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Czernuschka, Lisa-Marie; Podrou ek, Jan; Kratochvilova, Martina; Wan-Wendner, RomanEntwicklung der Kriech- und Schwindmodelle für Betontragwerke in Österreich und Deutschland - Bewertung der Modelle hinsichtlich der Sensitivität einzelner EingangsparameterBeton- und Stahlbetonbau11/2021821-836Aufsätze

Abstract

Beton ist eines der wichtigsten Baumaterialien in der modernen Ingenieurspraxis und findet sein Anwendungsgebiet im Brückenbau, Hochbau und Grundbau. Beton ist ein Material, welches sein Verhalten im Laufe der Zeit in Abhängigkeit von den Umweltbedingungen verändert. Besonders Verformungen, die durch Kriech- und Schwindprozesse hervorgerufen werden, spielen eine wichtige Rolle in der Bemessung von Betonstrukturen - vor allem im Hinblick auf die Gebrauchstauglichkeit. Bereits in den ersten Bemessungsnormen für Betonstrukturen gab es Regelungen hinsichtlich der Berücksichtigung dieser Einflüsse. Zu Beginn wurden die Verformungen mit einem äquivalenten Temperaturgradienten berücksichtigt. Erst mit der Herausgabe der Spannbetonnormen wurden entsprechende Kriech- und Schwindmodelle vorgeschlagen und bis hin zur heute gängigen Praxis weiterentwickelt. Im ersten Teil dieser Arbeit wird ein Überblick über die historische Entwicklung dieser Modelle bis zu den heute geltenden Eurocode 2 und fib Model Code-Richtlinien gegeben. Im zweiten Teil wird eine Sensitivitätsanalyse der Eingangsparameter der aktuellen Modelle präsentiert, gefolgt von einem Anwendungsbeispiel, in dem die Unterschiede der verschiedenen Modelle auf Basis einer Durchbiegungsberechnung veranschaulicht werden.

Evolution of creep and shrinkage models for concrete structures in Austria and Germany - Evaluation of the current models regarding the sensitivity of the input parameter
Concrete is one of the most important materials in civil engineering structures and finds its application in the building of bridges, ground engineering and building construction. As widely known, concrete is a material which changes its behavior with time in dependency of the environmental conditions. Especially the long-term deformations caused by creep and shrinkage processes are of great interest regarding the durability and sustainability of infrastructures. Early versions of design codes gave some guidelines on how to treat deformations caused by these sources. In the beginning, these deformations were taken into account by an additional temperature drop. The first sophisticated approaches on how to deal with these long-term processes were published in the standards for prestressed concrete structures. These early models were updated and improved to today's prevailing documents such as the Eurocode 2 and the fib Model Code. The first part of this paper gives an overview of the historical development of creep and shrinkage models in Austria and Germany until today's guidelines. The second part presents a sensitivity study of the current models regarding the input parameters for some given scenarios. Lastly, the differences between the models are highlighted based on an application example taken from literature.

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Cramer, Jonas; Empelmann, MartinEinfluss des Schwindens auf Rissbreiten unter Lastbeanspruchung in StahlbetonbauteilenBeton- und Stahlbetonbau9/2021677-688Aufsätze

Abstract

Rissbreiten in Stahlbetonbauteilen vergrößern sich unter Langzeitbeanspruchung durch das zeitabhängige Materialverhalten des Betons. Während viele Rissbreitenansätze das Betonschwinden bei der Rissbreitenberechnung vernachlässigen, zeigen Untersuchungen in der Literatur, dass Schwinddehnungen bei der Rissbreitenberechnung berücksichtigt werden sollten. Um die einzelnen Effekte aus Schwinden, Verbund- und Betonkriechen auf das Rissbild einzuordnen, wurden die verschiedenen Anteile unter einer Lastbeanspruchung analysiert. Hierzu wurde im abgeschlossenen Rissbild eine Parameteranalyse mit einem an experimentellen Versuchen validierten rheologischen Modell durchgeführt und der Anteil des Betonschwindens an der Rissbreitenvergrößerung festgestellt. Ein Vergleich mit dem Rissbreitenansatz des Model Code 2010 zeigt, dass der Ansatz des Model Code 2010 potenziell gut geeignet ist, zeitabhängige Effekte wirklichkeitsnah abzubilden, diese allerdings überschätzt. Aus diesem Grund wurde der Ansatz modifiziert, sodass in einem weiteren Vergleich sehr gute Ergebnisse erzielt wurden, ohne die in der Praxis etablierte Rissbreitenformel grundlegend zu verändern.

Influence of concrete shrinkage on crack widths in reinforced concrete members under direct loading
Under long-term loading, crack widths in reinforced concrete members increase due to the time-dependent material behaviour of concrete. While many crack width approaches neglect shrinkage in crack width calculations, investigations in the literature indicate that shrinkage strains should be taken into account for crack width calculation. In order to classify the individual effects of shrinkage, bond creep and concrete creep on the crack pattern, they were analysed separately under direct loading. For this purpose, a parameter analysis was performed in the stabilized cracking stage with a rheological model validated by experimental tests to determine the proportion of concrete shrinkage on crack width increase. A comparison with the crack width approach of Model Code 2010 shows that the approach of Model Code 2010 is potentially well suited to describe time-dependent effects realistically, but overestimates them. For this reason, the approach was modified without fundamentally changing the established crack width formula. With that modification, very good results were achieved in a further comparison.

