AK0+799. 15 ramp bridge has a total length of155.3m. The whole bridge is connected with 6 holes, and the span combination is 24.24+4 * 25+24.94m.. The superstructure of this bridge is prestressed concrete continuous box girder, and the substructure is single-column pier, ribbed slab bridge abutment and bored pile foundation. The height of prestressed box girder is 1.5m, the width of bridge deck is 15.5m, and the width of bottom plate is 3.6m The drawing design of this bridge is divided into three stages: 53.24+50+45.94m is tensioned three times.
During construction, the foundation of the bridge site shall be treated first, and then the bowl scaffold cast-in-place construction technology shall be adopted for construction. After concrete is poured at one time, the bottom plate is poured first, then the web plate, and then the wing plate and the top plate. The overall construction process is as follows:
Support foundation treatment → Support erection → Support static load preloading → Support pre-camber setting → Template production → Steel bar production → Concrete construction → Prestressed steel bar tensioning and grouting → Maintenance → Support removal → Bridge deck engineering.
1, support preloading
The setting of longitudinal pre-camber is that the maximum value is the middle of the beam span, and the abutment bearing, pier and box girder are consolidated to zero, which is calculated and determined by parabola or vertical curve. In addition, in order to ensure the quality of box girder construction, according to the distributed load of box girder concrete, soil bags are used for equal weight preloading. According to the pre-loading results, the values related to setting the pre-camber can be obtained, and the theoretical calculation values can be revised accordingly to determine a more suitable pre-camber. Settlement observation points shall be set every 5m in the longitudinal direction and every 2m in the transverse direction (mainly corresponding to the position of the web), and the preloading shall be carried out according to the structural weight, construction load and 1.2 times of the total weight of formwork and construction machines.
The preloading time reaches 24 hours, and the observation is made every 2 hours after the load is filled. Settlement observation is divided into foundation settlement observation and support settlement observation. Before the concrete pouring process, the preloading load is gradually removed and the formwork elevation is readjusted.
2, box girder formwork, steel bar
The internal and external formwork of box girder shall be made of high-strength bamboo veneer with a thickness of 1.8cm, and the external formwork shall be fully paved and then reinforced. The internal mold is manufactured in sections and blocks, and assembled and formed in sections and blocks. Steel bars are bent centrally at the processing site, the positions of steel bars and corrugated pipes are accurately laid out on the steel bar grid, and corrugated pipes are accurately positioned according to the designed positions.
When installing the template, please pay special attention to the following questions:
(1) The formwork for prestressed anchor head position and supporting formwork at the beam end and beam position shall be made at the specified angle and shape according to the design requirements and supporting shape, and the concrete surface at the anchor head position shall be perpendicular to the tangent of the steel strand here.
(2) Attached vibrators shall be installed on the exposed bottom plate and side formwork, especially on the prestressed tensioning end formwork, so as to ensure the concrete pouring quality.
(3) All exposed formwork joints should adopt release agent to ensure that the formwork is flat and dense.
(4) In the middle two places near the tensioning end, the roof formwork shall be provided with working holes with appropriate areas for prestressed tensioning.
⑤ The embedded pipes of vent holes, grouting holes, drainage holes and bridge deck drainage pipes are all fixed in place according to the design drawings, with no missing embedded parts, firm installation and accurate positioning.
3, prestressed steel strand production and installation
① check
Prestressed construction is the key to continuous beam construction, so it is necessary to check prestressed reinforcement, anchorage, fixture and tensioning equipment.
② Inspection of prestressed steel strand, anchorage and fixture.
Each batch of prestressed steel bars shall be clearly marked to prove the manufacturer, performance, size, furnace number and date, and each batch of prestressed steel bars shall be accepted in batches to check whether its quality certificate, packaging method and marks are complete and correct; Whether the surface quality and specifications of steel meet the requirements, and whether there is any damage, corrosion or oil stain that affects the bonding with cement after transportation and storage. In order to ensure the engineering quality, the mechanical properties of prestressed reinforcement, anchorage and fixture were tested.
Anchor, fixing device and steel strand:
Inspection must be carried out after entering the site, and installation can only be carried out after passing the inspection.
③ Tension equipment inspection
Tension equipment and anchorage should be used together, using YCD beam-slab series jacks. Before tensioning, the jack and the pressure gauge should be calibrated together, and the calibration equipment should be sent to the metrological department recognized by the state for calibration, so as to determine the relationship curve or linear regression agenda between the tension and the pressure gauge reading. So as to calculate the reading of the pressure gauge relative to each tension control stress, and a special person is responsible for the use, management and maintenance.
④ Prestressed installation
After the placement of ordinary steel bars is basically completed, the plane and height of prestressed steel bars (relative to the bottom die) should be released and clearly marked. After the lofting is completed, put on the corrugated pipe, and wrap the gap at the joint of the corrugated pipe with tape paper to prevent the cement slurry from infiltrating. For the embedding of tensioning end anchor pad, the angle and end template meeting the requirements of design drawings shall be made first, and the anchor pad shall be fixed on the end template with bolts.
