The Current State of Active Time-Reverse Imaging: Defect Localization in Digital Concrete Physics

Ensuring the structural integrity of concrete, particularly in critical infrastructures like bridges, requires reliable methods for identifying and localizing small-scale defects, such as fractures or damage. This study introduces a non-destructive evaluation technique that leverages the principles of both active ultrasonic testing and passive seismic methods to enhance defect localization. The method, active time-reverse imaging (A-TRI), compares ultrasonic waveforms recorded from concrete specimens in their intact and altered states to pinpoint damage.

Unlike conventional time-reverse imaging approaches that primarily rely on passive signals, A-TRI utilizes an active ultrasound source to generate wavefields. These wavefields propagate through the medium, with multiple receivers capturing the resulting signals. The experimental workflow includes two main steps: (1) Ultrasonic waves are emitted into the concrete specimen from an active transducer while the signals are recorded by an array of receivers; (2) The experiment is repeated under identical conditions, but the concrete specimen includes a predefined defect in the case of one or more arbitrary localized inclusions. The differential signal—representing the changes introduced by the defect—is then reversed in time and used as the input for a subsequent simulation. This backpropagated wavefield converges at the location of the damage, effectively visualizing the defect.

The numerical experiments in this study utilize a heterogeneous concrete model with inclusions that mimic localized changes in material properties. We highlight the current limitations of the method, including scatter size and shape, the number of receivers, and signal length, among others. Imaging conditions are applied to evaluate the precision and success of defect localization.

The results demonstrate the capability of A-TRI to accurately identify and localize multiple defects, even in complex heterogeneous media. By combining active ultrasound methods with time-reverse principles, this approach offers a robust tool for the detailed assessment of concrete structures.