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Schmidt, Boso; Marx, Steffen; Schneider, Sebastian; Betz, ThorstenEinfluss der Druckfestigkeitsstreuung auf den Ermüdungswiderstand von druckschwellbeanspruchtem BetonBeton- und Stahlbetonbau8/2021575-586Aufsätze

Abstract

Experimentell ermittelte Bruchlastwechselzahlen druckschwellbeanspruchter Betone weisen im Allgemeinen große Streuungen auf. Als Hauptursache wird die Streuung der tatsächlichen Probekörperdruckfestigkeit vermutet, da die im Ermüdungsversuch aufgebrachten Ober- und Unterspannungen stets auf die mittlere Druckfestigkeit bezogen werden. Demnach versagen Probekörper, die tatsächlich höhere Druckfestigkeiten besitzen, bei höheren Lastwechselzahlen als Probekörper mit geringeren tatsächlichen Druckfestigkeiten. Um diese Vermutung zu überprüfen, wird im folgenden Artikel ein Konzept zur stochastischen Berücksichtigung der Druckfestigkeitsstreuung bei der Herleitung von Versuchswöhlerkurven präsentiert. Das Konzept wird auf Grundlage umfangreicher Ermüdungsuntersuchungen an Betonen der Festigkeitsklassen C100/115 und C80/95 für Windenergieanlagentürme erarbeitet und führt in den durchgeführten Untersuchungen zu nahezu glatten streuungsbereinigten Versuchswöhlerkurven. Das deutet darauf hin, dass die ursprüngliche, in den Wöhlerkurven inhärente Streuung tatsächlich allein auf die Streuung der Druckfestigkeit innerhalb der untersuchten Betonchargen zurückgeführt werden kann. Die Anwendung dieses Konzepts grenzt sich zunächst auf die Randbedingungen ein, unter denen die experimentellen Untersuchungen durchgeführt wurden. In ergänzenden Untersuchungen werden die unter Betrieb herrschenden Feuchte-, Temperatur- und Frequenzbedingungen in einem Windenergieanlagenturm beleuchtet und mit den Bedingungen verglichen, unter denen die Laborergebnisse erzielt wurden. Abschließend wird für die untersuchten Betone ein Bemessungskonzept formuliert, welches die Anwendung der Wöhlerkurven gemäß fib Model Code 2010, inklusive einer angepassten Formulierung für die Bemessungsdruckfestigkeit bei Ermüdungsbeanspruchungen fcd,fat, empfiehlt.

Influence of compressive strength scatter on the fatigue resistance of concrete under compression
Experimentally determined load cycles to failure of concrete specimens under compression generally show a large scatter. The main cause is assumed to be the scatter of the real compressive strength, since the upper and lower stresses applied in the fatigue test are always related to the average compressive strength. Accordingly, specimens that really have higher compressive strengths fail at higher numbers of load cycles than specimens with lower real compressive strengths. In order to verify this assumption, the following article presents a concept for the stochastic consideration of compressive strength scatter in the derivation of S-N curves from tests. The concept is developed on the basis of extensive fatigue investigations on concrete of strength classes C100/115 and C80/95 for wind turbine towers and leads to almost smooth, scatter-corrected S-N curves. This indicates that the original scatter inherent in the S-N curves can be explained by the scatter of the compressive strength. The application of this concept is limited to the boundary conditions under which the experimental investigations were carried out. In supplementary investigations, the prevailing humidity, temperature and frequency conditions in a wind turbine tower during operation are investigated and compared with the conditions under which the laboratory results were obtained. Finally, a design concept is formulated for the investigated concretes, which recommends the application of the S-N curves according to fib Model Code 2010 including an adapted formulation for the design compressive strength at fatigue loads fcd,fat.

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Zeuli, Vincenzo; Maier, Christof; Diaz, José Manuel Suarez; Saeed, Fahed Ahmed; Tharamapalan, Jayapregasham; Germani, Carlo; Bayer, Lars; Atzl, GeorgStructural design of precast segmental lining reinforced by steel fibresGeomechanics and Tunnelling4/2021356-366Topics

Abstract

Dubai Municipality awarded to Porr Besix JV the Project for the Main Tunnel component of the Deep Storm Water System. The tunnel will collect both rainwater and groundwater from approximately 500 sq. km and transfer the captured flow to the sea. The Design Builder JV selected COWI as Designer of the entire Project and IC Consultant as Design Checker for the Tunnels. The Project includes approximately 10.3 km of 10-meter-inside diameter tunnel in rock, three construction shafts and one drop shaft. The main tunnel will convey stormwater and groundwater flows from the EXPO 2020 area near the intersection of Sheikh Mohammed Bin Zayed Road and Jebal Ali Lehbab Road to the sea close to the EGA facility. The tunnel will follow beneath the road easement along Jebal Ali Lehbab Road and along Sheikh Zayed Road and continue to the pumping station. The tunnel traversed through the Barzaman and Fars formation with an overburden of 33 m with maximum water pressure of 4.4 bar and was excavated by EPB TBMs. This project is characterized by its dimensions with an internal diameter of 10 m and 350 mm of segment thickness, and by the use of steel fibre reinforced concrete in the precast segmental lining. The use of fibres aims to reduce the CO2 footprint obtaining an optimized design from the environmental point of view. These facts are associated to a complex design of precast segments, in order to ensure their structural competence and their integrity according to the durability requirements, under large thrust forces (temporary loads) and permanent load. Hence, considering such complexities, the structural design has been carried out producing a 3D structural model by means of a sophisticated FEM structural software. Results of the model allow to identify areas of the segment where spalling and bursting stresses are generated along circumferential joints and maximum value of those stresses in the temporary load cases. Moreover, a structural design verification of the segment has been undertaken considering the contribution of steel fibres class 4c, as it is set up in the FIB model code, aiming to ensure that the precast segments are structurally competent and fulfil the durability requirements of the Project. The article details the design approach and the independent checker design verification approach. The experience gained during construction is also reported, describing challenging aspects of the Tunnel execution and an analysis of the lining damages.

Statischer Entwurf einer mit Stahlfasern verstärkten Tübbingauskleidung
Die Stadtverwaltung von Dubai vergab an die Arbeitsgemeinschaft Porr Besix das Projekt DS233/2 Deep Storm Water System - Main Tunnel. Der Tunnel wird sowohl Regen- als auch Grundwasser ableiten und fast 40 % des gesamten Stadtgebiets von Dubai entwässern. Das Projekt zeichnet sich durch seine Dimensionen mit einem Innendurchmesser von 10 m und einer Tübbingdicke von 350 mm sowie durch den Einsatz von stahlfaserverstärktem Beton in der vorgefertigten Tübbingauskleidung aus. Die Verwendung von Fasern zielt darauf ab, den CO2-Fußabdruck zu reduzieren, um ein aus ökologischer Sicht optimales Design zu erhalten. Um die statische Funktion und Integrität gemäß den Dauerhaftigkeitsanforderungen aufgrund der großen Vortriebspressenkräfte (temporäre Lasten) und unter permanenter Belastung zu gewährleisten, wurde ein 3D-Strukturmodell mithilfe einer FE-Software erstellt. Die Ergebnisse des Modells ermöglichen es, die Bereiche des Segments zu identifizieren, in denen Abplatzungen und Spaltzugspannungen entlang der Umfangsfugen entstehen, sowie den maximalen Wert dieser Spannungen in den temporären Lastfällen. Darüber hinaus wurde ein statischer Nachweis des Segments unter Berücksichtigung des Beitrags von Stahlfasern der Klasse 4c durchgeführt, wie es im FIB-Modellcode festgelegt ist, um sicherzustellen, dass die vorgefertigten Segmente die Anforderungen des Projekts an die statische Tragfähigkeit und Dauerhaftigkeit erfüllen. Der Artikel beschreibt detailliert den Entwurfsansatz und den Ansatz der unabhängigen Prüfung des Entwurfs.

