Portable Air Compressors,Electric Air Compressor,Air Compressor small,Industrial air compressor AWLOP CO.,LTD , https://www.awlop.com
1. The role of cement in the pumping process The pumping pressure of the concrete is transferred by the liquid phase of the material. The liquid phase material carries the solid phase material together to complete pumping. There are two aspects of the role of cement, one is cementation, so that the concrete in the pumping to maintain the solid phase material is surrounded by liquid phase material;
The second is that the lubricating action reduces the frictional resistance between the concrete and the mechanical parts of the pump, the conveying pipe and the concrete and has good fluidity. 2. The effect of cement type on the pumpability of concrete The cement should have good water retention performance, so that it is not easy for the concrete to pump out water during pumping. Ordinary portland cement and pozzolanic cement have better water retention, while slag cement has poor water retention. If it is used to mix pumped concrete, it is necessary to increase the amount of cement and increase the sand rate or add a part of fly ash. Low slump. When fly ash cement is used, the fluidity of the concrete is good, but the early water exudation is large, and it is preferable to use a lower slump and pump it as continuously as possible.
3, cement dosage should be appropriate. If the amount of cement is not enough, the volumetric efficiency of pumping is reduced, and the transport resistance is significantly increased. Moreover, this concrete has poor water retention, and it is prone to bleeding, segregation, and clogging. If the amount of cement is too large, the concrete will have excessive viscosity and increase the transport resistance. Large, but the pumping volume efficiency does not change much. Compared to gravel aggregates with the same particle size and gradation, gravel concrete has a larger amount of cement. For fine aggregates, the use of artificial crushing sand uses more cement than natural sand. The smaller the pipe diameter and the longer the pipe, the higher the fluidity, lubricity, and water retention of the concrete, so the amount of cement should be increased. If the aggregate size is appropriate, the larger the aggregate size, the smaller the amount of cement used: but when the aggregate size exceeds 30 mm, the super-diameter aggregate will destroy the continuity of the concrete, and it is difficult for the cement slurry to fully fill these aggregates. Containment, so the amount of cement can no longer be reduced. Fines smaller than 0.2 mm in fines also act like cement. However, these fines should have a certain proportion. If the content is insufficient or the average particle size is too small, the amount of cement should be increased. If pumping lightweight aggregate concrete, the amount of cement should be increased. This is due to the fact that lightweight porous aggregates absorb water under high pressure and release water under reduced pressure, so that it is easy to make the concrete hard and easy to separate during pumping. Therefore, there should be more cement slurry in the light aggregate concrete, and the amount of cement should not be less than 340kg/m3.
Second, aggregate
1. Fine aggregates Fine aggregates can be classified into river sand, sea sand, mountain sand, and artificial broken sand according to their sources. The former adopts concrete with better pumpability; artificial sand has a rough surface, and the sand shape is not good. It is necessary to add part of natural sand to improve the mixing ratio. However, artificial sand has better water retention and can reduce bleeding and segregation of concrete. According to the size of the average particle size, fine aggregate can be divided into three types of fine sand in coarse sand. The concrete prepared with medium sand has the best pumpability and its average particle size is 0.25-0.3 mm. The average particle size is too small (ie too much fine sand) is not good, will increase the amount of concrete water and cement consumption. If measured by the modulus of fineness of the sand, the particle size of the fine aggregate with an FM value between 2.4 and 3.0 is sufficient for pumping. In pumped concrete, the fine aggregate material has a large relationship with the voidage of the coarse aggregate. The cement mortar must be filled with voids of coarse aggregate, the sand rate is insufficient, and the voids are filled with cement, which will increase the amount of cement, otherwise the aggregates will be easily segregated; if the sand rate is too high, the cement must fill the gaps of the sand, and the amount of cement must also increase. Otherwise, the lubricating effect of cement mortar will be greatly reduced, and the pumping resistance will increase significantly. Therefore, under certain conditions, there is an optimal sand rate. If the gradation of coarse aggregates is reasonable, the larger the aggregate size, the lower the optimal sand rate. The sand rate is related to the average grain size or fineness modulus of the sand. The smaller the grain size, the lower the sand rate. The sand rate also decreases with the increase of the cement dosage. In the case of high sand rates, as long as the amount of cement increases, there is no significant effect on the pumpability of the concrete.
2. Coarse aggregate pumping concrete can use pebbles, gravel or aggregates mixed with gravel and gravel.
The pebble aggregate concrete has the best pumpability, followed by the mixture and the worst aggregate of gravel.
