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- 7 Bauwerkssicherheit (62)
- 8 Zerstörungsfreie Prüfung (55)
- 3 Gefahrgutumschließungen; Energiespeicher (44)
- 9 Komponentensicherheit (43)
- 9.3 Schweißtechnische Fertigungsverfahren (32)
- 4 Material und Umwelt (20)
- 2 Prozess- und Anlagensicherheit (18)
- 5 Werkstofftechnik (17)
- 7.2 Ingenieurbau (17)
- 6 Materialchemie (15)
This study aims at investigating the use of coal fly ash-based alkali activated mortars as passive fire protection system for steel structures. These systems are used to slow down the temperature rise of the steel substrate in case of fire. In addition, the protective system should guarantee the ability to prevent and/or mitigate steel corrosion phenomena. The behavior of a light-weight mortar was compared to that of a normal-weight mortar. Density and porosity were measured to better characterize the physical properties of the mortars. The degree of protection in case of fire was assessed by performing medium-scale fire tests. Acoustic emission measurements were conducted to analyze cracking phenomena during the high temperature exposure. The corrosion process was evaluated using an electrochemical approach in order to monitor the durability of the developed material. Preliminary results show that a 20 mm-thick layer of light-weight mortar is able to protect the steel substrate from reaching the critical temperature of 500 °C for 38 minutes in case of cellulosic fire. In addition, alkali activated mortars provide protection for carbon steel in presence of aggressive environment (i.e. presence of chlorides). The corrosion resistance is strictly related to the physical properties of the developed mortars.
Corrosion of steel reinforcement in concrete is one of the major deterioration mechanisms limiting the service life of reinforced concrete structures. While for conventional (Portland cement-based) concretes a great amount of experience exists in this regard, the factors that determine the onset of reinforcement corrosion in alkali-activated materials are incompletely understood yet. One aspect of corrosion protection is leaching and the accompanying changes of the concrete pore solution. In the present study, alkali-activated fly ash mortars with embedded carbon steel rebars were exposed to de-ionised water for periods up to 330 days, and the electrochemical response of the steel (free corrosion potential, polarisation resistance), the alteration of the mortar (ohmic resistance, mechanical strength, pore size distribution) as well as the pore solution composition were monitored. Although substantial alkali leaching was observed, the pH of the pore solution remained at values sufficient to protect the embedded steel from depassivation. The mortar did not exhibit indications of significant deterioration. Thus, the present results suggest that leaching is not critical for protection of steel reinforcement in alkali-activated fly ash mortars and concretes.
The production of Portland cement causes a substantial environmental impact, since the calcination of limestone results in high emissions of carbon dioxide. The use of supplementary cementitious materials such as calcined clays as partial replacement for Portland cement offers a solution to limit this environmental impact. This paper investigates four clays from deposits in central Germany with the aim of obtaining pozzolans for the production of Portland-pozzolana cement. The results obtained show that the calcined clays possess pozzolanic properties, which differ depending on calcination temperature and the relative amounts of Kaolinite and 2:1 clay minerals.
Mortars for application on concrete, e.g. repair mortars or protective coatings, need to have a durable bond to the substrate. This bond is determined by the adhesion between the two materials and by the differential deformations of the mortar and the substrate. In the present contribution, the hygric deformations (shrinkage/expansion) of novel one-part alkali-activated mortars and their bond to concrete substrates are studied. Shrinkage of the mortars was studied at 50 % r.H., while expansion was studied on mortars stored over an open water surface (> 99% r.H.). The bond behaviour was studied by pull-off tests according to DIN EN 1542 and by optical microscopy.
The alkali-activated mortars exhibit hygric deformations much lower than the deformations of an established, commercial mortar for sewer maintenance that was tested as reference in parallel with the alkali-activated mortars. The bond behaviour of the alkali-activated mortars depends strongly on their mix-design and curing. Optical microscopy showed that in the
mortars with lower bond strength, cracks developed in the mortar during curing. Mortars with appropriate mix-design and curing did not exhibit cracking, and their pull-off strength (up to >3 MPa) conformed to the requirements of relevant standards.