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Schmidt, Philipp; Kueres, Dominik; Claßen, Martin; Hegger, JosefDurchstanztragverhalten von Flachdecken mit variierenden DurchstanzbewehrungsgradenBeton- und Stahlbetonbau1/202013-25Aufsätze

Abstract

Mit der bauaufsichtlichen Einführung des Eurocode 2 in Deutschland wurde die Durchstanzbemessung von Flachdecken neu geregelt. Seitdem ergibt sich die Durchstanztragfähigkeit mit Durchstanzbewehrung aus der Summe eines konstanten Betontraganteils und eines Stahltraganteils, der unabhängig vom Bauteil aus einem 33°-Fachwerk resultiert. Für die Berechnung des Stahltraganteils wird gemäß Eurocode 2 eine reduzierte Stahlfestigkeit in Abhängigkeit von der statischen Nutzhöhe angesetzt, wodurch die Verankerungsqualität der Durchstanzbewehrung berücksichtigt wird. Auf Basis einer Datenbankauswertung und eines Vergleichs des Sicherheitsniveaus mit der damaligen DIN 1045-1 wurden im Deutschen Anhang zum Eurocode 2 (EC2+NA(D)) zusätzliche Erhöhungsfaktoren für die ersten beiden Durchstanzbewehrungsreihen ergänzt, die nahezu zu einer Verdopplung der Durchstanzbewehrungsmenge führen. Während zur Bewertung der Bemessungsgleichungen für die Ermittlung der Durchstanztragfähigkeit ohne Durchstanzbewehrung und auf dem Niveau der maximalen Durchstanztragfähigkeit zahlreiche Versuchsergebnisse vorliegen, sind bislang nur sehr wenige Versuchsreihen an Flachdeckenausschnitten mit einem Durchstanzversagen innerhalb des durchstanzbewehrten Bereichs vorhanden.
Zur systematischen Untersuchung des Durchstanztragverhaltens von Flachdecken mit Bügeln als Durchstanzbewehrung wurden daher insgesamt drei Versuchsserien mit variierenden Durchstanzbewehrungsgraden durchgeführt. Das Versuchsprogramm umfasste drei Referenzversuche ohne Durchstanzbewehrung und acht Versuchskörper mit geringen und mittleren Durchstanzbewehrungsgraden. Dabei erfolgte die Variation der Durchstanzbewehrungsmenge jeweils ausschließlich über den Bügeldurchmesser. Weitere untersuchte Einflussparameter waren die Schubschlankheit und die statische Nutzhöhe. Im vorliegenden Beitrag werden die durchgeführten Versuche vorgestellt und die Versuchsergebnisse diskutiert. Durch den Vergleich der experimentell ermittelten Bruchlasten mit den rechnerischen Durchstanztragfähigkeiten nach Eurocode 2 und EC2+NA(D) können die bestehenden normativen Regeln bewertet werden. Darüber hinaus werden die Bemessungsregeln nach Model Code 2010 in die Bewertung mit einbezogen.

Punching strength of flat slabs with varying shear reinforcement ratios
With the introduction of Eurocode 2 in Germany, the punching shear design provisions of flat slabs were revised. Since then, the punching shear strength with shear reinforcement is calculated from the combination of a constant concrete contribution and a steel contribution, which results from a strut inclination of 33°. Besides, a reduced steel strength depending on the effective depth of the flat slab is assumed for the calculation of the steel contribution to consider the anchorage conditions of the shear reinforcement elements. Due to a database analysis and an evaluation of the level of safety compared to the former DIN 1045-1, additional factors have been implemented in German Annex to Eurocode 2 (EC2+NA(D)) for the first and second row of shear reinforcement leading to almost twice the amount of required shear reinforcement. Various test series on reinforced concrete flat slabs without shear reinforcement and with high amounts of shear reinforcement are available and can be used for the evaluation of the current provisions. Nevertheless, the evaluation of the code equations for the design of the shear reinforcement (failure inside the shear-reinforced zone) is still not possible since only very few systematic punching tests with a varying amount of shear reinforcement have been conducted.
To investigate the punching shear behavior of reinforced concrete flat slabs with stirrups as shear reinforcement, three systematic test series (eleven specimens) with varying shear reinforcement ratios were performed. Each test series included one reference test without shear reinforcement. In the tests, the shear reinforcement ratio was varied by changing the stirrup diameter only. Further investigated influences were the shear span-depth ratio and the effective depth. In this paper, the results of the tests are presented, discussed and compared to the predictions according to Eurocode 2 and EC2+NA(D). Moreover, the punching design provisions according to Model Code 2010 are evaluated.

