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- 8.6 Faseroptische Sensorik (7)
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- 7.4 Baustofftechnologie (5)
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Der Beitrag berichtet von einer aufwändigen Messkampagne vor Ort, mit der die Emissionen direkt an einer Rohzementmühle, die Transmissionen im Baugrund und die Immissionen in benachbarten Bebauungen genauer untersucht wurden. Im Ergebnis der ausgewerteten Messungen sowie strukturdynamischer Berechnungen zeigte sich, dass der Baugrund zwar aus statischer Sicht sehr gut tragfähig, aber dynamisch als eher ungünstig zu beurteilen ist. Weiterhin wurde festgestellt, dass in dynamischer Hinsicht ein vorgenommener Bodenaustausch tendenziell eine Verschlechterung der Aufstellbedingungen darstellte. Für das betroffene Bürogebäude konnte nachgewiesen werden, dass durch den Betrieb der Zementmühle die Anregung der Deckeneigenfrequenzen zu deutlich spürbaren Resonanzschwingungen führte. Aufgrund der tieffrequenten Anregung durch den Mühlenbetrieb sind Maßnahmen zur Minderung der Schwingungsemissionen generell und insbesondere nach erfolgter Aufstellung der Mühle schwer bzw. gar nicht zu realisieren. Im Beitrag wird daher darauf eingegangen, auf welche Aspekte bei der Planung zur Aufstellung derart großer Mühlen zwingend zu achten ist.
Synopsis: The paper presents the results of a study on the influence of agitation on the structural build-up of fresh cement pastes using a penetration test. It first presents results about the influence of the penetrating shape’s geometry on the influence on the cement paste specimen. Then, results are shown for the load-deflection curves depending upon time and agitation before testing. Based on the observation of the force required to penetrate the sample, conclusions on the structural build-up can be made. The observations were made over the course of time with samples that were left at rest and partly agitated before testing at different time steps. The setup allows to identify the contribution of the chemical reaction to the structural build-up process. The presence of superplasticizer obviously reduces the load that cement paste can withstand, while the loss of thixotropy is found notably lower than that of neat cement pates
Synopsis: Lately, there has been rising attention to superplasticizers (SP) based on polyphosphate esters. However, the influence of the molecular structure of the polyphosphate polymers on time-dependent properties such as structural build-up has not been examined yet intensively. To investigate this effect, three comb polyphosphate superplasticizers with different charge densities were synthesised by free radical polymerisation. Our findings indicate that SP with the lowest and medium charge densities extend the induction period more strongly than the SP with the highest charge density. The reduction of the structural build-up rate is linearly dependent on the dosage and concentration of the functional group of polyphosphate SP in the cementitious system. This study proposes a mathematical equation expressing the relationship between the structural build-up rate during the induction period and the molecular structure of the polyphosphate SP.
Influence of the hydroxypropylation of starch on its performance as viscosity modifying agent
(2022)
Synopsis: Starch is a commonly used viscosity modifying agent (VMA). The performance of starch as VMA depends on its origin (e.g. potato, corn, cassava, etc.) and corresponding molecular properties, such as molecular weight, ratio between amylose and amylopectin etc. Depending upon the application, the efficiency of starch can be enhanced by hydroxypropylation. The maximum degree of substitution (DoS) cannot be greater than 3.0, which is the number of hydroxy groups per glucose monomer in the polymer. In the current research three potato starches exhibiting the DoS of 0.4, 0.6 and 0.8 were utilised. The influence of the modified starch on the rheological properties and hydration of cement paste, as well as the viscosity of the pore solution were investigated. Our findings show that the starch with the highest DoS increases the dynamic yield stress the most, while the plastic viscosity is less dependent on the DoS. Additionally, starch with the highest DoS retards hydration to lower degree than other starches.
Usually, geometric irregularities are considered as the main cause of ground vibrations from trains. A varying stiffness of the track, the track support and the soil can also generate ground vibrations. The regular stiffness variation of the track on and between the sleepers results in a deterministic dynamic axle load. The random stiffness variation of the track support yields also dynamic axle loads which are generated by the acceleration of the unsprung mass (from the varying wheel displacements under the static axle load). The random stiffness variation has a second effect. The pulses from the passage of the static axle loads are superposed regularly to the quasi-static response, but also irregularly to yield a “scattered” part of the axle pulses. The same holds for a random variation of the soil stiffness. All these effects of stiffness variations have been calculated by wavenumber-domain multi-beam track models, a random finite-element soil model and the superposition of axle impulses in a stochastic simulation. The results are confronted with many measurements at different sites. It is concluded that the stiffness variation of the track and the soil generate an important ground vibration component near railway lines.
A simple and fast prediction scheme is presented for train-induced ground and building vibrations. For the emission, finite-element boundary-element or multiple-beam-on-continuous-soil models of the track have been analysed and approximated by faster track-on-Winkler-soil models. The vehicle-track interaction due to irregularities yields the excitation forces. For the transmission of waves in the soil, the wavenumber integral of the compliance of layered soils has been evaluated. The calculation time is reduced for the prediction by using the solution of a homogeneous half-space with a frequency-dependent wave velocity (the dispersion) of the soil. For the immision, many 2 and 3-dimenisonal finite-element building models have been investigated, and a good approximation has been established by a 1-dimensional soil-wall-floor model. In addition, the axle sequence of the train, the quasi-static and the “scattered” response of the soil, and the wave propagation from a tunnel to a pile foundation of a building have been included.
Vibration measurements have many causes and many technical and natural sources. Problems can sometimes be solved by short-term measurements, but in many cases, a long-term measurement is necessary. In long-term measurements of days, weeks, months and even years, it is easy to collect a huge quantity of raw data, but at the end, the post-processing of these data can be exhausting (for example one-year vibration data of a wind energy tower). A software has been developed which con-sists of measuring and evaluation routines where the measuring routines can operate different meas-uring systems and different measuring cards. The main advantage of this software is the fact that the interesting evaluations can be integrated in the measuring process so that the characteristics of the vibration can be extracted without storing all the raw data. Only important time segments are stored, for example train passages. The overall concept of the software and the main evaluation routines will be described in some details. Examples of our measurement experience will illustrate the capabilities of the software. 1) Surveying construction work in nearby sensitive buildings (for example an old wind tunnel), including a stable alarm system and meaningful vibration limits. 2) Prediction of train-induced vibration for a planned building to prevent annoyance and to improve the building design. 3) Modal analysis and long term measurements of several single- or multi-span, concrete or steel bridges 4) Modal and wave analysis of coupled floors in a historical building (“Neues Palais” at Potsdam). 5) Soil properties of various measurement sites (different routines to evaluate the dispersion). Moreover, from many projects, amplitudes, frequencies, and attenuation laws have been collected and analysed for the different sources such as vibratory or impact pile driving and ground compaction, demolition work with different machines, blasting in quarries and in tunnel works, bomb and mine clearing.
