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Organisationseinheit der BAM
Assessment of existing steel structures - A guideline for estimation of the remaining fatigue life
(2007)
In many countries and regions, traffic infrastructure projects suffer from low funding. The budget is tight for new infrastructure building and, thus, the importance of inspection, maintenance and assessment of the existing traffic infrastructure increases. A new fatigue assessment guideline for the estimation of the remaining fatigue life of steel bridges has been written by technical committee 6 of the European Convention of Constructional Steelwork (ECCS). It will be a useful tool for the complementation of bridge management systems, used commonly for condition assessment.
Design specifications and rules are harmonised throughout Europe. They are under constant development, but there is still a lack of forwarding and concentrating experiences as well as developing rules for the fatigue assessment on existing steel structures. This paper presents a guideline with a proposed fatigue assessment procedure for existing steel structures embedded in information about old materials and non-destructive testing methods for the evaluation of details (ECCS 2004, Assessment of existing steel structures). Particular attention is paid on remedial measures which are proposed for weak details and damages caused by fatigue. The developed fatigue assessment procedure can be applied to existing steel structures under cyclic loading in general, but the guideline concentrates on the existing traffic infrastructure made from old steel, because of the public importance. The proposed procedure summarizes, regroups and arranges the knowledge in the field of assessment on existing steel to be applied by practicing engineers. The procedure is a milestone in knowledge transfer from a state of scientific knowledge to state-of-the-art.
Die zunehmende Bedeutung der Nachhaltigkeit im Bauwesen wird in Zukunft das Bauen im Bestand gegenüber Neubauten in den Vordergrund rücken. Dies gilt auch für den Brückenbau, wo das Qualitätsmanagement seit langem Brücken von ihrem Entstehungsgang über den Betrieb bis in die Entsorgung begleitet.
Die starke Vergrößerung der Frachtvolumina für Straße und Schiene und die zunehmende Überalterung des Brückenbestandes fragen danach, ob die bestehenden Brücken für die veränderten Betriebsbedingungen noch geeignet sind und wie sie und ob sie dafür instandgesetzt werden könnten.
Für die erforderlichen Instandhaltungs- und Instandsetzungsmaßnahmen oder einen eventuellen Ersatz durch einen Neubau ist aber die Frage der Restnutzungsdauer von entscheidender Bedeutung. Es ist daher nicht von ungefähr, dass sich Baulast-träger auf nationaler und internationaler Ebene frühzeitig zusammengetan haben, um nach geeigneten Methoden der Bestimmung der Sicherheit und Restnutzungsdauer bestehender Brücken zu forschen. Auch die internationalen technisch-wissenschaftlichen Verbände wie EKS und fib sowie die Standard-Organisationen ISO und CEN sind der Frage eines internationalen Austauschs und der Vereinheitlichung der Methoden nachgegangen.
Für Stahlbrücken hat insbesondere das technische Komitee6 - Ermüdung - der EKS Anregungen aufgegriffen, aus den verschiedenen nationalen Erfahrungen und dem vorhandenen Wissen technische Empfehlungen für die Beurteilung bestehender Stahlbrücken zu entwickeln [1], die sich als Grundlage für die von CEN/TC 250 - das CEN-Komitee für die Bearbeitung der Eurocode - geplante Erweiterung der Eurocode zur Instandhaltung und Instandsetzung des Brückenbestandes eignen.
Das Ergebnis ist ein technisch-wissenschaftlicher Bericht, der im Rahmen der Zusammenarbeit der europäischen Kommission (DG-Research - Joint Research Centre JRC) mit der EKS als Basispapier für eine zukünftige Eurocode-Erweiterung veröffentlicht wurde und schon jetzt als Empfehlung für eine Vorgehensweise nach dem Stand der Technik verwendet werden kann.
Der vorliegende Beitrag behandelt die Grundzüge dieses Berichtes [1], der vom e-bookshop der Kommission kostenfrei herunter geladen werden kann.
