Modern buildings have increasingly higher requirements for concrete materials. Due to changes in floor materials and environment, it has become inevitable to use concrete admixtures to improve the workability of cement, which is crucial to the performance of concrete materials. There are many kinds of admixtures, and their effects in concrete are also different. Concrete water-reducing agents (naphthalene-based high-efficiency water-reducing agents, polycarboxylic acid high-performance water-reducing agents), as representative products of admixtures, have the largest dosage and occupy a major position. It is crucial for the concrete mixture to have good constructability (fluidity, stability, homogeneity and volume stability). The water-reducing agent achieves water reduction and improves the strength of the cementitious material through the physical dispersion of cement cementitious materials. Dispersion uniformity and good construction performance of concrete mixture. Let’s discuss how to choose concrete admixtures?
There are no more problems in the application of admixtures than in three aspects: First, the admixture technology itself cannot well meet the concrete performance requirements; second, the actual The quality of the produced admixtures is unstable, or the admixtures produced are inconsistent with the test samples due to cost and price reasons; third, the quality of concrete raw materials fluctuates greatly. Solving problems in the application of admixtures requires returning to the basic level, which is to select appropriate admixtures based on specific performance requirements and the concrete component materials used, and strictly control the quality stability of the admixtures and concrete component materials.
For the application of concrete admixtures, there is no best, only the most suitable product. The admixtures used well in mixing station A may not necessarily be used well in mixing station B. Only by selecting products that meet the performance requirements and have stable quality can the admixtures play their due role. The sources and properties of the constituent materials of concrete are different. The final optimal dosage may not be the same as the dosage recommended by the supplier. If necessary, the concrete mix ratio and admixture formula need to be adjusted. Therefore, the selection of admixtures needs to be based on specific concrete performance requirements and specific concrete constituent materials.
The selection of admixtures should be based on the technical performance and sensitivity of the admixture, quality stability, compatibility with cement cementitious materials, credibility of the admixture manufacturer, technical support capabilities, logistics support capabilities and costs, etc. aspects to consider.
1. The selection of admixtures should first determine the concrete performance requirements based on the type of engineering concrete structure, concrete strength grade, concrete construction method and construction conditions, durability and other special requirements. Such as strength, workability and workability retention time, setting time, etc. Determine the initial mix ratio of concrete based on the concrete performance requirements and the specific cementitious materials and sand and gravel materials that can be used, such as water-cement ratio, cementitious material dosage, water consumption, sand rate, etc., as well as the technical performance requirements of admixtures, such as water reduction, workability, etc. Maintain sex, retard coagulation or promote coagulation, etc.
It should be pointed out that the optimal dosage should not be near the saturation point. Most water-reducing agents are very sensitive near the saturation point, and there are small fluctuations in concrete production. May cause significant changes in concrete properties. If the final additive content is very close to the saturation point, it means that the product is not a suitable product. Therefore, in addition to concrete performance tests, sensitivity tests of admixtures and mix ratios are also needed to determine the optimal working range of admixture dosage and the allowable fluctuation range of concrete mix ratios.
Different varieties or different manufacturers of the same variety of admixtures may have different optimal dosage ranges. Within this range, the relationship between properties (such as water reduction rate, setting time, air content, etc.) and dosage should be close to a linear relationship, that is, changes in concrete properties with changes in admixture dosage are predictable. Beyond this range, the performance changes of concrete are very sensitive to the dosage of admixtures, and various problems may occur at any time, such as segregation, bleeding, excessive setting time, or excessive air content, etc. The smaller the optimal working range, the more difficult it is to operate and manage concrete production.
For admixtures with retarding effects, in addition to measuring the relationship between dosage and setting time, it is also necessary to measure the sensitivity of setting time to temperature. Temperature sensitivity can determine the application range of admixtures to seasonal temperatures.
Another important factor is the sensitivity to concrete mix proportions, especially high slump or high flow concrete. In actual production, there will always be certain normal fluctuations in the constituent materials and ingredients. In particular, it is very common for the actual water consumption of concrete to fluctuate between 5kg/m3 and 10kg/m3 due to fluctuations in the moisture content of sand and gravel. After determining the amount of admixture and the proportion of concrete through performance testing, it is necessary to conduct a water sensitivity test, that is to say, add or subtract 5 kg/m3 to 10 based on the determined concrete mix proportion and admixture amount. kg/m3 water, check whether other phenomena such as segregation and bleeding occur in addition to appropriate changes in slump or fluidity in concrete workability.
