Detection and visualization of internal defects in shotcrete specimens using SSE analysis considering local noise for noncontact acoustic inspection

Noncontact acoustic inspection method has been proposed to detect and visualize internal defects in shotcrete in a non-contact, non-destructive manner. The measurement surface is vibrated by airborne sound waves, and the vibration velocity distribution is measured by a laser Doppler vibrometer. SSE analysis is an analysis using spatial spectral entropy and can detect the resonance frequency of internal defects. Shotcrete is more difficult to analyze than ordinary concrete. The reason is due to its characteristics, in particular, the unevenness of the surface. This problem is solved by performing SSE analysis after performing statistical preprocessing. Circular cavity defects with diameters of 200, 300 mm were embedded in shotcrete specimen at depths of 40, 60, 80 mm. Internal defects were detected and visualized well.