What is the construction scheme of cast-in-place box girder?

Construction scheme of equal section cast-in-place continuous box girder

1, design introduction

The superstructure of this bridge is a 4×25m cast-in-place prestressed concrete box girder, with beam height 1.40m, box room height 1.0m, total length 100m, bridge width 15.0m and single width of 7.5m The rear part is located at A=60. Abutment adopts single and double GPZ3DX basin bearings, pier No.2 adopts pier beam consolidation, and piers 1 and No.3 adopt single GPZ6DX basin bearings. The geology under the bridge is 4m thick loam, 5m thick clayey pebble and siltstone.

2. Overview of construction scheme

(1) Supporting foundation

The constructable bridge site shall be cleaned, leveled, backfilled with 1m slag, compacted, and then backfilled with 1m siltstone slag under the beam. When filling, it shall be paved and compacted in layers with a thickness of 40cm. During the filling process, the settlement pile shall be buried every three days for settlement observation, and the settlement for three consecutive times shall not exceed 3mm until one month after the filling is completed. After testing meets the requirements, river pebbles with a thickness of 10cm and C20 plain concrete with a thickness of 10cm are finally laid as supporting foundations. See attached figure 1 for details.

(2) bracket installation

According to the design scheme, the cast-in-place construction of full-house support is adopted, and the frame is divided left and right before and after construction. After the bracket foundation construction is completed, the plane position of box girder bracket shall be determined by setting out. During erection, according to the predetermined position, the vertical and horizontal spacing of the vertical steel pipe is 80cm×80cm, the vertical and horizontal spacing of the bracket at the web is 40cm×40cm, and the vertical and horizontal spacing around the pier is 40cm×40cm. In order to increase the integrity of the support, each vertical steel pipe is horizontally connected with the longitudinal and transverse steel pipes. The vertical spacing of horizontal steel pipes is 120cm, and the vertical spacing of horizontal steel pipes at the top of the support is adjusted to 40cm. In order to ensure the overall strength, stiffness and stability of the full-house support, at the pier, 9 steel tube bracing are set at every 3m in the longitudinal direction of 1/8 span, 3/8 span and middle span, and 5 bracing are set in the transverse direction of each span.

Erection requirements: each vertical pole should be vertical and use a single steel pipe. After the vertical pole is erected, the longitudinal and transverse plane steel pipes shall be fixed in time to ensure that the full-house support has sufficient strength, rigidity and stability. After the full-house steel pipe support is erected, the height of each steel pipe is determined by measuring and lofting (the height of each steel pipe is calculated by subtracting the thickness of structural formwork and the thickness of square wood and wedge from the height of the beam bottom where it is located), and the steel pipe is marked, and the steel pipe with higher part is cut by electric welding machine to ensure that the height of the whole support is consistent and meets the design requirements. At the top of the bracket, a transverse steel pipe is set in the direction of the transverse bridge (square wood and wooden wedge are directly set on it to pave the formwork), and a longitudinal steel pipe is fastened at the lower part of the transverse steel pipe fastener, which requires that the transverse steel pipe fastener be attached to the longitudinal steel pipe fastener, and then a reinforcing fastener be attached to the longitudinal steel pipe fastener, so as to ensure that the fastener connection between the transverse steel pipe and the vertical steel pipe has sufficient strength to bear the construction load. For the convenience and safety of construction, pedestrian work ladders shall be set outside Platform 0 and Platform 4 respectively, and work and inspection platforms with the width of 1.2m shall be set on both sides of the support, and guardrails with the height of 1.2m shall be installed on both work ladders and platforms. (See attached figure 2 for bracket layout)

(3) Determination and setting of construction camber

When the continuous box girder is cast on the support, the superstructure will sink and deflect to some extent during the construction process and after the support is removed. In order to ensure that the superstructure can meet the design requirements after the support is removed, appropriate pre-camber shall be set during the support and formwork construction. When determining the pre-camber, the following factors are mainly considered: a, the elastic deflection δ1caused by the self-weight of the structure and half live load;

B. Inelastic deformation Δ 2 caused by the extrusion of the rod joint and the compression of the unloading equipment after the support bears the load;

C, the elastic deformation δ 3 caused by the bearing bearing construction load;

D, inelastic settlement δ 4 of the supporting foundation after loading;

E. Deflection δ5 of statically indeterminate structure caused by shrinkage, creep and temperature change of concrete.

After calculation, it is1.8cm.

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, sand bags are used to preload the whole bridge before pouring. According to the preloading results, the relevant values of pre-camber can be obtained, and the theoretical calculation values can be revised accordingly to determine a more suitable pre-camber.

