Segregation of concrete is a phenomenon in which the cohesion between the components of the concrete mixture is insufficient to resist the sinking of the coarse aggregate, and the components of the concrete mixture separate from each other, resulting in uneven internal composition and structure. It usually manifests as the separation of coarse aggregate and mortar from each other, for example, dense particles are deposited at the bottom of the mixture, or coarse aggregate is separated from the mixture as a whole.
Chinese name: Concrete segregation properties, methods, characteristics, advantages among the materials of the concrete mixture, coarse aggregates are separated from the mixture as a whole
Causes of segregation
The cause of segregation may be pouring, Improper vibration, the maximum particle size of the aggregate is too large, the proportion of coarse aggregate is too high, the content of cementitious materials and fine aggregate is low, the density of coarse aggregate is too high compared with fine aggregate, or the mixture is too dry Or too thin, etc. The tendency to segregation can be reduced by using mineral admixtures or air-entraining agents.
The hazards of segregation
Affect the pumping performance of concrete, cause sticky tanks, blocked pipes, affect the construction period, etc., and reduce economic benefits.
Affects the apparent effect of the concrete structure, such as sand patterns, exposed aggregates, and exposed steel bars on the surface of the concrete.
The strength of the concrete will be greatly reduced, which will seriously affect the load-bearing capacity of the concrete structure, destroy the safety performance of the structure, and in severe cases will cause rework and huge economic losses.
The poor homogeneity of concrete results in inconsistent shrinkage of various parts of the concrete, which easily causes shrinkage cracks in the concrete. Especially when constructing concrete floor slabs, due to concrete segregation, the surface cement slurry layer thickens, shrinkage increases sharply, and serious cracks occur. It greatly reduces the durability properties of concrete such as concrete impermeability and frost resistance.
Influencing factors
Causes and countermeasures of concrete segregation
As we all know, ordinary concrete is mainly composed of cement, water, sand and gravel coarse and fine aggregates, admixtures and other materials; pumped concrete is On the basis of ordinary concrete, it is mixed by adjusting the coarse and fine aggregate gradation and sand rate of sand and gravel and adding a certain amount of fly ash. In addition to the mixing method and mixing time, concrete segregation is generally related to the quality of the cementitious materials of the concrete mixture, excessive water consumption, poor gravel gradation, and excessive amount of water-reducing agent. The relationship is closer.
Cement
Cement is the most important cementing material in concrete. The stability of cement quality directly affects the stability of concrete quality. Changes in cement quality will cause segregation of concrete, and there are many factors in cement that affect the performance of concrete mixtures.
Changes in cement fineness
As we all know, the higher the fineness of cement, the higher its activity, the greater the water demand of cement. At the same time, the greater the fineness of cement, the greater the impact of its cement particles on concrete water-reducing agents. The stronger the adsorption capacity, the greater the water-reducing effect of the water-reducing agent. Therefore, in actual production, when the fineness of cement is greatly reduced, the water-reducing effect of concrete admixtures will be enhanced. When the dosage of admixtures remains unchanged, the water consumption of concrete will be greatly reduced. The decrease in cement fineness can easily lead to excessive amounts of concrete admixtures, causing concrete to segregate. And this kind of segregation usually occurs in high-strength concrete grades with higher amounts of water-reducing admixtures.
Changes in the alkali content in cement
The alkali content has a great influence on the adaptability of cement and admixtures. The alkali content of cement decreases and the water-reducing effect of water reducing agent increases. Therefore, when the alkali content of cement When obvious changes occur, it may have a greater impact on the viscosity and fluidity of concrete.
The impact of cement storage time
Cement is a hydraulic cementitious material. If it is not stored well, it is easily affected by moisture. After the cement gets damp, the water demand will decrease; at the same time, the longer the cement is stored, the temperature of the cement itself will decrease. The cement fine powder particles condense into larger particles through adsorption, which reduces the surface energy of the cement particles and weakens the adsorption of the water-reducing agent by the cement particles. In the concrete test, the water-reducing agent often appears. The water-reducing effect is enhanced, and the new concrete mixture appears to bleed out and sink to the bottom. In actual production, if cement stored for a long time is used and the mix ratio is not adjusted, it is very easy to cause concrete segregation. Of course, the impact of cement storage time on different types of cement is inconsistent, which needs to be understood through experiments. To sum up, it can be seen that there are many factors in cement that affect the workability of concrete, and they are also relatively complex. However, no matter what the factors are, the results are the same, that is: (1) The cement needs Changes in water volume; (2) Adaptive changes in cement and admixtures. Therefore, if the segregation of concrete is caused by cement, the following measures can generally be taken to solve the problem: (1) After the cement enters the factory, it must be tested according to the required test items, paying special attention to the water demand of the cement, if any abnormal water demand is found When required, conduct a timely adaptability test of cement and currently used admixtures. If necessary, re-do the concrete mix test. (2) On the premise of ensuring that the water-cement ratio of the concrete remains unchanged (basically ensuring the 28-day strength of the concrete), appropriately adjust the amount of water-reducing agent. (3) On the basis of ensuring strength,�Use a mix ratio with a larger amount of fly ash and other admixtures for production (commercial concrete companies should have different mix ratios of the same strength level), which must be based on experiments. (4) Substituting a small amount of cement with I and II fly ash will be able to well control the segregation phenomenon of concrete and improve the workability of concrete.
