In modern bridge construction, both cast-in-place box girder and precast box girder are widely used. In a previous article, we explored the key construction technologies for cast-in-place box girder. In this article, we will focus on precast concrete box girder. With its significant advantages in construction efficiency and quality control, precast box girder has become the preferred structural form for many bridge projects. The construction process involves several complex and crucial technical aspects, each of which directly impacts the overall quality and safety of the bridge. Let's explore the key technologies involved in precast box girder construction.

1. Rebar Binding and Corrugated Pipe Installation

First, fabricate the steel bar installation jig required for steel bar installation, carefully check the positions of the steel bars on the jig according to the construction drawings, and rationally and scientifically formulate the binding sequence, generally from top to bottom and from inside to outside. When installing the corrugated duct joints, use corrugated ducts of the same type but one size larger in specification, and tightly bind them with wide adhesive tape to prevent grout leakage at the joints. Ensure that the corrugated ducts are connected straight without any curvature, and take good fixing measures to prevent rotation and displacement during pouring.

The welded or tied rebar joints should be placed in areas with lower internal forces and arranged staggered. For tied joints, the joint distance should not be less than 1.3 times the lap length. For welded joints, within the joint length section, a single rebar should not have more than one joint. The rebar's joint cross-sectional area should not exceed 50% of the total cross-sectional area in the tension zone, and there are no restrictions in the compression zone; for tied joints, the cross-sectional area should not exceed 25% in the tension zone and 50% in the compression zone.

2. Formwork Fabrication and Installation

Modular steel molds are used for the outer form, and all processing is concentrated in the factory. The end mold should be the same as the end of the box girder. The internal molds use disassemblable steel plate forms, and the bottom plates are made with a notch for easy assembly and disassembly. After the bottom plate concrete is poured, the bottom piece should be installed, followed by pouring the web concrete. To prevent the inner mold from floating during pouring, a 16mm steel beam should be installed every 2 meters, and a pull ring should be pre-embedded in the concrete site, with both ends of the lifting bar connected to the pull ring for fixation.

 3. Concrete Pouring

The pouring process should follow the principle of "horizontal layering, vertical segmentation," pouring in the sequence of the bottom plate, web, and top plate strictly. The pouring process is as follows: once the bottom plate concrete reaches about 6 meters, its compaction should be tested by a designated person. If the compaction does not meet the standard, vibration should be carried out inside the core mold. The pouring on both sides of the web should follow a symmetrical approach in terms of material feeding and vibration. Pouring should be done in three layers and the hopper should be controlled by specialized personnel, moving slowly and ensuring that the pouring thickness is 30cm. A pouring segment should be 1.5 to 2 meters long. When the continuous pouring exceeds 1 meter, a type 30 vibrator should be used, inserted vertically at 30 cm intervals, ensuring that "quick insertion, slow withdrawal" is maintained and the concrete reaches the bottom of the corrugated pipe and is compacted.

(1) The surface of the formwork and platform should be uniformly coated with a release agent, but care should be taken not to contaminate the steel strands or other rebar.

(2) The pouring process should start at the end of the box girder and gradually progress to the other end, with measures in place to prevent segregation of the concrete.

(3) The pouring process should proceed symmetrically to prevent displacement or floating of the internal mold.

(4) When using vibrators, the distance should not exceed 1.5 times the vibrator's effective radius and the vibrator should maintain a 5-10 cm distance from the side template, inserting into the concrete 5-10 cm below the surface.

(5) The surface of the box girder should be roughened transversely to facilitate the bond between the deck concrete and the beam.

(6) During pouring, designated personnel should monitor the stability of the formwork, rebar, and embedded parts, addressing any movement, deformation, or misalignment immediately.

(7) The outer formwork should only be removed once the concrete reaches a strength of 2.5 MPa or higher.

4. Prestressing Construction

(1) Tensioning: Tensioning can be performed using an intelligent control system once the beam body has reached design strength and the curing period exceeds 10 days. Before tensioning, adequate equipment such as limited plates, computers, and sunshades should be prepared, and a thorough inspection should be conducted according to the system checklist. The construction personnel should receive detailed instructions on how to use the equipment. After the installation of the tensioning system, the sequence and components should be checked for any issues, and the theoretical elongation values should be accurately calculated before proceeding with the tensioning operation.

(2) Intelligent Grouting: The grouting pipeline should be placed close to the grouting vehicle, ensuring that the grouting and return pipes are as short as possible. The optimal distance between the control panel and the grouting vehicle is 5-50 meters.

To Wrap Up

The key technologies involved in pre-stressed box girder construction, such as rebar binding and corrugated pipe installation, formwork fabrication and installation, concrete pouring, and prestressing, are interconnected and indispensable. The strict control and scientific application of these technologies are crucial for ensuring the quality of pre-stressed box girder, which in turn guarantees the safety and stability of the bridge project. As construction technology continues to evolve and innovate, we believe that pre-stressed box girder construction techniques will continue to improve and play a more significant role in future infrastructure projects, contributing to the creation of more efficient and durable transportation networks.

If you have unique insights or experiences regarding these key technologies, feel free to share with us. Let's work together to advance bridge construction technology.