Generally, concrete is required to be air entrained event if it is not exposed to freezing and thawing due to important benefits that concrete enjoys in other ways such as improving concrete workability.
Concrete air entrainment is not an easy process since it is influence by several factors. These factors shall be properly investigated to be able to obtain designated air entrainment.
Fig.1: Air Entrained Concrete
Factors Affecting Air Content of Concrete
- Water content
- Fine aggregates grading
- Coarse aggregates
- Temperature of concrete
- Mixing action of concrete
- Use of admixtures other than air entrained admixture
- Cement content of concrete
- Fly ash content
- Vibration or compaction of concrete
1. Effects of Water Content on Air Content of Concrete
Increase of water content in the mixture will lead to increase concrete air content and vice versa. This is because the increase of water content would increase fluid mixture into which air bubbles could be integrated easily through mixing operation.
Hard water such as well or quarry water, which contain minerals, would reduce air content if they are used to dilute air entraining admixture.
2. Effects of Fine Aggregate Grading on Air Content of Concrete
Air entrainment will be carried out easily as the percentage of fine aggregate is increased. Fine aggregate sizes ranges from sieve No. 30 to sieve No. 100 will create small voids that can contain air bubbles.
However, other fine aggregate sizes will require greater quantity of air entraining admixture to reach the same air content.
3. Effect of Coarse Aggregate on Concrete Air Content
Presence of dust on coarse aggregate surface decreases air content. Crushed aggregate would entrain lesser air compare with gravel aggregate.
4. Effects of Temperature on Concrete Air Content
For constant amount of air entraining admixture, the increase of temperature will lead to decline concrete air content. So, if temperature varies considerably during mixture production, it would be required to adjust air entraining admixture to achieve designated air content.
Temperature reduction from 21C to 5C will increase air content by 40% whereas increase of temperature from 21C to 38C will decline air content by 25%.
5. Effects of Mixing Action on Air Content of Concrete
Air content increase by mixing for up to 15 minutes and mixing beyond this time will decrease air content. Entrained air will vary with type, physical condition, speed of the mixer and quantity of concrete that is being mixed.
Seriously worn mixer will increase difficulty of air entrainment operation. Similarly, mixer with significant amount of hardened concrete buildup on mixer blades or in a drum increase air entrainment operation.
6. Effects of Admixtures other than Air Entrained Admixture on Air Content of Concrete
There are number of admixtures that have air entraining capacity for example retarding admixture and water reducing admixture. So, smaller quantity of air entraining admixture will be needed when these admixtures are also provided in the mixture.
7. Effect of Cement on Concrete Air Content
Air content of concrete reduces with the increase of cement fineness. For the same air content, concrete produced using Type I cement need considerably lesser quantity of air entrainment admixture compare with concrete made with Type III cement.
8. Effect of Fly Ash on Air Content of Concrete
Air content reduces as the fineness or surface area of fly as increases. Increase of fly ash per unit concrete reduces air content of concrete and increase in fly ash carbon content reduces concrete air content.
8. Effect of Vibration on Air Content
Vibration reduces concrete air content. For example, half of air content will be lost if vibration is applied for more than three minutes.