Admixture in Concrete

An Admixture is a compound which is usually a liquid that is added to a batch of concrete during mixing in order to adjust the properties of fresh or hardened concrete in a required manner.

Most admixtures benefit concrete by decreasing the quantity of free water required for a given level of consistence, often in addition to some other specific improvement. Permeability is thereby reduced and durability increased.

There are occasions when the use of an admixture is not only desirable but also essential. Because admixtures are added to concrete mixes in small quantities, they should be used only when a high degree of control can be exercised.

Incorrect dosage of an admixture – either too much or too little – may adversely affect the strength and other properties of the concrete. BS 5075 specifies requirements for the main types of admixtures.

Various types of admixtures that are available in market and widely used in civil engineering applications are as follows.

Normal water-reducing

The cement paste is better ‘lubricated’, and hence the amount of water needed to obtain a given consistence can be reduced.

Accelerating water-reducing

Accelerators increase the initial rate of chemical reaction between the cement and the water so that the concrete stiffens, hardens, and develops strength more quickly. They have a negligible effect on consistency, and 28-day strengths are seldom affected.

Retarding water-reducing

These are chemicals that slow down the initial reaction between cement and water by reducing the rate of water penetration to the cement and slowing down the growth of the hydration products.

Air-entraining

The main reason for using an air-entraining admixture is that the presence of tiny air bubbles in the hardened concrete increases its resistance to the action of freezing and thawing, especially when aggravated by the application of de-icing salts and fluids.

Superplasticizing / high range water reducing.

These are chemicals that have a very great plasticizing effect on concrete. They are used to increase greatly the consistence of a mix so that ‘flowing’ and to produce high-strength concrete by reducing the water content.