The pullout behaviour of single steel fibres embedded in a concrete matrix is investigated for various configurations of fibre types and embedment lengths and angles by means of laboratory tests and analytical models. Laboratory tests for fibre pullout are performed to investigate the fibre‐matrix bond mechanisms. Parameters influencing the fibre pullout response, such as fibre shape, fibre tensile strength, concrete strength and fibre inclination angle are systematically studied. The effects of these parameters on the pullout force versus displacement relationship, fibre efficiency and fibre/matrix failure response are analysed based on the experimental results. For the analytical modelling of the fibre pullout behaviour of straight fibres, an interface law is proposed for the frictional behaviour between fibre and matrix. In the case of inclined fibres, the plastic deformation of the fibre and the local damage to the concrete are also considered. For hooked‐end fibres, the anchorage effect due to the hook is analysed. Combining these sub‐models allows the pullout response of single fibres embedded in a concrete matrix to be predicted. In addition, numerical simulations of pullout tests are performed to obtain insights into the local fibre‐concrete interactions and to provide supporting information for the analytical modelling. The models are successfully validated with the experimental results.

Stahlfaserbetone finden verstärkt auch in konstruktiven Betonbauteilen Eingang. Ein Beispiel sind Tübbinge für den Tunnelausbau, wobei hier die Besonderheit hinzukommt, dass dafür meist Betone mit Festigkeiten in der Größenordnung von 70 bis 90 N/mm2 verwendet werden. Damit weisen diese Betone ein vergleichsweises sprödes Verhalten mit geringer Bruchzähigkeit auf, sodass es des Öfteren zu Schädigungen in Form von Rissbildung und Abplatzung im Eck‐ oder Kantenbereich kommt. Die Zugabe von Stahlfasern kann hier erheblich zur Verbesserung der Duktilität beitragen. Dabei ist von wesentlicher Bedeutung, inwieweit die Stahlfaser im gerissenen Zustand des Betons Zugkräfte über den Riss hinweg übertragen kann bzw. inwieweit sie aus der Matrix ausgezogen wird. Um das charakteristische Verbundverhalten zwischen Stahlfaser und Beton genau zu untersuchen, wurden Auszugsversuche an Einzelfasern durchgeführt, die in höherfesten Betonen eingebettet waren. Gezielt wurden dabei Stahlfasern bezüglich Form, Geometrie, Festigkeit und Oberflächenbeschaffenheit sowie Einbettungswinkel und Einbindelänge variiert. Basierend auf den experimentellen Ergebnissen wurden die Auswirkungen dieser Parameter auf das Auszugsverhalten von Stahlfasern im Hinblick auf das Kraft‐Verschiebungs‐Verhältnis, die Arbeit beim Ausziehen und den Ausnutzungsgrad der Faserzugfestigkeit sowie die Versagensbilder von Faser und Beton analysiert und diskutiert. Experimental investigation of the pullout behavior of steel fibers in high‐strength concrete Steel‐fiber‐reinforced concretes are used more and more also in structural concrete elements. Examples for these are among others tunnel lining segments. In this case there is a specific characteristic, that concretes for such elements usually have a compressive strength between 70 and 90 N/mm2. These concretes are very brittle with only low fracture toughness, so that damages in form of cracking and spalling often occur in the periphery of the segments. By the addition of steel fibers the ductility of the concrete can be considerably improved. The primary question is, to which extend the stresses can be transferred in concrete cracks by the fibers bridging these cracks. To investigate the bonding behavior between steel fiber and concrete matrix, single fiber pullout tests were performed in high‐strength concrete matrix. Various parameters such as fiber shape, geometry, tensile strength, surface properties, inclination angle and embedment length were investigated. Based on the experimental results, the effect of those parameters on pullout force versus slip relationship, pullout work, fiber utilization and fiber/matrix failure mode were analyzed and discussed.

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According to the objectives of the research group 1498, this paper deals with degradation effects in concrete structures that are caused by cyclic flexural loading. The goal is to determine their influence on the fluid transport processes within the material on the basis of experimental results and numerical simulations. The overall question was, to which extent the ingress of externally supplied alkalis and subsequently an alkali‐silica reaction are affected by such modifications in the microstructure. Degradation in the concrete microstructure is characterized by ultrasonic wave measurements as well as by microscopic crack analysis. Furthermore, experiments on the penetration behavior of water into the investigated materials were performed. The penetration behavior into predamaged concrete microstructures was examined by the classical Karsten tube experiment, nuclear magnetic resonance method, and time domain reflectometry techniques. In order to create an appropriate model of the material's degradation on the water transport, the Darcy law was applied to describe the flow in partially saturated concrete. Material degradation is taken into account by an effective permeability that is dependent on the state of degradation. This effective permeability is obtained by the micromechanical homogenisation of the flow in an Representative Elementary Volume (REV) with distributed ellipsoidal microcracks embedded in a porous medium. The data gained in the microscopic crack analysis is used as input for the micromechanical model. Finite element simulations for unsaturated flow using the micromechanical model were compared with the experimental results showing good qualitative and quantitative agreement.

Auf Grundlage der Zielsetzung der Forschergruppe 1498 beschäftigt sich dieser Beitrag mit den Auswirkungen einer zyklischen mechanischen Belastung im Vierpunktbiegeversuch auf das Transportverhalten in Betongefüge. Hierzu wurde zunächst die Degradation des Mikrogefüges mittels Ultraschallmessungen sowie rissmikroskopischen Untersuchungen an Dünnschliffen charakterisiert. Mit dem Ziel der numerischen Modellbildung wurden Untersuchungen zum Wassereindringverhalten durchgeführt. Es wurden u. a. das Wassereindringverhalten über die Zeit und der Einfluss von Vorschädigungen experimentell geprüft. Basierend auf den gewonnenen Erkenntnissen zu den Einzelprozessen des Ionentransports in poröses Gefüge wurde ein mikromechanisches Mehrskalenmodell entwickelt, welches es ermöglicht, die Wirkung der Vorschädigung auf gekoppelte Feuchte‐ und Ionentransportprozesse vorherzusagen. Das Modell berücksichtigt die Topologie und räumliche Verteilung der Mikrorisse und deren Einfluss auf die Ionendiffusivität. Die numerische Simulation liefert bei anisotroper Verteilung der Mikrorisse eine erhöhte Alkali‐Eindringtiefe. Degradations in concrete due to cyclic loading and its effects on transport processes with regard to ASR damage According to the goals of the research group 1498, this paper deals with the effects of cyclic flexural loading in a four‐point bending test on the fluid transport processes within a concrete structure. Therefore, the degradation of the microstructure is characterized through ultrasonic wave measurements as well as microscopic crack analysis. In order to numerically model these processes, experiments on the penetration behavior of water into the concrete were carried out. The penetration behavior over time as well as the influence of degradation on the water transport were investigated. To predict the influence of concrete degradation on alkali diffusivity, a multi‐scale continuum micromechanics model is incorporated into the numerical model, which accounts for the topology and the three‐dimensional distribution of microcracks. As expected, the numerical simulation predicts larger alkali‐penetration in pre‐damaged concrete. Regarding the micro‐crack distribution, an anisotropic distribution of micro‐cracks tangential to the direction of the alkali and water flux increases their penetration depth.