Ultrasonic imaging of multi-layer concrete structures

Ultrasonic imaging is a powerful nondestructive evaluation technology for examining the condition of concrete structures through focused images obtained with the Synthetic Aperture Focusing Technique (SAFT). However, its application has been limited to constant-velocity concrete structures; it has not been used with multi-layer concrete structures with different wave propagation velocities, such as bridge decks with overlays. Advancements of SAFT have theoretically allowed ultrasonic imaging for multi-layer concrete structures. In this study, SAFT is combined with the Multi-Layer Delay-And-Sum (MLDAS) method to develop a data analysis method for multi-layer concrete structures. The SAFT-MLDAS method is first verified with the experimental data from a three-layer structure with point-like scatters. The method is then further examined with finite element simulation data from a concrete specimen with a 50-mm overlay, and with experimental data from a concrete specimen with a 38-mm silica fume modified concrete overlay. Results indicate that the SAFT-MLDAS method enables improved ultrasonic imaging for multi-layer concrete structures. Its application in imaging bridge decks with overlays improves the accuracy of the testing results, when the estimation of the shear-wave velocity of each layer is accurate.