Characterization of Ultrasonic and Radar Reflector Types in Concrete by Phase Evaluation of the Signal and the Reconstructed Image

Echo methods combined with imaging evaluation allow for investigation of the inner structure of concrete members. Ultrasonic echo and impulse radar data are analysed with reconstruction calculation procedures like 3D-SAFT (Synthetic Aperture Focusing Technique) and 3D-FT-SAFT (Fourier-Transform SAFT). From the representations of the 3D-data sets (B-scans and C-scans) the location of rein- forcement bars, tendon-ducts, air voids and honeycombing can be deduced. In order to localise ungrouted areas (air inclusions) in tendon ducts or to decide whether a reflector measured with radar is metallic (reinforcement) or air-like (void), up to now the interpretation of the results is based on the intensity of the re- flection. This criterion is non unique in most cases, because the intensity of the backscattered waves is also influenced by the coupling conditions of the probes or by local variations of the concrete properties. A new approach is to take the phase information into account. The phase in- formation contained in the impulse-like data or in the complex type reconstructed image can help to decide whether the wave pulses pass from concrete to a denser material (e.g. steel) or to a less dense material (e.g. air), even layer thicknesses can be characterized. Since the evaluation of the data usually does not consider single A- scans but reconstructed data, the phase analysis has to be integrated in the recon- struction procedure. In order to investigate and improve the method, a specimen with planar reflec- tors was used containing four steel-plates of different thickness partly covered with polystyrene (symbolising air-like reflection) and the wave propagation was simu- lated by the FIT (Finite Integration Technique) method.