8.2 Zerstörungsfreie Prüfmethoden für das Bauwesen
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Digitaler Zwilling: Verwaltungsschale BBox als Datenablage über den Lebenszyklus einer Brücke
(2021)
Auf dem Weg zur digitalen Brücke existieren bereits erste Lösungen, die den Lebenszyklus einer Brücke abbilden können. Für die Planung, den Bau und den Unterhalt stehen unterschiedlichste Werkzeuge, z. B. BIM, DIN 1076, SIB-Bauwerke, Monitoring etc. zur Verfügung, die jeweils mit unterschiedlichen Datenformaten arbeiten. Für ein intelligentes Erhaltungsmanagement müssen aber alle Daten mit den verschiedenen Datenformaten zusammengeführt, abgelegt und so verwaltet werden können, dass über den gesamten Lebenszyklus einer Brücke die Abbildung eines ganzheitlichen digitalen Zwillings eines Bauwerks möglich ist. Die Autoren haben dafür mit BBox den Prototyp einer Verwaltungsschale (VWS) auf Grundlage von Industrie 4.0 entwickelt. Damit wird das physikalisch-ingenieurtechnische Modell zur Zustandsbewertung der Brücke in den Mittelpunkt gestellt und der gesamte Lebenszyklus einer Brücke kann unabhängig von Datenformaten digital erfasst werden. Da der Aufbau der VWS durch die Granularität optimal strukturiert ist, bietet die Ablage und Einspeisung von Messdaten sowohl die Grundlage eines Live-Monitorings als auch den Grundstein für maschinelles Lernen (ML). Der Datenzugriff via S3-Schnittstelle erleichtert die Entwicklung von eigenen Prognosemodellen mit Informationsmustern (SHIP – Structural Health Information Pattern). Am Beispiel der Heinrichsbrücke Bamberg wird die praktische intelligente Umsetzung des Bauwerksmonitorings inkl. VWS mit Informationsmustern und ML gezeigt.
Ultrasound measurements in concrete are a well-known technique in civil engineering and non-destructive testing.
For consistent monitoring of a concrete structure, the common techniques, using external sensors can often not provide the appropriate degree of repeatability, as the surface of structures changes, and comparable coupling conditions cannot be guaranteed when a measurement is repeated after some time. By embedding ultrasound transducers in concrete, we aim to develop a strategy for long-term monitoring of infrastructure, especially bridges, as a supplement and extension to other techniques. Applying the so-called coda wave interferometry to these measurements we can detect subtle changes in the medium far beyond the Resolution limit of traditional time of flight methods. A smart sensor layout enables cost-efficient sensing of the entire area of interest.
Embedding the transducers might remove uncertainties like coupling or positioning changes, while other challenges remain.
Temperature and moisture content influence the structure and the transducers. These drifts need to be recorded and removed and good coupling must be ensured while not being able to visually inspect the sensor. In a multidisciplinary research group funded by the German Research Foundation, we aim to solve these problems on the way towards an ultrasound monitoring system for reinforced concrete structures. In various experiments in the lab and field, we determine the influence of temperature variations on the measurements and the equipment. As the monitoring task is the detection of irreversible damages - not reversible changes - a smart system requires a smart way of discrimination between permanent damages and reversible changes. With the data collected in these experiments, we present an approach to an environmental correction to ultrasound data to avoid a
misinterpretation of these environmental changes as damage indicators.
Corrosion of concrete reinforcement is one of the major damage mechanisms affecting both the load-bearing capacity and the serviceability of re-inforced concrete structures significantly. When externally discernible damages are observed during visual inspections on the structure, the extent of the damage inside the concrete is often already significant. Corrosion caused by carbonation often leads to severe discoloration of the surface or even large-area spalling of the concrete cover. In contrast, chloride-induced corrosion is usually difficult to observe visually but can cause much more serious damage in less time. The effect occurs locally and can lead to weakening of the cross-section of the reinforce-ment. This, in turn, can cause sudden structural collapses without prior notice.
In the meanwhile, various non-destructive and minimally invasive testing methods are available to evaluate the resistance to penetration of corrosion-pro-moting pollutants and to detect active corrosion. In this paper, a bridge crossing the river Regen is used as a case-study to demonstrate how the information ob-tained applying different testing methods can be combined and evaluated in the context of structural reassessments. Both the results of the permeability testing and the electrical resistance measurement are considered, as well as active corro-sion areas are localized using the half-cell potential mapping combined with the concrete cover measurement with the eddy current method and ground penetrat-ing radar. The results are evaluated using drill cores and in addition laser-induced breakdown spectroscopy was applied to obtain information about possible chlo-ride ion transport into the 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.
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.
The Alpine Fault zone in New Zealand marks a major transpressional plate boundary that is late in its typical earthquake cycle. Understanding the subsurface structures is crucial to understand the tectonic processes taking place. A unique seismic survey including 2D lines, a 3D array, and borehole recordings, has been performed in the Whataroa Valley and provides new insights into the Alpine Fault zone down to ∼2 km depth at the location of the Deep Fault Drilling Project (DFDP)-2 drill site. Seismic images are obtained by focusing prestack depth migration approaches. Despite the challenging conditions for seismic imaging within a sediment filled glacial valley and steeply dipping valley flanks, several structures related to the valley itself as well as the tectonic fault system are imaged. A set of several reflectors dipping 40°–56° to the southeast are identified in a ∼600 m wide zone that is interpreted to be the minimum extent of the damage zone. Different approaches image one distinct reflector dipping at ∼40°, which is interpreted to be the main Alpine Fault reflector located only ∼100 m beneath the maximum drilled depth of the DFDP-2B borehole. At shallower depths (z < 0.5 km), additional reflectors are identified as fault segments with generally steeper dips up to 56°. Additionally, a glacially over-deepened trough with nearly horizontally layered sediments and a major fault (z < 0.5 km) are identified 0.5–1 km south of the DFDP-2B borehole. Thus, a complex structural environment is seismically imaged and shows the complexity of the Alpine Fault at Whataroa.
To conduct a reliable, repeatable and accurate LIBS analysis, the optimization of the experimental setup is an important task. Hardware parameters of lasers and spectrometers used in the setup as well as additionally required components such as optics or process gas pipes must be carefully aligned and adjusted because factors like (i) focal conditions of the optics, (ii) alignment of the sample, (iii) process purge gas (types, flow rate) or (iv) measurement settings (integration time, accumulation of pulses) have a big impact on the signal quality. Therefore, in most cases the effect of different factors is evaluated empirically due to changing one factor at a time while keeping the overall configuration the same. In the end, the optimal configuration is selected based on the best parameters for each influencing factor. During optimization, a configuration of the experimental setup is aimed at, which allows e.g. the highest signal intensity or the lowest variation. In most cases, cross-correlation, interference and interaction among the various factors are not considered. For this reason, the possibilities of using Design of Experiment (DoE) to optimize a LIBS experiment will be shown and advantages of (i) reduction of testing plans using a feature space, (ii) identifying and considering cross-correlations and interactions, (iii) evaluating individual impacts on the measurement (e.g. contour and surface plots) as well as (iv) multivariate models for prediction of impacts will be presented.
In civil engineering, clear regulations and standards, such as the European standard DIN EN 206 apply to ensure that the existing infrastructure is sufficiently resistant to a wide range of exposure conditions. Despite these regulations, in practice concrete structures often show severe damage during their service life. One of these damages is pitting corrosion of the reinforcement, which can be caused by chloride ingress into the structure. Therefore, determining the distribution and depth of chloride ingress is important in predicting the service life. LIBS provides an alternative method to conventional wet chemistry in civil engineering. Despite many advantages, the use of LIBS has been severely limited due to a lack of regulations. Together with project partners from research and industry, we are currently working on a leaflet that will regulate chloride analysis in civil engineering using LIBS. This year, an international round robin test was organized to evaluate the performance of LIBS for chloride analysis in cement pastes. No specifications were given for the experimental LIBS setup and data evaluation. The preliminary results are presented.
This dataset contains raw data resulting from Impact-Echo measurements at the reference conrete block "Radarplatte", located at BAM (German Federal Institute for Materials Research and Testing). This specimen has been described in detail by Niederleithinger et al. (2021), who applied muon tomography, ultrasonic echo measurements, radar and X-ray laminography to visualize its internal structure.
The Impact-Echo method is based on the excitation of the zero-group-velocity frequency of the first symmetric Lamb mode of a plate-like structure, in order to assess its thickness. Numerous publication elaborate on Impact-Echo theory, examples are (Gibson and Popovics 2005, Schubert and Köhler 2008 , Abraham and Popovics 2010).
The measurements have been conducted using a setup that contains only commercially available components. The setup consists of an Olson CTG-2 concrete thickness gauge (Olsen Instruments, USA) for actuation and sensing and an 8-bit NI USB-5132 digital storage oscilloscope (National Instruments, USA) combined with the Echolyst software (Schweizerischer Verein für technische Inspektionen (SVTI), Switzerland) for data acquisition.
Measurements were conducted using a grid of 23x23 points with a spacing of 50 mm. At each point 8192 samples were recorded at a sampling rate of 1 MS/s.
The dataset contains the (X,Y) location in mm of the individual measurement points as well as the raw measurement data at those points.
The data is provided in the formats *.mir/*.mhdr (Echolyst), *.npy (Python) and *.mat (Matlab) and *.csv to ease the import in various post-processing tools.
Die Alterung bestehender Brückenbauwerke fällt mit dem sich verschlechternden Zustand und zunehmenden Anforderungen an die Bausubstanz zusammen, die eine Folge von steigenden Verkehrszahlen und der Zulassung des Schwerlastverkehrs sind. Der daraus erwachsende Bedarf an Beurteilungen von bestehenden Brücken ist national und international fortwährend groß. Die Grundlage für Nachrechnungen sind die über das System verfügbaren Informationen. Sowohl die rechnerische Zuverlässigkeit eines Tragwerks als auch die Validität der Ergebnisse hängen im Wesentlichen vom Zustand, also von der Qualität und der Relevanz dieser Informationen ab. Die gezielte Messung von gütebewerteten, relevanten Informationen kann die Urteilsfähigkeit des Beurteilenden über die Zuverlässigkeit eines Tragwerks signifikant steigern. Wünschenswert ist die Erhebung derartiger Informationen ohne signifikante Eingriffe in die Bausubstanz.
Das Ziel der vorliegenden Arbeit ist die Entwicklung und Erprobung einer Vorgehensweise für die probabilistische Beurteilung von bestehenden Bauwerken mit zerstörungsfrei gemessenen Daten und einer Methodik zum objektiven Nachweis der Eignung von zerstörungsfreien Messverfahren zur Anwendung im Zuge einer Nachrechnung. Herausgestellt wird die Eignung der im Guide to the Expression of Uncertainty in Measurement (GUM) publizierten Regeln zur Bewertung der Qualität der gemessenen Informationen, die später im statischen Nachweis verwendet werden sollen. Die Anforderungen an die Basisvariablen, d. h. an die Eingangsgrößen im probabilistischen Nachweis, werden aus den Vorschriften zur Zuverlässigkeitsanalyse mit dem probabilistischen Näherungsverfahren FORM, der First Order Reliability Method, abgeleitet. Darauf basiert die Entwicklung der Schnittstelle zwischen der Messtechnik und der Nachrechnung. Im Kern werden die messdatenbasierten Basisvariablen explizit als Zufallsgrößen bei der Zuverlässigkeitsanalyse berücksichtigt. Die entwickelte Vorgehensweise wird am Beispiel von zwei Spannbetonbrücken und vier Basisvariablen erprobt. In die Nachweise der Biege- und der Querkrafttragfähigkeit sowie in einen Nachweis der Dekompression fließen geometrische Messergebnisse ein, die mithilfe von Radar- und Ultraschallmessungen generiert werden. Ein wichtiger Bestandteil ist die Bildung von Modellen zur Auswertung der Messungen und zur Berechnung der Messergebnisse.