(4) Template production and installation

Box girder bottom, web, vertical plate and internal formwork are all made of 15mm thick bamboo veneer.

Bottom formwork installation: erect a longitudinal arc steel pipe on the longitudinal steel pipe at the top of the steel pipe support, and horizontally erect a square timber of 5cm×8cm×2.5m on it. Nodes are staggered, with a pitch of 25cm. Longitudinal steel pipe and square wood joint are adjusted with wooden wedge to ensure the straightness of bottom die. The bamboo veneer of the bottom die is directly paved and nailed at the flat-fell seam of the square bamboo veneer, and spliced with a 45 inclined plane. Under the flat-fell seam, square wood is added, so that the flat-fell seam is right in the middle of the square wood, double-sided adhesive tape is sandwiched in the middle of the flat-fell seam, and the flat-fell seam surface is sealed with paraffin. Before laying the bottom formwork, place basin supports, and punch holes in the bottom formwork according to the size of the beam bottom wedge at the supporting position, and support the template of the wedge at the opening. The bottom die of the wedge is also drilled according to the size of the embedded steel plate, and the bottom die of the embedded steel plate and the wedge is sealed with high-strength mortar.

Installation of side formwork of web and bottom formwork of wing plate: after the bottom formwork is laid, re-calibrate the central axis of bridge, set out the plane position of box girder, and mark the side formwork of web, internal formwork, edge of wing plate and reinforcement position on the bottom formwork. The side formwork of the web adopts high-strength plywood, with square timber and back reinforcement timber every 25cm, and the vertical back reinforcement timber is directly placed on the horizontal square timber supported, and wedged tightly with a wooden wedge. During construction, the strength and rigidity of formwork support must be ensured, and the side formwork of box girder and the bottom formwork of wing plate must be integrated.

The inner web is also made of bamboo plywood, and a certain number of positioning steel bars are set to ensure that the side formwork is firmly fixed on the main reinforcement and web stirrups of the box girder. Accurately determine the position of formwork, and set φ 14 round steel bars on the web of box girder. The rib spacing of the internal mold web is 25cm, and the rib spacing between the top plate and the bottom plate is 40cm. Vertical square timber supports with longitudinal spacing of 40cm and horizontal spacing of 60cm are set between the top plate and the bottom plate, and upper and lower horizontal supports with vertical spacing of 60cm are set horizontally. The horizontal support and vertical support form a combined "#"-shaped bracket, which is pre-nailed. The bottom plate and top plate of the internal mold are set to be movable, and before binding the top plate reinforcement. When pouring the bottom plate, dismantle the combined "#" frame, open the top plate and bottom plate of the internal model, close the bottom plate of the internal model after pouring the bottom plate, put it into the combined "#" frame for fixing, and finally close the top plate of the internal model.

When installing formwork, special attention should be paid to the following problems: the formwork at the prestressed anchor head of beam end and cross beam and the formwork at the bracket should be made into specified angles and shapes according to the design requirements and the shape of the bracket, and the concrete surface at the anchor head should be ensured to be perpendicular to the tangent of the steel strand there.

According to requirements, attached vibrators shall be installed on the exposed bottom formwork and side formwork, especially on the prestressed tensioning end formwork, so as to ensure the concrete pouring quality.

All exposed formwork joints shall be treated with a new paraffin coating process to ensure that the formwork is flat, dense and does not leak slurry.

In the middle two parts near the tensioning end, the roof formwork shall be provided with working holes with appropriate areas for prestressed tensioning.

The embedded pipes of exhaust holes, grouting holes, drainage holes and bridge deck drainage pipes are all fixed in place according to the design drawings, and the embedded parts are embedded without omission, firmly installed and accurately positioned.

See attached figure 3 for formwork support.

(5) Preloading bracket

Pre-loading: after the bottom formwork of box girder is laid, the whole bridge bracket and formwork are pre-loaded. The preload shall be considered as 1 10% of the total weight of newly poured concrete, steel bars, construction personnel and equipment. The preload in concrete construction is 1.2 times of the box girder weight, that is, the total half-width preload is1200 t.

Pre-pressing method: sand bags are used for pre-pressing, that is, within the half-width range of the whole bridge (according to the span of the beam), the bridge formwork and support are pre-pressed for 7 days with sand bags equivalent to the self-weight of the beam 1 10% and about 1200 tons. Before, after and during preloading, observe the deformation of 1/4 beam at any time with instruments, and check the stress of each fastener of the support, verify the reliability of the set value of construction camber, and determine the reasonable set value of support camber in the next step.

(6) steel processing and binding

A, steel inspection

Steel bars must be accepted in batches according to different types, grades, brands, specifications and manufacturers, and stacked separately, and shall not be mixed, and signs shall be erected for identification. Steel bars shall be protected from corrosion and pollution during transportation and storage, and shall be stacked in the steel shed.

After the rebar enters the site, it is required to provide the qualification certificate of this batch of rebar manufacturers, indicating the batch number and relevant mechanical performance test data for ex-factory inspection. Each batch of steel bars entering the site shall be sampled and tested according to JTJ055-83 "Metal Test Specification for Highway Engineering", and those that fail the test shall not be used in this project.

B, steel production, binding

Box girder reinforcement is processed in the reinforcement processing shed according to the design drawings; The longitudinal reinforcement shall be welded by flash butt welding, and the welded joint shall meet the requirements of JGGJ18-96 Code for Welding and Acceptance of Reinforcement. Welded joints should not be located at the maximum pressure, and the joints should be staggered. The area of joint reinforcement in the same welded joint in tension area shall not exceed 50% of the total area of reinforcement in this part. Reinforcement layout should be based on design drawings. First, bind the bottom plate reinforcement on the bottom die, install the outer die of the web plate and the bottom die of the wing plate, then bind the web plate reinforcement, and finally bind the top plate and the wing plate reinforcement.

In order to ensure the thickness of the protective layer of steel bars, triangular mortar pads are set between the steel bars and the formwork, fastened with embedded iron wires and staggered with each other.

In order to facilitate the operation and consider the removal of the internal model in the future, manholes are opened at the beam-slab spacing of each span 1/4, so the longitudinal reinforcement of the roof here must be disconnected from the upper and lower floors in the middle 1 1, and the transverse reinforcement of the roof should be disconnected by four lanes. In the case of stirrups, the circumferential seam of stirrups should be adjusted to be broken, and the lap length of these broken steel bars should be considered so as to expose the manhole edge in the future.

C. Installation of prestressed pipes and embedded parts

The buried position of prestressed pipeline determines the stress and stress distribution of prestressed steel bars in the future, so the pipeline should be buried in strict accordance with the design drawings, pay attention to the position of plane and elevation, weld φ 12 steel bars into a "#" frame to clamp the pipeline, and fix it on stirrups and erection poles by spot welding. During installation, strictly check the position of the pipeline point by point, and adjust it immediately if there is any problem. Before pouring, check the tightness of corrugated pipe and the firmness of each joint, do a leak test with irrigation method, and blow out the residual water in the pipe with high-pressure wind after the leak test.

Before pouring, carefully check the drawings (including the general drawing), and pay attention to the embedding of embedded parts such as bearing embedded steel plates, prestressed equipment, drainage holes, guardrail base reinforcement, ventilation holes in the box, expansion joints, etc. This must not be missed, and attention should also be paid to the size and position of embedded parts when embedding.

(7) Fabrication and installation of prestressed steel strand

First, check

Prestressed construction is the key to continuous beam construction, so it is necessary to check prestressed reinforcement, anchorage, fixture and tensioning equipment.

B, prestressed steel strand, anchorage, fixture inspection

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 of the bridge were tested.

A. Anchors and fixtures: visual inspection: 65,438+00% but not less than 65,438+00 sets of anchors shall be selected from each batch, and their visual dimensions shall be inspected. When there is a group of surface cracks or exceeds the product standard, double the number of anchors should be taken for reinspection. If there is another set that does not meet the requirements, don't use it, and don't check it one by one. Only those that pass the test can be used.

Hardness test: 5% but not less than 5 anchor clips shall be taken from each batch, and at least 5 anchor clips shall be taken from each group. Each part should be tested at three points, and its hardness should be within the design requirements. When a part is unqualified, it shall not be used or inspected one by one. Only qualified parts can be used.

B steel strand: prestressed steel strand shall be accepted in batches, each batch is made of steel strand of the same grade, specification and production process, and the mass of each batch is not more than 60 tons. Three strands were selected from each batch for surface quality, diameter deviation, relaxation test and mechanical properties test (breaking load, yield load and elongation). If one of the test results is unqualified, it will be scrapped as unqualified disk. Take twice as many samples from the untested steel strand for reinspection. If there is still one unqualified item, it is judged as unqualified.

