Water-reducing admixtures reduce the quantity of mixing water required to produce concrete of a given consistency, with greater economy for a given strength. These products make the concrete more uniform and predictable. Retarders are used to delay setting time to facilitate placing and finishing operations. Accelerators hasten cement hydration resulting in shortened setting times and increased early age strengths. They reduce time required for curing and protection. Accelerators work particularly well in cooler temperatures.
Water reducers come as Low Range, Mid Range, and High Range Superplasticizers. There are enough different admixtures available that it is possible to select one that meets the need of a particular project whether it is tall columns that need a mix that pumps easily, or an easy-to-finish durable floor slab. In general, they provide the required slump with less water in the mix, and may provide higher strength concrete without increasing the amount of cement. However, the change in slump usually lasts only about 30 to 60 minutes depending upon the brand and dosage rate.
Admixtures containing microsilica (condensed silica fume) are used to meet high strength and low permeability requirements. Benefits include reduced permeability, increased compressive and flexural strengths, and increased durability. Silica fume can be used in concrete to produce very high compressive psi strengths under jobsite conditions.
Fly ash is a valuable additive that makes concrete stronger, more durable and easier to work with. Fly ash aids the formation of cementitious compounds to enhance the strength, impermeability and durability of concrete. It reduces bleeding and segregation in early concrete. In hardened concrete it increases ultimate strength, reduces drying shrinkage and permeability, lowers heat of hydration and reduces creep.
High Early Concrete which, through the use of high-early-strength cement or admixtures, is capable of attaining specified strength at an earlier age than normal concrete.
Air entrainment is the intentional creation of tiny air bubbles in concrete. The bubbles are introduced into the concrete by the addition to the mix of an air entraining agent. The air bubbles are created during mixing of the plastic (flowable, not hardened) concrete, and most of them survive to be part of the hardened concrete. The primary purpose of air entrainment is to increase the durability of the hardened concrete, especially in climates subject to freeze-thaw; the secondary purpose is to increase workability of the concrete while in a plastic state.
