Continuous box girder cantilever casting prestress loss control and secondary tensioning construction method

Abstract

The invention discloses a continuous box girder cantilever casting prestress loss control and secondary tensioning construction method which is suitable for large-span prestressed concrete continuous box girder cantilever casting construction. According to the method, the material quality is controlled through special acceptance inspection of steel strands, anchorage devices and corrugated pipes; positioning steel bars are adopted to fix the corrugated pipes and seal the joints, so that hole friction loss is reduced; after the steel strands are woven and penetrated, the tensioning equipment is calibrated in a matched mode; after the concrete reaches 90% of the design strength and the age is larger than or equal to 7 days, longitudinal and transverse pre-stress one-time tensioning is conducted firstly, and then vertical pre-stress one-time tensioning is conducted; after the vertical prestress lags behind the three beam sections, the anchorage device is monitored to retract, and then secondary tensioning is conducted through the H-shaped supporting angle and the connector to eliminate retraction loss; and finally, pore passages are compacted by adopting a vacuum grouting process. The method can effectively control prestress loss caused by duct friction resistance, anchorage device retraction and the like, the performance and durability of a continuous box girder structure are improved, and the linear precision of a girder body is guaranteed.

Description

Technical Field

[0001] This invention relates to the field of bridge engineering construction technology, specifically to a method for controlling prestress loss and secondary tensioning in the cantilever casting of continuous box girders. Background Technology

[0002] In the cantilever casting construction of long-span continuous box girders, the prestressing system is the core to ensure the load-bearing capacity, crack resistance and linear quality of the beam, while prestress loss is a key issue affecting the prestressing effect and is directly related to structural safety and service life. In existing technologies, the prestress loss in cantilever casting of continuous box girders mainly comes from several aspects: First, duct friction loss. Positioning deviations during the installation of plastic corrugated pipes, poor joint sealing, or residual debris in the ducts can lead to an increase in the duct friction coefficient (usually above 0.2), far exceeding the design value of 0.17, resulting in actual tension stress lower than the design value. Second, anchorage retraction loss. Vertical prestressing often adopts a single tensioning process, and the retraction of anchorage wedges is usually 6-8mm, which cannot be eliminated by a single tensioning, resulting in significant vertical prestress loss and easy vertical cracks in the web. Third, steel strand slack loss. In current construction, improper control of the steel strand storage environment or failure to grout in time after tensioning can easily lead to a steel strand slack rate exceeding the design value of 0.03, further aggravating prestress loss. Fourth, grouting incomplete compaction loss. Traditional grouting processes lack vacuum assistance, and air can easily remain in the ducts, forming voids, leading to steel strand corrosion and continuous prestress loss during long-term use.

[0003] Furthermore, existing construction methods have numerous shortcomings: for example, corrugated pipe positioning relies solely on simple supports without a unified spacing standard, resulting in pipe axis deviations exceeding 1cm and increased duct friction; tensioning equipment is not calibrated according to specifications or the calibration period is too long, leading to tension force errors exceeding 5%; there is no hysteresis monitoring after vertical prestressing tensioning, making it impossible to detect anchorage retraction problems in a timely manner; grouting is delayed by more than 24 hours, and prolonged exposure of steel strands makes them prone to corrosion. These problems collectively result in the actual prestress value of continuous box girder cantilever casting being only 85%–90% of the design value, making the beam susceptible to excessive deflection, web cracking, and other hidden dangers, reducing structural service life and increasing subsequent maintenance costs.

[0004] To address the shortcomings of existing technologies, a systematic method for controlling prestress loss is urgently needed. This method, combined with targeted secondary tensioning techniques, should control losses throughout the entire process, from materials and construction to monitoring, to ensure the accuracy of prestress application in the cantilever casting of continuous box girders and safeguard structural performance. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for controlling prestress loss and secondary tensioning during cantilever casting of continuous box girders, thus solving the problems mentioned in the background section.

[0006] To achieve the above objectives, the present invention provides a method for controlling prestress loss and secondary tensioning during cantilever casting of continuous box girders, comprising the following steps:

[0007] S1. Steel strands, anchorages, and plastic corrugated pipes shall be graded and inspected. Steel strands shall pass tests for tensile strength, yield strength, elongation, and relaxation rate. Three coils shall be randomly selected from each batch for mechanical property tests. Anchorages shall be tested according to type for wedge hardness, anchor ring compressive strength, and anchoring efficiency coefficient. Five sets shall be selected from each batch for static load tests. Plastic corrugated pipes shall pass tests for wall thickness, ring stiffness, and 1-hour water pressure. Different specifications shall be sampled and inspected separately.

[0008] S2. For straight sections of longitudinal pipes, use HRB400Φ16 positioning bars with a spacing of 30-40cm, and for curved sections, use a spacing of 15-20cm, which are welded and fixed to the main bars. For transverse pipes, arrange them along the top slab with positioning bars spaced 0.5m apart and tied to the skeleton. For vertical pipes, arrange them along the web, with a spacing of 0.5m when the beam height is ≤3m and 0.3m when it is >3m, and weld the bottom to the bottom slab reinforcement. For joints, use a larger joint pipe, with 3 layers of sealing tape spirally wrapped around it, and a Φ20 vent hole at the highest point.