C, tensioning equipment inspection

Tension machine and anchorage should be used together. YCD beam-slab series jacks should be used. Before tensioning, jack and pressure gauge should be calibrated together. The calibration equipment should be sent to the metrological department recognized by the state for calibration, and the running direction of the jack piston should be consistent with the actual tensioning working state, 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.

D, layout and placement of prestressed reinforcement

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 the steel bars should be 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.

The following preparations should be made before threading: (a) Remove all kinds of sundries and redundant bellows on the anchor head.

(b) Flush the pipeline with high-pressure water.

(d) The steel wire rope on the crane should be replaced with a new one and carefully checked for any damage.

(e) Before weaving, comb steel beams with special tools to prevent stranded wires from being twisted together.

(f) 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.

If the channel of prestressed beam is curved, it is more difficult to pass through the beam manually. Usually, the steel wire rope is tied to the high-strength steel wire. First, the high-strength steel wire is pulled through the catheter manually, and then the wire rope head is used. The semicircular steel ring of 12 is welded with the steel beam head. Start the winch to slowly pull the steel beam through the hole, and when the steel beam head enters the hole, it will enter smoothly with manual assistance. If the steel beam is blocked in the process of penetration, stop immediately, find out the blocking position, cut the concrete to remove the blocking debris in the pipe, and continue to drag the steel beam through the hole with the hoist.

(8) Concrete pouring and vibrating

Before concrete pouring, carefully check the support, formwork and embedded parts, remove the sundries in the formwork, and carefully wash the formwork with clear water. In order to prevent the concrete itself from shrinking and the construction time is too long, retarder should be added to the concrete. During the pouring process, the back reinforcement of the bottom plate is vibrated by an internal vibrator, and the top plate is vibrated by a flat vibrator. Be careful not to break the bellows of prestressed beam to prevent the bellows from being blocked by cement slurry. In the pouring project, it is necessary to knock the ends of the steel strand back and forth frequently to prevent the slurry from leaking and blocking the pipeline during pouring.

Before box girder concrete pouring, the safety of the supporting system must be comprehensively inspected, and pouring can only be carried out after self-inspection and supervision inspection.

Box girder concrete pouring is divided into three batches of parallel operations. The first batch of bottom plates should be poured. When the pouring length of the bottom plate is 1.5m, close the bottom plate of the internal model, fix the combined "#" frame, close the top plate of the internal model, and then close the second batch of webs. When the web casting length reaches 1.5m, the third batch of roof and wing plates shall be cast, and the interval between the three batches of casting shall be kept above 1.5m for parallel operation. Concrete pouring shall be carried out in a layered order with a certain thickness and direction, with a thickness of 30cm. Attention must be paid to pouring the upper concrete before the initial setting or remodeling of the lower concrete. When the upper and lower layers are poured at the same time, the pouring distance of the upper and lower layers should be kept above 1.5m, and the internal vibrator should be used for vibrating, and the moving distance should not exceed 1.5 times the action radius of the vibrator, and the distance from the side formwork should be 5~ 10cm. When vibrating, insert the lower concrete 5~ 10cm, and slowly lift out the vibrator after each vibration. Avoid vibrating rod templates, steel bars, etc. When vibrating; Each vibrating part must be vibrated until the concrete in this part is dense, that is, the concrete stops sinking, no longer bubbles, and the surface is smooth and pulpy. In the process of pouring, each type of work should be arranged to check the changes of steel bars, supports and templates, and the situation should be handled in time. The order of concrete pouring is: bottom plate, web plate → top plate and wing plate.

When pouring, pay attention to leave a manhole of 1/4 at each span 1.2m (horizontal) ×0.5m (vertical). After the internal formwork is dismantled and reinforced, the bottom plate is hoisted with iron wire to make up for the pouring of manhole concrete.

The concrete is stirred by a forced mixer and pumped into the mold. In order to prevent the internal mold from shifting, symmetrical balanced casting is adopted. Plug-in vibrator is used for concrete vibrating. The selection of concrete raw materials and additives and the design of mixture ratio must meet the requirements of technical specifications for concrete construction to ensure the quality of beams.

After the concrete pouring is completed, it should be cured as soon as possible after the initial setting, and the concrete surface should be covered with sacks or other objects. The water spraying curing time of concrete is 10 day, and the concrete surface should be kept in a wet state every time.

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.

During the maintenance period, it is forbidden to use the bridge deck as a construction site or pile up raw materials.

