Application of vacuum assisted grouting technology in prestressed engineering

VSL Vacuum Assist Method 2.1 Principle Before grouting, vacuum chest is used to pump the air in the prestressed channel to make the vacuum in the channel more than 80%. Then the squeegee is used at the other end of the channel. A positive pressure greater than 0.7 MPa presses the cement slurry into the prestressed tunnel. Because there are only a few air holes in the channel, it is difficult to form bubbles; at the same time, due to the positive and negative pressure difference between the channel and the squeegee, the plumpness and density of the hole paddle are greatly improved. In the cement slurry, the water-cement ratio is reduced, special additives are added, the fluidity of the cement slurry is increased, and the shrinkage of the cement slurry is reduced.

2.2 Construction equipment vacuum suction device see. In addition to the traditional press frame construction equipment, vacuum assisted grouting also requires the following equipment: vacuum pumps, pressure gauges, and control disks; pressure bottles that act as protective barriers to prevent the slurry mix from entering the vacuum pump and damaging the vacuum pump; clean reinforced bleeding pipes , Can withstand large negative pressure, airtight valve and airtight anchor cap.

2.3 Process Flow A typical process flow involves the sealing of the valve at the exit and population of the cement slurry. A vacuum pump is connected to the non-grouted end and a grouting pump is connected to the propeller end.

Close all exhaust valves (except connected to the vacuum pump) before pressure paddle and start vacuum pump to negative pressure O.IMPa. If this data is not satisfied, it means that the bellows is not completely sealed and inspection is required.

While maintaining the vacuum pump operation, grouting is started until the cement slurry reaches the three-phase valve above the pressure bottle.

The operating valve isolates the vacuum pump from the grout and directs the grout to the waste container. Keep squeezing until the slurry is even.

Close the vacuum pump and close the valve located at the squeezing pump outlet.

Open the valve on the grouting cap and open the valve in the squeezing pump until the grout is in uniform shape. Close the valve on the grouted cap and keep the pressure. 4MPa continue grouting for 0.5min. Close the valve located at the squeezing pump outlet and close the squeezing slurry.

2.4 Cement slurry design Cement slurry design is the key to the grouting process. Appropriate cement paddles should be: 1 good workability; 2 low porosity after hardening, low permeability; 3 has a certain degree of expansion to ensure that the hole filling is dense 4 high compressive strength; 5 effective 144 bond strength; 6 durability; 7 temperature should be controlled below 25t: otherwise the slurry is prone to segregation. When the Nanjing No. 2 Bridge site was constructed, it was during the high-temperature season in August and September in Nanjing. In order to prevent the slurry from overheating, a water slurry with a temperature of no more than 7 T was used to make cement slurry, and ice was added when necessary.

In order to prevent the cement slurry from producing water during the infusion process and cracking after hardening, a small amount of additives may be added to reduce the water-cement ratio (0.30-0.35). The cement slurry used by VSL in some projects is shown in Table 1. These were determined after several tests before construction and achieved success in actual construction.

A large number of tests and engineering studies have shown that the cement slurry used in the vacuum suction method must meet the following requirements: Portland cement should be used for cement, with no less than 425; in the 1.725L hopper, the consistency of cement should be 10%. ~15s, up to no more than 18s. Table 1 Cement slurry ratio for some projects Project Name Portland cement Water superplasticizer Retarder Water-cement ratio Model dosage (kg West Nanjing West China On the No. 50 of the upper tower of the Yangtze River Bridge No. 50, the hole forming of the vacuum-assisted grouting process on the No. 50 column of the tower tower adopts the embedded high-density polyethylene plastic corrugated pipe PT-PLRS. Unlike the traditional metal bellows, the plastic corrugated pipe is not only sealed. Good performance, good corrosion resistance, added a solid barrier for anti-corrosion of prestressed tendons, and compared with steel pipes and metal bellows, the friction resistance of PT-PLRS plastic bellows is small, Table 2 is the application of this kind of project in Nanjing Second Bridge Project. Comparing the measured results of plastic bellows.

Table 2 Comparison Results of Nanjing No. 2 Bridge Project Comparison of buried pipe type k Value of measured P value Friction loss rate of fixed end hole Hydraulic pressure sensor Force sensor Oil pressure meter force sensor Total pressure sensor Total Directional Mean Metal Bellows Positive Direction Inverse Direction Plastic Tube Total Mean Positive Direction Inverse Direction From the data in the table, the PT-PLRS plastic bellows has a smaller friction coefficient than steel tubes and metal bellows.

