Bridge prefabricated cantilever assembly construction includes processes such as segment transportation, assembly, and closure. It has similar advantages to cantilever casting construction. However, the cantilever pile assembly process uses cranes to assemble prefabricated beam segments one by one. Therefore, the cantilever assembly method also has its unique advantages: the prefabrication of beam bodies can be carried out simultaneously with the construction of bridge substructures, and parallel operations shorten the bridge construction period; the concrete curing age of prefabricated beams is longer than that of cantilever casting; the prefabrication of beam segments in prefabrication yards or factories is conducive to quality control of the overall construction.

Assembly methods

The cantilever beams of prefabricated segments can be assembled using different splicing methods according to on-site and equipment conditions. When the bridge is close to land or is a land bridge and the bridge span is not high, self-propelled cranes or gantry cranes can be used for assembly. For bridge openings in rivers, floating cranes can also be used for assembly. If the piers are very high or the water flow in the water is very fast and construction on land or water is not feasible, high-altitude construction can also be carried out with the help of various cranes. Commonly used assembly methods include floating crane assembly method, cantilever crane assembly method, continuous member assembly method, cable crane assembly method, and mobile guide beam assembly method. Among them, the floating crane assembly method and the cantilever crane assembly method are the most commonly used.

Assembly construction

(1) Joint treatment forms There are three forms of beam segment splicing: full-section hinged, partial hinged and partial wet joint, and wet joint. Different joint forms are often used in different construction stages and different construction parts. The full cross-sections of two beam segments are bonded by epoxy resin to form a full-section hinged joint and are designed as shear hinges. In partial hinged and partial wet joint, the webs are hinged, and the top slab and bottom slab are connected by protruding reinforcement and then cast with concrete. In wet joint, a 10cm to 20cm concrete section is poured between adjacent beam segments as a joint. It is used to subsequently adjust the position of the reference beam segment so as to more accurately control the installation accuracy of subsequent beam segments. Both full-section hinged joints and partial hinged joints have their own advantages in engineering. Compared with partial cross-section hinged joints, full-section hinged joints have the advantages of fast cantilever assembly speed, fewer construction procedures, no need for special cement, and easy grouting of ducts. However, partial cross-section hinged joints have the advantages of less epoxy resin usage, easy construction adjustment, and good bridge integrity.

(2) Measures to improve joint quality In order to compensate for the shrinkage and early strength of concrete at wet joints, early-strength micro-expansion concrete should be preferentially used for pouring in engineering. Generally, in an ambient temperature of around 20°C, the design strength can be reached in 24 hours. Before the full-section hinged or partial hinged assembly construction, multi-functional epoxy resin should be applied on the contact surface first to ensure the water tightness of the connection surface and use the thickness of the epoxy resin layer for leveling. In order to enable the prefabricated segments to be installed accurately and quickly during assembly, positioners (also known as positioning pins) are evenly arranged on the top slab. Some positioners can not only play a role in fixing the position but also bear shear force. This kind of positioning device is called anti-shear wedge or anti-slip wedge. During segment prefabrication, in addition to paying attention to embedding the positioner device, attention should also be paid to embedding the duct former and lifting point device (lifting ring or vertical prestressed reinforcement) at the correct positions. At the same time, in order to improve the shear resistance of the splicing surface of the two segments, the splicing surface should be made into a toothed shape, that is, shear teeth. In order to improve the fit degree of the splicing surface of the beam segments, the beam segments to be poured and the spliced surface of the poured beam segments should generally be closely poured on the long-line pedestal. The middle is separated by epoxy resin without hardener as an isolation layer. The prestressed tendon holes are separated by metal bellows or can also be separated by partition plates. The deviation of the splicing surface should be controlled within the specified range. The template thickness deviation should not exceed 0.5mm, the tooth plate deviation should not exceed 1mm, and the front and rear concave-convex tooth deviation should not exceed 0.5mm.

(3) Temporary consolidation of supports In order to ensure the balance and stability of the segmented assembly construction of continuous beams, usually due to the same cantilever casting method, the T-shaped structure supports are temporarily consolidated. When the temporary consolidated supports cannot meet the requirements of cantilever assembly, generally consider adding temporary supports on both sides or on one side of the pier. After the cantilever assembly is completed and the T-shaped structure is closed, it can be restored to its original state and the supports can be removed.

(4) Assembly process The first segments on both sides of the bridge pier column are generally connected to the beam segment at the upper end of the pier column in the form of wet joint. The first segments on both sides of the pier column are the reference segments of the cantilever beam and are the key to quality control of the entire span installation. During cantilever assembly construction, the key to preventing upward warping and downward deflection lies in the accurate positioning of the first segments on both sides of the pier column. Therefore, various methods must be used during construction to ensure the accurate positioning of this segment. After accurate positioning, the first segment can be supported by suspension of a crane or by a temporary bracket set below. In order to facilitate the splicing of pipeline joints at the joints, the welding of joint reinforcement and the vibration of concrete, the width of the wet joint is generally set to 0.1 to 0.2m. For large-span T-shaped steel structure bridges, due to the very long cantilever, a cast-in-place box girder diaphragm is often set in the middle of the cantilever and a wet joint is also set. In addition to increasing the structural stiffness of the box girder, this wet joint can also adjust the assembly position. During the assembly process, if there is a large upward warping error during assembly and it is difficult to remedy by other methods, a wet joint can be added to adjust. However, it should be noted that the width of the added wet joint must be provided by chiseling the segment surface. After other beam segments are lifted and basically positioned, first insert the temporary prestressed tendons, install the connectors, then start gluing and closing, tension the temporary prestressed tendons so that the compressive stress of the glued joint before curing is not less than 0.3MPa, and then the hook can be released. Subsequently, threading and tensioning construction can be carried out.

(5) Grouting Pipeline grouting is to ensure that the prestressed tendons are not corroded. At present, the commonly used method is to first use high-pressure water to check the smoothness of the pipeline, the tightness of the matching surface, and the end sealing situation, and then carry out formal grouting until the slurry overflows from the outlet. Close the outlet and hold pressure for a few minutes to ensure that the cement slurry fills the pipeline as much as possible. Grouting is carried out after partial end sealing. If the end sealing has not been carried out, the end sealing cement mortar is very prone to shrinkage and cracking, resulting in slurry leakage during grouting, directly affecting the grouting effect; and the cement slurry will shrink in the pipeline, making the grouting quality difficult to control. Therefore, in addition to ensuring the quality of end sealing, if an appropriate amount of micro-expansion agent is added to the cement slurry and an appropriate mix ratio is selected, it can not only make the grouting work proceed smoothly but also make the solidified cement slurry fill the pipeline as much as possible and discharge the water and air in the pipeline as much as possible to avoid corrosion of the tendons.

 Conclusion

 The prefabricated cantilever assembly process of bridges can break down large-span beam components into smaller parts and transform many high-altitude constructions into ground constructions. The factory-based prefabrication mode is conducive to the control of overall construction quality and makes the operation safer. Therefore, in bridge construction, the cantilever assembly method should be preferred. Reasonable arrangement of the construction process and simultaneous construction on multiple work surfaces will help shorten the construction period and accelerate the construction progress.