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Landler, Josef; Fischer, OliverSteigerung der Durchstanztragfähigkeit und Duktilität durch die Zugabe moderner HochleistungsstahlfasernBeton- und Stahlbetonbau9/2019663-673Fachthemen

Abstract

Seit der Veröffentlichung normativer Vorschriften zur Bemessung von Stahlfaserbeton in Deutschland und Europa gewinnt dessen Einsatz für tragende Konstruktionen des Hoch- und Ingenieurbaus stetig an Bedeutung. Nach Eintreten der Rissbildung im Beton ermöglichen Makrofasern aus Stahl eine effektive Übertragung von Zugspannungen über den Riss hinweg, was sich in Bezug auf das Durchstanztragverhalten als besonders vorteilhaft erweist. Während für die Durchstanztragfähigkeit stahlfaserbewehrter Flachdecken mit Plattendicken bis 150 mm zahlreiche Versuchsergebnisse vorliegen, fehlen Untersuchungen an praxisrelevanten Plattenstärken von 200 mm bis 300 mm nahezu gänzlich. Zudem versprechen Fortschritte in der Leistungsfähigkeit moderner Stahldrahtfasern ein verbessertes Trag- und Verformungsverhalten des Stahlfaserbetons.
Zur Untersuchung des Durchstanztragverhaltens stahlfaserbewehrter Flachdecken mit praxisrelevanten Plattendicken wurden acht Durchstanzversuche an Flachdeckenausschnitten mit unterschiedlichen Plattenstärken durchgeführt. Als weitere Einflussparameter standen der Stahlfasertyp und insbesondere der Stahlfasergehalt im Vordergrund. Der vorliegende Beitrag liefert einen Überblick über die durchgeführten Versuche und diskutiert die gewonnenen Ergebnisse. Anhand eines Vergleichs der experimentell gewonnenen Versagenslasten mit den rechnerischen Durchstanztragfähigkeiten nach der Richtlinie “Stahlfaserbeton” des Deutschen Ausschusses für Stahlbeton (DAfStb) sowie nach fib Model Code 2010 erfolgt eine Bewertung der Bemessungsansätze.

Increase of the punching shear capacity and structural ductility achieved through utilisation of modern high-performance steel fibres
Since the introduction of normative regulations for the design of steel fibre reinforced concrete in Germany and Europe, the application is recently growing in concrete engineering. After crack formation has occurred in the concrete, macro fibres made of steel allow an effective transfer of tensile stresses across the crack, which proves to be particularly advantageous regarding punching shear failure. While there are numerous test results available for the punching shear capacity of steel fibre reinforced concrete slabs with slab thicknesses of up to 150 mm, there is an almost complete lack of tests on practically relevant slab thicknesses. In addition, advances in the performance of modern steel fibres promise improved load-bearing and deformation behaviour of the steel fibre concrete.
Eight punching shear tests were carried out on flat slab specimens with different slab thicknesses to investigate the punching shear behaviour of steel fibre reinforced flat slabs with practically relevant slab thicknesses. Further investigated influencing parameters were the steel fibre type and in particular the steel fibre content. This article provides an overview of the tests carried out and discusses the results obtained. Based on a comparison of the experimentally obtained failure loads with the calculated punching shear capacities according to the DAfStb-Guideline “Steel fibre reinforced concrete” of the German Committee for Structural Concrete (DAfStb) as well as according to fib Model Code 2010, an evaluation of the design approaches will be provided.

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Schmidt, Boso; Schneider, Sebastian; Marx, SteffenBetonermüdung - Sicherheiten und Entwicklungspotenziale aktueller Nachweiskonzepte - 22. Dresdner Baustatik-Seminar - "Realität - Modellierung - Tragwerksplanung"Bautechnik4/2019329-337Aufsätze

Abstract

Das tatsächliche Sicherheitsniveau bei der Ermüdungsbemessung von druckschwellbeanspruchtem Beton ist derzeit unbekannt. Für die Ermüdungsnachweise werden Sicherheitsbeiwerte verwendet, die aus der statischen Bemessung übernommen wurden, und weitere, teilweise nicht wissenschaftlich begründete Sicherheitselemente implementiert. In diesem Beitrag werden Sicherheiten in aktuellen Nachweiskonzepten nach DIN EN 1992-1-1(/NA), DIN EN 1992-2(/NA) und fib Model Code 2010 diskutiert und Entwicklungspotenziale aufgezeigt. Außerdem werden Ergebnisse bestehender Ermüdungsuntersuchungen stochastisch ausgewertet und mit Wöhlerlinien nach fib Model Code 2010 verglichen. Die vergleichenden Untersuchungen deuten auf ein höheres Sicherheitsniveau des Ermüdungswiderstands im Vergleich zur statischen Bemessung hin.

Concrete fatigue - safety and development potential of current design concepts
The safety level for the fatigue design of concrete under compression is currently unknown. This is the result of safety factors taken from the static design and other safety elements, which are in some cases not scientifically justified. In this paper, the safety aspects of current fatigue design concepts according to DIN EN 1992-1-1(/NA), DIN EN 1992-2(/NA) and fib Model Code 2010 are discussed and potentials for further developments are shown. In addition, results of existing fatigue tests are evaluated stochastically and compared with S-N curves according to fib Model Code 2010. The comparative investigations indicate a higher safety level of the fatigue resistance compared to the static design.

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Kueres, Dominik; Schmidt, Philipp; Hegger, JosefDurchstanztragverhalten von Einzelfundamenten mit variierenden DurchstanzbewehrungsgradenBeton- und Stahlbetonbau4/2019231-241Fachthemen

Abstract

Mit der bauaufsichtlichen Einführung des Eurocode 2 in Deutschland wurde die Durchstanzbemessung von Fundamenten und Bodenplatten neu geregelt. Für die Bemessung der Durchstanzbewehrung wird im Eurocode 2 unabhängig vom Bauteil ein 33°-Fachwerk mit konstantem Betontraganteil vorgeschlagen. Aufgrund von Sicherheitsbedenken wurde im Deutschen Anhang zum Eurocode 2 (EC2+NA(D)) davon abweichend ein Aufhängefachwerk ohne Betontraganteil für die Bemessung der Durchstanzbewehrung in Fundamenten und Bodenplatten eingeführt. Während zur Bewertung der Bemessungsgleichungen für die Ermittlung der Durchstanztragfähigkeit von Fundamenten ohne Durchstanzbewehrung bzw. der maximalen Durchstanztragfähigkeit verschiedene Versuchsergebnisse vorliegen, fehlen bislang systematische Versuchsreihen an Fundamenten mit einem Durchstanzversagen innerhalb des durchstanzbewehrten Bereichs (Bemessung der Durchstanzbewehrung).
Zur systematischen Untersuchung des Durchstanztragverhaltens von Fundamenten mit Bügeln als Durchstanzbewehrung wurden insgesamt drei Versuchsserien (elf Versuche) mit variierendem Durchstanzbewehrungsgrad durchgeführt. Dabei erfolgte die Variation der Durchstanzbewehrungsmenge über den Bügeldurchmesser. Weitere untersuchte Einflussparameter waren die Schubschlankheit und die statische Nutzhöhe. Im vorliegenden Beitrag werden die durchgeführten Versuche vorgestellt und die Versuchsergebnisse diskutiert. Durch den Vergleich der experimentell ermittelten Bruchlasten mit den rechnerischen Durchstanztragfähigkeiten nach Eurocode 2 und EC2+NA(D) können die bestehenden normativen Regeln bewertet werden. Darüber hinaus werden die Bemessungsregeln nach Model Code 2010 in die Bewertung mit einbezogen.