Ultrasonic Coda Wave interferometry has the potential to detect minute changes in scattering materials like concrete. By permanently installing ultrasonic transducers in concrete, DFG Research unit CoDA aims to develop methods for concrete damage assessment in Germany's aging infrastructure. To test the methods developed in simulations and laboratory experiments on a large scale, we have implemented several ultrasonic transducers at the Gänstorbrücke Ulm, one of Germany's most monitored road bridges. Since fall 2020 we are monitoring parts of the center of the Bridge, as well as an abutment, and compare the results to the commercial monitoring system. All data is recorded with a self-made data collection device, the so-called W-Box, and analyzed with different coda wave-based algorithms to detect signal and volumetric velocity changes. The long-term measurements show that the influence of temperature changes on strains and therefore ultrasound velocity changes calculated with coda waves can be monitored. The capabilities and limitations of the coda wave-based monitoring system are tested in a controlled experiment. Static loading using a truck with varying loads at several positions allows the calibration of the system to improve the detectability of possibly damaging loads and changes induced by this loading. A map of velocity change analyzing data from this load experiment shows that the influence of load on the material and strain distribution can be detected with array measurements.
Earth buildings and adobe construction have been in use for thousands of years. Loam is locally available, environmentally friendly and has a small CO2 footprint mainly resulting from extraction and transport. Because of these reasons loam is one of the building materials of the future.
Water resistance is an important aspect when using loam as a construction material. If the structural option of building protruding roofs cannot be implemented, the water resistance of clay can be improved by stabilization.
Positive properties of the material should not be diminished by stabilization. For example, the desired regulation of air humidity, which however cannot be fully preserved by cement stabilization.
For an alternative and more appropriate stabilisation, different mixtures with calcined clays with alkali activator were examined. The calcination temperature for clays is usually lower than for cement and starts at 600° C. The calcined clays therefore also have a lower CO2 footprint compared to cement. To keep the mixture environmentally friendly potassium was used as alkali ion. It could be proven that the alkali activated polymer enhances water resistance of the loam mixture. Provided some further optimization the material shows potential to replace cement without compromising on the positive properties of clay.
The low calcination temperature for clays (600° C) and a low CO2 footprint compared to cement make them an interesting material. To find a stabilization method for loam using calcined clays, activation by alkalis was necessary. Therefore (and to keep the mixture environmentally friendly) KOH was used as alkali activator. It could be proven that the alkali activated polymer enhances water resistance of the loam mixture.
The retroreflective corner echo is used, for example, in ultrasonic non-destructive testing of metals to find fatigue cracks in tubes or shafts. If the much weaker crack tip signal is additionally detected, the crack length can also be determined. A corner reflection occurs in cases of surface breaking cracks with predominantly perpendicular orientation to the surface. The intensity of the corner reflection depends on the angle of incidence and on the ultrasonic wave mode used. For the reliable detection of vertical surface breaking cracks in metals, transversal waves are commonly used, which propagate at an angle of 37° to 53° to the inspection surface. As shown in this contribution, the wide spread low frequency ultrasonic arrays with dry point contact sources available for ultrasonic testing of concrete also allow to receive corner echoes. These devices generate transversal waves in concrete structures with a large divergence of the sound field. A series of experiments was carried out with such dry point contact arrays on concrete specimens with artificial test defects and controlled induced cracks of different depths. The ultrasonic time-of-flight signals were recorded, exported and reconstructed utilising the SAFT (Synthetic Aperture Focusing Technique) algorithm. The SAFT reconstruction parameters were adjusted to visualize the corner echo indication. As will be shown, with this targeted processing, the reproducible detection of surface breaking cracks in concrete is possible. The retroreflective corner echo can thus be exploited in civil engineering for non-destructive inspection of concrete.
Turning maneuvers of rail vehicles involve demanding situations for the infrastructure because of the steering forces generated and the resultant lateral load transfers. Such forcing situation involves increased tangential forces that consume kinetic energy from the vehicle. While much attention has been paid in the literature to the damage in such infrastructure segments, the energy that is dissipated in these maneuvers, has not been explicitly addressed. In this paper, a specialized scale-down infrastructure is used to characterize the effect of the initial speed of the vehicle and its mass, on the magnitude of both the rail forces developed and the amount of dissipated energy during turning maneuvers. The outputs of the experiments suggest that most of the original kinetic energy is dissipated at the first portion of the turn. Also, that the magnitude of the wheel forces and the amount of dissipated energy, correlate. That is, a greater rail damage would correspond to greater energy dissipation.
This contribution summarizes actual developments and draft fundamental teaching topics in the field of nondestructive testing in civil engineering (NDT-CE). It is based on the first memorandum on teaching and research in the field of NDT-CE at German speaking universities and provides an overview of the academic education and highlights possible focuses, especially in teaching but also takes into account noteworthy developments and topics in research in the field of NDT-CE.
Suggestions are given for the development and advancement of the teaching curricula in regards to a comprehensive and sound professional education of students in civil engineering and adjacent disciplines.
The Infrastar training school provides lectures and hands-on training to Master and PhD students, early-stage researchers and (young) professionals on all aspects of asset management of civil infrastructures with respect to fatigue of materials. It is organized yearly since 2019 and up to 20 profiles are selected to attend the 3.5-day training.
A team of 7 teachers provides insight in multi-disciplinary and intersectoral basic concepts in three core fields, ranging from the design to the dismantling of the structures (bridges and wind turbines):
1. Monitoring and auscultation,
2. Structural and action models,
3. Reliability, risk and decision analyses.
Each year, a recognized expert is invited to deliver a keynote lecture to raise interest in a specific aspect of one of the fields covered. The presentation is recorded and published on the training school website.
