Post-tensioned pre-stressed concrete tunnel vacuum grouting process and its application

Post-tensioned pre-stressed concrete tunnel vacuum grouting technology and its application Wang Chaofeng (China Railway Tunnel Group Co., Ltd., Luoyang 471009, Henan, China) Reinforced tunnel grouting compaction to achieve the purpose of improving the safety and durability of prestressed concrete structures. This article mainly introduces the principle and equipment of vacuum grouting. Paste ratio. Process flow and its application in Nanyou Highway.

Post-tensioned prestressed concrete structures generally adopt pressure grouting. Because the slurry contains air bubbles, it tends to corrode the tendons after water seepage, which will weaken the effect of pre-stressing the structural concrete through steel grouting. In severe cold areas, these waters will form ice and expand components, causing serious consequences. In addition, the cement slurry will easily segregate, dewater, dry and shrink, and the water will produce pores, resulting in insufficient strength and poor adhesion. The project left hidden dangers.

1 Project Overview The box beam of Nanyou Highway C13 Contract Section is prefabricated by post-tensioning method. The structural form is single tank single room with a span of 30m beam height of 150mm. Prestressed rib configuration *15.24 (75) High-strength low-relaxation steel strand, The strength is 1860MPa arrangement as shown.

In order to prevent the prestressed tendons from being corroded, improve the safety and durability of the structure, and eliminate the common quality problems of traditional pressure grouting, the vacuum grouting process developed by Liuzhou Ohm Construction Machinery Co., Ltd. was adopted for the post-tensioned prestressed beam tunnel grouting of this project.

2 Basic Principles Vacuum grouting is the use of vacuum at the end of the tunnel to pump the air in the prestressed tunnel, so that it produces a vacuum of about 0.lMPa, and then at the other end of the tunnel with a grout pump to optimize the slurry from the tunnel. Fill one end until it fills the entire hole and apply a positive pressure of >0.7MPa to increase the fullness and compactness of the grouting in the prestressed hole, thereby improving the safety and durability of the post-tensioned prestressed concrete structure.

3 The construction equipment adopts a special vacuum grouting equipment developed and produced by Liuzhou Haiweim Construction Machinery Co., Ltd. It is mainly composed of an air pipeline system, a stirring system, a grouting system, and the like, and the main equipment is as shown.

The average of 2%%94 blocking test results is 2 MPa).

/ SZ-type water ring vacuum pump, a vacuum pressure gauge, QSL-0 type air filter 1, 15kg about one scale.

The prestressed box girder adopts a curved reinforcement method. Although the original design of the 0 metal bellows can meet the requirements of the small radius reinforcement, the metal pipe has no permanent corrosion resistance, and the pressure joints are not tight enough. Resisting the leakage of water and reaching the slurry and the prestressed tendons, the plastic bellows that better reflects the superiority of vacuum grouting and has better protection in terms of strength and corrosion resistance. At the same time, the plastic corrugated pipe is extruded, and the joint is connected with a ferrule that is sealed with an inner gasket, and the entire pipe can be made airtight.

4 Mixing ratio test of cement slurry The study of the mix ratio of cement slurry directly affects the mortar strength and the perfusion density, especially for vacuum grouting, which is a key part of the construction process. The mixing ratio mainly follows the principle of low water-cement ratio and multi-component, so as to achieve the purpose of reducing the shrinkage deformation of voids, bleeding, and cement slurry during the setting and hardening process.

The vacuum grouting slurry mix test was carried out using the recommended operating procedure of HyWeem.

41 performance requirements of cement paste 140. The initial set time is 3~4h. In the 1.725L hopper, the consistency of the cement paste should be no more than 50s for 15~45s. () The rate of bleeding is low after infusion and is less than the initial volume of cement slurry. The cement slurry can absorb itself.

The cement slurry should have a certain expansion before solidification, so that the slurry fills the entire hole. In order to overcome the shortcomings of pre-stressed longitudinal, oblique, and up-curved parts, the grouting is not full or dense. The slurry should have a slight expansion in the mid-hardening period (about 4 days) to compensate for the natural shrinkage of the cement slurry in the middle and later stages.

The slurry should have sufficient compressive strength and cohesive strength, and no less than 30 MPa is best matched with the beam concrete to satisfy the effective stress transfer between the prestressed steel reinforcement and the concrete components.

