Ultra-high performance fiber-reinforced concrete (UHPFRC) is usually applied for thin-walled and lean constructions because of its excellent mechanical properties. However, these lightweight constructions such as bridge girders, masts, towers, and railway tracks, for example, are susceptible to oscillations due to cyclic loading. A model describing the fatigue behavior of UHPFRC is required. Therefore, the material behavior of all mixture components and the composite itself, each subjected to cyclic tensile loading, is studied experimentally and numerically on the mesoscale at iBMB, Division of Concrete Construction and Institute of Structural Analysis both at TU Braunschweig. In this paper, first findings concerning bond processes of single and multiple micro steel fibers embedded in ultra-high performance concrete as well as a new geometrically and physically nonlinear bond model are presented. K E Y W O R D Sbond stress-slip behavior, damage processes, micro steel fiber, nonlinear bond model, ultra-high performance concrete

Zusammenfassung Zur Erfassung des Trag- und Verformungsverhaltens von ultrahochfestem Stahlfaserbeton (UHPFRC) unter zyklischer Zugbeanspruchung wurde das Ermüdungsverhalten der einzelnen Komponenten („nackte“ Stahlfaser, faserfreier UHPC, einbetonierte Stahlfasern) untersucht. Dazu wurden zyklische Zugversuche an der „nackten“ Mikrostahldrahtfaser sowie am faserfreien UHPC mit variierenden Schwingbreiten durchgeführt und Wöhlerdiagramme aufgestellt. Durch verschiedenartige Pullout-Versuche mit unterschiedlichen Anordnungen von Mikrostahldrahtfasern im UHPC konnten wesentliche Erkenntnisse zum Auszugverhalten unter monoton steigender und zyklischer Belastung gewonnen werden.

Bei der Bemessung von Stahlfaserbeton legt der Planer auf Grundlage der DAfStb‐Richtlinie „Stahlfaserbeton” die Leistungsklassen des Stahlfaserbetons (Nachrissbiegezugfestigkeit L1 und L2) als grundlegende Eigenschaften fest. In der Praxis stellt es sich oft als schwierig heraus, den Zusammenhang zwischen Leistungsklassen und erforderlichem Stahlfasergehalt in Abhängigkeit von der Stahlfaserart und der Betongüte herzustellen. Um sowohl für die Hersteller als auch für Planende und Überwachende eine Abschätzung der voraussichtlich notwendigen Stahlfasergehalte zu ermöglichen, existieren verschiedene empirische Ansätze zur näherungsweisen Bestimmung der Nachrissbiegezugfestigkeit von Stahlfaserbeton. Deren Herleitung erfolgte jedoch auf Basis unterschiedlicher Versuche und fast ausschließlich auf Grundlage von wenigen bzw. „eigenen” Versuchsergebnissen der jeweiligen Forschungsstellen, weshalb nur eine beschränkte Gültigkeit des jeweils vorgeschlagenen Näherungsansatzes vorliegt. Mithilfe der Biegebalken‐Datenbank „Stahlfaserbeton” wurden diese Ansätze systematisch analysiert und auf Basis dessen ein verbesserter Ansatz zur Bestimmung der Nachrissbiegezugfestigkeit von Stahlfaserbeton entwickelt, worüber im Beitrag berichtet wird.

The objective of the contribution is to understand the fatigue bond behaviour of brass-coated high-strength micro steel fibres embedded in ultra-high performance concrete (UHPC). The study contains experimental pullout tests with variating parameters like load amplitude, fibre orientation, and fibre-embedded length. The test results show that fibres are generally pulled out of the concrete under monotonic loading and rupture partly under cyclic tensile loading. The maximum tensile stress per fibre is approximately 1176 N/mm2, which is approximately one third of the fibre tensile strength (3576 N/mm2). The load-displacement curves under monotonic loading were transformed into a bond stress-slip relationship, which includes the effect of fibre orientation. The highest bond stress occurs for an orientation of 30° by approximately 10 N/mm2. Under cyclic loading, no rupture occurs for fibres with an orientation of 90° within 100,000 load changes. Established S/N-curves of 30°- and 45°-inclined fibres do not show fatigue resistance of more than 1,000,000 load cycles for each tested load amplitude. For the simulation of fibre pullout tests with three-dimensional FEM, a model was developed that describes the local debonding between micro steel fibre and the UHPC-matrix and captures the elastic and inelastic stress-deformation behaviour of the interface using plasticity theory and a damage formulation. The model for the bond zone includes transverse pressure-independent composite mechanisms, such as adhesion and micro-interlocking and transverse pressure-induced static and sliding friction. This allows one to represent the interaction of the coupled structures with the bond zone. The progressive cracking in the contact zone and associated effects on the fibre load-bearing capacity are the decisive factors concerning the failure of the bond zone. With the developed model, it is possible to make detailed statements regarding the stress-deformation state along the fibre length. The fatigue process of the fibre-matrix bond with respect to cyclic loading is presented and analysed in the paper.

The paper presents a method to establish a performance-based fibre design of high-strength micro steel fibres for ultra-high-performance concrete (UHPC). The performance-based fibre design considers effects of fibre layout, fibre orientation, and type of loading (quasi-static and cyclic) and expands the current approach using experiences and suitability testing results. The performance-based fibre design is based on a so-called utilization rate, which is determined via pullout tests of high-strength micro steel fibres in UHPC under quasi-static as well as high cyclic loading with varying orientations and embedment depths. The utilization rate for a straight fibre pullout is 0.27 on average considering the measured tensile strength of the fibre and 0.50 considering the manufacturers specifications. For inclined fibres, additional bending stresses occur at the exit point of the fibre channels, leading to a significant increase in local tensile stress. Therefore, the utilization rate of inclined fibres under quasi-static loading is approximately 60–70% higher than in the case of straight embedded fibres (comparing it to the measured tensile strength). Comparing the utilization rate to the manufacturer’s specification, it increases to approximately 1.00. Under cyclic loading, the additional bending stresses in inclined fibres result in a local increase of the load amplitude, leading to a reduced fatigue resistance and premature fibre rupture, underlining the feasibility of a performance-based fibre design.