Detection of Hidden Cracks in Concrete Structures Using Reverse Time Migration of Ultrasonic Echo Data

Ultrasonic echo measurements are widely used in the field of non-destructive testing (NDT). In civil engineering, concrete structures are evaluated by this technique. Currently, Synthetic Aperture Focusing Technique (SAFT), a group of migration algorithms, is state of the art in ultrasonic data processing. Reverse Time Migration (RTM), recently introduced to NDT, shows significant improvements in mapping complex structures, like vertical steps. Modelling a concrete test specimen, synthetic experiments confirm that RTM can be used to map notches and crack-like structures in concrete. With introducing heterogeneous synthetic models, the influence of the migration velocity of RTM was investigated. Furthermore experiments on real concrete structures were conducted. Thereby ultrasonic echo data is acquired on a test specimen with a known vertical notch. A commercial ultrasonic tomograph and a scanner system, developed at BAM, are used. The data from both systems is processed using SAFT and compared to results of RTM of the scanner data. Results from the SAFT migration do not reveal any information about the vertical notch, other than an indication of the lateral position. Data migrated using RTM shows the side wall of the notch and affirms its potential for such purposes. Processing the data from the commercial ultrasonic device using RTM does not show any improvements compared to the initial SAFT result. The fixed aperture of the device is not suitable for RTM. An additional measurement is performed on a second specimen using the scanner system. The specimen shows several fine cracks of which only outcrops at the sides are visible. Results of RTM indicate mapped signatures from those cracks. Their lateral positions along the specimen as well as an estimate about the crack height can be determined within the final images. This study thereby introduces the applicability of RTM for detecting cracks within concrete structures.

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