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Recent progress in scanning laser detection of pulse-echo ultrasound in concrete

: Koehler, B.

Wiggenhauser, H. ; Bundesanstalt für Materialforschung und -prüfung -BAM-, Berlin; Deutsche Gesellschaft für Zerstörungsfreie Prüfung e.V. -DGZfP-, Berlin:
Non-Destructive Testing in Civil Engineering 2003. CD-ROM : NDT-CE, Berlin, September 16-19, 2003
2003 (DGZfP-Berichtsbände 85-CD)
ISBN: 3-931381-49-8
International Symposium on Non-Destructive Testing in Civil Engineering (NDT-CE) <2003, Berlin>
Conference Paper
Fraunhofer IZFP ()

Scanning laser detection of ultrasound has proven to be a very powerful tool to detect echoes from inhomogeneities in concrete structures (e.g. back-wall, gravel pockets, voids, tendon ducts, etc). This results from the use of a large synthetic aperture as well as from the possibility to apply synthetic aperture focusing technique (SAFT). The most important disadvantage of the method so far is the very long measurement time accounting for the relatively bad signal-to-noise ratio (SNR) of laser detection. In this paper we present an improvement of the underlying measurement strategy by optimizing the measurement point density, the number of averaging cycles, as well as the size of the synthetic aperture. In doing so the first goal was to significantly reduce the measurement time by keeping the signal-to-noise ratio in the SAFT reconstruction unchanged. As a result of the investigations a reduction of measurement time from several hours to several dozens of minutes was reached. A second goal was to further improve the SNR by introducing more acoustic energy into the specimen via the transmitting transducer. This can be done by using coded signals together with pulse compression. Up to now chirp signals have mainly been used for this purpose. In the paper various other coded signals are compared and the best choice for concrete testing is presented.