Concrete structures are invaluable assets to a society and managing them efficiently and effectively can be supported by information gathered through structural health monitoring (SHM). In this paper, a combined approach based on passive, i.e., acoustic emission (AE), and active, i.e., ultrasonic stress wave (USW) monitoring techniques for application to concrete structures is proposed and evaluated. While AE and USW are based on the same underlying physics, i.e., wave motion in solids, they differ fundamentally with respect to the nature of the source. For the former, external stimuli such as mechanical loads or temperature cause the rapid release of energy from initially unknown locations. As a result, AE events are unique and cannot be repeated. For the latter, a known source at a known location is employed at a specified time. This approach is thus controlled and repeatable. It is
argued that a combination of these two techniques has the potential to provide a more comprehensive picture of ongoing fracture processes, damage progression, as well as slowly occurring aging and degradation mechanisms.
This combined approach does thus promise new opportunities to support condition assessment of concrete structures. After providing an overview and comparison of the two techniques, results, and observations from a full-scale laboratory experiment and an in-service bridge monitoring study are discussed to demonstrate the promise of the proposed combined monitoring approach. Finally, suggestions for further work are presented.
Muon tomography applied to assessment of concrete structures: First experiments and simulations.
(2022)
Non-destructive techniques for reinforced or prestressed concrete inspection 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 new fields of application. As a first step towards practical application in civil engineering 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. In a second step, we have performed simulations to assess the potential for a set of important testing problems such as grouting defects in tendon ducts. The next steps include the development of mobile detectors and optimizing acquisition and imaging parameters.
To improve ultrasonic imaging of concrete structures, we transferred a seismic migration technique, the Reverse Time Migration (RTM), to non-destructive testing. A 2D elastic RTM algorithm was tested on synthetic ultrasonic echo data. Compared to the typically used synthetic aperture focusing technique (SAFT) as well as our acoustic RTM algorithm, the presented elastic RTM results show an enhancement in imaging vertical reflectors and complex features inside the 2D numerical concrete model.
Corrosive geothermal brines are a major challenge to geothermal power-plants. For cost reasons, plant designers favorize low alloyed steels, e.g., carbon steel, which are susceptible to uniform and localized corrosion when exposed to geothermal brines having acidic and saline properties. To solve such problem, coatings or inhibitors would be a protective solution as an alternative to the use of high alloyed materials.
This study investigated a coating system consisting of polyaniline/silicon dioxide basing on locally available resources. Protection against corrosion of carbon steel is shown by long-term exposure and electrochemical tests of coated carbon steels, performed in an artificial acidic and saline geothermal brine, comparable to real conditions at a site in Indonesia.
Therefore, an integrated coating system is presented for corrosion protection, combining the electro-chemical functionality of polyaniline and the physical advantages of silica.
In this paper, a cost-efficient distributed fiber optic measurement system based on Rayleigh scattering is presented. The distributed digital incoherent optical frequency domain reflectometry (I-OFDR) method is developed for detection of significantly large strain in the range from 3 % up to 10 % as required by end users. For this purpose, a vector network analyzer used in the I-OFDR is replaced by a compact and cost-effective digital data acquisition system. This digital emitting/receiving unit enables the recording of the complex transfer function carrying information about the local deformations along the entire sensing fiber.
Investigating the spalling behaviour of a concrete mixture using large scale members is complex and expensive. Thus, the susceptibility of concrete to spalling is investigated by means of fire tests on small scale specimens. However, the reduction of the fire exposed surface increases the influence of boundary effects. Macrocracking and the water loss via the lateral surfaces reduce the impact of the thermomechanical and thermohydraulic damage mechanisms and therefore, lower the risk of spalling. Thus, the influence of different restraints on the spalling behaviour of intermediate scaled specimens (Ø=0.47 m; h=0.29 m) was investigated for two ordinary concrete mixtures. Both mixtures had the same composition except the type of aggregate. One mixture contained only quartzitic aggregates, whereas the other mixture was made with basalt grit as coarse aggregates. Applied steel rings restrained the thermal expansion of the specimen and prevented the loss of water, whereas applied steel sheet primarily reduced the loss of water during the fire test. Additionally, the fire exposed surface of one specimen of each mixture was pre-dried under controlled climate conditions and fire tested with applied steel rings. The results show a higher spalling volume for the ring restrained specimens compared to the other covering types. Further, the pre-dried boundary zone leads to delayed and decreased occurrence of spalling compared to the non-dried specimens for the concrete with quartzitic aggregates and to a prevention of spalling for the mixture with basalt aggregates. Additionally, the results show that the influence of the thermomechanical behaviour of the coarse aggregates increases with increasing restraint. The restrained conditions of the specimens with applied steel ring are comparable to the conditions in the centre of a large scale member. This test series is a further step for possible concept of steel ring restraint concrete specimens as “screening-tests” in upcoming projects.
Existing concrete structures were usually designed for lifetimes of several decades. The current and urgently required efforts to increase sustainability and protect the environment will likely result in extended service lives up to 100 years. To achieve such objectives, it is required to assess structures over their entire lifecycles. Non-destructive testing (NDT) methods can reliably support the assessment of existing structures during the construction, operational, and decommissioning phases. One of the most important and safety-relevant components of a prestressed concrete structure are the tendons. NDT methods such as the ultrasonic echo method are suitable for both the detection and the localization of the tendons, i.e., the measurement of their geometrical position inside the component. The uniqueness of structures, concrete heterogeneity, and varying amounts of secondary components such as the reinforcement represent obstacles in the application of these methods in practice. The aim of this contribution is to demonstrate a practicable procedure, that can be used in the field to determine the parameters required for the measuring data analysis without extensive knowledge about the investigated components. For this purpose, a polyamide reference specimen is used to show which steps are required to obtain reliable imaging information on the position of tendons from the measurement data. The procedure is then demonstrated on a concrete test specimen that covers various relevant and practice-oriented test scenarios, such as varying tendon depths and component thicknesses.
