Experimental study on shear performances of ultra-high performance concrete deep beams

This study presents an experimental study on the shear behaviors of fourteen ultra-high performance concrete (UHPC) deep beams. The effects of different parameters, including shear span to effective depth ratio λ, concrete strength s, steel fiber volume fraction (UHPC strength) Vf, longitudinal reinforcement ratio ρs, horizontal and vertical web reinforcement ratio (ρsh and ρsv), were considered. The other two deep beams of normal strength concrete (NSC) and high strength concrete (HSC) were also tested for comparison. Experiment results indicate that the shear strength of the UHPC specimens increased with the concrete strength, longitudinal reinforcement ratio, stirrup ratio, and horizontal web reinforcement ratio, and decreased with increasing ratio of shear span to effective depth. The UHPC specimens are ideally reinforced, while NSC and HSC specimens are over-reinforced, therefore, the strength of longitudinal reinforcement is easier to be fully utilized in the former than that in the latter. Both “beam action” and “arch action” coexisted in the UHPC deep beams. Before the applied load reached 50%∼70% of the ultimate load, the UHPC deep beams were dominated by the “beam action”; after that, they were dominated by the “arch action”. All deep beam specimens failed in shear, 13 UHPC specimens failed in shear compression, one UHPC with a lower shear span to effective depth ratio (λ = 0.554) as well as NSC and HSC specimens failed in diagonal compression, which indicated that the failure modes were affected by the ratio of shear span to effective depth λ and the concrete strength s. For the shear strength of the UHPC deep beams, 0.739 is the threshold value of λ. When λ is greater than it, the shear strength decreases with the increase of λ, while when λ is less than that, the shear strength approximately remains constant.