The industrial use of ultrashort laser pulses has made considerable progress in recent years. The reasons for this lie in the availability of high average powers at pulse repetition rates in the several 100 kHz range. The advantages of using ultrashort laser pulses in terms of processing precision can thus be fully exploited. However, high laser intensities on the workpiece can also lead to the generation of unwanted X-rays. Even if the emitted X-ray dose per pulse is low, the accumulated X-ray dose can become significant for high-repetition-rate laser systems so that X-ray exposure safety limits must be considered. The X-ray emission during ultrashort pulse laser processing was investigated for a pulse duration of 925 fs at 1030 nm wavelength and 400 kHz repetition rate. Industrially relevant materials such as steel,aluminum and glass were treated. Tungsten served as reference. X-ray spectra were recorded, and X-ray dose measurements were performed for laser treatment in air. For laser intensities > 2 × 10^13 W/cm2, X-ray doses exceeding the regulatory exposure limits for members of the public were found. Suitable X-ray protection strategies are proposed.
This work presents the testing of a developed elec¬tronic nose for outdoor odour nuisance monitoring. The sensor system consists of a sampling system, a measuring cell equipped with commercially available low-cost gas sensor elements and a data acquisition system. The sensor testing showed that the developed prototype is able to distinguish, identify and partially quantify individual odorous substances like ethanol, ammonia and hydrogen sulphide. Ethanol and ammonia could even be detected below or near their odour threshold.
Heute werden in Stahlkonstruktionen zunehmend hochfeste Baustähle (Streckgrenze ≥ 960 MPa) eingesetzt. Daher sind auch in Schweißnähten solche hohen Festigkeiten gefordert. Jedoch können hohe Eigenspannungen aufgrund geringerer plastischer Dehnungsreserven in diesen Schweißnähten die Sicherheit der Bauteile vermindern. Diese sind insbesondere im Zusammenhang mit Wasserstoff und Härtegefügen risskritisch. Niedrige Bauteilbeanspruchungen und Kosteneinsparungen lassen sich heutzutage mit modernen fokussierten Lichtbogenvarianten und engeren Nahtspalten erreichen. Die vorliegende Studie zeigt allerdings, dass diese modernen Schweißprozesse und die angepasste Nahtgeometrie höhere Wasserstoffkonzentrationen und signifikant gesteigerte oberflächennahe Zugeigenspannungsniveaus bedingen. Anhand mehrlagiger Schweißversuche an S960QL unter Variation von Wärmeführung, Lichtbogenvariante und Nahtöffnungswinkel wurden die Effekte und Wechselwirkungen unter Zuhilfenahme von Eigenspannungsanalytik mittels Röntgendiffraktometrie und Wasserstoffanalytik mittels Trägergasheißextraktion systematisch untersucht. Das Schweißgefüge führte unter kritischer mechanischer Beanspruchung und erhöhtem Gehalt an diffusiblem Wasserstoff insbesondere bei geringem Nahtöffnungswinkel und fokussiertem Lichtbogen zu einer deutlichen Ausbildung von Mikrorissen. Die Mikrorisse führten teils zu einer makroskopischen Ausprägung. Durch Analysen zur Nachwärmung aus der Schweißwärme heraus konnten suffiziente Wärmeführungsparameter für eine effektive Prävention solcher Risse erarbeitet werden. Die vorliegende Arbeit zeigt anhand des Interaktionssystems bei der wasserstoffunterstützten Kaltrissbildung Gefüge-Beanspruchung-Wasserstoff, welche Wärmeführungs- und Schweißparameter sich für die hochfesten Feinkornbaustähle zur Kaltrissvermeidung eignen.
Vor nun 30 Jahren im Jahre 1989 wurde die erste Zulassung für eine Kunststoffdichtungsbahn von der BAM auf der Grundlage des „Niedersächsischen Dichtungserlasses“ erteilt. Vor ungefähr 10 Jahren wurde die BAM mit dem Erscheinen der Deponieverordnung (DepV) mit der Zulassung von Geokunststoffen, Polymeren und Dichtungskontrollsystemen beauftragt. Wir möchten dies zum Anlass nehmen, an dieser Stelle den viele ehrenamtlichen Experten des Fachbeirats und seiner Arbeitsgruppen für ihren Einsatz und die Unterstützung zu danken.
