What should be paid attention to when using concrete admixtures?

1. Choose suitable admixture according to engineering characteristics

Admixtures can be used for almost all kinds of concrete, but suitable admixtures must be selected according to engineering needs, construction conditions and construction technology. For example, ordinary superplasticizer is mainly used for general concrete, superplasticizer is used for early strength and high strength concrete, water reducer with high air entraining property is mixed with water reducer when air temperature is low, single retarding superplasticizer is generally not used, compound early strength superplasticizer is often used, in order to improve the water resistance of concrete, waterproofing agent is used, pumping agent should be used when pumping concrete is used in high-rise buildings, in order to give full play to the characteristics of various admixtures. Should not be substituted for each other. At the same time, the admixture has the problem of adaptability to different cement, which must be tested and controlled in use.

2. The quality of admixtures

Admixtures must be tested according to the standard before use, and only the admixtures that meet the requirements of the standard can be used to prepare concrete.

3. The types of cement

Among the raw materials, cement has the greatest influence on the admixture, and the water-reducing and strengthening effects of superplasticizers are different with different types of cement, especially on the water-reducing effect. Superplasticizer is more selective to cement, and the water reducing rate of different cement varies greatly. Cement mineral composition, admixture, pour point regulator, alkali content, fineness and so on will affect the effect of superplasticizer. For example, the cement mixed with anhydrite will produce quick hardening for some concrete mixed with superplasticizer or greatly shorten the initial setting time of concrete, in which naphthalene superplasticizer has little effect, molasses will cause quick hardening, and wood calcium will prolong the initial setting time. Therefore, the effect of the same kind of superplasticizer is obviously different because of different cement under the same dosage. In some cement, some superplasticizer will cause abnormal setting phenomenon. For this reason, when cement is available, the cement that is more suitable for superplasticizer should be selected to improve the effect of superplasticizer. When the superplasticizer is available, the more suitable superplasticizer for construction cement should be selected. in order to make the superplasticizer play a better effect, cement selection test should be carried out in combination with the project before use.

4. Carry out the test before use

In order to ensure the engineering quality, according to the existing standards, for example, the homogeneity test should be carried out before the superplasticizer is used, and the water reducing rate and solid content (moisture content) should be determined, and then the concrete should be tested, such as testing the performance of the superplasticizer concrete, slump, water reducing rate, air content and compressive strength should be measured.

5. Pay attention to the content

Each admixture has a suitable amount, even if the same admixture, different uses have different appropriate amount. If the mixing amount is too large, it is not only economically unreasonable, but also may cause quality accidents. For the superplasticizer with air entrainment and retarding effect, special attention should be paid not to over-dosage. If the amount of wood calcium is more than 0.5% of the weight of cement, excessive air will be introduced to slow the initial setting and reduce the strength of concrete. If the dosage of superplasticizer is too small, it will lose its high efficiency, while if the dosage is too large, the quality will be affected by water secretion. The limitation of chloride salt is well known, and excess will cause steel bar corrosion. The amount of antifreeze is related to temperature, and it is stipulated according to the effect of strength. in short, there are many factors that affect the amount of admixture, such as the method of adding water reducer, the variety of cement, the initial popularity of the mixture and the curing system, etc.

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