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- 2015 (11) (entfernen)
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- Concrete (3)
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- Acoustic emission (2)
- Civil engineering (2)
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- Deconvolution (2)
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Time reversal techniques are used in ocean acoustics, medical imaging, seismology, and non-destructive evaluation to backpropagate recorded signals to the source of origin. We demonstrate experimentally a technique which improves the temporal focus achieved at the source location by utilizing deconvolution. One experiment consists of propagating a signal from a transducer within a concrete block to a single receiver on the surface, and then applying time reversal or deconvolution to focus the energy back at the source location. Another two experiments are run to study the robust nature of deconvolution by investigating the effect of changing the stabilization constant used in the deconvolution and the impact multiple sources have upon deconvolutions focusing abilities. The results show that we are able to generate an improved temporal focus at the source transducer using deconvolution while maintaining the robust nature of time reversal. Additionally, deconvolutions costs are negligible due to it being a preprocessing step to the recorded data. The technique can be applied for detailed investigation of the source mechanisms (e.g. cracks) but also for monitoring purposes.
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.
Sealing and strengthening of the subsoil by injection is a major issue in the field of geotechnical engineering. This involves also jet grouting, which allows creating columns of grouted soil by eroding and mixing the in-situ soil with a thin cement suspension. A general difficulty of this method is to predict the column diameter and its material strength. Here, we present two-dimensional finite-difference numerical simulation results of a promising non-destructive quality assurance testing tool to evaluate the diameter of jet grout columns. This approach incorporates crosshole and downhole seismic measurements. Preliminary tests showed that this tool is applicable under real site conditions.
Sealing and strengthening of the subsoil by injection is a major issue in the field of geotechnical engineering. One commonly applied method is jet grouting, which allows creating columns of grouted soil by eroding and mixing the in-situ soil with a thin cement suspension. A general difficulty linked with this method is to predict the resulting column diameter and its material strength. In this paper we illustrate the application of a newly developed non-destructive quality assurance testing tool used to determine the diameter of jet grout columns. This approach incorporates standard crosshole and downhole seismic measurements. To demonstrate its effectiveness, we tested the new approach within two-dimensional finite-difference numerical simulations. Additional field tests showed that this tool is also applicable in real site conditions. For this purpose, three jet grout columns were produced with different process parameters in a depth between 3.0 and 10 m. The evaluated diameters were within 1.0 and 1.5 m, slightly deviating from the previously predicted range by the jet grouting contractor. Moreover, we were able to detect the base of the columns at 10 m depth with no significant difficulties. On the other hand, unsaturated, partly unconsolidated sands between ground water level and surface considerably affected the seismic data, hence complicating the detection of the top of the columns.
Jet grouting is a geotechnical method of ground improvement to increase shear strength and stiffness of soils. The method is typically used to construct in-situ geometries of grouted soil such as panels or columns. The diameter of grouted columns and its material strength depend on various process parameters and the subsurface soil properties. It is only vaguely possible to predict the final column diameter. Therefore, it is a general practice to excavate a test column and perform a visual examination. However, an excavation to control the in situ diameter is often impossible, especially under complex site conditions, such as a high ground water table. Therefore, as part of a research project, borehole seismic measurements (crosshole, downhole and tomography) were tested as a quality control to verify the extent of the column and to monitor the influence of the jet grout injection on the soil over time. The field surveys were conducted before and after the jet grouting process at different time intervals. The acquired seismic data show clear traveltime differences which allow the determination of the specific column depth and diameter. The tomogram measured in the natural soil and the tomograms of the measurements after the injection process were used to visualize the time dependent effects of the jet grout injection on the soil.
Challenging new constructions and ageing infrastructure are increasing the demand for permanent monitoring of loads and condition. Various methods and sensors are used for this purpose. But the technologies available today have difficulties in detecting slowly progressing locally confined damages. Extensive investigations or instrumentations are required so far for this purpose. In this study we present new sensors and data processing methods for ultrasonic transmission, which can be used for non-destructive long term monitoring of concrete. They can be mounted during construction or thereafter. Larger volumes can be monitored by a limited number of sensors for changes of material properties. The principles of ultrasonic transmission and influencing factors are presented. This latter include load, damages as well as environmental parameters as temperature or moisture. Various methods for data processing, e. g. coda wave interferometry are introduced. They allow the detection of very small changes in the medium. The embedded sensors are shown including mounting and operation. Application examples so far include small scale laboratory freeze-thaw experiments, localizing loads in larger concrete models, monitoring load effects on real structures as well as detecting acoustic events. Some sensors are operating already for several years. The sensors can be used as transmitter or receivers or switched between both roles. While most of the previous experiments have been active (at least one sensor serving as transmitter), new studies show that the sensors are useful as well for passive measurements, e. g. in acoustic emission or time reversal experiments. Besides application in civil engineering our setups can also be used for model studies in geosciences.
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.
The legend continued: The International Symposium NonDestructive Testing in Civil Engineering (NDT-CE) had returned to Berlin. The Bundesanstalt für Materialforschung und -prüfung (BAM) and the Technical University of Berlin (TUB) hosted this prestigious event in the industrial landmark building Peter-Behrens-Halle and the Test Site Technical Safety (TTS) in Horstwalde in September 2015. Almost 300 participants from more than 35 countries gathered to present recent research, exchange knowledge and to lest the newest equipment in NDT-CE. This symposium, held regularly in various locations all over the world, was organized for the third time in Berlin and is by far the largest of its kind.
Recently developed new transducers for ultrasonic transmission, which can be embedded right into concrete, are now used for non-destructive permanent monitoring of concrete. They can be installed during construction or thereafter. Large volumes of concrete can be monitored for changes of material properties by a limited number of transducers. The transducer design, the main properties as well as installation procedures are presented. It is shown that compressional waves with a central frequency of 62 kHz are mainly generated around the transducer's axis. The transducer can be used as a transmitter or receiver. Application examples demonstrate that the transducers can be used to monitor concrete conditions parameters (stress, temperature,
) as well as damages in an early state or the detection of acoustic events (e.g., crack opening). Besides application in civil engineering our setups can also be used for model studies in geosciences.
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.