In road and bridge engineering, cast-in-place construction is currently the main construction technology for box girder structures, which are composed of steel box girders and reinforced concrete box girders. These structures are stable and their quality is controllable. However, the current box girder casting methods face many problems during construction, and accurate understanding and strict control of the construction process are required to truly improve construction quality. In response to this, this article focuses on analyzing the construction techniques for road and bridge projects.  

1. Foundation Treatment  

Before constructing a cast-in-place box girder, foundation treatment is necessary. During actual construction, workers will first construct drainage ditches along the extension direction outside the bridge to optimize the overall site drainage environment, quickly drain accumulated water, and ensure smooth construction. After improving the construction site, the foundation is leveled. Then, the foundation is reinforced with lime and compacted using equipment such as vibratory rollers to enhance its stability and ensure the alignment meets project specifications. In field construction, soft soil conditions are inevitable and can directly affect foundation stability. In such cases, the foundation can be reinforced through backfilling, soil replacement, and reinforcement techniques to achieve long-term stability. The foundation surface is reinforced with C20 concrete casting, and a reasonable cross slope is set according to actual construction conditions to reduce the possibility of water accumulation. Foundation compaction imposes higher requirements on construction: drainage ditches are set in the foundation structure, with special attention to the design of ditches on both sides to prevent water from affecting the foundation. Technicians strictly control and analyze operations according to standards to ensure qualified foundation treatment, addressing existing issues or incompatible elements to create favorable conditions for subsequent girder construction.  

2. Scaffolding Erection Technology  

Scaffolding erection is also a key technology in cast-in-place box girder construction for road and bridge projects, requiring workers to strictly adhere to construction standards. First, the center line of the alignment, the vertical projection line of the box girder wings, and the box girder center line are marked using a total station. The scaffolding is installed symmetrically in horizontal and vertical directions by two teams of workers, with components such as base plates, vertical rods, diagonal braces, horizontal rods, and shear braces installed in sequence. Before concrete casting, the scaffolding must undergo pre-treatment to improve its stability.  

When installing the cast-in-place box girder scaffolding, ensure uniform force distribution by laying base plates at the bottom of each scaffold and positioning the scaffold at the center of the base plate to ensure alignment. Second, carry out scaffolding installation according to engineering design. During scaffold design, in addition to considering project-specific requirements, diagonal braces should be placed at the nodes of the entire framework to ensure the casting girder scaffolding meets construction requirements and the reliability of the entire steel pipe scaffold design. Generally, horizontal and vertical rods are installed from bottom to top. When installing diagonal braces, control components are placed at framework nodes to further stabilize the scaffold structure, while high-quality diagonal braces are connected. During this process, ensure the verticality of the braces meets design requirements to guarantee the stability of the cast-in-place box girder scaffolding, achieve the expected installation effect, and ensure the safety of the road and bridge project. Additionally, foundation settlement can affect the stability of steel pipe structures, so scientific and reasonable girder installation is required, with effective control of vertical height and shear fixing distance. Third, road and bridge construction projects should minimize the use of inelastic rod structures to reduce uneven settlement issues. After installing the longitudinal and transverse beams, sandbags are used to preload the bottom of the box girder. The preloading load and sequence during preloading are similar to the concrete load, starting from the middle and grading toward both sides. The preloading time exceeds 24 hours. After measuring the scaffold deformation, if the deviation exceeds the standard range, the scaffold design must be adjusted.  

3. Scaffold Preloading Test

To ensure the smooth progress of subsequent construction and safe operation, the installed scaffold must undergo a preloading test to determine its durability limit and avoid collapse due to overloading during construction. The preloading test uses sandbags placed on the support beams, with the total reactor weight being 1.2 times the total weight of the box girder. Typically, sandbags are added every 10 minutes. After completing the load increase, the load is maintained for over 24 hours. Once the load is reached, unloading is carried out gradually. After pile load preloading, the formwork camber must be adjusted.

4. Steel Strand Installation

A grinding wheel cutting machine is used to cut steel strands. Before installation, steel bars must be prepared for cutting and kept ready for use during the cutting process. During steel bar storage, spacers must be placed on the ground. Before using steel bars, a protective layer must be applied to their surface. When each bundle of steel strands is tied, numbered, and threaded through the corrugated pipe, the front end should be wrapped with sealing tape to prevent the corrugated pipe from being cut by the steel strands. When inserting into the corrugated pipe, ensure the steel strands are not tensioned to avoid stress loss during the stressing process. When installing steel bars, reserved sections must be positioned to ensure accuracy and strength. If the position of the steel bars conflicts with the anchor position, the steel bar positions should be adjusted accordingly. When welding steel bars, take care to prevent damage to the metal pipes. After steel bar installation, strictly control their quality, and address any issues promptly to avoid safety accidents. The following steps should be noted during girder passing:

（1）Before passing the girder, thoroughly inspect the anchor plate and anchor hole to ensure they are in the correct position.

（2）Ensure the inner hole is smooth and undamaged, the wire bundle is secure, and the ends are free of blockages or rotations.

（3）Check the girder length during construction to ensure it meets the standard requirement, i.e., the tensioned end is 80 cm long.