2.6 Advantages of vacuum-assisted pressure paddle technology With the aid of vacuum, the original air and water in the tunnel are eliminated. At the same time, bubbles and excess free water mixed in the cement paddle are also eliminated, which enhances the density of the slurry.

The micro foam slurry and the paddle in the slurry flow under the vacuum negative pressure to the first pressure bottle. After the thick slurry flows out, the consistency of the paddle in the channel can be maintained, and the density and strength of the slurry can be guaranteed.

Vacuum assisted grouting is a continuous and rapid process that shortens the grouting time.

Under the condition of vacuum, the channel reduces the head difference formed by the slurry itself due to the high and low curvature of the channel, and facilitates the filling of the entire channel by the frame body, especially the key positions of different shapes. Branch bend, U-shaped and vertical prestressed tendons can better reflect the superiority of vacuum grouting.

Because the hole pressure test (vacuum or positive pressure) is performed before grouting, this guarantees the hole seal.

As a comprehensive technology, vacuum-assisted grouting requires a high level of quality at the construction site, including high-level management personnel and operating teams. In this way, because of the nature of this method itself, it has a high level of quality control.

With proper mix design and proper grouting procedures, vacuum suction can provide a uniform, void-free slurry and eliminate harmful water that accumulates in the vicinity of prestressed tendons. In combination with other measures such as plastic prestressed catheters, extremely high protection levels can be guaranteed.

3 Applied research In recent years, VSL has successfully adopted vacuum assisted grouting technology in many international and domestic large projects. As shown in Table 2, the following introduces some of the major projects conducted in Hong Kong, France, Switzerland and China. Happening.

3.1 A 26-kilometer high-speed railway linking Hong Kong Island and the new airport constructed by Hong Kong’s public railway transport company (MTRC) has adopted vacuum-assisted grouting technology, and another project is the western immersed tunnel, which is 1260m long and requires 34,000m of conduit grouting. . Before the construction, a vacuum-assisted grouting test was carried out. The length of the test piece was 30m. The other dimensions were the same as the actual dimensions. Sampling results show that the hole filling is quite good. Of the four groups of specimens, only one group has some small pores. 3.2 France uses more vacuum suction than any other country. The French National Railway (SNCF) specification and other major departments now require vacuum suction technology. In the past 10 years, VSL has adopted eight major vacuum pumping technologies in France, and in the A14 project, a large number of grouting mixes were studied before construction.

Table 3 The major projects of the VSL Group with vacuum-assisted grouting technology National Project Name Owner Year Features New Airport High-speed Railway Gan Mei France's largest concrete concrete barge in the world, with vertical prestressing ribs Switzerland uses vacuum-assisted grouting technology to exceed 400 meters The prestressed tendons were used to complete the two bridges on the Swiss National Road N1. The electrical insulation VSL-CS super prestressed system was adopted. The Yangtze River Second Bridge in Nanjing, China used the vacuum assisted grouting technology for the first time and large-tonnage large-curvature U-preforms. The Stress Relief Zhejiang Provincial People's Court is building the first domestic application of this technology in housing construction. The construction of the cable-stayed main tower, the lower beam, and the anchorage prestressing of the Hubei Yinghuang Bridge, 3.4 China VSL in Nanjing Second Bridge Tower The tower column uses a vacuum assisted grouting method, which is the first time that this new technology has been used in China. In order to ensure the quality of grouting, a 1:1 model test was conducted prior to construction. The results showed that the slurry in the plastic bellows was densely packed; the transfer beam of the Zhejiang Provincial People’s Court used a vacuum assisted grouting technique to ensure the quality of the project. This is the first time that this technology has been applied to housing construction in China.

4 Conclusion VSL has successfully mastered and applied vacuum-assisted grouting technology over the past 20 years. Through a large number of tests and engineering practices, it has been demonstrated that by using a suitable ratio of cement slurry in combination with a high level of quality control methods, vacuum assisted grouting can well solve the voids and imperfections caused by air bubbles in traditional grouting methods. It is a powerful means of ensuring quality grouting, thus ensuring a dense, impervious layer of prestressed tendons.

In China, a large number of cable-stayed bridges, suspension bridges, railways and highway bridges, and prestressed structures for houses are all constructed using traditional grouting methods. The development and promotion of vacuum squeezing techniques have led to the development of prestressed engineering technology in China, and the quality of levitation projects has been eliminated. The hidden danger of quality and the increase of economic benefits are of great significance.

5 2 Nanjing Second Bridge Construction Command, Hunan Highway and Bridge Construction Corporation, Southeast University. Application of annular prestressed bundles and vacuum assisted grouting technology in the construction of the Nanjing No. 2 Bridge Pylon Project, 1999

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