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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.
After successful laboratory analysis on the cm-scale, cylindrical and cuboid specimens on the m-scale are produced from classical salt concrete and a specifically developed alkali-activated material. A comprehensive multi-sensory monitoring scheme is applied to compare the setting process of both materials and to demonstrate the sensors’ resistance to highly alkaline environments. Besides 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 8 months. Passive sensor systems based on radiofrequency identification technology (RFID) embedded in the concrete allow for wireless access to temperature and humidity measurements and are compared to conventional cabled systems. After hardening and removal of the outer casings of the specimens, further non-destructive evaluations using ultrasonic echo and thermographic measurements are conducted. Preliminary results clearly highlight differences between the tested materials, particularly showing lower acoustic emission activity for the newly developed alkali-activated material potentially indicating less phase changes or cracks.
Complementary, ultrasonic methods are improved to be used for quality assurance to detect obstacles, potential cracks and delamination at in-situ sealing structure scale. A unique large aperture ultrasonic system (LAUS) with depth penetration as large as 9 m has already successfully been applied at the test site in Morsleben, Germany, of the federal company for radioactive waste disposal (BGE). Modelling studies help to further optimize the measurement layout. Advanced imaging techniques applied to the modelled and measured data will further improve the obtained images of internal structures. Additionally, an ultrasonic borehole probe is developed and constructed using phased arrays to further enhance the detection of potential cracks. Modelling and preliminary results from laboratory specimens prove the feasibility and potential of the directional response even in heterogeneous material such as concrete. Final investigations under in-situ conditions at the test site of the sealing structure are planned.
Overall, the project SealWasteSafe improves the construction material, multi-sensory monitoring, and ultrasound for quality assurance to allow for the development of safe nuclear sealing structures. Although the techniques are tailored for sealing structures within salt as a host rock, they are transferrable to a wider field of applications and alternative disposal conditions.
Additive manufacturing of concrete structures is a novel and emerging technology. Free contouring in civil engineering, which allows for entirely new designs, is a significant advantage. In the future, lower construction costs are expected with increased construction speeds and decreasing required materials and workers. However, architects and civil engineers rely on a certain quality of execution to fulfil construction standards. Although several techniques and approaches demonstrate the advantages, quality control during printing is highly challenging and rarely applied. Due to the continuous mixing process commonly used in 3D concrete printing, it is impossible to exclude variations in the dry mixture or water content, and a test sample cannot be taken as a representative sample for the whole structure. Although mortar properties vary only locally, a defect in one layer during printing could affect the entire integrity of the whole structure . Therefore, real-time process monitoring is required to record and document the printing process. At the Bundesanstalt für Materialforschung und -prüfung (BAM) a new test rig for the additive manufacturing of concrete is built. The primary purpose is measuring and monitoring the properties of a mortar during the printing process. The following study investigates an approach for calculating yield stress and plastic viscosity based on experimentally recorded pressure data. The calculations assume that fresh mortar behaves as a Bingham fluid and that the Buckingham-Reiner-equation is applicable. A test setup consisting of rigid pipes with integrated pressure sensors at different positions is utilized. Monitoring the printing process with different sensors is crucial for the quality control of an ongoing process.
Ultrasonic coda wave interferometry can detect small changes in scattering materials like concrete. We embedded ultrasonic transducers in the Gänstorbrücke Ulm, a monitored road bridge in Germany, to test the methodology. Since fall 2020, we've been monitoring parts of the bridge and comparing the results to commercial monitoring systems. We calculate signal and volumetric velocity changes using coda waves, and long-term measurements show that the influence of temperature on strains and ultrasound velocity changes can be monitored. Velocity change maps indicate that different parts of the bridge react differently to environmental temperature changes, revealing local material property differences. A load experiment with trucks allows calibration to improve detectability of possibly damaging events. Our work focuses on measurement reliability, potential use of and distinction from temperature effects, combination with complementary sensing systems, and converting measured values to information for damage and life cycle assessment.
By tracing sustainability impacts along the supply chain, certification schemes and labels are crucial to unlock the potential of bio-based systems in achieving several SDGs. However, the rapid proliferation of certification schemes and labels led to questioning their effectiveness and robustness and to what extent they deliver important outcomes, in line with sustainability policy priorities. The STAR4BBS project develops indicators and a monitoring system for assessing the effectiveness and robustness of existing certification schemes and labels, applicable to biological feedstock and bio-based materials and products.
