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Organisationseinheit der BAM
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Eingeladener Vortrag
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Im Jahr 2011 hat der Bundestag in Deutschland den Ausstieg aus der Kernenergie beschlossen. Der angefallene Abfall wird mehr als hunderttausend Jahre eine derartig hohe Aktivität besitzen, dass er eine erhebliche Gefahr für nachfolgende Generationen darstellen kann. Dieser radioaktive Abfall soll in tiefen geologischen Formationen (z.B. Salz, Ton oder Kristallin) sicher endgelagert werden. Die Endlager werden mittels sogenannter geotechnischer Abdichtbauwerke (Schachtbzw. Streckenverschlüsse) verschlossen, wofür u.a. Zement- oder Sorel-basierte Baustoffe (Betone) in Frage kommen. Die Beschreibung des Dichtvermögens dieser Bauwerke ist unabdingbar. In der Forschungsphase werden verschiedene Werkstoffe untersucht und Methoden für die zerstörungsfreie Untersuchung der Bauwerke als Mittel der Qualitätsprüfung entwickelt. Hierbei ist der Nachweis der Rissfreiheit bzw. die Detektion möglicher Risse ein vorrangiges Thema. In den letzten Jahren konnten bereits umfangreiche Erfahrungen mit der Untersuchung von in-situ-Versuchsobjekten in Realmaßstab gesammelt werden. Die Messungen wurden im Endlager für radioaktive Abfälle Morsleben am in-situ-Versuch “Abdichtbauwerk im Steinsalz” und in der Grube Teutschenthal am Großversuch GV2 durchgeführt. Es ist gelungen mit einem neu entwickelten Ultraschall-Messsystem Eindringtiefen bis ca. 9,0 m zu realisieren. Die Messungen mit dem Large Aperture Ultrasonic System in Kombination mit der Rekonstruktion-Methode Synthetic Aperture Focusing Technique zeigen, dass die zerstörungsfreie Prüfung von Abdichtbauwerken möglich und die Nutzung des Messsystems zur Qualitätssicherung bei der Erstellung der Verschlussbauwerke vielversprechend ist.
Within the project SealWasteSafe, we advance construction materials and monitoring concepts of sealing structures applied for underground disposal of nuclear or toxic waste. As these engineered barriers have high demands concerning integrity, an innovative alkali-activated material (AAM) is improved and tested on various laboratory scales. This AAM has low reaction kinetics related to a preferential slow release of the heat of reaction in comparison to alternative salt concretes based on Portland cement or magnesium oxychloride cements. Hence, crack formation due to thermally induced strain is reduced. After successful laboratory scale analysis (Sturm et al., 2021), the AAM is characterised on a larger scale by manufacturing test specimens (100–300 L).
Conventional salt concrete (DBE, 2004) and the newly developed AAM are compared using two specimen geometries, i.e. cylindrical and cuboid. A comprehensive multisensor monitoring scheme is developed to compare the setting process of AAM and salt concrete for these manufactured specimens. The analysed parameters include temperature and humidity of the material, acoustic emissions, and strain variations. Passive sensor systems based on radiofrequency identification technology (RFID) embedded in the concrete, enable wireless access to temperature and humidity measurements and are compared to conventional cabled systems. Additionally, fibre-optic sensors (FOS) are embedded to record strain, but also have potential to record temperature and moisture conditions. Part of this project aims at demonstrating the high reliability of sensors and also their resistance to highly alkaline environments and to water intrusion along cables or at sensor locations. Further technical improvements were implemented so that first results clearly show the scalability of the setting process from previous small-scale AAM experiments and particularly the high potential of the newly developed approaches.
Furthermore, ultrasonic methods are used for quality assurance to detect obstacles, potential cracks and delamination. On the one hand, both active and passive ultrasonic measurements complement the results obtained from the multisensor monitoring scheme for the produced specimens. On the other hand, the unique large aperture ultrasonic system (LAUS) provides great depth penetration (up to nearly 10 m) and can thus be applied at in situ sealing structures built as a test site in Morsleben by the Federal Company for Radioactive Waste Disposal (Bundesgesellschaft für Endlagerung, BGE) as shown by Effner et al. (2021). An optimised field lay-out identified from forward modelling studies and advanced imaging techniques applied to the measured data will further improve the obtained results. To characterise the inside of the test engineered barrier and achieve a proof-of-concept, an ultrasonic borehole probe is developed to enable phased arrays that can further improve the detection of potential cracks. Modelling results and first analysis of semispherical specimens confirmed the reliability of the directional response caused by the phased arrays of the newly constructed ultrasonic borehole probe.
Overall, the project SealWasteSafe improves the construction material, multisensor monitoring concepts and ultrasonics for quality assurance. This will help to develop safe sealing structures for nuclear waste disposal. The outcomes are particularly valuable for salt as a host rock but partly also transferrable to alternative conditions.
