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How to Improve the Cohesion of Low and Medium Strength Concrete?

Thomas Huang posted this article in June 30, 2020

The workability of concrete includes many indicators such as fluidity and cohesiveness. Concrete production practice often encounters the problem of poor cohesiveness of low and medium strength concrete. It is often manifested as the separation of exposed stone and mortar, and the water retention is poor. Sometimes, when the amount of water-reducing agent is normal, the concrete has no segregation, but the exposed stone phenomenon is more obvious, that is, the slurry cannot wrap the crushed stone well. The problem cannot be solved fundamentally by simply increasing the sand rate. When the amount of water-reducing agent is reduced by 0.1% to 0.2%, the fluidity of the concrete mixture becomes poor, and the slump loss is accelerated. The slump loss exceeds 60mm after the concrete is discharged for 30 minutes. , Causing on-site construction difficulties.
1. Sand screen analysis
The quality of fine aggregate and particle gradation will affect the workability of concrete, especially the fine particle content in fine aggregate of 0.30mm and 0.15mm or less, which is very important for the cohesion and water retention of concrete. Both domestic and foreign standards It is recommended to reach 15% and above 5% respectively. For low- and medium-strength concrete, the sand fineness modulus should be about 2.5, and the particle size should be less than 0.30mm and the particle content should be 20%. When the concrete mixture has poor cohesiveness, the fine sand can be properly mixed to increase the content of particles below 0.30mm to improve the cohesiveness. When there is no fine sand, the amount of fly ash can be increased.
2. Slurry volume
The fine aggregate in the concrete fills the voids of the coarse aggregate, and the slurry fills the voids of the aggregate. The slurry is composed of glue, water, admixture and gas content, which should be included in the contribution of concrete gas content to the slurry slurry. Under normal conditions, the volume of the single-side slurry of medium and low-strength concrete is 0.28-0.33m3, and the content of the slurry is insufficient. If it cannot be effectively filled and wrapped with aggregate, the cohesion will be deteriorated.
For the poor cohesiveness of concrete caused by insufficient slurry, you can keep the water-binder ratio unchanged and increase the amount of water, increase the amount of slurry or add air entraining components and additives to increase the gas content of the concrete, but it should be noted that the gas content is increased by 1%. Concrete strength will be reduced by about 4%.
3. Compatibility of additives and gelling materials
Admixtures can improve the working performance of freshly mixed concrete, but in production practice often encounter the phenomenon of poor compatibility of admixtures with the raw materials of concrete. The compatibility of additives with concrete raw materials is an important factor that cannot be ignored, especially the compatibility of additives with cement and mineral admixtures. Concrete mortar is easy to separate, easy to expose stone, and the amount of admixture is relatively sensitive. When the amount of admixture is low, the fluidity of the concrete is poor, and the slump loss is accelerated. After increasing the amount of admixture, it is easy to cause the separation of the mortar and bone, and the cohesiveness is worse. It is a typical manifestation of the poor compatibility of the admixture with other raw materials.
 In case of poor concrete cohesion due to poor compatibility of the admixture with the raw material of the concrete, the adaptability of the admixture should be adjusted. Commonly used admixtures usually consist of water-reducing agent mother liquor and compound "small materials". Adjusting the types and dosage of small materials can sometimes achieve the effect of "four or two pounds".
Scheme 1: The components of the mother liquor of the water reducing agent remain unchanged, and the air-entraining components and thickening components (such as dextrin, cellulose ether, etc.) are appropriately increased";
Option 2: Adjust the components of the superplasticizer mother liquid, such as polycarboxylic acid superplasticizer with slump retention and water reduction, etc. By adjusting the type and combination ratio of the polycarboxylic acid mother liquid, the best The optimal ratio can solve the compatibility problem. The sensitivity of the concrete mixture to the water consumption and the admixture content has also been improved. Even if the water consumption is increased by 10kg/m3 during trial mixing, the admixture content will increase and decrease by 0.1% respectively, and there will be no separation of the mortar and bone .
4. Sand rate
The sand rate has an important impact on the workability of concrete. Generally speaking, as the sand rate increases, the cohesion of the concrete increases first and then decreases. There is an optimal sand rate. When the actual sand rate is lower than the optimal sand rate, the void ratio of the mixed aggregate composed of gravel aggregates is large, and more slurry is required to fill the voids. The thickness of the slurry wrapped on the surface of the aggregate is thin and the cohesion is poor . However, when the sand rate is greater than the optimal sand rate, the amount of fine aggregate sand is too large, resulting in an increase in specific surface area, and the void rate is also greater than the optimal void rate, which also causes the thickness of the slurry wrapped on the surface of the aggregate to become thinner and the concrete to stick together Sex becomes worse. In production practice, the sand rate can be adjusted according to the actual situation.
