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The ultrasonic echo technique is widely used in non-destructive testing for investigation and damage analysis of concrete constructions. Important applications include thickness measurements, geometry determination, the localization and characterization of built-in components as well as the detection of quality issues (cracks, honeycombing, low concrete strength).
To improve ultrasonic data imaging of complicated structures in concrete, we transferred a seismic migration technique, the Reverse Time Migration (RTM), to non-destructive testing in civil engineering. In a preliminary study, we tested a 2D acoustic RTM algorithm on measured ultrasonic echo data acquired at a concrete foundation slab. Compared to the conventional used synthetic aperture focusing technique algorithms (SAFT) for ultrasonic data reconstruction, our acoustic RTM results showed a significant improvement in imaging the interior structure of the concrete slab. Vertical reflectors were reconstructed which was not possible by traditional imaging.
In contrast to SAFT, RTM is a wavefield-continuation method in time and uses the full wave equation. RTM is, thus, able to include multiple reflections and to handle multi-pathing as well as many other complex situations. As a drawback RTM requires extensive computing power and memory capacity. Nevertheless, due to progresses in parallel processing and other computational technologies RTM has become appealing for the application in the field of non-destructive testing.
An RTM algorithm, which uses the full elastic wave equation instead of the full acoustic one (as applied in our preliminary work) has the potential to optimize the imaging results even further. This is due to the fact, that our ultrasonic measurement data are generated by exciting elastic waves. Thus, in a first step, two 2D elastic RTM algorithms were tested on synthetic ultrasonic echo data generated with a concrete model consisting of several steps and circular shaped air inclusions. In addition, two imaging conditions were evaluated to reduce migration artifacts. Our synthetic elastic RTM results showed an enhancement in imaging the features inside the test model compared to acoustic RTM and SAFT. In a second step, we acquired ultrasonic measurement data at a concrete test specimen consisting of three steps and four air filled tendon ducts. The evaluation of the real data with our elastic RTM code was also successful and the reconstruction of the geometries of the steps and tendon ducts could be improved. With our study we have shown that elastic RTM is a step forward for ultrasonic testing in civil engineering.
To improve ultrasonic imaging of concrete structures, we transferred a seismic migration technique, the Reverse Time Migration (RTM), to non-destructive testing. A 2D elastic RTM algorithm was tested on synthetic ultrasonic echo data. Compared to the typically used synthetic aperture focusing technique (SAFT) as well as our acoustic RTM algorithm, the presented elastic RTM results show an enhancement in imaging vertical reflectors and complex features inside the 2D numerical concrete model.
For decades, the low-strain impact integrity testing using a hammer blow is well established as a method of quality assurance for various pile types. However, this method has its limitations. Our research and development focuses on improving the excitation signal using a shaker system in contrast to the standard hammer method. Another approach is to increase the amount of sensors used during testing. The purpose is to identify the direction of wave propagation which gives advantages under difficult conditions, such as piles below structures.
Pile integrity testing (PIT) using a shaker system was performed on two 11 m long piles of 90 cm in diameter. While one pile was intact, the other one showed a flaw at approx. 3.5 m below pile top, which was confirmed by standard PIT in 2012. A logarithmic sweep between 500 Hz and 1 KHz of 0.1 s was used as the input signal, being vertically injected into the pile. Prior to that, simulations on similar pile geometries showed that the depth of the pile toe as well as flaws within the pile can be extracted by applying regularized deconvolution. The result is the impulse response in the time domain.
The application of deconvolution on the measured signals shows that it is possible to identify the pile length but it is more difficult to clearly extract the flaw’s position in the pile. Additional digital signal processing techniques and the improvement of the regularized deconvolution method as well as the experimental setup need to be investigated.
Another way to improve the PIT method is to use a multichannel sensor arrangement. By arranging several accelerometers vertically along the accessible part of the pile shaft, it is possible to distinguish between downward and upward traveling waves. Furthermore, it is possible to estimate the unknown wave speed, which gives the possibility of more accurate pile length calculations. The method was evaluated successfully during a measurement campaign of a slab foundation with subjacent piles. In 20 of 28 cases the pile length could be detected accurately.
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.
Advanced ultrasonic imaging for concrete: Alternative imaging conditions for reverse time migration
(2018)
Ultrasound echo is a widely used NDT technique for determining the internal geometry of structures. Reverse-time migration (RTM) has been recently introduced to NDT applications, as an imaging method for ultrasound data, to overcome some of the limitations (e.g. imaging steeply dipping reflector) experienced by the Synthetic Aperture Focusing Technique (SAFT), the most commonly used imaging algorithm for these measurements.
The standard implementation of RTM also experiences some drawbacks caused by its imaging condition, which is based on the zero-lag of the cross-correlation between source and receiver wavefields and generates high-amplitude low-frequency artifacts. Three alternative imaging conditions, developed for seismic data applications, were tested for their ability to provide better images than the standard cross-correlation: illumination compensation, deconvolution and wavefield decomposition. A polyamide specimen was chosen for the simulation of a synthetic experiment and for real data acquisition. The migrations of both synthetic and real data were performed with the software Madagascar. The illumination imaging condition was able to reduce the low-frequency noise and had a good performance in terms of computing time. The deconvolution improved the resolution in the synthetic tests, but did not showed such benefit for the real experiments. Finally, as for the wavefield decomposition, although it presented some advantages in terms of attenuating the low-frequency noise and some unwanted reflections, it was not able to image the internal structure of the polyamide as well as the cross-correlation did. Suggestions on how to improve the cost-effectiveness of the implementation of the deconvolution and wavefield decomposition were presented, as well as possible investigations that could be carried out in the future, in order to obtain better results with those two imaging conditions.
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.
Defect detection in concrete pile using impulse response measurements with sine sweep excitations
(2015)
For pile integrity inspection, a low cost and portable shaker was used to create the sine sweep signal for pile excitation. The impulse response function, calculated by the deconvolution of pile response from the sine sweep excitation, was proposed to identify the echoes in the piles due to the pile’s impedance changes. The proposed methodology has been evaluated and validated both numerically and experimentally. Based on the results from the simulations and experiments, it was found that the impulse response measurement with sine sweep excitation could be an effective tool to detect the echoes of the pile toe and the defects in the pile.
Low strain pile integrity testing is an established method in QA of foundation piles. The technique is very effective and well accepted for larger flaws and length determination. It is part of standards and recommendations. Challenges exist for more complicated structures (e. g. pile walls, diaphragm walls, and pile under structures) and small flaws. Interpretation is subjective in many cases. Possible solutions, e.g. the use of vibrators instead of a hammer or application of several sensors instead of one have been proposed decades ago, but not used much in practice. In several projects we are working on the extension and optimizations of these ideas, based on input from other engineering disciplines. In the frame of the project PileInspect we are working with an international consortium on the use of vibrators instead of a hand held hammer. This allows the full control of input signals as well as the use of (semi)automatic classification routines from machine diagnosis. At BAM we are using a low cost vibrator and deconvolution routines to improve the results of classical low strain testing. In another project we are working on multichannel measurements with sensor placement along the pile axis. This can be used to determine the travel direction of certain waves (downwards or upwards) to improve the interpretation of measurements on piles below constructions or pile walls. The use of ideas adapted from geophysics („vertical seismic profiling“) are helpful. Both concepts are proven by simulations results and first field tests.
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.