Punching strength of footings with varying shear reinforcement ratios
With the introduction of Eurocode 2 in Germany, the punching shear design provisions of footings and ground slabs were revised. According to Eurocode 2, the design of the shear reinforcement is performed by means of a design model considering a strut inclination of 33° and a constant contribution of concrete. Due to safety related concerns, in German Annex to Eurocode 2 (EC2+NA(D)), a different concept was introduced. In this context, the design of the shear reinforcement in footings and ground slabs is conducted without consideration of a concrete contribution. Various test series on reinforced concrete footings without shear reinforcement and with high amounts of shear reinforcement are available and can be used for the evaluation of the current provisions. Nevertheless, the evaluation of the code equations for the design of the shear reinforcement (failure inside the shear-reinforced zone) is still not possible since systematic test series on footings with a varying amount of shear reinforcement have not yet been conducted.
To investigate the punching shear behavior of reinforced concrete footings with stirrups as shear reinforcement, three systematic test series (eleven specimens) with varying shear reinforcement ratios were performed. In the tests, the shear reinforcement ratio was varied by changing the stirrup diameter only. Further investigated influences were the shear span-depth ratio and the effective depth. In this paper, the results of the tests are discussed and compared to the predictions according to Eurocode 2 and EC2+NA(D). Moreover, the punching design provisions according to Model Code 2010 are evaluated.

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Oettel, Vincent; Matz, Henrik; Empelmann, MartinBestimmung der zentrischen Nachrisszugfestigkeit von UHPFRC mithilfe gekerbter 3-Punkt-BiegezugversucheBeton- und Stahlbetonbau4/2019255-264Fachthemen

Abstract

Sowohl die zentrische Matrixzugfestigkeit als auch die zentrischen Nachrisszugfestigkeiten von faserbewehrten Betonen werden in der Regel indirekt über Biegezugversuche ermittelt. Dazu muss eine in Versuchen ermittelte Kraft-Verformungs-Kurve in eine Zugspannungs-Dehnungs-Beziehung überführt werden. Für ultrahochfesten Faserbeton (UHPFRC) gibt es in Deutschland zurzeit mangels gültiger Normen und Richtlinien kein geregeltes derartiges Umrechnungsverfahren. Zur Untersuchung des Zugspannungs-Dehnungs-Verhaltens von UHPFRC wurden am iBMB, Fachgebiet Massivbau der TU Braunschweig gekerbte 3-Punkt-Biegezugversuche nach DIN EN 14651 durchgeführt. Unter Berücksichtigung der Versuchsergebnisse und in Anlehnung an den Model Code 2010, der normal- und hochfesten Faserbeton regelt, wurde ein Umrechnungsverfahren für UHPFRC entwickelt. Zur Validierung des Verfahrens wurde die Finite-Elemente-Methode (FEM) hinzugezogen.

Tensile Stress-Strain Relationship for UHPFRC based on notched 3-point-bending tests
Both the tensile strength and the residual tensile strengths of fibre reinforced concrete are generally determined indirectly by flexural tests. For this purpose, a force-deformation curve has to be converted into a tensile stress-strain curve. For ultra-high performance fibre reinforced concrete (UHPFRC) there is currently no regulated conversion procedure due to the lack of valid standards and guidelines in Germany. To analyse the tensile stress-strain behaviour of UHPFRC, notched 3-point-bending tests according DIN EN 14651 were carried out at iBMB, Division of Concrete Construction of the TU Braunschweig. A conversion procedure for UHPFRC based on the Model Code 2010, which regulates normal- and high-strength fibre reinforced concrete, was developed taking into account the test results. The finite element method (FEM) was applied for validation.

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Empelmann, Martin; Busse, DanielVergleich von normativen Modellen zur Berechnung der RissbreiteBeton- und Stahlbetonbau4/2018298-306Fachthemen

Abstract

Mit dem Ziel, einen Überblick über aktuelle normative Modelle zur Berechnung der Rissbreite im Stahlbetonbau zu erhalten, wurden diese am iBMB, Fachgebiet Massivbau der TU Braunschweig zusammengestellt und verglichen. Um einen unabhängigen Vergleich der Modelle sicherzustellen, wurde eine Datenbasis mit Versuchswerten zu Rissbreiten zusammengestellt. Der Modellvergleich wurde auf zentrische Zugversuche, bei denen charakteristische Werte der gemessenen Rissbreiten (95 %-Quantilwerte) angegeben waren bzw. ermittelt werden konnten, beschränkt. Durch die Vergleichsberechnungen konnte gezeigt werden, dass zwischen den unterschiedlichen Modellen signifikante Abweichungen auftreten. Während der Eurocode 2 die im Versuch gemessenen Rissbreiten tendenziell überschätzt, werden die Rissbreiten mit den Nationalen Anhängen für Deutschland bzw. Österreich überwiegend unterschätzt. Mit dem Modell nach Model Code 2010 werden die Versuchsergebnisse im Mittel ebenfalls überschätzt, die Ergebnisse liegen jedoch zwischen denen nach Eurocode 2 sowie den Nationalen Anhängen für Deutschland bzw. Österreich. Des Weiteren kann mit den Modellen der Nationalen Anhänge für Deutschland und Österreich sowie des Model Code 2010 eine, gegenüber dem Eurocode 2, erhöhte Vorhersagegenauigkeit erreicht werden.

