The cantilever casting method is a widely used construction method for prestressed concrete continuous beam bridges. Taking the completed pier top segment (usually referred to as the "pier table") as the starting point, through the suspended form traveler, the construction processes, such as formwork erection, concrete pouring, and prestressed reinforcement tensioning are carried out. The bridge is symmetrically closed towards the mid-span on both sides section by section to form the entire bridge.

1. Form Traveler

The forms of form travelers for the cantilever casting construction of prestressed concrete continuous beams are diversified. The structure of the form traveler is becoming lighter; the force distribution is becoming more reasonable; the construction is becoming more convenient; and its application is becoming more widespread.

2. Support and Formwork

For the construction of prestressed concrete continuous box girder bridges by cantilever casting, a method combining cantilever casting with the form traveler and cast-in-place construction of the beam end segments on the support is commonly adopted. The assembly and disassembly of the support must strictly adhere to the construction specifications and design requirements, and the construction operations should be conducted with safety in accordance with the support design drawings. The following points should be taken into consideration during the assembly and disassembly of the support.

(1) When part of the formwork and support needs to be removed in advance, the impact of load redistribution on support must be considered.

(2) During the closure of the side span, a large frictional resistance is generated between the support and the bottom of the beam. It is required to ensure that the support can slide relatively to the bottom of the beam before the concrete is poured. The purpose is to enable the cast-in-place segment to freely expand and contract with the cantilever casting segment during the closure of the side span, avoiding excessive tensile stress in the concrete. Especially when the support has high rigidity and cannot deform together with the beam body, a large tensile force will be applied to the concrete of the beam body.

(3) When prestress is applied, if the support does not slide, or the beam cannot slide on the support, or the support restricts the expansion and contraction of the beam due to resistance, it will be impossible to apply the specified tensile force to the beam.

(4) When prestress is applied, attention should also be paid to the rebound height of the support. If the elastic elevation of the support is large, it will not only be difficult to remove the support, but also the beam body will be affected by the upward force generated by the support. Sometimes, measures such as lowering the support should be taken while applying prestress.

The formwork used must ensure accurate dimensions of the concrete components, a smooth surface of the cast shape, easy turnover, and be made of materials that ensure operational safety. The commonly used formwork includes wooden formwork, plywood, and metal formwork.

3. Concrete Pouring

The cantilever casting construction is typically divided into four parts: the pier table is the pier top beam segment (usually 5-10m), the symmetrical cantilever casting segments (which can be divided into several segments, generally 3-5m per segment), the side hole cast-in-place segment on the support (usually 2-3 times the length of the cantilever casting segment), and the closure segment (usually 1-3m).

For the side hole cast-in-place segment on the support, a temporary support platform can be erected beside the pier. A full support or assembled with universal members, Bailey trusses, etc., and segmental pouring is carried out on it. The pouring sequence of the cast-in-place segment is to pour the part close to the side pier first and gradually approach the closure segment, gradually adjust the elevation of each beam segment of the cast-in-place segment to ensure that the elevation difference between the cast-in-place segment at the closure opening and the cantilever end is not too large. The pier table at the pier top has a complex structure, and special attention should be paid to the concrete pouring construction. Whether using a bracket or a temporary support for the cast-in-place concrete of the pier table, due to factors such as the elasticity of the support and bracket, gaps at the connections of the members, and settlement of the foundation, the support and bracket may sink, causing cracks in the concrete beam segment. Therefore, before pouring the concrete, the bracket or support should be pressure-tested. The pressure-testing method can repeatedly use a water tank for multiple pressurizations, or use a jack to tension the anchor cables for pressurization to eliminate the inelastic deformation. Measure the elastic deformation value to provide parameters for the reserved height of the outer formwork and check the safety of the bracket. The concrete pouring of the pier table should be carried out in two or three layers depending on its structural form and height. The construction joint should be left above the bottom plate rib or 20cm below the web support. The concrete of the bottom plate and web should be poured in layers using a tremie tube. After the pier table concrete reaches the specified design strength, the prestressed reinforcement should be tensioned. The general sequence is to tension transversely first, then longitudinally, and finally vertically; the longitudinal cables should be tensioned symmetrically to the center line; tension the web first, then the top plate, from bottom to top, and the long cables first and the short cables later; grout in a timely manner after tensioning to avoid prestress loss caused by the relaxation of reinforcement.

4. Key Points of Closure Segment Construction

(1) Before the closure, the elevation and axis of the top surface of the box girder should be jointly measured, and the changes in the relative elevation of the beam body and the axis offset under the influence of temperature should be continuously observed, as well as the change in the length of the beam body of the closure segment under the influence of temperature. The continuous observation time should be no less than 48 hours, and the observation interval should be determined based on the temperature change and the structure of the beam body, generally once every 3 hours.

