How to use concrete admixtures correctly

At present, the variety of additives, the role of different in the concrete, including water reducing agent dosage is the largest, occupying a major position. The following is a personal experience of the application of water reducing agent.

1 admixtures in the application of concrete problems

Concrete mix good constructability (fluidity and its stability, homogeneity and volume stability) is essential, the role of water reducing agent is mainly through the physical dispersion of cementitious materials to achieve water reduction and improve the uniformity of the dispersion of cementitious materials, to obtain a good construction performance of concrete mix. Admixture in the application of the problem in several aspects: First, the admixture technology itself can not well meet the requirements of concrete performance; Second, the test admixture technical performance can meet the requirements, but the actual production of admixture quality fluctuations, or due to the cost and price of the production of admixtures and test samples are not consistent; Third, the quality of raw materials for concrete fluctuations.

To solve the problem of admixtures in the application needs to return to the basic level, is for the specific performance requirements and the use of concrete composition materials to choose the right admixture, and strict control of the quality and stability of admixtures and concrete composition materials.

2 how to choose the right admixture

For admixture application, there is no best, only the most suitable products. In the mixing station A with good admixture does not necessarily in the mixing station B with good, high efficiency water reducing agent is not necessarily better than ordinary water reducing agent. Only the choice of products to meet the performance requirements of stable quality, the admixture can play a proper role. Concrete composition of the material source, performance is not the same, the final determination of the best admixture and the supplier's recommended admixture is not - necessarily the same, if necessary, need to adjust the concrete ratio and admixture formula. Therefore, the choice of admixtures need to be specific to the specific concrete performance requirements and specific concrete composition materials.

Selection of admixtures should be from the admixture technical performance and sensitivity, quality and stability, and cement cementitious materials compatibility, admixture manufacturer credibility, technical support capabilities and logistical support capabilities and costs and other aspects to consider.

Selection of admixtures should first be based on the type of project concrete structure, concrete strength level, concrete construction methods and construction conditions, durability and other special requirements to determine the concrete performance requirements. Such as strength, workability and workability to maintain the time, setting time and so on. According to the concrete performance requirements and the specific cementitious materials and sand and gravel materials can be used to determine the initial concrete ratio, such as the water-cement ratio, the amount of cementitious materials, water consumption, sand rate, etc., as well as the technical performance requirements of the admixture, such as water reduction, workability retention, slow setting or promotion of setting.

It should be noted that the best admixture should not be near the saturation point admixture, the vast majority of water reducing agents in the saturation point near the admixture is very sensitive, concrete production towel small fluctuations are likely to cause great changes in concrete performance. If the final determination of the admixture is very close to the saturation point admixture, indicating that this product is not the right product. Therefore, in addition to the concrete performance test, but also need to carry out the admixture and the sensitivity of the ratio test, in order to determine the best admixture working range and the concrete ratio allows fluctuations in the range.

Different varieties or the same variety of different manufacturers of admixtures may have different optimal admixture working range. In this range of performance (such as water reduction rate, setting time, gas content, etc.) and the relationship between the admixture should be close to a linear relationship, that is, the change in concrete performance with the admixture is predictable. Beyond this range, the change in concrete performance is very sensitive to the admixture admixture, various problems may occur at any time, such as segregation, water secretion, or set time is too long, or gas content is super high, etc.. The smaller the optimal range of work, the more difficult to manage the operation of concrete production.

For the slow-setting admixture, in addition to the determination of the amount of admixture and the relationship between the setting time, but also to determine the sensitivity of the setting time to temperature. Through the temperature sensitivity can determine the admixture to the seasonal temperature range.

Another important factor is the sensitivity to the concrete ratio, especially high slump or large flow concrete. In actual production, there will always be some normal fluctuations in the composition of materials and ingredients, especially the fluctuation of sand and gravel moisture caused by the actual water consumption of concrete Shi5kg/m3, 10kg/m3 fluctuations is very common. After determining the amount of admixture and concrete proportioning through performance testing, it is necessary to conduct a water alligator sensitivity test, which means adding or subtracting 5 kg/m3 to 10 kg/m3 of water on the basis of the determined concrete proportioning and admixture admixture, to see whether the concrete workability, in addition to the appropriate changes in slump or flow, occurs, such as segregation, water secretion and other phenomena.

