Browsing by Author "Ožbolt, Joško (Prof. Dr.-Ing. habil.)"

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Open Access
Auswirkungen der Bewehrungskorrosion auf den Verbund zwischen Stahl und Beton
(2012) Fischer, Christian; Ožbolt, Joško (Prof. Dr.-Ing. habil.)
Die carbonatisierungs- und chloridinduzierte Bewehrungskorrosion beeinflusst das Tragverhalten von Stahlbetonbauwerken und verkürzt infolgedessen deren Lebensdauer. Dies kann im Wesentlichen durch folgende drei Punkte geschehen: (i) durch den Verlust des Stahlquerschnittes, (ii) durch den Verlust des Betonquer-schnittes aufgrund von Rissbildung und Abplatzungen sowie (iii) durch den Verlust der Verbundwirkung zwischen Stahl und Beton. Die vorliegende Arbeit beschränkt sich auf die Untersuchungen zum Einfluss der chloridinduzierten Bewehrungskorrosion auf das Verbundverhalten zwischen Stahl und Beton. In diesem Zusammenhang stellt sich die Frage, welche Komponenten, die den Verbund gewährleisten, in welcher Art und Weise durch die Bewehrungskorrosion beeinflusst werden? Zur Lösung dieser Fragestellung wurden in der Vergangenheit vorrangig experimentelle Untersuchungen an unterschiedlichen Probekörpergeometrien durchgeführt. Dazu zählen zentrische und exzentrische Ausziehkörper sowie Versuche an Biegebalken. Es wurden sowohl Untersuchungen mit als auch ohne Querbewehrung, die in dieser Arbeit einheitlich als Bügel bzw. Bügelbewehrung bezeichnet wird, durchgeführt. Aufgrund der langen Zeiträume sowohl für die Depassivierung der Betondeckung als auch für den Fortschritt der Bewehrungskorrosion, wurden nahezu alle bisherigen Untersuchungen diesbezüglich beschleunigt. In vorangegangenen Untersuchungen wurde jedoch festgestellt, dass der Grad der Beschleunigung einen Einfluss auf den Zeitpunkt der Erstrissbildung und auf die Verbundfestigkeitsänderung hat. Sowohl diese Tatsache, als auch die große Vielzahl an unterschiedlichen Probekörpern hat zu einer großen Streuung der Verbundfestigkeitswerte in Bezug auf die Korrosion der Bewehrung geführt. Aus diesem Grund ist es bisher schwierig, die geometrischen und korrosionsbeeinflussenden Parameter bestimmten Auswirkungen auf das Verbundverhalten zuzuordnen. Als Beitrag zur Entflechtung dieser Problematik wurden experimentelle Untersuchungen an Balkenend-Probekörpern durchgeführt, welche als Basis für die grundlegenden Mechanismen des Verbundes unter korrosivem Angriff dienen. Die Korrosion der Bewehrung wurde potentiostatisch beschleunigt, jedoch erfolgte dies im Vergleich zu den bisherigen Untersuchungen auf die schonendste Weise mit lediglich fünffach überhöhter maximal in der Natur auftretender Korrosionsrate. Zu den Untersuchungsschwerpunkten zählten die Art und die Verteilung der Korrosionsprodukte, der Zusammenhang zwischen Korrosionsabtrag und messbarer Rissbreite in der Betondeckung und die Auswirkungen der Bewehrungskorrosion auf die Stabendverschiebung und die Verbundfestigkeit. Die Untersuchungen erfolgten getrennt nach Probekörpern mit und ohne Bügelbewehrung, wobei die Probekörper mit Bügelbewehrung nochmals in zwei Gruppen unterteilt wurden. Zum einen waren die Bügel elektrisch mit den Längsstäben verbunden, sodass auch die Bügel beschleunigt korrodierten. Zum anderen wurden die Bügel elektrisch von den Längsstäben entkoppelt, wodurch lediglich die Längsstäbe beschleunigt korrodierten. Auf diese Weise konnte eine qualitative Aussage zum Einfluss geschwächter Bügel auf das Verbundverhalten getroffen werden. Anschließend wurden die ermittelten Ergebnisse im Kontext von Ergebnissen anderer Wissenschaftler diskutiert. Daraus konnten u. a. neue Erkenntnisse zur Abhängigkeit der Verbundfestigkeit von der Beschleunigung der Bewehrungskorrosion gewonnen werden. Weiterhin konnten qualitative Aussagen zum Einfluss von Geometrieparametern wie Stabdurchmesser und Betondeckung auf die Verbundfestigkeitsänderung getroffen werden. Genauere Aussagen diesbezüglich wurden mithilfe numerischer Untersuchungen an FE-Modellen ermöglicht. Im Rahmen der numerischen Untersuchungen, die weitere, vor allem geometrische Parametervariationen abgedeckt haben, wurde der Einfluss von unterschiedlichen Stabdurchmessern mit jeweils verschiedenen Betondeckungen auf die Verbundfestigkeit zu unterschiedlichen Korrosionsstadien untersucht. Außerdem konnte mittels der numerischen Untersuchungen eine quantitative Aussage zur Auslastung der Bügelbewehrung getroffen werden. Mit Hilfe der so gewonnenen Erkenntnisse wurde ein Modell erstellt, mit dessen Hilfe eine Abschätzung der Restverbundtragfähigkeit möglich ist. Das Modell setzt sich aus der Berechnung der sog. Referenzverbundfestigkeit ohne Korrosionseinwirkung, einer Beschreibung der Abnahme der Umschließungswirkung des Betons und der Wirkung einer ggf. vorhandenen Bügelbewehrung zusammen, welche ebenfalls durch die Korrosionswirkung geschwächt wird. Dieses Modell grenzt sich einerseits gegen rein empirische Modelle ab, die aus der Regression von Versuchsdaten entstanden sind. Gegenüber den sehr komplexen analytischen Modellen besticht das vorgestellte Modell durch seine Simplizität und anwenderfreundliche Form.
