Svenja Höper

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.

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.

Ultra‐high performance concrete with a compressive strength of more than 140 N/mm2 reacts very brittle without the addition of fibres. At designing economical and resource‐efficient building components as well as performance‐optimized materials, the supporting effects of the additional microfibres must be considered. Prospectively, cyclic loading and the fatigue behaviour of high performance materials and slim components will come to the fore. Therefore, the influence of cyclic deterioration on the load bearing capacity of the interface transition zone connecting microfibres and the surrounding concrete surface must be investigated. As part of the DFG Priority Programme 2020, the damage mechanisms and processes regarding the deterioration of UHPFRC subjected to cyclic tensile loading are explored in an Experimental‐Virtual Lab. Using the concepts of elasto‐plasticity and continuum damage mechanics, the material models for the components of the composite material UHPFRC are improved and enhanced to be combined on a mesoscale with the help of a non‐linear bond model. This bond model is discretized by interface elements with curved surfaces in 3D space and describes the interaction of fibres and concrete in the bond zone in terms of adhesion and friction effects. For validation and verification, single and multiple fibre pullout tests are simulated in 3D‐finite element analyses. These FE‐simulations and preliminary findings are supposed to provide the essential basis for future numerical degradation prediction.