Air-Coupled Generation and Detection of Ultrasound in Concrete

It is well known that liquid coupling agents used to couple an ultrasonic transducer to a solid specimen cause a number of problems including inconsistency in results and slowness of the inspection. Especially when the specimen surface is rough such as those in-field concrete structures, the long surface preparation time that it takes to polish every single point of inspection makes it impractical to apply the traditional contact methods to the inspection of these structures. To address this issue, a fully noncontact air-coupled measurement setup in mid and high ultrasonic frequencies (50-150 kHz) is presented. The setup generates and detects bulk and Rayleigh surface waves in this frequency range with a sufficiently high signal-to-noise ratio (SNR), which enables performing a fast scan with a small number of signal averages. Using this setup, ultrasonic velocity and attenuation in a concrete specimen are measured. Also the possibility to detect discontinuities such as steel reinforcement bars or cracks in a concrete sample is explored.