Research on the shear performance and ductility collaborative design method of prefabricated U-shaped UHPC permanent formwork composite beams

Precast concrete non-removable formwork can effectively improve construction efficiency. Ultra-high performance concrete (UHPC) has become an ideal material for lightweight permanent formwork due to its excellent mechanical properties and formability. To study the mechanical behavior and shear mechanism of composite beams composed of precast U-shaped UHPC formwork and post-cast normal concrete (NC) under concentrated shear forces, this paper carried out shear loading tests on six test beams through a combination of model tests, finite element simulations and theoretical analyses, with the thickness of the UHPC formwork and the internal stirrup ratio as the main parameters. The results show that the UHPC permanent formwork can significantly increase the cracking load and ultimate bearing capacity of the composite beam, delay crack propagation and improve the structural ductility. Based on the truss-arch theory, a shear model considering the residual stress of UHPC and the material softening effect was established, which incorporated the shear contributions of the UHPC shell, the core concrete and the stirrups into a unified analysis framework. The ratio of the experimental value to the calculated value of the bearing capacity is 1.03, indicating a good agreement. A ductility control index based on the total shear contribution ratio of the UHPC shell and the stirrups was proposed, and a matching relationship among the formwork thickness ratio, the material strength ratio and the stirrup ratio was established. Verified by a full-scale model of the bent cap of a supporting project, the shear ductility coefficient of the structure is 2.75, and the bearing capacity meets the design requirements. The research results can provide a theoretical basis and method reference for the shear ductility design and engineering application of composite beams with U-shaped UHPC permanent formwork.