Chemical Admixture

Modern concretes almost always possess additives, either in the mineral form or chemical form. Particularly, chemical admixtures such as water reducers and set controllers are invariably used to enhance the properties of freshand hardened concrete.
 A ‘Chemical Admixture’ is any chemical additive to the concrete mixture that enhances the properties of concrete in the fresh or hardened state. This does not include paints or coatings. ACI 116R [2000] defines the term admixture as ‘a material other than water, aggregates, hydraulic cement, and fiber reinforcement, used as an ingredient of concrete or mortar, and added to the batch immediately before or during its mixing’. A number of types of chemical admixtures are used for concrete [Ramachandran, 2002; Rixom and Mailvaganam, 1999]. The general purpose chemicals include those that reduce the water demand for a given workability (called ‘water reducers’), those entraining air in the concrete for providing resistance to freezing and thawing action (called ‘air entrainers’), and those chemicals that control the setting time and strength gain rate of concrete (called ‘accelerators’ and ‘retarders’). Apart from these chemicals, there are others for special purposes – namely, viscosity modifying agents, shrinkage reducing chemicals, corrosion inhibiting admixtures, and alkali-silica reaction mitigating admixtures.
A water reducing chemical, as the name implies,is used to reduce the water content of a concrete mixture while maintaining a constant workability. The resultant effect of the reduced water content is the increased strength and durability of concrete. However, water reducers may also be employed to ‘plasticize’ the concrete, i.e. make concrete flowable
In this case, the water content (or water to cement ratio) is held constant, and the addition of the admix- tures makes the concrete flow better, while the compressive strength (which is a function of the water to cement ratio), is not affected. Another use of water reducers is to lower the amount of cement (since water is proportionately reduced) without affecting both strength and workability. This makes the concrete cheaper and environmentally friendly, as less cement is consumed.
The Masterbuilder – January 2009

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