Imaging of inclusions in concrete with enhanced low-frequency ultrasound tomography

Abstract

Ultrasonic Testing (UT) is widely used to identify surface and sub-surface defects/flaws in various materials. Especially for a concrete structure, the visualization of the interior using ultrasound tomography allows for the quantitative description of defects. However, imaging error errors in concrete are inevitable due to the nature of ray-trace-based tomography and material, whose detectability can be influenced by many factors (e.g., wave propagation, the existence of aggregates, pores, heterogeneity, etc.). In order to enhance the imaging quality, a proper selection of abnormal path imaging methods based on the average ultrasonic velocity is proposed. The defective and imperfect zones in concrete can be identified by plotting intersection zones formed with multiple paths of ultrasound propagation. To illustrate the proposed strategy, both numerical and experimental analyses were conducted; furthermore, the reliability and efficiency have been confirmed accordingly. Compared with the conventional tomography method, the path optimization imaging method can effectively enhance the detectability and reduce the deployment of transducers and the number of measurements. In addition, an optimization method for determining the time of flight (TOF) is presented to obtain a more accurate arrival time for each ultrasound path with a noise outlier-based method. Low-frequency ultrasonic testing schemes were proposed to ensure sufficient ultrasonic energy penetration and reduce signal attenuation. This work offers a practical solution for high-quality ultrasonic imaging in concrete and further provides an idea for cost-effective and in-situ evaluations.