Darüber hinaus soll über aktuelle Arbeitsthemen und Entwicklungen im Fachbeirat berichtet werden. So soll in einer Sondersitzung des Fachbeirats über die Entlassung aus der Nachsorgephase beim Einsatz von Geokunststoffprodukten in Böschungen diskutiert werden. Es ist geplant, in einer Unterarbeitsgruppe eine Sitzung über die Fremdüber¬wachung in der Produktion abzuhalten. Darüber hinaus werden Änderungen in den Zulassungsrichtlinien für Kunststoff Dichtungsbahnen und Kunststoff Dränelemente kurz vorgestellt. In der Zwischenzeit wurde auch die abschließende Zulassungsrichtlinie für Geogitter verabschiedet. Neue Fragestellungen sind hier entstanden. Unter welchen Voraussetzungen können Geogitter in Zwischenabdichtungen eingesetzt werden. Bisher können diese Produkte formal nur in Oberflächenabdichtungen eingesetzt werden. Auch auf diese beiden Themen soll im Folgenden in Kürze eingegangen werden.
Metallene Konstruktionswerkstoffe reagieren infolge ihres thermodynamisch instabilen Zustandes mehr oder weniger stark mit Stoffen aus der Umgebung. Eine große Rolle bei diesen Reaktionen spielt das Feuchtigkeitsangebot an der Metalloberfläche, welches nicht zwingend mit einem sichtbaren Wasserfilm einhergehen muss. An der Atmosphäre bilden sich an sauberen Metalloberflächen bei etwa 80 % relativer Luftfeuchte, bei versalzten Oberflächen auch schon ab etwa 25 % r. F., Feuchtfilme aus. Im Feuchtfilm laufen grundsätzlich die gleichen elektrochemischen Reaktionen ab, wie diese von wässrigen Medien bekannt sind. Dabei geht das Metall zunächst in Ionenform und in nachgelagerten Reaktionen in einen thermodynamisch stabilen Zustand in Form unterschiedlichster Oxid- und Hydroxidverbindungen über. Die so entstehenden Korrosionsprodukte können im Feuchtigkeitsfilm begrenzt löslich oder unlöslich sein und lagern sich auf den Oberflächen der Konstruktionswerkstoffe häufig als feste Korrosionsprodukte ab bzw. wachsen dort auf. Je nachdem wie die Korrosionsprodukte strukturell zum Untergrund passen, können sich dichte, sehr gut haftende Schichten ausbilden oder poröse, dicke Schichten ohne nennenswerte Haftung aufwachsen. Wachsen dichte, porenfreie Schichten mit guter Haftung auf, die im umgebenden Medium unlöslich sind, wird die weitere Auflösung des Metalls stark gehemmt. Man spricht dann von der Ausbildung schützender Deckschichten bzw. von Passivschichten auf welcher der Korrosionsschutz und damit die Dauerhaftigkeit vieler Konstruktionswerkstoffe beruhen. Kaesche bezeichnete die Passivität sehr treffend als „optimalen Grenzfall des Korrosionsschutzes durch Schichten von Korrosionsprodukten“. Aus technischer Sicht hält Kaesche eine Unterscheidung von Deckschichten bzw. Passivschichten für nicht zwingend erforderlich, da beide die Korrosionsgeschwindigkeit in gleichem Maß herabsetzen können. Die Übergänge zwischen den meist dickeren und dadurch sichtbaren Deckschichten und den meist dünnen, nicht sichtbaren Passivschichten sind fließend.
