An air-entraining agent is a chemical admixture that forms very small, evenly distributed air bubbles in concrete or mortar during mixing. The bubbles measure from tenths of a micrometre to a few hundred micrometres and remain as voids in the hardened cement paste.
How do air bubbles protect against frost?
Concrete always holds water in its pores. When that water freezes it expands and creates internal pressure that cracks the cement paste — the familiar scaling of paving slabs, kerbs and balcony floors after a hard winter.
The entrained bubbles act as expansion vessels: freezing water can move into the neighbouring air-filled void and the pressure never builds up. The critical point is that the bubbles must be close together, so the water never has far to travel.
Where it is used
- Concrete exposed to frost and de-icing salts: bridges, decks, pavements, driveways and balconies
- Concrete blocks and paving units
- Masonry mortar used in winter conditions or that stays permanently damp
- Mixes whose workability needs to improve without adding more water
Side effects
| Effect | Explanation |
|---|---|
| Frost resistance | Improves substantially — the main reason for using it |
| Workability | Improves; the mix becomes smoother and easier to pump |
| Segregation and bleeding | Both reduce |
| Compressive strength | Falls, because voids carry no load |
The loss of strength is why you never simply add more. A widely used rule of thumb in concrete technology is that each additional per cent by volume of air costs several per cent of compressive strength. For frost and de-icing salt resistant concrete an air content in the order of four to six per cent by volume is usually targeted; the exact figure follows from the exposure class and the concrete specification.
Practical points
- Dosing is not a DIY job — the quantity and bubble size depend on temperature, water content, cement type, mixing time and compaction. It belongs in the mix design, not in the bucket on site
- Do not over-vibrate — prolonged vibration drives the bubbles back out and cancels the protection
- Measuring — air content is checked on site on fresh concrete with a pressure-method air meter
- Pouring it yourself? — then order ready-mixed concrete in the correct exposure class from a batching plant; it is already designed for frost resistance
Related terms
- Admixture
- Fresh concrete
- Ready-mixed concrete
- Concrete quality
- Frost resistance of concrete
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