Application of vacuum auxiliary grouting in post-tensioned prestressed structure

1 Selection of grouting plan During the construction of highway and bridges, the issue of post-tensioned pre-stressed grouting has been listed by the Ministry of Communications as one of the top ten diseases in the construction of highway bridges. The solution to the safety and durability of prestressed structures is to ensure the compactness of the grouting of the prestressed channels. The vacuum assisted grouting is to ensure the compactness of the grouting of the channels and to develop the safety and durability of prestressed structures. The new grouting process.

2 Working principle of vacuum-assisted grouting The new technology of vacuum-assisted grouting has three major improvements compared to conventional grouting: vacuum assistance, optimization of slurry properties, and use of new pipe materials.

The basic principle is to make the grouting pipeline form a certain degree of vacuum before grouting. Vacuum equipment is provided at one end of the pipeline and grouting equipment is at the other end to make the vacuum equipment-pipe-grouting equipment form a tandem whole, through orderly Construction to complete the grouting process.

The specific process is: before the grouting, the pipeline is sealed, one end is connected to a vacuum pump, and the vacuum pump is started to carry out suction, and the vacuum is controlled at about 0.08 MPa so that the vacuum degree in the pipeline reaches 80 or more. While maintaining the vacuum pump in the pipeline, At the other end, the grouting machine is started to press the cement slurry that meets the design requirements into the pipeline. The pressure of the grouting machine is controlled at 0.5 MPa to 0.7 MPa, and the slurry is continuously pressed into the pipeline until the evacuation of the vacuum slurry reaches a certain target. Vacuum end valve, press-in side continues squeezing to keep pressure holding for 2 min, grouting is over.

3 Control points for empty auxiliary grouting in construction 3.1 Design of cement slurry The design quality of cement slurry directly affects the construction quality. The design of cement slurry is the key to the grouting process. In the design, the strength, workability, bleeding rate, consistency, and durability of the slurry should be comprehensively considered so that they can be integrated into the best state.

The water-cement ratio is the most critical part of a series of indicators for slurry design. Water-cement ratio is the main factor that affects the shrinkage, porosity, bleeding rate, consistency, strength and durability of cement slurry. Larger water-cement ratio results in more fluidity and is more conducive to construction. However, the large water-cement ratio results in high concrete porosity. The porosity accounts for 25-40 of the volume of the slurry, especially where the capillary pores account for a large part, and the pores are the channels through which moisture, various erosion media, oxygen, carbon dioxide, and other harmful substances enter the slurry. If the porosity is too high, the impermeability is insufficient, resulting in insufficient durability, and the slurry strength is also greatly reduced due to excessive porosity.

In addition, the greater the water-cement ratio, the greater the bleeding rate, the greater the shrinkage of the slurry, and the longer the time required for bleeding to drain.

According to the actual construction conditions at the construction site: 2) Samples shall be sealed, and sample construction shall be carried out according to relevant operational procedures before the construction of the process is fully carried out. Check its quality, decompose its operating procedures, and set specific requirements for its key process control standards.

3) After the material enters the market, it shall be stored in time. The polystyrene board and plastic-coated alkali-resistant glass fiber cloth shall have moisture-proof measures and be within the shelf life.

4) External wall insulation The cold bridge of the partitioned window floor and the like is its advantage, so the window side of the outdoor side of the plane according to the relevant atlas, should paste insulation materials, in order to avoid installation conflict with the window frame, should be in the main construction window hole Predetermined height difference between the inner and outer sides of the window frame for the construction of the insulation layer.

5) The polystyrene board should be checked for contamination on its surface before installation, and interface treatment is performed on the surface, especially because the adhesive strength of the polymer mortar and its surface directly affects the construction quality and its durability. In addition, the thickness of the surface polymer mortar should not be too thick so as to avoid hollow cracking.

6) During the construction, it should be noted that the ambient temperature should be above 5°C, and the wind force should be greater than 5 or the construction cannot be carried out on rainy days.

7) The patch seam width between polystyrene boards is greater than 2 mm, and the corresponding polystyrene board plugs shall be used.

The length of the bulging bolt used for fixing the polystyrene board should be greater than 4 cm.

8) After completion of construction, product protection should be strengthened to avoid bumping.

4 Conclusion After the construction of the project is completed, the thermal insulation wall is basically no cracking phenomenon, the level of surface smoothness in line with the relevant national acceptance specification. As long as the strict requirements, master engineering construction points, and careful construction, polystyrene wall external insulation construction can meet the requirements of residential insulation and energy saving.

Shanxi building 1) The pipe bending angle θ is small and the vertical height difference is small 2) The length of the pipeline is small, L = 29.58 m, the number of pipelines is small, only 3 beams per beam, so the filling time is short, no more than 30 min. A large number of experiments have designed a slurry with a relatively small water-cement ratio and a relatively large consistency. In order to satisfy the fluidity and workability of the slurry, an appropriate amount of an admixture is added to the slurry design. The details are as follows: 1) The water is tested and the components that are not harmful to prestressing tendons and cement are not included. 2) The cement is made of ordinary Portland cement with a strength class of 42 .5(R), and the fineness modulus and storage time of the cement are controlled. 3) Additives do not contain chemical substances that are harmful to prestressed tendons and cement. 5) The rate of bleeding is controlled at 2 hours after cement slurry mixing. 2) The bleeding should be sucked back by slurry within 24 h. 6) The consistency of cement slurry is 1.725 mL. In the meter, the consistency of the cement slurry is 25 s to 35 s.