Es wird gezeigt, dass die Einbeziehung der zerstörungsfrei generierten Messergebnisse die Unsicherheit in den Berechnungsmodellen reduzieren und den Grad der Annäherung der Modelle
an die Realität steigern kann. Die Messergebnisse sind zuverlässig und vergleichbar. Umfassende Sensitivitätsanalysen geben Aufschluss über sinnvollerweise zu messende Bauwerksparameter, die einen maßgeblichen Einfluss auf die Tragwerkszuverlässigkeit ausüben. Auf diese Weise wird die Relevanz der Informationen ermittelt. Das Potential derartiger Analysen für die gezielte Planung und Beauftragung von Messungen sowie für die bedarfsgerechte Zuschärfung von initial unzureichenden Berechnungsmodellen erscheint groß. Die messdatenbasierten Modelle können darüber hinaus die Robustheit einer Tragwerksanalyse steigern. Die Berücksichtigung von Messergebnissen ist dann besonders nützlich, wenn zur Beurteilung erforderliche Informationen fehlen oder Zweifel an den verfügbaren Informationen aufgekommen sind. Im besten Fall resultiert daraus die Verlängerung von rechnerischen Restnutzungsdauern, die Steigerung der Verfügbarkeit der Infrastruktur, die Optimierung des Ressourcenverbrauchs und insgesamt eine realitätsnahe Bauwerksbewertung.
Amorphous, porous materials represent by far the largest proportion of natural and men-made materials. Their pore networks consists of a wide range of pore sizes, including mesoand macropores. Within such a pore network, material moisture plays a crucial role in almost all transport processes. In the hygroscopic range, the pores are partially saturated and liquid water is only located at the pore fringe due to physisorption. Therefore, material parameters such as porosity or median pore diameter are inadequate to predict material moisture and moisture transport. To quantify the spatial distribution of material moisture, Hillerborg’s adsorption Theory is used to predict the water layer thickness for different pore geometries. This is done for all pore sizes, including those in the lower nanometre range. Based on this approach, it is shown that the material moisture is almost completely located in mesopores, although the pore network is highly dominated by macropores. Thus, mesopores are mainly responsible for the moisture storage capacity, while macropores determine the moisture transport capacity, of an amorphous material. Finally, an electrical analogical circuit is used as a model to predict the diffusion coefficient based on the pore-size distribution, including physisorption.
The duraBASt area at the highway intersection “Köln-Ost” is a large test field for road testing.
It includes the duraBASt test bridge which is approximately 66 m long and 14.25 m broad. This bridge is part of the “road in the 21st Century” project and is equipped with several sensors for structural health monitoring. This feasibility study is carried out to investigate the prerequisites necessary for the constraction of intelligent bridges in the foture. Embedded sensors are able to measure quantities such as temperature, strain, moisture, corrosion, etc. In the presented work, the focus is set on passive RFID-based sensors. The communication and energy supply are realised by a mobile transmitter through the concrete cover. Hence, no cables or batteries are required. The sensors are designed to withstand the alkaline environment in concrete. Thus, moisture and corrosion values are measurable for several decades.
In the framework of non-destructive-testing advanced seismic imaging techniques have been applied to ultrasonic echo data in order to examine the integrity of an engineered test-barrier designed to be used for sealing an underground nuclear waste disposal site. Synthetic data as well as real multi-receiver ultrasonic data acquired at the test site were processed and imaged using Kirchhoff prestack depth migration reverse time migration (RTM). In general, both methods provide a good Image quality as demonstrated by various case studies, however deeper parts within the test barrier containing inclined reflectors were reconstructed more accurately by RTM. In particular, the image quality of a specific target reflector at a depth of 8 m in the test-barrier has been significantly improved compared to previous investigations using synthetic aperture Focusing technique, which justifies the considerable computing time of this method.
Air-coupled ultrasonic (ACU) testing has proven to be a valuable method for increasing the speed in non-destructive ultrasonic testing and the investigation of sensitive specimens. A major obstacle to implementing ACU methods is the significant signal power loss at the air–specimen and transducer–air interfaces. The loss between transducer and air can be eliminated by using recently developed fluidic transducers. These transducers use pressurized air and a natural flow instability to generate high sound power signals. Due to this self-excited flow instability, the individual pulses are dissimilar in length, amplitude, and phase. These amplitude and angle modulated pulses offer the great opportunity to further increase the signal-to-noise ratio with pulse compression methods.
In practice, multi-input multi-output (MIMO) setups reduce the time required to scan the specimen surface, but demand high pulse discriminability. By applying envelope removal techniques to the individual pulses, the pulse discriminability is increased allowing only the remaining phase information to be targeted for analysis. Finally, semi-synthetic experiments are presented to verify the applicability of the envelope removal method and highlight the suitability of the fluidic transducer for MIMO setups.
We present a preliminary investigation on distributed humidity monitoring during the drying process of concrete based on an embedded polymer optical fiber (POF). The water dissipated into the POF changes several properties of the fiber such as refractive index, scattering coefficient and attenuation factor, which eventually alters the Rayleigh backscattered light. The optical time Domain reflectometer (OTDR) technique is performed to acquire the backscattered signal at the wavelengths 650 nm and 500 nm, respectively. Experimental results show that the received signal increases at 650 nm while the fiber attenuation factor clearly increases at 500 nm, as the concrete dries out. In the hygroscopic range, the information retrieved from the signal change at 650 nm agrees well with the measurement result of the electrical humidity sensors also embedded in the concrete sample.
While contact and immersion ultrasonic testing are established methods in non-destructive testing (NDT), generating high power air-coupled ultrasound remains a challenging task. Solutions often involve setups that are restricted to lab environments. When field measurements are required, such as in NDT for civil engineering, a handy, robust and safe transducer is needed. For this purpose, an ultrasonic transducer based on a fluidic switch has been developed. A sonic air flow inside the device is switched rapidly so that an ultrasonic signal is generated. Both theory and previous flow simulations suggest that the control flow pressure ramp has only little influence on the switching time of the device. This publication gives an overview over the operating principle of the fluidic ultrasonic transducer and investigates the influence of control tube length and pulsing repetition rate on the ultrasonic pressure amplitude. High repetition rates are found to reduce the signal amplitude, whereas long tubing has only little negative influence on the amplitude while improving signal quality.
"Die effiziente Verwendung von Messergebnissen bei der Nachrechnung und von Ergebnissen aus zerstörungsfreien Prüfungen im Speziellen setzt voraus, dass die Relevanz der messbaren Informationen beziffert, die Qualität der gemessenen Information berechnet und die Anforderungen an die Genauigkeit der Inspektionen aus der statischen Berechnung abgeleitet werden können. Der Vergleich der Anforderungen an eine zu messende Information mit der Qualität der gemessenen Information wird in diesem Aufsatz als Validierung eines Messverfahrens aufgefasst. Es wird gezeigt, wie zerstörungsfreie Prüfverfahren für die Nachrechnung validiert werden können. Die Methodik wird anhand einer mit der Ultraschallecho- und Impulsradarmethode untersuchten Spannbetonbrücke demonstriert."
Damage assessment of civil structures using wave propagation analysis and transmissibility functions
(2021)
This presentation shows the applicaiton of seismic interferometry to vibration recordings. The Normalized-Input-Output-Minimization (NIOM) method to reconstruct the virtual impulse wave propagation is introduced. We obtain wave velocities and transmissibility functions and consider them as reference values for the damage detection method. At the end, the evaluation results are presented by two examples: a 14-story RC building and a 64 m long pedestrian bridge.
Motivation:
• Air-coupled ultrasound (AC-US) enables faster measurements in non-destructive testing for civil engineering
• Current state of the art: piezo and capacitive transducers are mostly used for AC US
• Currently >99.9% sound intensity loss due to impedance mismatches
Innovation:
• Fluidic transducers generate US (30-60 kHz) by rapid switching of a supersonic jet
• Fluidic transducers rely on external components such as solenoid valves and tubing
--> How do repetition time and tube length influence the signal?
Ultrasound measurements in concrete are a well-known technique in civil engineering and non-destructive testing. For consistent monitoring of a concrete structure, the common techniques, using external sensors can often not provide the appropriate degree of repeatability, as the surface of structures changes, and comparable coupling conditions cannot be guaranteed when a measurement is repeated after some time. By embedding ultrasound transducers in concrete, we aim to develop a strategy for long-term monitoring of infrastructure, especially bridges, as a supplement and extension to other techniques. Applying the so-called coda wave interferometry to these measurements we can detect subtle changes in the medium far beyond the resolution limit of traditional time of flight methods. A smart sensor layout enables cost-efficient sensing of the entire area of interest. Embedding the transducers might remove uncertainties like coupling or positioning changes, while other challenges remain. Temperature and moisture content influence the structure and the transducers. These drifts need to be recorded and removed and good coupling must be ensured while not being able to visually inspect the sensor. In a multidisciplinary research group funded by the German Research Foundation, we aim to solve these problems on the way towards an ultrasound monitoring system for reinforced concrete structures. In various experiments in the lab and field, we determine the influence of temperature variations on the measurements and the equipment. As the monitoring task is the detection of irreversible damages - not reversible changes - a smart system requires a smart way of discrimination between permanent damages and reversible changes. With the data collected in these experiments, we present an approach to an environmental correction to ultrasound data to avoid a misinterpretation of these environmental changes as damage indicators.
The internal project "SealWasteSafe" of the Federal Institute for Materials Research and Testing is dedicated to the further improvement in the construction of sealing systems for underground repositories of radioactive waste. Particularly, a multi-sensory concept is developed to ensure quality assurance and continuous long-term monitoring on the engineered barriers. Therefore, beside other measures, the temperature and equilibrium moisture of the construction shall be monitored and respective multi-sensoric monitoring concepts are tested first on the laboratory scale. One focus in this project is on embedded application in alkali-activated mortars (AAM), which results in special requirements for the resistance of the sensors to strongly alkaline environments with up to pH 14. In addition, the ingress of water along cables or at sensor positions is critical for sensors embedded in concrete, as the cable access of the sensor housing is a major weak point. It is therefore advisable to equip wireless high frequent radio frequency identification-based sensors, so called HF RFID sensors (short: RFID), that do not require cable access and thus have a stable and leak-proof sensor housing for long-term monitoring. In this contribution, the results from temperature, moisture and transmitted power monitoring of a AAM and a M2 specimens by means of HF RFID sensor systems are presented.
The internal project "SealWasteSafe" of the Federal Institute for Materials Research and Testing is dedicated to the further improvement in the construction of sealing systems for underground repositories of radioactive waste. Particularly, a multi-sensory concept is developed to ensure quality assurance and continuous long-term monitoring on the engineered barriers. Therefore, beside other measures, the temperature and equilibrium moisture of the construction shall be monitored and respective multi-sensoric monitoring concepts are tested first on the laboratory scale. One focus in this project is on embedded application in alkali-activated mortars (AAM), which results in special requirements for the resistance of the sensors to strongly alkaline environments with up to pH 14. In addition, the ingress of water along cables or at sensor positions is critical for sensors embedded in concrete, as the cable access of the sensor housing is a major weak point. It is therefore advisable to equip wireless high frequent radio frequency identification-based sensors, so called HF RFID sensors (short: RFID), that do not require cable access and thus have a stable and leak-proof sensor housing for long-term monitoring. In this contribution, the results from temperature, moisture and transmitted power monitoring of a AAM and a M2 specimens by means of HF RFID sensor systems are presented.
Ultrasonic measurement technology has become indispensable in NDT. In order to reduce measurement time and extend the application to other materials, contactless ultrasound is the subject of many different research groups. Department 8 has been researching successfully in this field for years. A novel approach is based on so-called fluidic devices. These devices can be used to perform binary logic operations with the help of natural flow instabilities. Hence the abbreviated name, Fluidic (FLUID+LogIC). Only a pressure reservoir of the used fluid is required as energy supply. This enables the production of very robust actuators that generate ultrasonic signals in an extremely energy efficient way.
