Ultrasonic imaging of concrete using scattered elastic wave modes

Two different techniques for ultrasonic imaging of concrete are presented, namely a weighted sum technique and a statistical technique. The techniques utilize both the scattered elastic compressional and the mode converted shear wave fields. Since the conventional imaging techniques utilize either the compressional or the shear wave mode to generate the subsurface image, information contained in the scattered ultrasonic field is partially utilized. The proposed techniques intend to use more information contained in the scattered field leading to an improvement in the final image. Synthetic full matrix data is generated using the finite difference in time domain scheme for simulation of elastic wave propagation in concrete with embedded rebars and aggregates. The proposed imaging techniques are then used to generate the subsurface image of the medium. Experiments were also performed to generate full matrix data on two concrete slabs with different geometry, sizes and distribution of rebars. The applicability of the proposed imaging techniques is demonstrated on the experimentally acquired data. The images when compared to the ones generated by the conventional single mode approach, show improvement in resolution and distinction between the embedded features, e.g. closely spaced rebars. It is also observed that the weighted sum technique performs slightly better in comparison to the statistical technique with respect to suppression of background noise.