Entrapped Air vs. Entrained Air in Concrete: What's the Difference?

Entrapped air consists of larger, irregular voids naturally occurring in concrete, typically making up about 1% to 1.5% of the total volume. Entrained air, however, is intentionally introduced as microscopic, uniformly distributed bubbles, primarily to enhance concrete's resistance to freezing and thawing.

Entrapped air is undesirable, accidental voids in concrete, typically 1-1.5% of volume, while entrained air is beneficial microscopic bubbles intentionally added to improve freeze-thaw resistance.

Entrapped air refers to the larger, irregular air pockets that naturally become part of the concrete mix during batching, mixing, and placement. These voids are generally undesirable, as they can reduce the concrete's strength and overall durability. In most ordinary concrete, these entrapped air voids account for approximately 1% to 1.5% of the hardened concrete's total volume. These larger bubbles can be partially reduced through proper vibration during placement, though normal vibration, not exceeding about 15 seconds, primarily eliminates these larger entrapped air bubbles rather than significantly reducing intentionally entrained air.

What is Entrained Air?

Entrained air, on the other hand, is a system of microscopic air bubbles intentionally introduced into the concrete mix using special admixtures. This process creates billions of tiny, closely spaced bubbles uniformly distributed throughout the cement paste. For example, a single cubic yard of air-entrained concrete can contain as many as 500 billion such bubbles. The presence of these small, spherical air voids significantly improves the concrete's ability to resist deterioration caused by repeated cycles of freezing and thawing, as they act as tiny reservoirs for expanding water.

Benefits of Entrained Air

The primary benefit of entrained air is increased freeze-thaw resistance. When water in concrete freezes, it expands. Without entrained air, this expansion creates internal pressure that can crack and spall the concrete. The microscopic air bubbles provide space for this expanding water, preventing damage. This is particularly important for exterior concrete in regions with freeze-thaw cycles, though less critical in Southern California's generally mild climate, unless in high-elevation areas or specific applications requiring enhanced durability against moisture. Concrete subject to moderate or severe weathering generally requires air entrainment, often with a minimum total air content of 5 percent. Some building codes specify an air content between 5% and 7% by volume for air-entrained concrete.

Beyond freeze-thaw resistance, entrained air can also improve the workability of the fresh concrete mix. It reduces the rate of settlement and, because of reduced bleeding, allows the concrete to be finished sooner than non-air-entrained concrete. When high strength is required, for instance in some prestressed concrete applications, using air-entrained concrete might make it more challenging to achieve the specified strength, as each 1% of added air can reduce 28-day compressive strength by about 5% compared to a similar mix without entrained air.

Factors Affecting Entrained Air Content

Several factors influence the final air content in a batch of air-entrained concrete. The most obvious is the dosage of the air-entraining admixture used. However, the type and condition of the mixer, the volume of concrete being mixed, and the mixing rate also play significant roles. The temperature of the concrete is also a factor, with higher concrete temperatures typically leading to less air being entrained. The use of other admixtures, like high-range water reducers, can also impact air content, potentially requiring adjustments to the air-entraining admixture dosage.

> Pro Tip: While Southern California generally has mild weather, understanding air entrainment is important for specialized pours, such as precast elements or specific applications that might be exposed to colder conditions or moisture. Always discuss project-specific needs with your concrete supplier and contractor.

Common Mistakes

• Over-vibration: While vibration helps remove entrapped air, excessive vibration can start to remove intentionally entrained air, reducing its benefits. Normal vibration should not exceed about 15 seconds.
• Ignoring temperature: Pouring air-entrained concrete on very hot days can reduce the actual amount of air entrained, compromising its performance.
• Misjudging admixture interactions: Some admixtures, like certain high-range water reducers, can reduce the amount of entrained air, requiring careful adjustments to achieve the desired air content.

When planning a concrete project in Southern California, understanding the role of air in your concrete mix ensures optimal durability and performance for your specific application. Western Concrete recommends discussing your project's environmental exposures and structural needs with your contractor to determine the appropriate concrete mix design. You can explore different concrete options and design ideas in our Design Center or use our Concrete Project Calculator to help plan your pour.