Verschlussbauwerke sind für die untertägige Endlagerung im Salzgestein als eine wichtige technische Barriere vorgesehen. Diese Bauwerke müssen hohen Anforderungen an Sicherheit und Integrität genügen, weshalb im Projekt SealWasteSafe Werkstofftechnik, Prüfverfahren und Überwachungsmethoden für sichere Verschlussbauwerke verbessert werden. Dabei zeigen die untersuchten alkaliaktivierten Materialien (AAM) langsamere Reaktionskinetik bei der Erhärtung im Vergleich zu Salzbeton, was potenziell geringere Rissbildung erwarten lässt. Die Erhärtung wird an Probekörpern (100-300 l) multisensorisch über einen Zeitraum von mindestens 28 Tagen überwacht. Die Parameter Temperatur und Feuchtigkeit werden einerseits kabelgebunden, andererseits mittels drahtloser Radio Frequency Identification (RFID)-Technik aufgezeichnet. Zusätzlich kommen Schallemissionsmessungen sowie Dehnungsmessungen mittels verteilter faseroptischer Sensorik (FOS) zum Einsatz. Die Überwachung zeigt verschiedene Charakteristika beim Erhärten der Materialien mit geringerer Temperaturentwicklung des AAM. Zusätzlich werden Ultraschallmessungen genutzt, um Einbauteile und Störstellen wie Risse und Delaminationen im Rahmen der Qualitätssicherung am Probekörper und am Verschlussbauwerk zu detektieren. Dafür werden sowohl ein Ultraschallmesssystem mit großer Apertur (LAUS) als auch eine Ultraschall-Bohrlochsonde eingesetzt. Durch Erhöhung des Schalldrucks und Schallfeldbündelung im Beton wird die Aussagekraft der von der Bohrlochsonde aufgezeichneten Messsignale verbessert, was in Modellierungen und ersten Labortests gezeigt werden kann. Die Auswertung der Ultraschallmessungen zeigt das große Potenzial der Methode mit Eindringtiefen bis zu 9 m, wobei anspruchsvolle Abbildungsverfahren zu einer verbesserten Abbildung der internen Strukturen führen. Insgesamt verbessern die im Projekt SealWasteSafe entwickelten Materialien und Methoden die Möglichkeiten für sichere Verschlussbauwerke nuklearer Endlager. Obwohl die Konzepte speziell für Bauwerke im Salzgestein entwickelt werden, sind sie partiell gut auf andere Wirtsgesteine übertragbar.
Die B1 Brücke am Altstädter Bahnhof in Brandenburg an der Havel wurde im Dezember 2019 aufgrund von während einer turnusmäßigen Inspektion entdeckten Rissen und Hohlstellen entlang der vorgespannten Längsträger gesperrt und im Mai 2021 abgebrochen. In der Zwischenzeit wurde die Brücke detailliert überwacht. Vor dem Abriss wurden zudem umfangreiche Untersuchungen zur Bestandsaufnahme und Schadensanalyse sowie Tests moderner Sensorik vorgenommen. Dabei konnte sowohl die richtige, zuvor nicht sicher bekannte Anzahl von Spanngliedern in den Querträgern sicher ermittelt werden als auch durch moderne Varianten der Schwingungsmessungen und der faseroptischen Sensorik zusätzliche Kenntnisse püber das Bauwerksverhalten ermittelt werden . In dem Beitrag werden die Verfahren mit ihren Möglichkeiten und Grenzen vorgestellt, die Ergebnissee an der Brücke in Brandenburg erläutert und zukünftige Einsatzmöglichkeiten diskutiert.
This contribution introduces an investigation of a large-scale prestressed concrete bridge model (“BLEIB” structure at the BAM-TTS open air test site) by means of on-site cooperative measurements. This bridge has an external post-tensioning system and has been instrumented with the ultrasonic transducers, temperature sensors and optical fibers for Distributed Acoustic Sensing (DAS). Our experiment was designed to test the suitability of the novel 6C sensors developed within the framework of the GIOTTO project – the IMU50. The IMU50 sensor enables vibration measurements in translation along three axes and rotation around three axes. The geophone sensors were considered for complementary measurements of vertical velocity response. In the experiment, several perturbations were achieved by controlling the external influence factors such as loading and prestressing changes. The aim of the integrated measurement strategy was to fully observe the results of the condition change and to verify the effectiveness of multiple sensors for bridge monitoring.
A novel ultrasonic borehole probe is developed for the quality assurance of sealing structures in radioactive waste repositories using existing research boreholes. The goal is to examine the sealing structures made of salt concrete for possible cracks, delamination, and embedded objects. A prototype probe uses 12 individual horizontal dry point contact (DPC) shear wave transducers separated by equidistant transmitter/receiver arrays of six individual transducers each. It is operated with a commercially available portable ultrasonic flaw detector used in the civil engineering industry. To increase the generated acoustic pressure of the borehole probe, the number of transducers in the novel probe is increased to 32. In addition, a timed excitation of each probe directs a focused beam of sound to a specific angle and distance based on calculated time delays. This limits the sensitive test volume and improves the signal-to-noise ratio of the received signals. This paper presents the validation of the newly developed phased array borehole probe by investigating the directivity through beam computation in the CIVA software and experimental investigations on a semi-cylindrical test specimen. It is expected that an optimised radiation pattern of the probe in combination with geophysical reconstruction methods will improve the signal quality and thus increase the reliability of the imaging results. This is of great importance for the construction of safe sealing structures needed for the disposal of radioactive or toxic waste.