The presentation includes the research results of the ZIM innovation project OsciCheck. The original idea will be presented and its application on different building materials is validated. Beyond this, the possible application areas are much larger and a detailed outlook is given to discuss the future potential of fluidic ultrasonic actuators.
A joint project of partners from industry and research institutions for the research and construction of an analysis system for an automated, sensor-supported sorting of construction and demolition waste will be presented. This is intended to supplement or replace the previously practiced manual sorting, which harbors many risks and dangers for the staff and only enables obvious, visually detectable differences for separation. The method of laser-induced breakdown spectroscopy is to be used in combination with hyperspectral sensors. Due to the jointly processed information (data fusion), this should lead to a significant improvement in the separation of types. In addition to the sorting of different materials (concrete, main masonry building materials, organic components, glass, etc.), impurities such as SO3-containing building materials (gypsum, aerated concrete, etc.) could also be detected and separated.
The subsequent recycling and sales opportunities are examined, such as the use of recycled aggregates in concrete, the recycling of building materials containing sulphate as a gypsum substitute for the cement industry or the agglomeration of synthetic lightweight aggregates for lightweight concrete or as a substrate for green roofs. At the same time, it is investigated whether soluble components (sulfates, heavy metals, etc.) can be detected by LIBS without a wet chemical analysis and what impact the recycling materials have on the environment.
The entire value chain is examined using the example of the Berlin location in order to minimize economic / technological barriers and obstacles on a cluster level and to sustainably increase the recovery and recycling rates.
CASPAR - FKZ: 19F2178A "Plattform für die beweissichere und rückführbare Datennutzung im Bauwesen"
(2021)
Bestandsunterlagen von älteren Bauwerken sind oft nicht in ausreichendem Umfang verfügbar. Soll jedoch der Zustand von z.B. Gebäude oder Brücken bewertet werden, stellen Bestandsunterlagen die Grundlage dafür dar. Ansonsten muss die Bauweise des Bauwerkes kostenintensiv nachvollzogen und mit nachträglichen Untersuchungsverfahren bewertet werden.
Heute spielt die digitale Planung im Bauwesen eine immer größere Rolle und Bauvorhaben werden in sogenannten BIM Modellen (Building Information Modelling) geplant. Diese Daten müssen langfristig und manipulationssicher gespeichert werden.
In diesem Projekt erarbeiten wir die technologischen Grundlagen, um Daten effizient und sicher zu speichern. Wir erarbeiten eine digitale Schnittstelle, um Daten aus unterschiedlichsten Quellen sicher zusammenzuführen und Informationen semantisch zu verknüpfen. Um den Nachweis für die Nutzbarkeit unserer Technologie zu erbringen, entwickeln wir im Rahmen des beantragten Projekts eine prototypische objektbezogene Datenablage in Form eines Demonstrators, in die digitale Informationen einer realen Brücke überführt werden.
CASPAR - FKZ: 19F2178A "Plattform für die beweissichere und rückführbare Datennutzung im Bauwesen"
(2021)
Bestandsunterlagen von älteren Bauwerken sind oft nicht in ausreichendem Umfang verfügbar. Soll jedoch der Zustand von z.B. Gebäude oder Brücken bewertet werden, stellen Bestandsunterlagen die Grundlage dafür dar. Ansonsten muss die Bauweise des Bauwerkes kostenintensiv nachvollzogen und mit nachträglichen Untersuchungsverfahren bewertet werden.
Heute spielt die digitale Planung im Bauwesen eine immer größere Rolle und Bauvorhaben werden in sogenannten BIM Modellen (Building Information Modelling) geplant. Diese Daten müssen langfristig und manipulationssicher gespeichert werden. In diesem Projekt erarbeiten wir die technologischen Grundlagen, um Daten effizient und sicher zu speichern. Wir erarbeiten eine digitale Schnittstelle, um Daten aus unterschiedlichsten Quellen sicher zusammenzuführen und Informationen semantisch zu verknüpfen. Um den Nachweis für die Nutzbarkeit unserer Technologie zu erbringen, entwickeln wir im Rahmen des beantragten Projekts eine prototypische objektbezogene Datenablage in Form eines Demonstrators, in die digitale Informationen einer realen Brücke überführt werden.
Corrosion of concrete reinforcement is one of the major damage mechanisms affecting both the load-bearing capacity and the serviceability of reinforced concrete structures significantly. The challenge of detecting corrosion is that the corrosion process in its various forms is not immediately visible, especially in the corrosion initiation phase inside the concrete. When externally discernible damages are observed during visual inspections on the structure, the extent of the damage inside the concrete is often already significant. Corrosion caused by carbonation often leads to severe discoloration of the surface or even large-area spalling of the concrete cover. In contrast, chloride-induced corrosion is usually difficult to observe visually, but can cause much more serious damage in less time. The effect occurs locally and can lead to weakening of the cross-section of the reinforcement. This, in turn, can cause sudden structural collapses without prior notice. Therefore, it is important to investigate whether there is protection against corrosion of the reinforcement in the concrete and to detect active corrosion in the structure at an early stage.
In the meanwhile, various non-destructive and minimally invasive testing methods are available to evaluate the resistance to penetration of corrosion-promoting pollutants and to detect active corrosion. In this paper, a bridge crossing the river Regen (Germany) is used as a case-study to demonstrate how the information obtained applying different testing methods can be combined and evaluated in the context of structural reassessments. Both the results of the permeability testing (Torrent tester) and the electrical resistance measurement (Wenner probe) are considered, as well as active corrosion areas are localized using the half-cell potential mapping combined with the concrete cover measurement with the eddy current method and ground penetrating radar (GPR). The results are evaluated using drill cores and in addition laser-induced breakdown spectroscopy (LIBS) was applied to obtain information about possible ion transport in the concrete.
In this contribution, an approach is outlined to process non-destructively gath-ered measurement data in a comparable way in order to include the measured information in probabilistic reliability assessments of existing structures. An es-sential part is the calculation of measurement uncertainties. The effect of incor-porating evaluated NDT-results is demonstrated by means of a prestressed con-crete bridge and GPR measurements conducted on this bridge as a case-study. The bridge is assessed regarding SLS Decompression using the NDT-results.
This data article presents characteristic acoustic and flow data of a fluidic ultrasonic transducer as well as acoustic data of a commercial piezoelectric ultrasonic transducer used in non-destructive testing for civil engineering. The flow data has been acquired using hot-wire anemometry and a Pitot tube. The three-dimensional acoustic data of both devices has been acquired using a calibrated microphone. The distribution of characteristic acoustic properties of both transducers are extracted and given in addition to the raw data. The data presented in the article will be a valuable source for reference and validation, both for developing fluidic and alternate ultrasound generation technologies. Furthermore, they will give additional insight into the acoustic-flow interaction phenomena of high speed switching devices. This article is accompanying the paper Experimental Analysis of the Acoustic Field of an Ultrasonic Pulse Induced by a Fluidic Switch (Bühling et al., 2021) published in The Journal of the Acoustical Society of America, where the data is interpreted in detail and the rationale for characteristic sound properties of the fluidic transducer are given.
The development of prevention methods and the detection of moisture related damages in buildings and transport infrastructure at an early stage are current issues
in the field of non-destructive testing in civil engineering. Especially the subject of partial saturation requires further research since it is more likely to occur than full saturation of the material. In fact, partial saturation in porous media is even more complicated because both fully and partly saturated pores (i.e. pores in which the surfaces are covered with thin water layers) are present.
As the non-destructive method nuclear magnetic resonance
(NMR) enables the measurement and quantification of relative low moisture contents, it is suitable for the investigation of partly saturated porous building materials. Nevertheless, the differentiation between partly and fully saturated pores is still a challenge. Therefore, in this study, we investigate two sandstones types (Bozanov and Schönbrunner sandstone) at various defined saturation states by using NMR. Furthermore, we measure the relative humidities within the samples and compute the water layer thicknesses (WLT) along the pore walls of all pore sizes to calculate the corresponding degree of pore saturation. To finally assign the NMR signals to pore sizes and to differentiate between partly and fully saturated pores, the water content distribution obtained from the WLT calculation
is used for calibration of the relaxation-time distribution.
The development of prevention methods and the detection of moisture related damages in buildings and transport infrastructure at an early stage are current issues in the field of non-destructive testing in civil engineering.
Especially the subject of partial saturation requires further research since it is more likely to occur than full saturation of the material. In fact, partial saturation in porous media is even more complicated because both fully and partly saturated pores (i.e. pores in which the surfaces are covered with thin water layers) are present.
As the non-destructive method nuclear magnetic resonance (NMR) enables the measurement and quantification of relative low moisture contents, it is suitable for the investigation of partly saturated porous building materials. Nevertheless, the differentiation between partly and fully saturated pores is still a challenge. Therefore, in this study, we investigate two sandstones types (Bozanov and Schönbrunner sandstone) at various defined saturation states by using NMR. Furthermore, we measure the relative humidities within the samples and compute the water layer thicknesses (WLT) along the pore walls of all pore sizes to calculate the corresponding degree of pore saturation. To finally assign the NMR signals to pore sizes and to differentiate between partly and fully saturated pores, the water content distribution obtained from the WLT calculation is used for calibration of the relaxation-time distribution. In this extended abstract, selected results only for Schönbrunner sandstone are presented.
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.
Making optimal decisions about the reliability of existing structures requires that the information used in assessment adequately represents the properties and the condition of the structures. The knowledge gap regarding a structure to be assessed can be successively filled by individually purposeful observations on site. This paper gives an overview of an approach for utilizing nondestructively gathered measurement results in reliability assessment of existing structures. An essential part of measurement-based stochastic modeling of basic variables is the calculation of measurement uncertainties, which serves to establish confidence in measurement, to ensure the comparability of unambiguously expressed measurement results, and to quantify the quality of the measured information. Regarding the current discourse on how to treat information collected on-site in the context of assessment, the authors recommend that measurement uncertainty becomes an uncertainty component mandatorily to be represented in measurement-based stochastic models. The main steps of the proposed concept are presented, and the advantages of its application are emphasized by means of a prestressed concrete bridge as case study. The bridge is assessed regarding the serviceability limit state decompression using ultrasonic and radar data measured at the structure.
The aim of this work is to improve the current structural health monitoring (SHM) methods for civil structures. A field experiment was carried out on a two-span bridge with a built-in un-bonded prestressing system. The bridge is a 24-metre long concrete beam resting on three bear-ings. Cracks were formed subsequently when a prestressing force of 350 kN was changed to 200 kN, so that different structural states could be demonstrated. The structural assessment of this reference bridge was accomplished by the non-destructive testing using ultrasonic devices and vibration measurements. The ultrasonic velocity variations were investigated by using the coda wave interferometry method. The seismic interferometry technique was applied to the vi-bration recordings to reconstruct the wave propagation field in the bridge. This investigation shows that the wave velocity is sensitive to the current structural state and can be considered as the damage indicator. Overall, the implementation of coda cave interferometry and seismic interferometry technique facilitates structural health monitoring (SHM) in civil engineering.
Nowadays, civil engineers have applied prestressing technique to long span bridges to enhance the structural strength and to improve the structural performance in their life time.
However, it‘s not easy to avoid prestress loss during the construction or in service time. This problem will influence the function of the structures and worse make our life in danger.
Therefore, it‘s important to focus on the topic „bridge monitoring“ to make sure the structural property is "health". This presentation introduces the work which fulfills the research goal - birdge monitoring. A large-scale prestressed concrete bridge model was considered in this work. We obteined various damaged condition due to the different prestressed levels which we can change every time in the experiment.
The experimental measurement in two scales were demonstrated by ultrasonics and natural vibration of the bridge.
