Inspection of concrete using air-coupled ultrasonic pulse velocity

The ultrasonic pulse velocity (UPV) method has been used for decades to characterize concrete structures. In this method, ultrasonic transducers are attached to the concrete surface using a coupling agent. The coupling process is both time and labor intensive and, in some cases, may limit the ability to collect data. This paper describes the development of a fully contact-less (air-coupled) UPV method. By adding a matching layer between the transducer crystal and air and using signal processing methods, air-coupled through-thickness compression wave measurements in concrete are made possible. A scanning test setup was proposed and applied to a concrete test specimen that had different thicknesses and contained internal defects. The thicknesses of the test specimen represent realistic values for concrete elements. Experimental results show that defects and thickness variations within the concrete were visualized when the UPV data were presented in a two-dimensional (2D) scan image. A data interpretation algorithm was used to accurately locate the embedded defects within the test specimen.