TwelveUHPCbeams with different combinations of steel fibers, longitudinal and transverse reinforcement were investigated experimentally under pure torsion. Steel fibers having 17mm length and 0.15mm diameter with volumetric ratios of 0.5%and 0.9%were used. The average compressive strength of the reference UHPC standard cylinderswas about 205MPa.The UHPC beams with steel fibers only showed a ductile behavior. It was found that the addition of either longitudinal or transverse reinforcement to the steel-fiber UHPC beams added much to the ductility and very little to the ultimate capacity. The steel fibers having a volumetric ratio of 0.9 % together with the longitudinal or transverse reinforcement accomplished an effective load carrying mechanism after cracking. Only the case of both longitudinal and transverse reinforcement in addition to steel fibers provided a very high increase both in ultimate torsion capacity and ductility. An analytical model based on the well known ”thin-walled tube” analogy was developed. The comparison between the experimental and analytical results shows very good agreement.
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