Common problems in concrete pumping works for super high-rise buildings
According to a large amount of data and practice, the blockage of pump pipeline is the most common problem in super high-rise structure concrete pumping.
The main reason for the pipe blockage in pumping is the poor pump ability and working performance of the concrete mixture, and the mix ratio and materials do not meet the construction requirements. The specific manifestations are as follows:
1, The high viscosity of high-strength concrete results in poor fluidity and cannot meet the pumping requirements of super high-rise buildings, so it is easy to cause pipe blockage.
2, When the fluidity of concrete is too large, its anti-segregation and bleeding ability will decrease. Under the action of pump pressure, the segregation of concrete will cause the aggregate and mortar to separate, without the lubrication of the mortar, the coarse aggregate and the pump direct contact with the tube will increase friction and cause tube blockage.
3, It takes a period of time for ready-mixed concrete to be transported from the mixing station to the construction site, which will reduce the working performance of the concrete and deteriorate the pumpability. The concrete manifestation is the reduction of the expansion value and the slump value, which will eventually lead to the blockage of the pump tube.
4, Whether the strength grade of high-fluidity concrete meets the design strength.
Effects of materials on concrete pumping performance
The coarse and fine aggregates in the concrete mixture have no flow properties, and their surface must be wrapped with cement paste and move through the movement of the cement paste.
In the concrete mixture, the cement slurry wraps the aggregates and fills the gaps between the aggregates, forming a slurry layer on the surface of the aggregates. The thicker the slurry layer that wraps the aggregates, the smaller the resistance to the movement of the aggregates.
Increase the amount of slurry, the fluidity of the concrete will increase, and a certain thickness of grout layer will be formed on the inner wall of the pumping pipeline. The grout layer can lubricate the inner wall of the pipeline, thereby reducing the friction resistance between the mixture and the pipeline, and make the mixture Easy to pump.
Cement type, mineral composition, fineness, etc. are also important factors that affect pumping performance.
In addition to the slurry, there are coarse and fine aggregates in the main components of pumped concrete, and aggregates occupy the largest volume in concrete. Aggregate gradation, particle shape, maximum particle size, and water absorption performance will all affect the pumping performance of concrete.
(1) Aggregate gradation
The aggregate-graded concrete has a small void ratio of the mixture, and the pumping performance can be improved under the condition of the same amount of cementing material and admixtures.
(2) The shape of aggregate particles
If the surface of the aggregate particles is smooth, with few edges and corners, and low porosity, the slurry needed to fill the gaps and wrap the particles will be less, and additional slurry will be added correspondingly, which can thicken the lubricating layer of the pipeline. With the thickness and the thickness of the coating layer, the flow performance of the mixture will also be enhanced, and the friction resistance of the concrete will be reduced, which further improves the pumping performance of the concrete.
The pin-sheet shape of the aggregate has a greater impact on the pumping performance, and its content should be strictly controlled.
(3) Maximum particle size
When pumping concrete for conventional buildings, it is required that the ratio of the maximum particle size of the aggregate to the inner diameter of the pipe should not be 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, segregation is prone to occur. Therefore, in the construction of super high-rise buildings, it is necessary to select aggregates with a smaller particle size, and the ratio of the maximum aggregate particle size to the pipe diameter is less than 1:5.
(4) Water absorption performance of sand and gravel
Due to the presence of sunken holes on the surface of the sand and gravel, the unsaturated sand and gravel will cause the water in the concrete to penetrate into the cavities under the action of the pumping pressure, which will affect the pumping performance of the concrete.
For sand and gravel with high water absorption, when preparing pumped concrete, it should be wetted to make the cavities on the surface saturated with water.
(5) The influence of sand rate (Sand coarse aggregate ratio)
Under the premise of a certain amount of slurry, if the sand ratio is too large, there will be more sand than stones, and the total surface area of the aggregate will become larger, and the slurry covering the aggregate will become less and thinner, and the frictional resistance between it and the inner wall of the pipeline will also become Large, resulting in poor fluidity of the mixture.
If the sand ratio is too small, there will be less sand than stones, and the gaps between the stones are difficult to be filled with sand. It is necessary to fill the excess gaps with slurry, which will reduce the amount of slurry wrapped on the surface of the aggregate and make the gap between the aggregates The increased frictional resistance will also cause the fluidity of the mixture to deteriorate.
During concrete pumping, the gaps between the coarse aggregates will increase, and the slurry will travel through the gaps of the coarse aggregates under the action of the pumping pressure, and the coarse aggregates are difficult to flow due to excessive frictional resistance. Will produce concrete bleeding and segregation phenomenon.
In the low-strength machine-made sand concrete, as the stone powder content and MB value increase, the pumping resistance gradually decreases. When the stone powder content is 20%, the pumping resistance is the least; fly ash and air-entraining agent can effectively reduce the pumping resistance.
In high-strength machine-made sand concrete, with the increase of the stone powder content and MB value, the pumping difficulty increases. When the MB value is 0.25, the 5%~7% stone powder content does not affect the pumping of high-strength machine-made sand concrete; And slag powder, silica fume can effectively reduce the pumping resistance of high-strength machine-made sand concrete.
Super absorbent resin is an external admixture that effectively reduces the pumping resistance of machine-made sand concrete.
If the machine-made sand concrete mix design is carried out according to the rules of natural sand, the workability of the concrete will be deteriorated, bleeding will easily occur, and the pumping performance of machine-made sand concrete will be seriously affected.
To design the mix ratio of concrete according to its own characteristics of machine-made sand, it is necessary to adjust the sand ratio of the mix ratio and rationally select the type and amount of mineral admixtures to prepare machine-made sand concrete with better working performance.
The cementitious material accumulation system is mixed with mineral admixtures that are finer than cement. These mineral admixtures will fill the voids between the cement particles, so that the original filling water used to fill the voids is reduced. Under certain circumstances, the excess filling water can be turned into surface layer water, which will increase the thickness of the water film.
As the specific surface area of the mineral admixture increases, more surface water is needed to wrap the surface, and the thickness of the water film will also be affected.
Therefore, the water-reducing effect of mineral admixtures should consider water filling and surface absorption.
Water-reducing and set-retarding admixtures
Adding superplasticizers can comprehensively improve the performance of concrete and is the main method for preparing high-performance concrete. The double mixing method of superplasticizer and high-quality mineral powder admixture can improve the water retention and fluidity of high-performance concrete, and comprehensively improve the pumpability of high-strength and high-performance concrete.
Water plays a key role in whether the concrete mixture can be pumped successfully, and various materials in the concrete components can be glued into a whole through water.
Insufficient amount of water will increase the viscosity of the concrete mixture (even the various materials of the components cannot be glued into a whole and are in a loose state), the workability of the mixture will become poor, and the pumpability of the concrete will not be realized.
However, excessive water consumption will make the concrete easy to segregate and cause the pump to block, and it is also extremely unfavorable for the strength growth and durability of the concrete (After the concrete is poured, excessive moisture will evaporate, which will cause the concrete to shrink and crack, which directly affects the durability and strength of the concrete.).