Untersucht wurde der Einfluss der Wassersättigung auf die statische und dynamische Druckfestigkeit von Beton. Hierzu wurden zwei verschiedene Probengeometrien mit definierten Sättigungsgraden getestet. Für die dynamischen Druckfestigkeitsversuche wurde der Split‐Hopkinson‐Bar verwendet. Es wurde festgestellt, dass die Druckfestigkeit von Beton mit steigendem Sättigungsgrad abnimmt, sowohl im statischen als auch im dynamischen Belastungsfall. Der Betrag der Abminderung der Druckfestigkeit mit steigender Sättigung ist im statischen und dynamischen Fall gleich. So kann festgehalten werden, dass der Einfluss der Wassersättigung unabhängig von der Dehn‐ und Belastungsgeschwindigkeit ist. Die trockenen Proben versagten explosionsartiger mit einer erhöhten Riss‐ und Bruchstückanzahl im Vergleich zu den wassergesättigten Probekörpern.

The ductile behavior of strain-hardening cement-based composites (SHCC) under direct tensile load makes them promising solutions in applications where high energy dissipation is needed, such as in earthquakes, impacts by projectiles, or blasts. However, the superior tensile ductility of SHCC due to multiple cracking does not necessarily point to compressive and shear ductility. As an effort to characterize the behavior of SHCC under impact compressive and shear loading relevant to the aforementioned high-speed loading scenarios, the paper at hand studies the performance of a particular SHCC and its constituent, cement-based matrices using the split-Hopkinson bar method. For compression experiments, cylindrical specimens with a length-to-diameter ratio (l/d) of 1.6 were used. The selected length of the sample led to similar failure modes under quasi-static and impact loading conditions, necessary to a reliable comparison of the observed compressive strengths. The impact experiments were performed in a split-Hopkinson pressure bar (SHPB) at a strain rate that reached 110 s−1 at the moment of failure. For shear experiments, a special adapter was developed for a split-Hopkinson tension bar (SHTB). The adapter enabled impact shear experiments to be performed on planar specimens using the tensile wave generated in the SHTB. Results showed dynamic increase factors (DIF) of 2.3 and 2.0 for compressive and shear strength of SHCC, respectively. As compared to the non-reinforced constituent matrix, the absolute value of the compressive strength was lower for the SHCC. Contrarily, under shear loading, the SHCC demonstrated higher shear strength than the non-reinforced matrix.

An analytical model is presented in this paper which, based on the maximum crack velocity, provides a hypothesis for one of the reasons of the increase in tensile strength of concrete under high loading rates. Due to the fact that the formation of cracks needs a certain time to pass through the cross section and not happens suddenly, stresses can still be transmitted over the remaining uncracked cross section during this time. The hypothesis is that at high loading rates, the increase in externally induced stresses can be greater than the decrease in the load-bearing cross-sectional area due to limited crack propagation velocity, which results in an externally measurable increase in strength. This measured strength increase depends on the stress distribution in the crack plane. In this paper two variants of this stress distribution during the failure process are described, and their effect on the increase in strength is mathematically evaluated.

The determination of crack propagation velocities can provide valuable information for a better understanding of damage processes of concrete. The spatio-temporal analysis of crack patterns developing at a speed of several hundred meters per second is a rather challenging task. In the paper, a photogrammetric procedure for the determination of crack propagation velocities in concrete specimens using high-speed camera image sequences is presented. A cascaded image sequence processing which starts with the computation of displacement vector fields for a dense pattern of points on the specimen’s surface between consecutive time steps of the image sequence chain has been developed. These surface points are triangulated into a mesh, and as representations of cracks, discontinuities in the displacement vector fields are found by a deformation analysis applied to all triangles of the mesh. Connected components of the deformed triangles are computed using region-growing techniques. Then, the crack tips are determined using the principal component analysis. The tips are tracked in the image sequence and the velocities between the time stamps of the images are derived. A major advantage of this method as compared to the established techniques is in the fact that it allows spatio-temporally resolved, full-field measurements rather than point-wise measurements. Furthermore, information on the crack width can be obtained simultaneously. To validate the experimentation, the authors processed image sequences of tests on four compact-tension specimens performed on a split-Hopkinson tension bar. The images were taken by a high-speed camera at a frame rate of 160,000 images per second. By applying the developed image sequence processing procedure to these datasets, crack propagation velocities of about 800 m/s were determined with a precision in the order of 50 m/s.

