Selection of main raw materials for concrete
1, the requirements of water quality
Any drinkable water can be used to mix and maintain concrete; when using other water sources, sampling and testing should be conducted. Untreated industrial wastewater, sewage and swamp water and seawater can not be used.
2、The requirements of cement
The cement used for pumping concrete is generally ordinary silicate cement, the construction site temperature is high and large volume concrete (such as the elevator floor of the project (i.e. joint pile bearing platform) is suitable for cement with lower heat of hydration, such as slag silicate cement. This project uses Conch cement, 32.5R cement for C30 and below; 42.5R cement for C30 to C45.
The amount of cement should not only meet the strength requirements of the structure, but also ensure a certain amount of cement slurry as lubricant to reduce the resistance of the conveying process. But too much cement amount, but will increase the viscosity of the concrete, increasing friction, so the amount of cement is controlled at 300kg.
3, the requirements of coarse aggregate:
the ratio of the maximum size of the aggregate to the inner diameter of the pipe: pumping height < 50m, should not be greater than 1:3.
4, the requirements of fine aggregate:
pumping concrete should be appropriate than ordinary concrete to increase the sand rate to meet the needs of pipeline transport; but the sand rate is too high, it will reduce the compatibility, so try to reduce the sand rate under the premise of meeting the pumpability. It is desirable to use sand, Zone II sand, through the 0.315?L sieve particle content of 20 to 30%, including through the 0.16?L sieve of fine powder content of about 8 to 20%.
5、The requirements of the admixture
A large number of experimental studies and engineering practice shows that the concrete mixed with a certain amount of high-quality fly ash, not only can replace part of the cement, and because the fly ash particles are spherical with a ball effect, play a lubricating effect, can improve the fluidity of the concrete mix, cohesion and water retention, thereby improving the pumpability.
Particularly important effect is the addition of the original or finely ground fly ash, can reduce the heat of hydration of cement in concrete, reduce the adiabatic conditions of the temperature rise, so suitable for high temperature season and bulk concrete construction.
6、The requirements of admixtures
Pumping concrete requires high fluidity of concrete, and make concrete can be pumped, mainly the use of concrete admixtures, mainly concrete pumping agent. Add a certain amount of admixtures with water reduction, plasticization, slow setting and other effects in the concrete to improve the fluidity of the concrete mix, water retention, reduce the heat of hydration, delay the appearance of the thermal peak time.
7、Water-cement ratio, slump
(1) pumping concrete water-cement ratio is too large, the concrete consistency is reduced, good fluidity, pumping pressure decreases; but if the concrete is too thin, poor water retention, prone to segregation and clogging pipes, so the water-cement ratio should be limited to 0.4 ~ 0.6.
(2) the effect of temperature on slump loss. When the temperature rises, on the one hand, the hydration reaction of cement will be accelerated and the slump loss will increase; on the other hand, the volatilization of water caused by the temperature increase will also lead to slump loss. Pumping concrete slump to 8 ~ 15 is appropriate.
Pumping process of concrete
1、Pumping construction preparation
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, which can avoid the damage caused by high temperature to the concrete structure. In addition, the construction company should arrange for watering and maintenance to ensure that the concrete level is sufficiently wet. In low temperature weather construction to choose cover or insulation material wrapping. Improve the inspection of the pump machine, must ensure that each equipment is normal before it can be put 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 can affect the performance of use.
(1) the choice of concrete delivery pump.
At present, there are two kinds of concrete pumps used in China, one is a pump truck with a fabric rod 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 arranged reasonably 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 installation site of the pump.
2、Concrete pumping
When pumping, you should start at a slow speed and check whether the pumping machine parts are running normally and whether the conveying pipeline is leaking and firm, and only start to accelerate to normal speed after the systems are running normally. If abnormalities are found, stop the machine 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 pipeline should be cleaned.
Quality control of pumped concrete construction in construction projects
In the process of concrete quality control, accurate measurement is the top priority. 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 percent. Clear the beams, columns, foundations, pits in the water: before construction, flush the steel, wetting the 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 the sudden drop, not deep too large when more likely to appear off the wash phenomenon, and lead to sand trap layer, stone trap layer, so the water must be removed clean.
Due to the large fluidity of concrete, when the formwork has more than 2cm2 after the hole, it is very easy to cause leakage. Pulp leakage will make the concrete produce honeycomb, hemp surface, serious enough to cause local loose, resulting in loss of strength, must be knocked out and re-poured. The way and time of mechanical pounding should be carried out according to the specific provisions of the construction manual. When pouring columns, bulky foundations and beams, gravel (cleaned) should be sprinkled at the end stage of construction 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, the floating pulp should be removed before the next process. Strengthen the maintenance because of fly ash and plasticizer, it must strengthen the maintenance. In particular, it should be kept wet within 14 days after casting and molding, so that the fly ash and Ca (OH) 2 produce a chemical reaction to form a cementitious component, thus improving the strength of concrete.
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 properties 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.
The use of the flat layer method to complete the pouring, with strict control of the pouring thickness, generally at 30-50 cm, from a distance.
The height of concrete discharging should be controlled within 1M, and it is strictly forbidden to direct the fabric to the inner side of the formwork or to direct the 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 the fabric horizontal movement, generally to perpendicular to the mold counter fabric; vibrating pumping concrete process, on the vibrating rod moving spacing strictly controlled at 40cm, the pounding time then lasted 15-30 seconds, strictly prohibited pounding time is too long, otherwise it will affect the use of concrete performance.
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 needs can be iron roller pressure more than 2 times, so as to avoid the formation of shrinkage cracks.
After the end of concrete pouring, to cover the insulation material on top, and water sprinkling wet maintenance.
Conclusion
To sum up, with the continuous development of China’s construction industry forward, the requirements for concrete are getting higher and higher. Modern construction not only has a high strength of concrete materials, waterproof, heat-resistant, acid-resistant and other performance requirements, accordingly, the concrete construction technology also has higher requirements, pumped concrete is a new concrete construction process under such conditions.