Crack-Depth Estimation in Concrete Elements Using Ultrasonic Shear-Horizontal Waves

Determination of the depth of surface-opening (visible) cracks is of critical importance for evaluating the structural safety of concrete elements. Accurately determining the crack depth with traditional nondestructive testing methods, however, is challenging. This study developed a new method using ultrasonic shear-horizontal (SH) waves to detect the depth of surface-opening cracks in concrete. The method is based on the diffraction of ultrasonic SH waves at the bottom edge of a crack. An off-the-shelf ultrasonic imaging device was used with dry-point contact transducers having a set of transmitters and a set of receivers performing in a pitch-catch configuration. SH waves are superior to other waves in data interpretation because the SH waves have a higher signal-to-noise ratio as the only wave type generated by their transmitter and without mode conversion after diffraction. Thus, accurately identifying the travel time of diffracted SH waves in the time domain is very achievable. The crack depth can be calculated with the two-way travel time of SH waves, the spacing between the transducers, and the SH-wave speed. The method was validated with a-finite element model and experimental data from two case studies. Results indicated that this method can significantly improve the accuracy of determinations of the depth of surface-open cracks compared with the ultrasonic longitudinal wave method.