1. Foundation treatment

According to the requirements of box girder section size and bracket type for foundation, the foundation treatment method is determined. If the support span is large and the requirements for foundation are high, the treatment form of foundation will be strengthened, otherwise it will be relatively weakened. Foundation treatment forms include ① foundation replacement and compaction; (2) concrete strip foundation; ③ Pile foundation and concrete beam, etc.

When the foundation is treated, the drainage of the foundation should be done well to prevent the influence of rain or dripping water on the foundation during concrete pouring and maintenance.

2. Bracket

The supports used in concrete box girder construction mainly include: ① assembling with universal bars; (2) The combined system of strong pillars, trusses and I-beams; ③ Bailey assembly; ④ Lightweight portal support; ⑤ Other steel-wood mixed structures. It is necessary to determine which support to use according to the existing materials and equipment and engineering characteristics. At present, light portal supports are widely used.

The layout of the support is determined by calculation according to the size of the beam section to ensure that the strength, stiffness and stability meet the requirements. In addition to the concrete weight of beam, the weight of formwork and support, construction load (personnel, materials, machinery, etc. ), the calculation should also consider the wind force acting on the formwork and support, as well as other possible loads (such as snow load to ensure the load of facilities).

The support should be pre-pressed according to the requirements of technical specifications, and the deformation data of the support and foundation should be collected as the basis for setting the pre-camber. When setting the pre-camber, the influence of tension on the camber should be considered. Pre-camber is generally set according to quadratic parabola.

Support unloading equipment can choose to use wooden wedge, sand cylinder, jack, U-shaped jacking, etc. According to the support form, the unloading equipment should pay special attention to having sufficient strength.

Step 3: Template

The formwork consists of bottom formwork, side formwork and internal formwork. Generally, components are made in advance and then assembled when they are used. Template is given priority to with steel template, tooth plate, plug or corner using wooden template.

The rafters of the formwork shall be square steel, channel steel or square wood, and the spacing shall be about 75cm. The specific layout should be determined according to the box girder section size, and the strength and stiffness of the formwork should be checked by calculation.

Formwork should be firmly supported, and the wing plate or roof should be supported by frame wood. For a box girder with one-time concrete pouring, the internal formwork frame is supported by precast blocks arranged on the bottom formwork, and the concrete strength of the precast blocks is the same as that of the beam. For the web template, a pull rod should be set according to the height of the web, and the pull rod should use a plastic sleeve to facilitate the pull rod to be taken out, and the pull rod should not be cut off by gas cutting. Box girder concrete is fair-faced concrete, so attention should be paid to the appearance of concrete, and all kinds of joints should be tight and not leak slurry. If necessary, the joints should be encrypted. Clean engine oil, soapy water or other reliable quality release agents should be used as concrete release agent, and waste engine oil should not be used.

According to the needs of construction, manholes are set on the top plate of box girder and the clapboard of formwork, so as to dismantle the internal formwork. Due to the installation of box girder bottom and side formwork, there are many processes such as reinforcement, prestressed reinforcement and internal formwork, and the operation time is relatively long. Often when pouring concrete, there are many sundries in the formwork, which need to be cleaned by air compressor, and a movable plate can be arranged at the adaptive position of the bottom die to clean it.

4. Ordinary reinforcement and prestressed reinforcement

After the installation and adjustment of bottom formwork and side formwork, the binding of ordinary steel bars at the bottom and web and the pre-embedding of prestressed pipes are started. When the concrete is poured once, the internal formwork shall be installed after the bottom and web reinforcement and prestressed pipeline are completed, and then the roof reinforcement and prestressed pipeline shall be bound. When the concrete is poured for the second time, the concrete is poured for the first time after the bottom, web reinforcement and prestressed pipeline are completed. After the final setting of concrete, support the internal formwork roof, bind the roof reinforcement and prestressed pipeline, and carry out the secondary pouring of concrete.

Ordinary steel bars and prestressed steel bars shall be inspected according to the specification requirements and submitted to the engineer for approval. Laying shall be carried out in strict accordance with the requirements of design drawings. For web members, they should be welded into steel skeleton in advance according to their lifting capacity, and then tied or welded after lifting. The binding and welding of steel bars shall meet the requirements of technical specifications.

Prestressed pipes are made of galvanized steel strip, and the position of prestressed pipes is accurately arranged according to the design requirements, and fixed with positioning ribs every 50 cm. The joint should be smooth, wrapped with adhesive tape, and vent holes should be set at the high point of the pipeline.

Before installing the anchor pad, check whether the geometric size of the anchor pad meets the design requirements, and the anchor pad should be firmly installed on the template. To make the pad and the channel strictly centered, and perpendicular to the end of the channel, and shall not be misplaced. The spiral ribs and reinforcing ribs under the anchor shall be set in strict accordance with the drawings, and the mouthwash shall be connected smoothly and sealed with corrugated pipes. Grouting holes on anchor pads should be properly plugged to prevent slurry leakage and plugging holes when pouring concrete.

