Admixtures have long been used to enhance concrete performance. In choosing an admixture, the first step is to set goals that define what you want to accomplish. Since there is more than one way to reach some concrete performance goals, specifications should be flexible enough to permit adjustments in admixture choices as jobsite conditions warrant. The following are various types of admixtures:
WATER-REDUCERS: Adding water reducers to concrete without making other adjustments results in higher slump, equal or higher compressive strength, and depending on amount and chemical composition, slightly longer set time.
SUPERPLASTICIZERS: Type F superplasticizers keep concrete flowable only for a fairly short period of time. Type G superplasticizers keep concrete flowable longer. Tremendous early strength develops when you combine superplasticizers with a water-reducing retarder and use accelerated steam curing.
AIR-ENTRAINING AGENTS: Entrained air makes concrete more resistant to damage from cyclic freezing and thawing. It also makes concrete more watertight.
SILICA FUME: This pozzolan enhances concrete's strength-producing ability and greatly lowers the permeability of concrete, thus protecting concrete from seawater and deicers.