As we all know, ordinary concrete is mainly mixed with cement, water, sand and gravel coarse and fine aggregates, admixtures and other materials; pumped concrete is based on ordinary concrete to adjust the sand and gravel coarse and fine aggregate gradation, sand rate and mixing ratio. It is made by 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.
2.1 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.
2.1.1 Changes in the fineness of cement.
It is well known that the higher the fineness of cement, the higher its activity and the greater the water demand of cement. At the same time, the greater the fineness of cement, the stronger the adsorption capacity of its cement particles to concrete water-reducing agents, which is greatly The water-reducing effect of the water-reducing agent is weakened. 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.
2.1.2 Changes in alkali content in cement.
Alkali content has a great influence on the adaptability of cement and admixtures. As the alkali content of cement decreases, the water-reducing effect of water-reducing agent increases. Therefore, when the alkali content of cement changes significantly, it may lead to Concrete has a greater impact on viscosity and fluidity.
2.1.3 The influence of cement storage time.
Cement is a kind of water -hard gel material. If it is not stored well, it is vulnerable to moisture. The amount of water after the cement will be reduced. They 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 concrete tests, the water-reducing effect of the water-reducing agent is often enhanced, and the concrete is newly mixed. The material appears to secrete pulp 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 kind of factors are affected, the results are the same, that is:
1 .Changes in cement water demand,
2. Adaptability changes of 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 site, it must be tested according to the required test items, paying special attention to the needs of the cement. If the water demand is abnormal, conduct a timely adaptability test of the cement and the currently used admixtures. If necessary, re-do the concrete mix test.
2. Under the premise of ensuring that the water-cement ratio of the concrete remains unchanged (basically ensuring the 28d strength of the concrete), appropriately adjust the amount of water-reducing agent. 3. On the basis of ensuring strength, use a larger mix ratio of fly ash and other admixtures for production (commercial concrete companies 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.
2.2 Admixtures
The admixtures used in concrete are mostly multi-functional products composed of water-reducing agents and other products such as air-entraining agents, retarders, plasticity retaining agents, etc. It is an indispensable and important material for pumping concrete. 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, but will continue to work during the concrete transportation process. As a result, the slump of the concrete at the site is greater than the slump when it leaves the machine. This situation can easily cause serious segregation of concrete. And it is manifested in high-strength 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 proportion appropriately to reduce the amount of water reducing agent; 2. Add a certain amount of thickening agent to the concrete admixture; 3. A certain amount of air-entraining agent combined with the agent can enhance the cohesion of concrete and improve the segregation resistance of concrete; 4. During the trial mixing of concrete, the concrete should have a slump loss of 20~30mm under static conditions ( 1h), concrete is not prone to segregation in actual production.
2.3 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, compactness and strength properties of concrete. The fineness of high-quality fly ash is in the range of 20 to 8, which 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. Similarly, 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 admixture is relatively weak compared to the cementing material. The dosage has actually been increased.
So the segregation of concrete will occur and the following measures should be taken for fly ash:
1. Strengthen testing. It is best to test every fly ash entering the site and resolutely deal with unqualified materials. Cannot enter the venue 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 should be strengthened. Or choose a mix ratio with a lower additive content for production.
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