Imaging of vertical surface-breaking cracks in concrete members using ultrasonic shear wave tomography

Surface-breaking cracks in concrete pose a significant threat to the durability of built structures. They create pathways for an accelerated moisture intrusion, and the transport of environmental and chemical agents such as chloride ions, which in turn, exacerbate reinforced concrete deterioration. Estimating the extent of crack propagation beneath the surface helps in determining the safety, durability and reliability of a structure. This information can be used to devise appropriate repair methodologies. Developments in Dry Point Contact (DPC)-based ultrasonic arrays have enabled the detection and localization of defects in concrete through high-resolution imaging. This work proposes the use of Half-Skip travel modes in the time-domain pulse-echo data to be deployed for imaging vertical cracks in concrete. In contrast to the standard Synthetic Aperture Focusing Technique (SAFT), which is widely used for ultrasonic imaging of concrete, the Half-Skip Total Focusing Method (HSTFM) configuration uses the target-scattered signals after a reflection from the back wall. The technique has been evaluated on both simulated and experimentally measured ultrasonic data on specimens fabricated with notches and cracks of different depths. Crack sizes were estimated by measuring the length of thresholded reflection signatures obtained in the images. The presented solution using half-skip modes serves as a supplementary technique to standard SAFT, in order to estimate the depth of surface-breaking cracks in concrete.