Comparison of code provisions for the calculation of crack widths
In order to give an overview of current code provisions for the calculation of crack widths, these provisions were compiled and compared at the iBMB, Department of Concrete Construction of TU Braunschweig. In order to ensure an independent comparison, a database with test values for crack widths was compiled. The comparative calculations were limited to tensile tests in which characteristic values of the measured crack widths (95 %-quantile values) were or could be determined. As a result, it can be stated that significant deviations occurred for the different code provisions. While the provisions by Eurocode 2 tend to overestimate the test values, the National Annexes for Germany and Austria mainly underestimate the measured crack widths. Model Code 2010 also overestimates the test results on average, but the results are between those obtained with Eurocode 2 and the national annexes for Germany and Austria. Furthermore, the models in the national annexes for Germany and Austria as well as the Model Code 2010 achieve a higher prediction accuracy than Eurocode 2.

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Heek, Peter; Look, Katharina; Minelli, Fausto; Mark, Peter; Plizzari, GiovanniDatenbank für querkraftbeanspruchte Stahlfaserbetonbauteile - Bewertung der Bemessungsansätze nach DAfStb-Richtlinie und fib Model Code 2010Beton- und Stahlbetonbau3/2017144-154Fachthemen

Abstract

Der Einsatz von Makrofasern aus Stahl gewinnt seit der bauaufsichtlichen Einführung der Richtlinie “Stahlfaserbeton” des Deutschen Ausschusses für Stahlbeton (DAfStb) auch in tragenden Betonkonstruktionen zunehmend an Bedeutung. Insbesondere beim Nachweis ausreichender Querkrafttragfähigkeit erweist sich die Fasertragwirkung als günstig, da eine Querkraftbewehrung auch bei Balken rechnerisch vollständig durch Fasern gebildet sein kann - Bügel also entfallen. Haupteinflussparameter ist die Nachrisszugfestigkeit, die aufgrund vielfältiger interagierender Einflussparameter hohen Streuungen unterliegt und nach Ansätzen der DAfStb-Richtlinie bzw. den Modellen des Model Codes 2010 deutlich unterschiedlich einfließt. Der Beitrag vergleicht die Prognosegenauigkeiten rechnerischer Querkrafttragfähigkeiten, indem rechnerische Bruchlasten experimentellen Daten von Schubversuchen aus der Literatur gegenübergestellt werden. Die aufgebaute Schubdatenbank umfasst dabei über 250 Versuche mit praxistypischen Konfigurationen, d. h., dass Querschnittsgeometrie, Längs- und Bügelbewehrungsgehalt, Nachrisszugfestigkeit des Stahlfaserbetons sowie Betondruckfestigkeit variable Parameter sind.

Database with shear tests on steel fibre reinforced concrete girders
The application of macro fibres made of steel is recently growing in concrete engineering since valid standards like the DAfStb-Guideline “Steel fibre reinforced concrete” of the German Committee for Structural Concrete (DAfStb) are available. In particular, it is meaningful to take into account fibre's effect in shear design to considerably reduce the amounts of stirrups. The most significant parameter is the steel fibre's post-cracking tensile strength that exhibits pronounced scattering due to several interacting parameters. It is differently considered in the design approaches of DAfStb-Guideline and fib Model Code 2010, respectively. The paper investigates accuracies of predicted shear resistances by comparing calculative and experimental ultimate loadings. For this purpose, a shear database is build up comprising more than 250 test results from the literature that differ in cross-section's geometry, ratios of longitudinal rebars and stirrups, fibre amounts and concrete compressive classes, respectively.

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Gunkler, Erhard; Marx, Johann Jakob; Blum, CarstenShear resistance of reinforced masonry beams with and without additional concrete or prestress / Querkrafttragfähigkeiten von bewehrten Mauerwerksbalken mit und ohne Aufbeton oder VorspannungMauerwerk3/2017155-167Fachthemen

Abstract

This article investigates the transferability of the Simplified Modified Compression Field Theory (SMCFT) [2], which is known in reinforced concrete design and included in the fib Model Code for Concrete Structures 2010 (Volume 3) [1], to reinforced or prestressed masonry beams (RM beams) with or without an additional layer of concrete. The investigation for this work is the obsolete shear design concept that has been used until now for reinforced masonry under shear loading, which does not adequately reflect the actual load-bearing behaviour of significant areas of masonry. The fundamentals of the SMCFT are explained and the transferability of the theory to RM beams is examined, taking into account in particular the different material properties of masonry compared to reinforced concrete. A first approach for future application is represented by the equations presented here for the determination of the shear force capacity of RM beams. The verification is performed through a comparison of the shear resistances determined experimentally (exp.) and by calculation (calc.).

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Qi, Jia-Nan; Ma, Zhongguo John; Wang, Jing-Quan; Liu, Tong-XuPost-cracking shear strength and deformability of HSS-UHPFRC beamsStructural Concrete6/20161033-1046Technical Papers

Abstract

Eleven T-beams, reinforced with high strength steel, were tested to failure to investigate the effect of shear span to depth ratio, fibre ratio, fibre type, concrete strength and stirrup ratio on the shear behaviour, especially post-cracking shear strength and deformability, of ultra-high performance fibre reinforced concrete (UHPFRC) beams. Test results indicated that fibres were efficient not only in enhancing the post-cracking shear strength, but also in improving the post-cracking deformability of UHPFRC beams. In addition, fibres could bridge the cracks and help in redistributing and homogenizing the concrete stress beside the cracks, allowing more short fine diagonal shear cracks with small spacing to develop around the existing cracks. A moderate amount of stirrups can effectively restrain shear cracks and allow more parallel diagonal shear cracks to develop and propagate thoroughly within the shear span. The stiffness of the UHPFRC beams at ultimate state was about 50 % of initial beam stiffness, which was considerable in strength calculations and ductility analysis, especially in seismic performance evaluation. Lastly, the current shear provisions were evaluated using the experimental results.

