Concrete admixtures are chemical materials used to improve the performance of newly mixed or hardened concrete. Common types of concrete admixtures include: water reducers, retarders, accelerators, air-entraining agents, quick-setting agents, waterproofing agents, and rust inhibitors. Admixtures can be divided into four categories based on their functions: (a)those that improve the mixture and rheological properties of concrete, (b)those that adjust the setting time and hardening properties of concrete, (c)those that improve the durability of concrete, (d)those that improve other properties of concrete.
The admixtures used by concrete companies are usually multi-component admixtures, and because these products are composite, they often bring some problems to users. Here are some practical tips on how to correctly select and use admixtures.
I Fully understand the characteristics of each component of the admixture
A single type of admixture usually only plays a major role in a single aspect and is difficult to fully meet the various requirements of concrete strength, durability, workability, etc. Therefore, it is necessary to use two or more different types of admixtures in combination to achieve the effect of 1+1≥2 and meet the performance requirements of concrete.
Concrete production companies generally use Concrete admixtures that are mainly composed of water reducers, retarders, air-entraining agents, water-retaining agents, thickeners, etc. When using admixtures, it is important to understand the role and characteristics of each component in the concrete. This is the prerequisite for correctly using admixtures. For example, the retarder component not only slows down the setting of concrete but can also appropriately increase the water reduction rate and extend the slump loss. However, excessive use of retarders can cause concrete bleeding and serious retardation. Air-entraining agents can improve the frost resistance, durability, and flowability of concrete, but excessive use can cause a decrease in concrete strength. Thickening components in admixtures can improve the cohesiveness of concrete and reduce bleeding, but excessive amounts can cause a decrease in flowability.
In the process of concrete production and construction, it is important to fully consider the interaction between admixtures and other raw materials, mix proportions, climate, and construction conditions in order to formulate concrete that meets performance requirements. In fact, many problems with concrete mixtures are related to the compatibility of admixtures and other raw materials to some extent. For example, abnormal phenomena such as segregation, bleeding, excessive air bubbles, rapid loss of slump, and abnormal setting time in concrete mixtures are mostly caused by poor compatibility of certain components of admixtures with the cementitious materials.
The dosage of concrete admixtures is expressed as the percentage of admixture mass in the total mass of cementitious materials in the concrete. During use, the dosage of admixtures is not constant, and should be adjusted accordingly when the quality of other raw materials changes or the concrete mix proportion changes. Generally, the water-reducing rate of admixtures increases with the increase of admixture dosage, and when the dosage reaches saturation point, the water-reducing rate index no longer significantly increases with further increase of dosage. However, excessive use of other components of admixtures, such as air-entraining agents and retarders, can have a negative impact on concrete.
In the process of concrete production, it is appropriate to use a rapid compatibility test method for Concrete admixtures to identify the reasons for incompatibility with raw materials. However, when selecting or using admixtures, concrete testing should be taken as the standard, in order to observe the flowability, cohesion, water retention, workability, and slump loss of concrete. Regardless of which admixture is used, compatibility testing is an essential item, and only when the admixture and other materials are compatible can the expected effect be achieved.
Strict control of admixture quality is the premise and foundation for rational use of admixtures. The incoming inspection of admixtures should be tested according to the quality indicators specified in the contract. When signing the admixture procurement contract, the method of resolving quality disputes should be agreed upon in order to avoid confusion.
Upon arrival at the plant, admixtures should be strictly inspected, and products that do not meet the quality requirements should not be allowed in. Secondly, admixtures should be stored according to their categories, with clear labeling, to avoid engineering quality accidents caused by mixing or confusing different types of admixtures. Thirdly, there are two main methods for testing admixtures, one is the cement paste method and the other is the concrete method. When using the cement paste method, the flowability should be controlled between 220-260 mm (water-cement ratio of 0.29). If the flowability is too large, the concrete is prone to segregation and bleeding, and if it is too small, the flowability of the concrete is poor. The cement paste flow test method only tracks the changes in cement, and when other raw materials change, it cannot reflect the changes in a timely manner. For the concrete method, a certain mix proportion is usually taken as the basis, and concrete is mixed and observed for 1-2 hours. If the concrete meets the requirements, it is considered qualified, otherwise the admixture should be adjusted. The concrete method should consider the influence of environmental temperature and be consistent with the actual use environment as much as possible. When there are discrepancies in the results of admixture testing, such as large fluctuations in the water-reducing rate of concrete, previous qualified samples of admixtures should be taken out for comparison and analysis to determine if the change is due to the admixture or other raw materials.
When the dosage of admixtures is too high, the concrete will exhibit segregation, bleeding, and difficulty in consolidation. If the water content is reduced to prevent bleeding, the strength of the concrete will increase. If the dosage of admixtures is too low, the slump will be too small. Increasing the water content to improve the flowability of concrete will result in lower concrete strength.
Each concrete strength grade has an appropriate dosage of admixture that will result in the desired slump and strength. For example, when pumping C30 concrete, the dosage of admixture is 2%, and the concrete is suitable for use. If the same dosage of admixture is used for C15 concrete, it will generally bleed severely. When designing concrete mix ratios, the difference in admixture dosage should be considered. For C30, C25, C20, and C15, the dosage of admixture can be designed as 2%, 1.8%, 1.6%, and 1.4%, respectively. This makes it easier to quickly infer the dosage of admixture for various types of concrete.
Adjusting the dosage of admixture is a very important task in concrete production control. The factors that affect the adjustment of admixture dosage often change due to variations in raw materials, different construction sites, and environmental changes.
The following are common situations encountered in the production of concrete, where "-" indicates a decrease in the dosage of admixture:
(a) When large stones are used instead of small ones, the dosage of admixture decreases by 0.1%;
(b) When the fineness modulus of sand increases by 0.3, the dosage of admixture decreases by 0.2%;
(c) When the clay content in sand decreases by 3%, the dosage of admixture decreases by 0.2%;
(d) When the temperature drops by more than 15°C, the dosage of admixture decreases by 0.1% to 0.2%;
(e) When the required water content of cement, slag, or fly ash decreases, the dosage of admixture should also be decreased;
(f) When the flowability of cement paste is between 240-260 mm, the dosage of admixture decreases by 0.2%. When the flowability is between 260-280 mm, the dosage of admixture decreases by 0.4%;
(g) For inclined roofs, the dosage of admixture decreases by 0.2%.
The above empirical data is for reference only. To prevent quality accidents, it is important to quickly identify the cause of quality problems and promptly adjust the dosage of admixture accordingly. Also, note that adjusting the dosage of Concrete admixtures may require adjustments to other raw materials as well.
