Study on non contact acoustic imaging method for concrete structures

The hammer method is widely used for the inspection of concrete surface of tunnels or bridge girders. With this method, however, it is difficult to inspect the places where people cannot reach. Therefore, we are researching about the non-destructive inspection method by non-contact acoustic method for a concrete structure. This is the technique of having combined a scanning laser Doppler vibrometer (SLDV) and a long range acoustic device (LRAD). It is suitable for searching a comparatively narrow region precisely taking advantage of plane excition ability of LRAD, and the two-dimensional scanning capability of SLDV. Although the directivity of the LRAD itself are excellent in 1 kHz with ±15 degree, there are problems, such as angular dependence to the measurement surface and environmental noise to the circumference at the time of considering practical use. Then, the combination of a strong ultrasonic sound source and a single point LDV as the 2nd construction of the non-contact acoustic inspection method was examined. From the experimental results, it became clear that the prototype sound source can generate sound pressure for non-destructive inspection of concrete even with a 5 m distance. Therefore, the strong ultrasonic sound source will occupy a very important position in this method in the future.