Ultrasound sensors should be embedded into concrete for monitoring concrete properties. These new longitudinal wave sensors with a center frequency of 60 kHz were examined regarding their suitability for ultrasonic measurements in concrete structures in terms of emission characteristics, sensitivity and frequency ränge.
For the measurement of the radiation patterns, the sensors were embedded vertically and horizontally in concrete cylinders. The directivity pattern was measured using a laser vibrometer.
The sensitivity of the sensor was determined in water using different sensors of the same type. It shows changes in the signal amplitude as well as variations in the frequency ränge for different transmitter-receiver combinations.
The attenuation of the concrete affects the achievable resolution of the measurements and thus, the maximum possible spacing of the sensors within a concrete element. Experimental tests helped optimizing the distances with respect to the required resolution and the effort of embedding the sensors. The signal attenuation in the concrete was measured in the frequency ränge of 60 kHz in response to various degrees of reinforcement and grain size. For this purpose, the sensors were cast at different distances in the specimens studied. The recorded Signals were evaluated for their amplitude and frequency spectrum.
The European infrastructure asset has developed historically and is characterized by nation-specific construction processes. Inspection, condition assessment, and maintenance procedures differ from country to country. Because of historical and political circumstances, national infrastructure assets are maintained at different levels, too. Since the budget for maintaining the bridge infrastructure less and less meets the demands of a growing bridge stock, bridge inspection, maintenance, and life-cycle considerations gain higher importance. The need exists to develop effective diagnosis tools for early detection of construction faults, defects, and deterioration processes during inspection, to keep the bridge infrastructure at an acceptable level, from structural safety and economic viewpoints. An overview on the latest research projects and integrated bridge management systems in Europe is given. The potentials of nondestructive testing (NDT) are presented, with special focus on technical advances of NDT applications to reinforced concrete (RC) and posttensioned concrete bridges. Although NDT is not regularly integrated in these processes, the application brings valuable information on the current condition of the inner structure in called-in special inspections. NDT-automation and the application of imaging echo methods, combined with advanced data processing, produce a surprising level of information about the inner structure of massive RC slabs up to a depth of about 60 cm. Detected inhomogeneity and scatterers of acoustic or electromagnetic waves can be visualized in vertical or horizontal slices through the structure or animations. The fusion of different three-dimensional data sets of processed data improves the interpretability and accuracy of the results.
Complex special inspection of an old masonry arch bridge according to the Guideline on Inspection and Condition Assessment of Railway Bridges and numerical analysis of the structure are presented. The guideline summarises recommendations for the step-by-step investigation of railway bridges applying enhanced methods developed during the EU-funded project Sustainable Bridges. For the investigation of the arch barrel, the ballast parameters and the inner structure of the backfill behind the arch barrel a number of various advanced non-destructive and minor-destructive testing methods were applied. Deformation of the structure during load tests was measured using three independent measuring systems: laser vibrometer, LVDT and microwave radar. Results of calculations performed with 2D and 3D models based on FEM are compared with the field load tests. Sensitivity of the ultimate load of the structure to investigated parameters is studied in FE model. Some general conclusions according to methods of testing and modelling of masonry arch bridges are presented and discussed.
Complex special inspection of an old masonry arch bridge according to the Guideline on Inspection and Condition Assessment of Railway Bridges and numerical analysis of the structure are presented. The guideline summarises recommendations for the step-by-step investigation of railway bridges applying enhanced methods developed during the EU-funded project Sustainable Bridges. For the investigation of the arch barrel, the ballast parameters and the inner structure of the backfill behind the arch barrel a number of various advanced non-destructive and minor-destructive testing methods were applied. Deformation of the structure during load tests was measured using three independent measuring systems: laser vibrometer, LVDT and microwave radar. Results of calculations performed with 2D and 3D models based on FEM are compared with the field load tests. Sensitivity of the ultimate load of the structure to investigated parameters is studied in FE model. Some general conclusions according to methods of testing and modelling of masonry arch bridges are presented and discussed.