（4）After installing the steel strands, check the position of the corrugated pipe to ensure accuracy and tightness.

5. Formwork Construction

During design and construction, ensure complete connection between the formwork and the box girder. After formwork installation, subsequent pressure treatment should be carried out effectively; preloading treatment can significantly prevent inelastic deformation during formwork construction. After unloading, ensure the formwork height meets construction requirements and engineering standards. When laying steel beam reinforcement materials, direct dragging of reinforcement on the formwork top is prohibited to avoid severe scratches on the formwork surface. When welding steel bars, special insulating materials must be used between the bars and the formwork to effectively prevent material damage from high-temperature welding. The following points must be noted during formwork installation:

First, comprehensively inspect the quality of formwork materials; formwork parameters must be correct before use in construction. Ensure the formwork surface is smooth, flat, and free of adhesive residues. All formwork used must be clean, and installation intensity must be controlled to avoid affecting the overall formwork quality. This task is typically performed manually or using mechanical equipment, with strict on-site control to meet formwork installation standards.

Second, when installing side modules, ensure they overlap properly with adjacent formwork to avoid leakage. After installing side formwork, fix it securely to prevent deflection during casting. Confirm the position of all formwork components and adjust them reasonably to meet box girder construction standards. Record parameters during formwork setup as important references for subsequent work.

Third, when installing end modules, place flexible pipes in the holes of the end modules. During installation, inspect the quality to ensure compliance with standards and maintain tight connections between components. Additionally, strictly follow construction drawings when installing end modules to position pre-installed components and prevent leakage.

6. Prestressing Construction Technology

To ensure the effective use of cast-in-place box girders in road and bridge projects, in addition to orderly drainage according to construction standards, pre-treatment of the box girders is required after basic construction to overall enhance their load-bearing capacity and ensure project safety and stability. Prestressing treatment of cast-in-place box girders in road and bridge projects can be carried out as follows:

First, carefully inspect the specifications of steel strands according to actual conditions and strengthen inspection efforts. To prevent damage to steel strands, they are typically tied manually. Wire bundle construction should be completed in accordance with design drawings. The length of steel strands should not exceed 2 cm, and the anchor belt length of steel strands should be maintained between 90–100 cm with waterproofing. The pipeline centerline should be designed according to drawings, with a spacing of 50 cm during pipeline deployment.

Second, inspect tensioning equipment to ensure concrete quality during tensioning and adjust tensioning according to pipeline test data. After pressure work is completed, promptly check whether the pressure meets standard requirements. For safety, tensioning of steel strand longitudinal and transverse beams must strictly adhere to construction specifications. At the end of pre-tensioning, longitudinal beams must be deployed from bottom to top. During prestressing, beams must be deployed from top to bottom.

Finally, conduct environmental temperature tests to avoid temperature impacts on the construction process, adjust construction time, and ensure scientific and reasonable operations.

7. Concrete Casting Technology

First, technicians strictly inspect material quality and analyze the performance of concrete materials. If quality issues are found, address them promptly to prevent improper casting, particularly by focusing on removing obviously segregated concrete materials. The casting process is generally divided into two stages: first casting the web and bottom plate, then casting the top plate and wing plates, and finally completing the box girder structure. This casting work requires optimizing construction joints to prevent obvious cracks. Technicians strictly supervise each layer of casting to ensure continuity and prevent long interruptions during concrete casting. Vibration compaction is highly emphasized in concrete work; technicians focus on the casting area to control compaction quality, ensuring full utilization of vibration rods to achieve a smooth, uniform, and bubble-free box girder structure. Additionally, pay attention to the overall coordination of concrete to minimize impacts from steel structures or formwork collisions and ensure effective concrete compaction.

After casting the cast-in-place box girder, construction personnel should strengthen curing work to improve the stability of the concrete structure, prevent damage during the setting process, target the prevention and control of concrete cracks, and enhance construction quality. The key to daily curing of cast-in-place box girder structures lies in regulating temperature and humidity. Close attention must be paid to changes in the girder structure itself and the external environment, with corresponding adjustment measures taken to achieve satisfactory setting effects. To improve curing efficiency, no equipment or personnel are permitted to enter until the cast-in-place box girder reaches the design strength. After reaching the design strength, formwork removal can be carried out in the order of "first installed, last removed." Handle formwork with care to prevent damage to the cast-in-place box girder structure due to improper removal.

To Wrap Up

In summary, the construction technology for cast-in-place box girders in road and bridge projects covers multiple key links, including foundation treatment, scaffolding erection, preloading tests, steel strand installation, formwork construction, prestressing construction, and concrete casting. Controlling the technical points of each link is crucial. Only by strictly following construction specifications, accurately implementing technical requirements for each link, strengthening construction process supervision and management, and performing post-casting curing work can the construction quality of cast-in-place box girders be effectively guaranteed, ensuring the structural stability and safety of road and bridge projects and laying a solid foundation for the sustainable development of transportation. In the future, with continuous technological advancements, cast-in-place box girder construction technology will continue to optimize and upgrade, bringing more innovations and breakthroughs to road and bridge construction.