Detection of defect in concrete slab using Rayleigh waves

Concrete is a heterogeneous cement-based material and when ultrasonic waves propagate through it, the waves exhibit a high degree of attenuation and scattering. This makes ultrasonic damage detection in concrete a difficult task. However, despite these difficulties, evaluation of concrete structures, using ultrasonic compressional waves, is quite popular and widespread. In this paper, we explore the feasibility of detection of near-surface damage such as radial cracks emanating from rebars, using Rayleigh waves. For this purpose, wedged ultrasonic transducer elements, having a center frequency of 100 kHz are used. An artificial delamination, originating from a rebar, is embedded inside a laboratory based concrete slab, in order to simulate a radial crack, approaching the slab surface. The Synthetic Aperture Focusing Technique (SAFT) is used for image reconstruction, using the scattered surface wave field data, acquired on the slab surface. Two operations are performed for combining the SAFT images corresponding to individual sources: a) the Summation approach and b) the Multiplication approach. The results show that the Multiplication based algorithm generates better results in comparison to the conventional Summation algorithm. The difficulties, associated with Rayleigh wave imaging of the near surface defects, are also discussed in the paper.