The Berlin Congress Hall was a gift of the United States of America to Germany for the Berlin World Exhibition in 1957. The elegantly double-curved roof was made from an advanced prestressed concrete construction. The construction followed the design by the American architect Hugh Stubbins with special structural modifications to comply with German construction rules. No early indications for failure initiation were detected before a sudden partial collapse. On May 21st, 1980, the Southern external roof overhang collapsed. In December 1980, Jörg Schlaich and his co-authors published a summarizing expert opinion about structural causes of failure. The present paper mainly introduces results of cause analysis made at the Federal Institute for Materials Research and Testing (BAM), mandated by the Public Prosecutor at the Berlin District Court to specify the causes of the sudden failure. This paper refers to BAM-publications about analyses that were performed under this mandate and published – most of them in German language – within the first years after failure. The expert's opinions comprise structural considerations materials investigations, metallographic analyses and corrosion. Nowadays experts have learnt from the failure and built a slightly modified roof in the original shape at the 750th birthday of the city of Berlin and re-opened the former Berlin Congress Hall to the public on May 9th, 1987. The Hall is now serving as the House of the Cultures of the World, following the initial intention of the hall.
Adolf Martens (18501914) was a pioneer of materials engineering. After his studies in Berlin he joined a railway company being responsible for the technical equipment. His interest for the basic mechanisms of mechanical strength and fracture of metallic materials led him to the development of the basic concepts for metallography; his papers published between 1878 and 1889 laid the fundament of this technique. In 1880 Martens changed to the Royal Industrial Academy in Berlin and by 1884 he was designated as director of the Materials Testing Laboratory at the Technical University Berlin. Under his guidance this institution acquired an excellent reputation in all relevant areas of the German industry, forming the nucleus of the Bundesanstalt für Materialforschung und -prüfung (BAM) – the Federal Institute for Materials Research and Testing. Martens developed equipment in many different areas as mechanical testing of metallic materials and of paper, characterization of the wear behavior of materials and viscosity of lubricants. His overwhelming contribution to materials engineering was recognized as early as 1895 by Floris Osmond, who denominated martensite a metallographical constituent resulting from quenching of steels. In 2000 ISO designated the value obtained from the instrumented indentation method as Martens Hardness – HM.
Following the long tradition of the Federal Institute for Materials Research and Testing (BAM) since Adolf Martens has created first routines in failure analysis (Ruske, 1971), BAM has been frequently called-in by the Berlin Traffic Association (BVG) to carry out root-cause analyses of the Berlin suburban train line (Frahm, 1902), reported in Helmerich and Herter (1999), Helmerich (2000), Nega and Winkler (1998), Helmerich et al. (2002) and Herter et al. (2002). This was also the case in the 1990s, when BVG-inspectors found cracks in hanging, barrel-like shaped ballast plates of the Berlin underground steel viaducts during regularly scheduled inspections. Cracks were located parallel to the riveted connection between the ballast plates and the upper chord of the viaduct cross girder steel profiles. For safety reasons, the operator BVG immediately stopped the cracks by means of drilling stop holes at the crack tips. As intermediate measures, longitudinal steel profiles were spanned below the rail axes between the cross girders affected to stabilize the track in longitudinal direction. The inspection period was shortened from years to few weeks. BAM was mandated to measure strains under regular train traffic to analyze the cause of the cracks. Strains were measured in identical connections as the damaged details, which did not suffer from cracks at the time of the measurement. The traffic-induced strain cycles and thus the resulting strain differences in the questionable cross sections were higher than expected and resulted in stresses of max. 85.8 MPa. Calculations showed that the credible remaining fatigue life for this particular structural detail was exceeded after 68 years according to nowadays standards. Extensive discussions, further field and laboratory tests followed to develop a rehabilitation plan for retrofitting the structure with minimum interference of the traffic. Finally, a method with minimum intervention to the structure was elaborated by a consortium of the operator BVG, BAM and producers based on further laboratory and field tests at BAM. Now, the viaduct is saved for the future.
