Application of vacuum assisted grouting in Nanjing Changqiao Third Bridge

Hole. For curved, U-shaped, vertical prestressed tendons can better reflect its superiority.

2 Application of vacuum assisted grouting process in Nanjing Sanqiao The design of the box beam with beam height 2.8111 and standard span 5111 was adopted in the 01th Contract Section of Nanjing Sanqiao Approach Bridge. ¥15-19, ¥115-12, -9, YMB15-4 (flat bellows) and other types of prestressed beams. The longest beam is 58.4m. The main prestressed materials that satisfy the vacuum assisted grouting process include plastic bellows and anchors with good airtightness. The main prestressed materials used in this project are shown in Table 1. Table 1 Prestressed Material Usage Table 1 Prestressed Steel Beam Model Bellows Model Anchorage Model Production Enterprise Model Production Enterprise Hefei Geological Anchorage Factory Hefei Ground Anchorage Factory Hefei Geology Factory Hefei Geological Broadcasting Plant Note: 1 The material model is the internal model of the production enterprise.

2 Anchors are specially manufactured by manufacturing companies for supporting vacuum grouting.

The hole-forming material for the prestressed steel strand used in this project is the PT-PLUS plastic bellows produced by VSL China Ltd. (VSL China). Compared with the metal bellows, it has the following advantages: the frictional resistance is small, and its friction coefficient /X is generally in the range of 0.12. The deviation coefficient of the pipeline k is 0.0008. Corrosion resistance, no rust, and it is conducive to the protection of steel strands.

Non-conducting, it can protect the pre-stressed tendons against stray current and prevent steel strands from hitting fire.

The FT-PUJS dipping bellows has high strength and stiffness and is not easily broken by vibrating rods.

Solderable connection, no need for additional fittings, low cost, low material loss, remaining stub tubes are still available.

The prestressed steel beam anchorage used in this project is Hefei geological anchorage plant, which is specially produced for this project. It has good air tightness. In order to achieve good air tightness, a cap () is added at the end of the anchor seat. An O-ring seal is installed between the anchor seat and the cap.

The assembly relationship of each part is as shown.

Selection of vacuum-assisted grouting equipment The important equipment that satisfies the vacuum-assisted grouting process are vacuum pumps () and grouting pumps (). The vacuum pump requires a vacuum capacity of -0.08-O.IMPa. The grouting pump adopts a screw type grouting pump with good airtightness. Its motor power is 3kW, the maximum pressure is 0.8MPa, and the grouting capacity is 3m3/h. The grouting pump can continuously press the slurry into the pipeline so as to avoid the entry of air. Piston type squeegee pump is easy to put air into the slurry, so it is generally not used.

a) Tensile end caps; b) Lateral beam (flat anchor) caps; c) Connector anchor caps (half) 3S cap Clip anchors Seals Seals Strand anchors Helicoid joints Ripple Sudden Vacuum pump grout The design of cement slurry is the key to vacuum assisted grouting. Improper design of the slurry will cause grouting failure or air bubbles in the slurry within the pipeline and are not dense. The preparation of vacuum auxiliary pressurizing slurry must improve the performance of the cement slurry, reduce the water-cement ratio, reduce porosity, bleeding, eliminate the phenomenon of segregation, and reduce and compensate the shrinkage deformation of cement slurry in the process of condensation to prevent the production of cracks. Has a high compressive strength and effective bond strength.

In order to achieve the above-mentioned slurry properties, the HF type cement slurry special additive developed by VSL was selected. This additive has functions of reducing water, retarding and micro-expansion (does not contain aluminum powder), and improves fullness. . At the same time, good plasticizers in the admixture allow the slurry to maintain a sufficiently colloidal liquid state so that the slurry has excellent resistance to segregation, resistance to bleeding and shrinkage resistance. The slurry prepared at Nanjing Sanqiao Approach Bridge D1 contract section uses a water-cement ratio of 0.33 to 0.34 and an admixture of 4 to 5%. The cement used is P.042.5 cement produced by Nanjing Conch Cement Plant. The configuration of the mud body has the following characteristics: -2000 stipulates that the water-cement ratio of ordinary grouting slurry generally takes 0.4 to 0.45) fluidity: 13S18S; within 45 minutes, the consistency of the slurry should not change more than 2S. Bleeding rate: After the slurry is mixed for 3 hours, its bleeding rate is less than 2%; and the bleeding should be completely absorbed by the slurry within 24 hours.

