Air Entrainment in Concrete: How to Improve Workability and Freeze-Thaw Resistance

Air entrainment is the intentional creation of tiny air bubbles in concrete during mixing, achieved by adding a surfactant-based air-entraining agent. These microscopic voids—typically between four and seven per cent of the total concrete volume—improve both fresh and hardened concrete properties. If you work in climates with freeze-thaw cycles or need better pumpability on-site, understanding how air entrainment functions will help you specify and handle the mix correctly.

Approximately 85 per cent of concrete manufactured in the United States contains air-entraining agents, which are now considered the fifth ingredient in concrete technology alongside cement, aggregates, water, and admixtures. The agent—often a natural wood resin, soap, vegetable fat or oil, or alkali salt of sulphonated or sulphated organic compounds—creates stable bubbles while the concrete slurry is still liquid. Most of these bubbles survive to remain in the hardened matrix.

What Does Air Entrainment Do in Fresh Concrete?

In the plastic state, entrained air improves workability: the tiny bubbles act as ball bearings, reducing internal friction and making the mix easier to handle, place, and pump. This also helps prevent bleeding—the unwanted rise of excess water to the surface—and segregation, where heavier aggregates settle out. For site teams, this translates to smoother pumping, reduced labour during placement, and fewer surface defects such as laitance formation.

How Air Voids Protect Hardened Concrete from Freeze-Thaw Damage

The primary durability benefit is freeze-thaw resistance. Even hardened concrete is porous—typical porosity runs between six and twelve per cent by volume—due to capillaries left when excess water evaporates. A water-to-cement ratio of approximately 0.38 (38 lbs. of water per 100 lbs. of cement) is required for full hydration; most workable mixes use 0.45 to 0.60, leaving surplus water that later evaporates and creates pores.

When environmental moisture infiltrates these pores and freezes, it expands. Without relief zones, the internal stress cracks the matrix. Entrained air bubbles, spaced closely and evenly throughout, provide pressure-relief zones: expanding ice pushes into the voids rather than fracturing the paste. Properly air-entrained concrete can remain durable for many years even under severe freeze-thaw exposure.

Additional Performance Benefits

Trade-Offs You Must Manage

Air entrainment is not without costs. For every one per cent increase in air content, compressive strength decreases by two to six per cent. You must balance durability needs against structural design requirements—air-entrained concrete works best in applications such as dams, bridges, tunnels, and road pavements in cold climates or sulphate-bearing soils, where freeze-thaw and chemical exposure outweigh the strength penalty.

Other limitations include increased susceptibility to drying shrinkage when the concrete sets, and reductions in modulus of elasticity and abrasion resistance. If your project demands high surface abrasion resistance—for example, industrial floors—you may need to adjust mix design or surface treatments accordingly.

Air Entrainment vs. Foam Concrete

Do not confuse air-entrained concrete with foam concrete. Foam concrete introduces larger, stable bubbles via a separate foam agent, producing a low-density material used for insulation or void-filling. Air-entrained concrete, by contrast, has evenly distributed microscopic voids that enhance durability and workability without significantly reducing density or compromising mechanical properties for structural applications such as prestressed bridges or roller-compacted pavements.

On-Site Handling and Quality Control

Air content must be measured accurately at the point of placement. Variations in mixing time, temperature, aggregate grading, and admixture dosage can all affect the final air volume. Prolonged mixing, high temperatures, or carbon-containing fly ash can interfere with bubble stability and reduce entrained air. Always verify air content with a pressure meter or volumetric method before pouring, and adjust dosage if necessary to meet specification—typically in the range of four to seven per cent by volume.

Recent research has explored superabsorbent polymers as an alternative mechanism for internal curing and freeze-thaw protection, though air entrainment remains the dominant, proven technology for durable concrete in freeze-thaw environments.

Practical take-away: Specify air entrainment whenever your concrete will face freeze-thaw cycles, de-icing salts, or aggressive chemical exposure. Verify air content on-site, and accept the modest strength trade-off in exchange for long-term durability. For guidance on classification and exposure classes, consult exposure class standards for concrete.