Radiation of Surface Waves into Concrete by Means of a Wedge Transducer: Design and Optimization

Spectral Analysis of Surface Waves (SASW) has been used for two decades to determine the stiffness profile of soils. The application of this technique is currently evaluated for inspecting subsurface damage in concrete structures in the 100 kHz frequency range. Accurate measurements of phase velocity and attenuation require an appropriate source to efficiently generate wideband Rayleigh waves. This paper presents the design and testing procedure of a variable angle wedge transducer. The performance of this transducer is evaluated in terms of the peak amplitude and frequency of the transmitted signal, and it is compared with the performance of the fixed angle wedge transducer used so far. In mortar, the new design is slightly more efficient than the previous one. In other materials (steel and Plexiglas), it allows to generate Rayleigh waves, whereas the fixed angle transducer is completely inefficient. The newly designed transducer can thus be considered as a suitable source of Rayleigh waves for a wide range of materials. Further optimisation requires the wave path in the wedge to be minimized. Introduction: This paper presents the design and evaluation of a variable angle wedge transducer. This transducer is intended to be used to investigate subsurface defects in the concrete cover of civil structures. The testing technique is the Spectral Analysis of Surface Waves, or SASW (Krstulovic-Opara et al., 1996). It consists in transmitting pulsed Rayleigh waves along the surface of the structure and, by picking up the signals received at several locations, to evaluate the phase velocity dispersion. As the penetration depth of the Rayleigh wave is proportional to the wavelength, this measurement provides results related to the elasticity and damage profile as a function of depth. As deterioration of concrete is due to penetration of chemical agents (moisture, CO2, nitrates, sulfates,...) through the cover, such information is of major importance to predict the long-term durability of the structure. The implementation of the SASW technique in heterogeneous materials, like concrete, requires efficient generation and reception of Rayleigh waves. In our application, the subsurface area to be investigated is a few centimeters deep, so a frequency range from 150 to 700 kHz approximately is required to achieve sufficient spatial resolution. Several excitation sources can be used: impact sources, point-contact, comb and wedge (or angle beam) transducers (Viktorov, 1967). Impact sources, such as the impact hammer (Cho and Lin, 2001), are not repetitive and limited to a lower frequency range. In addition, the level of spurious signals associated with the generation of volume wave is high. Dry point contact piezoelectric transducers have been recently designed (Shevaldykin et al., 2003), but their frequency range is still below 200 kHz. Comb transducers are efficient sources of Rayleigh waves, but they are restricted to narrowband applications. Finally, the wedge method is the most widely employed for Rayleigh wave excitation, and it complies with the requirements of our specific application: wide bandwidth, high frequency range and high sensitivity. The principle of the wedge method is to convert the volume wave (longitudinal or shear) in the wedge into a Rayleigh wave in the tested sample, by causing total reflection of volume waves. According to Snell-Descartes laws of geometrical acoustics (Kinsler and Frey,1982), the angle of refraction of a plane wave incident with angle θ1 from the wedge material is given as: