Water Reducers and Air-Entraining Agent
Air-entraining agents are used to incorporate air into the concrete mix, usually for the purpose of improving resistance to freezing and thawing or salt scaling, but often they are added to improve workability. Since air itself provides no strength, a small amount of additional cement is often required to compensate for the strength that might otherwise be lost. Incorporation of air makes it possible to reduce slightly the water-cement ratio needed to produce a given slump, however. For this reason not so much extra cement is needed as would be if the water-cement ratio were to remain the same. Air-entraining agents should conform to ASTM C 233. Water-reducing admixtures have a more pronounced effect on the amount of water needed for a given slump. They are supplied in three common forms: normal, retarding and accelerating. Normal water-reducing admixtures are added for any or all of many reasons. The most common are to permit use of less cement by reducing water-cement ratio and to improve workability. Water-reducing admixtures frequently entrain some air also and may be used to help control the air content within limits. Retarding water-reducing admixtures perform the same functions as those just described but they also delay rate of set and for this reason find use in hot weather or when longer working times are needed. Accelerating water-reducing admixtures also perform the same functions as the normal admixtures but in addition they accelerate the rate of set and rate of hardening. They are useful in cold weather or where high early strength is needed. Water-reducing admixtures should conform to ASTM C 494.