Commercial concrete is a mixture of cement, mineral admixtures, sand, stone, water and admixtures. Good workability of concrete mixture is the prerequisite and basis for ensuring the quality of concrete. This requires that concrete must have good fluidity as well as excellent cohesion and water retention.
Cohesion, water retention and fluidity are interrelated and contradictory. Concrete has good cohesion and often has good water retention, but it may have poor fluidity. ; When the fluidity of concrete gradually increases, the cohesion and water retention tend to become worse, and the probability of bleeding and segregation increases. Reducing bleeding and segregation in the production process of commercial concrete is a problem that technicians have to face. Regarding bleeding and segregation, the author briefly talks about some of my superficial opinions for discussion by colleagues.
1. The concepts of segregation and bleeding
1.1 Bleeding
After the concrete mixture is poured and vibrated, during the setting process, part of the mixed water floats to the surface of the concrete along with the sinking of the granular materials, which is called Bleeding water. A small amount of bleeding on the surface of the concrete is normal and has no effect on the performance of the concrete mixture and hardening. However, excessive bleeding will bring many adverse consequences.
1.2 Segregation
Due to the particle size and The difference in density leads to the separation and unevenness of components, which is called segregation. This uneven distribution of components will lead to differences in the performance of each part of the concrete, which can easily cause some defects inside or on the surface of the concrete, affecting the performance and normal use of the concrete. Therefore, it is necessary to take effective measures to prevent segregation and ensure that the concrete mixture has good cohesion and homogeneity, thereby ensuring the consistency of the performance of each part of the concrete.
There are two types of segregation in concrete. One is that the coarse aggregate in the concrete is easy to separate and fall off, and the other is that the cement slurry is easy to separate from the mixture during mixing. . Cement slurry with higher viscosity is helpful to prevent segregation, so when the concrete water-cement ratio is small, the segregation tendency is small; while the concrete mixture with a small amount of cementitious material is prone to segregation of coarse aggregate when the fluidity is high; large Fluid concrete is prone to cement slurry separation.
2. Causes of concrete bleeding and segregation
2.1 Effect of concrete raw materials on bleeding , The influence of segregation
(1) Admixtures
Admixtures used in concrete, Most of them are multi-functional products composed of water-reducing agents and other products such as air-entraining agents, retarders, plastic retaining agents, etc. They are indispensable and important materials for pumping concrete. The addition of admixtures can greatly improve concrete. Mixture performance, but improper use of admixtures may lead to concrete segregation. (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.
(2) Cement reasons
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 greater the impact of its cement particles on the concrete water-reducing agent. 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 alkali content in cement
Adaptability of alkali content to cement and admixtures The impact is great. The alkali content of the cement decreases and the water-reducing effect of the water-reducing agent is enhanced. Therefore, when the alkali content of the cement changes significantly, it may have a greater impact on the viscosity and fluidity of the concrete.
The impact of cement storage time
<p style="text-indent: 2emAs long as the surface is closed in time, it will not have any adverse effects on the concrete. On the contrary, bleeding will reduce the water consumption of concrete and reduce the water-cement ratio, which can improve the strength.4. Countermeasures against bleeding and segregation during commercial concrete production
Commercial concrete After segregation or bleeding occurs in the mixture, test personnel should be organized in time to analyze the causes of bleeding and segregation, find out the main factors, and solve the problem in a targeted manner.
(1) Check whether the quality of raw materials fluctuates
The quality fluctuations of raw materials cause concrete mixing The direct factor of compound fluctuation is that after segregation and bleeding, personnel should be organized to review the raw materials in a timely manner. Factors such as the standard consistency water consumption of cement, the adaptability of cement and admixtures, the water reduction rate of admixtures, the water demand (fluidity) ratio of mineral admixtures, and the water content and mud content of sand and gravel. Carefully analyze the experimental results of various raw materials, compare them, and find out the reasons.
(2) Check whether the concrete measuring equipment is abnormal
Failure of concrete mixing equipment can also cause concrete One of the reasons for bleeding and segregation of the mixture is that in many cases it is hidden and should not be discovered. Check whether there is a measurement failure in the admixture scale and water scale, resulting in excessive weighing and segregation of concrete; check whether there is insufficient loading of cement, mineral admixtures and sand; check whether there are other failures that affect the measurement of raw materials.
For example, during the production of a certain mixing station, it was discovered that the slump of the first concrete load produced every morning or when the machine was not used for a long time was too high. , accompanied by bleeding. After searching for the cause, it was found that the admixture valve had not been replaced for a long time and was severely worn. The admixture leaked and dripped into the mixing water meter. As the interval lengthened, the amount of admixture dripping also increased, resulting in the amount of admixture in the concrete. If it is too large, the concrete will segregate and bleed. After finding the cause, the admixture valve was replaced and production returned to normal.
(3) Adjust the concrete mix ratio
The direct cause of concrete bleeding and segregation It means that there is too much free water in the concrete mixture, and the concrete cannot retain the excess water, so it is released in the form of bleeding. The method of adjusting the concrete mix ratio for concrete bleeding and segregation is generally reflected in two aspects:
On the one hand, it reduces the impact of concrete free water, and then controls Concrete bleeds. For example, reducing water consumption, reducing the amount of admixtures, thereby reducing the water reduction rate, and reducing the relative amount of free water in the concrete system;
On the other hand, improving the strength of concrete Water retention, reducing the chance of bleeding. For example, increase the amount of cementitious material, increase the sand rate or use sand with a smaller fineness modulus, increase concrete air-entraining agent, and improve the water retention of concrete.
5. Conclusion
Concrete bleeding and segregation are common problems in production practice Phenomenon, the reasons for this phenomenon are many and complex. The author talked about some of his rough opinions on this for everyone's criticism and guidance. In the process of concrete production, it is necessary to strictly control the quality of raw materials, strictly follow the mix ratio for production, strictly control parameters such as water consumption, water-cement ratio and sand rate, regularly inspect production equipment, and strengthen personnel education and management.