Open Access
Behavior of concrete structures subjected to static and dynamic loading after fire exposure
(2021) Lacković, Luka; Ožbolt, Joško (Prof. Dr.-Ing. habil.)
The resistance of concrete structures exposed to extreme loading conditions such as explosion, impact, industrial accidents, tsunami, earthquake or their combination represents one of the major topics in research today. Such loading conditions are characterized with high loading rates often acting in conjunction with fire exposure. Especially vulnerable are the structures located in the seismically active areas with high level of urbanization and proximity to HAZMAT landfills, which additionally exacerbate fire conflagrations. The behavior of concrete changes significantly when exposed to elevated temperatures resulting in the decrease of its mechanical properties. Reinforced concrete (RC), when exposed to high temperature culminates in a simultaneous thermal behavior of its two constituents, steel and concrete, that should be considered in the analysis. It is also known that the resistance, crack pattern and failure mode in concrete are strongly influenced by the loading rate. The dynamic response of RC structures previously exposed to fire changes significantly when compared to initially undamaged RC structures. The main objective of the present work is to further improve the existing rate sensitive thermo-mechanical model for concrete through the following: (i) the implementation of the experimentally obtained thermal dependence of concrete fracture energy in the thermo-mechanical model, (ii) the calculation of concrete thermally dependent mechanical properties by means of nonlocal (average) temperature and (iii) to perform parametric study on fastening elements and RC frames in order to investigate the interaction between the thermally induced damage and mechanical behavior of structures. The experimental investigations in the present work indicated that the concrete fracture energy has a declining tendency with the temperature increase, measured on small and mid-sized concrete beams. This is implemented in the thermo-mechanical model and it is indicated that the decrease of fracture energy has a relatively mild influence on reaction values in terms of loading rate. However, its effect on the fracture patterns and reaction-time histories can be considered as more significant. The influence of the nonlocal temperature is validated against the experimental results carried out on RC frames which had been thermally pre-damaged and subsequently loaded with impact. Currently there are almost no models that can realistically predict the structural behavior at this level of complexity. Furthermore, a parametric study is carried out to show the influence of preloading of single-headed stud anchor and anchor group with two and four studs, on the residual concrete edge failure capacity after fire exposure. The anchors are exposed to fire and loaded in shear, perpendicular to the free edge of the concrete member up to failure, in both hot and cold state (after cooling). The influence of different geometry configurations and initial conditions such as the edge distance, embedment depth, anchor diameter and duration of fire on the load-bearing behavior of anchors is investigated. It is demonstrated that the preloading has a strong negative influence on the residual load-bearing capacity of the concrete. Finally, the numerical parametric study is performed to investigate the influence of fire duration and the loading rate on the resistance of RC frames. The response of the RC structures strongly depends on whether it was loaded in hot or residual (cold) state, i.e. after being naturally cooled down to ambient temperature. Furthermore, an extensive numerical investigation on the influence of post-earthquake fire on the residual capacity of RC frames with and without ductile detailing is conducted. The numerical investigation encompassed the validation of the thermo-mechanical model in terms of temperature distributions, thermal deflections and load-bearing capacity against the test data and subsequent parametric analysis with different levels of fire exposure ranging from 15 to 120 min.
Open Access
Concrete failure of headed stud fasteners exposed to fire and loaded in shear : experimental and numerical study
(2019) Tian, Kaipei; Ožbolt, Joško (Prof. Dr.-Ing. habil.)
Open Access
Experimental and numerical study of chloride induced corrosion in reinforced concrete
(2017) Sola, Emiliano; Ožbolt, Joško (Prof. Dr.-Ing. habil.)