Periodic mapping of reinforcement corrosion in intrusive chloride contaminated concrete with GPR
(2014)
This paper presents an experimental effort for developing a novel reinforcement corrosion monitoring technique based on Ground Penetrating Radar (GPR). Experiments were carried out to periodically monitor the accelerated reinforcement corrosion process with GPR. The data were processed in both time and timefrequency domains to investigate the tendencies of GPR signal attribute changes related with corrosion, moisture and chloride contamination. Data processing methods were proposed to visualize the reinforcement corrosion and chloride distribution with target specified signal attribute mapping. Half-Cell Potential (HCP) and Laser-Induced Breakdown Spectroscopy (LIBS) methods were employed to verify the GPR observations.
Die Langzeitüberwachung von Bauteilen (Structural Health Monitoring, SHM) ging in den letzten Jahren zunehmend von verkabelten Sensoren zu kabellosen Sensornetzwerken (WSN) über. Frühwarnsysteme sollen rechtzeitige Signale senden, wenn Schädigungsprozesse einsetzen, wenn ein Bauteil z. B. zu feucht wird (Deiche, Staumauern), oder wenn Feuchteprozesse in Bauteilen auftreten, die den Transport von schädlichen Stoffen ermöglichen. Mittels handelsüblicher multifunktionaler Sensoren wie einem Bluetooth Low Energy (BLE) wurden erste Machbarkeitsuntersuchungen zur Massenfeuchtemessung durchgeführt. Die untersuchten Parameter sind der sogenannte Received Signal Strength Indicator (RSSI) eines Bluetooth-Low-Energy(BLE)-Signals und die abgestrahlte Sendeleistung eines eingebetteten BLE-Moduls. Das BLE-Modul kommuniziert mit einem mobilen smarten Empfänger wie einem Tablet oder einem Smartphone als SmartHub. Die Veränderung des RSS-Indikators ist ein Maß für die Veränderung der Feuchte im Bauteil, während die ausgesandte Signalstärke konstant bleibt. Wenn das BLE-Signal einen feuchten Bereich im Baumaterial durchläuft, verändert sich der RSS-Indikator. Nach erfolgreicher Machbarkeitsstudie wurde nun ein aus dem Netzwerk MI4G (Modern Industry for Germany) zwischen der Bundesanstalt für Materialprüfung und dem Klein- und mittelständigen Unternehmen LinTech GmbH heraus beantragtes ZIM-Projekt bewilligt. Der Bericht zeigt erste Ergebnisse von Voruntersuchungen und gibt einen Ausblick auf weitere Forschungsaktivitäten.
A new method for periodically monitoring reinforcement corrosion in concrete with Ground-Penetrating Radar (GPR) is proposed and a ten-year long-term corrosion periodic monitoring experiment is reported. GPR historical data was taken in different years, under different conditions. In order to align and normalize the obtained images properly prior to effectively performing any automatic defect detection, image registration techniques based on mutual-information are employed, and a new signal processing scheme is proposed for normalizing the intensity of GPR images. Then, the processed image results can be used to compare and find out the change of GPR detection due to corrosion.
In this article, we present a laboratory experiment to monitor the accelerated corrosion in concrete using ground-penetrating radar (GPR). Four concrete test specimens were cast with rebars of different size and placed at different depths. The lifetime decades of reinforcement corrosion process were accelerated into 18 days by using the impress current technique. The electrochemical corrosion process was periodically monitored with GPR. Two control specimens were also prepared to investigate the influence of chloride contamination on GPR signal. The measured data were analysed both in time and frequency domains. In time domain, the peak-to-peak amplitude of a wave reflected by a rebar was calculated to investigate the relationship between an increase in signal amplitude and the degree of corrosion. In frequency domain, the time–frequency representations of the signal were computed by using S-transform. The results show that reinforce corrosion increased the amplitude of reflected signal in time domain but did not change the peak frequency in frequency domain while chloride contamination attenuates the signal to smaller amplitude and lower peak frequency. Based on the results, a novel process is finally proposed for GPR-based corrosion detection.