Mixing and curing of geopolymers and related alkali-activated materials without storage and handling of highly alkaline solutions possesses advantages regarding safety and economic viability. One possible approach is to produce these materials from solid silica feedstocks and solid sodium aluminate, and subsequent mixing with water. We present a comparison between geopolymers and aluminosilicate gel-zeolite composites synthesized by this route from different silica feedstocks (by-product silica from chlorosilane production, microsilica, rice husk ash) and with different SiO2/Al2O3 ratios, using results from XRD, NMR, SEM, thermal analysis, mechanical and acid resistance testing. The use of rice husk ash favors formation of a fully amorphous geopolymer with high strength. Utilization of the other silica feedstocks leads to formation of aluminosilicate gel-zeolite composites, the amount and kind of zeolites depending on the feedstock and the SiO2/Al2O3 ratio. These composites show beneficial dehydration behavior, viz. no distinct dehydration step of thermal strain, with the phase assemblage after heating to 1000°C controlled by the starting composition. Mortars produced from both, the geopolymers as well as the composites, exhibit high resistance to sulfuric acid attack, making them promising materials for the construction and the repair of industrial and sewer structures.
The process of ensuring reliability of NDT applications contains various aspects, such as determining the performance and probability of success, the uncertainty in measurement, the provision of clear and functional procedures and ensuring the correct application accordingly. Test specimens have become powerful elements in supporting many of these aspects. Within the committee for NDT in Civil Engineering (NDT-CE) of the German Society for Nondestructive Testing (DGZfP), the subcommittee on Quality Assurance (UA-QS) therefore addresses the design and the integration of test specimens in the quality assurance process. Depending on the specific purpose, the requirements on test specimens can vary significantly based on the defined simulated scenario. The most prominent purposes of test specimens might be seen in providing references for inspection systems in regard to function control, calibration and validation. Further aspects can be parametric studies, basic investigation of physical principles related to NDT or a simplified and therefore comprehensive demonstration of inspection concepts (e.g. for teaching purposes). The specific purpose of a test specimen dictates the requirements regarding its conception, including the exact design, the material or the fabrication accuracy and the conditioning. In the development of a general guideline by the UA-QS for application-specific procedures and their validation, the use of test specimens is addressed and specific concepts for the design of test specimens are made. This includes the analysis of the measurement process regarding any given application, deriving an adequate calibration approach for it and designing test specimens (calibration specimens) accordingly. Furthermore, it includes the validation of the procedure taking into account all conditions related to the specific application in the field. The validation requires a statistically sufficient number of trials. Thorough evaluation of each trial can only be established if the ground-truth is known. Therefore, test specimens providing a realistic but controlled simulation of the inspection problem are valuable and indispensable elements in the validation process. The requirement of being fully realistic will often not be possible to fulfill due to practical restrictions. Any aspect that cannot be included in the simulation realistically needs to be simulated conservatively. This again, requires a sufficient understanding of the inspection principle and technique to ensure conservativeness. Among other quality-assurance-related aspects, the UA-QS establishes concepts and guidelines regarding sound and efficient approaches for the specific purposes of test specimens. This subcommittee brings together representatives of different Groups along the entire value chain of NDT-CE, including researchers, practitioners, manufacturers and clients. They all work together in establishing a common understanding and level of quality assurance in the industry.
The field of non-destructive testing of civil structures (NDT-CE) has been continuously growing. Due to the complexity and diversity of civil constructions as well as the heterogeneity of concrete, specific standards or guidelines for the application of modern NDT-CE are still missing. The development of individual solutions is the current approach, which is just as challenging as it is common for NDT-CE.
With the increasing development and commercialization of NDT-CE technology, the group of practitioners is growing. To ensure a good level of quality in the industry, it appears necessary to establish adequate means.
Naturally, the performance of NDT-CE methods regarding a specific application is strongly dependent on choosing the most suitable inspection technique and applying it correctly, generally referred to as the inspection procedure in the field of NDT. There are well-defined guidelines regarding procedure documentation and handling in many fields of NDT (e.g. nuclear, aerospace or automotive) according to the high importance of procedures in assuring a successful and reliable application. For a long time, this has not always been the case with NDT-CE, which is still considered a unique discipline of NDT. Part of the reason for that might be the young development state of NDTCE, the heterogeneity of building materials like concrete, timber or masonry as a material and the diversity of civil structures. In consequence, NDT-CE procedure development is considered challenging.
