Nondestructive Evaluation of Rebar Corrosion-Induced Damage in Concrete through Ultrasonic Imaging

Corrosion of reinforcement in concrete is a billion dollar problem spread across the globe. Early detection of corrosion-induced damage preempts weakening of the structure and timely rehabilitation extends its life. Therefore, there is a need in the industry for a reliable nondestructive diagnostic tool for detection of corrosion-induced damage in concrete structures. This paper presents two ultrasonic wave-based imaging techniques for monitoring changes in the concrete subsurface during various stages of rebar corrosion. An accelerated corrosion setup was developed to induce corrosion in a rebar embedded in a concrete slab specimen. A pitch catch mode of ultrasonic scanning was performed on a set of grid points on the test specimen using compressional and Rayleigh wave transducer arrangements. In the first approach, the traditional synthetic aperture focusing technique (SAFT) was used to produce images in the horizontal and vertical planes using the compressional wave velocity information and incorporating corrections related to limited directivity of the transducers. In the second approach, the planar SAFT algorithm was used for detection of vertical corrosion cracks, using the scattered Rayleigh wave field. The study shows that with progress of corrosion, the rebar image disappears from the compressional wave-based SAFT images while the corrosion-induced surface breaking cracks appear in the planar SAFT images. The combination of these two approaches has potential to be a powerful qualitative nondestructive technique for identification and localization of damage leading to requisite repair and maintenance activities.