Traditional concrete bridge decks often incorporate steel mesh to ensure connection and prevent cracking. However, the cracking in the connecting layer, low bond strength, misalignment of steel mesh, and settling at the bottom often appear. In this study, fiber-reinforced concrete was used for the bridge deck overlay, and a horseshoe-shaped shear key was employed to connect it with the beam body, forming a robust composite bridge deck system. By optimizing the concrete composition and interface bonding methods within the system, a comprehensive investigation was conducted into the compressive and splitting tensile strengths of different composite systems. The findings showed that the horseshoe-shaped shear key enhances the splitting tensile strength of the composite structural system while maintaining its compressive strength, ensuring a certain level of structural integrity during failure. As the strength grade of the steel fiber-reinforced concrete in the deck overlay increases, the compressive and splitting tensile strengths of the composite system initially rise and then stabilize, with C40 being the optimal strength grade for the deck overlay concrete. Furthermore, the overall performance of the deck overlay concrete with steel fibers is superior to that with the POM and PP fibers. The application of the YJ-302 interface bonding agent at the connection between the deck overlay and the beam body concrete further enhances the mechanical properties of the composite system.

This study developed a high-performance composite mortar with a low water-to-binder (W/B) ratio to improve the mechanical strength and volumetric stability of preplaced aggregate concrete-filled steel tubes (PACFST). Silica fume was incorporated to optimize the interfacial transition zone (ITZ) between the matrix and coarse aggregates. The effects of the sand-to-binder (S/B) ratio, water-to-binder (W/B) ratio, and expansive agent content on the flowability, compressive strength, and volume stability of the composite mortar were systematically analyzed. Experimental tests were conducted using vibration-free molded specimens, and the influence of different S/B ratios (0.8–1.4), W/B ratios (0.26–0.32), and expansive agent dosages (0–8%) on mortar properties was evaluated. The results indicate that an optimal S/B ratio of 1.2 significantly enhances flowability and strength, whereas further increases offer limited improvement. Reducing the W/B ratio enhances strength, with a decrease from 0.32 to 0.28 leading to a 23.4% increase in 28-day compressive strength. Additionally, a 6% expansive agent dosage reduces 90-day shrinkage by 13.1% while maintaining high compressive strength. The optimized PAC achieved a 28-day compressive strength of 115.9 MPa, with an 11.6% increase in 7-day strength and a 51.2% reduction in 90-day shrinkage compared to conventional C100 concrete. These findings provide theoretical guidance for designing high-strength, low-shrinkage PAC, offering insights for bridge, tunnel, and high-rise building applications.