Chloride-induced corrosion is considered as one of the major concern for durability of reinforced concrete (RC) structures. Especially vulnerable, are structures located in coastal marine environment or highways and garages treated with de-icing salts during winter seasons (Tuutti, 1993; Cairns, 1998). Consequences of chloride-induced corrosion of steel reinforcement have negative effects on structural behavior and involve several aspects related to the life cycle of the structure, such as serviceability, safety and structural performance. Direct and indirect costs of maintenance and repair are relatively high and constitute nowadays a huge economic exertion. Therefore, challenging task is to develop and improve a numerical tool, which can realistically predict corrosion processes and the related mechanism of deterioration in RC structures, supporting the service life prediction of damaged and undamaged structures.The main objective of the present work is to validate the recently developed 3D Chemo-Hygro-Thermo-Mechanical model for concrete by means of an extensive experimental program, which includes tests under natural and laboratory controlled conditions. The comparison between numerical and experimental results is significantly important in order to quantitatively calibrate the parameters responsible for the computation of corrosion rate and distribution of rust in pores and cracks. To better describe the problem, hysteretic moisture behaviour is coupled with the transport of corrosion products in cracks and a relationship between diffusivity of corrosion compounds and crack width is proposed. It has been demonstrated experimentally that the production of corrosion compounds are strongly dependent on the environmental conditions and presence of chemical reactants. Therefore, type of corrosion products and corresponding distribution are in the present work investigated on concrete thin section by means of microscope analysis and Raman spectroscopy. Computation of corrosion rate and related corrosion induced damage is directly related to the assumed position of anode and cathode on the reinforcement surface and currently there is no algorithm which can predict the combination between anode and cathode surfaces that results to the highest corrosion induced damage. To investigate this influence, the expression for maximum entropy production, deduced from irreversible thermodynamics, is formulated. The entropy is considered produced by dissipative processes, which are in this special case the flow of ions through the electrolyte, the anodic and cathodic polarization and the diffusion oxygen process. Through several numerical examples, in which the size and position of anodic and cathodic surfaces are varied, is demonstrated that maximum entropy leads to maximum corrosion induced damage.
Open Access
Numerical and experimental study of concrete structures exposed to impact and fire
(2018) Ruta, Daniela; Ožbolt, Joško (Prof. Dr.-Ing. habil.)
During their service life concrete and reinforced concrete structures may be exposed to extreme loading conditions such as fire, explosions, impact, earthquakes and terroristic attacks. In particular situations, as in case of chemical industries where the probabilities of explosions are relatively high, combination of extreme loadings represents a major risk. To assure safety conditions in terms of cost and lives losses for the involved structure as well as for the surrounding buildings, it is important to take into account the effect of multi-hazard phenomena. The aim of this work is to study the dynamic concrete behaviour after thermal exposure analyzing the change of the material state and structural response, by means of experimental tests and numerical analysis. In the literature, few studies can be found on the behaviour of concrete and RC structures subjected to coupled thermal and dynamic loads. The results of the study are also useful to extend the experimental and numerical database available in the literature. Experimental and numerical investigations on fire exposed plain concrete (compact tension specimen) and full scale reinforced concrete structures (slabs and frames) under high loading rates are presented and discussed.
Open Access
Pryout capacity and bearing behavior of stocky headed stud anchorages
(2018) Jebara, Khalil; Ožbolt, Joško (Prof. Dr.-Ing. habil.)
Concrete pryout failure mode, which occurs for relatively short and stocky cast-in headed studs or post installed anchors, has been investigated only to a limited extent in the past. The increasing use of such anchorages in engineering practice emphasizes the need to clarify the pryout failure mechanism and to formulate design rules that assure safe and economical structural design. Currently used design provisions to predict the pryout capacity are conservative and based on the so-called indirect-tension model, which applys the CC-Method for anchors loaded in tension to predict the pryout capacity of anchors loaded in shear and fail in pryout. The aim of this dissertation is to get more understanding into the load bearing behaviour and the pryout mechanism of shallowly embedded anchors and anchor groups, which are likely fail in concrete pryout. Furthermore, it aims at extending the CC-method to account for the pryout failure mode and improving current design recommendations. This dissertation covers experimental and numerical investigations. The experimental investigation aims at clarifying the bearing bahavior and influencing parameter of stocky cast-in single welded headed studs, which fail in concrete pryout. On the other hand, the numerical investigation aims at parametric study and pryout failure simulation of single anchor and anchor group. The results of the experimental and numerical investigation are evaluated and discussed. Finally, based on the obtained results a pryout capacity prediction equation for anchors under pure shear load is presented. An extended CC-method for pryout failure mode, which accounts for single anchor and anchor group as well as for edge and corner influence, is proposed and compared with current design formula.