Ultrasonic imaging of concrete elements. State of the art using 2D synthetic aperture: Fid-Bau Portal

Ultrasonic imaging of concrete elements. State of the art using 2D synthetic aperture

Krause, M. / Mielentz, F. / Milmann, B. / Streicher, D. / Müller, W.

Ultrasonic 2D scanning methods in combination with 3D reconstruction by means of 3D-SAFT (Synthetic Aperture Focusing Technique) are a powerful tool for investigating concrete elements. In comparison to linear scanning and reconstruction they offer the possibility to visualise reflection and scatter versus depth in all desired directions (B-scan images), and parallel to the surface in a chosen depth (C-scan image). Thus the interpretation of the data can be more reliable than regarding only one projection. Different scanning systems are in use: Array of broad band transducers being moved over the surface, neighboured transmitting/receiving transducers, and scanning laser vibrometer as receiver. New developed point contacts transducers enable measuring without coupling agent. The different possibilities of measuring bistatic and multistatic are analysed and compared for different testing problems. The comparison of different imaging techniques is part of a co-operation in a research group of the German Research Council (FOR 384 of DFG). It has been demonstrated at foundation plates containing artificial defects, that the described imaging technique is able to localise compaction faults and honeycombing behind rather dense non prestressed reinforcement. Also the concrete cover of the lower reinforcement near the back wall could be measured. The imaging of cracks including measuring the crack depth is possible by measuring the forward scatter of ultrasonic waves at the crack tip. Here the scanning laser Doppler vibrometer acts as ultrasonic receiver. Since real cracks often contain sound bridges like unbroken aggregates and reinforcing, the crack tip can be identified as the lowest ultrasonic scatter. In order to indicate cracks going through the concrete element, the back wall echo is analysed. Grouting faults in tendon ducts were investigated with pressure-waves and shear waves. Applying shear waves, it was clearly demonstrated for the first time that the back side of a tendon duct containing tending wires is clearly imaged in case of good grouting. Since this it not the case, when there are air inclusions or voids in the duct, a qualitative parameter for indication of grouting defects is demonstrated. Analysing the intensity of the direct reflection intensity of the ducts, the localisation of artificial voids in tendon ducts by means of ultrasonic 3D imaging is feasible, but not yet reliable. Further research has to be done in order to analyse the influence of the conditions of the boundary layer and the air pores.