In this paper, we propose an auto-frequency-adaptive three-dimensional (3D) ultrasonic phased-array system for visualizing internal defects in concrete. Given the high attenuation and diversity of aging concrete structures, we use a piezoelectric and laser ultrasonic system combining broadband ultrasonic transmission (up to several hundred kHz) with two-dimensional scanning reception using a laser Doppler vibrometer (LDV). This system, originally developed for imaging less attenuative metallic samples, is adapted here for highly attenuative materials like concrete. During the propagation of ultrasonic waves in highly attenuative materials, such as concrete, the high-frequency components of a broadband incident wave are preferably attenuated. As a result, the frequency components of the surviving wave change depending on the attenuation of materials. Although the remaining frequency changes depending on concrete structures, the LDV with a broad reception bandwidth can receive such waves regardless of the degree of attenuation. This enables the automatic frequency adaptation in 3D imaging. After confirming the broadband transmission, we demonstrate the 3D imaging capabilities for different attenuative materials (i.e., mortar with delamination and carbon fiber-reinforced concrete with a slit). The proposed method would be useful for improving the maintenance management of aging concrete structures with unknown ultrasonic attenuation.
Fujikawa et al. (Mon,) studied this question.