1, when prestressed tendons are tensioned, the strength, elastic modulus and age of concrete should meet the design requirements. Before tensioning, the formwork that limits the displacement of members shall be removed.
2, prestressed tensioning sequence should comply with the design requirements. When the design has no requirements, it should be placed in stages, symmetrically and staggered.
3. The tensioning shall be carried out with a wedge or jack as a whole and meet the design requirements.
4, prestressed tensioning speed should not be too fast.
5. After tensioning, the sequence of cutting off prestressed tendons should start from Zhang Duan and then cut off at the other end.
Second, the tensioning of post-tensioned prestressed concrete members shall meet the following requirements:
1. Before prestressing, check the external dimensions of the members and the compactness of the concrete behind the anchor pad, and clean the mortar in the tunnel.
2, prestressed tensioning procedure should comply with the design requirements. The centralized prefabrication of concrete box girder should be divided into three stages: pre-tensioning, initial tensioning and final tensioning, and the centralized prefabrication of T beam should be divided into two stages: initial tensioning and final tensioning.
3. The strength, elastic modulus and age of concrete in each stage of prestress should meet the design requirements.
4, prestressed tensioning sequence should comply with the design requirements.
5, prestressed tensioning end settings should comply with the design requirements.
6. When prestressing, the anchor pad, anchorage and jack should be located on the same axis. When the two ends are tensioned, the two ends should be synchronized during the prestressing process, and the elongation at both ends is basically the same.
7. Prestressed tendons can only be anchored after the tension control stress is stable. After the anchorage is completed and inspected, the redundant prestressed tendons at the end can be cut. The redundant prestressed tendons at the cutting end shall comply with the provisions of Article 7.5.2, and the exposed length after cutting shall not be less than 1.5 times of its diameter and not less than 30 mm ..
8. The number of broken wires or slippage of prestressed tendons of post-tensioned prestressed members shall not exceed 5% of the total number of prestressed tendons. , and shall not be located on the same side of the structure, and there shall be no more than one broken wire in each bundle. (1) Preparation before prestress construction
(1) Preparation of prestressed materials: The products of A-controlled manufacturer that have been purchased for prestressed materials in this project have entered the site and passed the outsourcing test. Steel strand produced by Hebei Peng Jing Prestressed Steel Strand Co., Ltd., supporting anchorage produced by Baoding Tian Li Construction Machinery Manufacturing Co., Ltd., and channel grouting material produced by Shijiazhuang Yida Lian Heng Road and Bridge Materials Co., Ltd.
② Tension grouting equipment is in place: 2 YT-V CNC intelligent tensioning equipment produced by Hebei Yitie Electromechanical Technology Co., Ltd. are used for prestress tensioning equipment, and the supporting jack is 150t2. At present, the equipment matching calibration work has been completed, and the supporting tool anchor of tensioning equipment adopts the same anchorage and the supporting products of the same manufacturer; Grouting equipment adopts 1 SQL90 intelligent grouting equipment produced by Hebei Wuqiao Houde Construction Machinery Co., Ltd.
(3) After form removal, clean up the prestressed pipe and anchor mouth (redundant corrugated pipe and cement slurry) of box girder, ensure that the bell mouth of anchor mouth meets the design requirements, and at the same time check that the anchor pads at both ends should be perpendicular to the center of prestressed channel, so that the steel strand can pass through the beam.
(4) Calculate the theoretical elongation of each steel strand according to the test results of steel strands, and calculate the corresponding oil meter reading at each stage according to the calibration certificate of tensioning equipment.
(5) Before tensioning grouting operation, pre-job training of tensioning operators (intelligent tensioning equipment manufacturer training and on-site technical training) shall be completed, and they shall take up their posts after passing the examination.
2 Prestressed tensioning and grouting design and specification requirements
By mainly controlling the tensile stress and checking it with the actual elongation value, the tension of each grade can be accurately controlled, and the corresponding elongation can be measured and recorded. The difference between the actual elongation value and the theoretical value should be controlled within 6%, otherwise the tensioning should be suspended until the reason is found out and adjusted.
The tensioning sequence is N 1, N2 and N3, and the principle of symmetrical tensioning is adopted to prevent excessive stress on beam concrete, torsion and lateral bending of members and frame displacement.
(3) Processing and installation of steel strand
(1) The steel strand is cut by a grinding wheel cutter. The blanking length of steel strand is the length of channel plus the working length at both ends. The working length of both ends is 2000px, and the design length should be checked before blanking. (2) Binding: After the concrete strength of this project reaches 1 0,000% of the design strength, the steel strand bundle will penetrate into the tunnel (to prevent the steel strand bundle from rusting due to premature penetration). Bundle the steel strand bundle number before threading (every 3 meters with No.22 binding wire) to prevent the steel strand from intertwining in the tunnel. Before threading, grind the steel strand and wrap both ends with plastic tape to prevent the corrugated pipe from being damaged and blocked.
③ Protection of prestressed tendons after installation.
