz central frequency are generated into the samples by the wedge method. Phase velocity dispersion curves are obtained by broadband phase spectroscopy from signals received at various distances from the source. The signal processing is first validated on simulated signals with known dispersion law. Then, the measured dispersion curves are compared with the theoretical curve for a two-layered medium, following Haskell's approach. The measured curve displays the general behavior expected from theory. However, a three-layered, visco-elastic model would be necessary to get a better fit and to estimate more accurately the parameters of each layer.