The determination of crack propagation velocities can provide valuable information for a better understanding of damage processes of concrete. The spatio-temporal analysis of crack patterns developing at a speed of several hundred meters per second is a rather challenging task. In the paper, a photogrammetric procedure for the determination of crack propagation velocities in concrete specimens using high-speed camera image sequences is presented. A cascaded image sequence processing which starts with the computation of displacement vector fields for a dense pattern of points on the specimen’s surface between consecutive time steps of the image sequence chain has been developed. These surface points are triangulated into a mesh, and as representations of cracks, discontinuities in the displacement vector fields are found by a deformation analysis applied to all triangles of the mesh. Connected components of the deformed triangles are computed using region-growing techniques. Then, the crack tips are determined using principal component analysis. The tips are tracked in the image sequence and the velocities between the time stamps of the images are derived. A major advantage of this method as compared to established techniques is in the fact of its allowing for spatio-temporally resolved, full-field measurements rather than point-wise measurements and that information on crack width can be obtained simultaneously. To validate the experimentation, the authors processed image sequences of tests on four compact-tension specimens performed on a split-Hopkinson tension bar. The images were taken by a high-speed camera at a frame rate of 160,000 images per second. By applying to these datasets the image sequence processing procedure as developed, crack propagation velocities of about 800 m/s were determined with a precision in the order of 50 m/s.

In dieser Studie wurde der Einfluss verschiedener Lagerungsbedingungen auf die statische und dynamische Druckfestigkeit von drei Betonen (C20/25, C35/45 und C60/75) untersucht. Dabei wurden die Proben gezielt getrocknet und wassergesättigt und anschließend sowohl statisch als auch dynamisch geprüft. Die dynamischen Versuche wurden in einem Split‐Hopkinson‐Bar durchgeführt. Hierbei konnten je nach Betonfestigkeitsklasse Festigkeitssteigerungen von 200 bis 300 % bei Dehnraten im Bereich von 90 bis 160 1/s festgestellt werden. Im Vergleich zur Lagerung unter Umgebungsbedingungen sinkt die Druckfestigkeit durch die Trocknung aufgrund der Mikrorissbildung. Des Weiteren sinken die statischen und dynamischen Betondruckfestigkeiten von wassergesättigten Proben im Vergleich zu trockenen Proben. Dieser Abfall konnte sowohl unter statischer als auch unter dynamischer Beanspruchung beobachtet werden und ist unabhängig von der Dehnrate. Die trockenen Proben versagen explosionsartiger mit einer erhöhten Anzahl an Rissen und Bruchstücken im Vergleich zu wassergesättigten Proben. Weiterhin konnte am Beispiel zweier Fallturmversuche von einem trocken und einem wassergelagerten großformatigen Betonbauteil gezeigt werden, dass vorhandenes Porenwasser einen signifikanten Einfluss auf das Versagensbild und die Übertragung der Impaktbelastung in die Auflagerkonstruktion hat.Dieser Aufsatz wurde in ähnlicher Weise bereits in [1] veröffentlicht.

Untersucht wurde die Druckfestigkeit von zwei Betonen unter zweiaxialer dynamischer Druckbeanspruchung. Dabei wurde ein zweiaxialer Split‐Hopkinson‐Bar verwendet, welcher die dynamische Beanspruchung in Form zweier senkrecht zueinander verlaufender Belastungswellen auf den Probekörper aufbringt. Zur Referenz wurden die statischen Druckfestigkeiten unter biaxialer Belastung mit verschiedenen Spannungsverhältnissen der beiden Belastungsachsen in einer Triaxialprüfmaschine ermittelt. Die Ergebnisse deuten auf eine teilweise Überlagerung, jedoch keine vollständige Superposition der beiden festigkeitssteigernden Effekte aus mehraxialer Druckbeanspruchung und erhöhter Dehnrate hin. Insbesondere sind deutliche Unterschiede im Bruch‐ und Rissbild bei den unterschiedlichen Belastungsarten zu erkennen.

The ductile behavior of strain hardening cement-based composites (SHCC) under direct tensile load makes them promising solutions for applications where high energy dissipation is needed, such as earthquake, impact by a projectile, or blast. However, the superior tensile ductility of SHCC due to multiple cracking does not necessarily entail compressive and shear ductility. As an effort to characterize the behavior of SHCC under impact compressive and shear loading, relevant to the mentioned high-speed loading scenarios, the paper at hand studies the performance of a SHCC and its constituent cement-based matrices using the split-Hopkinson bar method. For compression experiments, cylindrical specimens with a length-to-diameter ratio (l/d) of 1.6 were used. The selected length of the sample led to similar failure modes under the quasi-static and impact loading conditions, which was necessary for a reliable comparison of the obtained compressive strengths. The impact experiments were performed in a split-Hopkinson pressure bar (SHPB) at a strain rate that reached 110 s-1 at the moment of failure. For shear experiments, a special adapter was developed for a split-Hopkinson tension bar (SHTB). The adapter enabled performing impact shear experiments on planar specimens using the tensile wave generated in the SHTB. Results showed a dynamic increase factor (DIF) of 2.3 and 2.0 for compressive and shear strength of SHCC, respectively. As compared to the non-reinforced constituent matrix, the absolute value of the compressive strength was lower for the SHCC. Contrarily, under shear loading, the SHCC yielded the higher shear strength than the non-reinforced matrix.