Air entrainment in concrete is necessary even when not exposed to freezing and thawing because it offers significant advantages, such as enhancing workability. However, achieving air entrainment in concrete is a complex process influenced by various factors. Thoroughly examining these factors is crucial for achieving the desired level of air entrainment.
Fig.1: Air Entrained Concrete
Factors Affecting Air Content of Concrete
Water content refers to the amount of water used in the concrete mixture. It plays a crucial role in determining the workability and strength of the concrete.
Fine aggregates grading refers to the particle size distribution of the fine aggregates used in concrete. The grading affects the workability and strength of the concrete mixture.
Coarse aggregates are the larger particles used in concrete, typically made of crushed stone or gravel. They provide strength and stability to the concrete mixture.
The temperature of concrete during mixing and curing is important for its development and strength. Extreme temperatures can impact the hydration process and the overall performance of the concrete.
The mixing action of concrete involves blending the ingredients thoroughly to achieve a homogeneous mixture. Proper mixing ensures uniform distribution of materials and improves the strength and durability of the concrete.
Admixtures, apart from air entrained admixtures, are additional materials added to concrete to enhance specific properties. They can improve workability, strength, durability, and other performance characteristics of the concrete.
The cement content of concrete refers to the amount of cement used in the mixture. Cement is the binder that holds the concrete together and contributes to its strength.
Fly ash content in concrete is the proportion of fly ash, a byproduct of coal combustion, used as a supplementary cementitious material. Fly ash can improve the workability, strength, and durability of the concrete.
Vibration or compaction of concrete involves the process of consolidating the concrete mixture to remove air voids and ensure proper compaction. This process enhances the strength and durability of the hardened concrete.
1. Effects of Water Content on Air Content of Concrete
Increasing the water content in the mixture results in an increase in the air content of the concrete, and the opposite is true as well. This occurs because a higher water content creates a more fluid mixture, facilitating the easy integration of air bubbles during the mixing process. When using hard water such as well or quarry water, which contains minerals, to dilute air entraining admixture, the air content is reduced.
2. Effects of Fine Aggregate Grading on Air Content of Concrete
Increasing the percentage of fine aggregate facilitates easier air entrainment. Fine aggregate sizes ranging from sieve No. 30 to sieve No. 100 create small voids capable of housing air bubbles. On the other hand, different fine aggregate sizes necessitate a larger quantity of air entraining admixture to achieve an equivalent air content.
3. Effect of Coarse Aggregate on Concrete Air Content
The presence of dust on the surface of coarse aggregate reduces the air content. Crushed aggregate tends to have lower air entrainment compared to gravel aggregate.
4. Effects of Temperature on Concrete Air Content
The air content in concrete can be affected by changes in temperature, particularly when a constant amount of air entraining admixture is used. In such cases, an increase in temperature can result in a decrease in concrete air content. Therefore, when the temperature fluctuates significantly during the production of the mixture, it becomes necessary to adjust the amount of air entraining admixture in order to achieve the desired air content. For instance, if the temperature is reduced from 21°C to 5°C, the air content will increase by approximately 40%. Conversely, if the temperature is increased from 21°C to 38°C, the air content will decrease by approximately 25%. These temperature-related variations underscore the importance of careful monitoring and adjustment of the air entraining admixture to maintain the specified air content in concrete.
5. Effects of Mixing Action on Air Content of Concrete
The air content in concrete can be enhanced by mixing it for a maximum duration of 15 minutes. However, if the mixing process continues beyond this time, the air content will start to decrease. The amount of entrained air in the concrete can vary depending on factors such as the type and physical condition of the mixer, the speed at which it operates, and the quantity of concrete being mixed. When dealing with a mixer that is seriously worn, the process of air entrainment becomes more challenging. Similarly, if the mixer blades or drum have a significant buildup of hardened concrete, it will also hinder the air entrainment operation.
6. Effects of Admixtures other than Air Entrained Admixture on Air Content of Concrete
When incorporating admixtures with air entraining capacity, such as retarding admixtures and water reducing admixtures, the required amount of air entraining admixture can be significantly reduced. This means that by utilizing these specific admixtures in the mixture, a smaller quantity of air entraining admixture will be necessary.
7. Effect of Cement on Concrete Air Content
The air content of concrete is inversely related to the fineness of cement. As the cement fineness increases, the air content in concrete decreases. When aiming for the same air content, concrete produced with Type I cement requires significantly less air entrainment admixture compared to concrete made with Type III cement.
8. Effect of Fly Ash on Air Content of Concrete
The air content of concrete decreases as the fineness or surface area of fly ash increases. When the amount of fly ash added to a unit of concrete increases, it results in a reduction of the concrete’s air content. Additionally, if the carbon content of the fly ash increases, it further decreases the air content of the concrete.
8. Effect of Vibration on Air Content
The application of vibration has a significant impact on the air content of concrete, leading to a reduction in its overall air content. For instance, if vibration is consistently applied for a duration exceeding three minutes, approximately half of the initial air content within the concrete will be lost. This phenomenon underscores the importance of carefully managing vibration techniques during the concrete preparation process, as it directly affects the air content and, consequently, the overall quality and properties of the concrete structure. Proper control and monitoring of vibration duration are crucial to maintain the desired air content levels and ensure the structural integrity and performance of the concrete.