Low-cost and ductile prestressed UHPC beams with hybrid reinforcement: Experiments and design methods

Ultra-High Performance Concrete (UHPC) is a modern class of cementitious composite materials. With superior mechanical properties and durability, it has gained increasing structural applications worldwide. The current design of prestressed UHPC beams commonly fails due to the fracture of prestressing strands quickly after the crack localization of UHPC, rather than after the crushing of the cementitious matrix as expected in traditional concrete beams. When failing quickly after crack localization, the high compressive strength of UHPC is not fully utilized and beams exhibit low structural ductility and inadequate safety warnings before failure (i.e., nearly invisible cracking and compressive damage). To address these challenges, this study develops a novel ductile design method for the prestressed UHPC beams by introducing secondary reinforcement (mild steel or fiber-reinforced polymer bars), forming a hybrid reinforcement scheme. Four-point bending tests are conducted on four full-scale beams: two beams represent the current practice in China and the state-of-the-art design in the US, while the other two represent the proposed new design method. Test results demonstrate that the proposed design presents a ductile failure after the formation of multiple localized cracks and can exhibit significant UHPC crushing, providing many failure warnings. Compared with currently common design, the proposed design can improve the peak load and deflection capacity by up to 27 % and 109 %, while reducing the factored cost-to-strength ratio by 28–39 %, respectively. A failure path prediction method is developed and validated for prestressed and non-prestressed UHPC beams with various types of longitudinal reinforcement.