(2) The bridge must be closed symmetrically and evenly at the same time. Unnecessary construction loads on the T member must be removed; the construction loads on the T member should be in a relatively balanced state, and the closure should also be carried out symmetrically and synchronously to avoid relative deformation at the end of the closure segment, generating shear stress displacement, and affecting secondary stress and closure accuracy.

(3) The design of the rigid support at the closure opening and the tension of the temporary tendons must be implemented strictly according to the design requirements.

(4) During the closure construction, additional stress during the construction of this segment should not be caused. Therefore, during the pouring process, the closure construction loads at the two cantilever ends need to be adjusted to make their deformations equal, avoiding the generation of vertical stress in the closure segment. The closure construction loads of the cantilever can be adjusted by setting up a water tank and injecting water for adjustment.

(5) The concrete of the closure segment should be one grade higher than that of the beam body, and early strength is required. It is best to use slightly expansive concrete, and a special mix design should be carried out.

(6) The concrete pouring time of the closure segment should be selected during the period with low daily temperature and small temperature change amplitude. It is advisable that the temperature starts to rise after the pouring is completed. Pay attention to the vibration during the concrete pouring and the maintenance after the pouring is completed to prevent the generation of early cracks.

(7) After the concrete of the closure segment reaches the design strength, the temporary consolidation restraint of the support at the other end should be released to complete the system transformation. After releasing the temporary consolidation restraint, pay attention to observing the settlement of permanent support and make records to check the transformation effect.

(8) The tensioning sequence of the continuous prestressed tendons should comply with the design regulations. Generally, tension the top plate first, then the bottom plate, and then the web, tension the long tendons first and the short tendons later, and carry out the tensioning symmetrically.

5. Structural System Transformation and Support Reaction Adjustment

When the prestressed concrete continuous beam is constructed by the cantilever casting method, the structural form of the beam needs to be transformed from a statically determinate structure to a statically indeterminate structure. Due to the unbalanced internal forces generated during the construction process of the beam body and after the continuity, as well as the relatively large influence of the construction load and system transformation on the stress of the beam body, it is sometimes necessary to adjust the support reaction. Taking the Guilin Lijiang Bridge as an example, the temporary supports are set on the bracket, with 4 at each pier, and 2500kN pot rubber supports are used as the balance fulcrums during the cantilever casting. Therefore, the force during the cantilever casting is very large. To prevent uneven force from causing large bending and torsion of the box girder, the support reaction should be adjusted after the tensioning and grouting of the pier table.

6. Prestress Tensioning and Duct Grouting

There are many ways to apply prestress, including one-end tensioning, two-end tensioning, symmetric tensioning, over-tensioning, batch tensioning, staged tensioning, sectional tensioning, and compensation tensioning, etc. There are always problems with the unsatisfactory effect of vertical duct grouting, such as blocked grouting passage, insufficient grouting, and the grouting failing to play the role of bonding and wrapping. Therefore, the grouting process scheme should be improved as much as possible. For example, ensure that the vertical grouting duct is unobstructed and set a pressure-maintaining device at the tensioning end. Weld a 6cm-long steel pipe on the anchor backing plate at both ends to enhance the bearing rigidity of the anchor backing plate, fix the corrugated pipe of the vertical reinforcement in the steel pipe, and seal it with adhesive tape. Weld a small steel pipe beside the fixed-end pipe, insert the grouting plastic pipe, and make the grouting inlet pipe firm. Drill a slanted hole on the upper anchor backing plate to communicate with the inside of the pipe as the grouting outlet, and insert a small plastic pipe to lead it out. The length of the plastic pipe is 80cm as the pressure-maintaining length. After tensioning and anchoring, seal the tensioning end with high-strength concrete.

To Wrap Up

The cantilever casting construction of the continuous beam of the extra-large bridge is a complex engineering system integrating technological innovation and practical experience. Every link, from the selection of the form traveler, the construction of the support to the concrete pouring and system transformation, is closely related. Any slight oversight may have a significant impact on the quality and safety of the bridge. Its unique construction advantages not only provide a reliable solution for crossing complex geographical environments but also demonstrate remarkable value in terms of economy and construction efficiency. However, the current construction technology still faces many challenges, such as the urgent need to break through the difficulties of vertical duct grouting, and the application of new materials and technologies needs further exploration. In the future, with the continuous upgrading of the demand for bridge construction, the cantilever casting construction technology of the continuous beam of the extra-large bridge will surely reach new heights through continuous innovation. Engineering technicians need to continuously summarize construction experience, strengthen technical research, deepen the control of construction details, deeply integrate theory and practice, and ensure that every bridge can become a safe, reliable, and high-quality transportation monument, injecting a continuous driving force into the vigorous development of infrastructure construction.