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Verbund von Beton und Bewehrungsstahl bei hoch-dynamischer Belastung
Abstract
Vor dem Hintergrund zunehmender terroristischer Bedrohung für Bauwerke in besonderen Gefährdungslagen ist es erforderlich, die Bauteilreaktion bei der Bemessung auch für die hohen Verzerrungsraten, wie sie bei Explosionen und Impakt auftreten, zu berücksichtigen. Voraussetzung hierfür ist eine wirklichkeitsnahe Beschreibung des Materialverhaltens. Dabei wird auf der Mesoebene das Zusammenwirken von Beton und Betonstahl durch die Verbundwirkung beschrieben. Um das Verhalten von Bauteilen, die Einwirkungen mit hohen Belastungsgeschwindigkeiten ausgesetzt sind, richtig beurteilen zu können, ist daher auch die Kenntnis des verzerrungsratenabhängigen Verbundverhaltens erforderlich. Im Rahmen des zur Zeit an der Universität der Bundeswehr München (UniBwM) laufenden Forschungsprojekts RS1510 werden Verbundversuche am Split‐Hopkinson‐Bar (SHB) in Zusammenarbeit mit dem Fraunhofer Institut für Kurzzeitdynamik Ernst‐Mach‐Institut (EMI) durchgeführt. Die Ergebnisse dienen als Basis für die Entwicklung numerischer Modelle für die Abbildung des Verbunds von Stahl und Beton bei dynamischer Belastung. Nach einem kurzen Überblick zum Verbundverhalten werden die Grundlagen der Versuchstechnik dargestellt und die Besonderheiten bei den Push‐in‐Versuchen erläutert. Die Ergebnisse der durchgeführten Versuche zeigen eine deutliche Tendenz in Richtung von der Verzerrungsrate im Beton abhängigen Anstieg der Verbundfestigkeit.
Bond of concrete and steel under high dynamic loading
Against the background of increasing terrorist thread for critical infrastructure, it is necessary to consider the high strain rates related with explosion and impact for the design of structural components. The precondition for this is a realistic characterization of the material behavior. In meso‐level the interaction of steel and concrete is described by bond. In order to assess the behavior of structures exposed to high rate loading correctly, even the knowledge of strain rate dependent bond behavior is required. Within the ongoing research project RS1510 at the University of the Bundeswehr München (UniBwM) tests on bond between steel and concrete with a split‐hopkinson‐bar (SHB) are currently performed in collaboration with the Fraunhofer Institute for High‐Speed Dynamics Ernst‐Mach Institute (EMI). The results will be used as a basis for the development of a numerical model for the simulation of bond between steel and concrete under high loading rates. After a short survey on bond behavior, the theoretical basis for the testing technology is given and the specific characteristics of the performed push‐in tests are explained. The results of the tests show a clear tendency for the rate‐dependent increase in bond strength.
... In this course, the bond stress-slip relationship has been widely established to describe the behavior of the bond. It is well investigated for quasi-static loading, while only little information on the bond behavior under high loading rates exists (Michal et al. 2016). Knowledge of the bond stress-slip relationship is important for constructions under dynamic loading scenarios like explosions, earthquakes and impact loadings, because many materials show changed behavior under dynamic compared to static loading (Bischoff and Perry 1991). ...
... Generally, dynamic experiments are subjected to particular restrictions with regard to the experimental set-up. Several authors (Michal et al. 2016;Solomos and Berra 2010) have thus modified the SHPB in different ways in order to determine dynamic bond stress-slip relationships between concrete and reinforcement. ...
Reinforced concrete (RC) structures are increasingly subjected to extreme loading events e.g. impacts, explosions, earthquakes. RC is a composite material. For the load transfer between its two components concrete and reinforcing steel adequate bond is required. The work presented herein provides insights into experimental testing and numerical modelling of bond behavior. It aims to create a better understanding of its changed characteristics under dynamic compared to static loading. Dynamic bond stress-slip relationships are obtained during especially designed push-in tests, since conventional pull-out tests are rather unsuitable for dynamic testing. The used experimental setup and the evaluation process are described. The influence of different slip measurement approaches on the overall bond stress-slip relationship is illustrated. In addition, the definition of loading rate based on the slip increase in time is proposed. The experimental results indicate an increase of maximal bond stress with increasing loading rate, which agrees with known results from literature. Insights into the local behavior in the vicinity of the bond zone are given by the numerical simulation in LS-Dyna. The numerical results match the experimental observations well.
