Concrete construction preparation
1 Concrete with the main raw materials
Cement: try to use low-heat or medium-heat cement, reduce the amount of cement. To reduce the heat of hydration of cement. Coarse aggregate: continuous gradation should be used, the content of needle-shaped particles should not be greater than 10%.
Admixture: commonly used admixture in concrete is fly ash, concrete mixed with a certain amount of high-quality fly ash, not only can replace part of the cement, and play a lubricating effect, can improve the fluidity of the concrete mix, cohesion and water retention, thus improving the pumpability. It can also reduce the heat of hydration of cement, which is conducive to the control of temperature cracks.
Additives: Adding a certain amount of admixtures with water-reducing, plasticizing and slow-setting effects in concrete can well improve the fluidity and water retention of concrete mixes, reduce the heat of hydration and delay the appearance of thermal peaks. To prevent shrinkage cracks, can also be mixed with suitable expansion agent.
2、Concrete ratio design
Concrete requirements: a variety of strength grades, from C20-C40; durability, to ensure the service life of the building up to 70 years or more; high fluidity, that is, to adapt to ultra-long-distance pumping; high workability, that is, convenient construction, easy to operate, after mechanical pounding to achieve a high degree of compactness.
Concrete laboratory with the design of previous construction experience from the concrete in the transport pipeline travel, collapse loss according to the length of the pipeline up to 10%-30%; based on the basic principle of concrete ratio design and strength standard control value, the factory collapse degree is initially set at 200-220mm, the export of the transport pipeline collapse degree is initially set at 140-160mm.
Concrete pumping process
Concrete is a comprehensive performance of the traditional building materials, its economy, plasticity and durability is particularly outstanding. With the development of society, both raw materials, mix ratio, production and transportation, construction technology and quality control, etc. are undergoing profound changes. Therefore, pumped concrete technology has been widely used. In the construction process construction enterprises only pay attention to pumping concrete technology, improve its technical level, in order to ensure the quality of construction projects, in order to maximize the economic benefits of enterprises.
1, the selection and arrangement of concrete conveying pump.
(1) the choice of concrete transfer pump. At present, there are two kinds of concrete pump machines used in China, one is a pump truck with a fabric bar that can walk, and the other is a traction type fixed pump. Pump truck is highly mobile, easy to move, but more expensive. Fixed pump mobility is poor, the pump needs to be reasonably arranged according to the construction site conditions, but the price is lower.
(2) The arrangement of the pump machine. When choosing the location of the pump, it is necessary to make the pump pouring place nearest, near the water source and lighting facilities, no obstacle near the pump in order to facilitate the mixing truck walking and feeding. It is better to install the pump in position, and it is better to pad the wood block at the bottom of the frame to increase the adhesion to ensure the stability of the pump. There should be some space around the pump to facilitate the operation of the personnel. A protective shed should be set up at the pump installation site.
(3) Match the pump with the mixer truck. The loading capacity of concrete mixer truck is 5 m3 and 6 m3. After the mixer truck is filled with concrete, the mixing cylinder does low-speed rotation with a certain value of speed, and then the concrete is delivered to the construction site. Since there is a delivery distance between the mixing station and the construction site, and there is a certain difference between the discharge volume of the mixing truck and the pumping machine delivery volume, there is a problem of matching the quantity of the pumping machine and the mixing and transporting truck.
2. Due to the special characteristics of pumped concrete itself, its fluidity is stronger than that of conventional concrete, and various distribution forms of aggregate and cement mortar will appear during the vibrating process.
Controlling the performance of pumped concrete can guide the constructors to properly control the construction process and maintain the stability of the concrete structure. Elements to be noted in pumped concrete construction.
Using the flat layer method to complete the pouring, strict control of the pouring thickness, generally in 30-50cm, pouring from a distance; concrete under the height should be controlled within 1M, strictly prohibited to the inner side of the formwork straight fabric or straight reinforcement skeleton; pouring vertical structure concrete process is strictly prohibited in the same place continuous fabric, should be in the range of 2 ~ 3M to fabric horizontal movement, generally to perpendicular to the mold counter fabric; vibrating pumping concrete process, the vibrating rod moving spacing is strictly controlled at 40cm, and the pounding time lasts 15-30 seconds, and it is strictly forbidden to pound for too long, otherwise it will affect the use performance of concrete.
