In the previous paper Core Process Analysis of Form Traveler Design and Concrete Pouring for Large-Span Segmental Cantilever Box Girder Construction, we systematically illustrated the core design system of form travelers for large-span cantilever box girders, optimization schemes under special working conditions, standardized concrete pouring procedures and key quality control points, and clarified the core technical standards for form traveler equipment design and concrete pouring construction.

Within the complete construction chain of cantilever box girders, prestressed anchorage and tensioning constitute the core procedure to guarantee stable structural stress of bridges, improve bearing capacity and durability of box girders. Meanwhile, standardized control of construction details is the key to avoiding potential safety hazards and eliminating quality defects. Building on the previous paper, this article elaborates the complete construction process of prestressed anchorage and tensioning in form traveler construction, and comprehensively sorts out all core precautions for large-span segmental cantilever box girder construction, providing complete technical basis for standardized on-site construction and refined management.

Complete Tensioning Procedures for Prestressed Anchorage in Form Traveler Construction

1 Preparation for Anchorage System Installation

Inspect anchorages and bearing plates; clean mortar residues off anchor bearing plates and keep grouting holes unobstructed. Check that conical holes of anchorages and tooth grooves of wedges are free of damage, and ensure tight contact between anchorages and bearing plates. Pre-tighten fine-rolled rebar; pre-tighten each temporary consolidation rebar one by one to eliminate installation gaps (operated manually or with small jacks). Connect rebar segments with couplers; the steel plate of the reaction frame shall have a thickness of no less than 3 cm to guarantee rigidity.

2 Prestressed Tensioning Process Flow

Concrete strength reaches ≥90% of the design value → Thread prestressing strands → Install working anchorages and wedges → Install limit plates and jacks → Graded symmetric tensioning → Load holding and anchoring

(1) Strand threading: Straighten each prestressing strand individually, and bind the strands every 1.5 m to prevent tangling; fit protective sleeves on strand ends for protection.

(2) Installation of tensioning equipment: Calibrate jacks, oil pumps and pressure gauges as matched sets to ensure consistency between tension force and gauge readings. Install working anchorage, limit plate, jack and tool anchorage in axial alignment; apply lubricant on tool wedges.

(3) Graded tensioning control: Tensioning sequence: Initial stress (10%σcon) → Double initial stress → Controlled stress (hold load for 5 min before anchoring). Adopt dual control principle: Take pressure gauge readings as the primary standard, with allowable strand elongation deviation ≤+6% and elongation difference between two ends ≤5%.

(4) Symmetric tensioning sequence:

   - Longitudinal tendons: Tension middle tendons first, then side tendons; synchronously tension left and right web tendons.

   - Vertical tendons: Prioritize web tendons over top slab tendons.

3 Duct Grouting and Anchor Sealing

(1) Duct grouting: Complete vacuum-assisted grouting within 48 hours after tensioning; maintain pressure at 0.5–0.6 MPa for 3 min once the grout consistency stabilizes.

(2) End sealing treatment: Chisel the anchor recess to expose 75% fresh concrete surface, and coat with polyurethane waterproof coating; bind end sealing reinforcement cages before pouring C50 shrinkage-compensating concrete.

4 Key Points for Special Working Conditions

(1) Closure segment tensioning: Construct during low-temperature periods, synchronously unload counterweight water tanks, and adopt micro-expansion concrete.

(2) Safety control: Keep personnel away from the rear of jacks during tensioning; use special steel pipes to tap wedges, and avoid heavy blows that may cause damage.

 Full Set of Precautions for Large-Span Segmental Cantilever Box Girder Construction

1 Safety Control for Form Traveler Construction

(1) Form traveler design and acceptance: The self-weight of the form traveler shall be controlled at 0.3~0.5 times the weight of cast-in-place concrete per segment, and shall not exceed 0.7 times under special circumstances. Prior to site entry, complete acceptance of factory certificates, trial assembly records and load test reports; conduct focused inspection on main truss welds and anchorage systems.

(2) Preloading requirements: Conduct graded preloading (using sandbags or water tanks) up to 1.2 times the construction load, measure elastic deformation values and eliminate inelastic deformation; prohibit personnel from staying under the form traveler during the whole preloading process.