[0009] S3. Use a grinding wheel cutter to cut the material. The length = net length of the duct + 2 × (anchor thickness + jack working length + limit plate thickness + 60mm). Tie the cut with No. 20 iron wire. When bundling, straighten the steel strands. Tie the straight sections with No. 20 iron wire. For curved sections and within 3m at both ends, increase the spacing to 0.5m. Tighten the iron wire by wrapping it twice. Clean the hole with a Φ12mm through hole tool before threading. For straight sections, thread the bundle manually. For curved sections, use a winch for traction. Install a conical guide cap at the front end.

[0010] S4. Use YDC5500Q type jacks for longitudinal direction and YDC240Q type jacks for horizontal and vertical direction, equipped with 0.4 grade pressure gauges; calibrate in the metrology certification laboratory according to 0, 25%, 50%, 75%, and 100% rated tension force, hold the load for 3 minutes at each level, and draw calibration curves. The cycle is ≤6 months or 200 times. Recalibration is required after maintenance.

[0011] S5. Concrete curing and testing: Cover with geotextile and water for curing for ≥14 days within 12 hours after pouring. Water for ≥6 times a day for the first 7 days, and ≥3 times a day after 7 days. Monitor the temperature (internal temperature ≤75℃, internal surface temperature difference ≤25℃, cooling rate ≤2℃ / d). Leave 3 sets of test blocks under the same conditions and 2 sets of elastic modulus test blocks. Tensioning is carried out after the strength reaches 90% of the design strength, the age is ≥7 days, and the elastic modulus reaches 90% of the design strength.

[0012] S6. Prestressed graded tensioning: Symmetrical tensioning at both ends longitudinally, in the following sequence: web first, then top slab; bottom first, then top; long tendons first, then short tendons. Program: 0→0.1 (measure) → 0.2 (measure) → 0.6 (measure) → (1339.2 MPa, measured) (Holding load for 5 minutes) → Anchoring, actual elongation Deviation ±6%; Lateral single-end staggered tensioning (from the centerline to both sides), program: 0→0.1 → (1302MPa, held for 5min) → Anchoring, 24h recheck for shrinkage ≤6mm; Vertical tensioning, procedure: 0 → 0.1 → (1209MPa, holding load for 5min) → Anchoring;

[0013] S7. Vertical prestressing lag monitoring: Lag 3 beam segment cycles, select 3 sections for each beam segment, install 1 HJ3401 stress sensor on each side of the web, and simultaneously measure the anchorage retraction with vernier calipers; record at 9 am every day when the environment is stable, for 3 consecutive days, and start secondary tensioning when the stress loss exceeds 10% or the retraction is >6mm.

[0014] S8. Vertical secondary tensioning: Install H-shaped support angles and fix them with M20 expansion bolts. Connect the anchor ring to the Φ32mm precision-rolled threaded steel tensioning rod using a No. 45 steel connector; tensioning sequence: 0→0.5 → →Tighten the nut→Unload, control the retraction ≤3mm;

[0015] S9. Vacuum grouting: Construction should be carried out within 24 hours after tensioning. First, flush the duct with water, then dry it with oil-free compressed air. Use P.O42.5 cement for the cement grout, add 3% high-efficiency water-reducing agent and 10% expansion agent, and stir at high speed for ≥8 minutes. Start the vacuum pump to -0.06~-0.1MPa and stabilize for 3 minutes, then grout. After the grout comes out of the high-end exhaust hole, stabilize the pressure for 4 minutes. Leave 3 sets of 40×40×160mm test blocks per shift.

[0016] S10. Anchor sealing construction: After grouting for 7 days, roughen the surface, tie HRB400Φ12 steel mesh, and weld it to the beam reinforcement; install 15mm bamboo plywood formwork, pour C50 concrete, vibrate with Φ30mm vibrator, cover with film + geotextile for curing for ≥7 days, remove the formwork when the strength reaches 75%, and the surface should be flush with the beam with a deviation of ≤3mm.

[0017] Optionally, the steel strand is a Φs15.2 high-strength, low-relaxation type with a nominal diameter of 15.2 mm, a cross-sectional area of ​​139 mm², and a weight of 2000 ± 50 kg per coil. The relaxation test is conducted under 1000 h and 0.7 Fpk conditions. The anchorage includes a working anchor, a tool anchor, and a limiting plate. The working anchor and tool anchor wedges must not be mixed. The groove depth of the limiting plate is 1-2 mm greater than the exposed length of the wedge. The outer wall of the plastic corrugated pipe has annular reinforcing ribs with a spacing of ≤500 mm, each 6 m long, with a positioning step at the interface. The low-temperature impact test is conducted at -20℃, and there is no cracking after the impact.