42 Slurry Raw Material Selection Grade 5 Portland cement, groundwater that meets technical standards. Taking into account the high summer temperature in Guangxi, the JN-retarding superplasticizer was used; the cement was mainly made of aluminum powder before coagulation, and UEA expansion agent was used after the coagulation.

43 Test methods The test methods for grouting material's bleeding rate and expansion rate are respectively referred to JTJ041-2000 "Technical Specifications for Highway Bridges and Culverts," appendices G-10, G-11; expansion test is referred to JG476-92 "Standard for Concrete Expansive Agents"; The compressive strength test refers to GB50203-98 Masonry Stone Construction and Acceptance Specifications.

44 Analysis of test results 441 Fluidity test 38 The effect of JN- on the fluidity of cement slurry at different dosages and the change in fluidity over time at 20*C and 40*C at 0.4% dosing were determined. The results are as shown.

The fluidity of mud, the greater the water-cement ratio, the greater the fluidity, the higher the bleeding rate, the longer the initial set time, the greater the addition of water reducer, the greater the fluidity, and the greater the rate of bleeding. JN- retarded high ambient temperature, the different temperature of the cement slurry, the impact on the fluidity of the slurry is small, but after a certain time of storage (about 40min), the fluidity decreased significantly, while the temperature of the machine The higher the flow rate is, the faster the drop will be. Therefore, the exact exit temperature is about 22*C, and the cement slurry is used up within 40 minutes.

JN3 retarding superplasticizer also has the retarding effect, which can make the cement paste under high temperature conditions have good plasticity.

442 Swelling test According to the use of aluminum powder and UEA-H two kinds of concrete expansion agent, they act on the pre-condensation and medium-term expansion of the slurry, respectively, so that the expansion rate and mid-expansion ratio of the slurry before condensation are greatly improved.

Before the condensation, the aluminum powder is added before cement is cemented. During the process of hydration of cement, carbon hydroxide is continuously precipitated. During the initial period of cement hydration, Ca(OH) in the slurry is in a saturated state, that is, it is in a high-alkaline state. The OH-reaction of the alkaline solution generates hydrogen gas, forming a number of uniformly dispersed bubbles, which causes the slurry to expand, thereby filling the bends of the grouting hole and the gaps in the strand.

The test uses different amounts of aluminum powder, measured 3h volume expansion rate, the required expansion rate before condensing is about 1%, because the 1% gas content of cement slurry strength loss. The test analysis shows that the aluminum powder content is 0.005%, and the aluminum powder reaction is basically completed in 3 hours, and the expansion is very small in the 2nd and 3h. Therefore, the cement slurry with aluminum powder must be poured immediately after the water mixing, otherwise the The expansion rate has a greater impact.

The medium-term expansion is the use of UEA added to the cement slurry to generate a large amount of swelling crystalline hydrated calcium sulfoaluminate (ie, calcium vanadium), so that the slurry expands properly, under the restraint of the rebar and concrete reserved pores, in the slurry A pre-compressive stress of 0.2-0.7 MPa is established in the structure. This expansion stress can substantially offset the shrinkage stress generated during the hardening process of the slurry, thereby preventing or reducing the shrinkage and cracking of the slurry and densifying the slurry and increasing the strength of the structure. Add paste and prestressed tendons for gripping. However, if the expansion rate is too high, the slurry may have a large expansion stress, which is not conducive to the overall structure, so it is necessary to strictly control the free expansion rate is less than 10%. The expansion rate of 0.04% ~ 0.06%, suitable for cement paste Compensation contraction function.

The UEA dosing is an internal blending method that replaces the cementitious material in equal amounts. This must be taken into account when the compounding ratio is used.

45 Selection of slurry mix ratio After indoor test, the final determined mix ratio and related properties are shown in Table 1. Table 1 Slurry mix ratio and related performance Material consumption/g Performance Cement water-cement ratio Consistency/s Bleeding rate Before condensation Medium expansion rate Compressive strength 5 Level 5 Construction process 51 Preparation before grouting After pulling is completed, cut the exposed steel strand (outer strand leakage 30~50mm), rinsing with clean water, blow with high pressure air, and then seal the anchor.

Clean the grouting holes on the anchor plate to ensure smooth grouting channels.