... This value is comparable to the DIF presented in literature. For instance Michal et al. [28] performed recently push-in tests in a split Hopkinson bar for concrete with a compressive strength f c = 50.5 N/mm² which is similar to the concrete used in this study. ...
Drop tower tests on beam specimens were carried out to evaluate the influence of different geometric features and different loading velocities on their response. Tested geometric characteristics are stirrup reinforcement in addition to longitudinal reinforcement, and notches. The goal is to quantify the differences in the global behaviour and the amount of strains and strain rates measured at different positions on the reinforcement. This gives an insight into strain distributions along the rebars from which bond stresses can be derived. It can be stated that stirrup reinforcement held the specimens together and thus increased the impact resistance. That was seen in the deflection and acceleration measurements, but it was also suggested by strain measurements, especially looking at residual strains. The maximum strains are more affected by global bending of the specimens than by the formation of a localized punching cone, which was the main failure mode.
Under dynamic loading conditions, the bond between reinforcing bars and concrete significantly affects the structural behaviour of reinforced concrete beam-column joints. Nonetheless, limited data are available with regard to the dynamic bond-slip relationship of steel reinforcement. This paper describes the dynamic bond-slip behaviour of steel rebars in concrete. In the experimental programme, a total of 63 specimens, with different compressive strengths of the surrounding concrete and embedment lengths of reinforcement, were tested under pull-out loads. The loading rate ranged from 0.1 mm/s to 100 mm/s. Besides the tension force and the measured slip, the strain profile of reinforcement was also monitored by using strain gauges mounted to the reinforcement along its longitudinal ribs, and the corresponding bond stress was calculated based on force equilibrium. Comparisons were made between static and dynamic bond stresses to shed light on the influence of loading rate on bond stress distribution. Moreover, an analytical method was developed based on test data, and the analytical stress-slip relationship, strain and bond stress distributions were obtained by using the method. The analytical results are in reasonably good agreement with the experimental data, suggesting that the model can be used to predict the bond-slip behaviour of reinforcing bars subjected to different loading rates.
The present paper describes essential results of comprehensive experimental investigations of the analysis of rc‐slabs under impact loads. The presented work was done in collaboration between the Fraunhofer Institute for High‐Speed Dynamics – Ernst‐Mach‐Institut (EMI) and the Technical University of Dresden (TUD) in the national research project “impact on concrete structures – experiment and simulation” sponsored by the DFG (Deutsche Forschungsgemeinschaft). In a comprehensive test series the loading of a so‐called hard impact – that means the kinetic energy is almost completely introduced in the structure – was studied for different velocity ranges (4.2–6.0 m/s; 44–67 m/s and 200–274 m/s). The resulting damage phenomena and the dynamic structural response were measured by extensive instrumentation (strain gauges, pressure sensors, laser and high‐speed cameras). The received data for low and high strain rates serves for the general understanding of the relations between local transient loading and global structural response. It is the basis for the adaption of existing material models, the development of engineering models and the validation of numerical calculations.
Bei dem Entwurf von Bauwerken wie Botschaften, militärischen Einrichtungen und Kernkraftwerken sind neben den planmäßigen auch außergewöhnliche Beanspruchungen wie Explosion und Impakt zu berücksichtigen. Diese Einwirkungen führen zu großen Verzerrungsraten in den betroffenen Baustoffen. Es ist bekannt, dass sich die mechanischen Eigenschaften von Stahl und Beton ebenso wie ihr Verbundverhalten mit der Belastungsgeschwindigkeit verändern. Die Kenntnis des ratenabhängigen Verbundverhaltens von Stahl und Beton ist die Voraussetzung für die nichtlineare Analyse von Stahlbetonbauteilen bei hochdynamischen Beanspruchungen, wenn Systemreserven durch plastische Lastumlagerungen ausgenutzt werden sollen, die Dichtigkeit gegen Flüssigkeiten oder Gase gewährleistet sein muss und Anforderungen an die Begrenzung der Rissbreite bestehen oder die auftretende Verformung begrenzt werden muss. Um das komplexe Zusammenwirken von Stahl und Beton bei hohen Belastungsgeschwindigkeiten zu erfassen, wurden geeignete experimentelle Methoden entwickelt. Auf Grundlage der Versuchsergebnisse werden Modelle vorgeschlagen, mit denen der Einfluss der Zeit auf den Verbund beschrieben werden kann.
