What should be paid attention to when using concrete admixtures(2)?

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1. Adopt a suitable mixing method

In the process of concrete mixing, the mixing method of admixture has a great influence on the effect of admixture. For example, the mixing method of superplasticizer is generally divided into first mixing method (adding before mixing water), same mixing method (adding at the same time with mixed water), stagnant method (adding superplasticizer lagging behind water 1~2min in the mixing process), and later mixing method (after mixing, it is added to the concrete mixture with a certain content for a certain time after a certain period of time, and then mixed for 2 or more times). Different mixing methods will bring different effects, and different varieties of superplasticizers have different mixing methods because of their different action mechanisms. For example, for naphthalene series superplasticizer, in order to avoid the selective adsorption of C3A and C4AF mineral composition of cement, the later mixing method is better, such as wood calcium superplasticizer, because its action mechanism is macromolecular protection, so the effect of different mixing methods is not significant. The main factors affecting the mixing method of superplasticizer are the variety of cement, the type of superplasticizer, the amount of superplasticizer, the time of adding and other compound admixtures, etc. Which method should be determined by the specific conditions of trial mixing and engineering.

2. Adjust the mix proportion of concrete

Generally speaking, the admixture has no special requirements on the mix ratio of concrete and can be designed according to the common method. However, in the case of reducing water or saving cement, the sand ratio, cement dosage and water-cement ratio should be properly adjusted.

Sand ratio

Sand ratio have great influence on the workability of concrete. Due to the great improvement of the properties after adding superplasticizer, the sand ratio can be reduced appropriately. The reduction is about 1% 4%. For example, the lower limit of wood calcium is 1% 2%, and the upper limit of air-entraining superplasticizer is 3% 4%. If the sand ratio is on the high side, the reduction range can be increased, and the excessive sand ratio not only affects the strength of concrete. it also brings some difficulties to the molding operation.

The amount of cement

The superplasticizers used in concrete have the effect of saving cement in varying degrees. The use of common superplasticizer can save 5%-10%, and efficient water-reducing can save 10%-15%. Using high-grade cement to prepare concrete, adding superplasticizer can save more cement.

Water-cement ratio

The water-cement ratio of concrete mixed with superplasticizer should be determined according to the water-reducing rate of the variety. It turns out that the water reduction rate of those with large water-cement ratio is also higher than that of those with low water-cement ratio. In order to keep the slump the same after saving cement, the water-cement ratio should be the same as that without saving cement or increase by about 0.0.1 to 0.03.

3. Pay attention to the construction

For example, in the process of stirring, the amount of superplasticizer and water should be strictly controlled, and the appropriate mixing method and mixing time should be selected to ensure that the superplasticizer works fully. There should be different points for attention for different mixing methods, such as the water reducer used in dry mixing should have enough fineness, the powder is too thick and the dissolution is uneven, the effect is not good; after mixing or dry mixing, the mixing time must be extended above 1min.

The slump loss of concrete mixed with superplasticizer is generally faster, so the transportation and parking time should be shortened, otherwise the slump protection measures should be increased.

In the process of transportation, attention should be paid to maintaining the homogeneity of concrete and avoiding delamination, and attention should be paid to the delay of initial setting time when adding retarding superplasticizer. For example, if the Chinese and foreign admixtures of steam-cured concrete are not used properly, there will be quality problems such as bulging, cracking and loosening on the surface of steam-cured concrete, and the strength will also decrease significantly, so we should pay attention to the following problems in steam-cured concrete: select suitable admixtures, such as air-entraining admixtures, not suitable for use, control the amount of admixtures, have a certain pre-curing period and rising temperature period, and determine the constant temperature and time through experiments.

4. The construction performance and hardening performance of concrete with strengthening curing and adding admixtures are very different from those of concrete without admixtures, some prolong the setting of concrete, and some increase rapidly in early strength. most admixtures increase the shrinkage of concrete, while expansive agents expand the volume of concrete. Therefore, concrete mixed with admixtures should pay more attention to curing. Keeping concrete curing in wet conditions is the key condition for concrete to meet the design requirements and an important step to avoid concrete cracking.

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