Punching Tests of Double-Hooked-End Fiber Reinforced Concrete Slabs

Ten (10) high-strength concrete slabs reinforced with new type of steel fibers, the double hooked-end steel fibers, were tested under punching shear loads. The strength of the concrete fc varied from 80 to 100 MPa. The fiber content Vf varied from zero to 1.2%. Two different values of flexural reinforcement ratios ρ (= As/bd) of 0.9% and 1.4% were chosen for this test program. The experimental results showed that the use of double hooked-end steel fibers in the concrete enhances slab performance significantly in many ways. As the fiber volume or fiber content Vf increased, the flexural stiffness of the slab throughout loading history also increased, while both the deflections and crack widths decreased considerably. At the ultimate load stage, the punching shear strength increased by up to 156% compared to non-fibrous concrete slabs. The increase in punching shear strength is significantly higher than the increase introduced by conventional single hooked-end steel fibers. The ductility of the slabs was also significantly improved.
Comparisons between design methods with experimental results show that the design method by Concrete Societys TR-34 performs very well. Another method that was based on the yield line theory overestimates the strengths of the slabs. Model Code 2010 method also overestimates the punching shear strengths. Finally, some relevant design recommendations are given.