Low frequency elastic wave diagnosis method for detecting the internal defects of concrete using metal magnetostrictive devices

For detecting the defects, existing in the concrete structures, a method using low frequency elastic wave and vibration has been proposed. The method employs metal magnetostrictive devices for generating and sensing low frequency acoustic signals ranging from 1 kHz to 10 kHz in the audio frequency band, which enables less attenuation and scattering in concrete. The chirp signal is used for exciting the harmonic oscillations of target structures. As a result of this excitation, flexural vibration arises at the surface structure of the shallower defects, and longitudinal vibration arises in the bulk between the surface and the deeper defects. The system utilizes a band of vibration energy for detecting the shallower defects, and the peaks of the frequency spectrum for the deeper defects. Thus, the depth from the surface to the defects and the location of the internal defects can be estimated quantitatively. This paper also describes a compact tool using impact response. The tool uses the magnetostrictive devices for sensing flexural vibration and quantifies the existence of the shallower defects. With the principle of the detecting algorithm, a number of results obtained at model tests and in-situ field tests in the actual concrete structures in service are presented.