Compared with the pouring construction of ordinary concrete, the pouring construction of pumped concrete has many advantages (such as saving manpower, material resources, shortening the construction period, etc.), and the single cost increase is not much, so in recent years, concrete construction has been widely used in high-rise buildings Pumped concrete pouring and the extensive use of pumped concrete have promoted the development of commercial concrete. The advancement of science and technology has provided guarantee for the preparation and construction of pumped concrete. However, pumped concrete generally has the characteristics of large slump, which to a certain extent increases the difficulty of concrete quality control. In particular, the segregation and sinking of concrete are very common, which seriously affects the apparent performance and structural function of concrete. . Therefore, how to suppress and solve the segregation problem of concrete is an important issue that needs to be researched in the current development of pumped concrete. In response to the above problems, this article mainly analyzes the causes of concrete segregation from the raw materials used in concrete, and proposes countermeasures.
1. Harm caused by segregation
Segregation refers to a phenomenon in which the cohesion between the materials of the concrete mixture is insufficient to resist the sinking of the coarse aggregate. The main manifestations are concrete aggregate separation, delamination, sinking, poor workability, etc. Concrete segregation will seriously affect all aspects of concrete performance.
1.1 It affects the pumping construction performance of concrete, causes sticky tanks, blocked pipes, affects the construction period, etc., and reduces economic benefits.
1.2 Affects the apparent effect of the concrete structure, such as sand patterns, exposed aggregates, and exposed steel bars on the surface of the concrete.
1.3 The strength of concrete will be greatly reduced, which will seriously affect the load-bearing capacity of the concrete structure and destroy the safety performance of the structure. In severe cases, it will cause rework and cause huge economic losses.
1.4 The poor homogeneity of concrete results in inconsistent shrinkage of various parts of the concrete and is prone to 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.
1.5 greatly reduces the durability properties of concrete such as concrete impermeability and frost resistance.
2. 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 grade 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.
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 cement fineness. As we all know, the higher the fineness of the cement, the higher its activity and the greater the water demand of the cement. At the same time, the greater the fineness of the cement, the stronger the adsorption capacity of the cement particles to the concrete water-reducing agent, which greatly weakens the reduction of concrete water-reducing agents. Water reducing effect of aqueous 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.
2.1.2 Changes in alkali content in cement. The 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 the water-reducing agent increases. Therefore, when the alkali content of cement changes significantly, it may cause changes in the viscosity and flow of concrete. have a greater impact.
2.1.3 The influence of cement storage time. Cement is a kind of hydraulic cementitious material. If it is not stored well, it is very susceptible to moisture. After the cement gets damp, the water demand will be reduced. At the same time, the longer the storage time of the cement, the temperature of the cement itself will decrease, and the adsorption between cement fine powder particles will occur. They condense into larger particles, which reduces the surface energy of the cement particles and weakens the adsorption of the water-reducing agent by the cement particles. During the concrete test, the water-reducing effect of the water-reducing agent is often enhanced, and the new concrete mixture appears to ooze slurry. , the phenomenon of sinking 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.
In summary, 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 it:
(1) After the cement enters the factory, the test items must be carried out according to the requirements Test, pay special attention to the water demand of cement, and take action in time when abnormal water demand is found.The segregation problem caused by excessive flake stone chips in the concrete cannot be solved by adjusting the sand rate alone. The concrete cementing materials (especially the amount of admixtures) should be increased and the amount of admixtures should be adjusted at the same time.
(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.
2.5 Others. Many causes of concrete segregation have been analyzed above, but the causes of concrete segregation are far more than these. Issues such as cement dosage, types of cement and admixtures, and measurement 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.
3. Summary
3.1 The key to preventing concrete segregation is prevention.
3.2 When segregation occurs in concrete, the quality of various raw materials should be carefully analyzed and checked. It is best to use comparative tests to check, use the elimination method to find the cause, and prescribe the right medicine.
3.3 Segregation is a common quality problem in pumped concrete. When a problem occurs, it should be tested in time. The problem is easy to solve.
<img src="/upfile/202304/2023041334528065.jpg"/
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