As a concrete mixing station, it is necessary to pre-test the compatibility of the admixture with other possible cements during the admixture selection stage. In most cases, the problem can be solved by appropriately adjusting the admixture dosage. If there is indeed incompatibility, additional additive formulas or varieties need to be determined to solve the problem.
The selection of admixtures should also determine the technical plan for possible but uncontrollable events, such as traffic jams, construction siteThe problem is not difficult, and it mainly depends on the technical means and level of the admixture manufacturer.
In practice, many so-called applicability problems of admixtures, such as insufficient water reduction, segregation, bleeding, etc., mostly come from the quality of concrete components (such as sand and gravel, quality of admixtures) and do not belong to The real technical compatibility problem is caused by management level and market reasons. In reality, mixing stations always hope that various concrete problems, such as segregation and bleeding caused by poor quality sand and gravel materials, can be solved by adjusting the admixture formula, saving worry and money. From a purely technical point of view, it seems feasible, but from the perspective of quality control and quality assurance systems, if it is used as a conventional quality control method, it is a bit putting the cart before the horse, and the mixing station is bound to often fall into a state of "firefighting" . What the mixing station needs to do is not to wait for problems to arise and then solve them, but to prevent and avoid problems as much as possible. Continuously adjusting the admixture formula to solve similar quality problems is only an emergency method, but it is impossible to prevent the occurrence of problems.
The problems that arise in the application of admixtures are ultimately due to changes or fluctuations beyond the allowable range in the quality of concrete components (including cement, aggregates and admixtures) or in their processing processes. If you do not control the factors that cause concrete quality problems at the source and only make adjustments when there are problems, then concrete quality problems will continue to occur. In addition, too frequent adjustments to the additive formula are empty words for the quality and stability monitoring of the additive. The compatibility of admixtures with cement cementitious materials or even sand and gravel materials should be resolved when selecting admixtures, and possible changes in material supply should be fully taken into account, and all possible problems should be solved in advance. Changing or adjusting the additive formula is a method to solve problems when new technical requirements (such as new projects) or uncontrollable circumstances arise, and should not be used as a method of daily quality control.
IV. The relationship between concrete mixing stations and admixture manufacturers
Under market economy conditions, the purpose of concrete mixing stations and admixture manufacturers is to obtain profits. When admixture manufacturers look at If there is less profit, they will either quit or resort to unfair means such as cutting corners or counterfeiting. The result is that the quality is not guaranteed. If the quality of admixtures is not guaranteed, the concrete mixing station needs to strengthen quality inspection methods, increase the testing items and frequency, and improve the safety factor of the concrete mix ratio. All of these will ultimately increase the operating cost of the concrete mixing station.
Although admixtures play a decisive role in the performance of concrete, the quality of the final concrete is affected by a series of factors such as composition materials, mix ratio, batching, mixing, transportation, pouring, maintenance, etc. Problems in any of these links may cause concrete quality problems. After all, admixtures are only one of many influencing factors.
5. Admixture standards and specifications
In practice, people often rely on standards and specifications to control the quality of admixtures and concrete. In fact, the relevant standards and specifications for concrete and concrete admixtures We can only provide some principle requirements or guidance and some detection methods. It is impossible to provide solutions to specific cases. The standard of admixture is just a product standard, used for the inspection of products before leaving the factory. The inspection standard of qualified products is that they are homogeneous, non-toxic and harmless, and the property indicators meet predetermined values. The indicators specified in the standard are all minimum requirements. Therefore, admixtures that meet the technical requirements of the standard only represent a certain degree of "qualification". There is no guarantee that concrete that meets the technical requirements and has stable quality can be formulated. Standards and norms are only external factors, and external factors must work through internal factors. The mixing station must determine the mix ratio and select appropriate additives based on its own technical level and available raw materials, and strictly control the quality stability of the constituent materials and the stability of the production process. Admixture manufacturers must start by improving their own technical level and quality management level, and provide admixtures with stable quality that can meet concrete production, construction and various technical performance requirements. This is the only way to reduce problems in admixture application and improve concrete quality.
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