Wuhan Junshan Yangtze River Highway Bridge is the main 1000 lines of the two national highways of Beijing-Zhuhai and Hu-rong. The bridge spanning the Yangtze River is one of the key projects of the state during the Ninth Five-Year Plan period. The project mileage is 4881.178m, of which the total length of the bridge is 2847m, and the main bridge is a (48+204+460+204+48)m five-span double-tower double-cable semi-floating steel box girder cable-stayed bridge, 964m long, and the bridge is bidirectional. Six lanes, with a width of 335m, is the widest cross-river bridge in China. The design speed is 12km/h; the design load is Car-Super 20, and Trailer-120. Using a separate inverted Y-shaped space tower, the tower column is divided into three sections: the lower tower column, the middle tower column, and the upper tower column, which is an asymmetric hollow thin-walled section, the bottom elevation of the tower column is 7m, and the tower top elevation is 1710m The total height of the tower is 1635m, and the section size is 7.0m (in the direction of the bridge) x 4.5m (in the direction of the bridge).
The upper tower pillar height 115595 164.933m of Wuhan Junshan Changjiang Highway Bridge is the cable tower anchorage zone. The large tonnage small radius circular prestressed system is adopted in the design, and the pre-stressing radius of the ring is 1.5m, which breaks through the reinforced concrete of the highway and Specification for stress concrete bridges and culverts design p minimum radius 4m. Because the cable tower anchorage zone adopts toroidal prestressing, each prestressed tendon is 12.15mm less than 12, and the strength level is 1860MPa low relaxation steel strand. The stranded steel pipeline is a small radius curve, after the strand is completed If traditional grouting is used, the grouting effect cannot be guaranteed. In order to enhance the corrosion resistance and durability of steel strands, a new process such as vacuum-assisted grouting has been adopted for the bridge.
2 Influences of traditional grouting process on prestressed steel strands In prestressed conditions, in order to prevent slipping of prestressed tendons and pre-stressed tendons from corrosion during long-term placement, the prestressed tendon must be drawn within a specified time after completion of tensioning. Grouting the holes, but the grouting effect has always been an issue that people are worried about. The traditional pressure paddle process has certain limitations, which are mainly manifested in that: the poured slurry often contains bubbles. When the mixture hardens, the place where the bubbles exist turns into pores, becomes a water infiltration accumulation place, and can easily cause corrosion of prestressed steel beams. At the same time, in cold regions, due to the low temperature, these waters will form ice and the components will burst, causing serious consequences. In addition, the cement slurry will easily segregate, excrete water, dry and shrink. Bleeding will produce pores, resulting in insufficient strength of the components and bad bonding, leaving hidden dangers for the project.
In order to prevent the prestressed tendons from being corroded and improve the safety and durability of the structure and ensure the quality of the project, it is necessary to improve the traditional grouting process, especially for the prestressed steel strands with large tonnage, small radius, and complex structures. Improve the construction process.
In the traditional grouting process, the water slurry ratio of the cement slurry should be at 1. 4 ~ .45, with appropriate amount of water reducing agent can reduce the maximum bleeding rate of cement slurry does not exceed 3%, 3h after mixing the bleeding rate should be controlled at 2%, bleeding should be completely absorbed by the plasma within 24h.
After the test, suitable swelling agent can be added into the cement slurry, but its free expansion rate should be less than 1%. 3 Vacuum assisted grouting principle The principle of vacuum assisted pressure is to use a vacuum pump to pump the prestressed pipe to a negative pressure state in the tunnel. At the other end, the grouting press is used to press the slurry into the prestressed pipeline under no pressure or low pressure, so as to increase the fullness of the grouting of the channel, reduce the air bubbles, and form a dense protective layer for the prestressing ribs. Durable protection. Make grouting more continuous and ensure the compaction and filling of the channel grouting.
The vacuum assisted pressure process features reduced channel resistance, accelerated slurry flow, and improved production efficiency.
It enhances the inertial flow and impact of the slurry, ensuring the compaction of the perforated channel.
Reduce free water in the slurry.
4 The ratio design and test of vacuum assisted grouting improve the properties of mortar, reduce the water-cement ratio, reduce porosity and bleeding, eliminate segregation, and increase the workability of cement slurry.
Reduce the hardened porosity and block the water seepage channel.
Reduce and compensate the shrinkage deformation of the mortar in the process of condensation hardening to prevent cracks.
