Air entrained concrete is a special type of concrete that contains tiny, uniformly distributed air bubbles introduced by air-entraining agents. These microscopic air voids, usually between 0.05 mm and 1.25 mm in size, significantly improve the durability, workability, and freeze-thaw resistance of concrete. In modern construction, air entrained concrete is widely used in pavements, bridges, dams, and structures exposed to harsh weather. By enhancing long-term performance, reducing cracking, and preventing damage from temperature changes, air entrained concrete has become a vital material in civil engineering.
What is Air Entrained Concrete?
Air entrained concrete is concrete in which controlled amounts of air are intentionally incorporated using air-entraining admixtures. Unlike natural entrapped air in conventional mixes, air entrained concrete contains millions of stable air bubbles that act as pressure relief valves within the hardened material. This prevents water expansion during freezing and reduces the risk of surface scaling and cracking.
The percentage of air typically ranges from 4% to 7% of the concrete volume, depending on environmental exposure conditions. The result is concrete that not only withstands freezing and thawing cycles but also offers better workability and reduced segregation during mixing and placement.
Importance of Air Entrained Concrete in Construction
Air entrained concrete matters in construction because it provides multiple benefits where ordinary concrete often fails. Its key importance includes:
- Improved freeze-thaw resistance for cold regions
- Reduced risk of scaling and cracking in pavements
- Better resistance to deicing salts and chemical attacks
- Enhanced workability and reduced bleeding during placement
- Extended service life of structures exposed to severe environments
By using air entrained concrete, engineers and contractors ensure durability, sustainability, and lower maintenance costs, which are essential for long-term infrastructure performance.
Properties of Air Entrained Concrete
Air entrained concrete has unique properties that distinguish it from conventional concrete.
Workability
The presence of air bubbles increases the cohesion of the mix, making it easier to place and finish.
Density
Due to the presence of air, density decreases slightly, usually by 3–10% compared to non-air-entrained mixes.
Strength
Air entrained concrete has slightly lower compressive strength than non-air-entrained concrete because air voids reduce the load-bearing area. However, this trade-off is acceptable for durability benefits.
Freeze-Thaw Resistance
Air entrained concrete provides excellent resistance to freeze-thaw cycles, making it suitable for regions with extreme weather.
Permeability
Although air increases voids, the microstructure remains stable, and the permeability does not significantly increase when designed properly.
Durability
By preventing scaling and cracking, air entrained concrete improves overall durability in harsh environments.
How Air Entrained Concrete Works
The mechanism of air entrained concrete lies in its air bubbles. When water inside concrete freezes, it expands by nearly 9%. In ordinary concrete, this expansion causes internal stresses and cracks. In air entrained concrete, the air bubbles act as “expansion chambers,” providing space for water to expand without damaging the structure.
During mixing, air-entraining agents reduce the surface tension of water, stabilizing millions of tiny air bubbles throughout the mix. These bubbles remain even after hardening, offering long-term protection.
Materials Used in Air Entrained Concrete
The production of air entrained concrete requires specific materials.
- Cement: Ordinary Portland Cement or blended cements compatible with admixtures
- Fine and Coarse Aggregates: Clean, well-graded aggregates
- Water: Potable water for mixing
- Air-Entraining Agents: Admixtures such as vinsol resin, synthetic detergents, fatty acids, or modern chemical-based agents
- Other Admixtures: Superplasticizers, retarders, or accelerators as needed for performance
Methods of Air Entrainment
Air entrainment in concrete can be achieved in different ways.
Using Air-Entraining Admixtures
The most common method is adding chemical air-entraining admixtures during mixing. These agents stabilize small air bubbles.
Using Air-Entraining Cement
Air-entraining cements already contain surface-active agents that produce air voids during hydration.
Mechanical Method
Air can be mechanically introduced during mixing, although this method is less common in large-scale construction.
Advantages of Air Entrained Concrete
Air entrained concrete provides several advantages:
- Increases durability under freeze-thaw conditions
- Reduces bleeding and segregation of concrete
- Enhances workability and finishing quality
- Reduces permeability and scaling from deicing salts
- Provides better resistance to weathering and chemical attack
- Extends the lifespan of pavements and exposed structures
Disadvantages of Air Entrained Concrete
Despite its benefits, air entrained concrete also has some drawbacks.
- Slight reduction in compressive strength (5–15%)
- Increased cost due to special admixtures
- Requires skilled supervision to maintain proper air content
- Not suitable for very high-strength concrete where maximum compressive capacity is required
Applications of Air Entrained Concrete
Air entrained concrete is widely used in civil engineering projects where durability and weather resistance are critical.
- Pavements and Highways: Reduces scaling and cracking under traffic and weather
- Bridge Decks: Provides freeze-thaw protection and durability
- Runways and Taxiways: Withstands deicing chemicals and heavy loads
- Marine Structures: Enhances resistance to saltwater and harsh environments
- Dams and Spillways: Improves long-term performance under hydraulic conditions
- Sidewalks and Driveways: Prevents cracking and scaling in cold climates
Mix Design for Air Entrained Concrete
Mix design depends on required strength, workability, and durability. Typical air content is maintained between 4–7%.
| Exposure Condition | Air Content (%) | Compressive Strength (MPa) |
|---|---|---|
| Mild | 4 | 20–25 |
| Moderate | 5 | 25–30 |
| Severe | 6 | 30–35 |
| Very Severe | 7 | 35+ |
Proper balance between strength and durability ensures effective use of air entrained concrete.
Difference Between Air Entrained Concrete and Non-Air Entrained Concrete
| Property | Non-Air-Entrained Concrete | Air-Entrained Concrete |
|---|---|---|
| Workability | Lower | Higher |
| Strength | Higher | Slightly lower |
| Freeze-Thaw Resistance | Poor | Excellent |
| Durability | Moderate | High |
| Resistance to Salts | Low | High |
| Cost | Lower | Higher |
Best Practices for Using Air Entrained Concrete
- Select proper air-entraining admixture compatible with cement
- Maintain recommended air content (4–7%) based on environment
- Avoid excessive mixing which may reduce air content
- Regularly test fresh concrete for air content using pressure methods
- Protect freshly placed concrete from rapid drying and extreme conditions
FAQs on Air Entrained Concrete
What is the purpose of air entrained concrete?
The main purpose of air entrained concrete is to improve freeze-thaw resistance, reduce cracking, and enhance durability in structures exposed to harsh weather.
How much air should be in air entrained concrete?
Typically, 4–7% air content is maintained depending on exposure severity and structural requirements.
Does air entrainment reduce strength?
Yes, compressive strength may reduce slightly (5–15%), but the durability benefits outweigh this drawback.
Where is air entrained concrete used most?
It is widely used in pavements, highways, bridge decks, dams, and structures exposed to freezing temperatures and deicing salts.
Can air entrainment be added to high-strength concrete?
Air entrainment is not preferred in very high-strength mixes where compressive capacity is critical, but it can be used in normal strength concrete requiring durability.
Conclusion
Air entrained concrete plays a crucial role in modern construction by ensuring durability, weather resistance, and long-term performance of structures. Although it slightly reduces compressive strength, its advantages in freeze-thaw resistance, workability, and protection against deicing salts make it indispensable in cold and aggressive environments. By understanding its properties, mix design, applications, and best practices, engineers and contractors can effectively use air entrained concrete to build safer and longer-lasting infrastructure.
