1 Processing of bottom plate and web reinforcement
1), storage and inspection of reinforcement
Steel bars are stored on special scaffolding and covered on the support above 0.5m from the ground. All kinds of steel bars shall be identified and stored according to the manufacturer, batch and model, and shall not be mixed.
2), steel processing:
Steel bar production is completed in the steel bar processing shed. Before steel bar setting-out, pay-off is carried out first, and then pay-off is carried out after being reviewed by technicians. The quantity, specification, length and quantity of reinforcement shall be indicated in the blanking sheet. Steel products and semi-finished products are stacked separately with complete marks. Cushions are stored in welded steel frames with a ground clearance of 750px. See Annex 2 for inspection methods and frequency of reinforcement processing and installation.
3), steel welding
When arc welding is used, the overlapping parts of two steel bars should be bent in advance to make the axes of the two steel bars consistent. The weld length of double-sided welding shall not be less than 5d+50px, and that of single-sided welding shall not be less than 10d+50px. The weld should be full to ensure the welding quality.
Do not cut off the steel bars that interfere with bellows, and make special-shaped steel bars according to the cross section of the beam.
The height and spacing of the diaphragm reinforcement shall be consistent, meet the design requirements, and ensure accurate alignment after installation.
4) Installation of bottom plate and web reinforcement
When box girder reinforcement is constructed, the bottom plate and web reinforcement shall be bound first. Before the bottom plate reinforcement is bound, tire molds shall be set at both ends of the bottom mold to control the position of the main reinforcement of the bottom plate. The main reinforcement spacing is 70mm, 70mm, 123mm and 100mm from outside to inside respectively. The welded joints of bottom reinforcement shall be staggered at the maximum bending moment of each hole as far as possible, and the joints of adjacent reinforcement shall be staggered for 35d.
After the installation of floor reinforcement is completed, vent holes are set according to the drawings, and PVC pipes with a diameter of 50mm are bound on the reinforcement skeleton. Both ends of PVC pipe are sealed, and the distance from the beam end is 6000px and 12250px respectively. The vent holes at both ends are symmetrically arranged, and each beam plate has four vent holes. PVC pipes are firmly bound on the low-plate steel bars, and the positions are correct. The pipes are filled with fine sand to avoid pipeline leakage and blockage when pouring concrete.
When binding the web reinforcement skeleton, the self-made fixture should be used to ensure the accurate spacing of reinforcement, the stirrup joints should be staggered, and all the steel bar intersections should be firmly bound with wire. The bound skeleton should have sufficient rigidity and stability to ensure that the position of reinforcement is not affected when concrete is vibrated. Every 1500px, a set of bottom plate sleeve reinforcement, and every 1500px, a set of web plate sleeve reinforcement.
Installation sequence of steel bars: first bind the bottom plate steel bar, then bind the web plate steel bar, set the prestressed pipe positioning steel bar, and wear corrugated pipe. Erect the side formwork and internal formwork of box girder, bind the roof steel bar after the acceptance of the supervision engineer, hang the roof steel bar for construction to ensure the steel bar is neat and consistent, and spot weld the roof shear bar on the roof sleeve steel bar in advance to ensure the same number and spacing.
5), the installation of the block
The cushion block adopts customized precast concrete cushion block, and the thickness of the cushion block is customized according to the drawing design, and the wire is embedded during prefabrication. The bottom cushion blocks are arranged in 6 quincunx shapes per square meter, and the bottom protective layer cushion blocks are divided into two layers, one between the steel bar and the bottom die, and the other between the steel bar and the core die. The cushion blocks of the web protection layer shall be arranged in a quincunx shape with four blocks /m2, and the cushion blocks of the web protection layer shall also be arranged in two layers, one layer is arranged between the steel bar and the mandrel, and the other layer is arranged between the steel bar and the side formwork, all of which are firmly bound. The protective layer cushion block is supported on the outer steel bar of steel mesh, and the cushion block is firmly bound with the steel bar to prevent the concrete from falling off during vibrating. The thickness of the protective layer is set in strict accordance with the requirements of the design drawings. Net protective layer of top plate 100px, web plate 50px and bottom plate 75px.