[0018] Optionally, in step S2, the corrugated pipe is pre-assembled before installation, extending 5-10mm into the anchor plate when connected, with an axial deviation ≤3mm; the anchor spiral reinforcement uses Φ12mm steel bars, with a diameter 1.5 times that of the anchor plate, ≥5 turns, a spacing of 50mm, and one end ≤50mm from the surface of the anchor plate; the allowable positioning deviation is: longitudinal ±10mm, transverse ±5mm, elevation ±5mm, and the coordinate deviation of the starting and ending points of the curved section ≤5mm. After installation, the total station is used to check each point.

[0019] Optionally, in step S4, the jack and pressure gauge must be matched one-to-one and must not be mixed. Before tensioning, check the oil level and the sealing of the oil pipe. The pressure gauge pointer must be zeroed. The calibration report includes the equipment number, date, validity period, curve equation and maximum error. The reading is converted on site according to the curve. If the jack leaks oil, the piston is stuck or the pressure gauge is abnormal, stop using it immediately and recalibrate.

[0020] Optionally, in step S6, the theoretical elongation of the longitudinal prestress is calculated using the following formula:

[0021] The formula for calculating the theoretical elongation of longitudinal prestress is as follows:

[0022] Formula 1: Theoretical elongation

[0023]

[0024] Formula 2: Average tension

[0025]

[0026] The meaning and values ​​of each parameter are as follows:

[0027] Theoretical elongation. For average tension, For the tensioning end, Let be the coefficient of influence of local deviation per meter of duct on friction, taken as 0.0015. The length of the duct from the tensioning end to the calculated cross-section. Let be the coefficient of friction between the steel strand and the duct wall, taken as 0.17. The angle of the duct plane curve from the tensioning end to the calculated section is the wrap angle. The cross-sectional area of ​​the steel strand is taken as 139 mm². 2 , The elastic modulus of the steel strand is taken as 1.95*10. 5 MPa;

[0028] When the deviation exceeds the limit, check the elastic modulus of the steel strand (test 3 strands per batch), the unobstructedness of the duct, and the equipment calibration value. If necessary, conduct a duct friction test.

[0029] Optionally, in step S8, the flatness of the H-shaped support angle is ≤2mm, and the contact part with the tension rod is milled flat; the connector thread accuracy is 6H / 6g, and grease is applied before installation; the beam segment elevation is measured simultaneously with a level during tensioning, and the process is paused when the single change exceeds 5mm; after completion, excess tension rods are cut off, and anti-corrosion coating is applied to the anchor head and exposed steel strands.

[0030] Optionally, in step S9, the cement grout must be injected into the duct within 30 minutes after mixing; otherwise, it shall be discarded. The grouting system includes a vacuum pump, a grouting pump, and a 3mm aperture filter screen. The length of the high-pressure hose shall be ≤30m, and the joints shall be fastened with clamps. If the pressure drops suddenly, check for blockages or leaks. The same duct shall not be grouted more than twice. The test block shall be cured in an environment of 20±2℃ and ≥95% humidity, and the compressive strength shall be measured at a loading rate of 5kN / s.

[0031] Optionally, in step S3, the steel strands should be stored ≥30cm off the ground, covered with a rainproof cloth, and stored for ≤6 months. Strands with rust exceeding 5% of their surface area are prohibited from use. When encountering resistance during stranding, use a hole clearer to unblock the blockage. Do not force tensioning. For multiple strands, follow the corresponding hole number to avoid mis-stretching. The guide cap should be wrapped with 3mm thick rubber to prevent scratching the inner wall of the corrugated pipe.

[0032] Optionally, in step S7, the stress sensor is installed at 1 / 2 of the web thickness, 30-50mm away from the vertical reinforcement, and tied to the skeleton. During pouring, the vibrator should be avoided from touching it. The monitoring data includes stress value, shrinkage, ambient temperature and beam segment number. The average value is taken for 3 consecutive days. When the data is abnormal, the monitoring frequency is increased to twice a day.

[0033] Optionally, in step S10, the base layer should be moistened to a depth of ≥10mm before sealing the anchor, the gaps in the formwork should be sealed with sealant, and the concrete surface should be smoothed. During the curing period, condensation should be kept inside the membrane, and the anchoring area should be protected from collisions. After demolding, the surface flatness should be checked, and if the honeycomb pitted area exceeds 1%, it should be repaired with epoxy resin.

[0034] This invention provides a method for controlling prestress loss and secondary tensioning during cantilever casting of continuous box girders, which has the following beneficial effects:

[0035] The prestress loss control and secondary tensioning construction method for cantilever casting of continuous box girders uses corrugated pipe sealing joints and precise positioning to control the duct friction coefficient to within 0.17, reducing friction loss; secondary tensioning eliminates vertical anchor shrinkage loss, ensuring the actual shrinkage is ≤3mm; vacuum grouting ensures duct compaction, avoiding long-term losses caused by steel strand corrosion; ultimately, the actual prestress values ​​of each part of the continuous box girder reach the design values, ensuring the full functioning of the prestressing system.

