Fully non-contact, air-coupled generation and detection of ultrasound in concrete for nondestructive testing

Abstract

It is well known that liquid coupling agents, which are commonly used in conventional ultrasonic testing to couple an ultrasonic transducer to a solid specimen, cause a number of problems including inconsistency in results and slowness of the inspection. This is especially true when the specimen surface is rough, such as those in field concrete structures; here the solution involves time-consuming surface preparation to polish every single point of inspection, making it impractical to inspect field structures with conventional, contact methods. To address this issue, this thesis proposes a new, fully non-contact, air-coupled measurement setup in the mid to high ultrasonic frequencies (50-150 kHz). This advanced setup and measurement technique is evaluated by calculating the signal to noise ratio for different numbers of signal averages. In addition, the effect of the lift-off distance of the transducer over the sample is also investigated. Ultrasonic waves are generated and detected 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, phase velocity and attenuation of Rayleigh surface waves in a concrete specimen are first measured. Then, the air coupled ultrasound technique is used to detect dicontinuities such as cracks at a concrete joint and reinforcement bars in a concrete block. Also, the capability of the proposed technique for measuring depths of surface-breaking cracks using air-coupled generated Rayleigh waves is demonstrated. Since this measurement setup directly generates Rayleigh waves, most of the disadvantages in the techniques based on the impact-echo method can be avoided and thus data processing is much simpler than that in the impact-echo based techniques. The results of the measurements show that this setup is highly promising and a big advancement towards the rapid ultrasonic nondestructive testing on large-scale field concrete structures.