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Imaging of cracks and honeycombing in concrete elements

: Krause, M.; Mielentz, F.; Milmann, B.; Müller, W.; Schmitz, V.

Arnold, W.:
Acoustical Imaging. Vol.27: Proceedings of the 27th International Symposium on Acoustical Imaging : Took place in Saarbrücken from March 24th to March 27th 2003
New York, NY: Kluwer Academic Publishers, 2004 (Acoustical imaging 27)
ISBN: 1-4020-2401-0
International Symposium on Acoustical Imaging <27, 2003, Saarbrücken>
Fraunhofer IZFP ()
ultrasonic inspection; concrete

For several years BAM (Federal Institute for Materials Reserach and Testing) and Fraunhofer Institute for non-destructive testing co-operate successfully in the area of ultrasonic concrete inspection (non-destructive testing in civil engineering; NDT-CE). BAM developed the equipment using low frequency probes in array and pitch and catch arrangement, and a scanning laser inferometer, respectively. The data are evaluated at IZFP using the Synthetic Aperture Focusing Technique for 3-dimensional imaging.
In this contribution we report about the possibilities of imaging cracks in concrete. Besides the detection of cracks the evaluation of the crack depth is an important task of the inspections to decide, whether the crack reaches the reinforcement bars or is even through In case of idealised cracks (notches) there exists a relatively simple method to measure the notch depth, but in case of real cracks, containing unbroken aggregates and reinforcements, the situation is much more complicated. It could be shown, that imaging cracks with the principle of synthetic aperture is a realistic way to characterise the cracks. Results of measurements of notches and real cracks are presented.
Another non-destructive testing problem is the imaging of honeycombing and compaction faults in concrete slabs. Normally such production faults can be localised relatively simply using Radar and acoustic methods like ultrasonic thickness measurement and impact echo. But the situation is more difficult, when the concrete elements are heavily reinforced. In this case acoustic imaging using 2D scanning and 3D-reconstruction enables to detect even relatively small defects in concrete (diameter 80 mm). Examples of successful experiments at foundation slabs (test specimens) are demonstrated.
Parts of the study have been sponsored by German Research Council (DFG, Research group FOR 384) and Deutsche Bahn AG.