Precast box girder features short construction cycle, mature technology and controllable cost, making it one of the most widely adopted superstructure forms for bridges. Its construction quality directly determines the safety, durability and service life of the bridge. This article introduces the construction technology of box girders.

1. Beam Yard Preparation

After the site of the beam yard is selected, the ground shall be leveled and compacted. Reinforcement layouts for girder-making pedestals, beam storage pedestals and gantry crane pedestals shall be determined through mechanical calculation. The girder-making pedestal bears the load of formwork and beam body and serves as the starting point of precision control. Deviations in pedestal elevation and camber will be amplified in the finished beam alignment. Therefore, the matching accuracy between pedestals and box girder steel formwork is the primary acceptance criterion for the first girder. Concrete is centrally supplied by the on-site mixing plant. Water pipes shall be pre-embedded on pedestals to connect automatic sprinkler systems, which maintain proper humidity during concrete pouring and curing to prevent dry cracking.

2. Reinforcement and Corrugated Pipe Installation

Reinforcing bars shall be cut, bent and tied into reinforcement meshes, main bars and stirrups in accordance with design drawings. The specification, quantity, position and concrete cover of reinforcement shall be checked item by item. Corrugated pipes shall be positioned as per design coordinates and fixed with iron wires or special clamps to prevent displacement and floating during concrete pouring. Misaligned ducts will cause abnormal tension friction and insufficient grout filling, which are hidden quality risks. The position and quantity of corrugated pipes shall be rechecked before concrete pouring.

3. Formwork Installation

The inner formwork of precast box girder consists of multiple modules, which shall be assembled in the specified sequence with tight and uniform joints. After assembly, the section dimensions of bottom slab and top slab shall be rechecked. Rust and laitance at joints shall be removed by angle grinders. Foam sealant shall be applied at joints and excess sealant scraped off to avoid voids caused by mortar leakage. Before installing the inner formwork, the concrete cover of bottom slab and side parts shall be inspected. Ventilation holes shall be reserved with sand-filled PVC pipes, and positioning rebars shall be placed. The outer formwork shall be adjusted with a spirit level to ensure design height. The tooth plates on the top of outer formwork shall be straight and fixed by flat clamps without covering the tooth gaps; otherwise, rebars for wet joints cannot be installed properly. Adjustable blocks shall be symmetrically installed according to beam length for uniform stress distribution. The lower edge of outer formwork shall closely fit rubber blocks on the side of pedestals to prevent grout leakage. When installing end formwork, rebars against the formwork can be slightly cut if necessary, but excessive cutting is strictly prohibited. Insufficient concrete cover or missing end rebars will lead to stress concentration after tensioning, which frequently causes cracking at beam ends.

4. Concrete Pouring

Before pouring, confirm that top slab reinforcement and embedded parts are complete and formwork supports are stable. Concrete shall be poured in the sequence of bottom slab → web slab → top slab, adopting longitudinal sectional pouring and horizontal layered pouring to reduce layered voids induced by excessive slump. 26 high-frequency vibrators are installed on each side of formwork, at heights of 0.5 m and 1.1 m above ground. They work in groups of six alternately for about 30 seconds each time to avoid over-vibration of formwork. For densely reinforced webs, 30 mm diameter immersion vibrators are used; 50 mm vibrators are adopted for beam ends and top surfaces. Both under-vibration and over-vibration shall be avoided, and vibration duration and spacing shall follow construction cards. After initial setting, the top surface shall be roughened. Water sprinkling curing shall be carried out after the concrete temperature drops. Exposed rebars shall be coated with cement paste for rust prevention after formwork stripping.

5. Roughening Treatment

After rebars for wet joints are installed, 8 cm wide non-roughening grout stop strips shall be attached to the inner side of tooth plates. Manual roughening is not required after side formwork stripping, greatly reducing labor cost for large-scale construction. For end diaphragms and intermediate diaphragms, vertical chalk lines shall be marked 3 cm inward from the edge as control lines for roughening. Manual roughening shall be performed after concrete hardening to guarantee bonding strength between old and new concrete and prevent interface delamination.

6. Tensioning

Intelligent numerical control tensioning equipment is adopted to precisely control tension force, duration and elongation. Steel strands shall be threaded straight without twisting. Tensioning is double-controlled by oil gauge reading and strand elongation, both of which shall fall within design ranges. After anchoring with clip-type anchors, the load shall be held for 5 minutes to stabilize prestress and prevent sudden tension loss.

7. Grouting

Duct grouting shall be completed within 48 hours after tensioning to form reliable bonding between steel strands and concrete. Grouting too early or too late will impair performance. Intelligent grouting machines are used to prepare grout. The grouting pressure shall be stabilized at 0.57～0.64 MPa. The interval from grout mixing to pumping into ducts shall not exceed 40 minutes. Grout shall fill the duct from one end to the other. The grout discharged from vent holes shall have the specified fluidity to ensure no voids inside ducts.

8. End Sealing

Prefabricated blanking plates shall be inserted into beam ends first. Temporary anchor mortar shall be chipped off, then end sealing rebars shall be tied to improve bearing capacity. After formwork closing, concrete is poured for end sealing to achieve tight integration between the sealing part and beam body.

9. Lifting and Beam Storage

Beams can only be moved from girder-making pedestals to storage areas after the concrete reaches the specified strength and age. Flexible cushion materials shall be placed between steel wire ropes and beam corners / wet joint areas to avoid beam damage. When lifted to 1 m height, pre-embedded vent holes at the beam bottom shall be dredged to prevent abnormal pressure buildup. The running condition of gantry crane wheels on rails shall be monitored during beam transfer to avoid beam swinging. The stacking layer of precast beams shall not exceed 2 layers. Sleepers shall be placed under wedge blocks during beam landing to ensure uniform bearing.

Conclusion

From pedestals and inner & outer formwork to vibration curing, tensioning and grouting, every procedure of precast box girder construction relies on precision and practical experience. Shandong Boyoun Heavy Industry specializes in bridge steel formwork, supplying complete sets of equipment including inner, outer and end formwork for precast box girders, girder-making pedestals, beam storage pedestals, rhombus form travelers, segmental box girder formwork, T-beam formwork, pier column formwork and tunnel lining trolleys. With standardized design, customized formwork matching and on-site technical service, we help beam yards improve construction efficiency and guarantee quality.

Stay tuned for our next article: Key Control Points of Precast Box Girder Construction Quality for Bridges.