When cutting the length of steel strand, the working length of tensioning end should be considered. When cutting, first tie it with lead wire at 5cm on both sides of the cutting mouth, and then cut it with a cutting machine. After blanking, braid the strands on the floor to make them straight. Each share in each share should be numbered and placed in order. At intervals of 1m, lead wires of 18 ~ 22 shall be woven, folded and bundled. After the corrugated pipe and anchor pad are installed and the steel strand is braided, you can wear the steel strand, and be careful not to puncture the corrugated pipe when wearing it. When installing prestressed pipes, prestressed steel beams should be inserted at the same time. After passing through the beam, use φ12 "#" positioning steel bars to firmly fix the corrugated pipe on the steel frame with a spacing of 50cm to ensure the accuracy of its plane position and height. When there is a conflict between prestressed steel bars and ordinary steel bars, the ordinary steel bars can be appropriately moved or cut off, and recycled at other locations. The exposed part of steel strand is wrapped with plastic film to prevent pollution.
Make the following preparations before threading:
A, remove all kinds of sundry and redundant bellows on the anchor head.
B, flush the channel with high-pressure water.
C, when cleaning the cement floor beam, to prevent steel beam pollution.
D, before weaving, comb the steel beams with special tools to prevent the stranded wires from being twisted together.
E, make the end of the steel beam into a cone, weld it firmly with electric welding, and smooth the surface with a grinding wheel to prevent the steel beam from causing the bellows to roll over at the bellows joint and blocking the channel.
4, concrete pouring
Concrete construction should be carried out in the order of bottom plate, secondary web plate, wing plate and top plate. According to the requirements, the concrete should use ZN50 and ZN30 plug-in vibrators. When vibrating, special attention should be paid not to collide with bellows and embedded parts.
Before the initial setting of concrete, the top surface should be polished for many times to eliminate cracks, and the surface should be roughened during the initial setting of concrete to facilitate the connection of bridge deck pavement concrete.
Pay attention to embedded parts such as expansion joints, guardrail seats and drainage pipes during concrete construction.
After concrete pouring is completed, it should be cured as soon as possible after initial setting, and the concrete surface should be covered with geotextile and watered for curing. The curing time of concrete by sprinkling water is 14 days, and the concrete surface should be kept wet every time sprinkling water. The concrete roof and wing plate shall be vibrated by flat vibrator and plug-in vibrator to ensure the quality of concrete. The concrete surface shall be chiseled after the first surface closing, and the construction shall be carried out in strict accordance with relevant documents.
The concrete strength test blocks used to control formwork removal and scaffold falling are placed in the box girder room, and the curing is carried out under the same conditions.
5, box girder prestressed tensioning, grouting
According to the design prestress, the overstretching method is adopted to control the tension and elongation, and the tensioning is carried out in strict accordance with the order specified in the design drawings. During tensioning operation, it is forbidden for the operator to stand opposite the jack and protect both ends to prevent accidents.
After tensioning is completed, all prestressed steel bars shall be grouted with piston-type water-mud pump immediately within 24 hours after tensioning is completed, and the slurry inlet of grouting pump shall be filtered with filter screen to prevent blockage. The sequence of hole grouting is to press down first, and then finish the last time. After the slurry with the same concentration as the slurry inlet appears at the slurry outlet, it will be closed immediately. Immediately after the hole grouting, the cement slurry at the beam end shall be washed clean, and the dirt on the bearing pad, anchorage and end face concrete shall be removed at the same time, and the end face shall be chiseled, and the end steel mesh shall be set, and the tensioned prestressed steel beam shall be anchored according to the design requirements.
C50 cement slurry is used for hole grouting. In order to make the grouting in the hole smooth and make the slurry contact well with the hole wall, the hole should be washed with pressure water before grouting, and finally the water in the hole should be removed with compressed air, and the hole should be dried. The grouting sequence is from bottom to top, and the holes of one section should be pressed at one time to avoid the leakage from blocking the adjacent holes. Connect the steel strand after the grouting strength reaches the design strength, and proceed to the next stage of construction.
6, the removal of stents and templates
After all prestressed tendons are tensioned, and the grouting strength of the pipeline reaches more than 100% of the design strength, the support can be unloaded only after receiving instructions from the supervisor. Dismantle the bracket and formwork, first dismantle the wing plate and side formwork, and then dismantle the bottom formwork and bracket. When falling, it should be carried out from mid-span to both ends according to the principle of full hole, multi-point, symmetry, slowness and uniformity, and each demolition is only allowed in one hole. When unloading the frame, special attention should be paid to the safety of construction work.
7. concluding remarks
Through the strict technical control of the above key processes, the qualified rate of cast-in-place construction quality of prestressed concrete continuous box girder of Ak0+799,438+05 ramp bridge overpass is 65,438+000%, which fully meets the design requirements.