2 Install bellows and reserve vent holes.
After the steel bar binding of the bottom plate and the web structure is completed, φ 8 steel bars shall be welded with bellows, and the steel bar grid shall be positioned according to the design requirements, and the steel bar grid shall be welded into a "well" shape. Strictly control the position of steel skeleton, set a curve segment every 1250px and a straight segment at 2500px, with accurate vertical and horizontal coordinates. The positioning steel tic-tac-toe frame is welded and fixed with the steel skeleton to prevent displacement during concrete pouring.
After the positioning tic-tac-toe frame is set, install bellows. Before installation, check the bellows one by one to ensure that there are no cracks and sundries in the pipeline. High-density polyethylene plastic corrugated pipe with an inner diameter of 50mm is adopted, and the pipe joint is connected with a larger corrugated pipe. The corrugated pipe joint is hammered flat at the corrugated pipe joint to prevent the corrugated pipe from turning over when passing through the steel strand, which will lead to pipe blockage in severe cases and make it difficult to pass through the steel bar. The connection length of the joint is 875px, and both ends are screwed symmetrically. Joints should be sealed to prevent cement slurry from entering the pipeline. Circular curve transition is adopted at the corner of corrugated pipe. In order to ensure the smooth flow of the pipeline and the accurate and firm position of the corrugated pipe, plastic pipes are added to the corrugated pipe and twitched during concrete pouring.
After the corrugated pipe is installed and positioned, the full-time quality inspector will conduct a comprehensive inspection in conjunction with the steel bars, and check the size of the steel skeleton of the bottom plate and the web plate, the spacing between the steel bars and the thickness of the protective layer; Check the bellows for holes, disconnection and deformation, and make adjustments at any time. After passing the inspection, report to the supervision engineer for inspection, and the formwork can be erected only after passing the inspection.
3. Install the mandrel and side form.
All prefabricated box girder formwork in this contract section adopts the shaped steel formwork specially made by the manufacturer according to the design drawings (the external formwork panel is the original flat plate with 6mm thickness). After the new template comes into play, try to assemble and remove rust, and then formally assemble the template. And ensure the correct sequence when splicing in the future. After the formal splicing is completed, the staggered joints of the formwork shall be polished and leveled to make the joints of the formwork transition smoothly. All formwork of box girder shall be painted with formwork paint before installation for demoulding.
Rubber pads consistent with the dental plate should be placed in the dental plate to prevent pulp leakage between the teeth.
The core mould is assembled by steel plates, and the inside is connected by angle steel ribbed bolts. Each joint is sealed with pull-back tape to prevent slurry leakage. After the installation is completed, adjust the elevation and size of the mandrel, and set a pressure bar at the top of the mandrel every 2 meters along the beam length direction to prevent the mandrel from floating up and moving, which will cause the inaccurate size of the bottom web. The compression bar consists of channel steel and fasteners. After pouring the roof concrete, dismantle the pressure bar and vibrate for the second time.
Adjustable legs are arranged below the side formwork to adjust the height of the side formwork. The top and bottom of the side formwork are supported by bolt holes along the beam length direction, and the bolt holes at the bottom correspond to the reserved holes of the bottom formwork support one by one. During installation, the left and right side forms are fixed with tie rod bolts through the reserved holes in the base, and a sponge strip slurry stop belt is inserted between the side forms and the bottom form to prevent slurry leakage. At the top, use φ 16 tie rod to pass through the steel pipe, and pull out and fasten the inner top. When demoulding, releasing the leg side form will automatically demould due to its own weight. According to the needs of construction, an attached vibrator is set on the side formwork to ensure the beam concrete is vibrated compactly.
After the formwork is installed, check the plane position, top elevation, node connection and vertical and horizontal stability to ensure that the formwork is not deformed, displaced or leaked during concrete pouring. When there is deviation beyond the allowable range, it should be adjusted in time. The joint between precast box girder and hollow slab core mould shall be sealed with pull-back adhesive tape. Effective measures should be taken to prevent the spindle from floating. The pressure bar to prevent the mandrel from floating should be tightened by the ground anchor and should not be directly fixed on the formwork.
Before installing the anchor pad, check whether its size is correct. The anchor pad shall be firmly installed on the end formwork, and the bolts of the positioning holes shall be tightened. The gasket should be strictly aligned with the channel and perpendicular to the end of the channel. Grouting holes of anchor pads shall be sealed with wooden plugs. The exposed part of the anchor pad outside the beam body should be sealed to prevent water, mud and other sundries from corroding the bellows.
