Enhancing the interfacial bond of steel tube confined ultra-high performance concrete: Synergistic effects of coarse aggregate and expansive agent

Ultra-high performance concrete (UHPC) combined with steel tubes offers enhanced ductility and compressive strength, making it a promising material for advanced structural applications. However, the high autogenous shrinkage of it can compromise its synergy with steel tubes, limiting the overall performance. To address this issue, this study explores the use of calcium oxide-based expansive agents and coarse aggregates to mitigate shrinkage and improve the compatibility of UHPC with steel tubes. A comprehensive approach combining microscopic analysis and macroscopic mechanical testing was employed to systematically evaluate the individual and synergistic effects of these additives on the properties of UHPC. The experimental results demonstrate that the expansive agent significantly optimized the pore structure, leading to a notable reduction in autogenous shrinkage. Furthermore, the inclusion of coarse aggregates contributed to additional shrinkage mitigation. Importantly, the synergistic interaction between the expansive agent and coarse aggregates not only further reduced autogenous shrinkage but also substantially enhanced the interfacial bond strength. Based on the experimental data, a quantitative relationship between interfacial bond strength and core concrete shrinkage was established, providing a predictive model for evaluating the interfacial performance. These findings provide novel insights into the optimization of the interfacial bond in steel tube-confined UHPC systems, offering significant potential for improving structural performance in engineering applications.