Compared with ordinary mortar, the main difference of this kind of high-performance ash in raw material ratio is low water-cement ratio and multi-component. The purpose is to increase the density of the mortar and improve the performance of the mortar so as to achieve high strength and durability.
(2) Slurry characteristics requirements and corresponding proportioning test fluidity requirements: The fluidity after mixing is less than 30 The fluidity at the outlet of the pipe is less than 15s. The fluidity is measured using a flow cone meter. The test results show that the mixing ratio and ZK gray Compared to different, fluidity is also different. The greater the water-cement ratio, the greater the fluidity. In order to meet the requirements for irrigation, pure cement slurry is used and the water-cement ratio should be determined.
Water release: less than 2% of the initial volume of the cement slurry; the average value of 4 consecutive test results is less than 1%; 24 hours after mixing, the bleeding of cement slurry should be able to be absorbed.
The test results show that when the water/cement ratio is 33, the water exudation rate is about 3.3%~5%; when the water/cement ratio is 0.3, the water exudation rate is about 0.02%. The volume change rate test results show that when the water/cement ratio is less than 0.3, the volume change The rate is less than 1 %. The water-cement ratio is greater than. At 33 o'clock, the volume change rate was greater than 4%, and the volume above was shrinking.
The test results show that the smaller the 7C gray ratio, the higher the strength, the use of No. 525 Portland cement, 7d maximum strength of 78MPa, this result greatly exceeds the required strength. The selection of Pugui cement No. 425, 7d maximum strength of more than 55MPa, so choose 425 Portland cement can meet the requirements.
5 Engineering Application The vacuum-assisted grouting process for the model construction of the Junshan Bridge is to cut off the exposed steel strands after completion of the pre-stressed beam tensioning, and remove the impurities in the mounting screw holes on the pressure-bearing plate, respectively at the anchorages at both ends. A valve switch is installed on the grouting tube (or evacuated tube), wherein the suction vacuum end is placed on a grooving hole on the anchor seat at a high position, and the grouting end is on an anchor seat placed at a low position.
After cement, expansive cement, water reducer, fly ash, and water are homogenized in an ash mixer according to a certain ratio, the fluidity within 3 seconds is better. In the model test, two exposed corrugated tubes were used outside the model for vacuum grouting test. From the stripped appearance, the entire corrugated tube was circular, and the entire slurry structure was also circular. The slurry saturation was 99.8. %, the slurry is denser. In the full-scale model, 28 hills after grouting were opened at the both ends of the slurry inlet and the slurry outlet to observe that the grouting effect was good and the slurry was dense.
The obtained slurry test results show that the slurry is less than 2% of the initial volume of the 7K slurry, the initial setting time is 3 hours, and the 7A age strength can be greater than 40 MPa. Through the bare tube slurry test and full-scale model test It is believed that the irrigating materials used fully meet the requirements.
Table 1 Vacuum-assisted grouting cement mix name cement fly ash expansive agent admixture water manufacturer Huaxin Cement Plant Wuhan Iron and Steel Fly Ash Products Co., Ltd. Shandong Laiwu Li Corporation Model Common Sulfuric Acid Pipe Cement Grade 1 Product FDN Water Reducing Agent Water Tap Water Weight ratio 5.1 Construction equipment and materials The main equipments are: a 52-2 water ring vacuum pump with a displacement of 2 "/, a vacuum pressure gauge, a QSL-20 air filter, and a balance of about 15kg; One grouting pump; one supporting high-pressure rubber pipe; two mandatory steel ash pipes with threaded 20 steel pipe at both ends; several tees, one connected live, one valve, several rubber pads, and other auxiliary tools.
5.2 Installation of grouting pipe and evacuation pipe The grouting hole and evacuation pipe are all installed on the anchor plate. When the anchor plate is installed, the grouting hole at one end of the same hole is placed at the bottom and used as a grouting hole, and the other end of the grouting hole is placed above. Use vacuum.
When the anchoring plate is embedded, the grouting holes of the holes arranged in the same (straight line or curve) direction are concentratedly installed in the same direction, which facilitates easy movement of the implements during construction.
5.3 Inlet and Outlet Control (1) The ball valve used is sealed with raw tape. When opened, the ball valve can be fully opened, reducing the resistance when entering the frame.
(2) Before the installation, the ball valve must be inspected to ensure that no failure occurs during the evacuation process.
