1. Overview and Construction Characteristics of Cantilever Casting Method

1.1 Theoretical Overview of Cantilever Casting Method

The essence of the cantilever construction method is to symmetrically extend the cantilever section by section towards both banks, with the pier as the center. The cantilever casting construction method for prestressed concrete continuous beam bridges evolved from the cantilever assembly of steel bridges. During construction using the cantilever casting method, the cantilever form traveler is the most crucial technical equipment. During the segmented cantilever casting of a single "T" of the prestressed concrete continuous box girder, the pier and beam need to be temporarily fixed. During the process of closing to form a continuous beam, the temporary fixation between the pier and beam needs to be removed one by one to achieve the system conversion and form a continuous beam.

1.2 Construction Characteristics of Cantilever Beam Bridges

The cantilever casting technology for prestressed concrete continuous beam bridges has the following characteristics:

(1) Due to the unloading effect of the negative bending moment at the support, the positive bending moment in the mid-span is greatly reduced, and both the dead load and live load have an unloading effect.

(2) In prestressed concrete continuous beam bridges, the pier and beam are separated, and the self-weight of the continuous box girder and the load on the beam are transmitted to the pier through the pier-top bearing.

(3) Since the balanced cantilever carriage, which is the core equipment of the cantilever casting method, has simple structure and low cost, making it easy to control the construction cost during the construction process.

(4) Most of the construction procedures for each segment of the cantilever casting method are carried out inside the balanced cantilever carriage, with little influence from external factors, ensuring the continuity of the construction process.

(5) The cantilever casting method's segmented construction is convenient for designing bridges with high safety performance, lightness, and aesthetics according to the actual situation of the beam bridge during the design process.

(6)Because this construction technology adopts the non-support construction method, the space used during the construction process is relatively small.

2. Key Technologies and Control Measures of Cantilever Casting Method

2.1 Key Technologies

The main construction procedures for cantilever casting method for prestressed concrete continuous beam bridges are as follows:

(1) Pouring of the pier table: Set up the pier table bracket, lay the bottom formwork, preloading, erecting the formwork, binding the reinforcement, laying the prestressed pipeline, pouring the pier table, and temporarily fixing the pier and beam of the continuous box girder, and tensioning the prestressed tendon bundle.

(2) Assemble the balanced cantilever carriage and preload it.

(3) Pour the 1# segment.

(4) Tension the prestressed tendon bundle.

(5) Move the balanced cantilever carriage forward, adjust and anchor it.

(6) Pour the next segment. Including: moving the form traveler forward, setting the top and bottom plate elevations based on the formwork elevation, binding the reinforcement, laying the prestressed pipeline, pouring the concrete, curing, threading the prestressed steel cable, and tensioning the prestressed steel cable.

(7) Complete the cantilever casting in sequence.

(8) Remove the cantilever forming traveler.

(9) Close the side span.

(10) Close the mid-span.

2.2 Control Measures

During the construction of the beam segment concrete by the cantilever casting method, the workers need to thoroughly inspect the center line of the cantilever form traveler, the elevation of the bottom formwork, and the prestressed tendon pipeline reinforcement before pouring the concrete. If the pouring needs to be carried out in batches, the workers need to follow the following procedures: for the second pouring, it is first from the bottom plate to the lower corbel of the web, and then the remaining part. For the third pouring, it is first from the bottom plate to the lower corbel of the web, then from the lower corbel of the web to the upper corbel of the web where the prestressed pipeline is dense, and finally from the upper corbel of the web to the top plate. The concrete pouring should first be carried out from the front section of the balanced form traveler. After the concrete has been poured, the workers should promptly use a through-hole device to check the inside of the pipeline. If there is a blockage, it needs to be solved in a timely manner.

During the closing of the prestressed concrete continuous beam, the key construction points that need to be controlled are: before closing, it is necessary to understand the air temperature and analyze the fluctuation of the air temperature, and do a good job in the preliminary preparation work for the closing work; select the time point with a relatively low and small fluctuation in the daily air temperature to pour the concrete of the closing section; when locking the closing opening, first weld one end of the outer rigid support and the embedded parts at the beam end, and then connect the other end of the outer rigid support and the beam correspondingly. It 's crucial to ensure rapid welding. The concrete at the closing opening should preferably be one grade higher than the beam body during construction. When the concrete reaches the corresponding strength, partially tension the prestressed steel cable, then remove the stiffening skeleton, and then tension the remaining prestressed tendon bundles according to the previous design requirements.

For the construction of the cast-in-place beam end of the support, the corresponding work can be carried out through the following procedures: (1) Set up temporary pile foundations; (2) Pour the cap; (3) Widen the cap and the embedded parts set; (4) Assemble the support; (5) Build a section steel platform; (6) Conduct loading pressure test; (7) Install the cast-in-place bottom formwork and side formwork; (8) Measure the elevation of the bottom plate; (9) Bind the bottom formwork reinforcement and vertical prestressed tendon; (10) Install the longitudinal prestressed pipeline of the bottom plate; (11) Install the end formwork and web formwork; (12) Conduct acceptance work; (13) Pour the concrete of the bottom plate and web; (14) Conduct curing; (15) Install the inner top formwork and bind the bottom reinforcement of the top plate; (16) Install the longitudinal and transverse prestressed pipelines; (17) Bind the top reinforcement of the top plate; (18) Conduct acceptance again; (19) Pour the concrete of the top plate; (20) Conduct curing again; (21) Tension the vertical prestressed tendon and the transverse prestressed tendon of the top plate; (22) Move the outer side formwork; (23) Finally, conduct the formwork removal work.

Conclusion

With the continuous development of the construction technology of prestressed concrete continuous beam bridges, its application range is further expanding. It is believed that in future development, the continuous improvement of technology will make the advantages of this technology more and more obvious and be more widely applied in bridge construction.