The mountain area is faced with complex conditions such as vertical and horizontal valleys, complex geological topography, many earthquake fault zones, high seismic intensity, fragile ecology, and changeable climate. The traditional concrete bridges have a heavy weight, high consumption of sand and gravel materials, large structural dimensions of steel box composite bridges, high steel consumption, and poor overall horizontal performance of steel plate composite beams, which are not suitable for mountain construction conditions. Therefore, based on the construction environment and application requirements of highway bridges in mountainous areas, the structural scheme of steel box-steel plate composite beam is put forward in this paper, and the design of a 25 m–50 m standard-span steel box-steel plate composite beam is studied. The results show that the steel box-steel plate composite beam structure scheme is adopted, which has excellent mechanical characteristics of steel box girder and simple structure of steel plate girder. It has the mechanical characteristics of “balanced transverse force and excellent longitudinal force”, so it is a reasonable choice for small radius curved beam bridge in a mountainous area, so it is a reasonable choice for small radius curved beam bridge in a mountainous area. The steel box-steel plate composite beam adopts the integrated technology of manufacture, transportation and installation, which can better meet the road transport conditions in the mountain area. The longitudinal assembly of the manufacturing unit as the installation unit can ensure the rapid and high-precision installation of the construction site, and the assembly construction of the bridge in the mountain area has been realized. The technical and economic comparison shows that the mechanical performance of steel box-steel plate composite beam is similar to that of steel box beam and obviously better than that of steel plate girder. The steel consumption and construction period of steel box-steel plate composite beam are similar to that of steel plate girder, and are significantly less than that of the steel box girder. Therefore, the steel box-steel plate composite beam is a good choice for mountain bridge construction.

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.

Beam bridges in mountainous and high-altitude transport corridors are frequently exposed to large diurnal temperature differences, intense solar radiation, low humidity, freeze–thaw action and repeated wetting–drying cycles. These coupled actions can accelerate concrete surface cracking and reduce the durability of prestressed concrete T-girder bridges, but field evidence linking crack morphology, crack depth, concrete cover and short-term environmental response remains limited. This study investigates a representative 40 m in-service prestressed concrete T-girder bridge on the Western Sichuan Plateau through field survey and four-month continuous monitoring. Crack location, length, width, depth and concrete cover thickness were measured, and representative crack-width responses to ambient temperature were analyzed. The results show that web cracks are dominated by reticular and irregular microcracks, bottom cracks are mainly longitudinal intermittent short cracks, and diaphragm cracks are concentrated near reticular zones and local corner discontinuities. The sunny side of the edge girder contained approximately 598 cracks, about 4.8 times the 123 cracks observed on the shaded side. Web and diaphragm crack widths were mainly 0–0.04 mm, while bottom-crack widths of 0–0.10 mm accounted for about 92.7%; most crack lengths were 2–20 cm. During monitoring, newly developed cracks accounted for about 6.0% of all recorded cracks, and only 3 of 721 existing cracks increased by 4–6 cm. Representative crack widths fluctuated by about 0.02 mm under −1 to 24 °C without sustained growth. Cracks wider than 0.20 mm generally exceeded the approximately 40 mm concrete cover. Such penetrating cracks should be prioritized in durability maintenance and long-term monitoring.

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.