How to prevent concrete ＂separation＂

Pump sent concrete of pouring construction with General concrete of pouring construction compared has many Shuo advantages (like can save human, and material, and shortened duration,), and unilateral cost improve not more, so in recent years senior building concrete construction widely using pump sent concrete pouring, pump sent concrete of of large using promote has commodity concrete of development, technology of progress for pump sent concrete of preparation and the construction provides has guarantees. But the characteristics of pump concrete universal slump, which to some extent increase the difficulty of the quality controlling of the concrete, particularly the segregation of concrete, sink is very common, seriously affects the apparent performance and structure features. So, how is this suppression, segregation of the concrete is needed for the current development of pump concrete research important issues. To solve the above problems, in this paper, concrete is used for raw materials was analyzed the reasons for segregation of concrete, and proposed responses.

1. the harm caused by segregation

Segregation refers to the concrete mix composition of cohesion between the material enough to resist sinking a phenomenon of coarse aggregate. Mainly for concrete aggregate segregation, stratification, bottom, poor workability. Segregation of concrete concrete aspects of performance will be seriously affected.

① concrete pumping construction performance, owing to effects tank, pipe, duration, reduced economic benefit.

Second, the apparent effect of concrete structure, concrete surfaces, sand, aggregate exposure and exposure of reinforcing bars, and so on.

③ concrete strength decreases significantly, seriously affecting the concrete structural bearing cage force, destroying the safety performance, caused serious rework, causing huge economic losses.

④ the homogeneity of the concrete difference, resulting in various parts of the shrinkage of concrete is inconsistent, easy to produce concrete shrinkage cracks. Especially in the construction of concrete slabs, due to segregation of concrete surface layer of cement slurry thickening and shrinkage increases rapidly, serious cracks.

II greatly reduces the concrete impermeability, frost resistance, durability of concrete.

2. reasons for segregation of concrete and countermeasures

It is well known that concrete is composed of cement, water, sand and coarse and fine aggregates, admixtures and other materials mixed together; pumping concrete was adjusted on the basis of ordinary concrete sand-gravel aggregate-level an elixir, and a certain amount of fly ash sand rate mixed together. Segregation in relation to concrete mixing method, mixing length related, generally speaking, in concrete mix of pros and cons of cementitious materials, water, and aggregate gradation worse, large amount of reducing agent such as closer.

3. cement. Cement is the primary cementing materials in concrete, cement quality directly affects the quality of concrete is stable. Quality changes will lead to segregation of concrete phenomena and factors of the cement concrete mixture properties.

4. changes of cement fineness. It is well known that higher cement fineness, the higher activity, greater water requirements of cement while cement fineness, the greater and its adsorption capacity of concrete superplasticizer on cement particle is stronger, the greatly reduced the water reducing effect of water reducing agent. Therefore, in practice, when the fineness of the cement significantly decreases, the admixture of concrete water reducing effect will be enhanced, in the case of additives for the same amount, the water consumption of concrete will be greatly reduced. Cement fineness drop, easily lead to excessive concrete admixtures, concrete segregation. And this separation usually occurs in water-reducing agent in high strength concrete with high dosage.

5. the alkali content of cement. Effect of alkali content on adaptability of cement and admixtures, alkali content of cement and water reducing effect of water reducing agent, so when the alkali content of the cement when a significant change occurs, possibly causing greater impact in terms of viscosity, fluidity.

Influence of cement storage time. Cement is a water hard rubber gel material, if store bad, very easy damp, cement damp Hou needed water will reduced; while cement store time more long, cement itself temperature has reduced, cement fine powder particles Zhijian by adsorption role each other condensation for larger particles, reduced has cement particles of surface can, weakened has cement particles on reduction agent of adsorption, in concrete test Shi often performance for reduction agent of reduction water effect enhanced, concrete new mix collection real appeared lactating pulp, and sink of phenomenon. In practice, if you use long time storage of cement and mixture ratio shall not be adjusted, easily lead to segregation of the concrete. Of course, the effect of storage time on different varieties of its cement-cement is inconsistent, this need to experiment to find out.

Above you can see, are many factors influencing the workability of cement, is more complicated, but no matter what kind of impact factors, the displayed result is the same, namely: (1) change of cement water requirement, (2) change adaptability of cement and admixtures.

Therefore, if the causes of cement concrete segregation, General can take the following measures:

(1) cement plant, the required test pilot projects, with special attention to cement water requirement of situations, when an exception is found water, cement and admixture of the suitability test. Necessary concrete mix ratio test.

(2) the premise of ratio for concrete, the same assurance (basic guarantees the 28 d strength), appropriate adjustment quantity of water reducing agent.

(3) ensuring strength based on using fly ash and other admixture of proportioning for production of a larger amount (ready-mixed concrete companies should have the same strength of different proportions), which must be based on the test.

ⅰ, ⅱ fly ash small amounts of cement, will be able to better control the segregation of concrete and improve concrete workability.

