Filtern
Erscheinungsjahr
Dokumenttyp
Schlagworte
- Monitoring (12) (entfernen)
Organisationseinheit der BAM
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.
The project "SealWasteSafe" of the Bundesanstalt für Materialforschung und -prüfung (BAM, Berlin) deals with sealing structures applied for underground disposal of nuclear waste from two perspectives: (1) material improvement for application in sealing constructions and (2) feasibility study regarding multi-sensor approaches to ensure quality assurance and long-term monitoring.
One specimen of 150 l made of alkali-activated material, which was found innovative and suitable for sealing constructions based on preliminary laboratory studies, and, for comparison purpose, another one made of salt concrete, are manufactured with an integrated multi-sensory setup for quality assurance and long-term-monitoring. The specimens were left in their cast form and additionally thermally insulated to simulate the situation in the repository. The multi-sensory concept comprises RFID technology embedded in the specimens suppling material temperature and moisture measurements, integrated fibre optic sensing allowing strain measurement and acoustic emission testing for monitoring possible crack formation. Overall, the suitability and the functionality of the sensors embedded into and attached to strongly alkaline (pH > 13 for the AAM) and salt corrosive (NaCl) environment was proven for the first 672 h.
First temperature measurement based on RFID succeeded after 626 h for the alkali-activated material and after 192 h for the conventional salt concrete. Strain measurement based on distributed fibre optic sensing turned out the alkali-activated material with > 1 mm m-1 undergoing approximately twice the compression strain as the salt concrete with strains < 0.5 mm m-1. In contrast, the acoustic emission first and single hits representing crack formation in numbers, was found for alkali-activated material half of that detected at the salt concrete.
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.
Innovative structural damage detection of bridges by least squares adjustment with constraints
(2016)
Long-term monitoring of bridges requires the early detection of spreading damage because very often the damage is not really visible. For many decades engineers and scientists relayed on dynamic methods especially modal ones. However, after fundamental tests by the U.S. FHWA in 1993, it became evident that more successful methods should be developed, but no innovation really happened. A substantial innovative detection method started four years ago from Lagrange Multiplier Testing which eventually resulted in experimental verification based on measurement of deflection and curvature of the bridge beam and together the methodological combination of both.
Mit einem auf Lagrange-Multipiikator-Tests basierenden Monitoring von Brücken ist es gelungen, in Belastungsversuchen an einer Holzbrücke (7,25 m Spannweite), an der relativ kleine Schädigungen künstlich herbeigeführt worden waren, die Zuverlässigkeit dieser Art des Monitorings zu bestätigen. Ausgangspunkte waren Messung von Durchbiegungen sowie Krümmungen und die Zusammenführung der jeweiligen Regression mit Nebenbedingungen, die den mechanischen Bedingungen gehorchen, unter Einschluss von Lagrange-Multiplikatoren, die sich als brauchbare Testparameter erwiesen.
Movements in geological areas are often caused by shear zones deep in the underground or by hardly observable slipping zones. Detection of movements reaching critical values is of crucial importance to avoid losses and harms. Optical sensor fibers with gauge lengths of several meters or even distributed fiber-optic sensors with gauge lengths of hundreds of meters allow detecting deformation of such zones with quite good resolution. The paper presents a new technology of detecting deformations and movements in soil or rock massifs. The core part of the sensing system is a tiny pultruded rod with an outer diameter of 3 mm to 6 mm. It contains two fiber-optic sensor systems. One sensor system is used for permanent online measurement and provides integral strain information along the whole length of the rod; in case of critical deformations, it provides a warning signal. The second sensing system uses fiber-optic Bragg grating (FBG) strain sensors located close to the expected shear zone. This measurement system will be activated after the warning signal has been initiated by the integral strain measurement system. The FBG strain sensor system records then resulting deformations in detail. The sensing rod containing both sensor systems can be installed in a borehole to detect beginning soil movements automatically. The electronic equipment is powered by a solar panel; the amount of deformation in soil or rock formations can be estimated from the strain change measured in the sensor fibers. The effectiveness of this warning system will be shown using the example of a critically sliding slope in an open brown coal pit in Germany.
Fibre-optic sensors need to be more established in the sensor market. Their advantages have unquestionably been verified by numerous demonstrations. However, there are some open questions leading now and then to restraints in the user's community. The paper dicusses examples where fibre-optic sensors provide outstanding knowledge about the structure's behaviour, but application is often challenging. Requirements are listed and open questions to be solved are discussed. Short outlook to standards useful for better design, characterization and application is given. Standards are the basis for establiching SHM systems, especially for safety-critical structural diagnostics.
Reinforced concrete piles are used in case of structures that are constructed on soft ground to transfer the loads into deeper strata with sufficient bearing capacity. In order to determine the pile’s behavior and possible damage, static and dynamic pile tests are carried out. Dynamic measurements taken from the pile head can show the bearing behavior and structural integrity by using the theory of wave propagation. In order to receive more precise information about the pile features, now, a string of sensors is embedded at different levels of the pile. A fiber optic strain wave sensor, based on the extrinsic Fabry-Perot interferometer (EFPI), has already been developed and tested in full-scale field tests by Schallert (2010). It was possible to detect the introduced deformation caused by the static load and the dilatational wave during dynamic loading. Although the full-scale tests were successful, the engineering design of the sensor body left room - from the economical point of view - to be optimized. After laboratory tests with the optimized sensor, a cast-in-situ bored pile has been built at the BAM Test Site Technical Safety in Horstwalde, South of Berlin. Additionally to the EFPI sensors, fiber Bragg grating (FBG) sensors, temperature sensors and resistance strain gauge (RSG) sensors are embedded in order to compare the signals with each other. In this paper, the modified sensor and the Setup of the cast-in-situ bored pile along with results of dynamic tests are shown.