The blanking length of prestressed reinforcement should be determined by calculation. The length of hole curve, anchor fixture length, jack length and exposed working length should be considered in the calculation. The cutting of prestressed tendons shall be carried out with a grinding wheel saw. When weaving prestressed tendons, they should be combed straight and tied firmly to prevent intertwining. After being tied up, they should be numbered and listed in a unified way, and they should be sorted and stacked neatly for use.

Before the prestressed reinforcement passes through the beam, the channel should be cleaned. When the steel beam is short, it can be sent manually from one end. If the steel beam is long, the metal mesh method can be used. First, use the lead wire reserved in the tunnel to pull the wire rope of the traction net into the tunnel, and then use manual or slow winch to slowly introduce the steel beam.

5. Concrete pouring

Before box girder construction, concrete mix design and various material tests should be carried out and reported to the engineer for approval. According to the actual situation, it is determined that box girder concrete should be poured once, twice or three times.

Box girder concrete has a large volume, so it is advisable to use mixing station to mix concrete, or use multiple mixers to form a mixing unit for mixing. Concrete tank car transportation, concrete pump truck pumping into the mold. Before concrete pouring, the mixing station, pump truck and other equipment must be carefully overhauled to ensure that the machine is in good condition. If necessary, emergency equipment shall be provided to prevent the concrete pouring process from being interrupted due to equipment obstacles.

When pouring concrete, the pouring sequence should be arranged, and the pouring speed should ensure that the lower concrete covers the upper concrete before initial setting. In general, in order to prevent cracks in the concrete of pier top beam due to uneven settlement of pier and bearing, continuous pouring should be carried out from mid-span to piers on both sides. When concrete is poured in stages, the first concrete on the contact surface should be roughened, and the laitance should be removed during the second concrete pouring.

The concrete shall be vibrated by an internal vibrator, and the moving distance of the vibrator shall not exceed 1.5 times of its action radius, and the concrete shall be inserted into the lower layer by 5- 10cm. For each vibrating part, it must be vibrated until the concrete in this part is dense, and excessive vibration is not allowed. When vibrating, avoid the vibrating rod colliding with the formwork and steel bar, especially the corrugated pipe, and do not use the vibrator to transport concrete. Special attention should be paid to the concrete under vibrating anchor and prestressed pipeline to ensure the compactness of concrete. Due to the dense steel bars and small gaps, vibrators should usually be small in diameter.

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Before tensioning operation, the jack and oil pump must be calibrated and checked regularly. When there are multiple tensioning devices, they should be used in groups, and the devices with different group numbers should not be mixed.

When the concrete strength of the beam reaches the tensile strength specified in the design (the specimen cured under the same conditions as the beam), it can be tensioned.

The prestressed tensioning of box girder adopts double control, that is, it mainly controls the tensioning force and checks the actual elongation of steel beam. The error between the measured elongation value and the theoretical elongation value shall not exceed the specification requirements, otherwise, the tensioning shall be stopped, and the reasons shall be analyzed. After the reasons are found out and adjusted, the tensioning shall be continued. The theoretical elongation of post-tensioned prestressed tendons is δL = PL/AyEg, and P is the average tension of prestressed tendons. When tensioning prestressed tendons, the initial stress should be adjusted first, and then the elongation value should be measured by tensioning. The actual elongation value consists of two parts, one is the measured elongation value from the initial stress to the tension control stress, and the other is the calculated elongation value at the initial stress. The actual elongation value is the sum of the two.

The tensioning procedure shall be carried out according to the requirements of technical specifications, generally as follows:

Hold the load for 5 minutes.

0——& gt； Initial stress->; 105σk——& gt； σk

Broken wires and slippery wires during tensioning shall not exceed the specification or design requirements. If it exceeds the requirements, the steel wire shall be replaced or other remedial measures agreed by the engineer shall be taken.

The tensioning sequence shall be carried out according to the requirements of the drawings. If there is no clear stipulation, tensioning shall be carried out according to the principle of subsection, batch and symmetry.

7. Grouting and anchoring

After tensioning is completed, hole grouting and anchor sealing should be carried out as soon as possible, and the strength, consistency, water cement ratio, bleeding rate and extrusion amount of expansion agent for grouting should be controlled according to the requirements of construction technical specifications and inspection standards. Generally, No.525 ordinary portland cement should be adopted, with water-cement ratio of 0.4-0.45, and the expansion agent is aluminum powder, the content of which is one ten thousandth of the cement weight, and the aluminum powder should be degreased.

Grouting is carried out slowly and evenly with piston grouting pump, and the maximum grouting pressure is generally 0.5-0.7 MPa. When the conduit is longer or the slurry conveying pipe is longer, the pressure can be higher, and vice versa. After each channel is grouted to the maximum pressure, it should have a certain stabilization time. Grouting shall fill the other end of the channel and grout it. And make the exhaust hole discharge cement slurry with the same consistency as specified until.

After grouting is completed, the surrounding area of grouting anchorage should be washed and roughened, steel mesh should be set up, and anchor sealing concrete should be poured.