The manner in which medium-frequency ultrasonic pulses travelling through concrete are generated, received, digitized and analysed is described. Due to the highly attenuating nature of this medium and its differential effects on the frequency composition of broad-band signals, signal analysis was performed by partitioning the signal into discrete windows in the time domain, corresponding to the emergence of individual wave phenomena within the medium. These windows were then transformed to the frequency domain for subsequent filtering and interpretation. Experimentation combined with theoretical modelling has shown that the appearance and decay of discrete frequency bands depends on both the composition of the concrete, termed the resonance phase, and its external importance with respect to the ultrasonic inspection of concrete and other such inhomogeneous materials.
Several experiments are described in which ultrasonic signals were transmitted and received through concrete containing steel rods and cables. Details of how the signals were processed to extract information relating to the condition of the steel itself are presented. In particular, the method deconvolution is employed in a manner which allows the impulse response and frequency response of the embedded steel alone to be uniquely isolated from the effects of the surrounding concrete, whilst still allowing the transducers to be mounted on the surface of the concrete.
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