Abstract
Concrete is a multi-phase composite material which is difficult to inspect using conventional ultrasonic techniques, including those that work well on relatively homogeneous materials such as metals. This paper summarizes recent research that makes use of signal processing techniques to overcome ultrasonic inspection difficulties in concrete. Basic findings from several new laboratory-based NDE techniques for concrete are reported. First, the application of split spectrum processing (SSP) is described. The SSP techniques obtains a frequency-diverse ensemble of narrowband signals through a filterbank and recombines them nonlinearly to improve the target visibility. Examples that demonstrate the capability of SSP to reduce coherent noise (clutter) in ultrasonic signals collected from concrete samples are presented. Next, a self-compensating procedure for practical one-sided surface wave transmission measurements on concrete structures is described. The utility of the technique is demonstrated by sensitivity to surface-opening crack depth in concrete slabs. Finally, an approach by which the setting process (stiffness change) in concrete is nondestructively monitored is described. The reflection factor of shear wave pulses at a steel-concrete interface is measured, from which the stiffness change (setting) of the concrete is inferred.