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The importance of concrete admixture to concrete quality

Release time:

2022-11-14

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Concrete is the main raw material of modern real estate construction project, because of its high strength, long service life and other advantages by the majority of workers. In the process of concrete mixing, it is necessary to add an appropriate amount of concrete admixture to improve the performance of concrete. Then, let's understand the importance of concrete admixture to concrete quality!

Brief introduction of concrete admixture

In general, substances added to concrete during mixing and before mixing to improve concrete properties are collectively referred to as admixtures. The quality of admixture added in concrete shall generally not exceed 5% of the cement quality. Retarder, accelerator and early strength agent can adjust the hardening performance and setting time of concrete. The main function of admixtures such as air entraining agent, concrete admixture and pumping agent is to improve the rheological properties of concrete mixture and facilitate pumping to the designated place. Admixtures such as water-proofing agents, air-entraining agents and rust-proofing agents have been used to improve the durability of concrete. In order to improve the other properties of concrete, concrete should be mixed with colorants, gas additives, antifreeze and expansion agent and other additives.

Influence of concrete admixture on concrete

1) Influence on concrete slump

Generally speaking, the staff in the concrete mixed with a certain amount of concrete admixture, the purpose is to improve the slump of concrete, so as to effectively improve the workability of fresh concrete.

Experience has shown that concrete without concrete admixtures loses less slump within 1 hour than concrete with concrete admixtures. The main reason for this "Hu San odor" phenomenon is that the mixing of concrete admixture needs to go through mixing, transportation, pouring and other processes. Therefore, it is necessary to add water to the concrete construction site to restore the slump of concrete mixed with concrete admixture, but this method will significantly reduce the cohesion and other properties, which may cause concrete cracking, abnormal hardening and other phenomena.

In order to effectively solve the problem of concrete slump loss of concrete admixture, the staff suggested that the method of batch concrete admixture addition and post-mixing method can not only effectively avoid the problem of concrete slump loss, but also control the production cost to a certain extent, so that the concrete construction benefit is greater.

2) Influence of excessive admixture on concrete

Relevant construction standards stipulate that the dosage of concrete admixture in concrete should be 0.25~0.75, but due to various adverse effects, the quality of concrete admixture mixed in concrete often exceeds the standard in the actual construction process, especially the test data of the dosage of concrete admixture exceeds the standard show that when the quality of concrete admixture in concrete reaches more than 1 times of the recommended quality, the hardening and setting time of concrete is greatly prolonged, the early strength of concrete can not meet the design requirements, and sometimes concrete can not be solidified.

In actual work, the staff shall act in strict accordance with the relevant national regulations on admixture of concrete. Before the formal admixture mixing work, the construction unit shall arrange professional personnel to carry out concrete trial mixing work, determine the admixture dosage, and ensure the concrete quality.

3) Gas introduction

After the concrete admixture is mixed into the concrete, the air content inside the concrete will rise significantly. Under normal circumstances, when the concrete contains a certain amount of uniform bubbles, the secretion of concrete will be greatly reduced, and the workability can be effectively improved, which can improve the impermeability and freeze-thaw resistance of concrete to a certain extent.

However, in recent years, it has been found that adding concrete admixtures to concrete can not only improve the performance of concrete, but also significantly reduce the flexural strength and compressive strength of concrete. The experimental results show that under the premise of a certain water-cement ratio of concrete, the flexural strength of concrete decreases by 2% ~ 3% and the compressive strength decreases by 4% ~ 5% for every 1 percentage point increase in the air content of concrete. Coupled with the concrete absorption of more than 6% of the critical point, the durability and strength of concrete and other properties will be significantly reduced, at this time the concrete can not meet the engineering requirements.

Therefore, the air-entraining concrete admixture has strong stability, of course, this does not mean that the air-entraining concrete admixture can not be used, the staff in the process of using the air-entraining concrete admixture as long as the attention to the concrete gas content control in the appropriate range.

4) Adaptability of cement and concrete admixtures

In a concrete mixing work, it is found that after a small amount of cement is mixed, the cement hardening state changes abnormally. After a few minutes, the cement solidification speed accelerates sharply, resulting in the so-called fast setting phenomenon. The gypsum, mixture, mineral raw materials and other substances in the cement are not suitable for concrete admixtures. It can be seen that after mixing qualified concrete admixtures, employees find that fluidity deteriorates and rapid setting occurs, and the construction method of forcibly adding water cannot be adopted, it is necessary to find a solution from the adaptability of cement and concrete admixtures, so that when different varieties of admixtures are used in combination, they should pay close attention to their compatibility to avoid the occurrence of excessive retarding, pseudo-setting or rapid setting of concrete, the slump that fails to meet the requirements, and the viscosity reduction, water holding capacity, fluidity and other adverse effects on the performance of concrete.

Key words:

Concrete admixture

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