Flexural Behavior and Design of Prestressed Ultrahigh-Performance Concrete Beams: Failure Mode and Ductility

Ultra-high performance concrete (UHPC) is well-known for its ultra-high compressive strength and sustained postcracking tensile ductility, making it an attractive choice for the construction of modern structures. However, prestressed UHPC members often fail quickly after crack localization accompanied by reinforcement rupture, which shows limited failure warnings (i.e., relatively small ductility, nearly invisible cracking, and negligible compressive damage). In contrast, UHPC members can also be designed to allow the gradual strain hardening of reinforcement to compensate for the load loss due to crack localization, and the final failure is attributed to the gradual crushing of UHPC prior to reinforcement rupture. Failure after gradual strain hardening is desirable since it brings warning signs through high ductility, visible cracks, and controlled spalling. To achieve a resilient structural design of UHPC members, this study aims to develop design methods to avoid early failure after crack localization and promote failure after gradual strain hardening. This study first establishes a three-dimensional finite-element analysis (FEA) model to simulate the flexural behavior of prestressed UHPC beams, which is validated against seven existing experimental beams. Then, parametric analyses are conducted to assess the influence of five key aspects, including the steel rebar-to-prestressing strand ratio, the postyield hardening of mild steel rebars, the pretensioned stress of prestressing strands, the tensile behavior of UHPC, and the web width to bottom flange ratio. Additionally, the authors propose a design method to predict the failure mode of prestressed UHPC beams, which can be used to guide the design of reinforcement configuration and promote ductile structural behavior. In particular, a threshold reinforcing ratio for mild-steel rebar is proposed to design for failure after gradual strain hardening.