Effect of Fiber Dosages and Stirrup Ratios on Torsional Strength of Ultrahigh-Performance Concrete Beams

Ultrahigh-performance concrete (UHPC) is a type of concrete that has gained attention from researchers for its potential use in various structural applications to improve the behavior of concrete structures. Despite this interest, there are limited experimental test results available on how the ratio of stirrups and the dosage of steel fibers affect the torsional behavior of UHPC beams. This study presents an experimental investigation and theoretical prediction of torque capacity (UHPC) of beams with different ratios of stirrups and dosages of steel fibers under pure torsion. One normal-strength concrete (NSC) beam and 10 UHPC beams with $150\times 200\text{-}\mathrm{mm}$ cross sections were tested. The experimental parameters were the dosages of steel fibers (1% and 2%) and the spacings of the stirrups of 0.0, 50,100, 150, and 200 mm. Including the UHPC high tensile strength and volume fraction of steel fibers, an expression for cracking and ultimate torques of UHPC was proposed and verified. Results showed that the UHPC beams have brittle failure modes compared to NSC beams. Results demonstrated that the UHPC beams showed higher initial cracking and ultimate torsional moments than the NSC beam. The additional steel fibers enhance the torsional properties, and this improvement was linearly at the effective stirrup’s ratio higher than 0.25%. The 1% or higher steel fiber dosage was confirmed to be enough to substitute the missing strength of the stirrups. Results of previous research and the suggested cracking and ultimate torque equations of UHPC agreed with the experimental results.