[0036] Symmetrical tensioning and elongation control of longitudinal and transverse prestressing ensure uniform stress distribution in the beam and reduce local stress concentration caused by prestressing deviations. Vertical secondary tensioning solves the problem of web cracking due to insufficient prestress. Combined with the synergistic effect of web reinforcement and prestressing, it significantly improves the crack resistance of the beam. Due to the precise application of prestressing during the closure stage, the beam alignment deviation is ≤3mm, avoiding excessive deflection in the later stage, ensuring the load-bearing capacity and stiffness stability of the continuous box girder, and meeting long-term operation requirements.

[0037] This invention strictly controls the storage environment of the steel strands and completes grouting within 24 hours after tensioning to prevent the steel strands from being exposed and corroded; the sealing concrete is tightly bonded to the beam to prevent rainwater from seeping into the ducts; secondary tensioning eliminates stress concentration caused by anchor retraction and reduces the generation of micro-cracks inside the beam; these measures together extend the service life of the continuous box girder, reduce maintenance work such as later crack repair and prestressing supplementation, and reduce the total life cycle cost.

[0038] Regular calibration and full-process monitoring of tensioning equipment prevent tensioning accidents caused by equipment errors or abnormal stress; standardized construction of corrugated pipes and steel strands reduces on-site rework; the lag period of secondary tensioning matches the construction progress of beam segments, without taking up additional construction time, ensuring safety while ensuring that the cantilever casting of continuous box girders proceeds as planned, thus improving overall construction efficiency. Detailed Implementation

[0039] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0040] This invention provides a technical solution: a method for controlling prestress loss and secondary tensioning during cantilever casting of continuous box girders, comprising the following steps:

[0041] S1. Full Acceptance of Prestressed Materials: Steel strands, anchorages, and plastic corrugated pipes are subject to graded inspection. Steel strands must pass tests for tensile strength (≥1860MPa), yield strength (≥1670MPa), elongation (≥3.5%), and relaxation rate (≤2.5%). Three strands from each batch are randomly selected for mechanical performance testing. Anchorages are tested according to type for wedge hardness (HV80-100), anchor ring compressive strength (≥500MPa), and anchoring efficiency coefficient (≥0.95). Five sets from each batch are selected for static load testing. Plastic corrugated pipes must pass tests for wall thickness (≥3mm), ring stiffness (≥8kN / m²), and 1-hour water pressure (0.1MPa without leakage). Different specifications (longitudinal 90 / 100mm round, transverse 60×19mm flat, vertical 50mm round) are randomly sampled for inspection.

[0042] The steel strand is Φs15.2 high-strength, low-relaxation type, with a nominal diameter of 15.2mm, a cross-sectional area of ​​139mm², and a weight of 2000±50kg per coil. The relaxation test is conducted under 1000h and 0.7fpk conditions. The anchorage includes a working anchor, a tool anchor, and a limiting plate. The working anchor and tool anchor wedges must not be mixed. The groove depth of the limiting plate is 1-2mm greater than the exposed length of the wedge. The outer wall of the plastic corrugated pipe has annular reinforcing ribs with a spacing of ≤500mm, each 6m long, with a positioning step at the joint. The low-temperature impact test is conducted at -20℃, and there is no cracking after the impact.

[0043] S2. Three-dimensional positioning installation of corrugated pipes: For straight sections of longitudinal pipes, use HRB400Φ16 positioning steel bars with a spacing of 30-40cm; for curved sections (R≤500m), use a spacing of 15-20cm, and weld them to the main reinforcement bars. For transverse pipes, arrange them along the top slab with positioning steel bars spaced 0.5m apart and tied to the frame. For vertical pipes, arrange them along the web; for beams ≤3m high, use a spacing of 0.5m; for beams >3m high, use a spacing of 0.3m. Weld the bottom to the bottom slab reinforcement bars. For joints, use a larger joint pipe (length 6 times the outer diameter), and spirally wrap 3 layers of sealing tape (overlap ≥1cm). A Φ20 vent hole (with a stop valve) is provided at the highest point.

[0044] The corrugated pipe is pre-assembled before installation, and when connected to the anchor plate, it extends 5-10mm inward, with an axial deviation of ≤3mm; the spiral reinforcement under the anchor uses Φ12mm steel bars, with a diameter 1.5 times that of the anchor plate, ≥5 turns, and a spacing of 50mm, with one end ≤50mm from the surface of the anchor plate; the allowable positioning deviation is: longitudinal ±10mm, transverse ±5mm, elevation ±5mm, and the coordinate deviation of the starting and ending points of the curved section is ≤5mm. After installation, it is checked point by point with a total station.

[0045] S3. Steel strand bundling and threading: Cut the strands using an abrasive wheel cutter. The length = net length of the duct + 2 × (anchor thickness + jack working length + limit plate thickness + 60mm). Tie the cut ends with No. 20 iron wire (≥5cm). When bundling, straighten the steel strands. Tie the straight sections with No. 20 iron wire (1.2-1.5m spacing). For curved sections and within 3m at both ends, increase the spacing to 0.5m. Tighten the wire by wrapping it twice. Before threading, clean the holes with a Φ12mm through-hole tool. For straight sections, thread the strands manually. For curved sections, use a winch for traction. Install a conical guide cap (diameter less than 5mm from the pipe) at the front end.