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Wieneke, Katrin; Kueres, Dominik; Siburg, Carsten; Hegger, JosefInvestigations of the punching shear behaviour of eccentrically loaded footingsStructural Concrete6/20161047-1058Technical Papers

Abstract

The punching shear design provisions according to various codes have been derived from the results of tests conducted on centrically loaded flat slabs. The application of these provisions for footings and ground slabs might lead to inconsistent results since more compact dimensions and soil-structure interaction lead to higher punching shear capacities. In this context, Eurocode 2 introduced a new design equation for column bases, which was derived from the evaluation of test results from centrically loaded footings.
Since centrically loaded footings represent an exception in practice, Eurocode 2 and ACI 318-14 consider load eccentricities by increasing the applied load, while the fib Model Code 2010 proposes a reduced length of the control perimeter to determine the punching shear resistance. The different approaches were derived from the evaluation of tests on eccentrically loaded flat slabs and have not been verified for footings yet.
Theoretical and experimental investigations on the punching shear behaviour of eccentrically loaded footings indicate a reduction of the multi-axial stress state along the column face with increasing load eccentricity. Based on punching tests on eccentrically loaded footings described in literature, non-linear finite-element simulations were performed and subsequently the influence of load eccentricities on the punching shear behaviour was examined in parametric studies. In this article, the results of the numerical simulations are presented and compared to experimental results and various code provisions.

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Buttignol, Thomaz Eduardo Teixeira; Colombo, Matteo; di Prisco, MarcoLong-term aging effects on tensile characterization of steel fibre reinforced concreteStructural Concrete6/20161082-1093Technical Papers

Abstract

The paper discusses the effect of aging on steel fibre reinforced concrete (SFRC) after 10 years. The aim is to observe the change in mechanical properties, especially of the residual post-cracking tensile strength, due to long-term aging. For this purpose, a comparison between the results of four-point bending tests (4PB) at the age of 1 year and 10 years was carried out and it indicates that aging affects the serviceability post-cracking residual strength, increasing fibre interfacial bond strength. Material classification is performed according to fib Model Code 2010 for 1-year old and 10-year old specimens. The objective is to estimate possible changes in the material class through the years. Three- and four-point bending test results on 10-year old specimens are described, together with a comparison between those tests. Both tests showed very similar results; slightly higher values were obtained with the three-point bending (3PB) test. The tensile constitutive law is obtained according to fib Model Code 2010 and is compared with results of direct tensile tests on cylindrical specimens and Double Edge Wedge Splitting tests on prismatic specimens. A plane section (PS) approach adopting the tensile constitutive law is applied to predict the bending behaviour in terms of nominal stress against crack mouth opening displacement and it is compared with the bending test results.

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Zimmermann, Thomas; Lehký, David; Strauss, AlfredCorrelation among selected fracture-mechanical parameters of concrete obtained from experiments and inverse analysesStructural Concrete6/20161094-1103Technical Papers

Abstract

The correlations among selected parameters of concrete were investigated for concrete mixes of the strength classes C20/25, C25/30, C30/37, C40/50 and C50/60. The focus was laid on correlations between basic mechanical parameters such as compressive strength, tensile strength and modulus of elasticity as well as parameters related to concrete fracture, represented here by specific fracture energy. Laboratory tests examining the fracture behaviour and mechanical properties were carried out in order to determine the fundamental concrete parameters. In particular, standard compression tests on test cubes and three-point bending tests on beams with central edge notch were performed. Additional material parameters were identified using the inverse analysis technique. Finally, correlation factors between different parameters of concrete were identified using the rank-order correlation method.

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Sakai, Koji; Shibata, Toshio; Kasuga, Akio; Nakamura, HikaruSustainability design of concrete structuresStructural Concrete6/20161114-1124Technical Papers

Abstract

Concrete has become the most used material on Earth over the 200 years following the invention of modern cement. The design concept has undergone a transition from the allowable-stress design method, limit-state design method, to the performance-based design method, in response to the evolution of materials, sophistication of experimental facilities, and advancement of computation skills. From the issues on resources and energy depletion, global warming, and resilience etc., it is necessary to create a new design framework taking into consideration the required performance beyond the conventional concept, in order to construct infrastructure and buildings in a more rational way. In other words, we should construct a design system that sets the continued existence of the diverse and rich global environment as its most important criterion of value. In this paper, we review the design and technology system developed in the past and discuss it based on the above-mentioned new viewpoint, while constructing and presenting a new design system for concrete structures, focusing mainly on the concept of sustainability, which is regarded as the most important factor in achieving conservation of Earth's rich resources as well as sound socio-economic activities of humankind in the future, and we examine its feasibility.

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von Greve-Dierfeld, Stefanie; Gehlen, ChristophPerformance-based durability design, carbonation, part 3: PSF approach and a proposal for the revision of deemed-to-satisfy rulesStructural Concrete5/2016718-728Technical Papers

Abstract

A performance-based safety factor durability design format is proposed and developed with respect to carbonation of concrete. Deemed-to-satisfy rules based on a partial safety factor design approach are developed for the carbonation of concrete. This design format follows the design procedure proposed in EN 1990 [1]. For the design format, the limit state equation for the carbonation is introduced in its probabilistic and safety factor format. The PSF approach has been used to derive design charts. Values for minimum concrete cover depending on material resistance and exposure class are proposed for critical environmental conditions and a design service life of 50 years.

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Di Carlo, Fabio; Meda, Alberto; Rinaldi, ZilaDesign procedure for precast fibre-reinforced concrete segments in tunnel lining constructionStructural Concrete5/2016747-759Technical Papers

Abstract

This paper presents a procedure for designing precast tunnel segments for mechanically excavated tunnel linings in fibre-reinforced concrete, without any traditional steel reinforcement. Both ultimate and serviceability limit states are considered as well as structural checks at different construction stages of the segment, including demoulding, positioning on floor, storage, transportation, handling and the final stage concerning the loads due to the ground pressure.
The structural checks are performed by means of bending moment-axial force interaction envelopes for both the considered limit states, once the constitutive relationship of the material is defined for each stage. Traditional interaction envelopes are drawn for the ultimate limit state check, whereas for the serviceability limit state check, envelopes obtained by limiting the maximum crack opening and maximum concrete compressive stress are proposed. The shear action is also accounted for by reducing the bending moment-axial force envelope. The possibility of having the assistance of a test procedure for particular loading situations is also proposed. Finally, a case study related to a precast steel fibre-reinforced concrete segment is analysed in order to clarify the procedure and show, practically, how to define the actions and evaluate the interaction envelopes.