For the limitation of the maximum size of coarse aggregates, please refer to the table below. Minimum diameter of conveying pipe Maximum diameter of coarse aggregate (mm) (mm) Pebble gravel 100 35 30 80 30 25 70 20 20 The maximum size of coarse aggregate for pumped concrete Limited by the minimum diameter of the conveying line. The maximum size of the pebbles should not exceed 1/3 caliber; the gravel should not exceed 1/4 caliber. A small amount of super-diameter aggregate is allowed to mix in. For example, in a 100-mm diameter delivery tube, a small amount of aggregate with a particle size of up to 50 mm can be passed, but this super-diameter aggregate must not exceed 1%, and they must be dispersed. of. Pumped concrete requires that the voidage of the coarse aggregate is as small as possible, and its gradation should be continuous and uniform, so that better pumpability can be achieved with a smaller amount of cement and sand. The gradation of the coarse aggregate should meet the requirements in the following table. Grading standard for coarse aggregates: Weight percentages of aggregates passing through standard sieves in various particle size ranges Particle size range Nominal size of mesh (mm) 50 40 30 25 20 15 10 5 2.5 30 -5 100 100-95 75-40 35-10 10-0 5-0 25-5 100 100-90 90-60 50-20 10-0 5-0 20-5 100-90 100-90 (80-55 ) 55-20 10-0 5-0 The pumpability of concrete is very sensitive to intermittent or non-uniform reaction of the aggregate gradation. The shortage of a component, or the appearance of localised, monolithic coarse aggregates in concrete, can result in pumping clogged accidents. The proportion of coarse aggregate in the total amount of aggregate has a great influence on the pumpability of the concrete, which is manifested in the changes in the pumping pressure and the volumetric efficiency of the concrete pump. In actual use, as long as the sand rate is controlled, the amount of coarse aggregate is naturally controlled.
Third, slump slump Slump in the concrete 10-15 cm dumped into the hopper, the surface is easy to become fragmented; slump below 10 cm, the concrete tilted completely broken, and with There is a flank edge, which is vertically broken into chunks. After the high slump concrete is dumped, there is no breakage on the surface. The transport resistance of concrete increases with the decrease of slump. The rate of change in slumping resistance of lightweight aggregate concrete is large, especially when the slump is less than 15 cm. As the slump decreases, the transportation resistance increases sharply. The volumetric efficiency of the fine stone concrete pump decreases as the concrete slump decreases, but this change is not significant when the slump is more than 10 cm. This range is 10-23 cm. Concrete with a slump of 10-20 cm has the best pumpability.
When the slump is too large, the pumpability is significantly reduced due to:
- When the concrete distribution valve is not tight, it is easy to leak the slurry;
- The concrete is easily segregated, causing blockages. Especially when conveying at a long distance, due to the action of gravity, the coarse aggregate gradually decreases, and then the cement slurry deposited on the upper part of the pipeline is squeezed out under pressure, and the suspended state of the coarse aggregate in the cement mortar is completely lost. Blocked.
Fourth, additives to improve the pumpability of concrete, it is necessary to increase the amount of cement, with a larger water-cement ratio and sand rate, more stringent aggregate grading, which means that the cost increase or quality reduction. A more economical and effective approach is to use admixtures to improve the pumpability of concrete. There are many kinds of additives, of which the admixtures that can improve the pumpability of concrete include air-entraining agents, water-reducing agents, retarders, and pumping agents. Water-reducing agents can change the agglomerate structure of the cement slurry into a disperse structure, thereby improving the fluidity. If the fresh concrete's fluidity is kept unchanged, the use of water-reducing agent can greatly reduce the amount of mixing water, thereby improving the stability of the concrete and improving the strength and compactness. If you keep the concrete strength, you can save the amount of cement. Common water-reducing agents include lignin sulfonate and naphthalene sulfonate formaldehyde condensate, such as calcium lignosulfonate, NNO, Jianyi, MF, and FDN. The air-entraining agent can make the concrete produce many fine air bubbles, improve the cohesion and water retention of the concrete, improve the fluidity, and can also improve the frost resistance and impermeability of the concrete, but affect the strength of the concrete. Commonly used air-entraining agents include rosin soap, sodium alkyl sulfonate, sodium fatty alcohol sulfate, and aluminum powder air-entraining agents. Retarder retards the coagulation and hardening of concrete, and can sometimes act as a water reducer. The role of retarder is to prolong the transport time of concrete, and it is important to pump in the hot season. Common retarders include molasses, calcium lignosulfonate, tartaric acid, citric acid, and boric acid. The pumping agent is a kind of lubricant. Like other admixtures, it is not necessary to change the mix ratio of concrete to significantly improve its fluidity, that is, it can expand the proportion design range of concrete. The commonly used pumping agents include turbid water type, aerated plastic, chemical type, and non-gasification plastic type, and there are many specific types. In order to improve the pumpability of the concrete, it is now possible to obtain the combined effect of improving the quality of the concrete or saving cement to use one or more admixtures. The effect, dosage, and method of addition of various admixtures shall be referred to the relevant regulations.
V. Adjustment of Pumping Concrete Mixing Ratio In construction, in the objective conditions, the concrete mix ratio to the original design cannot be fully realized, and some components in the concrete should be changed. To ensure the pumpability of the concrete, it is necessary to make corresponding adjustments to other components. The adjustment of water consumption and sand rate is common. According to a variety of different conditions of change, the adjustment method can be determined on its own, in order to achieve appropriate pumping.
First, cement