Retarding time: The initial setting time is not less than 3 hours and the final setting time should be greater than 17 hours.

Compressive strength: Under standard curing conditions, the strength at 7 days of age should not be less than 25MPa, after 28 days of age, the compressive strength of the slurry should be not less than 50MPa. 4 The construction operation has excellent cement paste, but also can not Ensure the success of vacuum-assisted pressure paddles. During construction, high-quality operators should also be selected, with detailed construction submissions and strict construction operations. Only strictly controlling every step of construction can achieve good grouting effect.

Preparation before construction: First, after the tensioning process is completed, the exposed strands are cut to ensure that the exposed length of the strands does not exceed 3 mm (determined by the anchors provided by each manufacturer). Then clean the anchor seat. In the sealing groove, evenly apply a layer of glass glue, install the seal ring and cap, screw the bolt into the screw hole and tighten, and place the exhaust hole vertically upwards.

Install grouting equipment and piping as shown in the figure (see the vacuum assisted grouting schematic diagram). The pipe connecting the paddle hole and the vacuum pump must be made of transparent plastic steel pipe. This will not only satisfy the vacuum condition, the tube will not be crushed, but also can easily observe the discharge of cement slurry. The length of the pipe from the slotted hole to the valve 1 (the position of the valve is shown in the vacuum auxiliary grouting principle diagram) is generally not less than 5m. This way, when the slurry is discharged, there is enough time to control the valve. Prevent the cement slurry from entering the vacuum pump.

Turn off valve 3, valve 4, open valve 1 and valve 2, and start the vacuum pump for vacuum. When the vacuum reaches -O.OSMPa, valve 4 can be opened, and the grouting pump is started to start grouting. If the vacuum level does not meet the requirements, the vacuum must be stopped and the connections should be checked to make the airtightness meet the requirements.

Keep the vacuum pump on, and when the transparent tee between valve 1, valve 2 and valve 3 is observed to have slurry passing, immediately turn off the vacuum pump and valve 2, and open the valve 3. Observe the access to the waste barrel Pipe out of the situation, when the consistency of the paste and the press-in side, turn off the valve 3, still continue to grouting, so that the pipeline has 0.5 ~ 0.7MPa pressure, holding pressure for 2 minutes, then turn off the valve 4. Will eliminate the valve 1. The parts other than the valve 4 are removed and cleaned, and then another group of holes is grouted.

The vacuum grouting in the hole is continuous and completed once. If there is a fault that cannot be eliminated in time, immediately remove the grouting pipe, flush the hole with high-pressure water, and re-press the slurry after troubleshooting. The slurry temperature during grouting and grouting should not be too high, and it is better to control it below 35*. When the pressure drop, the cement slurry discharged is consistent with the consistency of the cement paste at the pressing end. After the initial setting of the cement slurry, the slurry in the discharge pipe and the grouting hole is very full (). Through the calculation of the actual amount of cement and the theoretical amount is also very consistent.

5 Conclusion Vacuum assisted grouting can make the cement slurry in the prestressed pipeline plump and dense, so that the prestressed tendons are not rusted and the durability of the structure is enhanced. Vacuum assisted grouting is a kind of concrete construction technology. Its successful implementation is mainly controlled by the following factors.

a) Slurry inside the slurry pipe; b) Slurry in the grout hole. Complete and create the vacuum condition in the pipe. This requires the installation of a special anchorage and a plastic corrugated pipe with good performance. There must be no water in the pipeline.

The design of the slurry with good performance.

Special equipment; such as vacuum pumps, grouting pumps, airtight connection pipes and pipe connections.

There are correct and strict construction procedures as well as rigorous and skilled operators and managers.

Satisfying all conditions and strictly controlling every step of construction, vacuum assisted grouting will be very easy to implement. At the same time, only the strict control of operations can achieve the purpose of vacuum assisted sizing and can truly reflect its superiority.

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