Investigation on the Effect of Steel Fiber Property on the Torsional Performance of Ultra-high Performance Concrete Rectangular Beams without Steel Reinforcement

Steel fiber is the main reinforcing fiber of Ultra-high Performance Concrete (UHPC) and could significantly improve its tensile strength and toughness. As a result, the steel fiber has a significant effect on the structural behavior of UHPC components under pure torsion. To extensively investigate the effect of steel fiber property on the torsional performance of UHPC rectangular beams, a total of eight UHPC rectangular specimens (including one contrast specimen without steel fibers) were tested under pure torsion until failure. The parameters of the specimens considered for investigation included the volume fraction, type, dimension and hybrid effect of steel fibers. The failure modes, torque-twist curves, torque-strain curves, distribution of cracks of all specimens were acquired based on the torsion tests. Results show that the failure of the contrast specimen was very brittle in nature. In contrast, the failures of Ultra-high Performance Fiber Reinforced Concrete (UHPFRC) specimens were relatively slow with clear signs. The cracking and ultimate torques of UHPFRC specimens were significantly higher than those of the contrast specimen, with maximum increases of 79 % and 159 %, respectively. Increasing the volume fraction of steel fibers could increase the cracking and ultimate torques of UHPFRC specimens, and less diagonal cracks and smaller crack width were recorded. The specimen containing end-hooked steel fibers exhibited better torsional capacity and toughness than the one with straight steel fibers. The specimen adding steel fibers with higher length-to-diameter ratio had denser diagonal cracks, greater ultimate angle of twist and better toughness. The cracking and ultimate torques of the specimens adding hybrid steel fibers were higher than those of the specimens with single type of steel fibers, showing a positive hybrid effect. The type and dimension of the steel fiber had an effect on the post-cracking load-carrying capacity of UHPFRC beams. Specifically, the specimens adding short straight steel fibers (length-to-diameter ratio is 65) reached their ultimate torques quickly, and the ultimate-to-cracking torque ratios were between 1. 07 and 1. 18; the corresponding ratio of the specimen adding long straight steel fibers (length-to-diameter ratio is 100) was 1. 46, while the ratio of the specimen adding end-hooked steel fibers was 1. 34, which is between the specimens with short straight and long straight steel fibers. Finally, formulas were proposed for calculating the cracking and ultimate torques of UHPC rectangular beams. Comparisons between the results obtained from experiment and those from proposed formulas indicate that the proposed formulas have good calculation accuracy. © 2022 Xi'an Highway University. All rights reserved.