Der diesjährige Beitrag zeigt den Einfluss ausgewählter Versuchsrandbedingungen auf das Abplatzverhalten von Normalbeton im kleinskaligen Brandversuch. Im Rahmen der Untersuchungen wurde die Abplatzneigung von acht zylindrischen Betonprüfkörpern der Maße Ø47 cm x 29 cm aus Normalbeton analysiert. Vier Prüfkörper wurden aus Normalbeton mit quarzitischer Gesteinskörnung hergestellt, wohingegen die grobe Gesteinskörnung (>2 mm) der anderen vier Prüfkörper durch Basaltsplitt ersetzt wurde. Neben der Art der groben Gesteinskörnung wurden zusätzlich ausgewählte Versuchsrandbedingungen variiert. Dies umfasste zum einen die Vorlagerung der Prüfkörper bei verschiedenen Umgebungsfeuchten und zum anderen die Ummantelung der Prüfkörper mit zwei Ringen aus Stahl bzw. einem durchgehenden Stahlblech. Die Ummantelung der Prüfkörper mit Stahlringen im Brandversuch führt zu einer Behinderung der thermischen Ausdehnung der Prüfkörper und zum Aufbau thermomechanischer Zwangsdruckspannungen. Diese Zwangsdruckspannungen werden maßgeblich von der thermischen Ausdehnung der verwendeten Gesteinskörnung beeinflusst. Zusätzlich wird die Feuchteabgabe über die Mantelfläche behindert. Der Einsatz des Stahlblechs reduziert lediglich die Abgabe der Feuchtigkeit über die Mantelfläche und induziert vernachlässigbare Zwangsdruckspannungen im Beton. Die Brandbeanspruchung der Prüfkörper erfolgte einseitig nach der Einheits-Temperaturzeitkurve (ETK) über einen Zeitraum von 60 Minuten. Mit Hilfe eines Laser Scanners wurden vergleichende 3D-Messungen der brandbeanspruchten Oberfläche vor und nach dem Brandversuch durchgeführt. Aus diesen Messungen konnten das Abplatzvolumen, die Abplatztiefenverteilung und die maximale Abplatztiefe ermittelt werden. Während des Brandversuchs wurde die Temperaturverteilung im Prüfkörper mittels fünf eingebetteter Mantelthermoelemente in verschiedenen Tiefen bestimmt. Die Ergebnisse zeigen, dass die Abplatzungen an der brandbeanspruchten Oberfläche infolge der Prüfkörperummantelung mit Stahlringen bzw. Stahlblech zunehmen. Bei einer Ummantelung mit Stahlringen mit gleichzeitiger Vortrocknung der Betonrandzone platzt der Prüfkörper mit quarzitischer Gesteinskörnung ab, wohingegen der Prüfkörper mit basaltischer Gesteinskörnung keine Abplatzungen aufweist. Dies wird auf die unterschiedliche Ausprägung der thermomechanischen Schädigung infolge der unterschiedlichen thermischen Ausdehnungskoeffizienten von Quarzkies und Basalt zurückgeführt.
Die Ergebnisse der Brandprüfserie zeigen, dass die Variation der Prüfrandbedingungen bei Beibehaltung der Prüfkörpergröße einen maßgeblichen Einfluss auf das Abplatzverhalten haben. Die Randbedingungen sind daher bei kleinskaligen Brandversuchen, die zukünftig als „Screening-Tests“ ersatzweise für kostenintensive Bauteilversuche verwendet werden sollen, zu berücksichtigen.
Der Beitrag stellt experimentelle Untersuchungen zum Feuerwiderstand von Carbonzuggliedern vor. Die Anwendung von Zuggliedern aus Carbon ist durch die außergewöhnlich gute Ermüdungs- sowie hohe Zugfestigkeit des Werkstoffs motiviert. Ferner ergibt sich gegenüber Stahl eine deutliche Reduzierung des Querschnitts. Perspektivisch sollen damit Stahlzugglieder im Netzwerkbogenbrückenbau ersetzt werden. Durch die Materialsubstitution werden Ressourcen nicht nur beim Zugglied selbst, sondern auch am Gesamttragwerk der Brücke eingespart. Da zum Tragverhalten von Carbonzuggliedern im Brandfall bislang keine Erkenntnisse vorliegen, sollen mit Hilfe von Bauteilbrandversuchen die Versagensmechanismen sowie die grundsätzliche Gleichwertigkeit gegenüber konventionellen Stahlhängern untersucht werden.
Der Beitrag beschreibt die Durchführung und die Ergebnisse von Brandversuchen an mechanisch belasteten Carbonzuggliedern. Die realmaßstäblichen Versuche werden am unikalen Zugbauteil-prüfofen der BAM im Mai 2022 durchgeführt. Das Versuchsprogramm umfasst vier Carbonzugglieder mit kreisrunden Querschnitten in zwei verschiedenen Durchmessern, ⌀36 mm und ⌀51 mm. Aus Gründen der Vergleichbarkeit erfolgt die Brandbeanspruchung nach Einheits-Temperaturzeitkurve (ETK). Während des Brandversuches wird die axiale Prüfkraft von 750 kN bzw. 1500 kN bis zum Eintreten des Probenversagens konstant gehalten. Zusätzlich werden die Längsverformung sowie die Bauteil- und Brandraumtemperaturen aufgezeichnet.
In einem Vorversuch an einem unbelasteten Bauteilabschnitt eines Carbonzuggliedes konnte das Brand- und Erwärmungsverhalten des Verbundmaterials untersucht werden. Dabei zeigten sich Makrorisse und eine Zersetzung der Kunststoffmatrix sowie eine hohe Temperaturbeständigkeit der Kohlenstofffasern. Durch das Fehlen der schützenden Matrix reagierten die Carbonfasern sensibel auf Querbeanspruchung. Dies führte zu einem Bruch und Ablösen der oberflächennahen Fasern.
Intumescent coatings are used in civil engineering to improve the fire resistance of steel constructions. Due to the thin coating thickness and the profile-following application, the architectural appearance of the steel structure can be preserved. EN 13381-8 provides regulations to determine the contribution to the fire resistance of intumescent coatings applied to steel beams and columns. The scope of this standard excludes tension members and steel members with solid sections. For products that have already been successfully tested and assessed on beams and columns, EN 13381-10 offers the possibility based on unloaded fire tests to extend the scope of application of intumescent coatings to tension bars with solid sections. This approach contradicts the national safety level in Germany, where mechanically loaded testing is mandatory. Therefore, a new part of the test standard series EN 13381 is currently developed to enable the application of intumescent coatings on steel tension bars with solid section based on mechanically loaded fire tests. BAM has already carried out numerous fire tests on loaded steel tension members with intumescent coatings. Currently, in the BAM research project FIRESTEMIC, the influence of the steel bar orientation and the profile type on the thermal performance of intumescent coatings are investigated. Both questions concerning the thermal protection ability of intumescent coatings were analysed based on three different test sets, which were carried out in the tension furnace (Fig. 1a). The fire exposure corresponds to the standard temperature-time curve according to EN 1363-1.
