>> > On the other hand, very small bubbles are sometimes added
>> to the mix

>> Air Entrainment. Pretty important for outdoor concrete in
>> freezing climates.
>> http://en.wikipedia.org/wiki/Air_entrainment

> Ah, that's it.  I used to live in an area where that was important. 
> I don't
> like being cold.
>
> Done since 1930!  Some of those old guys were pretty smart.

Done *again* since the late 1930's, when it was discovered by mistake.

Done, it would appear, *originally* by the Romans, possibly by 
including blood or milk in the mix (!). Many Roman structures are 
concrete based. They considered it to be an unworthy material for 
display purposes and went to lengths to cover it with other materials.

Air void analysis of concrete using a flatbed scanner.
Goo discussion on what is required to make AE concrete frost 
resistant.
MSc civil Thesis
Mentions Roman air entrained concrete.

                 http://www.cee.mtu.edu/~krpeters/air_voids/thesis2001.pdf

The control of air in concrete
This monolog mentions the Roman method (foodscraps -> soap) but 
portrays it as an accident - in all relevant concrete apparently. 
Knowing the Romans just a little his explanation is highly suspect.

        http://www.lmcc.com/news/spring2004/spring2004-08.asp

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