Among other aspects, addressed in the subcommittee on Quality Assurance (UA-QS) within the committee for NDT-CE of the German Society for Nondestructive Testing (DGZfP), part of its work aims at establishing an adequate basis for NDT-CE procedure development. While some of the highly developed approaches from other industries are taken into consideration, they need to be analyzed regarding their suitability for NDT-CE and adapted accordingly. For a procedure to be as defined as possible, it needs to contain sufficient information, such as the scope and limitations regarding material, geometry and condition of the test object, inspection parameters, calibration, data acquisition, analysis criteria as well as requirements regarding the inspection personnel.
For a successful implementation in the field, it is important to define the specific procedure as precisely as possible. Despite the necessity of a great amount of information to be included, the procedure needs to be suitable for efficient field application.
The UA-QS is developing a guideline for NDT-CE procedures suitable for application in this field of NDT to ensure correct and reproducible application. To demonstrate and evaluate this concept, specific examples of procedures are also produced. In particular, the UA-QS has developed a procedure for the detection and positioning of tendon ducts using Ground Penetrating Radar (GPR). This procedure is tested regarding the practical applicability in a roundrobin on a defined type of reference test block.
The LAUS: First Applications of a New System for Ultrasonic Imaging of Very Concrete Structures
(2019)
The LAUS (Large Aperture Ultrasonic System) has been developed to image very thick concrete structures, which are not accessible for commercial systems. The device and the corresponding software is the result of joint Research of BAM, an ultrasonic instrument manufacturer and University of Kassel, Germany. It consists of 12 separate Arrays of 32 point-contact shear wave transducers each, which can be deployed in flexible configurations. Each array is combined with battery and transmitter, receiver and wireless communication electronics.
Three case histories are presented. First the system was deployed on a 5-m thick heavily reinforced foundation slab.
The reflection of the slab’s bottom was imaged clearly. In addition, a multiple reflection was registered, thus giving hope that even thicker elements might be imaged by the instrument. Second, the LAUS was used to investigate a massive bridge girder where a heavy rainstorm during concreting had led to imperfections that were visible after removing the formwork was removed. The LAUS could image tendon ducts in 1.8m depth and the backwall closely behind them. Some limited areas showed blurred reflections and were checked by drill holes; these areas were affected by diffuse damage which could be repaired by injections. Third, a large retaining wall was checked for thickness.
Meanwhile, the LAUS has been used in underground waste deposits (nuclear and other) for quality assurance of sealing plugs. A confirmed penetration depth of about 7 m has been reached.
Fatigue is one of the most prevalent issues, which directly influences the service life expectancy of concrete structures. Fatigue has been investigated for years for steel structures. However, recent findings suggest that concrete structures may also be significantly subjected to fatigue phenomena that could lead to premature failure of certain structural elements. To date, fatigue of reinforced concrete has been given little focus. Knowledge on the influence factors and durability/capacity effects on this material should be improved. Current technological means to measure fatigue in civil structures like bridges and wind turbines (both onshore and offshore) are outdated, imprecise and inappropriate.
Meanwhile, this topic has got much more attention as time-variant loading on concrete structures plays an increasing role, e.g. in bridges with increasing traffic and heavier trucks, and for wind turbines for renewable energy production, e.g. for offshore wind turbine support structures affected by wind and waves.
The European Innovative Training Networks (ITN) Marie Skłodowska-Curie Actions project INFRASTAR (Innovation and Networking for Fatigue and Reliability Analysis of Structures - Training for Assessment of Risk) provides research training for 12 PhD students. The project aims to improve knowledge for optimizing the design of new structures as well as for more realistic verification of structural safety and more accurate prediction of the remaining fatigue lifetime of existing concrete structures.
First, the INFRASTAR research framework is detailed. Then it will be exemplified through the presentation of the major results of the four PhD students involved in the work package dealing with auscultation and monitoring. This includes the development and improvement of Fiber Optics (FO) and Coda Wave Interferometry (CWI) for crack sizing and imagery, new sensor technologies and integration, information management, monitoring strategy for fatigue damage investigation and lifetime prediction.
