Effect of expansive agent on lightweight micro-expansive ultra-high performance concrete: Properties, microstructure, and interfacial bonding in concrete-filled steel tube

Debonding and voids between the steel tube and the core concrete, caused by the shrinkage of ultra-high performance concrete (UHPC), are the primary obstacles limiting the application of UHPC in concrete-filled steel tubes (CFST). To solve the shrinkage problem, this study developed a lightweight micro-expansive UHPC (LEUHPC) using lightweight aggregate (LWA) and expansive agent (EA). The effects of EA content on the mechanical properties, expansion rate, microstructure of LE-UHPC, and the interfacial bonding performance of CFST were systematically studied. The findings reveal that when the EA content is 8 %, LE-UHPC exhibits higher expansion and excellent mechanical properties, with 28d flexural and compressive strengths of 24.5 MPa and 122 MPa, respectively, and an interfacial bonding strength of CFST reaching 2.35 MPa. The internal curing effect of LWA reduces LE-UHPC shrinkage and enhances EA hydration, resulting in beneficial micro-expansion. The synergistic effect of LWA and an appropriate amount of EA can refine the pore structure of LE-UHPC and increase the density of the matrix. The LE-UHPC provided by this study can greatly enhance the interfacial bonding performance of CFST while also providing theoretical guidance for the application of UHPC in CFST.