Admixtures
Most of the admixtures used in concrete are multi-functional products composed of water-reducing agents and other products such as air-entraining agents, retarders, plastic retaining agents, etc. They are pumped Concrete is an indispensable and important material. The addition of admixtures can greatly improve the performance of concrete mixtures. However, improper use of admixtures may lead to segregation of concrete. (1) If the amount of concrete water-reducing agent is too large, the water-reducing rate is too high, and the water consumption of a single concrete is reduced, it is possible that the water-reducing agent will not fully play its role in the mixer, and continuous occurrences will occur during the concrete transportation process. The effect causes the slump of the concrete to the site to be greater than the slump when it leaves the machine. This situation can easily cause serious segregation of concrete. And it often appears in high-strength grade concrete, which is extremely harmful to concrete. (2) Excessive amounts of retarding components and plastic-preserving components in admixtures, especially excessive amounts of phosphates or sugars, can easily cause segregation in concrete. When concrete segregates due to admixtures, adjustments can be made from the following aspects: (1) Adjust the mix ratio and reduce the amount of water-reducing agent; (2) Add a certain amount of thickener to the concrete admixture; (3) Combining a certain amount of air-entraining agent with admixtures can enhance the cohesion of concrete and improve the segregation resistance of concrete; (4) During the trial mixing of concrete, the concrete should be allowed to have a temperature of 20 to 20% under static conditions. With a slump loss of 30 mm (1h), concrete is not prone to segregation in actual production.
Fly ash
Fly ash is one of the important admixtures of concrete. Although the dosage is very small, adding an appropriate amount of fly ash to concrete can greatly improve the workability and compactness of concrete. and strength properties. High-quality fly ash, with a fineness in the range of 20 to 8, is an ideal material for preparing concrete. It can replace 10% to 30% of the cement dosage, greatly reducing the cost of concrete production. If the quality of fly ash fluctuates greatly, it will increase the difficulty of concrete quality control and sometimes cause segregation of concrete. (1) When the quality of fly ash suddenly changes (for example, the fineness changes from 19% to 4%), the water demand of fly ash decreases greatly, which may easily cause sudden segregation of concrete; (2) When the same When the quality of fly ash suddenly deteriorates (for example, the fineness changes from 19% to 38%), since a large part of the weight of the fly ash has lost its function as a cementing material, the proportion of admixtures relative to the cementing material has actually increased. , so segregation of concrete will occur. The following measures should be taken for fly ash: (1) Strengthen testing. It is best to test each vehicle of fly ash entering the factory. Unqualified materials must not enter the site to play a preventive role. (2) Adjust the amount of fly ash and choose a lower mix ratio for production. (3) When the quality of fly ash is good, the water consumption can be appropriately reduced and stirring can be strengthened. Or choose a mix ratio with a lower additive content for production.
Sand and gravel aggregate
Sand and gravel are the most commonly used materials in concrete. The quality of sand and gravel directly affects the quality of concrete. Fluctuations in the quality of sand and gravel can easily cause segregation of concrete, and are factors that cause segregation. It’s multifaceted. (1) The increase in gravel particle size, deterioration of gradation, and single gradation can easily cause the segregation of concrete. (2) Excessive stone content in sand, especially excessive flake stone chips, will seriously affect the workability of concrete and lead to serious segregation of concrete. (3) The moisture content of sand and gravel is too high (especially the moisture content of sand is too high, greater than 10%), which will make it difficult to control the quality of concrete and prone to concrete segregation. Because the water content in the sand is too large, the water content in the sand is in a supersaturated state. When the concrete mixture is mixed in the mixer, the water content in the capillary tubes on the surface of the sand cannot be released in time, so it is easy to use too much mixing water during mixing. Large; at the same time, during the transportation of concrete, the water in the capillary tubes of the aggregate is continuously released, destroying the bond between the aggregate and the cement slurry, causing segregation and bleeding of the concrete. (4) Excessive mud content in sand and gravel will reduce the bonding force between the cement slurry and the aggregate, and the cement slurry’s ability to wrap the aggregate will decrease, leading to the separation of the aggregate and causing concrete segregation. For the phenomenon of concrete segregation caused by aggregates, the following measures can be taken to adjust. (1) In order to avoid segregation of concrete due to aggregate problems, we should first focus on prevention and strictly enforce the inspection system for the entry of aggregates to ensure the quality of the aggregates. (2) For the first reason above, the sand rate can be appropriately increased to adjust the concrete mix ratio to solve the segregation problem. (3) For the segregation problem caused by excessive flake stone chips in the aggregate, adjusting the sand rate alone cannot solve the problem. The concrete cementing materials (especially the amount of admixtures) should be increased, and the admixtures should be adjusted at the same time. Dosage. (4) The problem of concrete segregation caused by the problem of aggregate moisture content should be solved by extending the mixing time. Increasing the dosage of admixtures such as fly ash is also very effective in controlling this type of segregation phenomenon.
Others
The above has analyzed many causes of concrete segregation, but the causes of concrete segregation are far more than these. Issues such as cement dosage, cement and admixture types, measurement, etc. are all causes of concrete segregation. . In daily work, you should pay attention to accumulation, strengthen the management of details, and solve problems in a timely manner.
There are many reasons for concrete segregation, but the reasons for concrete segregation are far more than these. Issues such as cement dosage, cement and admixture types, measurement, etc. are all causes of concrete segregation. In daily work, you should pay attention to accumulation, strengthen the management of details, and solve problems in a timely manner.
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