There are many factors influencing the cohesion of concrete, sometimes it is a single factor, but sometimes it is a combination of multiple factors. Therefore, when this problem is encountered, one-sided analysis cannot be done, and the reasons and experimental arguments should be sought one by one from multiple angles.
5. How to improve cohesion
① Additive
Water reducers increase fluidity, you should know this.
Air-entraining agents can also increase fluidity, and can also improve cohesion and reduce bleeding.
Adding a small amount of thickener can also achieve the same effect, increase the cohesiveness of fluid concrete and reduce the bleeding.
② Admixture
Such as high-quality fly ash, silica fume and other ultra-fine particle admixtures. Adding a certain amount of admixture to mix, can also achieve the effect mentioned above, that is, increase fluidity, reduce bleeding, reduce segregation, and increase cohesion.
③ Aggregate
Generally speaking, the gradation of the aggregate is good, so the water retention and cohesion will not be bad.
④ Cement
Generally speaking, the influence of cement on the workability of fresh concrete is mainly the water demand and water permeability of cement. The fresh concrete made by cement with large water demand has less fluidity, but the general cohesion is better than the water retention. Well, of course, the water retention of concrete mixed with bleeding cement is poor.
The cohesiveness is not good, sand, cement, water retention agent, water consumption, which must be problematic.
Case: When mixing the C30 concrete mix ratio, by adjusting the sand rate, adjusting the amount of water-retaining agent in the water-reducing agent and other measures, the cohesiveness and water retention of the concrete has not been completely improved, and the viscosity is always slightly. Bleed. How can the cohesion and water retention of concrete be improved?
Is there a sand head in the sand?
Didn't the adaptability of cement and water-retaining agent be tested?
High water consumption?
Common methods:
1. Add sand and reduce stone
2. Add water retention agent to reduce water
3. Add ash and reduce stone
According to the analysis of the problem, the sand may not be good. It is recommended to use the third one, adding fly ash or cement or mineral powder, and reducing the stone. In addition, to see whether it is a problem of raw materials, whether the gradation of sand and stone is reasonable, and the gradation is not good, the water retention will not be good, and then adjust the sewage-cement ratio, the water-cement ratio may be too large. It may also be a mix ratio problem. Too much aggregate and less cement will also cause this kind of problem. Proper admixture can improve concrete performance.
6. Which ingredients in the admixture will affect cohesion?
In fact, the water-reducing component, air-entraining component and retarding component of the admixture have more or less influence on the cohesiveness of concrete. And it is closely related to its dosage. For example, if the air-entraining agent is added properly, it will improve the workability of the concrete, but if there are too many and the air bubbles are too large, the workability may also be poor. The same is true for the water reducing agent. The proper dosage and fluidity are good. Some small amount of sugar, reasonable mixing amount can improve the cohesion of concrete. For the sole purpose of improving cohesiveness, I think some thickeners are better, such as sodium polyacrylate.
7. How to accurately describe the fluidity, cohesiveness and water retention of concrete?
The method for evaluating cohesion is to gently tap the side of the collapsed concrete mix cone with a ram, if the concrete mix cone keeps the whole slowly and sinks evenly, it indicates good cohesion; if The sudden collapse of the concrete mix cone or the segregation of stones indicate poor cohesion. (The expression is good/poor)
The method of assessing water retention is to observe the bottom of the concrete mix cone. If there is more thin cement slurry or water precipitation, or the aggregate is exposed due to loss of slurry, it shows that the water retention is poor: such as the concrete mix cone There is no or only a small amount of cement slurry precipitated at the bottom of, indicating that the water retention is good. (The expression is also good/poor, plus "severe bleeding" or "slight bleeding" or "almost no bleeding")
The slump method is used to measure the fluidity of concrete mixtures under the action of gravity, and is suitable for concrete mixtures with high fluidity. During the measurement, the concrete mixture is loaded into the concrete slump cylinder according to the prescribed method, and the slump cylinder is lifted up vertically after scraping. The concrete mixture slumps due to gravity and the height of the slump mm is the on-site slump. The greater the slump, the greater the fluidity. This method is the most widely used in engineering, using concrete mixtures with a slump greater than or equal to 10 mm and a maximum particle size of less than 40 mm. (The expression is large/small, attach the slump value mm)
The Weibo consistency method is used to measure the fluidity of concrete mixtures under the action of mechanical vibration, and is used for concrete mixtures with low fluidity. When measuring, put the concrete mixture into the slump cylinder according to the prescribed method, and lift the slump cylinder vertically, and then place the specified transparent plexiglass disc on the top surface of the concrete mixture cone, Then turn on the vibrating table and record the time s when the bottom surface of the transparent disc is just filled with cement slurry, ie, the Weibo consistency. The greater the Weibo consistency, the lower the fluidity. This method is suitable for concrete mixes with a maximum particle size of less than 30 for 5 to 30 seconds. (The expression is big/small, with the Weibo consistency value s)

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