Die ZfP ist im Bauwesen bisher deutlich weniger reguliert (und auch akzeptiert) als in anderen Industriebereichen. Zum Beispiel gibt für die meisten Prüfverfahren weder Normen noch ein allgemein akzeptiertes Ausbildungs- und Zertifizierungssystem.
Dies hat auch Konsequenzen für die Digitalisierung. Im Gegensatz dazu wird im Bauwesen derzeit für viele Bereiche verbindlich ein neues Verfahren zur Unterstützung von Design, Planung, Herstellung und Betrieb eingeführt: BIM (Building Information Modeling). Der Vortrag beleuchtet das Spannungsfeld zwischen dem geringen Organisations- und Standardisierungsgrad und den neuen digitalen Möglichkeiten und stellt den neuen Arbeitskreis zur ZfP 4.0 im Bauwesen vor. Er beleuchtet auch, in welchen Bereichen die ZfP im Bauwesen von Entwicklungen aus „Industrie 4.0“ und der zerstörungsfreien Prüfung in anderen Anwendungsbereichen profitieren kann - z. B. bei Datenformaten, Datenarchivierung und Schnittstellen.
Die ZfP ist im Bauwesen bisher deutlich weniger reguliert (und auch akzeptiert) als in anderen Industriebereichen. Zum Beispiel gibt für die meisten Prüfverfahren weder Normen noch ein allgemein akzeptiertes Ausbildungs- und Zertifizierungssystem.
Dies hat auch Konsequenzen für die Digitalisierung. Im Gegensatz dazu wird im Bauwesen derzeit für viele Bereiche verbindlich ein neues Verfahren zur Unterstützung von Design, Planung, Herstellung und Betrieb eingeführt: BIM (Building Information Modeling). Der Vortrag beleuchtet das Spannungsfeld zwischen dem geringen Organisations- und Standardisierungsgrad und den neuen digitalen Möglichkeiten und stellt den neuen Arbeitskreis zur ZfP 4.0 im Bauwesen vor. Er beleuchtet auch, in welchen Bereichen die ZfP im Bauwesen von Entwicklungen aus „Industrie 4.0“ und der zerstörungsfreien Prüfung in anderen Anwendungsbereichen profitieren kann - z. B. bei Datenformaten, Datenarchivierung und Schnittstellen.
The effective gas diffusion coefficient describes the process of gas diffusion in porous materials. Several materials have a significant number of micropores in the lower nanometre range leading to a reduction of gas diffusion (Knudsen effect). In the case of partial pore saturation during adsorption, the available pore space is further reduced, as is the gas diffusion. In this study, the influence of partially saturated pores on the Knudsen effect and on the gas diffusion is quantified. Three different pore geometries are investigated (slit, cylindrical and spherical pores) and three different types of pore size distribution, including a broad equal distribution, three narrow normal distributions and two measured distributions of concrete.
Besides the intensive computation of the exact pore saturation, a simplified model with low computational requirements is suggested. This study shows that the influence of the water layer thickness on the effective diffusion becomes significant for pore radii below 50 nm and the assumed pore geometry is important. At the end, the overall effect is quantified for an amorphous material with most pore
radii below 30 nm. At a moisture level of 50% relative humidity, the effective diffusion is reduced by 35% due to partial saturation.
This article presents a unique method of installing a special type of embedded ultrasonic transducers inside a 36-m-long section of an old bridge in Germany. A small-scale load test was carried out by a 16 ton truck to study the temperature and load influence on the bridge, as well as the performance of the embedded transducers. Ultrasonic coda wave interferometry technique, which has high sensitivity in detecting subtle changes in a heterogeneous medium, was used for the data evaluation and interpretation. The separation of two main influence factors (load effect and temperature variation) is studied, and future applications of wave velocity variation rate Φ for structural health condition estimation are discussed. As a preliminary research stage, the installation method and the performance of the ultrasonic transducer are recognized. Load- and temperature-induced weak wave velocity variations are successfully detected with a high resolution of 10−4%. The feasibility of the whole system for long-term structural health monitoring is considered, and further research is planned.
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.
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.
Long term monitoring of concrete structures using innovative NDT and SHM approaches: NDT-CE 4.0
(2021)
In Germany, a large amount of reinforced and/or prestressed concrete structures are approaching the end of their life cycle. Many of them already show a significant reduction of operability or capacity. The issue is most obvious, but not limited to traffic or energy related infrastructure. These structures have to be inspected and assessed on a regular basis. More and more permanent monitoring systems are installed to follow the degradation and to give pre-warning before failure.
While the basic mandatory inspection (e. g. of bridges) is still limited to visual examination and tap tests, more advanced investigation tools as ultrasound or radar are used more and more often. Monitoring e. g. by acoustic emission systems is increasingly used e.g. for detection of wire breaks in prestressed structures. The presentation gives an overview on the state of the art and an outlook on innovative systems, including the integration in modern data processing, evaluation, display and archive systems.
Civil engineering industry is one of the most important industry sectors in the world-wide economy. It contributes significantly to the gross economic product and general employment. Even more important, it provides many of the basic needs of the society (e. g. housing, infrastructure, protection from natural hazards).
The concept of “Industry 4.0” or “Smart Production” has not yet made significant progress in the civil engineering industry. The design, build and operate processes are still widely dominated by the exchange of printed documents and drawings. Most objects (buildings and other constructions) are unique, and a large part of the production still requires a large amount of manual labor. As-built documentation and quality assurance are often neglected. Civil engineering is among the industries sectors with the lowest level of digitalization and the lowest gain in productivity.
However, this is going to change. In the past decade, several drivers have challenged the ways clients, contractors, and authorities currently operate. These drivers include but are not limited to an increasing demand for serialization and automatization, the mandatory introduction of “Building Information Modeling” (BIM) in public procurement, the availability of construction equipment with sensors and digital interfaces or emerging automated construction technologies such as 3D-printing.
NDE (referred to as NDT-CE in this sector), after a rapid technological development in the last two decades, plays an increasing role in quality assurance, condition assessment and monitoring of structures. However, with very few exceptions, applications are mostly non-standardized and performed only at selected sites. To change this, the NDT-CE community including manufacturers, service providers, clients and the scientific community must work consistently on open data formats, interfaces to BIM, standardization and validated ways for a quantitative use of the results in the assessment of constructions.
To provide better means for a safe and effective monitoring of cemented waste packages including prediction tools to assess the future integrity development during pre-disposal activities, several digital tools are evaluated and improved in the frame of the EC funded project PREDIS. Safety enhancement (e. g. less exposure of testing personnel) and cost effectiveness are part of the intended impact.
The work includes but is not limited to inspection methods such as muon imaging, wireless sensors integrated into waste packages as well as external package and facility monitoring such as remote fiber optical sensors. The sensors applied will go beyond radiation monitoring and include Proxy parameters important for long term integrity assessment (e. g. internal pressure). Sensors will also be made cost effective to allow the installation of much more sensors compared to current practice.
The measured data will be used in digital twins of the packages for specific simulations (geochemical, integrity) providing a prediction of future behavior. Machine Learning techniques trained by the characterization of older packages will help to connect the models to the actual data.
All data (measured and simulated) will be collected in a joint data base and connected to a decision framework to be used at actual facilities.
The presentation includes detailed information about the various tools under consideration, their connection and first results of our research.
The Alpine Fault at the West Coast of the South Island (New Zealand) is a major plate boundary that is expected to rupture in the next 50 years, likely as a magnitude 8 earthquake. The Deep Fault Drilling Project (DFDP) aimed to deliver insight into the geological structure of this fault zone and its evolution by drilling and sampling the Alpine Fault at depth. Here we present results from a seismic survey around the DFDP-2 drill site in the Whataroa Valley where the drillhole almost reached the fault plane. This unique 3D seismic survey includes several 2D lines and a 3D array at the surface as well as borehole recordings. Within the borehole, the unique option to compare two measurement systems is used: conventional three-component borehole geophones and a fibre optic cable (heterodyne Distributed Vibration Sensing system (hDVS)). Both systems show coherent signals but only the hDVS system allowed a recording along the complete length of the borehole.
Despite the challenging conditions for seismic imaging within a glacial valley filled with sediments and steeply dipping valley flanks, several structures related to the valley itself as well as the tectonic fault system are imaged. The pre-processing of the seismic data also includes wavefield separation for the zero-offset borehole data. Seismic images are obtained by prestack depth migration approaches.
Within the glacial valley, particularly steep valley flanks are imaged directly and correlate well with results from the P-wave velocity model obtained by first arrival travel-time tomography. Additionally, a glacially over-deepened trough with nearly horizontally layered sediments is identified about 0.5 km south of the DFDP-2B borehole.
With regard to the expected Alpine fault zone, a set of several reflectors dipping 40-56° to the southeast are identified in a ~600 m wide zone between depths of 0.2 and 1.2 km that is interpreted to be the minimum extent of the damage zone. Different approaches image one distinct reflector dipping at 40°, which is interpreted to be the main Alpine Fault reflector. This reflector is only ~100 m ahead from the lower end of the borehole. At shallower depths (z<0.5 km), additional reflectors are identified as fault segments and generally have steeper dips up to 56°. About 1 km south of the drill site, a major fault is identified at a depth of 0.1-0.5 km that might be caused by the regional tectonics interacting with local valley structures. A good correlation is observed among the separate seismic data sets and with geological results such as the borehole stratigraphy and the expected surface trace of the fault.
In conclusion, several structural details of the fault zone and its environment are seismically imaged and show the complexity of the Alpine Fault at the Whataroa Valley. Thus, a detailed seismic characterization clarifies the subsurface structures, which is crucial to understand the transpressive fault’s tectonic processes.
This poster is to introduce the research work in the GIOTTO project funded by Federal Ministry of Education and Research (BMBF) in the frame of “early warning of earthquakes” program. The aim of this work is to investigate the application of seismological noise-based monitoring for bridge structures. A large-scale two-span concrete bridge model with a build-in post-tensioning system, which is exposed to environmental conditions, is chosen as our experimental test structure. Ambient seismic noise measurements were carried out under different pre-stressed conditions. Using the seismic interferometry technique, which is applied to the measurement data in the frequency domain, we reconstruct waveforms that relate to wave propagation in the structure. The coda wave interferometry technique is then implemented by comparing two waveforms recorded in two pre-stress states. Any relative seismic velocity changes are identified by determining the correlation coefficients and reveal the influence of the pre-stressing force. The decrease of the wave propagation velocity indicates the loss of the pre-stress and weakening stiffness due to opening or event extension of cracks. We conclude that the seismological methods used to estimate velocity change can be a promising tool for structural health monitoring of civil structures.
Structural health monitoring is a big issue of the prevention measure for civil structures in the seismic hazard region. The GIOTTO project aims to develop of the real time monitoring in civil engineering by use of the 6 DOFs sensors. This presented work is part of the GIOTTO Project. BAM offered the idea test structure BLEIB bridge for experiment. This prestressed concrete bridge model benefits us to have the various structural condition with different pre-stress. The passive and active monitoring were carried out. The seismic interferometry technique was considered to extract the virtual wave propagation in the structure comparable to the real impulse response excited by the hammer impact. Meanwhile the ultrasonic measurement was performed during the monitoring. The coda wave interferometry (CWI) was applied to investigate the wave velocity variation in two kinds of scale recordings at different prestressed states. The experimental result approves that the seismological noise- based methods are the appropriate evaluation tool for the measurement data.