For the construction of large volume horizontal structure of concrete, after the closing of the warehouse before the initial set to re-vibrate, and in due course with a wooden batch grinding flat rubbing hair more than 2 times. According to the actual need can iron roller pressure more than 2 times, so as to avoid the formation of shrinkage cracks; after the end of the concrete pouring, to cover the insulation material on top, and water sprinkling wet maintenance.
3, for the construction during the high temperature,
before pouring concrete, the conveying pipe should be covered with wet cover cloth or wet straw bag to cool down, so as to avoid the damage caused by high temperature to the concrete structure.
In addition, the construction unit should arrange personnel to sprinkle water maintenance to ensure that the concrete level has enough before wetting. In low temperature weather construction to choose cover or insulation material wrapping. Improve the inspection work of the pump machine, must ensure that each equipment is normal before putting into use. After the pump is started, a certain amount of water should be pumped in to wet the parts of the hopper, piston and delivery pipe that are in direct contact with the concrete.
Then lubricate the concrete pump and conveying pipe with cement mortar of the same composition ratio as the pumped concrete. The laying of the conveying pipe is strictly inspected to prevent the pipe from loosening and other problems that affect the use of performance.
4、Pumping should first slow down, while checking whether the pumping machine parts are running normally and whether the conveying pipeline is leaking, firm, and only after the normal operation of the systems can start to accelerate to normal speed.
If abnormalities are found, the machine should be stopped immediately for inspection. If for any reason the pumping must be interrupted in the middle of the pumping, the interruption time shall not exceed the time allowed from mixing to completion of pouring.
When pumping concrete, if the concrete supply is not available or for other reasons the conveying pipe inhales air, the concrete in the pipe should be sucked out immediately to the hopper and mixed again and the air should be discharged before re-pumping.
When concrete is delivered to the pouring place, it should be poured into the mold immediately. If the concrete mix appears to be segregated or layered, the concrete mix should be mixed twice. After pumping, the concrete pump should be cleaned up and the pump and conveying pipe should be cleaned.
Quality assurance measures for pumped concrete in construction projects
1, accurate measurement: in the process of concrete quality control, accurate measurement is the most important. In particular, the amount of water, be sure to deduct the water content of sand and stone, the error value of the water-cement ratio is less than is %.
2, clear the beam, column, foundation, pit water: before construction, rinse reinforcement, wetting formwork and other operations or after the rain, in the above parts are usually easy to accumulate water, when the concrete flow to the part, it will be due to the local water-cement ratio is too large and the strength of a sudden drop, not deep too large when the more likely to appear off the wash phenomenon, and lead to sand trap layer, stone trap layer, so the water must be removed clean.
3, to prevent leakage: due to the large fluidity of concrete, when the formwork has more than 2cm2 after the hole, it is very easy to cause leakage. Leakage will make the concrete produce honeycomb, pockmarked, serious enough to cause local loose, resulting in loss of strength, must be knocked out and re-poured.
4, to prevent floating pulp too thick: mechanical pounding mode and time should be carried out in accordance with the specific provisions of the construction manual, when pouring columns, bulk foundation and beam, at the end of the construction stage should be sprinkled with gravel (cleaned) to equalize the content of the bone section. When the floating slurry precipitates water, it should also add the appropriate amount of cement of the same brand and batch number to improve the water-cement ratio. When the floating pulp is too thick, should be in the next process before the removal of floating pulp.
5、Strengthen the maintenance because of the admixture of fly ash and plasticizer, it must strengthen the maintenance. Especially within 14 days after casting and molding should be kept in a wet state, so that fly ash and Ca (OH) 2 chemical reaction, the formation of cementitious components, thereby improving the strength of concrete.
Conclusion
At present, pumped concrete technology has been popularized and applied. It can not only improve the construction performance of concrete, the thin-walled dense reinforcement structure less vibration or vibration-free construction, with improved seepage resistance, improved durability and other characteristics. At the same time, the limitations of pumped concrete aggregate grading, the use of a large amount of cementitious materials, generating a large amount of heat of hydration, resulting in the prevalence of temperature cracks. To a certain extent, it affects the impermeability and durability of the structure and should be given sufficient attention.