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Misunderstandings in the use of concrete admixtures

Concrete admixtures are an indispensable part of modern concrete and a means of modifying it. They can improve the physical and mechanical properties of hardened concrete, enhance the quality and durability of building components, save cement, reduce costs, and accelerate project progress. As modern architecture shifts toward high-rise buildings and concrete moves toward pre-mixed, centrally mixed concrete, the use of concrete admixtures is becoming increasingly widespread. However, if admixtures are not used properly, they often fail to deliver the expected results and can even lead to quality accidents.

❌️ Misconception 1:Additives are directly put into use without inspection

Before concrete construction, the construction unit and the supervision unit always send cement, sand, stone, and other building materials to inspection agencies for batch testing according to specifications, but often pay insufficient attention to performance testing of external concrete additive. In fact, many types of concrete admixtures have corresponding quality standards, and their performance directly affects their role in concrete, such as the water-reduction rate and chloride ion content.

Article 7.2.2 of the “Code for Acceptance of Construction Quality of Concrete Structures” (GB 50204) emphasizes that the quality and application technology of admixtures used in concrete should comply with relevant environmental protection regulations such as “Concrete Admixtures” (GB8076) and “Technical Specification for Application of Concrete Admixtures” (GB50119), and specifies the quantity and methods of inspection. The article description explains that this provision is mandatory; therefore, admixtures cannot be used in concrete without inspection.

❌️ Misconception 2: Improper use of admixtures without considering the actual situation of the project

The selection of additives should consider their intended use

When selecting additives, they should be determined through trial mixing tests based on the engineering materials and construction conditions. Various additives have their own characteristics, and their use should be based on the characteristics of the concrete additive, combined with the purpose of use, such as considering comprehensive indicators such as saving cement, improving concrete performance, and accelerating formwork turnover; that is, determining the type of additive to be used through technical and economic comparisons.

Analysis of Several Concrete Application Admixtures

The purpose of using additives in waterproof concrete is to improve the impermeability of concrete. To improve concrete impermeability, the key is to reduce internal pores, block leakage paths, and use water-reducing and air-entraining agents.
The key to the construction technology of large-volume concrete is to ensure good bonding between the layered poured concrete at the initial setting, reduce the temperature rise during the concrete hardening process, and avoid concrete cracking. The use of retarding water-reducing agents can meet the above requirements, namely retarding and reducing the temperature rise during the concrete hardening process, thereby improving the quality of concrete.
In large-span prestressed concrete structures, the self-weight of the structure is often the main contradiction. Therefore, additives are used to produce high-strength concrete, reduce the structure’s weight, and effectively utilize the high-strength steel bars’ stress. It is recommended to use high-efficiency water-reducing agents, which can significantly reduce unit water consumption and improve concrete strength.

❌️ Misconception 3: Neglecting the Adaptability of Admixtures and Cement

What is the workability of additives and cement?

The adaptability issue between concrete admixtures and cement can be explained as adding a tested admixture that meets relevant standards to cement-based concrete and using it according to regulations. If it produces the desired effect, the cement is compatible with the admixture.

Examples of unsuitable applications of additives and cement

Compatibility issues exist between almost all concrete additive types and cement. If several types of ordinary Portland cement are mixed with a certain high-efficiency water-reducing agent (which has been tested to meet the quality standards of high-efficiency water-reducing agents) to prepare concrete, and under the same preparation conditions, the concrete prepared with one type of cement has serious deficiencies in water reduction rate, it indicates that this type of cement is not suitable for the high-efficiency water-reducing agent. In contrast, the other cement types are suitable for use with high-efficiency water-reducing agents. For example, if a certain type of cement is mixed with concrete that meets the relevant quality standards but does not achieve rapid setting, or if a retarder is added but does not achieve retarding, the admixture and cement can be considered incompatible.

In terms of water-reducing agents, different types of water-reducing agents have different effects on the dispersion, water reduction, and reinforcement of cement; For the same type of water-reducing agent, due to differences in cement mineral composition, mixed material variety and dosage, alkali content, gypsum variety and dosage, the water-reducing and strengthening effect varies greatly. For example, wood ash, calcium and sugar syrup water-reducing agents set normally in cement concrete with dihydrate gypsum as a setting agent. Still, when used in cement with hard gypsum (CaSO4) as a setting agent, they cause setting (false setting). Some cement may set rapidly due to insufficient gypsum content (while still meeting standard performance) and incompatibility with water-reducing agents.

Adaptability testing of additives and cement

The adaptability issue between additives and cement is complex and unavoidable, affecting application effectiveness and sometimes leading to serious engineering accidents and incalculable economic losses. Before using admixtures, test their compatibility with cement by plotting a curve with dosage on the horizontal axis and flowability on the vertical axis. Admixtures with low saturation points, high flowability, and low economic losses have better adaptability to cement.

❌️ Misconception 4: The more additives added, the better the effect

Consequences of excessive use of concrete admixtures

Excessive use of retarders can not only cause severe retardation of concrete, resulting in long-term failure to achieve final setting, but also increase the air content significantly and cause excessive loss of concrete strength. In severe cases, concrete strength may fall below design requirements, leading to engineering accidents.
Excessive addition of high-efficiency water-reducing agents can cause air bubbles, which can affect the development of early concrete strength.
When the dosage of chloride salt additives is high, it will reduce the chemical resistance and wear resistance of concrete, increase the early shrinkage of concrete, and reduce the frost resistance and flexural strength of concrete.
Excessive dosage of quick setting agent results in an increase in setting time and a more severe decrease in the later strength of concrete.

Suitable dosage of concrete admixtures

The appropriate dosage of admixtures is reflected in the fact that when the dosage is small, the flowability of the concrete mixture increases less; When the dosage is too high, the fluidity does not increase proportionally. The optimal additive dosage is key to achieving the best technical and economic results. The principle is to use the optimal dosage while meeting the concrete’s performance requirements. The manufacturer’s product manual provides the dosage range for a given additive, and the user must determine the optimal dosage through trial mixing tests.

Conclusion-Misunderstandings in the use of concrete admixtures

The analysis above shows that to truly leverage admixtures in concrete, the following three points must be achieved.

Timely and accurate grasp of the relevant performance indicators of additives.
Select appropriate admixtures based on the engineering project’s performance requirements.
Determine the optimal additive dosage through experiments.