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Der durch Kräfte senkrecht zur Stabachse belastete Balken ist eines der Haupttragelemente des Stahlbetonbaus. Biegebalken finden unter anderem Anwendung in Form von Brückenträgern oder Unterzügen. Die bei maximaler Beanspruchung auftretenden Versagensformen unterteilen sich dabei in Biege- und Querkraftversagen (Schubversagen). Ein Versagen des Balkens kann je nach Versagensmechanismus mit oder ohne Vorankündigung erfolgen. Gebräuchliche Bemessungskonzepte der Baupraxis zur Dimensionierung entsprechender Tragelemente zielen auf ein Versagen mit ausreichender Vorankündigung (große Verformungen oder breite Risse) ab, wie es z.B. beim sekundären Biegedruckversagen entsteht. Für die Zustandsüberwachung von Bauwerken (z.B. Brücken) mittels zerstörungsfreier Prüftechniken sind fundierte Kenntnisse zu den bei Biegebeanspruchung auftretenden Schadensmechanismen erforderlich. Im Rahmen eines BAM-internen Projektes sollen durch praxisnahe Untersuchungen an einer brückenähnlichen Referenzstruktur unterschiedliche auf das Tragwerk einwirkende thermische und mechanische Belastungen untersucht werden und in die Entwicklung eines Monitoringsystems einfließen. Anhand der aus Biegeversuchen vorliegenden Ergebnisse wird der Einsatz unterschiedlicher Methoden der zerstörungsfreien Materialprüfung gezeigt und deren Anwendbarkeit für die Analyse des Verformungsverhaltens von Stahlbetonbalken diskutiert. Hierzu wurde das Verformungsverhalten eines Stahlbetonbalkens (L = 2,75 m) in einem Vierpunkt-Biegeversuch im Labormaßstab untersucht. Das Hauptaugenmerk liegt dabei auf der Charakterisierung der ausschlaggebenden Versagensmodi durch die einzelnen zerstörungsfreien Prüfverfahren und der Vergleich zu klassischen Messmethoden (z.B. Verformungsmessung mittels induktiver Wegaufnehmer (IWA)). Die Biegeversuche wurden dazu durch Schallemissionsanalyse (SEA) und optische Verformungsanalyse mittels Stereo-Fotogrammetrie (SF) und Bildkorrelationsverfahren (BKV) begleitet.
During the past 20 years, innovative developments in concrete technology have enabled the production of ultra-high performance fibre reinforced concretes (UHPFC) for the protection and preservation of concrete structures. The main characteristics of UHPFC comprise high compressive and flexural strength as well as high ductility compared to normal strength, normal weight concrete (NSC). This paper focuses on the analysis of crack formation and crack propagation in UHPFC under tensile loading under quasistatic conditions by using acoustic emission (AE) and optical deformation analysis (ODA).
AE is a non-destructive technique to monitor the development of micro structural damage processes caused by external forces. In this context, AE allows for a continuous monitoring of changes in the microstructure (cracks) over time and the corresponding localisation by appropriate algorithms. The AE measurements were performed along with ODA at the surface of the samples in order to analyse the deformation behaviour of the samples and the crack propagation.
For the tests, a UHPFC reference mixture was developed and tested with three different volume percentages of steel fibres (1.0 V.-%, 2.0 V.-%, and 3.0 V.-%). This paper shows that the application of AE is a suitable method to analyse the crack propagation in UHPFC specimens under tensile loading and to localise the crack initiation. Furthermore, insights into the failure mechanism of the fibres are offered by the analysis of the signals of the AE measurements, which allows for a better understanding of UHPFC and its general performance.
Ultra-high performance fiber-reinforced concretes (UHPFRCs) are most suitable for applications with extreme mechanical loads. These extreme conditions require ductile behavior under tensile loading, which is obtained solely by the working mechanism of steel fibers. Profound knowledge on the working mechanism of the steel fibers is necessary to optimize this material. Usually, this knowledge is obtained by means of classical destructive measuring techniques. Adopting measuring techniques from non-destructive material testing helps to analyze and to identify the different stages of the fracture mechanism of UHPFRC in detail. The application of different non-destructive measuring techniques is shown exemplary on tensile tests conducted on an UHPFRC mix and its applicability for analyzing the fracture behavior of such concretes is discussed. The main focus is on the characterization of the relevant failure modes under tensile loading by the different measuring techniques and the comparison with classical measuring techniques (e.g. extensometer). The tensile tests have been analyzed by optical deformation measurements using digital image correlation (DIC), acoustic emission analysis (AE), and 3D computed tomography (CT).
At present, Wind Turbine Generators (WTGs) operating in onshore and offshore wind farms are primary sources of renewable energy around the world. Cylindrical grouted sleeve connections are usually adopted in these WTG structures to connect the upper structure and foundation for ease of installation. These structures including grouted connections experience considerable adverse loading during their lifetimes. Settlements were reported inside similar connections used in energy structures especially oil and gas platforms, which were installed in last three decades. Thus, repair and rehabilitation of such connections in existing wind structures should also be planned ahead to keep them operating in the future. The nature of failure and crack generation in grouted connections are crucial prior to adopt a strengthening strategy. This pilot study is carried out to actualize the failure mechanism in the grouted connection, when subjected to axial loading. A novel reusable scaled cylindrical grouted connection with shear keys was designed and tested for its load bearing behaviour. The mechanical test was accompanied by classical measuring techniques (e.g. displacement transducer) as well as non-destructive measuring techniques (e.g. digital image correlation (DIC), acoustic emission analysis (AE)). The failure mechanism incorporating slippage of the shear keys and cracking of the grout was investigated. The capacity and applicability of such test mould were also discussed. The knowledge is expected to pave way towards repair of deteriorated grouted connections with similar geometry and failure pattern.
Remediation of Cracks Formed in Grouted Connections of Offshore Energy Structures under Static Loads
(2018)
The future energy demand necessitates the exploration of all potential energy sources both onshore and offshore. Global trend has shifted towards offshore energy, which can be obtained from either carbon intensive or renewable options, hence requiring structures such as rigs, platforms, and monopiles. Most of these structures adopt easily installable construction techniques, where lower foundation need to be connected with the super structure by mean of grouted composite joints. Generally, these composite connections have exterior sleeve, interior pile and infill grout. Being located in remote offshore conditions, connections can experience considerable adverse loading during their lifetimes. Degradations were reported inside similar connections, which were installed in last three decades. Besides, grouting in the offshore sites may often be proven difficult, which eventually leads to reduced capacity of connections in the long run. Thus, repair and rehabilitation of such connections should be planned ahead to minimize operational delays and costs in the future. This study aims at characterizing the nature of crack generation in grouted connections and thereby identifying the potential of repair using suitable repair material. Scaled grouted joints were manufactured using a novel mold, and connections were loaded under static load to visualize the main failure pattern. The failure mechanism and loading capacity are found compatible to previous results from earlier literature. Grouted connection was then repaired using cementitious injectable grout. The effectiveness of the repair system is also discussed.
The interaction mechanism of ultrasonic waves with surface-breaking cracks in reinforced concrete has been studied. For this, a detailed numerical analysis of the medium was first conducted using the spectral finite element method. Subsequently, measurements were conducted on a reinforced beam sample before and after mechanical-load-induced cracking. It was concluded that for the considered configuration, waves passing through the cracks dominated the tip-diffracted ones. In addition, substantial changes in the travel time and amplitude of the signals were observed when cracks were present.