Entrained air refers to microscopic air bubbles intentionally introduced into a material, most commonly concrete, during mixing. These bubbles are tiny, spherical, and evenly distributed, ranging from about 0.01 to 1 mm in diameter. Their primary purpose is to protect concrete from cracking and scaling when water inside it freezes and expands. The concept also appears in fluid dynamics and HVAC systems, where fast-moving air or fluid draws in surrounding air through pressure differences.
How Entrained Air Works in Concrete
Concrete is porous. Water seeps into its tiny internal channels, called capillary pores. When temperatures drop below freezing, that water expands by about 9%, generating enormous internal pressure. Without somewhere for the expanding water to go, the concrete cracks from the inside out.
Entrained air solves this by creating billions of microscopic chambers spread throughout the concrete. As water in the capillary pores begins to freeze and expand, it travels a short distance into the nearest air bubble, which acts as a pressure relief valve. The bubble compresses slightly to absorb the expansion, preventing the kind of internal stress that leads to surface scaling, spalling, and structural damage. The key is that these bubbles are small enough and close enough together that water never has to travel far to find relief.
Entrained Air vs. Entrapped Air
All concrete contains some air, but not all of it is useful. The air naturally trapped during mixing, called entrapped air, forms irregular, randomly shaped voids that are typically larger than 1 mm. These pockets aren’t evenly distributed and don’t provide reliable freeze-thaw protection.
Entrained air is different in every meaningful way. The bubbles are spherical, between 0.01 and 1 mm across, and spread uniformly through the mix. This even distribution means the maximum distance from any point inside the concrete to the nearest air bubble is no more than about 0.2 mm. That short travel distance is what makes entrained air effective: water doesn’t need to move far before it reaches a pressure relief point. Entrapped air, with its random placement and large voids, can’t offer the same protection.
How Air Gets Entrained
Concrete producers add a chemical called an air-entraining admixture (AEA) during mixing. These are surface-active agents, similar in concept to soap, that lower the surface tension of the water in the mix. This allows tiny, stable bubbles to form and remain suspended rather than rising to the surface and popping. The mixing action whips air into the concrete, and the admixture stabilizes it into the right size and spacing.
To achieve effective freeze-thaw resistance, the target is roughly 9% air in the mortar fraction of the concrete (the paste and sand portion, not counting the larger stones). The overall air content of the finished concrete is lower, because the coarse aggregate takes up volume without containing air bubbles. Standards from ASTM C260 govern the admixtures used, while guidelines from the American Concrete Institute adjust the target air content based on exposure conditions and the size of the coarse aggregate in the mix.
The Spacing Factor
Getting the right total volume of air isn’t enough. What matters most is how closely the bubbles are spaced. Engineers measure this with a metric called the spacing factor: the maximum distance from any point in the concrete’s pore network to the surface of the nearest air bubble. For reliable freeze-thaw durability, the spacing factor should be no more than 0.2 mm.
Hitting that target at a typical 5% total air content means most bubbles need to fall between 0.1 and 0.3 mm in diameter. Some sources describe the smallest entrained bubbles as just 1 to 100 micrometers across, roughly the same size as individual cement particles. These tiny bubbles intersect the capillary network at regular intervals, ideally spaced no more than 0.4 mm apart, creating a reliable safety net throughout the entire volume of the concrete.
Factors That Affect Air Entrainment
Several variables can raise or lower the amount of entrained air in a concrete mix, sometimes in ways that catch producers off guard.
Temperature plays a significant role. Cold raw materials reduce air-entraining efficiency. Research on concrete mixed at high elevations, where stored materials were colder, found that lower temperatures reduced air content by about 1 percentage point. That may sound small, but it can push the spacing factor above the 0.2 mm threshold and compromise freeze-thaw protection.
Aggregate quality matters too. Clean sand has little effect on entrained air, but coarse aggregates often contain impurities like stone powder and clay. These substances absorb the air-entraining admixture to varying degrees depending on the chemical type of admixture used, which can reduce the number of stable bubbles that form. Concrete mixes with dirtier aggregates may need a higher admixture dose to hit the same air content.
Time after mixing also influences what you end up with. Entrained air bubbles gradually escape from fresh concrete. In one study, air content in mortar samples tested 45 minutes after mixing had settled to a narrow range of 8.2 to 8.6%, regardless of starting conditions. The longer concrete sits before placement and finishing, the more air it loses.
The Tradeoff With Strength
Air bubbles are not concrete. Every bubble displaces material that would otherwise contribute to compressive strength. As a general rule, each additional 1% of entrained air reduces compressive strength by roughly 3 to 5%. A mix designed for 6% air content will be noticeably weaker than the same mix with no entrained air.
This tradeoff is worth it in any climate where concrete faces freeze-thaw cycles. Without entrained air, concrete exposed to winter weather can deteriorate within just a few years, with surface scaling, pop-outs, and deep cracking. The modest strength reduction is a deliberate trade for dramatically longer service life. In mild climates where freezing isn’t a concern, air entrainment is often unnecessary and skipped to preserve full strength.
Entrained air also improves workability. The tiny spherical bubbles act almost like ball bearings in the fresh mix, making it easier to place and finish. This means producers can sometimes reduce the water content slightly, which partially offsets the strength loss from the air itself.
Air Entrainment Beyond Concrete
The term “entrained air” appears in other engineering fields, where the underlying physics is different but the basic idea is the same: fast-moving fluid pulls surrounding air along with it.
In fluid dynamics, air entrainment happens when a high-velocity stream creates a low-pressure zone that draws in ambient air. This is the Bernoulli principle at work. A Venturi device, for example, forces a gas or liquid through a narrow constriction, which increases its speed and drops its pressure. Side openings in the constriction allow outside air to be “entrained,” or pulled in and mixed with the primary flow. Medical oxygen delivery devices use this mechanism to blend pure oxygen with room air at precise ratios.
HVAC systems use the same principle in equipment like active chilled beams. Primary air from a central air handler is forced through small nozzles at high velocity, creating a pressure drop that induces room air to flow across a cooling coil before mixing with the supply air and discharging back into the space. The ratio of this induced room air to the primary supply air is called the induction ratio (sometimes called the entrainment ratio). Higher induction ratios mean more room air is being recirculated, which improves cooling capacity without increasing the volume of conditioned air from the central system.