Long-term creep behavior of expansive agent core concrete in full-scale concrete-filled steel tube from the world's largest span arch bridge study

The Tian'e Longtan Bridge, currently under construction and boasting the world's largest span arch at 600 m, employs a concrete-filled steel tube (CFST) as its primary structural component, forming the stiffness skeleton upon which an outer reinforced concrete arch ring is constructed. As the internal defects of CFST, once encased by outer concrete, cannot be remedied, it becomes imperative to prevent long-term debonding of the core concrete. In recent years, expansion agents have been extensively utilized in CFST arch bridge engineering to compensate for early autogenous and thermal shrinkage. However, a comprehensive comprehension of the creep behavior of core concrete expansion under extended steel tube confinement remains elusive. To address this concern, the radial expansion process of the core concrete can be segmented into two stages: debonding and restriction. We derive a long-term deformation model for the radial expansion of core concrete during the restriction stage, based on elastic mechanics and linear creep mechanics. We represent the expansion process of these two stages uniformly using piecewise functions. Subsequently, in conjunction with the ongoing construction of the Tian'e Longtan Bridge, we measure the core concrete's behavior in full-size CFST specimens (Phi 0.92 m x 12 m x 0.01 m) both with and without expansion agents for up to 114 d. This validates the practicality of the creep model and enables us to determine its relevant parameters. Our results reveal that a debonding gap of 0.142 m occurred before the initial setting of the core concrete. The core concrete underwent a radial expansion of 290.1 x 10-6, with 158.3 x 10-6 being used to address early debonding, and the remaining 131.8 x 10-6 generating self-stress on the steel tube. The creep model indicates that radial creep of the core concrete persisted for approximately six months under the hoop limitation of the steel tube, resulting in a residual expansion deformation of 26.4 x 10-6 and a residual self-stress of 0.119 MPa. Additionally, axial deformation results of CFST without expansion agents demonstrated a decreasing constraining force of the steel tube on the core concrete from the outer to the inner sections, attributable to local core concrete yield. Conversely, the inclusion of an expansion agent altered the stress state of the core concrete, maintaining consistent constraining forces within the same section. As a result, we derive an axial long-term expansion model for the core concrete under steel tube restriction. Finally, the introduction of laboratory specimen deformations enhances the practicality of our model, with results demonstrating strong alignment between measured data and the model. The experimental findings and theoretical models developed provide critical support for quantifying the expansion behavior of CaO and MgO-based compound expansive agents in the vault of CFST arch bridges.