A concrete type C20/25 was examined under biaxial dynamic compressive loading with several stress ratios and two different loading rates (75 and 150 1/s). The question to be answered is, if the two strength enhancing effects of biaxial compressive loading and elevated strain rate overlay under this combined loading condition. For these experiments a unique biaxial Split-Hopkinson-Pressure-Bar, developed at TechnischeUniversität Dresden, was used to load the cubic specimen with two compression pulses from two perpendicular directions, simultaneously. That means the pulses have to reach the specimen within a time span of maximum 25 μs to achieve a rising stress level in both spatial directions in the specimen. For comparison, further static uni- and biaxial trials, static-dynamic biaxial trials and dynamic uniaxial trials were conducted. After the trials, the particles of the fragmented specimens were analysed in size and shape. The results show clearly that the two examined strength increasing effects superimpose in case of a combined biaxial dynamic compressive loading, but not completely. Therefore the influence factor of each single effect was analysed, depending on the stress ratio and the strain rate. For the considered range of high loading velocities, the strain rate effect has the main influence on the strength enhancement. This effect is decreasing with a growing stress ratio, here the biaxiality gains more influence but still remains behind the strain rate effect.
Although the subject of wave propagation in solids has a long history, the classical theory of elastic waves having been developed in the nine teenth century by STOKES, POISSON, RAYLEIGH and KELVIN, the last two decades have seen a remarkable revival of interest in this subject among both theoreticians and experimenters. There' are a number of reasons for this; first, experimental methods for the generation and detection of high frequency mechanical waves have become available only with the advent of electronic techniques and of high speed photo graphic recording apparatus. Secondly, the appearan
Elastic wave propagation in the split Hopkinson pressure bar (SHPB) is discussed with an emphasis on the origin and nature of the oscillations that often trail the leading edge of the pressure wave. The authors show that in the conditions of the SHPB test the pressure bars vibrate in the fundamental mode and that elastic wave progapation can be fully described mathematically. Excellent agreement is found between experimental results and predictions of the mathematical treatment. This suggests that dispersion effects in the pressure bars can be removed from the strain gage records, which reduces the magnitude of the oscillations in the resulting stress strain curve. 12 references, 11 figures, 1 table.
This paper presents the theoretical basis and the main results of an analytical model that describes the pre- and postyield response of bond in reinforced concrete, as well as its influence on the behavior of structural members. The model is based on a number of reasonable assumptions that simplify the differential equation governing the phenomenon. In this way, a closed-form integration is feasible in certain cases, and relatively simple but accurate expressions can be worked out for the interface slip, the strain, and the stress in the reinforcement and the bond stress at the bar-concrete interface. Applications are made to different structural members pull-out specimens, tension ties, and beams in bending, whose behavior is well documented in the literature. The agreement between the available test data turns out to be quite satisfactory. Consequently, the proposed model can be usefully adopted for studying the serviceability behavior, the ductility and the postyield response of a variety of reinforced-concrete members. reasonable assumptions that lead to the description of bond me- chanics in long bonded members by means of a first-order differ- ential equation. This equation can be easily integrated for any bond law, with reference to a wide range of practical applications. Two bond-slip laws are considered in this study. The corre- sponding analytical expressions describing the strain and slip dis- tributions for long pull-out specimens and tension ties include the pre- and postyield behavior of the reinforcement. The resulting laws are simple enough to be used in practical cases, and satis- factorily fit the available test data.
The velocity of longitudinal waves in cylindrical bars may be expressed as the velocity at infinite wave-length times a function of two variables: Poisson's ratio, and the ratio of the diameter of the bar to the wave-length. This function is computed over the domain of the arguments which is of physical interest. Asymptotic values for the velocities at very short wave-lengths are deduced, and the variation of the displacement as a function of the radius is discussed. It is found that a similar analysis can be applied to torsional and flexural waves.
The dynamic behavior of the bond slip between a deformed reinforcing bar and plain concrete has been experimentally investigated
by employing Hopkinson bar techniques. Pullout tests with various specimen types (unconfined, confined, cast-in-place, post-installed
etc.) have been performed. Pullout of the steel rebar and splitting of the concrete cylinder have been the failure modes induced.
