Conceptual Design of a Novel UHPC-NC Hybrid Continuous Box-Girder System with Controllable Long-Term Deflection

Long-span prestressed concrete continuous bridges are prone to excessive deflection at the main span during long-term operation, which often interacts with cracking issues. To address this challenge, this paper proposes a novel and practical ultrahigh performance concrete (UHPC) and normal concrete (NC) hybrid continuous box-girder system by synergizing the advantages of UHPC and NC. This system employs a UHPC-NC composite structure at the upper portion of the pier, with the remainder of the structure consisting of prefabricated UHPC girders. Specifically, the new system utilizes a precast UHPC strip, known for its low shrinkage and creep coefficients, as a formwork for casting NC. The precast UHPC strip can effectively restrain the shrinkage and creep of NC, thus inducing the strain redistribution within UHPC-NC composite structure. This mechanism produces a beneficial sectional rotation, which is instrumental in managing the long-term deflection at the main span. This study focuses on the long-term behavior induced by creep and shrinkage. First, long-term tests on one NC beam and two UHPC-NC composite beams validated the core mechanism and feasibility of the system. Second, a theoretical model based on the age-adjusted effective modulus method was developed, and mainstream shrinkage/creep prediction models were evaluated to support finite-element (FE) analysis. Finally, taking the 235-m main-span Buliuhe Bridge as a case study, FE analyses and economic evaluations were performed for both pure UHPC and UHPC-NC schemes. The results demonstrate that the UHPC-NC scheme achieves the minimal long-term midspan deflection (outperforming both the original and pure UHPC schemes) and exhibits favorable economic potential over the full service life.