Admixtures. Using admixtures in concrete, mostly by reducing agent with other products, such as air-entraining agents, retardants, plastic additives and other composite multifunctional product, is an indispensable material of pump concrete and admixture admixture which greatly improve the performance of concrete mix, but improper use of additives can lead to segregation of the concrete.

(1) if the amount of superplasticizer for concrete and water-reducing rate is too high, concrete water use to reduce unilaterally, potentially reducing agent did not play a full part in the mixer, and continue to play a role during the transit in concrete, the slump of concrete to slump more than a machine. Such a situation can easily result in serious segregation of concrete. And is often shown in high strength grade in concrete, concrete is very harmful.

(2) in additives of retarding component, large amount of plastic components, phosphates or sugar, excessive, or are likely to cause concrete segregation.

When due to the additive causes concrete segregation when, can be adjusted from the following aspects:

(1) the adjustment ratio, reducing the amount of reducing agent;

(2) in the concrete admixture compound in a certain amount of thickening agent;

(3) in a certain amount of air-entraining admixture compound in agent, can enhance the cohesiveness of concrete and improve segregation of concrete;

(4) in the concrete timing, should make concrete under static conditions slump loss with 20~30 mm (1h), is less prone to segregation of concrete in the actual production.

Fly ash. Fly ash is one of the most important admixture of concrete, though the amount was small, but mixed with fly ash in the concrete can greatly improve the workability, compaction and strength properties. Within the scope of quality such as fineness of fly ash on the 20~8, is the ideal material for concrete mixing, can replace the 10%~30% cement, concrete production cost is greatly reduced. If fly ash quality volatility, increases the difficulty of concrete quality control, can sometimes cause a situation of segregation of the concrete.

When fly ash quality suddenly changed for the better, (as in size from 19% to 4%), reduced a lot of fly ash water demand, likely to cause sharp segregation of concrete phenomena;

Similarly when a sudden variation of quality of fly ash (the size from 19% to 38%), due to a large fraction of fly ash has lost weight Binder features, and admixtures cement material volume has actually increased, so there will be concrete with segregation.

For fly ash, you should take the following measures: (1) testing, preferably on every car into the plant's fly ash on test for unqualified materials must not enter play a preventive role. (2) adjust the amount of fly ash, selection volume lower mix ratio for production. (3) when the fly ash quality, it may be appropriate to reduce the amount of water, strengthen the mix. Or mix with low selection of admixture for production.

Sand and stone aggregate. Aggregate in concrete is the largest amount of material, quality directly affects the quality of concrete aggregate, sand quality fluctuation easily lead to segregation of concrete and a variety of factors contributing to segregation.

(1) gravel particle size, grading variations, single level are easy to create concrete segregation.

(2) stone in the sand too much, especially in the General amount of lozenge-shaped stone chips seriously affects the workability of the concrete, resulting in serious segregation of concrete.

(3) the high water content of sand (especially sand water content is too high, greater than 10%) will make it difficult for concrete quality control, prone to segregation of concrete. Due to sand in the containing water had big, sand containing water in had saturated, dang concrete mix collection material in mixer in the mixing Shi, sand surface capillary in the of containing water not can timely of release out, so in mixing Shi easy makes mix Shui dosage had big; while concrete in transport process in the, aggregate capillary in the of water constantly of to outside release, damage has aggregate and water mud of bond, caused concrete of segregation bleeding.

(4) causes the amount of sand and clay cement-aggregate bond reduction, reduced ability to package of cement aggregate, resulting in separation of aggregates, concrete segregation.

For aggregate causes concrete segregation phenomena, can be adjusted to take the following measures.

(1) to avoid the problems of concrete aggregates segregation issues, should be based on prevention, strict entry of aggregate system of checks, ensure the quality of aggregates.

(2) in view of the above reasons the 1th reason, sand ratio can properly adjust the mix to address segregation.

(3) the lozenge-shaped stone in the aggregate segregation problems caused by too large, adjust sand rate alone will not solve the problem, should increase concrete bonding material (especially the admixture dosage), and adjust the dosage of admixtures.

(4) for aggregate moisture problem on segregation of concrete means of extended mixing time should be taken to deal with it. Improve the amount of fly ash and other admixture to control this kind of segregation is also very effective.

Other. Many concrete reasons for segregation of the above analysis, but the reasons for segregation of concrete is far more than that, such as cement, cement and admixture, measurement problems are caused by the segregation of concrete reasons. In the work, attention should be accumulated, strengthening the management of details, problem solved.

Summary

1, most prevention is the key to prevent segregation of the concrete.

2, when there is a segregation of the concrete, should be carefully analysed to check the quality of raw materials, the best use of comparative test checks, use a process of elimination to find the causes and tackle.

3, segregation was more common quality problems of pump concrete, if a problem occurs, please test, the problem is easy to solve.