This presentation is to introduce the research work in the GIOTTO project funded by Federal Ministry of Education and Research (BMBF) in the frame of “early warning of earthquakes” program. The aim of this work is to investigate the application of seismological noise-based monitoring for bridge structures. A large-scale two-span concrete bridge model with a build-in post-tensioning system, which is exposed to environmental conditions, is chosen as our experimental test structure. Ambient seismic noise measurements were carried out under different pre-stressed conditions. Using the seismic interferometry technique, which is applied to the measurement data in the frequency domain, we reconstruct waveforms that relate to wave propagation in the structure. The coda wave interferometry technique is then implemented by comparing two waveforms recorded in two pre-stress states. Any relative seismic velocity changes are identified by determining the correlation coefficients and reveal the influence of the pre-stressing force. The decrease of the wave propagation velocity indicates the loss of the pre-stress and weakening stiffness due to opening or event extension of cracks. We conclude that the seismological methods used to estimate velocity change can be a promising tool for structural health monitoring of civil structures.
In den vergangenen Jahren ist die Entwicklung additiver Fertigungsverfahren branchenübergreifend fortgeschritten. In der Industrie ist die Verarbeitung von Polymeren wie z. B. Polylactide (PLA) oder Acrylnitril-Butadien-Styrol-Copolymer (ABS) am weitesten verbreitet, aber auch die Verarbeitung von Metallen nimmt zu. Umfragen des Fraunhofer-Institut für Produktionstechnik und Automatisierung IPA aus dem Jahr 2016 verdeutlichen, wie hoch das Potential additiver Fertigungsverfahren eingeschätzt wird und auch die Anzahl der jährlich angemeldeten Patente im Bereich des 3D-Drucks hat sich in den letzten Jahren vervielfacht.
Messen heißt wissen. Dieses Wissen spiegelt die Realität wider und soll den Statiker eines bestehenden Bauwerkes bei der Entscheidungsfindung über die Sicherheit und Zuverlässigkeit der Konstruktion unterstützen. Gegenüber dem Neubauentwurf können bei der Bewertung von bestehender Bausubstanz Sicherheitsreserven genutzt werden, da Unsicherheiten, die beim Bau auftreten können, besser bekannt oder nicht mehr vorhanden sind. Können solche Unsicherheiten genauer bewertet werden, z. B. durch die Feststellung der genauen Lage der Spannglieder bei einer Spannbetonbrücke, so können Sicherheitsbeiwerte reduziert werden, ohne dass es dabei zu Auswirkungen auf das normativ festgelegte Zuverlässigkeitsniveau kommt. Mittlerweile sind Prüfmethoden an Bauwerken wirtschaftlich einsetzbar und auch so leistungsfähig, dass die für die Tragfähigkeit und Gebrauchstauglichkeit wesentlichen Parameter identifiziert werden können. Ein wesentlicher Punkt ist hierbei die Integration derartiger Messergebnisse in Rechenmodelle, die für die Nachrechnung des Bauwerks verwendet werden. Dies können sowohl semiprobabilistische Rechenmodelle als auch vollprobabilistische Modelle sein. Bei semiprobabilistischen Methoden können aus den Messergebnissen Teilsicherheitsbeiwerte berechnet und abgeleitet werden, die dann in den bekannten Nachweisformaten gemäß den Eurocodes und der Nachrechnungsrichtlinie berücksichtigt werden. Bei vollprobabilistischen Nachweisen können die Messdaten in Form von Verteilungsdichtefunktionen mit gemessenen Variationskoeffizienten direkt in das Rechenmodell eingehen.
Air-coupled ultrasonic testing is widely used in the industry for the non-destructive testing of compound materials. It provides a fast and efficient way to inspect large concrete civil infrastructures for damage that might lead to catastrophic failure. Due to the large penetration depths required for concrete structures, the use of traditional piezoelectric transducer requires high power electric systems. In this study, a novel fluidic transducer based on a bistable fluidic amplifier is investigated. Previous experiments have shown that the switching action of the device produces a high-power broadband ultrasonic signal. This study will provide further insight into the switching behaviour of the fluidic switch. Therefore, parametric CFD simulations based on compressible supersonic RANS simulations were performed, varying the inlet pressure and velocity profiles for the control flow. Switching times are analyzed with different methods, and it was found that These are mostly independent of the slope of the velocity profile at the control port. Furthermore, it was found that an inversely proportional relationship exists between flow velocity in the throat and the switching time. The results agree with the theoretical background established by experimental studies that can be found in the literature.
Experimental analysis of the acoustic field of an ultrasonic pulse induced by a fluidic switch
(2021)
Ultrasonic inspection is a common tool for non-destructive testing in civil engineering (NDT-CE). Currently, transducers are coupled directly to the specimen surface, which makes the inspection time-consuming. Air-coupled ultrasound (ACU) transducers are more time-efficient but need a high pressure amplitude as the impedance mismatch between the air and the concrete is high and large penetration depth is needed for the inspection. Current approaches aim at eliminating the impedance mismatch between the transducer and the air to gain amplitude; however, they hardly fulfill the NDT-CE requirements. In this study, an alternative approach for ultrasound generation is presented: the signal is generated by a fluidic switch that rapidly injects a mass flow into the ambience. The acoustic field, the flow field, and their interaction are investigated. It is shown that the signal has dominant frequencies in the range of 35–60 kHz, and the amplitude is comparable to that of a commercial ACU transducer.
The workpackage 7 of the EC funded project PREDIS deals with cemented low and intermediate level radioactive waste. The focus is on NDT/monitoring technologies, digiutal twins and data/decision frameworks to optimize facilitay and waste package management. The talk describes the project setup and planned activities.
This paper summarizes the results of research aimed at assessing cracks in reinforced concrete structures using embedded ultrasonic sensors. The diffuse ultrasonic waves were considered to evaluate the health status of the tested structures. There are different algorithms used to detect cracks in the structure, but most studies have been performed on benchmark reinforced concrete (RC) structures and in laboratory conditions. Since there were difficulties with the validity of Damage detection in real structures in the presence of environmental changes and noises, the application of advanced signal processing methods was necessary. Therefore, the wavelet transform was applied to process ultrasonic signals acquired from multiple civil structures. It is shown that the ultrasonic sensors with an applied wavelet transform algorithm on collected signals can successfully detect cracks in the laboratory as well as in a real environment. Experimental results showed a perfect match for detecting damage and quasi-static load in the presence of environmental changes. The results were confirmed with other techniques. In addition, designing an extra filter for removing noises can be avoided by using the applied algorithms. The obtained results confirmed that diffuse ultrasonic sensor methodology with the proposed algorithm is useful and effective in Monitoring real RC structures, and it is better than traditional techniques.
Monitoring of reinforced concrete structures to ensure their stability and increase their service-life is a crucial element of a modern infrastructural concept. With classical methods of non-destructive testing and inspection, repeated measurements under comparable conditions are difficult to conduct. Therefore, DFG research unit FOR 2825 CoDA researches the assessment of concrete damage using ultrasound coda wave interferometry and embedded sensors. Embedding the sensors into the monitoring target reduces human and non-human factors influencing repeatability. Using Coda Wave Interferometry (CWI), small velocity changes in the material can be detected by comparison of repeated measurements. The technique is sensitive to damaging changes like cracking as well as to reversible influences like material temperature. The understanding of these different influences on the signal is crucial for the analysis of long-term monitoring data to make an educated assessment of the structure and its integrity. With several laboratory experiments in a climate chamber and a long-term experiment recording an annual cycle in a large model on an outdoor test site in Horstwalde close to Berlin, we try to understand the influence of temperature on the CWI results. The results show that the velocity change calculated by CWI does closely follow the trend of concrete temperature. After one year of data recording with the large model being exposed to environmental variations only, the calculated velocity change resembles the annual temperature curve. The data shows a linear dependency between velocity and temperature change in a range of -0.03 percent per °K to -0.06 percent per °K - regardless of specimen size. An approach to remove temperature influence from the yearly cycle recorded in the large-scale experiment using this linear relation is unable to remove high-frequency variations - especially daily influences. Low-pass filtering the data can eliminate these variations while preserving permanent shifts caused by damages. Although we have shown that the influence of temperature on long term monitoring can be removed to a significant extent, there is still an influence of environmental changes remaining in the data. Possible nonlinear effects and influences not related to temperature need to be investigated in the future.
Die zerstörungsfreie Ermittlung der Materialfeuchte in Baustoffen ist für das Bauwesen von großer Bedeutung, denn ein erhöhter Feuchteeintrag kann zu Schimmelbildung, Alkali-Kieselsäure-Reaktionen, Korrosion etc. führen und somit ein hohes Schadenspotential mit sich bringen. Auch wenn zur qualitativen Lokalisierung möglicher Schad- und Risikostellen bereits zahlreiche zerstörungsfreie Verfahren in der Praxis Anwendung finden, besteht ein Mangel an quantitativen Verfahren, die auch bei niedrigen Feuchtegehalten sensitiv sind. Eine Methode zur zerstörungsfreien und quantitativen Bestimmung des Feuchtegehaltes und der effektiven Porosität ist die 1H-Kernspinresonzanzmethode (NMR). Da NMR sensitiv für die molekulare Beweglichkeit von Wasserstoffprotonen ist, lassen sich Informationen über die Bindungsform und -stärke sowie der Umgebung (Porengröße und Mineralzusammensetzung) der Wasserstoffprotonen gewinnen. Obwohl es bereits seit über 20 Jahren mobile NMR-Messeräte gibt, wird NMR heutzutage immer noch weitestgehend im Labor eingesetzt. Dies liegt daran, dass sie aufgrund der verwendeten Permanentmagnete schwer sind und nur äußerst geringe Eindringtiefen ermöglichen. In einer derzeit laufenden Forschungsarbeit wird deshalb ein speziell für Bohrkerne entworfener NMR-Tomograph eingesetzt. Mit seiner minimalen Echozeit von 50 µs lässt sich (abhängig von der Oberflächenrelaxivität) Wasser bis in nanometergroßen Poren erfassen. Um die Sensitivität von NMR bei Teilsättigung zu untersuchen, wurden verschiedene Sandsteinproben in Exsikkatoren mit relativen Luftfeuchten im Bereich von 33 % bis 96 % gelagert. Als Referenzen dienen zum einen das NMR-Signal bei Vollsättigung und zum anderen die Porengrößenverteilung aus Quecksilberporosimetriemessungen (MIP). Um die NMR-Sensitivität zu bewerten, wird die Korrelation der NMR-Amplitude mit den gravimetrisch ermittelten Feuchtegehalten sowie ein Vergleich der Porosität aus NMR und MIP betrachtet. Zu beachten ist in diesem Zusammenhang, dass sowohl die Reduzierung der Wasserfilmdicke als auch die Verkleinerung von Porengrößen zu einer ähnlichen Verkürzung der T2-Relaxationszeit führt. Als Ausblick wird ein Lösungsansatz vorgestellt, bei dem die NMR-Signale mit der Wasserfilmdickenberechnung basierend auf der relativen Luftfeuchte im Material korreliert werden.
Since nuclear magnetic resonance with focus on 1H protons is highly sensitive to pore filling fluids, it is nowadays often applied for the investigation of porous media. Mainly in materials research and especially in the field of non-destructive testing in civil engineering it is increasingly used. Scientific questions about and based on NMR meanwhile cover a broad spectrum. To give an overview, we have reviewed various studies dealing with the determination of moisture contents and parameters such as the pore-size distribution, surface relaxivity, porosity, etc. In some papers, the monitoring of moisture transport in connection with degradation processes or admixtures was the main objective. In other papers, NMR was used for pore space analysis or even applied on site to assess the state of conservation of cultural heritage. Building materials that have been investigated in the presented studies are for example cement, concrete, woods, sandstones etc. In this paper, short descriptions and the significant results of the reviewed articles are summarized and their measurement problems and discrepancies are pointed out. A special feature of this review article is the concise tabular compilation of determined 𝑇1 and 𝑇2 relaxation times, as well as of surface relaxivity values for various materials and components. Finally, relevant aspects are summed up and conclusions about the increasing potential of NMR relaxometry for investigations of porous building materials are drawn, followed by an outlook about future applications and the need for technical development.
