Analysis of problems in the application of admixtures in commercial concrete!

The application of commercial concrete has been basically popular in our country. The application of admixtures has also accelerated the development of concrete technology. Admixtures have also solved various technical problems in concrete applications, such as : Long-distance and ultra-height transportation of concrete, self-compacting concrete forming, and crack control of ultra-large concrete structures. Admixtures have been widely used in ultra-high-strength and underwater-resistant concrete.

Due to the uneven distribution of concrete material resources in my country, green environmental protection has also limited the lack of sand and gravel aggregate resources in some areas, and many areas have to switch to ultra-fine sand. , machine-made sand and non-continuously graded aggregates, the commercial concrete cannot achieve the expected technical effect after using admixtures, and the quality of the concrete decreases. In addition, technical personnel of some concrete production companies have certain misunderstandings about concrete admixtures and improper application. Problems frequently occur after using admixtures, and even engineering accidents occur.

The author makes a simple analysis on the application of admixtures in commercial concrete and discusses with relevant experts.

1 Effect of water-reducing admixture on shrinkage cracking of commercial concrete

Water-reducing admixture is a An admixture added during the concrete mixing process that can significantly reduce or significantly reduce the water used for concrete mixing, improve the fluidity of concrete, and increase the strength of concrete. Practice has proved that after adding water-reducing agent to concrete, if there is no need to increase the strength, the amount of cement can be greatly reduced and the compactness of concrete can be improved. Therefore, water reducing agent is an indispensable additive material in commercial concrete.

In order to further improve the economic benefits of commercial concrete, concrete production companies like to use water-reducing agents with high water-reducing properties to help increase or significantly increase the strength of concrete. Reduce cement consumption and reduce production costs. In fact, this is a major misunderstanding. Although water reduction is beneficial to improving the compressive strength of concrete, excessive water reduction will also have a negative impact on the flexural strength of concrete. Although an appropriate amount of water reduction is beneficial to reducing the shrinkage of concrete, it must be noted that when designing the concrete mix ratio, the water-reducing function of adding water-reducing agent has been taken into consideration. Generally, the water-binder ratio is designed to be low. If it is significantly reduced, Water consumption will increase the drying shrinkage of concrete and increase the shrinkage rate of concrete.

When the amount of cement is greatly reduced, although the compressive strength of commercial concrete does not decrease, the tensile strength decreases as the volume of hardened cement stones in the concrete decreases. . Due to the reduction in cement content, the concrete cement slurry layer is too thin, which will also cause more micro-cracks in the concrete. Of course, micro-cracks have little effect on the compressive strength of concrete, but their impact on the tensile and other properties of concrete cannot be underestimated. The substantial reduction of cementitious materials will also affect the elastic modulus and creep of concrete, making concrete more susceptible to cracking.

In summary, the author believes that when producing commercial concrete, the water reduction rate of concrete and the amount of cementitious materials must be fully considered, and water reduction must not be unlimited Or excessive reduction of cementitious materials.

2 The significant technical and economic benefits of air-entraining agents used in commercial concrete

Air-entraining agents It is an admixture that can produce a large number of tiny, fine and stable bubbles when mixed into concrete. After being mixed in, it can not only improve the workability, pumpability, fluidity and cohesiveness of concrete, but also improve the frost resistance and Anti-permeability and other durable properties. The addition of air-entraining agents to commercial concrete can prevent secondary adsorption of cement particles dispersed in the concrete and improve the slump-preserving performance of commercial concrete. At present, air-entraining agent has become one of the indispensable components of commercial concrete admixtures (the others are water-reducing agents and retarders). In Japan and Western countries, there is almost no concrete without air-entraining agents. In Japan, concrete without air-entraining agents is called special concrete (such as permeable concrete, etc.).

In our country, although air-entraining agents have been widely used, there are still many concrete production companies that do not have enough understanding of the significant technical and economic benefits of air-entraining agents. Some are only used when there are technical requirements for pumping, anti-seepage, and anti-freezing. For example, an admixture factory in Guangdong Province uses more than 20 tons of air-entraining agents per month, while a company of the same size in a certain region in southwest China only uses a few tons per month. To understand the reason, it is mainly the technical staff of the factory. It is another misunderstanding that adding air-entraining agents will reduce the strength of concrete.

Air entrainment will affect the strength of concrete. This refers to the test results under the same water-cement conditions. When the air content increases by 1%, the strength of concrete will decrease by 4 % ~ 6%, and the incorporation of air-entraining agents will also greatly increase the water reduction rate of concrete. The author has tested the addition of naphthalene series high-efficiency water-reducing agent. When the concrete water-reducing rate is 15.5%, after adding a very small amount of air-entraining agent, the concrete water-reducing rate reaches more than 20%, that is, the water-reducing rate increases by 4.5%. Under normal circumstances, concrete For every 1% increase in water reduction rate, the strength of concrete will increase by 2% to 4%. Therefore, as long as the amount of air-entraining agent is strictly controlled, not only will the strength of concrete not be reduced, but it will be increased. Regarding the control of air content, many domestic tests have shown that the air content of low-strength concrete should be controlled at 5%, the air content of medium-strength concrete should be controlled at 4% to 5%, and the air content of high-strength concrete should be controlled at 5%.The �� part is adsorbed by the soil, and the remaining admixture is not enough to adsorb and disperse the cement particles. At present, polycarboxylic acid admixtures have been widely used. Due to the small dosage of this product, the above phenomenon is more serious when used to prepare concrete with high mud-containing sand and gravel. At present, in-depth research is being carried out in China on measures to solve the problem of mud resistance of concrete. The main solutions are:

(1) Increase the amount of admixtures. Although this method The effect is obvious, but since the amount of admixtures in concrete needs to be doubled or more, the cost of concrete manufacturing increases. It is difficult for production companies to accept it.

(2) Chemically modify the admixtures used to change the molecular structure of the admixtures. There are many related reports on this, but the author understands that these newly developed anti-mud admixtures still have adaptability to different soils.

(3) Develop new anti-mud functional admixtures and use them in combination with commonly used admixtures. We have seen an imported anti-mud agent in Chongqing and Beijing. This product has a large dosage and a high price. It is also difficult for general commercial concrete companies to accept it. In addition, this product also has adaptability problems to different soils.

The author believes that the following anti-mud measures can be used for research reference:

(1) In Commonly used admixtures are mixed with low-cost materials with a certain dispersion force to increase the components that can be adsorbed by the soil, which has a certain effect. This means that adding low-cost admixture components sacrifices its increased function.

(2) Mixing a certain amount of water-soluble low molecular polymer into the admixture has a certain effect.

(3) Use some dispersants, retardants and water reducing agents that are prone to bleeding. </p