As a concrete mixing plant, in the admixture selection stage need to pre-test the compatibility of admixtures and other possible use of cement, in most cases the appropriate adjustment of admixture admixture can be resolved. If there is indeed incompatible situation, it is necessary to determine another admixture formula or species to solve.

The choice of admixture should also determine the possible but uncontrollable events when the technical plan, such as traffic jams, site waiting time caused by the reduction of concrete workability, etc., can be considered to add admixture to restore the pumping and pouring performance. Prepare test data to determine the relationship between the amount added on site and the amount of slump increase, and the additional admixture added on site should not have a significant impact on the concrete setting time and early strength development. Therefore, the admixture added on site should be non-retarded setting type of high efficiency water reducing agent, these pre-programs should be submitted to the design and supervision units before the start of the project.

3 admixture quality and stability of the detection and control

For concrete enterprises, the main thing is the quality stability of the constituent materials and the stability of the production process. In reality, some manufacturers may also provide technical performance in the test, the actual cost of the sample is very good, so as to pass the performance test, but in the actual supply, due to price and cost reasons, often with the sample provided for the experiment does not match the situation. In order to ensure the stability of admixture quality in the application, the following points need to be noted:

First, the need to determine the benchmark quality indicators and quality testing methods for admixtures. The previous performance test should be provided by the manufacturer and in the final performance test to meet the requirements of the sample as the admixture benchmark sample. The benchmark quality index is determined according to the technical performance and homogeneity index of the benchmark sample, and the range of fluctuation of the index is determined. Benchmark quality indicators and allowable fluctuation range and testing methods are signed by both parties to confirm part of the supply contract supply contract also needs to specify the supply quality does not meet the supply contract processing methods and compensation standards. Benchmark quality indicators and allow fluctuation range is the benchmark of admixture quality control, to ensure that the actual supply of admixture quality and test sample quality consistent.

Secondly, determine the quality stability of admixture benchmark quality indicators actually do not need a lot. Daily production, admixture solid content and relative density of the test is sufficient to control the stability of the admixture, and solid content, relative density of the determination is very convenient and simple. These two indicators are used separately is not very meaningful, but the combination of the two can basically determine the stability of the quality of the admixture, even within the allowable range of fluctuations, the direction of the two fluctuations should be consistent. If one of the indicators of stability, another indicator change, or two indicators fluctuate in the opposite direction, even within the allowable range of fluctuations, but also may make the admixture in the proportion of raw materials or the quality of raw materials in the composition of some changes, such as naphthalene water reducing agent, when the solid content remains unchanged but the relative density increases, indicating that the naphthalene sulfonate masterbatch in the sodium sulfate content may be higher than the original, or mixed with other relative Higher density but lower cost materials (such as inorganic salts) to replace part of the naphthalene sulfonate masterbatch. When the solid content remains unchanged but the relative density is reduced, it indicates that the water reducing agent is likely to be mixed with other materials of lower relative density but lower cost to replace part of the naphthalene sulfonate material (such as lignin sulfonate).

4 only when there is a suspicion of change in the composition of the admixture, or a random sample test once or twice in a car.

Another common quality indicator is the net admixture flowability. One of the most important prerequisites for quality consistency testing is the stability of the benchmarks and methods used for testing. If a material or method with large quality fluctuations is used to test the stability of another material, this test has no meaning for quality stability. This is the reason why the national standard of the admixture cement net liquidity test shall be used in the benchmark cement. In practice, few people use the benchmark cement to do the net mortar test to test the quality and stability of the admixture. The common situation is to use the cement being used at the time to test the quality of the admixture or the suitability of the admixture to the cement. If the liquidity is not good, the quality of the admixture and the suitability of the cement is considered bad, then the admixture manufacturer needs to adjust the formula to meet the suitability of the cement. From personal experience and experience. The correlation between net slurry liquidity and concrete workability is not good, so the net slurry liquidity test should be a method of admixture product quality testing rather than a measure of performance testing.