Multi-sensory monitoring and ultrasound for quality assurance at underground sealing structures
(2023)
Within the safety concepts of underground disposal sites of nuclear waste, engineered barriers play an important role. As these sealing structures have high demands concerning integrity, we aim at advancing the available construction materials, monitoring, and inspection techniques within the project SealWasteSafe. A specifically developed alkali-activated material is compared to classical salt concrete. A comprehensive multi-sensory monitoring scheme is used at 150-340 l specimens to monitor setting and hardening of both materials. All sensors are demonstrated to resist the highly alkaline environments. Besides cabled and wireless temperature and humidity of the materials, strain variations using fibre optic sensors and acoustic emissions are recorded over periods of at least 28 days, partly for more than eight months. After hardening of the specimens, further nondestructive evaluations using ultrasonic echo and thermographic measurements are conducted.
Preliminary results proof the suitability of the tested sensors and clearly highlight differences between the tested materials. Particularly, the newly developed alkali-activated material shows lower acoustic emission activity indicating less cracking activity. Additionally, unique ultrasonic methods will enable better images of potential internal objects and cracks at in-situ sealing structures. A largescale ultrasonic system is optimised to reliably detect objects at a depth exceeding 9 m while still obtaining a good resolution. Modelling studies show the potential of further increasing the distance between individual transducer arrays. Additionally, a new ultrasonic borehole probe using phased arrays allowing for beam focussing is constructed and tested. Laboratory measurements at a halfcylindrical concrete specimen coincide well with the previous modelling. In total, the presented safe materials, detailed monitoring approaches and ultrasonic quality assurance methods will help to obtain safe sealing structures within salt as a host rock. The concepts can partly be transferred to sealing structures in alternative host rocks and will also be valuable for non-nuclear waste repositories.
Entwicklung einer Bohrlochsonde für Ultraschalluntersuchungen an Abschlussbauwerken in Endlagern
(2021)
Für die Qualitätssicherung von Abschlusswerken für Endlager wird eine Ultraschall-Bohrlochsonde entwickelt. Mithilfe dieser Sonde, die aus einer Vielzahl von einzelnen koppelmittelfreien Ultraschall-Punktkontaktprüfköpfen besteht, soll die Rissfreiheit der Versuchsbauwerke überprüft werden.
Ein erster Prototyp der Sonde wurde mit einem kommerziellen Ultraschallgerät betrieben und bestand aus 12 Transversalwellen-Punktkontaktprüfköpfen von denen jeweils sechs als Sender und sechs als Empfänger parallelgeschaltet waren. Um den erzeugten Schalldruck der Bohrlochsonde zu steigern, wurde bei der Neuentwicklung die Prüfkopfanzahl erhöht und zusätzlich die laufzeitgesteuerte Anregung der einzelnen Prüfköpfe vorgesehen. Für die Anregung der Prüfköpfe mit programmierten Zeitverzögerungen wurde ein neuartiger mehrkanaliger Sender für bipolare Rechtecksignale entwickelt.
Durch die Entwicklung der Bohrlochsonde soll eine dynamische Anpassung der Prüfkopf-Apertur sowie eine Fokussierung des Schallfeldes in Abhängigkeit der zu untersuchenden Tiefe erfolgen. Mithilfe entsprechender Anregungsfunktionen kann das Schallfeld der Bohrlochsonde unterschiedlich geformt werden, z. B. ist auch ein Schwenken des fokussierten Schallbündels möglich. Dadurch wird das erfasste Prüfvolumen begrenzt und auf diese Weise das Signal-Störverhältnis der Empfangssignale verbessert. In Kombination mit SAFT-Rekonstruktionsrechnungen wird durch eine optimierte Abstrahlcharakteristik der Sonde mit einem hohen Schalldruck unter verschiedenen Winkeln eine verbesserte Signalqualität und damit eine erhöhte Aussagesicherheit der Ergebnisse in der Objektabbildung erwartet.
Muon Tomography of the Interior of a Reinforced Concrete Block: First Experimental Proof of Concept
(2021)
Quality assurance and condition assessment of concrete structures is an important topic world-wide due to the aging infrastructure and increasing traffic demands. Common topics include, but are not limited to, localisation of rebar or tendon ducts, geometrical irregularities, cracks, voids, honeycombing or other flaws. Non-destructive techniques such as ultrasound or radar have found regular, successful practical application but sometimes suffer from limited resolution and accuracy, imaging artefacts or restrictions in detecting certain features. Until the 1980s X-ray transmission was used in case of special demands and showed a much better resolution than other NDT techniques. However, due to safety concerns and cost issues, this method is almost never used anymore. Muon tomography has received much attention recently. Novel detectors for cosmic muons and tomographic imaging algorithms have opened up new fields of application, such as the investigation of freight containers. Muon imaging also has the potential to fill some of the gaps currently existing in concrete NDT. As a first step towards practical use and as a proof of concept we used an existing system to image the interior of a reference reinforced 600 kg concrete block. Even with a yet not optimized setup for this kind of investigation, the muon imaging results are at least of similar quality compared to ultrasonic and radar imaging, potentially even better. The data acquisition takes more time and signals contain more noise, but the images allowed to detect the same important features that are visible in conventional high energy X-ray tomography. In our experiment, we have shown that muon imaging has potential for concrete inspection. The next steps include the development of mobile detectors and optimising acquisition and imaging parameters.
