An international research team led by Yoshikazu Ohara from Tohoku University's Graduate School of Engineering, in collaboration with Los Alamos National Laboratory in the United States, has made it possible to visualize hidden defects inside aging concrete infrastructure in 3D, potentially strengthening the efficiency of infrastructural maintenance.

Details of the research were published in the journal Applied Physics Letters on Janauary 26, 2026 and has been selected as a Featured Article.

Aging concrete in tunnels, bridges, and highways has led to a growing number of structural failures and accidents worldwide, with internal structural defects, typically undetectable by the naked eye, being responsible more often than not. Since visual inspection and hammer-sounding tests can only reveal surface defects and defects in shallow regions, structural engineers have turned to ultrasonic testing for inspections. This has been aided by the advance of ultrasonic phased array systems.

Ultrasonic phased array systems use multiple small ultrasonic transducers arranged in a single probe, allowing devices to steer and focus ultrasonic waves electronically. Originally designed for medical diagnostics, these systems create detailed cross-sectional images by sending pulses into a material and analyzing the echoes that return. Still, concrete exhibits extremely high ultrasonic attenuation, meaning the signal gets weaker the deeper you go, making it difficult for current phased-array systems to internally probe aging concrete structures.

Ohara and his team overcame this challenge by improving their previously developed PLUS system. Ohara says, "Our previous system combined a piezoelectric transmitter with a laser-based reception to create an ultra-multiple two-dimensional matrix array receiver. On this occasion, we advanced the technology by integrating a broadband transmission-reception system capable of automatically selecting the optimal frequency for inspection targets."

This "Auto-frequency-adaptive PLUS" successfully achieved three-dimensional visualization of a wide variety of internal defects in concrete.

This breakthrough makes it possible to visualize hidden defects inside aging concrete infrastructure in 3D, with the team expecting it to support the long-term sustainability of vital infrastructure. "By identifying hazardous regions that are otherwise undetectable, the technology enables repair efforts to be focused where they are most needed, improving maintenance efficiency," adds Ohara.

Publication Details:

Title: Auto-frequency-adaptive 3D ultrasonic phased-array imaging system for highly attenuative materials

Authors: Yuto Fujikawa, Yoshikazu Ohara, and Timothy J. Ulrich

Journal: Applied Physics Letters

DOI: 10.1063/5.0291949

Press release in Japanese