The application of air-coupled ultrasonic systems and signal processing to the interrogation of concrete
This thesis describes the application of ultrasound to the interrogation of concrete for the retrieval of quantitative information. In particular the use of air-coupled ultrasound is applied for the first time with recent improvement in ultrasonic technology making this possible. Broadband capacitance transducers are used in tandem with pulse compression to deliver and receive ultrasonic signals with greatly improved SNR’s. Pulse compression involves the cross correlation of a chirp signal to record accurate ultrasonic time of flights. This metric is used to makes structural inferences about concrete and to compare contact and non-contact ultrasonic systems. This comparison reveals that concrete strength estimation from ultrasonic pulse velocity (UPV), alone is inaccurate. Other metrics such as aggregate content and humidity should also be considered. A study in to the effect of humidity on the UPV is presented and a correction factor obtained that normalises UPV around a humidity that could be considered normal to a temperate climate. Images of reinforcement bars embedded in concrete are presented using the pulse compression technique. Time-frequency (t-f) analysis is applied to ultrasonic chirp signals. Extensive simulation is carried out and a comparison between three different methods presented. This ensures accurate tracking of the ultrasonic chirp signals, which allows for frequency scattering to be examined. T-f analysis is then applied to real ultrasonic signals and it is shown how frequencies spectrums of received chirps can be de-noised using the Hough transform. Images of embedded defects are then presented. The Superheterodyne technique is then described and applied to concrete interrogation. Although not overly successful it is shown how energy distributions of received tone burst signals vary with time and the need for further work is discussed.