钢纤维特性对无配筋UHPC矩形梁抗扭性能的影响

钢纤维能明显提升超高性能混凝土(Ultra-high Performance Concrete, UHPC)的抗拉强度与韧性，对UHPC构件的扭转行为有显著影响。为深入研究钢纤维特性对UHPC矩形梁抗扭性能的影响规律，以钢纤维体积掺量、类型、尺寸以及混杂效应等为变化参数，完成了8根UHPC矩形梁(含1根未掺钢纤维的对比梁)的纯扭试验；获得了各试件的纯扭破坏形态、扭矩-扭率曲线、扭矩-应变曲线、裂缝形态等关键数据。结果表明：对比梁为脆性破坏，纤维增强UHPC梁的破坏则是有征兆的；纤维增强UHPC梁的开裂和极限扭矩均明显大于对比梁，最大提升幅度分别达79%和159%；增加钢纤维体积掺量能提高开裂和极限扭矩，且斜裂缝数量更多、宽度更小；掺端钩纤维试件的抗扭承载能力和延性均优于掺圆直纤维试件；钢纤维长径比越大，试件的裂缝分布越密集，极限扭率越大，延性越好；2根混掺纤维试件的开裂和极限扭矩均大于单掺试件，正混杂效应明显；钢纤维类型和尺寸均会影响试件的裂后承载能力，掺长径比65的圆直钢纤维在开裂后迅速达到极限状态，极限与开裂扭矩之比为1.07～1.18，长径比为100时对应的比值为1.46，而掺端钩纤维则为1.34，介于两者之间。最后，提出了UHPC矩形梁开裂和极限扭矩计算公式；并对30根UHPC矩形梁进行了验证，结果表明计算公式精度良好。

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.