For almost 15 years, carbon fibre-reinforced polymer (CFRP) plates and sheets have been used for strengthening and repair of civil engineering structures. CFRP plates were applied to upgrade structures for change in use or increasing traffic load. CFRP laminates have also been used to repair deteriorated structures such as multi-storey parking or bridges. The execution quality and condition of the strengthening measure in terms of bond integrity were neither monitored nor tested systematically. Inspectors eventually applied simple hammer knocking during regular main inspection for subjective noise distinction as a measure for eventual insufficient bond. This paper presents the feasibility and the limitations of low-frequency transversal ultrasonic waves with dry-coupling point-contact sensors for inspection and defect detection between CFRP laminate and concrete surfaces. The German Centre of Competence in Civil Engineering (DIBt) has initiated a national research project [Helmerich et al. (2012). Condition analysis of bonded CFRP-applications on concrete structures by means of non-destructive testing [in German: Zustandsuntersuchung von CFK-Klebeverstärkungen an Betonbauteilen mittels zerstörungsfreier Prüfung (ZfP)]. Final, internal project report], carried out at the Federal Institute for Materials Research and Testing (BAM), Germany. The aim of the project was to present a potential non-destructive testing method for bond defect detection as a measure for the durability of the strengthening system more than 10 years after application.
Trans-European corridors for transport of people and goods require strengthening of bridges to carry increased axle loads in terms of absolute axle loads and of their frequency. Most of the bridges along highways are reinforced or prestressed concrete bridges.
Since the late 1990s, CFRP laminates have been applied to box girder and T-beam bridges to increase the overall capacity. Although regular inspections are carried out, no special method was available to test the bond condition of CFRP laminates after being in Service for more than ten years. The German Center of Competence in Civil Engineering is aware of this need and funded a research project to investigate the bond condition by means of advanced non-destructive testing (NDT).
Structural health monitoring contributes to early damage detection in the built infrastructure. During the last two decades, sensor networks transferred from wired to wireless sensor networks. Several methods exist to measure moisture in building materials. Most of the introduced commercial moisture measurement methods provide information about the local or near surface moisture. A feasibility study is presented to demonstrate, how the Received Signal Strength Indicator (RSSI) of a Bluetooth Low Energy (BLE) signal, transmitted from the BLE-module embedded in building materials, correlate with changing moisture content. The feasibility of this influence was investigated in the presented study. The maximum and minimum RSSI were systematically recorded and analysed.
The vulnerability of low-quality concrete of some bridges to ageing and deterioration while they are exposed to environmental conditions during their service life, influence of growing population and climate changes, is a well-known effect and requires survey. For survey, the development of a low cost, cost-efficient sensor network to be embedded for continuous or attached to the surface for periodic monitoring is essential.
Changing ground water conditions, flood or even leakage in fresh or disposal of waste water can lead to in-creased moisture content in building materials and structural elements as walls or floors. Moisture allows the transport of other adverse chemicals that might reduce the durability of structures. The paper presents a feasibility study about the capability of microwave modules as bluetooth® or ZigBee with frequency range at 2,4 GHz to create advanced Wireless Sensor Networks (WSN) for estimation and visualization of moisture distribution in infrastructure assets. It was found that the value of the received signal strength indication (RSSI) depends on the moisture content and can be the basis for the moisture distribution inside a massive structure.
The microwave modules may be used to compose networks outside and inside of building materials or structural elements. The higher the number of connections between BLE modules, the more precise and the more distant information can be obtained from the network. Mesh networking was applied to increase the quantitative information.
Within the presented research project, experimental and numerical investigations were performed to develop a thin-shelled, modular, mobile element system made of a micro-reinforced ultra-high-performance ductile concrete (DUCON®). Material parameters were experimentally determined to adapt the material model within the numerical analysis applying the Drucker-Prager relationship. Afterwards, for validation of the numerical models, quasi-static and high-velocity impact tests were performed on plate-like structures. Finally, a suitable geometry of transportable barrier elements will be designed, which provides a maximum of resistance against impact by a minimum of weight and a maximum of mobility.