Test results comprise peak pullout forces and complete bond stress–slip diagrams. They clearly show that the dynamic pullout
forces and curves are well above the static ones, and that the pullout work of bond failure is considerably greater for the
dynamic impact loading. Confinement, provided by a steel tube, leads to improved bonding; peak loads increase up to 2.5 times.
The effects of bond length and concrete strength have also been put into evidence. Finally it has been verified that post-installed
rebars, depending upon the particular adhesive employed, can achieve the same bond resistance as the cast-in-place ones.
To study the behavior of concrete under dynamic loads, a Hopkinson-Bar was set up and used. Cylindrical concrete specimens were positioned at the end of the incident bar and the spall event was studied. The purpose of this contribution is to explain the measurement of the tensile strength and the specific fracture energy. To determine the tensile strength, the measured free surface velocity at the end of the specimen is used. The method is known from plate impact experiments and was adapted to Hopkinson-Bar experiments. The measurement of the specific fracture energy is more difficult in spall experiments. It cannot be measured directly as it can be done in direct tension tests. A method is proposed where the fracture energy is calculated from the change of the fragment velocities while cracking takes place.The experimental results of the investigation complete the data of the literature in regard to higher strain rates. In former investigations conducted by Weerheijm (PhD thesis. Delft University of Technology: Delft University Press; 1992), an increase of the specific fracture energy with the strain rate or the crack opening velocity was not seen. The experiments performed within this contribution consider the fracture behavior at higher strain rates. A sharp increase in the specific fracture energy at this strain rates was measured. The following paper describes the method and the experiments to measure the tensile strength and the specific fracture energy in spall experiments.
Die vorliegende Arbeit setzt sich mit der Schädigung von Beton durch stoßartige Beanspruchung auseinander. Experimentell untersucht wird die Schädigung von Beton unter hoher Druck- und dynamischer Zugbeanspruchung. Mit einem Split-Hopkinson-Bar werden in Spallationsversuchen die Zugfestigkeit und die spezifische Bruchenergie gemessen. Die experimentellen Erkenntnisse bilden die Basis für die Ableitung einer neuen Schädigungsentwicklung innerhalb eines vorhandenen Betonmodells (RHT-Modell). Das neue Entwicklungsgesetz wird validiert durch die Simulation einer Kontaktdetonation und einer zweistufigen Penetration.
The rate dependency of concrete in tension is studied in a combined experimental and computational research program. The aim is to develop a physically realistic material model describing the response and failure mechanisms at meso-level. This paper presents the applied experimental techniques and obtained results for three loading rates at three moisture levels for one type of concrete. New and unique data is obtained for (i) the dynamic fracture energy at high loading rates, (ii) the moisture distribution in the gel- and micro pores and its effect on rate dependency and (iii) data on crack distribution and width of the fracture zone which is obtained by microscopic research.
München, T. H., F. f. Bauw., Diss. v. 2. Mai 1958 (Nicht f. d. Aust.).
Theorie und Experiment der Werkstoffcharakterisierung für dynamisches Materialverhalten sind zwei Welten, die sich vielfach getrennt voneinander entfalten. Zusammen mit der numerischen Methodenentwicklung sind dies die drei obligatorischen Bausteine einer physikalisch sinnvollen Simulation von Crash- und Impaktvorgängen. Im Automobilcrash, beim Impakt von Flugkörpern auf harte oder gepanzerte Strukturen wie auch beim Einschlag von Mikrometeoriten in Raumfahrzeugen treten kurzzeitige Belastungen der jeweiligen Werkstoffe auf, die mit großen zeitlichen Änderungen der Druck-, Dichte- und Verformungszustände einhergehen. Ziel der vorliegenden Arbeit ist es, für diese drei Anwendungsbereiche die theoretischen Grundlagen sowohl für die Formulierung mathematischer Modelle als auch für die Definition und Durchführung von Experimenten zur Ermittlung von Materialparametern zusammenzustellen. Die Arbeit spiegelt eine wesentliche Bestrebung des Autors in seiner Tätigkeit am Ernst-Mach-Institut wider, die bestehenden Forschungsteams zur dynamischen Materialprüfung und zur numerischen Methodenentwicklung zusammenzuführen. Grundlagen und ausgewählte Ergebnisse der gleichzeitigen experimentellen und numerischen Untersuchung der Strukturdynamik bei Crash und Impakt werden dargestellt.
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