This paper summarises the main findings from the fire tests conducted in the FIRESTEMIC project. The results serve as an experimental background for the proposal of the new standard. Regarding the bar orientation, a new test setup was developed and proposed for the new standard. The two tested commercial water-based intumescent coatings with applied dry film thickness from 1.5 mm to 2.5 mm showed only a slight dependence on the bar orientation. In terms of the steel profile type, circular and rectangular solid sections with identical section factor and applied dry film thickness were tested. It was observed that the circular solid bars with diameter 30 mm heat up faster compared to the solid rectangular bars with dimension 30×30 mm (Fig. 1b). Also at larger steel bars, i.e. diameter 40 mm and dimension 40×40 mm, the same trend occurred. Thus, it is recommended for the new test standard to allow a transfer of the test results from circular to rectangular solid sections. In addition, the paper will describe and explain the test and assessment procedure proposed for the newly developed standard.
Bei einer turnusmäßigen Inspektion der B1 Brücke am Altstädter Bahnhof in Brandenburg an der Havel wurden im Dezember 2019 Risse und Hohlstellen entlang der Längsträger festgestellt. Als Ursache wurde das Versagen eines signifikanten Anteils der Spanndrähte der konzentrierten Spannglieder vermutet. Die Brücke wurde umgehend für den Verkehr gesperrt und nach weiteren, umfassenden Untersuchungen im Mai 2021 abgebrochen.
Um die Sicherheit des unter der Brücke geführten Verkehrs zu gewährleisten, wurde die Brücke unter anderem mit einem Schallemissionsmesssystem überwacht. Mit diesem Verfahren ist es möglich, Spanndrahtbrüche zuverlässig zu detektieren und zu lokalisieren. In dem gut einjährigen Überwachungszeitraum wurden mehr als einhundert Spanndrahtbrüche detektiert, die in einigen Bereichen der Brücke besonders konzentriert auftraten.
Vor dem Abbruch der Brücke war es möglich, den Nachweis der Detektion von Spanndrahtbrüchen mit der Schallemissionsanalyse und ihre Ortungsgenauigkeit anhand von künstlich erzeugten Drahtbrüchen zu validieren. Weiterhin konnte das sichtbare Schadensbild, Längsrisse und Hohlstellen über den Spannkanälen, auf die lokale Schädigung der Spanndrähte zurückgeführt werden. Derzeit wird ein Merkblatt erarbeitet, in dem die Erfahrungen in der bisherigen Anwendung der Schallemissionsanalyse zum Monitoring von Spanndrahtbrüchen zusammengefasst werden.
In diesem Beitrag werden die Grundlagen der Schallemissionsanalyse und die Ergebnisse der Validierungsversuche an der Brücke in Brandenburg dargestellt. Weiterhin werden die Vorteile und Grenzen der Brückenüberwachung mittels Schallemissionsanalyse sowie Maßnahmen zur Sicherstellung der Qualität und der Zuverlässigkeit diskutiert.
The oxidation of sulfur in aqueous solutions is an important process in several geochemical and industrial contexts. In concrete technology, it is particularly relevant for the protection of steel reinforcement in slag cement-based concretes, as the presence of sulfides affects the passivation of the steel and corrosion initiation in the presence of chlorides. However, the knowledge about the kinetics of sulfide oxidation in alkaline solutions, such as concrete pore solutions, is incomplete. To address this issue, we prepared solutions with 0.80 M NaOH, and nominally 10, 100 and 350 mM HS−, respectively, and followed the evolution of the sulfur species during bubbling with oxygen-containing gas (concentration 100% or 22%) using X-ray absorption near-edge structure (XANES) spectroscopy at the sulfur K-edge. Polysulfide, thiosulfate, sulfite and sulfate were identified and quantified via linear combination fitting of the spectra, using previously measured standards. A preliminary analysis of the results shows that the initial rate constant of sulfide oxidation depends strongly on the HS− concentration, while it less strongly depending on the oxygen concentration, in line with previous results for lower pH values. Approaches to fit the data to obtain the rate constants of all relevant reactions will be discussed.
Brückenbauwerke, die im Spannblockverfahren hergestellt wurden, können bauartbedingt wasserstoffinduzierte Spannungsrisse über den gesamten Querschnitt eines Spannkanals aufweisen. Das Risswachstum kann infolge dynamischer Beanspruchung über die Nutzungsdauer fortschreiten und zu Spanndrahtbrüchen führen.
This study aims to develop a real time structural health monitoring method by ultrasonic tests combined with advanced six component (6C) translation and rotation measurements. Conventionally, the investigation of the velocity and acceleration response in the translation direction is used to obtain the eigenfrequencies of structures. Recently the measurement of rotation has been considered to fully characterize the dynamic behavior of structures. This research undertakes the evaluation of a novel 6C sensor (IMU50-iXblue) with components originally developed for navigation for the purpose of bridge monitoring. However, as for all vibration recordings, there is a certain influence of environmental conditions (mainly temperature) which may affect evaluation and the results of structural assessment. We propose applying the cross-correlation function to the 6C ambient vibration signals to reconstruct wave propagation and using coda wave interferometry (CWI) to obtain internal velocity variation from waveforms. A field experiment on a large-scale prestressed concrete bridge model is presented. To verify that we are able to identify the pre-stress loss even in presence of temperature effects, we perform measurements in two different scales: the ultrasonic and output-only, vibration measurements. The change in the structural properties due to the pre-stress loss should be detected by the pulse velocity change. The results reveal both the performance and advantages of ultrasonic techniques and the capabilities of 6C sensors. To conclude, the application of CWI to wave signals contributes to a comprehensive assessment for bridge monitoring.
