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Highlights:Graphical/Tabular Abstract  Effect of the different hook geometries on the flexural behaviour of high strength concrete  Static and impact loading flexural strength of steel fibre reinforced concrete  Dynamic increase factor of steel fibre reinforced concreteIn recent years, steel fibre producers have proposed new generation steel fibres with high strength and multiple hooked-end geometry (4D and 5D) to the construction market. In this study, the effects of the dosage and the aspect ratio of 4D and 5D fibres on the flexural strength and fracture energy of high strength concrete under static and impact flexural loading conditions have been investigated comparatively with the conventional singlehook-end 3D fibres. Figure A. Typical load-deflection curves for concrete with 5D 65/60 steel fibre under static and impact loading conditions Figure B. Flexural strength DIF values Purpose: To compare the effects of hook-end geometry of fibres on the flexural strength and fracture energy of steel fibre reinforced high strength concrete under static and impact flexural loading conditions.Theory and Methods: Notched-prismatic samples (100×100×600 mm) were used in determining the flexural strength and fracture energy of fibre-reinforced high strength concrete under quasi-static and low-velocity impact loading conditions. Quasi-static flexural tests were performed by using a closed loop deflection-controlled testing machine. Low-velocity impact loading on simply supported notched beams was provided by an instrumented drop-weight test machine allowing the free-fall of a hammer from 2 m height onto the mid-span of the beams. The flexural load-deflection curves of the beams under impact loads were obtained by using a data acquisition system that simultaneously collecting the reaction forces with two piezoelectric load cells attached to the supports and the mid-span deflection of the beam with a noncontact laser displacement sensor. Results:The flexural strength and the fracture energy of fibre reinforced high strength concrete significantly improved with the increase in the fibre volume and using multiple hooks-end steel fibres with sufficiently high tensile strength. As compared to static loading, 1.4 to 2.6 times higher flexural strength and 1.2 to 3.0 times higher fracture energy values were obtained under impact loading. The best performance under flexural impact loading has been obtained from steel fibre reinforced high strength concretes with 5D fibres having aspect ratio of 65 and 4D fibres having aspect ratio of 80. Conclusion:The new generation steel fibres with high strength and multiple hooked-end geometry (4D and 5D) have a significant positive effect on flexural behaviour of fibre reinforced concrete under static and impact loads.On the other hand, increasing the fibre aspect ratio and the number of fibre hook ends without increasing the tensile strength of the fibre does not contribute significantly to the mechanical properties of high strength fibre concrete. Similar flexural behaviour with single-hook...