Construction technology of precast box girder
Cleaning the bottom formwork, setting out the construction → binding the bottom and web reinforcement → installing the bottom web prestressed pipe → installing the side formwork → installing the internal formwork → binding the roof reinforcement → installing the roof prestressed pipe → fixing the anchorage → installing the end formwork → pouring the beam concrete → curing the beam → tensioning grouting → moving the beam → sealing the anchor and plugging the beam end concrete.
1, construction preparation
After the concrete of beam-making pedestal reaches a certain strength, steel plates shall be laid, and the box girder can be prefabricated only after the supervision engineer has passed the inspection. Prepare reinforcement, formwork, anchorage, steel strand, concrete (concrete for commercial mixing station), etc.
2, clean the bottom die, construction lofting
Clean the bottom die, the surface of the bottom die has no concrete residue, the line shape is smooth and the surface is flat. Measure the length of the bottom beam, daub the release agent, which must be uniform, and there must be no oil pollution to the steel bar. Soft plastic pipes with a diameter of not less than 3cm should be installed on the contact surfaces on both sides of the bottom die and the side die. The plastic pipe should be parallel to the bottom die, and the joint should be smooth to prevent slurry leakage.
3, steel binding and prestressed channel reserved
The protective layer of steel bar adopts special concrete cushion block, which is arranged in plum blossom shape.
Post-tensioned prestressed channel adopts plastic corrugated pipe pore-forming. When arranging corrugated pipe, its longitudinal and transverse curve parameters must meet the design requirements. The joint of corrugated pipe shall be screwed with larger corrugated pipe, and the Mao Kou must be polished smooth before screwing the inner mouth of the joint to ensure smooth threading. The outer opening of bellows joint must be wrapped with adhesive tape to prevent slurry leakage. When the corrugated pipe is buried, attention should be paid to the accuracy of the tension end position, and it is required to be naturally connected with the anchor plate horn tube, with a firm interface and no slurry leakage. When the corrugated pipes are embedded, they shall be fixed with H-shaped positioning steel bars, one for straight lines every 50 cm and one for curves every 30 cm. The coordinates of prestressed corrugated pipe must be constructed in strict accordance with the design requirements of the drawings.
4, template installation
(1) template composition
The external formwork adopts customized large steel formwork. The side formwork is composed of a plurality of unit formwork fans, and the unit formwork fans are composed of panels, horizontal ribs, vertical ribs supporting the panels, brackets, top pull rods, bottom pull rods, corners for fixing the formwork fans, and vibration brackets installed on the side formwork.
The core die adopts custom assembled movable steel die. In order to facilitate the disassembly and assembly of the formwork in the box girder cavity, the core formwork adopts the combination of skeleton and panel. The skeleton is connected with the panel by bolts, and the panel of the mandrel is divided into four pieces (one on both sides of the web and one on the top chamfer) on the whole section, and the template inside the web is connected with the template on the top chamfer by fixed skeleton.
(2) Template installation
Clean the laitance on the surface of the side formwork and polish it with a wire brush, including the cleaning of the end formwork of the side panel and flange plate. Template installation seam is smooth, tight, error-free, the length, width and height in the mold meet the requirements of design drawings and construction specifications, and the screws are complete, tight and stable. Between the side form and the bottom form, and between the side form and the side form, the joints are not tight to ensure that the formwork joints do not leak slurry.
Template installation and reinforcement installation into one. The box beam mandrel is supported after the bottom and web reinforcement are bound and formed. The side formwork of box girder adopts the form of wrapped wall bottom, which is formed by binding the bottom and web reinforcement. After the prestressed (N 1 ~ N4) holes are arranged, the mandrel is installed in place. After the template is in place, adjust the edge and verticality of the template, and tighten the bolts between the templates to make the top edge of the template straight.
5, concrete pouring
Box girder concrete is mixed by commercial mixing station, transported by concrete truck and poured by 25-ton crane. Before pouring, the supports, templates, steel bars and embedded parts shall be inspected and recorded.
(1) Before prestressed concrete pouring is ready for concrete pouring, the position of the reserved channel should be accurately checked, and the end formwork should be close to the side formwork and bottom formwork and perpendicular to the axis of the channel. The fixed position of the conduit shall be marked with the coordinate position, and the anchor plate shall be numbered, so that the steel strands can be arranged accordingly. At the same time, the position of embedded parts in the beam should be accurate, especially the anchor pad should be close to the end formwork, and translation or rotation is not allowed. Grouting holes should be provided with slurry leakage prevention measures, and air holes should be installed to check whether slurry leaks. Finally, check whether the number of prestressed channels, fixed spacing and joints are sealed, and check whether the prestressed pipes are straight.
(2) Prestressed concrete pouring sequence Concrete pouring shall be carried out in layers according to a certain thickness, order and direction, and the upper pouring shall be completed before the initial setting of the lower concrete. When the upper and lower layers are poured at the same time, the pouring distance between the upper and lower layers should be kept above1.5m. When pouring concrete on the inclined plane, it should be expanded and raised from the lower part step by step, and horizontal stratification should be maintained. The concrete pouring sequence requires that the bottom plate and the web plate are poured together, and finally the top plate, from one end of the beam to the other end, is poured in layers. (3) Prestressed concrete pouring
1) Formwork, steel bar, prestressed pipe, anchorage and prestressed steel bar can only be poured after being inspected and approved by the supervision engineer.
2) When pouring concrete, prestressed pipes and anchor pads should be kept stable.
3) After the concrete starts pouring, the pouring process should be continuous.
4) Box girder concrete shall be vibrated with plug-in vibrating rod, with plug-in vibrating as the main method and attached vibrating as the auxiliary method.
5) When pouring in layers, insert 5 ~ 10 cm of lower concrete. After each vibration, slowly push the vibrator while vibrating. When vibrating, it is forbidden to touch the template, corrugated pipe and anchor pad with vibrating rod.
6) Concrete pouring shall be carried out continuously. In the process of pouring, designated persons should check the stability of supports, templates, steel bars and embedded parts. If looseness, deformation or displacement is found, it should be handled in time.
7) In the process of concrete pouring, specimens with strength of 7d and 28d shall be cured under the same conditions according to the requirements of specifications, so as to facilitate construction control.
8) In the anchorage zone at the end of box beam, in order to ensure the compactness of concrete, internal vibrator should be used to strengthen vibration, and the aggregate particle size should not exceed half of the clear distance between two steel bars or embedded parts.
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