The shape of the weld pool in laser beam welding plays a major role to understand the dynamics of the melt and its solidification behavior. The aim of the present work was its experimental and numerical investigation. To visualize the geometry of the melt pool in the longitudinal section a butt joint configuration of 15 mm thick structural steel and transparent quartz glass was used. The weld pool shape was recorded by means of a high-speed video camera and two thermal imaging MWIR and VIS cameras. The observations show that the dimensions of the weld pool vary depending on the depth. The regions close to the surface form a teardrop shaped weld pool. A bulge-region and its temporal evolution were observed approximately in the middle of the depth of the weld pool. Additionally, a transient numerical simulation was performed until reaching a steady state to obtain the weld pool shape and to understand the formation mechanism of the observed bulging phenomena. A fixed keyhole with an experimentally obtained shape was used to represent the full-penetration laser beam welding process. The model considers the local temperature field, the effects of phase transition, thermo-capillary convection, natural convection and temperature-dependent material properties up to evaporation temperature. It was found that the Marangoni convection and the movement of the laser heat source are the dominant factors for the formation of the bulging-region. Good correlation between the numerically calculated and the experimentally observed weld bead shapes and the time-temperature curves on the upper and bottom surface were found.
Damage mechanisms analysis of reinforced concrete beams in bending using non-destructive testing
(2019)
Beams that are loaded in transverse direction are one of the main structural elements used in reinforced concrete structures. Bending beams are used, for example, in bridge girders or joists. The strength related failure modes at maximum loading can be divided into bending and shear failure that could occur with or without indication. Conventional design concepts used in practice for designing structural elements subjected to bending aim at failure modes with sufficient indication (e.g. large deflections or cracks), as it occurs in the case of secondary flexure-compression-failure. These indicators can also be used to identify structural changes of civil infrastructure systems (e.g. bridges) using non-destructive testing methods for Structural Health Monitoring (SHM). However, profound knowledge on the determining failure modes of bending beams is required for the reliable application of structural health monitoring. Therefore, resulting effects of different thermal and mechanical loads on the response of a structure are investigated by means of a bridge demonstrator. The results of this investigation are used for developing a monitoring system. Different non-destructive testing (NDT) methods have been used for analyzing the deformation behavior of a reinforced concrete beam in bending. The suitability of the different measuring techniques will be discussed by means of the obtained experimental results. For this purpose, the deformation behavior of a reinforced concrete beam having a span of 2.75 m was investigated in a four-point bending flexural test at laboratory scale. The focus is on the characterization of determining failure modes by the NDT methods and the comparison with classical measuring techniques (e.g. deformation measurements by displacement transducers). The bending tests have been accompanied by Acoustic Emission analysis (AE), optical deformation measurements using Stereophotogrammetry (SP) and Digital Image Correlation (DIC). The conducted tests have shown that the three-dimensional detection of deformations and resulting strains with optical techniques is beneficial for analyzing the damage mechanism of loaded structures and related crack formation and propagation. Optical techniques can be supplemented by acoustic emission testing which gives detailed information on crack formation and progressing damage. The combination of both techniques offers the application as efficient monitoring system for larger structures.
Identifying optimal inspection and repair strategies for offshore jacket structures is a challenging task. We pre-sent an approach, which is based on recent developments in the field of risk-based operation and maintenance planning at the structural system level. The approach utilizes heuristics to define inspection and repair strate-gies at the system level and to reduce the search space of possible strategies. For each defined strategy, the expected service life cost of inspection, repair and failure is evaluated based on simulated inspection and re-pair histories. Subset simulation is applied to compute the conditional repair and failure probabilities required for this analysis. It also forms the basis for simulating inspection and repair histories. The strategy that mini-mizes the expected service life cost is the optimal one in the set of pre-selected strategies. The underlying condition and performance model accounts for the stochastic dependence among the deterioration states of the different structural elements and the structural redundancy. The approach is demonstrated in a case study considering a jacket-type frame. In this study, we essentially vary the inspection interval, the minimum num-ber of inspected components and the target reliability, and identify the combination that minimizes the ex-pected total service life cost.