4. Tie the roof reinforcement, and install negative bending moment, corrugated pipe and anchorage.
After the installation of side formwork and core formwork is completed, the top plate reinforcement shall be bound, and the top plate reinforcement and web reinforcement shall be bound one by one. The binding requirements are the same as those of the bottom plate and web reinforcement. The exposed steel bars of the wing plate are evenly spaced and the length meets the design requirements of the drawings. At the same time, negative bending moment bellows and anchors have constraints, and their requirements are the same as those of positive bending moment bellows and anchors. Install negative bending moment tensioning reserved groove template, the upper longitudinal main reinforcement at the reserved groove template is disconnected and bent upward, after negative bending moment tensioning grouting, adjust the reinforcement and weld it, and the reserved groove is tensioned and poured together with the wet joint.
The corrugated pipe with negative bending moment is spot-welded on the main reinforcement of the roof with U-shaped reinforcement to ensure the height accuracy of the corrugated pipe. The negative bending moment pipe penetrates into the PVC protection pipe to prevent the pipe from being deformed or blocked, and the two ends are exposed 250px. Before the initial setting of concrete, it shall be pulled out by special personnel to prevent pipe plugging. After the binding of roof reinforcement is completed, set the roof protective layer pad, and the thickness of the protective layer shall be controlled according to the drawing requirements, that is, 100px.
Check whether the position and quantity of embedded steel bars in bridge deck pavement are accurate. Check whether the corrugated pipe is straight, whether the anchorage is closely connected with the corrugated pipe, whether the grouting hole of the anchor pad is plugged with a cork, and whether the position of the reserved groove is accurate.
After the anchorage comes into play, check its factory certificate and quality certificate, and take samples for appearance, hardness and static load anchorage performance test. Only after passing the inspection and being confirmed by the supervision engineer can it be used.
Before installing the anchorage, check whether its size is correct, whether the anchorage model is consistent with the drawing, and whether the bell mouth of the anchor plate is blocked. During installation, the corrugated pipe is inserted into the anchor plate and sealed to prevent cement slurry from entering and blocking the pipeline. The position of spiral rib is accurate, the center is concentric with corrugated pipe, and the end is close to anchorage. The reinforcement under the anchor should be attached to the corrugated pipe and anchorage, and tied firmly.
After the self-inspection is qualified, it shall be reported to the supervision engineer for inspection and acceptance, and the concrete can be poured only after it meets the design and specification requirements.
5 pouring concrete
Before concrete pouring, concrete trial mixing shall be carried out, and the construction mixture ratio shall be adjusted according to the theoretical mixture ratio approved by the director's office and the actual situation. The adjustment method is as follows:
See table 1 for the theoretical mixture ratio of C50 mixture ratio design in XJ3 contract section of Shi' an expressway reconstruction and expansion project:
Unit: Kg/m3 table 1 fine aggregate and coarse aggregate of cement, fly ash and slag powder.
(10-20:5- 10=70:30) Water reducer 367 86 24 657168 7.16148 When mixing in the mixing station, the coarse aggregate is the aggregate washed by washing equipment. After the test, the construction mixture ratio is adjusted to sand ratio of 38%, 10-20:5- 10=80:20, and the construction mixture ratio is shown in Table 2:
Unit: Kg/m3 Table 2 Fine aggregate and coarse aggregate of cement, fly ash and slag powder
(10-20:5- 10=80:20) Water reducer 367 86 24 72311317.1694 Before concrete pouring, check the template installation size. Check whether the quantity and position of various embedded parts are accurate. Clear the number of precast beams, and gradually advance the construction from one end of the beam to the other. The upper and lower floors should be poured at the same time, and the pouring distance between the upper and lower floors should be kept between1.5 ~ 2.0m. The concrete pouring of beam and slab must be completed within 4h.
C50 concrete for box girder pouring is supplied by concrete mixing station. After the approval of concrete mixture ratio, the performance indexes of cement, aggregate and mixing water used in concrete meet the specification requirements, and all kinds of weighing instruments in the mixing station have verification certificates, which are accurate in measurement. Check the slump of concrete at any time during mixing, keep good workability, and control the slump at 180-200mm.
The concrete is transported to the box girder pouring site by the mixer truck, and the concrete in the hopper is hoisted to the pouring site by the gantry crane. Ensure that the sum of concrete transportation, pouring time and intermittent time does not exceed 180min.
