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Ultrasonic echo testing is widely used in non-destructive testing in civil engineering to investigate concrete structures, to measure thickness, and to locate and characterise built-in components or inhomogeneities. Currently, synthetic aperture focusing techniques are mostly used for imaging. These algorithms are highly developed but have some limitations. For example, it is not possible to image the lower boundary of built-in components like tendon ducts or vertical reflectors. We adopted reverse time migration for non-destructive testing in civil engineering in order to improve the imaging of complicated structures in concrete. By using the entire wavefield, including waves reflected more than once, there are fewer limitations compared to synthetic aperture focusing technique algorithms. As a drawback, the required computation is significantly higher than that for the techniques currently used.
Simulations for polyamide and concrete structures showed the potential for non-destructive testing. The simulations were followed by experiments at a polyamide specimen. Here, having acquired almost noise-free measurement data to test the algorithm, we were able to determine the shape and size of boreholes with sufficient accuracy. After these successful tests, we performed experiments at a reinforced concrete foundation slab. We obtained information from the data by reverse time migration, which was not accessible by traditional imaging. The imaging of the location and structure of the lower boundary of the concrete foundation slab was improved. Furthermore, vertical reflectors inside the slab were imaged clearly, and more flaws were found. It has been shown that reverse time migration is a step forward in ultrasonic testing in civil engineering.
Ultrasonic echo testing is widely used in non-destructive testing to investigate concrete structures as well as to locate built-in components or inhomogeneities. Currently, Synthetic Aperture Focusing Technique algorithms (SAFT) are used for imaging. These algorithms are highly developed but have some limitations. It is not possible to image the lower boundary of tendon ducts or vertical reflectors. We adopted a geophysical imaging technique, the Reverse Time Migration (RTM), to non-destructive testing in order to improve the imaging of complicated structures in concrete. By using the entire wavefield there are fewer limitations compared to SAFT.
In a first step, simulations for polyamide and concrete structures were performed by using a 2D acoustic finite difference code. The simulations were followed by experiments at a polyamide specimen. Here we were able to determine shape and size of boreholes with a sufficient accuracy. After these successful tests we carried out experiments at a reinforced concrete foundation slab. The reconstruction of the structure of the lower boundary of the slab was improved and vertical reflectors inside the slab were imaged clearly. These tests on polyamide and concrete showed that RTM is a step forward for ultrasonic testing. However we observed migration artifacts and difficulties in imaging 3D structures.
In a second step we implemented a 2D elastic RTM code, since for our ultrasonic measurements elastic waves are emitted. The modeling code is included in the Madagascar software package. The required computing power for performing elastic RTM is significantly high. Thus we need appropriate computer hardware to obtain meaningful results within an adequate time frame. Using our hardware RTM of an ultrasonic data set takes far too long (3 months).
We applied the elastic code to ultrasonic data acquired on a concrete specimen which contains vertical reflectors. A comparison of the acoustic RTM results with those obtained by elastic RTM showed an improvement in the image quality.
Future work includes the analysis of RTM artifacts and the expansion of the algorithm to three dimensions. Another topic to be addressed is to how to account for the size of the ultrasonic transducer arrays which we are using.
The ultrasonic echo technique is a frequently used method in non destructive testing for geometry Determination of concrete building elements. Important tasks are thickness measurements as well as the localization and characterization of built-in components and inhomogeneities.
Currently mainly the synthetic aperture focusing family of techniques (SAFT) is used for imaging. These algorithms have difficulties in imaging steeply dipping interfaces and complicated structures such as steps and lower boundaries of voids. As an alternative two geophysical migration methods, pre-stack Kirchhoff depth migration and pre-stack Reversetimemigration (RTM)were tested in this paper at a reinforced concrete foundation slab. The slab consists of various reinforcement contents, different thicknesses and two pile heads.
In a first step, both methods were evaluated with synthetic 2D data. In the second step, ultrasonic measurement data recorded with shear wave transducers on a line profile on the foundation slab were processed. The use of an automatic scanner simplified the measurements. A comparison of the geophysical migration results with those of SAFT shows, in particular for RTM, a significant improvement in the Imaging of the geometry of the foundation slab. Vertical borders were reconstructed and the location and structure of the lower boundary of the foundation slab were reproduced better. Limitations still exist in imaging the piles below the slab.