[0046] When storing steel strands, they should be suspended ≥30cm off the ground and covered with a rainproof cloth. Storage time should be ≤6 months. Strands with rust exceeding 5% of their surface area should be prohibited. When encountering resistance during stranding, use a hole clearer to unblock it. Do not force tensioning. When stranding multiple strands, follow the corresponding hole number to avoid mis-stretching. The guide cap should be wrapped with 3mm thick rubber to prevent scratching the inner wall of the corrugated pipe.

[0047] S4. Tensioning equipment calibration: Use YDC5500Q type jacks (5500kN) for longitudinal direction and YDC240Q type jacks (240kN) for transverse and vertical direction, equipped with 0.4 grade pressure gauges (range 1.5-2 times the maximum tension force); calibrate in a metrology certification laboratory according to 0, 25%, 50%, 75%, and 100% rated tension force, holding the load for 3 minutes at each level, and plotting calibration curves (R²≥0.999). The cycle should be ≤6 months or 200 times. Recalibration is required after maintenance.

[0048] The jack and pressure gauge must be matched one-to-one and must not be mixed. Before tensioning, check the oil level (3-5cm from the top of the oil tank) and the oil pipe sealing. The pressure gauge pointer must be zeroed. The calibration report includes the equipment number, date, validity period, curve equation and maximum error. On-site, convert the reading according to the curve (accurate to 0.1MPa). If the jack leaks oil, the piston is stuck or the pressure gauge is abnormal, stop using it immediately and recalibrate.

[0049] S5. Concrete curing and testing: Cover with geotextile and water for curing for ≥14 days within 12 hours after pouring. Water for ≥6 times a day for the first 7 days, and ≥3 times a day after 7 days. Monitor the temperature (internal temperature ≤75℃, internal surface temperature difference ≤25℃, cooling rate ≤2℃ / d). Leave 3 sets of test blocks under the same conditions and 2 sets of elastic modulus test blocks. Tensioning is carried out after the strength reaches 90% of the design strength, the age is ≥7 days, and the elastic modulus reaches 90% of the design strength.

[0050] S6. Prestressed graded tensioning: Symmetrical tensioning at both ends longitudinally, in the following sequence: web first, then top slab; bottom first, then top; long tendons first, then short tendons. Program: 0→0.1 (Measure L1) → 0.2 (Measure L2) → 0.6 (Measure L3) → (1339.2 MPa, measured at L4, held for 5 min) → Anchoring, actual elongation = (L4-L1)+(L2-L1), deviation ±6%; transverse single-end staggered tensioning (from the centerline to both sides), program: 0→0.1 → (1302MPa, held for 5 min) → Anchoring, recheck after 24 hours: shrinkage ≤ 6mm; Vertical tensioning (from root to end), procedure: 0 → 0.1 → (1209MPa, holding load for 5min) → Anchoring;

[0051] The theoretical elongation of longitudinal prestress is calculated using the following formula:

[0052] The formula for calculating the theoretical elongation of longitudinal prestress is as follows:

[0053] Formula 1: Theoretical elongation

[0054]

[0055] Formula 2: Average tension

[0056]

[0057] The meaning and values ​​of each parameter are as follows:

[0058] Theoretical elongation (mm);

[0059] Average tension (N);

[0060] Tension force at the tensioning end (N);

[0061] The coefficient of influence of local deviation per meter of duct on friction is taken as 0.0015;

[0062] : Length of the duct from the tensioning end to the calculated section (m);

[0063] The coefficient of friction between the steel strand and the duct wall is taken as 0.17.

[0064] : Angle of wrap of the duct plane curve from the tensioning end to the calculated section ;

[0065] Cross-sectional area of ​​steel strand (mm²), taken as 139mm. 2 ;

[0066] : Elastic modulus of steel strand (MPa), taken as 1.95*10 5 MPa;

[0067] When the deviation exceeds the limit, check the elastic modulus of the steel strand (test 3 strands per batch), the unobstructedness of the duct, and the equipment calibration value. If necessary, conduct a duct friction test.

[0068] S7. Vertical prestressing lag monitoring: Lag for 3 beam segment cycles (approximately 21 days), select 3 sections for each beam segment, and install 1 HJ3401 stress sensor (range 0-20MPa, accuracy ±0.2MPa) on each side of the web. Simultaneously measure the anchorage retraction using a vernier caliper (0.02mm accuracy); record at 9 am daily when the environment is stable, for 3 consecutive days. If the stress loss exceeds 10% or the retraction is >6mm, initiate secondary tensioning.

[0069] The stress sensor is installed at 1 / 2 of the web thickness, 30-50mm away from the vertical reinforcement, and is tied and fixed to the skeleton. Avoid contact with the vibrator during pouring. The monitoring data includes stress value, shrinkage, ambient temperature and beam segment number. The average value is taken for 3 consecutive days. When the data is abnormal, the monitoring frequency is increased to twice a day (9 am and 3 pm).

