Effect of the steel fibre hook geometry on the flexural properties of high strength steel fibre reinforced concretes under static and impact loading

In recent years, steel fibre producers have produced new generation steel fibres with high strength and multiple hooked-end geometry (4D and 5D). The studies on the mechanical properties of steel fibre reinforced concretes containing 4D and 5D fibres under the static loading conditions are very limited and there is no published study on the performance of these new generation fibres under flexural impact loading yet. In this study, the effects of the dosage and the aspect (length/diameter) 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 single-hook-end 3D fibres. The experimental results showed that the flexural strength and the fracture energy of fibre reinforced high strength concrete significantly improved with the increase in the fibre volume, and by using multiple hooks-end steel fibres with sufficiently high tensile strength. For the fibres with aspect ratio of 65, significantly higher flexural strength and fracture energy values have been obtained for 5D hook geometry as compared to conventional 3D fibres, while 4D fibres did not improve these engineering properties probably due to their relatively insufficient fibre strength for this aspect ratio. It has been revealed that the effect of the 4D steel fibre on the mechanical properties of high strength concrete was more pronounced in higher aspect ratio fibres. The increase of fibre aspect ratio for 3D steel fibres negatively affected the fracture energy of concrete. 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.