air-entrained concrete

Have read about "air-entrained" concrete. How does the air get in there? Does one buy a bag of "air-entrained" concrete or does one add the air somehow to ordinary Portland?
Thanks.
Dennis
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Geez...very quick Google reveals...
http://www.portcement.org/cb/concretebasics_airentrained.asp
ms
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Yes !!!
Chemical additive to concrete while mixing. Purchased with chemical already in dry portland.
########## Keep the whole world singing. . . Dan G (remove the 7)
Dennis wrote:

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