5.4 Vacuum assisted grouting process Cut off the exposed strand and pay attention to the design requirements of the exposed strand.
Clean the impurities in the mounting screw holes on the pressure plate.
Use a wire brush to clean the grout on the underside of the anchor seat.
Installation of grouting pipe, evacuated pipe, ball valve and quick-change joint on both ends of the anchor seat must be checked and ensure that all installation valves can be safely opened and closed.
The suction vacuum end and the squeezing end are identified, wherein the suction hole on the suction vacuum anchor seat is upward and the squeezing hole on the grouting end anchor seat is downward.
Before grouting, the hole must be flushed and moistened. If there is water, it should be removed by a blower and the anchor head must be sealed. The anchor must be sealed to prevent air leakage.
The upper outlet of the three-way valve connected to the vacuum pump is connected to the quick-change joint of the vacuum end by means of a pressure rubber hose.
Before vacuum grouting is officially started, vacuum is tested with a vacuum pump.
(1) The following matters should be paid attention to when stirring the cement slurry: Before mixing the cement slurry, it is required to add water to idle for several minutes, drain the accumulated water, make the inner wall of the mixer fully wet, and mix the good mortar to achieve basic depuration, and pour out all the ash. Do not put unmixed materials before, and you can't use the method of feeding material while discharging.
Strictly control the ratio of slurry, feeding sequence, mixing time and so on.
The water must be added strictly according to the ratio. Otherwise, the added water will be completely secreted, which may cause voids at the top of the pipeline. It is forbidden to add water to increase the fluidity of the cement slurry that has not been used in time and has reduced the fluidity.
Mixing time should ensure that the slurry is mixed uniformly, observe the consistency of the cement slurry, and during the grouting process, the stirring of the cement slurry shall not be interrupted. If the indirect pipe is stopped, the cement slurry shall be ensured to circulate between the mixer and the grouting pump until the chestnuts are sent. until.
Officially began vacuum assisted grouting. Start the vacuum pump and open the valve on the discharge side. Close the valve into the slurry, the suction vacuum requirements to achieve negative pressure of -0.09 ~ 0.lMPa.
Start the squeezing machine and press out the water and air bubbles remaining in the squeezing machine and the high pressure rubber pipe, and check the consistency of the cement slurry discharged. After the slurry meeting the requirements is discharged from the high pressure rubber pipe, the pressurizing machine is suspended and The grouting and pressing rubber tube is connected to the squeezing pulp spine of the anchor seat through the quick-change joint.
Keep the vacuum pump activated, open the squeezing valve, and inject pressure into the pipe. When grouting, the prestressed hole should be pressed by the low grouting hole and excluded by the high vacuum hole. During the entire period of flooding, the irrigated irrigated parts were vacuumed.
When the cement slurry is pressed out from the squeezing rubber tube that is connected to the vacuum pump at the discharge end, the consistency of the pressed cement paste is checked. When the consistency is consistent and the flow is smooth, close the slurry valve. Close the valve on the vent and pause the press.
Open the feed valve on the grouting end and actuate the press. Until the cement slurry flows out of the outlet hole, when the consistency of the slurry to be discharged is uniform and the flow is smooth, the squeezing machine is suspended and the valve on the outlet hole is closed.
Start the pressure and thank 3 Zhong Lian Huan difficult to break the hook min. Close the pressure Xie mushroom yak ç“¿ ç“¿ è‡ è‡ è‡ è‡ è‡ è‡ è‡ After each hole grouting is completed, remove the air filter, cleaning the pipeline, grouting pumps, mixers and other equipment, Until clear water appears. The ball valve installed at the grouting end and the pulping end can be dismantled and infiltrated in about 24 hours after grouting.
6 Conclusion Wuhan Junshan Yangtze River Highway Bridge cable tower anchorage zone using a circular prestressed tendons and plastic bellows vacuum pressure relief ã²é“¡N type 涫V-beam, vertical beam hole grouting provides a new idea: energy Provides a homogenous, dense slurry, which can ensure the uniformity and fullness of the grouting; it can eliminate the generation of air bubbles and form a dense, impervious protective layer, thereby effectively reducing engineering risks. However, because the vacuum assisted grouting technology is a new process in China's bridge industry, the vacuum grouting technology has higher requirements on the mix ratio and temperature of the cement slurry. At present, there are no mature technical standards that can be implemented, and further research is needed. Demonstration. It is believed that the vacuum assisted grouting process will be more and more widely used in the project.
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