In actual construction, the ratio of cement slurry used is: Water: Cement: Water reducer: Expansion 3.2 Pipeline sealing 1) Selection of sealing method: Pipe sealing is commonly used to seal and use sealing cover matching with anchor plate. Non-shrinking cement mortar seals were used in the construction.

2) Inspection of sealing performance: Pipe seals must wrap all anchor plates, clips, and exposed steel strands. Cover thickness is greater than 3 cm and grouted within 24 h to 28 h after sealing.

When the vacuum pump is started, the pressure gauge must be taken into account. If the pressure is not reached, the seal is considered to be insufficient. Check the cause and re-seal.

When the grouting pressure gauge on the grouting press does not reach the fixed value, there will be leakage of the seal at the sealing anchor, indicating that the slurry shall not be closed and the grouting shall be stopped. The slurry in the pipeline shall be flushed with pressure water in time, and then sealed and grouted again.

3.3 Cement slurry construction 1) Grouting conditions: When the temperature of the structural concrete is not less than 5°C during the grouting and within 48 h after the grouting, when the temperature is higher than 35°C, grouting should be prohibited.

2) Consistency control: After the cement slurry is mixed according to the designed mix ratio, the consistency should be checked with a consistency meter in time. When the consistency of the consistency and the design ratio is too large, the cause should be promptly searched to recheck the fineness modulus of the cement. In addition, a consistency meter is also used to check the consistency of the grout at the outlet of the grouting pump to ensure the quality of the paste injected into the pipeline.

3) Mixing quality: The raw materials must be measured with the exact measuring device in accordance with the design mix ratio during mixing so as to ensure the mixing quality. The mixing must be uniform and the mixing time is longer than 2 min. The mixed cement slurry should pass through a 1.2 mm mesh screen when entering the holding container. The slurry in the container should be agitated frequently to prevent bleeding and segregation due to prolonged storage. Blasting and segregating grouts should be disabled.

3.4 Equipment and Pressure Gauges The main equipment for vacuum assisted squeezing is vacuum pumps and pressure pumps. Maintenance and maintenance should be carried out frequently. Pre-use inspections and post-use cleaning should be performed to prevent construction equipment from adversely affecting the construction and causing construction quality problems. .

The degree of vacuum depends on the pressure gauge reading of the vacuum pump. The sizzle pump depends on the pressure gauge reading of the grouting pump. The compactness and plumpness of the slurry in the pipeline is derived from the work of the pressure gauge, so the pressure gauge is used before use. It must be calibrated and recalibrated after a period of use to ensure the accuracy of the gauge.

The pressure gauge must be strictly controlled during construction to prevent overpressure from causing safety accidents.

The squeezing end pressure gauge must satisfy the holding requirement after closing the vacuum end valve, and the voltage regulation period is not less than 2 minutes, so that the slurry filling degree in the pipeline meets the requirements.

3.5 The wall thickness of the plastic corrugated pipe is 2 mm, and the corrugation of the circular corrugation is about 5 mm thick. Under the same inner diameter, the outer diameter of the plastic corrugated pipe is much larger, which is very unfavorable to the construction and it is corrugated when the concrete is poured. The tube is prone to voids and other quality problems, so the pipe diameter should be adjusted according to the construction needs.

4 Advantages of vacuum-assisted grouting 4.1 Material advantages Vacuum-assisted grouting The use of plastics as pipe material has the following advantages: 1) The pipe has a low coefficient of friction and reduces stress loss during tensioning. 2) Small bending radius, high strength, and high stiffness , Sealing performance, etc., can simplify the construction operation, and more conducive to ensure the construction quality 3) Corrosion resistance, durability, ensure the quality of the use of the structure 4) Strong grip, better work to form a unified body 5) Non-conductive, can prevent Stray current corrosion 6) Conducive to transportation and storage during construction.

4.2 Pulp Optimization Vacuum assisted grouting can better optimize the slurry. For example, the water-cement ratio can be reduced, an appropriate amount of additives can be added, the fluidity of the slurry can be increased, the shrinkage of the cement slurry can be reduced, and the consistency can be increased.

4.3 Process Advantages 1) After the vacuum pump has been in operation for a certain period of time, the original air and water in the pipeline are eliminated. The air in the pipeline is thin and bubbles are not easily formed. The air bubbles and water mixed in the cement slurry under vacuum are removed and the slurry is increased. The compactness of the body ensures the quality of the grouting.

2) The most basic principle of vacuum assisted grouting is the formation of a positive and negative pressure difference. Under positive and negative pressure, the slurry and micro-foam in the slurry are sucked out earlier due to the small density, thereby ensuring uniform consistency of the consistency of the slurry, and better Improves the plumpness and density of the slurry in the pipeline. Tests have shown that the vacuum-assisted grouting can reach 99%.

4.4 Construction Advantages 1) Vacuum assisted sizing simplifies the construction process, shortens the construction time, and greatly reduces the bleeding of cement due to storage time.

2) Vacuum assisted grouting has correspondingly increased construction safety.

3) Vacuum assisted grouting improves the quality of the construction environment and is conducive to environmental protection.

5 Conclusion Vacuum-assisted grouting is a new type of grouting process. Some indicators have no data in the technical specifications for highway bridges and culverts. They should meet basic regulatory requirements during construction and require multiple reference, multiple tests, and multiple conclusions to ensure that the project is completed. Quality reaches new heights.

Liu Yuzhu. Vacuum assisted grouting in the application of highway prestressed concrete bridge [J].

Chen Yu et al.: Application of vacuum assisted grouting in post-tensioned prestressed structures

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