A, pipe opening should be sealed to prevent moisture from entering. When steam curing is used, tendons should not be installed before curing is completed. B, in any case, when welding near the components installed with prestressed tendons, all prestressed tendons and metal parts shall be protected to prevent the brittle fracture damage of steel strand caused by spatter of welding slag or electric arc.
C, tensioning and grouting operations shall be carried out in time after beam threading. In order to overcome the prestress loss of the anchor ring mouth, the project overstretched by 3%.
Prestressed tensioning process flow: threading steel strand → installing working anchor → installing supporting limit plate → installing jack → installing supporting tool anchor → symmetrically tensioning at both ends → tensile force 15%→ tensile force 30%→ tensile force 100%→ tensile force 103%→ anchorage with load for 2min → check the elongation value.
(1) After the concrete of box girder is dismantled, the anchor plate mouth shall be effectively closed in time to prevent curing water and sundries from entering.
⑵ Calculate the theoretical elongation of each steel beam according to the elastic modulus of the steel strand test report (attached with the theoretical elongation calculation), and at the same time calculate the oil meter reading corresponding to the stage tension according to the jack calibration result, which is convenient for checking the tension of the tensioning equipment during the tensioning process.
(3) After the box girder concrete is sprayed and cured for 7 days and the concrete strength reaches 100% of the design strength (the physical strength of the box girder is controlled by the concrete specimen cured under the same conditions), the tensioning operation shall be carried out.
(4) The tensioning sequence shall be carried out symmetrically at both ends of the beam at the same time according to the design requirements of N 1→N2→N3.
5] According to the number of steel beams, install working anchor rings and clips, tighten and pull out the corresponding working clips with φ20 steel pipes one by one, and install supporting limit plates, jacks and tool anchors.
[6] When installing the jack, ensure that the jack, working anchor and anchor pad are concentric and perpendicular to the anchor pad.
First, after tensioning, check the tightness between the working anchor and the anchor pad at any time, and adjust it in time after suspension when problems are found.
Being single beam tensioning immediately after the completion of the inspection of the actual elongation, and theoretical elongation deviation should be within the scope of plus or minus 6%, after confirmed with load anchorage for 2min. If it exceeds the range of 6%, the tension should be stopped and the reason should be found before continuing.
Levies after the completion of single beam tensioning, check that the top surface of the clamping piece should be flush, and at the same time, carefully check whether there is any sliding wire or broken wire, and proceed to the next beam tensioning after it is confirmed to be qualified.
⑽ hole grouting shall be carried out immediately after the completion of single box girder tensioning, and shall not exceed 24h at the latest.
⑾ In order to ensure the construction safety, the steel beam cutting in the working section should be carried out after the hole grouting is completed and the strength reaches 80%. Cutting should be done with a grinding wheel saw or a toothless saw. After cutting the steel strand, the anchorage shall be controlled at 3- 125px, and arc cutting is strictly prohibited. In order to ensure the compactness of hole grouting, secondary grouting technology is adopted in this project.
(1) Before grouting, flush the pipeline with high-pressure water, and dry the channel with oil-free high-pressure air.
(2) Before grouting, carefully block the anchorage and around the clip with putty to prevent slurry leakage around the clip and affect the grouting density of the channel.
(3) This project adopts the channel grouting material with special prestressed channel, and the water-binder ratio is 0.28. Use a high-speed mixer to stir (the rotating speed is not lower than 1000 rpm), and the stirring time is not less than 8 minutes. The index of cement slurry should meet the following requirements: the bleeding rate should not be greater than 2% within 3 hours after mixing, and finally not greater than 3%. The cement slurry will absorb all exudation within 24 hours, and the consistency will be controlled at 65438.
(4) The piston grouting pump is used for grouting, and the slurry mixed before grouting must meet the slurry dosage of one channel to ensure continuous grouting. Grouting should be carried out from the low end to the high end, and the grouting control pressure should be 0.5-0.7MPa. After the fully thick slurry flows out of the slurry outlet, close the slurry outlet valve, maintain the pressure not less than 0.5MPa for not less than 5 minutes, and then close the slurry inlet valve for the next hole grouting.
(5) After an interval of 30-45 minutes, carry out secondary pressure compensation to ensure that the mud in the hole is dense.
[6] After grouting is completed, it is forbidden to shake or disturb the beam body before the slurry strength reaches 80%.
After the slurry strength reaches 80%, cut the steel strand in the working section with a toothless saw, and reserve a length of 3- 125px outside the anchorage.
Being box girder prestressed duct grouting according to each beam lien not less than three groups of standard maintenance specimens, used to test the strength of grouting. End beam anchor sealing should be completed before hoisting. When sealing the anchor, it must be supported by shaped steel formwork and vibrated with concrete. When sealing the anchor, strictly control the length of beam and slab. The position, height and angle of embedded steel bars in the expansion joint of end beam should meet the design requirements to ensure the good connection between expansion joint and embedded steel bars; Template requirements are firm, no deformation, no mildew; Otherwise, it must be reworked.