[0070] S8. Vertical Secondary Tensioning: Install H-shaped support angles (Q235 steel, 20mm thick), fix with M20 expansion bolts (torque 300N·m), connect the anchor ring to the Φ32mm precision-rolled threaded steel tensioning rod (tensile strength ≥830MPa) through a No. 45 steel connector (HB220-250); tensioning sequence: 0→0.5 (Hold the load for 1 minute) → (Hold the load for 3 minutes) → Tighten the nut (torque 400 N·m) → Unload, control the retraction ≤ 3 mm;

[0071] The flatness of the H-type support angle is ≤2mm, and the contact part with the tension rod is milled flat; the connector thread accuracy is 6H / 6g, and grease is applied before installation; during tensioning, the beam section elevation is measured simultaneously with a level (±1mm accuracy), and the process is paused when the single change exceeds 5mm; after completion, excess tension rods are cut off (exposed ≤50mm), and the anchor head and exposed steel strands are coated with anti-corrosion paint.

[0072] S9. Vacuum grouting: Construction should be carried out within 24 hours after tensioning. First, flush the duct with water (0.3MPa), then dry it with oil-free compressed air (0.5MPa). Use P.O42.5 cement (water-cement ratio 0.36-0.38) for the cement grout, add 3% high-efficiency water-reducing agent and 10% expansion agent (flowability 18-22s), and mix at high speed (≥1500r / min) for ≥8min. Start the vacuum pump to -0.06~-0.1MPa and stabilize for 3min, then grout (0.5-0.7MPa). After the grout comes out of the high-end vent hole, stabilize the pressure for 4min. Leave 3 sets of 40×40×160mm test blocks (28d strength ≥30MPa) per shift.

[0073] The cement grout must be injected into the duct within 30 minutes after mixing; otherwise, it will be discarded. The grouting system includes a vacuum pump (ultimate vacuum ≤ -0.1MPa), a grouting pump (maximum pressure 1.5MPa), and a 3mm pore size filter screen. The high-pressure hose length is ≤30m (working pressure ≥1MPa), and the joints are fastened with clamps. Check for blockages or leaks when the pressure drops suddenly. The same duct should not be grouted more than twice. The test blocks should be cured in an environment of 20±2℃ and ≥95% humidity, and the compressive strength should be measured at a loading rate of 5kN / s.

[0074] S10. Anchor sealing construction: After grouting for 7 days, roughen the surface (depth ≥ 5mm), tie HRB400 Φ12 steel mesh (200×200mm), and weld it to the beam reinforcement (weld spacing ≤ 300mm); install 15mm bamboo plywood formwork (apply release agent), pour C50 concrete (slump 180±20mm), vibrate with Φ30mm vibrator (≥ 50mm from the formwork), cover with film + geotextile for curing for ≥ 7 days, remove the formwork when the strength reaches 75%, and the surface should be flush with the beam with a deviation ≤ 3mm;

[0075] Before sealing the anchor, the base layer should be moistened to a depth of ≥10mm (without standing water). The gaps in the formwork should be sealed with sealant, and the concrete surface should be smoothed. During the curing period, keep the membrane with condensation and avoid collisions with the anchor sealing area. After demolding, check the surface flatness (deviation ≤3mm). If the honeycomb pitted area exceeds 1%, repair it with epoxy resin.

[0076] Example

[0077] Taking the construction of the (36+4×60+36)m continuous box girder of Beixi Bridge in Chaozhou City as an example, the method of this invention is applied as follows:

[0078] The main span of the bridge is a single-box, single-cell, straight-web box girder with a single span width of 16.5m. It adopts a three-dimensional prestressing system: the longitudinal prestressing consists of 15-19 strands of Φ15.2 steel strands. =1339.2MPa; the transverse direction consists of 3 strands of Φs15.2 steel strands. =1302MPa; vertically, it consists of 3 strands of Φ15.2 steel wire. =1209MPa.

[0079] During construction, the corrugated pipes used longitudinally are of type SBG-90Y / SBG-100Y, transversely are of type SBGB-60×19, and vertically are of type SBG-50Y. Tensioning equipment includes 550t jacks (YDC5500Q) and YDC240Q type jacks, with pressure gauges of accuracy class 0.4. Grouting uses ZYJ-400 type intelligent grouting equipment.

[0080] After the first vertical prestressing tensioning, a second tensioning was performed after a delay of 3 beam segments (approximately 21 days). Monitoring showed that the anchorage shrinkage decreased from 6.5 mm to 2.8 mm. After grouting of the ducts, ultrasonic testing was conducted, and the compaction rate reached over 98%. There were no vertical cracks in the web of the beam, and the elevation deviation at the closure joint was ≤3 mm, meeting the design requirements.