(3) General safety specifications for operation: When moving the form traveler, the anti-overturning safety factor shall be ≥2, the anchorage system safety factor shall be ≥2, and the maximum deformation shall not exceed 20 mm. Forbid moving the form traveler under severe weather such as thunderstorms and strong winds. Strictly control offset during all working stages including positioning, preloading, concrete pouring and form traveler relocation to ensure construction safety.

2 Quality Control of Concrete Pouring

(1) Precision control of formwork and reinforcement: Recheck elevation and plane position after formwork installation, with allowable deviations ≤3 mm and ≤5 mm respectively; paste double-sided adhesive tape on formwork joints to prevent mortar leakage. The positioning deviation of prestressing ducts shall not exceed 5 mm; set fixing supports every 1 m for straight ducts and every 0.5 m for curved ducts, and conduct duct penetration inspection before pouring concrete.

(2) Key pouring technical points: Control concrete slump within 160–200 mm, with concrete placement temperature ranging from 5°C to 30°C; forbid adding water to concrete on site. Adopt layered symmetric pouring with single layer thickness ≤40 cm, and control the concrete height difference between left and right webs within 30 cm.

3 Critical Matters for Prestressed Construction

Full attention shall be paid to the sequence and symmetry of tensioning. For tensioning of prestressing tendons in segmental box girders, follow the priority order: longitudinal tendons first, then transverse tendons; long tendons first, then short tendons; web tendons first, followed by top slab tendons and bottom slab tendons, and conduct tensioning symmetrically to guarantee structural stability and safety.

(1) Protection of ducts and prestressing strands: Seal corrugated pipe joints to prevent mortar leakage; wrap and reinforce damaged duct sections. Cover prestressing strands during electric welding to avoid damage from welding slag.

(2) Tensioning conditions and operation: Conduct tensioning only when concrete strength, elastic modulus and curing age meet design requirements, and calibrate all tensioning equipment before use. Implement tensioning strictly in accordance with the designed sequence, and complete duct grouting within 48 hours after tensioning.

4 Control of Key Concrete Parameters to Reduce Non-Structural Cracks

Strictly control three critical links for form traveler box girder pouring: mud content of concrete aggregates, concrete curing measures and concrete slump. These technical measures serve as important means to reduce non-structural cracks of box girders constructed with form travelers.

5 Refined Site Management with Two "One Table, One File" Systems Implemented in Construction Practice

 Adopt two standardized management tools to improve management efficiency and refine construction operations for construction teams:

(1) Create a concise mnemonic table for layout and quantity of prestressing tendons in segmental box girders, enabling construction teams to implement tensioning sequences accurately.

(2) Establish separate filing tables for different types of prestressing tendons in segmental box girders, classified according to dual control requirements of strand elongation and tension force, to facilitate precise execution by construction teams.

6 Key Construction Points for Closure Segments

(1) Counterweight and temperature control: Apply balanced counterweights at cantilever ends before closure construction. Arrange concrete pouring during low-temperature periods (night or early morning) to reduce temperature-induced stress.

(2) Closure precision requirements: The elevation difference of the closure gap shall not exceed 15 mm, and the axis deviation shall not exceed 10 mm; deploy total stations for real-time monitoring throughout construction.

Conclusion

Form traveler construction technology serves as the core support for segmental cantilever box girder construction. The rationality of form traveler design, standardization of operation procedures and precision of process control directly determine the quality, safety and economy of bridge projects.

Cantilever box girder construction using form travelers constitutes a systematic project with highly integrated links including equipment design, concrete pouring technology, prestressed construction and detailed process control. If scientific form traveler design and standardized concrete pouring lay the foundation for engineering quality, precise and standardized prestressed tensioning technology as well as all-round control of construction details act as the core key to ensuring bridge structural safety, extending engineering service life and eliminating common quality defects.

Shandong Boyoun Heavy Industry has accumulated years of experience in R&D of bridge steel structure equipment and on-site technical service, with deep research on optimization of form traveler cantilever construction technology. We can provide customized form traveler equipment, complete technical guidance and construction solutions for all types of large-span bridge projects, supporting standardized and high-quality project delivery.