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Since several years ultrasonic imaging techniques are used for testing concrete elements. Combined with a reconstruction calculation they lead to an acoustic imaging of scatterers and reflectors from the inside of the concrete element. For several testing problems it is an advantage to use a two dimensional synthetic aperture and to combine it with a three-dimensional reconstruction calculation (3D-SAFT, Synthetic Aperture Focussing Technique).
This enables the analysis of the tested volume by means of B-scans in two directions and C-scans (projections parallel to the surface), respectively. Using the 2D-scanning technique, especially the disturbing influence of the reinforcing rebars can be minimised.
In this contribution the state of the art of this method is demonstrated and analysed. Examples of the application are presented, concerning
Localisation of honeycombing and compaction faults
Characterisation of cracks (especially measuring the crack depth)
Investigation of tendon ducts
The results presented are obtained in the frame of a research initiative Non-destructive Evaluation of Concrete Structures Using Acoustic and Electromagnetic Echo-Methods supported by the German Science Council (FOR 384 of DFG, Deutsche Forschungsgemeinschaft). The methods are assessed and improved in cooperation with the partners participating in the project.
Für die Strukturaufklärung von Betonbauteilen existieren mehrere bildgebende Ultraschallecho-Verfahren, u. a. mit Nutzung des Prinzips der synthetischen Apertur. Die Ergebnisse werden mit unterschiedlichen Verfahren, mit oder ohne Anwendung von Rekonstruktionsrechnungen, ausgewertet und in der Regel als B- bzw. C-Bilder dargestellt.
Ziel ist es, die unterschiedlichen Verfahren anhand von Messreihen an Testkörpern mit bekannten Prüfobjekten in Hinblick auf Ihre Leistungsfähigkeit quantitativ zu fassen und zu vergleichen, um den Einsatz der Prüfverfahren bei Untersuchungen unbekannter Objekte zu optimieren. Dazu wurden drei Kenngrößen definiert. Als erster Schritt erfolgte die Anwendung der Definitionen auf vorliegende Messergebnisse zu einer in einem 50 cm dicken Betontestkörper parallel zur Messfläche eingebrachten Bohrung.
Non-destructive Testing of a Post tensioned Concrete Bridge Applying an Automated Measuring System
(2004)
Ultrasonic investigation is an established non-destructive testing method for detection of defects and material characterisation. In the last years scanning ultrasonic echo methods were developed especially for the application in civil engineering.
The different scanning ultrasonic echo methods are modified with respect to the type of the transducer applied, the number and the arrangement of transducers and the frequency used. Also different algorithms are used to analyse the measured data, for instance SAFT (Synthetic Aperture Focusing Technique) reconstruction. The results are typically presented as B- and C-scans, obtained from raw data or the SAFT reconstructed data.
Since 2001 several German research institutes co-operate within the scope of the research initiative Non-destructive Evaluation of Concrete Structures Using Acoustic and Electromagnetic Echo-Methods (FOR 384) supported by the Deutsche Forschungsgemeinschaft (DFG). One of the tasks is to investigate these ultrasonic echo methods for special applications like the detection of cavities in concrete structures or faults in tendon ducts.
Concrete test specimens with intentional faults have been manufactured and investigated. Parameters were defined to assess the results more objectively. Currently three parameters -signal/noise-ratio, lateral edge resolution and uniformity- are considered. For the first time results of different working groups on the same imaged objects are compared quantitatively. It should help to get more knowledge about the ultrasonic inspection of typical faults in civil engineering structures.