[0081] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for controlling prestress loss and secondary tensioning during cantilever casting of continuous box girders, characterized by: Includes the following steps: S1. Steel strands, anchorages, and plastic corrugated pipes shall be graded and inspected. Steel strands shall pass tests for tensile strength, yield strength, elongation, and relaxation rate. Three coils shall be randomly selected from each batch for mechanical property tests. Anchorages shall be tested according to type for wedge hardness, anchor ring compressive strength, and anchoring efficiency coefficient. Five sets shall be selected from each batch for static load tests. Plastic corrugated pipes shall pass tests for wall thickness, ring stiffness, and 1-hour water pressure. Different specifications shall be sampled and inspected separately. S2. For straight sections of longitudinal pipes, use HRB400Φ16 positioning bars with a spacing of 30-40cm, and for curved sections, use a spacing of 15-20cm, which are welded and fixed to the main bars. For transverse pipes, arrange them along the top slab with positioning bars spaced 0.5m apart and tied to the skeleton. For vertical pipes, arrange them along the web, with a spacing of 0.5m when the beam height is ≤3m and 0.3m when it is >3m, and weld the bottom to the bottom slab reinforcement. For joints, use a larger joint pipe, with 3 layers of sealing tape spirally wrapped around it, and a Φ20 vent hole at the highest point. S3. Use a grinding wheel cutter to cut the material. The length = net length of the duct + 2 × (anchor thickness + jack working length + limit plate thickness + 60mm). Tie the cut with No. 20 iron wire. When bundling, straighten the steel strands. Tie the straight sections with No. 20 iron wire. For curved sections and within 3m at both ends, increase the spacing to 0.5m. Tighten the iron wire by wrapping it twice. Clean the hole with a Φ12mm through hole tool before threading. For straight sections, thread the bundle manually. For curved sections, use a winch for traction. Install a conical guide cap at the front end. S4. Use YDC5500Q type jacks for longitudinal direction and YDC240Q type jacks for horizontal and vertical direction, equipped with 0.4 grade pressure gauges; calibrate in the metrology certification laboratory according to 0, 25%, 50%, 75%, and 100% rated tension force, hold the load for 3 minutes at each level, and draw calibration curves. The cycle is ≤6 months or 200 times. Recalibration is required after maintenance. S5. Concrete curing and testing: Cover with geotextile and water for curing for ≥14 days within 12 hours after pouring. Water for ≥6 times a day for the first 7 days, and ≥3 times a day after 7 days. Monitor the temperature (internal temperature ≤75℃, internal surface temperature difference ≤25℃, cooling rate ≤2℃ / d). Leave 3 sets of test blocks under the same conditions and 2 sets of elastic modulus test blocks. Tensioning is carried out after the strength reaches 90% of the design strength, the age is ≥7 days, and the elastic modulus reaches 90% of the design strength. S6. Prestressed graded tensioning: Symmetrical tensioning at both ends longitudinally, in the following sequence: web first, then top slab; bottom first, then top; long tendons first, then short tendons. Program: 0→0.1 (measure) → 0.2 (measure) → 0.6 (measure) → (1339.2 MPa, measured) (Holding load for 5 minutes) → Anchoring, actual elongation Deviation ±6%; Lateral single-end staggered tensioning (from the centerline to both sides), program: 0→0.1 → (1302MPa, held for 5min) → Anchoring, 24h recheck for shrinkage ≤6mm; Vertical tensioning, procedure: 0 → 0.1 → (1209MPa, holding load for 5min) → Anchoring; S7. Vertical prestressing lag monitoring: Lag 3 beam segment cycles, select 3 sections for each beam segment, install 1 HJ3401 stress sensor on each side of the web, and simultaneously measure the anchorage retraction with vernier calipers; record at 9 am every day when the environment is stable, for 3 consecutive days, and start secondary tensioning when the stress loss exceeds 10% or the retraction is >6mm. S8. Vertical secondary tensioning: Install H-shaped support angles and fix them with M20 expansion bolts. Connect the anchor ring to the Φ32mm precision-rolled threaded steel tensioning rod using a No. 45 steel connector; tensioning sequence: 0→0.5 → →Tighten the nut→Unload, control the retraction ≤3mm; S9. Vacuum grouting: Construction should be carried out within 24 hours after tensioning. First, flush the duct with water, then dry it with oil-free compressed air. Use P.O42.5 cement for the cement grout, add 3% high-efficiency water-reducing agent and 10% expansion agent, and stir at high speed for ≥8 minutes. Start the vacuum pump to -0.06~-0.1MPa and stabilize for 3 minutes, then grout. After the grout comes out of the high-end exhaust hole, stabilize the pressure for 4 minutes. Leave 3 sets of 40×40×160mm test blocks per shift. S10. Anchor sealing construction: After grouting for 7 days, roughen the surface, tie HRB400Φ12 steel mesh, and weld it to the beam reinforcement; install 15mm bamboo plywood formwork, pour C50 concrete, vibrate with Φ30mm vibrator, cover with film + geotextile for curing for ≥7 days, remove the formwork when the strength reaches 75%, and the surface should be flush with the beam with a deviation of ≤3mm.