Zerstörungsfreie Prüfverfahren liefern wichtige Informationen zur Beurteilung von Bauwerken. Mit ihnen können Materialeigenschaften bestimmt oder die innere Struktur von Bauteilen beschrieben werden. Für letzteres werden in der Regel Volumenverfahren angewandt. Einen besonderen Stellenwert haben in der Praxis die Radar- und die Ultraschall-Echoverfahren.
Ihre Anwendung erfordert lediglich eine einseitige Zugänglichkeit
zum Bauteil. Zudem lassen sich zahlreiche unterschiedliche Prüfaufgaben lösen. Zum Stand der Technik gehören beispielsweise die Ortung von Bewehrung und Spannkanälen, die Bestimmung der Dicke von Bauteilen oder die Fehlerlokalisierung.
In diesem Beitrag wird gezeigt, wie mithilfe von Ultraschall-Echomessungen gesammelte Messwerte in ein quantitatives Messergebnis überführt werden können, das wiederum in ingenieurtechnische Berechnungen einfließen kann. Einleitend werden die Prüfmethode und die Messtechnik erläutert. Anschließend wird das Vorgehen von der Auswertung der Messergebnisse bis zur Bestimmung der bautechnischen
Kenngröße – Bauteildicke – gezeigt. Abschließend wird die Methodik an einem Referenzprüfkörper aus Polyamid demonstriert und den Ergebnissen von Messungen an Beton gegenübergestellt.
With a goal to improve our understanding of the relationships between microstructural features and ultrasonic signal behavior, concrete specimens of varying water-to-cement ratio were imaged using X-ray computed tomography (CT), and subsequently subjected to ultrasonic testing. From the CT scans, measurements were made of cement paste density and number of interfaces. Ultrasonic signals produced using a through-transmission configuration were analyzed and fit to a diffusion model to separate absorption from scattering attenuation. The results showed that at the frequencies tested, ultrasonic dissipation rate correlated weakly with paste density, while diffusivity correlated well with number of interfaces, but only if entrained air is considered separately. Cement paste density was found to be very well predicted by diffusivity, leading to a clear power-law relationship between diffusivity and compressive strength.
News from NDT@BAM
(2021)
The presentation summarizes some recent research at BAM in the field of NDT in civil engineering. the firste xample decribes lab experiments on the degardation of concrete during tunnel fires. Radar measuremenst were sucessfully used to detect interior damage. The second example decribes the first ever experiment on using muon imaging to evaluate concrete constructions.
Reinforced concrete bridges are iconic parts of modern infrastructure. They are designed for a minimum service life of 100 years. However, environmental factors and/or inappropriate use might cause overload and accelerate the deterioration of bridges. In extreme cases, bridges could collapse when necessary maintenance lacks. Thus, the permanent monitoring for structure health assessment has been proposed, which is the aim of structural health monitoring (SHM). Studies in laboratories have shown that ultrasonic (US) coda wave interferometry (CWI) using diffuse waves has high sensitivity and reliability to detect subtle changes in concrete structures. The creation of micro-cracks might be recognized at an early stage. Moreover, large-volume structures can be monitored with a relatively small number of US transducers. However, it is still a challenge to implement the CWI method in real SHM practical applications in an outdoor environment because of the complex external factors, such as various noise sources that interfere with the recorded signals. In this paper, monitoring data from a 36-m long bridge girder in Gliwice, Poland, instrumented with embedded US transducers, thermistors, and vibrating wire strain gauges, is presented. Noise estimation and reductionmethods are discussed, and the influence of traffic, as well as temperature variation, are studied. As a result, the relative velocity variation of US waves following the temperature change with a very high precision of 10−4% is shown, and a good bridge health condition is inferred. The influence of lightweight real traffic is negligible. The study verified the feasibility of the implementation of the CWI method on real bridge structures.
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.
Subfloors are layered structures, consisting largely of porous building materials, such as screed. They are often suffering damage from tap water leakage, which is a typical problem in buildings, and which has largely contributed to repair costs of almost 3 billion Euro in 2018 alone in Germany. In this context, especially mould plays a role, which is both destroying the structure and posing severe health risks.
To determine the damaging effects of moisture, it is necessary to know the respective processes occurring in building materials, especially to quantify the amount of moisture and its progress in the material. In this study, humidity sensors are used to derive the material moisture experimentally.
Capacitive sensors recording the relative humidity are embedded into the screed and in the insulation materials such as expanded polystyrene, extruded polystyrene, perlite and glass wool. For the application in screed, the sensors need to be shielded against the aggressive alkaline materials. To ensure an appropriate exchange with the environment, a permeable membrane is requested. Different membrane materials have been investigated regarding their robustness and their permeability.
In the first experimental setup, two humidity sensor arrays with seven individual sensors are embedded in homogeneous screed samples. The measured corresponding relative humidity of the screed is converted to the material moisture based on the approach of Hillerborg. In a second experimental setup, a layered structure of a complete subfloor is built in a box of 0.8 m times 0.8 m. The humidity sensors are positioned in the different insulation materials of various thicknesses. By adding water, leakage damage is simulated and its progress and effect is investigated experimentally.
The investigations point at the question if the observed moisture is able to generate damage such as mould. The moisture and corresponding humidity values are discussed. It will be shown that this low-cost hygrometric approach can be used easily for moisture monitoring of screed and insulation materials as well
Due to the importance of reinforced concrete structures for modern society, damage assessment during the entire life-cycle of such structures has become a special interest in non-destructive testing. Using embedded ultrasound sensors in combination with other measurement methods, numerical modeling and self-made data collectors, tailored specifically for monitoring tasks, the German research group DFG FOR CoDA aims to investigate and develop novel methods for damage detection and rapid model updating in reinforced concrete structures. In the first stage of the project, besides the development of custom-built, low-cost data collectors, ultrasonic transducers are embedded in a large, reinforced concrete specimen on a BAM test site near Berlin. In this experiment, the influence of changing environmental conditions (mainly temperature) on the ultrasound signal is investigated using coda-wave interferometry. The results show a correlation between changes in temperature and ultrasonic velocity. Such changes must be taken into consideration in a long-term monitoring setup to distinguish between reversible and permanent changes. By correcting the data using a linear relation between concrete temperature and velocity change to remove the seasonal trends and by low-pass filtering the data to remove daily variations can remove most of the temperature influence on the ultrasound measurements.
The purpose of this contribution is to introduce and to apply the developed approach of incorporating non-destructively gathered measurement results (instead of deterministic information and assumptions) into a reassessment model of a typical prestressed concrete road bridge and to outline the advantages. An essential part is the quality evaluation of the non-destructively measured information, that deals primarily with two questions. Could the object or parameter to be obtained reliably detected and if, how accurate are the inspection results achieved? Therefore, the importance of the combination of a probability of detection (POD)-approach and measurement uncertainty calculations is emphasized. With regard to the introduced case-study it is shown, for which structure parameters an assumption deviating from the actual (and measurable) situation has a particularly strong (and possibly arithmetically unfavorable) influence on the structural reliability. Measurements on such parameters are particularly beneficial for a reliable and robust reassessment. In conclusion, the individual reassessment results without consideration and with consideration of evaluated non-destructive inspection results are compared.
Due to increasing ageing, deterioration and loading , several prestressed concrete bridges bulit in the 1950ies and 1960ies need monitoring. Recently several bridges have sucesssfully equipped with acoustic emission sensing system. BAM works together with several universities on an add onn: Active ultrasonic monitoring combined with coda wave interferometry will help to detect subtle changes and precursors fo failure earlier than other methods.
Durch die Einbeziehung von gemessenen Daten bei der Bewertung von bestehenden Bauwerken können Unsicherheiten bei der Beurteilung des Tragverhaltens reduziert werden, ohne das normative Zuverlässigkeitsniveau einzuschränken. Informationen sind die wesentliche Grundlage für die Analyse der Zuverlässigkeit von Tragwerken. Umso wichtiger ist die Kenntnis über die Präzision und die Richtigkeit der verwendeten Informationen. Aus messtechnischer Sicht kann ein Messwert, dem keine Messunsicherheit beigeordnet wurde, als wertlos erachtet werden. Liegt ein vollständig dokumentiertes Messergebnis vor, so ist die Qualität der gemessenen Information bekannt und deren Vergleichbarkeit gewährleistet. In diesem Beitrag wird am Beispiel einer Spannbetonbrücke und zweier Grenzzustände gezeigt, wie Messergebnisse aus der zerstörungsfreien Prüfung in probabilistische Nachweise von Bestandsbauwerken einfließen können und welche Auswirkungen die Implementierung von gemessenen Daten bei der Bewertung von Bestandsbauwerken haben kann.
The analysis of the chemical composition of cement-based materials like concrete is a crucial task in civil engineering. Concrete is a multi-phase material made of cement, water, aggregates, additives and admixtures. During the damage assessment of concrete structures, the external ingress of harmful species like chlorine, alkalis or sulfates needs to be determined. In order to provide a fast and precise analysis of concrete the application of LIBS offers unique possibilities as the technique can detect all elements at air atmosphere.
To determine the condition of concrete, the information about the chemical composition, the ingress of harmful species as well as the evaluation of characteristic values like the used cement type or the type of aggregates are needed. This work presents the two-dimensional evaluation of concrete samples using LIBS and multivariate data analysis of atomic and molecular emission. For the evaluation of the main components different classification methods will be applied to identify the heterogeneity of the samples. With respect to their specific phase, different species will be quantified using multivariate regression (PLS) and important figures of merit will be presented. All results will be verified using Standard procedure of wet chemistry (XRF, potentiometric titration and ICP-OES).
Rund ein Jahr haben DIN und DKE in einem gemeinsamen Projekt mit dem Bundesministerium für Wirtschaft und Energie und zusammen mit ca. 300 Fachleuten aus Wirtschaft, Wissenschaft, öffentlicher Hand und Zivilgesellschaft an der Normungsroadmap Künstliche Intelligenz gearbeitet. Eine hochrangige Steuerungsgruppe unter dem Vorsitz von Prof. Wolfgang Wahlster hat die Erarbeitung koordiniert und begleitet.
Ziel der Roadmap ist die frühzeitige Entwicklung eines Handlungsrahmens für die Normung und Standardisierung, der die internationale Wettbewerbsfähigkeit der deutschen Wirtschaft unterstützt und europäische Wertmaßstäbe auf die internationale Ebene hebt.
New possibilities for concrete analysis 4.0 with the Laser-Induced Breakdown Spectroscopy (LIBS)
(2020)
In civil engineering the damage assessment of concrete infrastructures is an important task to monitor and ensure the estimated life-time. The aging of concrete is caused by different damage processes like the chloride induced pitting corrosion of the reinforcement. The penetration depth and the concentration of harmful species are crucial factors in the damage assessment. As a highly cost and time-consuming standard procedure, the analysis of concrete drill cores or drilling by wet-chemistry is widely used. This method provides element concentration to the total mass as aggregates and binder are homogenized. In order to provide a method that is capable to detect the element concentration regarding the cement content only, the laser-induced breakdown spectroscopy (LIBS) will be presented. The LIBS method uses a focused pulsed laser on the sample surface to ablate material. The high-power density and the laser-material interaction causes a laser-induced plasma that emits elemental and molecular line emission due to energy transition of the excited species in the plasma during the cooling phase.
As each element provides element-specific line emission, it is in principle possible to detect any element on the periodic table (spectroscopic fingerprint) with one laser shot. In combination with a translation stage the sample under investigation can be spatially resolved using a scan raster with a resolution up to 100 µm (element mapping). Due to the high spatial resolution, the element distribution and the heterogeneity of the concrete can be evaluated. By using chemometrics the non-relevant aggregates can be excluded from the data set and the element concentration can be quantified and referred to a specific solid phase like the binding matrix (cement) only. In order to analyze transport processes like diffusion and migration the twodimensional element distributions can provide deep insight into the transport through the pore space and local enrichments of elements. As LIBS is a multi-elemental method it is also possible to compare the ingress and transport process of different elements like Cl, Na, K, S, C, and Li simultaneously and evaluate cross-correlations between the different ions. Furthermore, the element mapping allows to visualize the transport along cracks. This work will show the state of the art in terms of hardware and software for an automated LIBS system as well as different application for a concrete analysis 4.0. Focus will be the application of LIBS for a fast concrete analysis.
