Comparison of sensors for contactless detection of void behind concrete using stress waves

Detecting voids in pipe surroundings is essential to structural condition assessment of concrete sewer pipelines. Impact-echo is a non-destructive testing method that can be used for this purpose. This method works based on exciting the surface of concrete and using a contact-based sensor to monitor the propagation of the resulting stress waves. However, the presence of deposits and humidity inside the sewer pipe makes establishing a contact between the sensor and the pipe wall very difficult. Therefore, the goal of this study is to compare the performance of contactless sensors for this application. Specifically, we assess how microphones, laser vibrometers, and particle velocity meters support void detection. To this end, we first investigate the requirements for excitation of stress waves in the concrete in terms of impact duration and energy. Next, we suggest a data analysis method for void detection based on the difference in the acoustic impedances of concrete, sand, and air. Both numerical modeling and experimental results show the supremacy of microphones in detecting voids behind concrete. We suggest that future studies conduct in-situ experiments to explore how pipe wall reflections and noise influence the performance of a microphone in detecting voids surrounding the concrete sewer pipes.