How to improve the pumping performance of concrete in high-rise buildings
Common problems in concrete pumping project of super high-rise building
According to a large amount of data and practice, pumping pipe clogging is the most common problem in concrete pumping for super high-rise structures.
The main reason of pumping pipe blockage is the poor pumping ability and working performance of concrete mixes, and the ratio and materials do not meet the construction requirements. Specific performance is as follows.
1, whether the strength level of high-fluidity concrete meets the design strength.
2, high strength concrete viscosity, poor fluidity, can not meet the pumping requirements of super tall buildings, easy to cause pipe blockage.
3、When the fluidity of concrete is too large, its ability to resist segregation and water secretion will be reduced. Under the action of pump pressure, the segregation of concrete will cause the separation of aggregate and mortar, and in the case of mortar without lubrication, the coarse aggregate will increase friction with the pump in direct contact with the pipe, causing the pipe to be blocked.
4, ready-mixed concrete from the mixing plant to the construction site takes a certain amount of time, which will reduce the workability of the concrete and make the pumpability worse. This is manifested by the decrease in expansion value and slump value, which eventually leads to the clogging of the pump pipe.
The influence of materials on the pumping performance of concrete
The coarse and fine aggregates in the concrete mix are not mobile, and their surfaces must be wrapped in cement paste and move with the movement of the cement paste.
In concrete mixes, the cement paste wraps the aggregates and fills the gaps between them, forming a slurry layer on the surface of the aggregates. The thicker the slurry layer wrapped around the aggregate, the less resistance to the movement of the aggregate.
By increasing the amount of slurry, the fluidity of concrete will increase and a grout layer of a certain thickness will be formed on the inner wall of the pumping pipe. The grout layer can lubricate the inner wall of the pipe, thus reducing the frictional resistance between the mixture and the pipe and making the mixture easy to pump.
Cement type, mineral composition, fineness, etc. are also important factors that affect the pumping performance.
In addition to the slurry, there are coarse and fine aggregates among the main components of pumped concrete, of which the aggregate accounts for the largest volume in the concrete. Aggregate gradation, particle shape, maximum particle size and water absorption properties will affect the pumping performance of concrete.
(1) Aggregate grading
Aggregate grading concrete mix void ratio is small, in the case of the same amount of cementitious materials and admixtures, can improve pumping performance.
(2) Maximum particle size
When pumping concrete in conventional buildings, the ratio of the maximum particle size of the aggregate to the inner diameter of the pipe is not greater than 1:3.
In the concrete construction of super high-rise buildings, the pumping height is high, and the required pumping pressure is also large. If the aggregate particle size is too large, it will be easy to deflection. Therefore, in the construction of super high-rise buildings, it is necessary to choose aggregates with smaller particle size, and the ratio of maximum aggregate particle size to pipe diameter is less than 1:5.
(3) The shape of aggregate particles
If the surface of aggregate particles is smooth, with few angles and low porosity, less slurry is required to fill the gap and wrap the particles, and the slurry can be added accordingly to thicken the lubrication layer. Pipe. As the thickness and thickness of the coating increases, the flowability of the mixture will also be enhanced, and the frictional resistance of the concrete will be reduced, thus further improving the pumping performance of concrete.
Aggregate needle plate shape has a greater impact on pumping performance, and its content should be strictly controlled.
(4) the impact of sand rate (sand coarse aggregate ratio)
In a certain amount of mud premise, the sand ratio is too large, sand than stone, the total surface area of the aggregate becomes larger, the mud covering the aggregate becomes less and thinner, and its friction resistance with the inner wall of the pipe also becomes larger, resulting in poor mobility of the mixture.
The sand ratio is too small, there is less sand than stone, and the gaps between the stones are difficult to fill with sand. It needs to fill the extra gaps with slurry, which will reduce the amount of slurry wrapped around the surface of the aggregate, making the frictional resistance of the gaps between the aggregates increase, and also making the mixture less fluid.
Concrete pumping process, the gap between the coarse aggregate will increase, the slurry will go through the gap of the coarse aggregate under the action of pumping pressure, the coarse aggregate is difficult to flow due to excessive frictional resistance. It will produce the phenomenon of concrete water secretion and segregation.
(5) The water absorption of sand and gravel
Due to the existence of sunken pores on the surface of sand and gravel, unsaturated sand and gravel will make the water in the concrete seep into the cavity under the action of pumping pressure, affecting the pumping performance of concrete.
For sand and gravel with high water absorption, when preparing pumping concrete, it should be wetted so that the surface cavity is filled with water.
In the low strength mechanism sand concrete, with the increase of stone powder content and MB value, the pumping resistance gradually decreases. When the content of stone dust is 20%, the pumping resistance is the smallest; fly ash and air-entraining agent can effectively reduce the pumping resistance.
In high-strength mechanism sand concrete, with the increase of stone powder content and MB value, the pumping difficulty increases. 5%~7% of stone powder content does not affect the pumping of high-strength mechanism sand concrete when MB value is 0.25; while slag powder and silica powder can effectively reduce the pumping resistance of high-strength mechanism sand concrete.
High-absorbent resin is an admixture that effectively reduces the pumping resistance of the mechanism sand concrete.
If the mechanism sand concrete ratio design is carried out according to the law of natural sand, it will make the concrete's compatibility deteriorate, and water secretion will easily occur, which will seriously affect the pumping performance of mechanism sand concrete.
According to the mechanism sand's own characteristics of the design of concrete ratio, the need to adjust the sand ratio of the mix, a reasonable choice of mineral admixture type and amount, in order to prepare a better performance of the mechanism sand concrete. Performance.
Water-reducing and slow-setting admixtures
Adding water reducing agent can improve the overall performance of concrete, is the main method of preparing high-performance concrete. The use of high efficiency water reducing agent and high quality mineral admixture of double mixing method can improve the water retention and fluidity of high performance concrete, and improve the pumpability of high strength high performance concrete in all aspects.
The cementitious material buildup system is mixed with mineral admixtures that are finer than cement. These mineral admixtures will fill the voids between the cement particles, thereby reducing the amount of filler water that would otherwise be used to fill the voids. In some cases, the excess fill water will become surface water, which will increase the thickness of the water film.
As the specific surface area of the mineral admixture increases, more surface water is required to wrap the surface and the thickness of the water film is affected.
Therefore, the water reduction effect of mineral admixture should consider water filling and surface water absorption.
Water plays a key role in the success of pumping concrete mixtures, and the various materials in a concrete member can be bonded into a single unit by water.
Insufficient water addition will increase the viscosity of the concrete mixture (even the various materials of each component cannot be bonded into a whole and are in a loose state), the compatibility of the mixture becomes poor, and the pumpability of the concrete becomes poor. This cannot be achieved.
However, too much water will make the concrete easy to segregate and cause pumping blockage, which is also extremely detrimental to the strength growth and durability of concrete (after concrete is poured, too much water will evaporate, causing concrete shrinkage and cracking, which directly affects the durability and strength of concrete.)