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Yu, Qiang; Le, Jia-Liang; Hubler, Mija H.; Wendner, Roman; Cusatis, Gianluca; Ba ant, Zden k P.Comparison of main models for size effect on shear strength of reinforced and prestressed concrete beamsStructural Concrete5/2016778-789Technical Papers

Abstract

This paper presents a critical comparison of the existing code provisions for the shear strength of concrete beams. The comparison is based on the computerized filtering-out of the inevitable statistical bias from the available multivariate database on shear strength, on an examination of the predicted size effects on shear strength and their underlying hypotheses and on the results of recent high-fidelity numerical simulations of shear failure. In addition to examining the existing models, the present comparison also provides several key considerations for testing the scientific soundness of any model of shear failure in concrete beams, which is necessary for future revisions to the design code provisions.

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Yang, Yuguang; den Uijl, Joop; Walraven, JoostCritical shear displacement theory: on the way to extending the scope of shear design and assessment for members without shear reinforcementStructural Concrete5/2016790-798Technical Papers

Abstract

This paper presents a new theory for the shear capacity of reinforced concrete members without shear reinforcement. While recognizing that there are multiple failure mechanisms, the theory attributes the opening of a critical flexural shear crack as the lower bound of the shear capacity. It proposes that the shear displacement of an existing flexural crack can be used as the criterion for the unstable opening of the critical flexural shear crack. Based on the theory, the paper presents a simplified shear evaluation model. Compared with the current shear provisions in the design codes, the model is characterized by good accuracy and a solid physical background. It demonstrates a great flexibility for dealing with complex design conditions. As an example, the paper discusses the possibility of extending the theory to the shear resistance of higher-strength concrete. The suggested method provides a more logical and fluent transition from normal- to high-strength concrete and shows good agreement with experimental observations.

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Figueira, Diogo; Sousa, Carlos; Calçada, Rui; Neves, Afonso SerraDesign recommendations for reinforced concrete interfaces based on statistical and probabilistic methodsStructural Concrete5/2016811-823Technical Papers

Abstract

A concrete interface is a material discontinuity that requires special care with respect to structural design and assessment. Therefore, the definition of design expressions based on experimental testing data must ensure the necessary reliability depending on the type of structure and its use. The present work describes a new proposal for the design of concrete interfaces subjected to shear loading for different roughness profile types. The proposal is characterized by three linear branches (for monotonic loading) and an S-N curve (for cyclic loading) and is the result of a parametric analysis of existing experimental data (obtained by the authors and also from an extensive literature search) based on statistical and probabilistic methods. Design expressions were defined in order to minimize the dispersion and variability of the safety factor values for each experimental test considered and also to assure that those values are within a target range (defined according to reliability considerations). These improvements became clearer when the new proposal was compared with the most common design code recommendations.

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von Greve-Dierfeld, Stefanie; Gehlen, ChristophPerformance-based durability design, carbonation part 2 - Classification of concreteStructural Concrete4/2016523-532Technical Papers

Abstract

At present, prescriptive regulations with regard to concrete cover and composition are applied to provide sufficient durability of reinforced concrete members under exposure conditions with different degrees of severity. In view of current knowledge on deterioration mechanisms and their modelling, it is planned to change from these deemed-to-satisfy specifications to a performance-based design approach in future standards. In such specifications, concrete durability design is based on the statistically characterized performance of concrete, determined in standardized tests with respect to defined classes of concretes with similar performance.
This paper presents the results of a study in which concrete mixes were tested and analysed with respect to their carbonation resistance. Compositions with similar performance are grouped into carbonation resistance classes. These classes are described statistically and requirements for performance testing are given. In addition, composition requirements are introduced in order to determine concrete performance depending on mix composition prescriptively. Finally, an example is given for the assessment of concrete performance with regard to carbonation.
This work was carried out at the request of JWG under CEN TC 250/SC2 and CEN TC 104/SC1 as an input and starting point for the ongoing committee work to implement the methodology from the fib Model Code for Concrete Structures 2010 in the next generation (2021) of European concrete standards.

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di Prisco, Marco; Martinelli, Paolo; Parmentier, BenoitOn the reliability of the design approach for FRC structures according to fib Model Code 2010: the case of elevated slabsStructural Concrete4/2016588-602Technical Papers

Abstract

This paper focuses on the reliability of the design approach proposed in the fib Model Code for Concrete Structures 2010 for estimating the ultimate capacity of fibre-reinforced concrete (FRC) elevated slabs on the basis of different tests for material characterization. The fracture properties of the material are determined through three-point bending tests on notched beams and through double edge wedge splitting (DEWS) tests carried out on cylinders cored in the full-size test structure. As a case study, an FRC elevated flat slab 0.2 m thick is considered which consists of nine bays (panels) measuring 6 × 6 m (overall size 18.3 × 18.3 m) and is supported by 16 circular concrete columns. The ultimate bearing capacity of the slab determined experimentally is compared with the design value predicted by means of a procedure based on limit analysis following fib Model Code 2010. The results show that the method proposed in fib Model Code 2010 using the characteristic values and the classification is reliable. Even if the tests are affected by a significant standard deviation and the two experimental campaigns with three-point bending tests give a significant difference between class “5c” and class “3e”, the structural test results in a loadbearing capacity that is always larger than the predicted one, which considers a safety coefficient for the material &ggr;F = 1.5.

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Hümme, Julian; von der Haar, Christoph; Lohaus, Ludger; Marx, SteffenFatigue behaviour of a normal-strength concrete - number of cycles to failure and strain developmentStructural Concrete4/2016637-645Technical Papers

Abstract

The fatigue behaviour of concrete is gaining new relevance against the backdrop of continuous developments in concrete construction. Modern types of concrete are achieving ever higher strengths; hence, concrete structures are becoming increasingly attractive for new fields of application such as onshore and offshore wind turbines. The fatigue of concrete has a special relevance for these cyclically loaded structures and knowledge of the number of cycles to failure is no longer sufficient for their design. There are further questions concerning strain and stiffness development and the combination of fatigue loading and maritime environmental conditions which have been investigated with new testing methods at Leibniz Universität Hannover within the scope of the “ProBeton” research project. The first results of this project, which is supported by the Federal Ministry for Economic Affairs and Energy, are presented here.

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