Flexural Behavior and Design of Ultrahigh-Performance Concrete Beams

Abstract

Beams made of ultrahigh-performance concrete (UHPC), a fiber-reinforced concrete with high compressive strength and tensile strain-hardening characteristics, exhibit flexural behaviors that are different than those traditionally associated with steel-reinforced conventional concrete beams. These behaviors necessitate the development of new predictive design tools that reflect the effect of the material-level properties on the flexural behavior. The research presented in this paper assessed the flexural behavior of UHPC beams through the displacement-controlled testing of a prestressed UHPC bridge girder to failure. The girder was 18.90 m (62 ft) long and contained a total of 26 17.8-mm- (0.7-in.)-diameter steel strands and no mild steel reinforcement. The testing focused on capturing the intermediate and final behaviors, including first cracking, yielding of strands, moment capacity at the development of a single dominant crack, and rupture of strands. Building on observations from this study and prior research by the authors and others, a flexural design framework, founded on the concepts of equilibrium and strain compatibility, is proposed for beams made with UHPC and reinforced with conventional steel reinforcing bars, prestressing strands, or both. The proposed framework includes considerations to avoid the local straining and subsequent hinging of UHPC beams and to address the ductility of flexural members. The framework is verified by comparing the experimental results of flexural tests performed by the authors and others to the analytical predictions, which predominantly relied on input material parameters obtained from independent material tests.