The first piece of precast box girder concrete adopts φ50 and φ30 plug-in vibrating rods, and attention should be paid to make the bottom concrete dense when vibrating the bottom plate. When vibrating, be careful not to collide with the bellows, so as not to damage the bellows and leak slurry. When pouring concrete, check the positions of embedded steel plates and embedded steel bars at any time, whether the holes are grouted, the degree of formwork fastening, whether there is slurry leakage, whether the core mould floats, and timely block and reinforce the slurry leakage. Vibrating time shall be subject to concrete no longer sinking, no bubbles and smooth and pan-slurry on the surface, and special personnel shall be appointed for compound vibrating, and problems shall be solved at any time when found.
In the process of pouring web and roof concrete, always pay attention to the size of various embedded parts and templates, especially the position of core mold. Concrete pouring requires strict adherence to the principle of "layered pouring and oblique propulsion". The concrete is poured evenly on both sides of the web from one end to the other, and then vibrated at the same time. The pouring thickness of each layer of concrete shall not exceed 750px.
In order to ensure the compactness of web concrete and improve the strength of concrete, attached vibrators and vibrators are used to complete all the vibrating work in the construction, and the attached vibrators are arranged along the beam length direction at the interval of1.2m. When vibrating, the attached vibrators are started at regular intervals in batches according to the concrete pouring position, and the vibrating time is controlled within 15s, and plug-in vibrators are arranged on them for auxiliary vibrating. The vibrating of web concrete should be carried out symmetrically at the same time, so as to prevent the internal mold from shifting to the other side when vibrating on one side. In the process of vibrating, the formwork, steel bar and corrugated pipe shall not be touched, and the insertion point shall be determined evenly. When pouring the roof, we should strictly control the depth of the vibrating rod inserted into the web, and pay attention to the compactness of the concrete at the joint with the web. Because the roof is thin, be careful not to touch the formwork, negative bending moment corrugated pipe and steel bar when vibrating, and pay attention to the position of embedded steel bar. Strictly control the roof thickness, which cannot exceed the specification requirements. After the roof concrete is poured, the slurry shall be plastered at appropriate intervals. After the slurry is solidified, it shall be plastered with wood for three times and steel for two times.
During the construction, four groups of specimens were made under the witness of the supervisor, two groups were cured under the same conditions as the box girder, and two groups were sent to the curing room for standard curing.
6. Form removal, hole sealing and maintenance.
Form removal:
(1) formwork removal is determined according to the change of external temperature and concrete strength. Only when the concrete strength can ensure that the surface and edges and corners are not damaged by formwork removal can the side formwork be removed. Generally, the formwork shall be removed after the beam concrete reaches 2.5Mpa and the concrete is poured 17h. In actual construction, the time of formwork removal will be flexibly controlled according to the temperature conditions during construction and the strength of the specimen under the same conditions.
(2) Only when the roof concrete surface does not collapse or crack can the core mold be removed. After form removal, indicate the production unit, bridge name, beam number, pouring date, etc. In order to avoid confusion during hoisting, it needs to be installed on one side of the box girder. After the formwork is removed, clean the bottom vent to prevent blockage.
Sealing hole:
When sealing the hole, install the plug plate at the connection end of the box girder to ensure the accurate position of the plug plate, and then fill the gap between the plug plate and the precast box girder with high-grade cement mortar.
Health:
After concrete pouring, it should be covered with moisture and covered with frame-type movable curing shed, and the concrete surface should not be damaged or polluted when covered. At the same time, hang a health care sign, indicating the pouring date, beam plate number and health care days. Spraying facilities are pre-embedded pressurized pipes in the precast beam yard, and the maintenance of each beam can be controlled independently. The nozzle can be adjusted at any time according to different beam lengths. PVC pipes, tees and metal nozzles are used in the pipeline, and there is a spray pipe on each side of the beam. After the construction is completed, it is forbidden to spray directly for maintenance. The beam should be covered with geotextile for spraying and wetting, and can be directly sprayed and maintained after the strength reaches 40%. Spraying curing time is 7 days.
7 haircut
The concrete strength of beams and slabs can reach 100% of the design strength standard value before roughening. The joint surfaces of new and old concrete, such as wet joints at beam ends, diaphragms, flange plates, etc., must be inked, and 37.5px should be reserved at the edges and corners to prevent the edges and corners from being damaged and affect the appearance. Chisel with a mechanical chisel to completely chisel off the concrete surface until all aggregates are exposed.