Introduction of a monitoring system for Bingham fluids in additive manufacturing with concrete
(2022)
Freeform additive manufacturing of concrete structures is a rising technology in civil engineering with several fascinating advantages. Nonetheless, to ensure reliability and structural integrity, standards and quality control are required in the future to bring this technology into the market. As the concrete is manufactured continuously, continuous quality control of the printing process is also required, i.e. comprehensive process monitoring. At BAM, a test rig will be installed, enabling the printing of concrete structures with a maximum size of 2 m x 1 m x 1 m (l x w x h). Here, process monitoring is the focus of the test rig. In this study, we show the results of the first pump tests, including the measurement of several parameters such as temperature and pressure along the supply system, i.e. from the concrete pump to the printer head.
Die B1 Brücke am Altstädter Bahnhof in Brandenburg an der Havel wurde im Dezember 2019 aufgrund von während einer turnusmäßigen Inspektion entdeckten Rissen und Hohlstellen entlang der vorgespannten Längsträger gesperrt und im Mai 2021 abgebrochen. In der Zwischenzeit wurde die Brücke detailliert überwacht. Vor dem Abriss wurden zudem umfangreiche Untersuchungen zur Bestandsaufnahme und Schadensanalyse sowie Tests moderner Sensorik vorgenommen. Dabei konnte sowohl die richtige, zuvor nicht sicher bekannte Anzahl von Spanngliedern in den Querträgern sicher ermittelt werden als auch durch moderne Varianten der Schwingungsmessungen und der faseroptischen Sensorik zusätzliche Kenntnisse püber das Bauwerksverhalten ermittelt werden . In dem Beitrag werden die Verfahren mit ihren Möglichkeiten und Grenzen vorgestellt, die Ergebnissee an der Brücke in Brandenburg erläutert und zukünftige Einsatzmöglichkeiten diskutiert.
This contribution introduces an investigation of a large-scale prestressed concrete bridge model (“BLEIB” structure at the BAM-TTS open air test site) by means of on-site cooperative measurements. This bridge has an external post-tensioning system and has been instrumented with the ultrasonic transducers, temperature sensors and optical fibers for Distributed Acoustic Sensing (DAS). Our experiment was designed to test the suitability of the novel 6C sensors developed within the framework of the GIOTTO project – the IMU50. The IMU50 sensor enables vibration measurements in translation along three axes and rotation around three axes. The geophone sensors were considered for complementary measurements of vertical velocity response. In the experiment, several perturbations were achieved by controlling the external influence factors such as loading and prestressing changes. The aim of the integrated measurement strategy was to fully observe the results of the condition change and to verify the effectiveness of multiple sensors for bridge monitoring.
To reveal the corrosion resistance of casing steel/mortar interface in CO2 injection condition, sandwich samples were prepared and exposed up to 20 weeks in aquifer fluid under 10 MPa and 60 °C. Cross section analysis revealed the crevice corrosion as main mechanism instead of pitting corrosion, which would be expected to happen in the extremely high Chloride concentration. Detailed analysis using EDS line scan shown the slow diffusion of Chloride, suggesting why pitting did not happen after 20 weeks. To mimic the passivated steel surface, the steel coupon was passivated in simulated pore solution having pH 13.5 for 42 days. The passivated coupon was further exposed to NGB solution for 28 days. Electrochemical characterization was performed along the exposure processes to reveal the change in impedance, indicating the corrosion resistance of steel casing/mortar interface.
Damage Assessment of Civil Structures Using Wave Propagation Analysis and Transmissibility Functions
(2022)
A common damage detection method in civil engineering is to monitor dynamic characteristics such as natural frequencies and modal shapes, which are directly related to the structural stiffness. However, the discrepancy in measured natural frequencies may result from environmental changes, the soil-structure interaction or effects caused by earthquakes. This makes it difficult to decide whether the local change in structural properties is caused by damage or other factors. To address this challenge and thus improve the current damage detection method, the wave propagation analysis method and the transmissibility relationship were considered. In our study, the wave propagation field in structures was reconstructed by applying the Normalized-Input-Output-Minimization (NIOM) method to vibration recordings. Wave velocities and transmissibility functions were considered as reference values for the damage indicators. This paper demonstrates the evaluation of local property changes in two examples of large-scale structures: a 14-story RC building and a 64 m long pedestrian bridge. The proposed damage indicators show a clear correspondence to structural changes.
Recently, non-destructive testing in civil engineering (NDT-CE), in particular of concrete components, has successfully mastered the leap from research to practice. Several methods have been established for field inspections to determine the concrete cover of reinforcement or to estimate the compressive strength as well as other parameters related to the concrete material. In addition, the application of nondestructive testing is indispensable, if information about the inner structure - such as the location of rebars and tendon ducts or the damage-related condition assessment to detect grouting defects, honeycombs, delamination, or corrosion - is required. Besides the selection of a suitable NDT method and an appropriate inspection system, the reliability of the results depends largely on the person who applies the non-destructive inspection technique and evaluates the inspection results. To ensure a high quality of non-destructive concrete evaluation as well as to keep the uncertainty caused by the inspection personnel to a minimum, structured, consistent and regulated theoretical as well as practical training of inspection personnel is essential. To close this gap, the subcommittee of education (UA-A) within the committee for NDT-CE of the German Society for Nondestructive Testing (DGZfP) has been reactivated in 2018 to establish uniform training standards for nondestructive concrete inspections in the long term. The subcommittee consists of scientists, practitioners, authorities, and clients. So far, the national standard DIN 4871 “Non-destructive testing - Qualification and Certification of NDT personnel in Civil Engineering (NDT-CE)” was developed and is currently under review. This standard considers the civil-industry-specifics, for example, that standards for NDT of concrete, as well as related product standards with a few exceptions, still do not exist at the moment. Within this presentation, the concept, the connection to ISO 9712 and other standards as well as an overview of the developed German standard DIN 4871 will be presented.
Structural health monitoring (SHM) intends to improve the management of engineering structures. The number of successful SHM projects – especially SHM research projects – is ever growing, yielding added value and more scientific insight into the management of infrastructure asset. With the advent of the data age, the value of accessible data becomes increasingly evident. In SHM, many new data-centric methods are currently being developed at a high pace. A consequent application of research data management (RDM) concepts in SHM projects enables a systematic management of raw and processed data, and thus facilitates the development and application of artificial intelligence (AI) and machine learning (ML) methods to the SHM data. In this contribution, a case study based on an institutional RDM framework is presented. Data and metadata from monitoring the structural health of the Maintalbrücke Gemünden for a period of 16 months are managed with the RDM system BAM Data Store, which makes use of the openBIS data management software. An ML procedure is used to classify the data. Feature engineering, feature training and resulting data are performed and modelled in the RDM system.