2. The method according to claim 1, characterized in that, In step S1, the steel strand is a Φs15.2 high-strength, low-relaxation type with a nominal diameter of 15.2 mm, a cross-sectional area of ​​139 mm², and a weight of 2000 ± 50 kg per coil. The relaxation test is conducted under conditions of 1000 h and 0.7 fpk. The anchorage includes a working anchor, a tool anchor, and a limiting plate. The working anchor and the tool anchor wedges must not be mixed. The groove depth of the limiting plate is 1 to 2 mm greater than the exposed length of the wedge. The outer wall of the plastic corrugated pipe has annular reinforcing ribs with a spacing of ≤500mm, each pipe is 6m long, and the interface has a positioning step. The low temperature impact test is carried out in an environment of -20℃, and there is no crack after the impact.

3. The method according to claim 1, characterized in that, In step S2, the corrugated pipe is pre-assembled before installation, extending 5-10mm into the anchor plate when connected, with an axial deviation ≤3mm; the anchor spiral reinforcement uses Φ12mm steel bars, with a diameter 1.5 times that of the anchor plate, ≥5 turns, 50mm spacing, and one end ≤50mm from the surface of the anchor plate; the allowable positioning deviation is: longitudinal ±10mm, transverse ±5mm, elevation ±5mm, and the coordinate deviation of the starting and ending points of the curved section ≤5mm. After installation, the total station is used to check each point.

4. The method according to claim 1, characterized in that, In step S4, the jack and pressure gauge must be matched one-to-one and must not be mixed. Before tensioning, check the oil level and the oil pipe sealing. The pressure gauge pointer must be zeroed. The calibration report includes the equipment number, date, validity period, curve equation and maximum error. On-site, convert the reading according to the curve. If the jack leaks oil, the piston is stuck or the pressure gauge is abnormal, stop using it immediately and recalibrate.

5. The method according to claim 1, characterized in that, In step S6, the theoretical elongation of the longitudinal prestress is calculated using the following formula: The formula for calculating the theoretical elongation of longitudinal prestress is as follows: Formula 1: Theoretical elongation Formula 2: Average tension The meaning and values ​​of each parameter are as follows: Theoretical elongation. For average tension, For the tensioning end, Let be the coefficient of influence of local deviation per meter of duct on friction, taken as 0.0015. The length of the duct from the tensioning end to the calculated cross-section. Let be the coefficient of friction between the steel strand and the duct wall, taken as 0.

17. The angle of the duct plane curve from the tensioning end to the calculated section is the wrap angle. The cross-sectional area of ​​the steel strand is taken as 139 mm². 2 , The elastic modulus of the steel strand is taken as 1.95*10. 5 MPa; When the deviation exceeds the limit, check the elastic modulus of the steel strand (test 3 strands per batch), the unobstructedness of the duct, and the equipment calibration value. If necessary, conduct a duct friction test.

6. The method according to claim 1, characterized in that, In step S8, the flatness of the H-shaped support angle is ≤2mm, and the contact part with the tension rod is milled flat; the thread accuracy of the connector is 6H / 6g, and grease is applied before installation; the elevation of the beam segment is measured simultaneously with a level during tensioning, and the process is paused when the single change exceeds 5mm; after completion, excess tension rods are cut off, and anti-corrosion coating is applied to the anchor head and exposed steel strands.

7. The method according to claim 1, characterized in that, In step S9, the cement grout must be injected into the duct within 30 minutes after mixing; otherwise, it will be discarded. The grouting system includes a vacuum pump, a grouting pump, and a 3mm pore size filter screen. The high-pressure hose is ≤30m in length, and the joints are fastened with clamps. If the pressure drops suddenly, check for blockages or leaks. The same duct should not be grouted more than twice. The test block is cured in an environment of 20±2℃ and ≥95% humidity, and the compressive strength is measured at a loading rate of 5kN / s.

8. The method according to claim 1, characterized in that, In step S3, the steel strands should be stored ≥30cm off the ground, covered with a rainproof cloth, and stored for ≤6 months. Strands with rust exceeding 5% of their surface area are prohibited from use. When encountering resistance during stranding, use a hole clearer to unblock the blockage. Do not force tensioning. For multiple strands, follow the corresponding hole number to avoid mis-stretching. The guide cap should be wrapped with 3mm thick rubber to prevent scratching the inner wall of the corrugated pipe.

9. The method according to claim 1, characterized in that, In step S7, the stress sensor is installed at 1 / 2 of the web thickness, 30-50mm away from the vertical reinforcement, and tied to the skeleton. During pouring, avoid contact with the vibrator. The monitoring data includes stress value, shrinkage, ambient temperature and beam segment number. The average value is taken for 3 consecutive days. When the data is abnormal, the monitoring frequency is increased to twice a day.

10. The method according to claim 1, characterized in that, In step S10, the base layer must be moistened to a depth of ≥10mm before sealing the anchor, the gaps in the formwork must be sealed with sealant, and the concrete surface must be smoothed. During the curing period, condensation must be kept inside the membrane, and the sealing area must not be bumped. After demolding, the surface flatness must be checked, and if the honeycomb pitted area exceeds 1%, it must be repaired with epoxy resin.

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