Ultrasonic methods are used in concrete investigations since decades. While being limited to transmission testing in the laboratory for a while, in-situ echo measurements for structural investigations and condition assessment have made their way into practical application in the past 20 years. However, several challenges remain. On one side, there are technical issues as limitations in depth of penetration, resolution and imaging capabilities. On the other side there are still gaps in validation, standardization and certification, which are limiting the applicability in condition/load capacity assessment.
This review reports a couple of developments which will help to overcome these issues. This includes technical developments as new devices which are easier to handle on site or giving a much deeper penetration depth (e.g. the LAUS device at BAM) as well as improvements in imaging by hardware update (e. g. air coupled ultrasound or coded signals) or new software (e. g. RTM imaging). To foster the application in real world projects we are as well working on standardization by developing new reference specimen with international partners which will ensure world-wide comparability of ultrasonic and other methods and quality assurance codes. Further, non-destructive methods are being used to update probabilistic models used for the reassessment of existing structures to support the structural engineer’s decisions.
Die Coda transmittierter Ultraschallsignale in Beton enthält Informationen zu Struktur und Zustand des Materials bzw. deren Änderung. Relevant sind u. a. Temperatur, Stress, Feuchtigkeit und verschiedene Schädigungsmechanismen. Die zahlreichen Einflussfaktoren sind nur schwer zu trennen und die Messeffekte oft zu Beginn sehr klein.Die CWI ermöglicht die Detektion kleinster Geschwindigkeitsveränderungen durch den Vergleich, der in kleinen Schritten gestauchten oder gestreckten Signalen mit eine Referenzmessung. Andere Indikatoren für Änderungen im Material sind:
• Kreuzkorrelation
• Laufzeit direkte Welle
• Energie/Amplitude
• Änderungen im Frequenzspektrum
Die beschriebene Technik wird von uns bisher primär in klein- und großskaligen Laborversuchen genutzt. Die Forschung konzentriert sich auf die Untersuchung und Trennung verschiedener Einflusseffekte. Aber auch erste reale Bauwerke wurden schon instrumentiert, z. B. Brücken und eine Tunneldecke.
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.
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.
The Large Aperture Ultrasound System (LAUS) developed at BAM is known for its ability to penetrate thick objects, especially concrete structures commonly used in nuclear waste storage and other applications in civil engineering. Although the current system effectively penetrates up to ~9 m, further optimization is imperative to enhance the safety and integrity of disposal structures for radioactive or toxic waste. This study focuses on enhancing the system’s efficiency by optimizing the transducer spacing, ensuring that resolution is not compromised. An array of twelve horizontal shear wave transducers was used to find a balance between penetration depth and resolution. Systematic adjustments of the spacing between transmitter and receiver units were undertaken based on target depth ranges of known reflectors at depth ranges from 5 m to 10 m. The trade-offs between resolution and artifact generation were meticulously assessed. This comprehensive study employs a dual approach using both simulations and measurements to investigate the performance of transducer units spaced at 10 cm, 20 cm, 30 cm, and 40 cm. We found that for depths up to 5 m, a spacing of 10 cm for LAUS transducer units provided the best resolution as confirmed by both simulations and measurements. This optimal distance is particularly effective in achieving clear reflections and a satisfactory signal-to-noise ratio (SNR) in imaging scenarios with materials such as thick concrete structures. However, when targeting depths greater than 10 m, we recommend increasing the distance between the transducers to 20 cm. This increased spacing improves the SNR in comparison to other spacings, as seen in the simulation of a 10 m deep backwall. Our results emphasize the critical role of transducer spacing in achieving the desired SNR and resolution, especially in the context of depth imaging requirements for LAUS applications. In addition to the transducer spacing, different distances between individual sets of measurement positions were tested. Overall, keeping the minimal possible distance between measurement position offsets provides the best imaging results at greater depths. The proposed optimizations for the LAUS in this study are primarily relevant to applications on massive nuclear structures for nuclear waste management. This research highlights the need for better LAUS efficiency in applications such as sealing structures, laying the foundation for future technological advances in this field.