The suction vacuum end and the grouting end are determined. The suction vacuum end is located at the grouting hole on the anchor block at a high position, and the grouting end is placed at the grouting hole at the lower anchor seat.

52 According to the connection diagram of the vacuum grouting equipment, connect the components, check the function, and try vacuum.

53 test vacuum will be grout valve, exhaust valve are closed, evacuation valve, open the valve to open the slurry, start the vacuum pump vacuum, observe the vacuum gauge readings, vacuum to a 0.08 ~ -0.1MPa and remain stable, stop the pump The 1min pressure must remain constant.

54 Stir the cement slurry Stir the water before stirring the cement slurry for a few minutes to pour the stagnant water so that the inner wall of the mixer is fully wetted. When charging, first weigh well (after deducting the portion of water used to melt the aluminum powder and UEA) into the blender, pour it into the cement while stirring, and mix for 3 to 5 minutes until it is even. Dissolve the water-soluble additives aluminum powder, UEA and JN-3 water reducer into the mixer, stir for 5-15 minutes, and then discharge.

Mixing cement slurry should pay attention to: (1) After the cement slurry discharge should be pumped as soon as possible, otherwise it is necessary to continue stirring; (2) must strictly control the amount of water, otherwise more water is leaked out, easily lead to the top of the pipe (3) It is strictly forbidden to add water to the mortar to reduce the fluidity of the cement slurry that has not been used in time and to reduce the fluidity; (4) The temperature of the mixture cement slurry shall not exceed 7*C, and ice input shall be used when necessary. Water. 55 Grouting Add grout to the grout pump and make a slurry at the outlet of the high-pressure rubber tube of the grout pump. When the concentration of these grouts is the same as the concentration in the grout pump, turn off the grout pump and connect the high-pressure rubber tube to the grouting tube of the hole. On top, firmly. Turn off the grout valve, open the vacuum valve, open the slurry end valve, start the vacuum pump to make vacuum, make the vacuum degree reach 0.08 0.1MPa and keep stable, start the grout pump, open the grout valve, start grouting, when the slurry passes through the air filter Turn off the vacuum pump and vacuum valve and open the exhaust valve. Observe the discharge condition of the exhaust pipe, check the consistency of the pressed cement slurry, until the consistency is equal to the poured slurry and the flow is smooth, close the exhaust valve and the outlet valve, the grouting pump continues to work at >0.7MPa Next, hold the pressure for 2~3min. Close the grout pump and grouting end valve and complete the grouting. Remove external piping, accessories, air cleaners, and equipment covered with mortar. Grouting was performed in the order of 3N4*2N3*2N2*2N1.

56 Precautions () Strict control of the material mix ratio, the error can not exceed 1% (2) Before entering the grout pump mortar should pass 1.2mm sieve. () The vacuum pump should be lower than the entire pipe. When it is started, the water valve of the connected vacuum pump should be opened first, then the pump should be turned on; when the pump is off, the water valve should be opened first, and then the pump should be stopped. (4) The grouting work should be carried out within 30 to 45 minutes before the mortar fluidity declines. The grouting at the tunnel should be continuous.

6 Conclusions and experience The key to vacuum grouting is to reasonably determine the mix ratio of the cement slurry and stir it in strict accordance with the order of addition. During the grouting process, ensure that the slurry temperature is below 25°C. When the slurry is flowing under vacuum and the fast grouting port flows to the discharge port, when the vacuum device is turned off, the exhaust hole must be allowed to slurry for a certain period of time, and the concentration should be measured to achieve pulping. The slurry outlet valve can only be closed after the cement slurry concentration.

The condition for determining whether the vacuum grouting is successful is that both ends show a slurry with the same consistency as the input slurry, and there is no obvious bubble, and it is a standard that the pressure is maintained for 2 to 3 minutes under the condition of >0.7 MPa.

It is worth noting that the problem of rinsing with pressure water before grouting is that effective measures must be taken to eliminate water in the tunnel (high-pressure air). Otherwise, the water-cement ratio will increase at the initial stage of grouting. This step must not be ignored.

7 Application effect Nanyou Highway C13 contract section 30m post-tensioned prestressed box girder adopts vacuum grouting to carry out grouting in the tunnel, with a vacuum degree of 96%~98%. After the slurry solidifies, the slurry is flush with the top of the grouting pipe. Very dense.

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