The contribution discusses the processing and analysis of data generated on two different ways of investigations for impact damage in reinforced concrete structures. Damage investigations are essential to determine type and characteristics of damage and thus the residual capacity. Damage describing data is generated using two different types of investigation, a non-destructive tomographic as well as numerical examination. Subsequently, data of both sources was merged and analysed. Within the research project “Behaviour of structural components during impact load conditions caused by aircraft fuel tank collision” reinforces concrete plates were damaged by impact loading, see Hering (2020). Afterwards the damaged specimens were investigated tomographically as well as numerically using several methods and models. Aim of the presented research work was to specify an objective comparability of numerical data with experimentally determined damage patterns and based on this, to establish a quantitative damage evaluation.
Falltürme sind Bauwerke, die als Versuchsanlagen sehr speziellen, impulsartigen Belastungen ausgesetzt sind. Am Fallturm auf dem Testgelände Technische Sicherheit (TTS) der BAM wurde bei Routineinspektionen eine Abnahme der strukturellen Integrität in Form von Vorspannungsverlusten in den Bolzen der Stahlverbindungen festgestellt. Um ein Verständnis für die zugrundeliegenden Trag- und Schädigungsmechanismen zu erlangen, wurde ein umfassendes Structural Health Monitoring (SHM) System geplant und am Bauwerk installiert unter Nutzung von digitalen Bauwerksmodellen. Für die Auslegung des Überwachungssystems, insbesondere aber zur Unterstützung der Untersuchung des Schädigungsprozesses, wurden Finite-Elemente-Modelle erstellt. Um sicherzustellen, dass die FE-Modelle das reale Tragwerksverhalten mit ausreichender Genauigkeit abbilden, mussten sie jedoch in Bezug auf die gemessenen Antworten des Tragwerks kalibriert werden. Der vorliegende Beitrag beschreibt experimentelle und numerische Untersuchungen zur Identifizierung des strukturellen Systems des Stahlrohrgitterturms in Vorbereitung einer Überwachungskampagne. Die Auswertung von gemessenen Schwingungen unter ambienter Anregung ermöglichte die Identifizierung der Eigenfrequenzen mehrerer globaler Schwingungsmoden des Fallturms. Zur Modellvalidierung wurde zunächst eine Sensitivitätsanalyse durchgeführt, um die Parameter mit dem größten Einfluss zu ermitteln. Anschließend wurde ein evolutionärer Algorithmus (EA) zur Optimierung nach dem Prinzip der Minimierung der Differenzen zwischen gemessenen und simulierten charakteristischen Antworten eingesetzt. Das aktualisierte Modell wurde schließlich an der dynamischen Reaktion der Turmstruktur infolge einer realen Falltest-induzierten Stoßbelastung validiert. Die Ergebnisse zeigten eine gute Übereinstimmung zwischen numerischen und experimentellen Ergebnissen.
Reinforced concrete is a widely used material for power generation structures, where load scenarios like impact loadings need to be considered. In this context mechanical splicing systems for the connection of reinforcement bars are of specific interest and impact resistance for the splicing systems has to be verified. High speed tensile tests need to be performed on splicing systems for reinforcement bars to confirm the capability of the coupler to resist impact loading. Furthermore, the ability of the reinforcement steel to dissipate energy by ductile behaviour with pronounced plastic strains should be confirmed by these tests. During the last decades comprehensive experiences were developed at BAM performing high speed tensile tests on reinforcement bars as well as on several splicing systems. For the lack of available standards defining these tests in detail an appropriate test procedure was developed and continuously optimized during this period at BAM. The test procedure is partially based on testing principles adapted from available standards. The main intention behind this test procedure is to perform high-speed tensile tests with a specific constant strain rate generated at the specimen. Furthermore, main objective was to establish a procedure to guarantee the comparability of test results for different diameter of reinforcement as well as for different types of couplers. Besides the pure execution of the high-speed tensile tests, the test specification also declares how to evaluate the measurements and the test results. Finally, some typical results will be presented in this contribution.
Investigation of temperature effects on ultrasonic velocity in a prestressed concrete bridge model
(2022)
Ultrasonic measurements have recently been applied in the context of structural health monitoring in civil engineering. The ultrasonic method is very sensitive to environmental conditions and material property changes, which might be caused by prestress losses and loading effects on the bridge. However, it remains a challenge to identify the damage mechanisms causing structural property changes from the above mentioned factors influencing ultrasonics. The focus of this contribution is to investigate the correlation between ultrasonic velocities and temperature by the coda wave interferometry (CWI) analysis. Long-term monitoring of a prestressed concrete bridge model under field conditions was carried out to observe the effect of temperature. The homogeneous change in concrete caused by temperature was revealed by applying the CWI method. The threshold value of the best correlation coefficients for the linear regression of temperature and ultrasonic velocity changes was also studied. Experimental assessment will provide a ratio for temperature correction to evaluate the pre-stressing force decrease and its consequences on crack opening or extension.
Self-ignition Behaviour of Bulk Materials: How Reliable is the Extrapolation of Laboratory Tests?
(2022)
The self-ignition behaviour of bulks is usually determined with the aid of isoperibolic or adiabatic hot storage tests; the sample volumes here are typically in the range of approx. 100 cm³ to several litres. The extrapolation to technically relevant volumes holds, however, considerable uncertainties. To reduce these uncertainties, a test stand was set up that allows to investigation sample sizes of up to 1000 dm³. These allow the study of selfignition behavior closer to the conditions that prevail in practice. The suitability (or unsuitability) of the established methods (including dangerous goods classification tests) were to be demonstrated and influencing variables that cannot be measured on a laboratory scale were to be determined.
In addition, various analytical methods were used for the determination of reaction kinetic data of solid bulk materials and to predict the self-ignition behaviour of large storages. These are thermal analyses such as differential scanning calorimetry DSC, simultaneous thermal analysis STA as well as microcalorimetry for highprecision measurement of heat flows.