The application of a LIBS system is presented. A diode-pumped low energy laser (3 mJ, 1.5 ns, 100 Hz) and a compact NIR spectrometer are used. A scanner allows the two-dimensional element mapping. For the quantitative analysis calibration of the system is carried out with reference samples in a concentration range of chlorine of 0.05 wt.% to 2.5 wt.%. To determine the 2D distribution of harmful elements (Cl, C), concrete cores were drilled, split and analyzed directly. By comparing the chloride ingress and the carbonation, the interaction of both processes can be visualized in one measurement that takes less than 10 minutes with a drill core of 50 mm x 100 mm. Results obtained were compared and verified with standard measurements.
Schnelle ortsaufgelöste Chloridbestimmung mit der laserinduzierten Plasmaspektroskopie (LIBS)
(2020)
Im Zuge der Ist-Zustandserfassung von Stahl- und Spannbetonbauwerken spielt die chemische Analyse von Beton eine zentrale Rolle. Durch den externen Eintrag von schädigenden Substanzen wie Chlorid oder Alkalien aus Tausalzen oder Meerwasser, kann u.U. der Beton und/oder die Stahlbewehrung angegriffen werden. Bei Überschreitung kritischer korrosionsauslösender Chloridgehalte, z.B. bei Stahlbeton 0,4 M.-% (bzw. 0,5 M.-%) und Spannbeton 0,2 M.-% bezogen auf den Zement, erhöht sich das Risiko für eine Chlorid-induzierte Korrosion. Für die Beurteilung eines Instandsetzungsbedarfs und, sofern notwendig, der Erarbeitung eines Instandsetzungskonzeptes muss nach der Rili-SIB1 ein sachkundiger Planer herangezogen werden. Standardmäßig werden für die chemische Analyse Bohrkerne bzw. Bohrmehlproben aus dem zu untersuchenden Bauwerk entnommen und nach aufwendiger Probenvorbereitung nasschemisch analysiert (nach DAfStb Heft 401 oder DIN 14629). Dabei wird der Gesamtchloridgehalt bezogen auf die Einwaage angegeben. Eine Umrechnung auf den Zementgehalt erfolgt i.d.R. durch Schätzwerte. Der Einsatz der laserinduzierten Plasmaspektroskopie (LIBS) wird seit Mitte der 90er Jahren an der Bundesanstalt für Materialforschung und -prüfung erforscht, um den Chloridgehalt durch eine ortsaufgelöste Elementanalyse an einer Betonprobe möglichst genau zu ermitteln. Die zweidimensionale LIBS-Messung mit einer lateralen Auflösung von 100 µm x 100 µm erlaubt die Berücksichtigung der Heterogenität von Beton. Dabei lassen sich die Bindemittelmatrix und die Gesteinskörnung separat betrachten, wodurch sich die Aussagekraft der Ergebnisse und somit die Sicherheit der chemischen Zustandserfassung erhöht. Durch die stetige Weiterentwicklung von Komponenten wie Laser, Spektrometer und analytischer Auswerteroutinen sind mittlerweile automatisierte Systeme für die schnelle LIBS-Analyse auf dem Markt verfügbar. In diesem Vortrag wird der aktuelle Stand der Technik anhand von Praxisbeispielen präsentiert und entscheidende Vorteile gegenüber dem Standardverfahren herausgestellt. Aufgrund der Möglichkeit für eine simultane Multielementanalyse mit LIBS werden weitere Anwendungen, wie z.B. die Ermittlung der Karbonatisierungstiefe, Untersuchungen von Transportprozessen, Chlorideintrag entlang von Rissen oder Untersuchungen von Elementverteilungen im Kontext einer AKR vorgestellt.
Im Zuge der Ist-Zustandserfassung von Stahl- und Spannbetonbauwerken spielt die chemische Analyse von Beton eine zentrale Rolle. Durch den externen Eintrag von schädigenden Substanzen wie Chlorid oder Alkalien aus Tausalzen oder Meerwasser, kann u.U. der Beton und/oder die Stahlbewehrung angegriffen werden. Bei Überschreitung kritischer korrosionsauslösender Chloridgehalte, z.B. bei Stahlbeton 0,4 M.-% (bzw. 0,5 M.-%) und Spannbeton 0,2 M.-% bezogen auf den Zement, erhöht sich das Risiko für eine Chlorid-induzierte Korrosion. Für die Beurteilung eines Instandsetzungsbedarfs und, sofern notwendig, der Erarbeitung eines Instandsetzungskonzeptes muss nach der Rili-SIB1 ein sachkundiger Planer herangezogen werden. Standardmäßig werden für die chemische Analyse Bohrkerne bzw. Bohrmehlproben aus dem zu untersuchenden Bauwerk entnommen und nach aufwendiger Probenvorbereitung nasschemisch analysiert (nach DAfStb Heft 401 oder DIN 14629). Dabei wird der Gesamtchloridgehalt bezogen auf die Einwaage angegeben. Eine Umrechnung auf den Zementgehalt erfolgt i.d.R. durch Schätzwerte. Der Einsatz der laserinduzierten Plasmaspektroskopie (LIBS) wird seit Mitte der 90er Jahren an der Bundesanstalt für Materialforschung und -prüfung erforscht, um den Chloridgehalt durch eine ortsaufgelöste Elementanalyse an einer Betonprobe möglichst genau zu ermitteln. Die zweidimensionale LIBS-Messung mit einer lateralen Auflösung von 100 µm x 100 µm erlaubt die Berücksichtigung der Heterogenität von Beton. Dabei lassen sich die Bindemittelmatrix und die Gesteinskörnung separat betrachten, wodurch sich die Aussagekraft der Ergebnisse und somit die Sicherheit der chemischen Zustandserfassung erhöht. Durch die stetige Weiterentwicklung von Komponenten wie Laser, Spektrometer und analytischer Auswerteroutinen sind mittlerweile automatisierte Systeme für die schnelle LIBS-Analyse auf dem Markt verfügbar. In diesem Vortrag wird der aktuelle Stand der Technik anhand von Praxisbeispielen präsentiert und entscheidende Vorteile gegenüber dem Standardverfahren herausgestellt. Aufgrund der Möglichkeit für eine simultane Multielementanalyse mit LIBS werden weitere Anwendungen, wie z.B. die Ermittlung der Karbonatisierungstiefe, Untersuchungen von Transportprozessen, Chlorideintrag entlang von Rissen oder Untersuchungen von Elementverteilungen im Kontext einer AKR vorgestellt.
Es wird ein alternatives Verfahren zur schnellen zweidimensionalen Analyse der chemischen Zusammensetzung von Baustoffen vorgestellt. Dieses beruht auf der Kombination des Abtrags einer geringen Materialmenge durch einen gepulsten Laser, der Plasmaerzeugung und der Analyse der Lichtemission des Plasmas mittels optischer Spektroskopie. Es können prinzipiell alle Elemente gleichzeitig nachgewiesen werden. Damit ist es möglich einerseits Informationen über den Baustoff an sich als auch über den Eintrag potentiell schädigender Elemente (z. B. Cl, C, Na, K, S) zu gewinnen. Das Verfahren kann automatisiert im Labor oder als mobile Version direkt vor Ort eingesetzt werden. Es liefert zweidimensionale Bilder der Elementverteilung in Echtzeit. Es kann ohne große Probenvorbereitung z. B. direkt an der Querschnittsfläche eines Bohrkernes gemessen werden. Die Messung dauert nur wenige Minuten. Das Prinzip der Messung, die Vor- und die Nachteile, sowie die verfügbare Gerätetechnik für den Einsatz im Labor oder direkt am Bauwerk werden vorgestellt. Abschließend werden praktische Beispiele für die Anwendung des LIBS-Verfahrens zur Erfassung des Chlorid-Eintrages, der Karbonatisierung und der Verteilung von Schwefel präsentiert.
Es wird ein alternatives Verfahren zur schnellen zweidimensionalen Analyse der chemischen Zusammensetzung von Baustoffen vorgestellt. Dieses beruht auf der Kombination des Abtrags einer geringen Materialmenge durch einen gepulsten Laser, der Plasmaerzeugung und der Analyse der Lichtemission des Plasmas mittels optischer Spektroskopie. Es können prinzipiell alle Elemente gleichzeitig nachgewiesen werden. Damit ist es möglich einerseits Informationen über den Baustoff an sich als auch über den Eintrag potentiell schädigender Elemente (z. B. Cl, C, Na, K, S) zu gewinnen. Das Verfahren kann automatisiert im Labor oder als mobile Version direkt vor Ort eingesetzt werden. Es liefert zweidimensionale Bilder der Elementverteilung in Echtzeit. Es kann ohne große Probenvorbereitung z. B. direkt an der Querschnittsfläche eines Bohrkernes gemessen werden. Die Messung dauert nur wenige Minuten. Das Prinzip der Messung, die Vor- und die Nachteile, sowie die verfügbare Gerätetechnik für den Einsatz im Labor oder direkt am Bauwerk werden vorgestellt. Abschließend werden praktische Beispiele für die Anwendung des LIBS-Verfahrens zur Erfassung des Chlorid-Eintrages, der Karbonatisierung und der Verteilung von Schwefel präsentiert.
Integration of fibre reinforcement in high-performance cementitious materials has become widely applied in many fields of construction. One of the most investigated advantages of steel fibre reinforced concrete (FRC) is the deceleration of crack growth and hence it’s improved sustainability due to e.g. decrease of permeability of concrete by aggressive substances. Additional benefits are associated with the structural properties of FRC, where fibres can significantly increase the ductility and the tensile strength of concrete. In some applications, such as tunnel linings or industrial slabs, it is even possible to entirely replace the conventional reinforcement, leading to significant logistical and environmental benefits. Fibre reinforcement can, however, have critical disadvantages and even hinder the performance of concrete, since it can induce an anisotropic material behaviour of the mixture if the fibres are not appropriately oriented. For a safe use of FRC in the future, reliable non-destructive methods need to be identified to assess the fibres’ orientation in hardened concrete. In this study, ultrasonic material testing, electrical impedance testing, and X-ray computer tomography have been investigated for this purpose using specially produced samples with biased or random fibre orientations. This paper demonstrates the capabilities of each of these NDT techniques for fibre orientation measurements and draws conclusions based on these results about the most promising areas for future research and development using these techniques.
Recent years have seen extended use of ultrasonic techniques for concrete infrastructure assessement. They are applied for quality assurance and condition assessement at bridges, power plants, dams and other important objects. However, there are still a couple of significant limitations. They include, but are not limited to depth of penetration, imaging complex structures or early stage detections of distributed damage. The talk will give information on recent research in this area. Specifically, the application of the unique deep penetration system LAUS is shown. Ultrasonic monitoring using embedded transducers to check for subtle and sudden changes in the material is introduced as well. Comments on initiatives for validation, standardization and certification will be given.
