Air-entrained concrete contains a controlled system of microscopic air bubbles formed by an air-entraining admixture. The bubbles are deliberately created and distributed through the cement paste. They are different from large, irregular pockets of entrapped air caused by incomplete consolidation.
Entrained air versus entrapped air
| Feature | Entrained air | Entrapped air |
|---|---|---|
| Origin | Intentionally produced with an admixture or air-entraining cement | Unintentionally trapped during mixing, placing, or consolidation |
| Typical form | Very small, generally spherical and well distributed | Larger, irregular voids |
| Main effect | Improves freeze-thaw and deicer-scaling resistance when the complete air-void system is suitable | Usually reduces strength and quality without providing reliable durability protection |
Why entrained air protects concrete
Concrete usually contains moisture in its capillary pores. During freezing, ice formation can create hydraulic pressure in the surrounding paste. Closely spaced entrained bubbles provide nearby relief space for displaced water. This reduces internal stress during repeated freezing and thawing.
Total air content alone does not describe the complete protection. Bubble size, spacing, distribution, saturation, water-cementitious materials ratio, curing, aggregate durability, and exposure all matter. That is why a field air test is an acceptance check, not a complete measurement of the hardened air-void system.
Effects on fresh and hardened concrete
| Property | Typical effect | Field caution |
|---|---|---|
| Workability | Small bubbles can improve cohesion and make a harsh mixture easier to place | Do not use air entrainment to hide poor grading or excessive water demand |
| Bleeding and segregation | Often reduced when the mixture is properly proportioned | Actual response depends on all mixture materials |
| Density | Generally decreases as air content increases | Unit weight can help identify batching variation |
| Compressive strength | Usually decreases when air replaces solid paste without another mixture adjustment | Confirm the approved mixture by trial batches and strength tests |
| Freeze-thaw durability | Can improve substantially | Protection also requires low permeability, suitable curing, and durable aggregate |
How much air is required?
There is no universal air percentage for every concrete mixture. The specified target depends on exposure class, nominal maximum aggregate size, project tolerances, and the governing specification. Use the project documents and current code requirements rather than copying a single percentage from a general article.
Air can change between batching and final placement. Temperature, mixing time, haul time, pumping, vibration, carbon in some fly ashes, admixture compatibility, and finishing operations can all affect the measured result. Trial batching should reproduce the actual materials, sequence, equipment, and expected temperature range.
Testing fresh concrete
| Test | Use | Important limitation |
|---|---|---|
| ASTM C231/C231M pressure method | Fresh air content for applicable normal- and dense-aggregate concrete | Not suitable for every aggregate type |
| ASTM C173/C173M volumetric method | Fresh air content, including mixtures with lightweight or porous aggregate | Requires careful rolling, agitation, and reading |
| ASTM C138/C138M unit weight | Yield, density, and gravimetric air calculation | Depends on accurate batch quantities and theoretical density |
Placing and finishing cautions
Check air content near the point of placement when pumping or long transport may change it. Consolidate enough to remove large entrapped voids without excessive vibration. Do not add water or admixture at the site without the approved procedure and remixing requirements.
Air-entrained concrete should not automatically be selected for an interior hard-troweled floor. High air content can increase the risk of blistering or delamination under dense machine-troweled surfaces. Follow the slab specification and confirm whether freeze-thaw exposure applies.
Where air entrainment is normally used
- Exterior concrete exposed to freezing and thawing while moist.
- Pavements, bridge decks, curbs, sidewalks, and parking areas exposed to deicing chemicals.
- Other work where the project durability specification requires an air-void system.
Air entrainment does not make concrete immune to scaling, chemical attack, poor curing, weak surface paste, or unsuitable deicers. It is one part of the complete mixture, placement, finishing, and curing plan.