Air entrainers or air entraining admixtures are admixtures (additives) which we add in the concrete mix so as to introduce small air bubbles inside the concrete matrix.
The bet is to control the size, the number, the total quantity, the stability and the distance between the bubbles.Sounds quite complicate, and it is! For this reason only companies with a substantial know-how can be trusted!
ADVANTAGES OF AIR ENTRAINED CONCRETE
The entrained air bubbles act as a physical buffer against the cracking caused by the stresses due to water volume augmentation in freezing temperatures.
Traditional materials used as basis for the fabrication of air entrainers are:
- Air bubbles increase the cohesion of the fresh mix and this means less bleeding and segregation.
- Workability is increased due to the ball bearing effect of the bubbles.
- The most important: The resistance to freeze-thaw cycles can be substantially increased.
The entrained air bubbles act as a physical buffer against the cracking caused by the stresses due to water volume augmentation in freezing temperatures.
CHEMICAL BASES
Traditional materials used as basis for the fabrication of air entrainers are:
- vinsol resin and
- salts of fatty acids.
More recently, completely synthetic products have come to the front of the stage.These are mostly synthetic detergents or salts from petroleum acids.
HOW AIR ENTRAINERS ACT
Air entrainers are surface active materials which work to lower the surface tension of the water. Once this is achieved, the formation and introduction of air bubbles is easy. How these bubbles will be dispersed is a matter of the blending of the surface active chemicals.
Resistance to freeze-thaw cycles is better when the bubbles are uniform, small in diameter and don’t coalesce.
COMPATIBILITY WITH OTHER ADMIXTURES
Air entrainers are compatible with almost all the concrete admixtures. Because some of thhave additional air entraining properties, this should be taken into account (e.g. those based on lignosulfonates). Admixtures should be added in the mix separately.
DOSAGE
Usual dosages range between 0.15% - 0.30% but higher dosages may be needed for higher temperature because air entrainment changes inversely with the air temperature.
Overdosing is very dangerous. Severe overdosing could cause unacceptable reduction of compressive strength.
Typically for every 1% of entrained air, compressive strength will be reduced by about 5%.
Air entraining admixtures should be specified for all constructions exposed to freeze/thaw cycles.
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