Intensified Alkali-Silica Reaction (ASR) damage has occurred on German roadways in recent years, leading to requirements for compulsory pre-construction investigation of aggregate alkali sensitivity using concrete tests with external alkali supply. However, since these tests are time-consuming and cost-intensive, there is interest in replacing them with a solubility test on pure aggregate in 0.1 M KOH solution at 80°C with a defined NaCl content (1wt.-%). In this context, the influence of aggregate pore structure on SiO2 and Al2O3 solubility was investigated in this project. This paper compares the results of porosity studies with X-ray Computed Tomography (3D-CT) and the Brunauer-Emmett-Teller (BET) method on individual quarried and river gravel granules of both rhyolite and greywacke. For visualization and quantification of both externally accessible and fully enclosed surfaces of granules using X-ray 3D-CT, special software tools were developed. The results demonstrated that the river gravel granules had significantly larger externally accessible surfaces than the quarried granules. BET measurements on individual stones showed, as expected, that measured surfaces were about three orders of magnitude larger than those from the X-ray 3D-CT analyses due to the higher spatial resolution of BET. There was no apparent correlation between the X-ray 3D-CT and BET surface areas. Mercury porosimetry measurements indicate that this may be due to the presence of significant porosity below the spatial resolution of the X-ray 3D-CT. A comparison of SiO2 and Al2O3 solubility measurements with the X-ray 3D-CT and BET surface area data resulted only in weak, inconclusive correlations, indicating the need for further experimental investigation.
The moisture content of the subfloor has to be determined before installation to avoid damages of the floor covering. Only if the readiness for layering is reached, an installation without damages can be expected in all cases. In general, three different approaches exist to measure the residual water content: determination of the moisture content, determination of the water release, or determination of the corresponding relative humidity. All three approaches are tested in laboratory at eight different screed types including two different samples thicknesses in each case. The moisture content and the water release are measured by sample weighing, the corresponding relative humidity is measured by embedded sensors. All three approaches are compared and correlated to each other. The evaluations show only weak correlation and, in several cases, contradicting results. Samples are considered as being ready for layering and not-being ready for layering at the same time, depending on the chosen approach. Due to these contradicting results, a general threshold for the risk of damage cannot be derived based on these measurements. Furthermore, the experiment demonstrates that the measurement of corresponding relative humidity is independent of the considered screed type or screed composition. This makes the humidity measurement to very promising approach for the installation of material moisture monitoring systems in the future.
Monitoring of Concrete with Embedded Ultrasound Sensors, Coda Waves and a Novel Measurement Device
(2020)
Using embedded ultrasound (US) sensors in civil engineering structures and Coda Wave Interferometry (CWI), DFG research group CoDA aims for developing methods of concrete damage assessment by combination of Micro and macro scale simulations and experiments. BAMs goal within the project is the provision of small and durable measurement equipment on the one hand, and the investigation of damage sensitive parameters and development of an imaging algorithm on the other hand. In this study we focus on the development of the measurement system and the influence of temperature variations on the measurements.
Die Überwachung von Bauteilen aus Stahl- oder Spannbeton mit Ultraschall hat in Labor- und Technikumsversuchen schon vielversprechende Ergebnisse gezeigt. Besonders gute Resultate wurden dabei mit eingebetteten Ultraschalltrans-ducern und bei Auswertung der Daten mit der hochsensiblen Codawelleninterferometrie erzielt. Erfasst werden können neben Temperatur- und Feuchteeffekten auch Belastungszustände und jegliche Art von Schädigung, die mit Mikro- oder Makrorissbildung einhergeht.
Seit 2019 untersucht die DFG-Forschergruppe 2825 „CoDA“ (Sprecher: Prof. Christoph Gehlen, TU München) ver-schiedenste Aspekte dieser innovativen Technologie mit dem Ziel, Einflussgrößen quantitativ zu erfassen, Umweltein-flüsse zu korrigieren und 3D-Auswerteverfahren zu verbessern. Final soll eine am Bauwerk einsatzfähige Methode ent-stehen, die klassische Monitoringverfahren ergänzt und erweitert sowie Input zu einem Update des statischen Systems liefert.
Aufgaben der BAM in der Forschergruppe ist neben Verbesserung und Adaptierung der Messsystem und Sensorik auch Langzeitversuche an einem Großobjekt und Testinstallationen an Realbauwerken. Hierzu liegen erste Ergebnisse vor, die zeigen, dass die Technologie auch außerhalb des Labors einsatzfähig ist.
Die Überwachung von Bauteilen aus Stahl- oder Spannbeton mit Ultraschall hat in Labor- und Technikumsversuchen schon vielversprechende Ergebnisse gezeigt. Besonders gute Resultate wurden dabei mit eingebetteten Ultraschalltrans-ducern und bei Auswertung der Daten mit der hochsensiblen Codawelleninterferometrie erzielt. Erfasst werden können neben Temperatur- und Feuchteeffekten auch Belastungszustände und jegliche Art von Schädigung, die mit Mikro- oder Makrorissbildung einhergeht.
Seit 2019 untersucht die DFG-Forschergruppe 2825 „CoDA“ (Sprecher: Prof. Christoph Gehlen, TU München) ver-schiedenste Aspekte dieser innovativen Technologie mit dem Ziel, Einflussgrößen quantitativ zu erfassen, Umweltein-flüsse zu korrigieren und 3D-Auswerteverfahren zu verbessern. Final soll eine am Bauwerk einsatzfähige Methode ent-stehen, die klassische Monitoringverfahren ergänzt und erweitert sowie Input zu einem Update des statischen Systems liefert.
Aufgaben der BAM in der Forschergruppe ist neben Verbesserung und Adaptierung der Messsystem und Sensorik auch Langzeitversuche an einem Großobjekt und Testinstallationen an Realbauwerken. Hierzu liegen erste Ergebnisse vor, die zeigen, dass die Technologie auch außerhalb des Labors einsatzfähig ist.
The composition of concrete determines its resistance to various degradation mechanisms such as ingress of ions, carbonation or reinforcement corrosion. Knowledge of the composition of the hardened concrete is therefore helpful to assess the remaining service life of an existing structure or evaluate the damage observed during inspections. For example, for most existing concrete structures the type of cement originally used is not known and must therefore be determined afterwards. This paper presents a preliminary study on the application of laser-induced breakdown spectroscopy (LIBS) to identify the type of cement. For this purpose, ten different types of cement were investigated. For every type, three cement paste prisms were produced: (i) prisms dried, ground and pressed into tablets, (ii) prisms dried and (iii) prisms untreated. LIBS measurements were performed with a diode-pumped low energy laser (1064 nm, 3 mJ, 1.5 ns, 100 Hz) in combination with two compact spectrometers which cover the UV and NIR spectral range. A reduced subset of spectral features was used to build a classification model based on linear discriminant analysis. The results show that the classification of homogenized pressed cement powder samples provides a high accuracy, however, factors such as a different sample matrix and moisture content can affect the accuracy of the classification. The study demonstrates that LIBS is a promising tool to identify the type of cement.
Entwicklung eines luftgekoppelten Ultraschall-Echo-Prüfverfahrens mittels fluidischer Anregung
(2020)
In vielen technischen Bereichen werden Ultraschallverfahren zur zerstörungsfreien Werkstoffprüfung (ZfP) eingesetzt um auf Basis der Signalstärke und der Laufzeit Einbauteile und Beschädigungen zu orten. Luftgekoppelter Ultraschall spielt bisher in kommerziellen Anwendungen vor Allem im Bauwesen eine untergeordnete Rolle, da die Differenz der akustischen Impedanzen von Luft und Festkörpern immense Verluste beim Übergang des Schallsignals hervorruft.
Im Rahmen des Promotionsvorhabens soll die Eignung eines neuartigen Anregungsprinzip untersucht werden, mit dem ein Großteil dieser Verluste vermieden werden soll. Anstelle eines Festkörpers soll mit Hilfe einer fluidischen Düse Druckluft zur Signalerzeugung eingesetzt werden. Die Impedanzverluste zwischen Aktuatormembran und Umgebungsluft entfallen daher.
Die gezielte Schallerzeugung durch einen pulsierenden Freistrahl ist weitgehend unerforscht. Es ist daher notwendig, den so erzeugten Schallpuls in der Interaktion mit dem transienten Strömungsfeld zu untersuchen. Das kompressible Medium Luft und die geringen räumlichen Dimensionen einer hochfrequenten Pulsdüse werfen darüber hinaus einige Herausforderungen hinsichtlich der eingesetzten Messtechnik auf. Hier sollen geeignete Verfahren weiterentwickelt und validiert werden, um die Eignung des fluidisch erzeugten Pulses zu überprüfen.
In diesem Vortrag werden erste Messungen an einem fluidischen Schalter mit denen an einem kommerziellen Luftultraschallprüfkopf verglichen.
Due to the high sensitivity of coda waves to the smallest structural alterations such as strain, humidity or temperature changes, ultrasonic waves are a valid means to examine entire structures employing networks of ultrasonic transducers. In order to substantiate this ex ante assessment, the viability of measuring ultrasonic waves as a valid point of reference and inference for structural changes is to be further scrutinized in this work. In order to investigate the influence of mechanical strain on ultrasonic signals, a four-point bending test was carried out on a reinforced concrete beam at Ruhr University Bochum. Thus, measurements collected from a network of selected transducer pairings arranged across the central, shear-free segment of the test specimen, were correlated to their respective strain fields. Detected ultrasonic signals were evaluated employing Coda Wave Interferometry. Such analysis comprised the initial non-cracked state as well as later stages with incremental crack depth and quantity. It was to ascertain that the test specimen can in fact be qualitatively compartmentalized into areas of compression and tension identified via Relative Velocity Changes presented in Attribute Maps. However, since results did not entail a zero crossing, i.e., neither positive nor negative values were to be calculated, only relative changes in this work displayed staggered over the height of the object under test, are discussed. Under the given methodological premises, additional information is currently required to make quantitative assertions regarding this correlation of ultrasonic and strain results. This holds true for the comparability of the ultrasonic and strain results for both non-cracked and even the cracked state.
Dynamic stall often occurs under conditions of simultaneous unsteady pitching and surging (e.g., rotorcraft and wind turbines), butmanymodels employ a dimensionless time base that implicitly assumes that surging is superimposed, in a quasi-steady manner, on dynamic pitching. An unsteady wind tunnel was used to examine this assumption, where a technique was developed to quantify the unsteady effects of surging on a pitching NACA 0018 airfoil. The technique involved performing multiple harmonic pitching experiments under nominally steady freestream conditions that bracketed a corresponding 50% surging amplitude (1.25 ⋅ 105 ≤ Re ≤ 3.75 ⋅ 105). By interpolating these data, unsteady-pitching/quasi-steady-surging data sets were constructed and compared with de facto synchronous pitch and surging experiments, thereby isolating the unsteady effects of surging on a pitching airfoil. Both large and small poststall maximum angles of attack (αs + 5° and αs + 15°) were considered at multiple pitch-surge phase differences. During deep dynamic stall (αs 15°), with large-scale separation, surging was seen to have a secondary effect on the unsteady aerodynamics. However, at small poststall maximum angles of attack (αs + 5°), either light or deep dynamic stall behavior was observed depending upon the pitch-surge phase difference. This was attributed to Reynolds number history effects, exemplified by boundary-layer transition, and thus it can be referred to as “transitional” dynamic stall.
Reinforced concrete and several other building materials deteriorate during their service life. Almost all deterioration processes are related to moisture. For understanding those processes, knowing the exact amount of the moisture content and the moisture transport characteristics is crucial. In fact, the moisture transport in porous materials is a two-phase flow; consisting of the vapour and the liquid phase. The two corresponding parameters are the diffusion coefficient and the hydraulic conductivity. By means of an inverse approach, these two coefficients are experimentally determined. The moisture transport of eight different screed types during hydration and evaporation is monitored based on embedded humidity sensors. The measured humidity profiles are converted into a pore saturation by adapting the approach of Hillerborg to the measured pore volume distribution. The summation of all pore saturations yields the total moisture content including the ink-bottle effect during desorption. The pore volume distribution was determined by mercury intrusion porosimetry as well as by gas adsorption. The measurement of the humidity and the pore saturation in the entire sample enables to invert the mass balance in order to extract the diffusion coefficient and the hydraulic conductivity. The determined diffusion coefficients of the eight investigated screeds are between m2s-1 and m2s-1.