Complete test series using all mentioned test methods will be presented for crosslinked polyvinyl-pyrrolidone (PVP) and conclusions regarding the applicability of the extrapolation methods will be discussed. Hot storage tests up to semi-industrial scale and their combination with various thermoanalytical methods allow a much more precise prediction of the self-ignition behaviour of large bulks. A further promising approach to estimate their heat release rates, and hence, the risk of self-ignition under technical conditions, is the use of microcalorimetric methods.
Remote gas sensors mounted on mobile robots enable the mapping of gas distributions in large or poorly accessible areas. A challenging task however, is the generation of three-dimensional distribution maps from these spatially sparse gas measurements. To obtain high-quality reconstructions, the choice of optimal measuring poses is of great importance. Remote gas sensors, that are commonly used in Robot Assisted Gas Tomography (RAGT), require reflecting surfaces within the sensor’s range, limiting the possible sensing geometries, regardless of whether the robots are ground-based or airborne. By combining ground and aerial robots into a heterogeneous swarm whose agents are equipped with reflectors and remote gas sensors, remote inter-robot gas measurements become available, taking RAGT to the next dimension – releasing those constraints. In this paper, we demonstrate the feasibility of drone-to-drone measurements under realistic conditions and highlight the resulting opportunities.
Es wird ein faseroptisches Messsystem vorgestellt, welches auf Basis der Rayleigh-Rückstreumessung eine ortsauflösende Detektion von signifikant großen Dehnungen bei Bauvorhaben im von Anwendern geforderten Größenordnungsbereich von 3 % bis 10 % ermöglicht. Mit dem Verfahren der digitalen inkohärenten optischen Frequenzbereichsreflektometrie (I-OFDR) werden materialspezifische Vorteile robuster perfluorierter polymeroptischer Fasern (PF-POF) erschlossen. Für den angestrebten industriellen Einsatz für die Zustandsüberwachung im Bereich des Tunnel- und Spezialtiefbaus soll die spezifische Rückstreuzunahme in der PF-POF hochauflösend gemessen werden. Diese Veröffentlichung zeigt in ersten Projektergebnissen die Eignung des digitalen I-OFDR für eine Dehnungsdetektion von bis zu 10%.
Es wird eine gekoppelte Finite-Element-Randelementmethode zur Berechnung von Pfahlgrün-dungen in inhomogenen (geschichteten) Böden vorgestellt. Sie beruht auf den Greenschen Funktionen (Punktlastlösungen) für inhomogene Böden. Diese Lösungen können auch für die Wellenausbreitung in der Tiefe, zum Beispiel von einem Bahntunnel zu einem eingebetteten Gebäude, dem Kellergeschoss benutzt werden. Die Punktlastlösungen in der Tiefe werden mit der Halbraumlösung an der Bodenoberfläche und mit der Vollraumlösung verglichen und Gesetzmäßigkeiten für geschichtete Böden abgeleitet. Zu den Pfahlgründungen werden die Horizontalnachgiebigkeiten von Pfählen in geschichteten Böden dargestellt. Für den homogenen und den kontinuierlich steifer werdenden Boden werden Potenzgesetze für den Boden- und Pfahleinfluss aufgestellt. Der Vergleich mit dem Winkler-Modell der rein lokalen Bodenreaktion zeigt, dass die Winkler-Bettung in allen Fällen einen zu kleinen Bodeneinfluss ergibt.
Am Beispiel von mehreren Bauvorhaben wird untersucht, mit welchen Unsicherheiten bei der Anwendung von Korrelationen zwischen Sondierergebnissen und Scherwellengeschwindigkeit zur Beschreibung der dynamischen Bodeneigenschaften gerechnet werden muss. Die Auswertung von Korrelationen aus der Literatur zur Abschätzung der Scherwellengeschwindigkeit aus Drucksondierdaten zeigt, dass bei homogenen Sandböden die Verteilung der Scherwellengeschwindigkeit über die Tiefe sehr gut wiedergegeben werden kann. Die Korrelationen haben dabei die Tendenz, den tatsächlichen Wert zu überschätzen. In den vorliegenden Fällen haben sich Überschätzungen zwischen 20 % und 50 % ergeben. Mit zunehmendem Feinkornanteil werden die Unsicherheiten aus den Korrelationen jedoch größer.
In den letzten Jahrzehnten wurde die Degradation der polymeren Matrix von flammgeschützten Polymeren eingehend untersucht. Erst in den letzten Jahren jedoch hat sich die Frage der Lebensdauer der Funktionalität des Flammschutzes selbst als wichtige Fragestellung herauskristallisiert, z.B. bei Kabeln. Deshalb sollte diese Fragestellung durch systematisch-variierte künstliche Klima- und Bewitterungstests an flammgeschützten Polymerwerkstoffen untersucht werden.
In this contribution, we consider two applications in which probabilistic approaches can potentially complement or enhance the design and assessment of offshore wind turbine foundations. First, we illustrate in a numerical example that probabilistic modelling can be helpful in dealing with chang-es in turbine locations during the planning phase of an offshore wind farm. In this case, spatial probabilistic modelling of the ground conditions enables (a) an inference of the soil properties at the modified turbine location from field data collected at different locations across an offshore wind farm site and (b) an optimisation of further site investigations. Second, we discuss the uncer-tainties and risks associated with the installation of large diameter monopiles in soils with hetero-geneities such as strong layers and/or embedded boulders. Subsequently, we present a concept for modelling, understanding, and managing these risks based on a probabilistic model of the subgrade conditions, monopile, and subgrade-pile-interaction.
Integrated Coating System for Corrosion Protection of Carbon Steel in Artificial Geothermal Brine
(2022)
Corrosive geothermal brines are a major challenge to geothermal power-plants. For cost reasons, plant designers prefer to use carbon and low alloyed steels, which are susceptible to uniform and localized corrosion when exposed to geothermal brines having acidic and saline properties. To solve such problem, coatings or inhibitors would be a protective solution as an alternative to the use of high alloyed materials.
This study investigated a coating system consisting of polyaniline/silicon dioxide based on resources locally available in Indonesia. Protection against corrosion of carbon steel was shown by long-term (28 day) exposure and electrochemical tests of coated carbon steels, performed in an artificial acidic and saline geothermal brine, comparable to the conditions encountered at a site in Indonesia.
Therefore, an integrated coating system is proposed for corrosion protection, combining the electrochemical functionality of polyaniline and the physical advantages of silica.