(9) Prestressing of box girder

Tension control adopts "double control method". After the overall pouring of box girder is completed, after the concrete strength reaches more than 90% of the design strength and concrete curing 15 days, with the approval of the supervisor, the ends of prestressed steel strand will be tensioned in batches. The tensioning sequence shall be arranged in strict accordance with the design of prestressed steel beams, and the same row of steel strands shall be tensioned at the same time, and both ends shall be tensioned at the same time. The tensioning procedure of each steel beam is: 0 →10% Δ con →100% Δ con (holding load for 5 minutes) → oil return and anchoring.

During initial tensioning, the tensioning end of prestressed steel strand should be filled with oil in the main cylinder of jack to make the steel strand slightly taut, and at the same time, adjust the position of anchor ring and jack to make the axes of hole, anchorage and jack coincide, and pay attention to make each steel strand stress evenly. When the steel strand reaches the initial stress of10% Δ con, the elongation at both ends should be marked to observe whether there is wire slippage. Tension should be gradually pressurized. When the tension reaches the design control stress (100% Δ con), continue to supply oil to keep the tension unchanged and hold the load for 5 minutes. At the same time, measure the actual elongation at both ends and compare whether it is consistent with the calculated value. The error between the calculated elongation and the measured elongation should be within 6%. When the measured values and calculated values do not meet the requirements, the reasons shall be found out in time, reported to the supervisor, and the elongation value shall be adjusted and calculated before tensioning.

In case of slippage, broken wires, insufficient elongation, etc. during tensioning, it is necessary to analyze the reasons and re-tension after treatment.

Steel wire slippage may occur during tensioning, for the following reasons: (a) During tensioning, there may be sundries between the tapered hole of the anchorage and the clamping piece.

(b) There is oil stain on the steel strand, concrete and other sundries in the bell mouth of the anchor pad.

(d) The steel strand may have negative tolerance, and its mechanical properties do not meet the design requirements.

(e) The initial stress is small, which may lead to uneven stress on the steel strand in the steel bundle, resulting in shrinkage and deformation of the steel strand.

(f) When cutting the anchor head steel strand, it is too short, or no cooling measures are taken.

(g) The beam is stretched, the elongation is large, the oil roof stroke is small, and the steel beam is deformed due to repeated tensioning and anchoring.

(h) Insufficient hardness of plugs and anchors.

There are generally the following reasons for wire breakage during tensioning: (a) The steel beam bends in the tunnel, and part of the stress during tensioning is greater than the destructive power of the steel strand.

(2) There is something wrong with the quality of the steel strand itself.

If it is found that the elongation of steel beam is insufficient or too large, the reasons should be analyzed in time. The general pipeline layout is inaccurate, which increases the friction of the channel and causes great stress loss. Sometimes, the elastic modulus value of steel strand used in design calculation may be different from that actually used.

In short, in the process of tensioning, if it is found that the steel wire is slipping, broken and the elongation is not enough, it is necessary to find out the reasons in time and report to the supervisor to take corresponding measures before proceeding to the next construction.

Outside the anchorage (leave 3~5cm outside the anchorage), the redundant steel strand shall be cut by a grinding wheel cutter, and it is absolutely forbidden to use electric welding for burn cutting.

Within 24 hours after all prestressed steel bars are tensioned, hole grouting should be carried out, and the hole grouting sequence should be completed first and then completed. After the hole grouting, the cement slurry at the beam end should be washed immediately, and the dirt on the supporting plate, anchorage and end face concrete should be removed at the same time, and the end face should be chiseled off, and the end face steel mesh should be set up, and the end face should be closed with formwork pouring concrete.

(10) bracket removal

When the concrete strength of the beam reaches more than 90% of the design strength, and tension grouting is completed, the support can be unloaded only after the supervision instruction is obtained. Demolition sequence: abutment and pier → 1/4 span → middle span. There should be a certain time interval between unloading, and the loose wooden wedge should be tightened during the interval to ensure the implementation of the beam. When unloading the frame, special attention should be paid to the safety of construction work.

3. Personnel organization and equipment

(1) Organization arrangement of construction personnel

(a) Management organization of work area: director of work area: ×××

Technical responsibility: ×××××

On-site technical management: ×××

Safety management: ××××××

Equipment scheduling management: ×××××

Construction cooperation: ××××××

Civilized construction: ××××××

Material management: ×××××

Logistics support: ×××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××××

Engineering test: ×××××

Construction survey: ××××××

Documentary: ×××

(2) Site construction personnel arrangement

Responsible for construction: ××××××

On-site construction party: ××××××

Field tester: ××××××

On-site safety officer: ××××××

Electrician: ×××××

Steel bar production and installation: steel bar processing and production: 6 people.

Reinforcement installation: 12 persons.

Template production and installation: 6 template workers.