Torsional behavior of ultra-high performance concrete (UHPC) rectangular beams without steel reinforcement: Experimental investigation and theoretical analysis

Steel fiber is the main reinforcing fiber of ultra-high performance concrete (UHPC) and could greatly improve its tensile strength and toughness. As a result, steel fiber has a significant influence on the structural behavior of UHPC beams under pure torsion. However, related research in this regard is severely insufficient. To fill in this research gap, this paper presents an extensive investigation to explore the effect of steel fiber property on the torsional performance of UHPC beams without steel reinforcement. A total of 8 UHPC rectangular specimens (including one control beam without steel fibers) were tested under pure torsion until failure. The variables in the specimens included the volume fraction, type, dimension and hybrid effect of steel fibers. The failure modes, torque versus angle of twist curves, torque versus strain curves and distribution of cracks for all specimens were acquired based on the torsion tests. Results show that the addition of steel fibers significantly increased the cracking and ultimate torques of UHPC beams, with maximum increases of 79 % and 159 %, respectively. The volume content, shape and dimension of steel fibers exhibited significant influence on the torsional performance of UHPC beams, including torsional capacity, ductility and cracking pattern. The shape and dimension of steel fibers both affected the post-cracking load-carrying capacity of UHPC beams. The cracking and ultimate torques of the two UHPC beams with hybrid steel fibers were higher than those containing single type of steel fiber, both exhibiting positive hybrid effects. Based on the experimental results of this study, the hybrid use of 1 % end-hooked and 1 % long straight steel fibers is recommended in UHPC beams by comprehensively considering the torsional performance and the cost. Finally, by fully considering the excellent tensile behavior of UHPC, theoretical formulas were proposed for calculating the cracking and ultimate torques of UHPC beams without steel reinforcement based on the Bredt thin-tube theory. Comparison between the experimental and theoretical results on 36 UHPC torsional beams demonstrated the feasibility of the proposed formulas.