## 9 Conclusions and Outlook  
The experimental investigations were carried out within the framework of the sub-project \"Damage processes in ultra-high-performance fibre-reinforced concrete under cyclic tensile loading\" with the DFG SPP 2020. The contribution describes a novel approach to select a fibre design for UHPFRC-components considering effects of bending stresses in fibres and cyclic loading. The experimental background has been described in detail by [19], including the consideration of single fibre pullout tests and pullout tests on fibre groups using high-strength micro steel fibres in UHPFRC. The tests were performed under quasi-static and cyclic loading with varying parameters like embedded length and orientation. The stress state during a straight quasi-static fibre pullout is more or less clear. However, inclined fibres show an additional bending stress that occurs at the exit point of the fibre. The additional bending stress can be calculated by the formulae according to [20; 21]. Nevertheless, fibres did not rupture during the test execution under quasi-static loading, showing that the fibre design is applicable for UHPFRC. Under certain circumstances (cyclic loading and orientation), fibres embedded in UHPF could also rupture. A local overstraining of the fatigue strength of the fibre material causes the fibre rupture. Fibre rupture in UHPFRC-components could lead to a sudden failure, because the load from the ruptured fibre has to been borne by another fibre. The other fibres are then overstrained again and might also rupture leading to a so-called zipping effect. Based on the results of the experimental investigations, extensive theoretical considerations were carried out with regard to better methods for fibre design for UHPCR. The main findings can be summarized as follows:* The current fibre design is mainly based on empirical experiences and suitability tests. The effects from a fibre orientation and cyclic loading are not considered appropriately. * The chosen high-strength micro steel fibres were always pulled out of the UHPC under quasi-static loading. The respective degree of utilization was a maximum of 0.38. On average, the degree of utilization was approximately 0.27. Both values rest upon a measured tensile strength of approximately 3576 N/mm\({}^{2}\). However, the utilisation rate could rise to approximately 0.50, if simplifying respectively without additional testing of the manufacturer's data (2000 N/mm\({}^{2}\)). * At inclined fibres, additional bending stress occurred due to the deflection at the exit point, which must be added to the measured tensile stress in the fibre. The amount of bending tensile stress depends on the fibre orientation. The degree of utilization increased under quasi-static loading to approximately 0.50. If, again, the utilisation rate is related to the manufacturer's specifications, it rises from 0.50 to approximately 1.00. The fibre would then be almost completely utilized. This means that the chosen fibre would have a feasible fibre design under quasi-static loading tailored to the concrete strength and the manufacturer's specifications. * Under cyclic tensile loading, partial fibre rupture occurred, because the actual load amplitude in the fibre is locally higher due to the additional bending stress resulting from the deflection and reduces the fatigue resistance of inclined oriented fibres. It was not possible to give a quantitative indication, how the fibre is exploited under cyclic tensile loading. The fatigue resistance of the fibre's material and the fatigue bond behaviour are two opposing effects. Probably the above statement on the feasible fibre design, chosen in the own investigations and well-behaving under quasi-static loading, is no longer valid for cyclic loading. The test results show that, under quasi-static loading, a deterministic fibre design using utilization rates is possible.