Synthetic aperture imaging for flaw detection in a concrete medium

This paper investigates the performance of the Synthetic Aperture Focusing Technique (SAFT) for the detection of air void defects at various locations inside a reinforced concrete medium. Elastic wave propagation, generated by the Impact-Echo method is simulated in two dimensions, using the Finite Difference in Time Domain (FDTD) method. Waveforms received at various receiver locations on the surface are cross-correlated with a reference point scatterer response function to form an interior image of the concrete medium. Traditional SAFT employs the scattered compressional wave field to form an image of the interior of the medium. In this paper, we extend the conventional approach using all mode conversions of the scattered field to synthesize the image of the medium resulting in a more robust sensing algorithm.The SAFT is able to detect air voids (especially those which are linear in shape, such as long straight cracks) embedded in various locations within the subsurface of the concrete along with the steel reinforcement bars. The straight horizontal cracks are strong scatterers and appear as bright regions in the image of the medium, thereby indicating the presence of an anomaly. Simulation examples depicting a severe loss of structural integrity due to the lack of a bond between the concrete and steel and the associated deterioration are investigated where multiple air-voids surround the reinforcement steel bars. A bright area near the anomaly is observed. Imaging of cracks, located at shallow depths close to the surface is also considered. The ones that are located very close to the surface cannot be identified very well by SAFT because the flexural resonance of the layer between the crack and the surface dominates the response. However, with increasing depth of the crack, by isolating the first arrival of the multiple reflections of the compressional wave component between the crack and the top surface, an image can be formed with SAFT. Thus SAFT qualifies as a useful diagnostic tool for structural health monitoring purposes.

► Synthetic Aperture Focusing Technique (SAFT) is used to detect damage in concrete.
► Synthetic elastic wave scattering from defects is generated with the FDTD method.
► Multiple mode conversions of the scattered waves are used for damage detection.
► This approach is found to increase robustness of SAFT as an inversion algorithm.