Inferring cumulative crack widths in reinforced concrete structures using Coda Wave Interferometry of ultrasound

Abstract Whether concrete structures are cracked, how wide these cracks are, and whether they must be regarded as damaging, is a central issue in structural health monitoring. Ultrasonic measurements evaluated with the sensitive Coda Wave Interferometry (CWI) have the potential to detect cracks early—long before they become visible. This paper presents a novel method for inferring cumulative crack widths in reinforced concrete structures using ultrasonic measurements evaluated via CWI. Building on the acoustoelastic effect, a mathematical relationship is derived that links the relative velocity change of ultrasonic waves to the average reinforcing steel strain. This correlation is calibrated experimentally and embedded into an established engineering model for crack width calculation. The resulting approach enables the direct determination of cumulative crack widths—without visual access—from internal ultrasonic measurements. The method is validated on a reinforced concrete beam subjected to bending, using distributed fiber optic strain measurements and digital image correlation as reference techniques. The predicted crack widths agree closely with those obtained from conventional methods.