Detection and acoustical visualization of internal defects in shotcrete structures by spatial spectral entropy for noncontact acoustic inspection method

Deterioration of concrete structures is a serious problem in social infrastructures such as tunnels and viaducts. In order to solve this problem, we have examined the method to inspect the internal defects of concrete structure in a noncontact and non-destructive manner. In our noncontact acoustic inspection method, the concrete surface is vibrated by strong airborne sound waves, 2-dimensional distribution of vibration velocity is measured by a scanning laser Doppler vibrometer (SLDV). And the internal defects (up to a depth of about 10 cm from the surface) is detected from a long distance (about 5-30m). It was made possible to detect internal defects of concrete with smooth surface in a released space such as a viaduct or a tunnel. However, in a closed space surrounded by concrete, such as an underground cavity, the S/N ratio was lowered due to reverberation from the surroundings. In shotcrete with uneven surface, the reflectance of laser light on the surface varies. And it is more difficult to detect internal defects. To detect internal defects of concrete, we have proposed the defect detection algorithm combining acoustic features (vibrational energy ratio and spectral entropy). At a measured point of internal defect, vibrational energy tends to be high and spectral entropy tends to be low. Since acoustic excitation is used, in a closed space, more intense resonance occurs in a head of SLDV due to reverberation from the surroundings. Since resonance frequency of a laser head and its frequency range can be detected by spatial spectral entropy (SSE) proposed previously, the resonance frequency peaks can be removed in data analysis. In addition, the influence of shotcrete surface was reduced by detecting resonance frequencies of internal defects on the measured surface by SSE, selecting frequency bands to analyze, and statistically evaluating healthy part by two acoustical features.