What is Admixtures?
Admixtures are material used in concrete to modify its properties. Admixtures are materials other than cement, aggregate, and water that are added to concrete either before or during its mixing to alter its properties such as workability, curing temperature range, set time or colour. Some admixtures have been in use for a very long time, such as calcium cloride to provide a cold-weather setting concrete. Other are more recent and represent an area of expanding possibilities for increased performance. Not all admixtures are economical to employ on a particular project. Also, some characteristics of concrete, such as low absorption, can be achieved simply by consistently adhering to high-quality concreting practice. Some of the important admixtures are discussed below.
1. Accelerating Admixtures,
2. Retarding Admixtures,
3. Water Reducing Admixtures,
4. Air-entraining Admixtures.
These are used to accelerate the rate of development of strength at an early age. Calcium chloride, fluorosilicates and tri-ethanolamine are the most commonly used accelerators.
These are used to retard setting and thereby reduce the evolution of heat. Common types of retarders are starches and cellulose product, sugar and hydroxyl carboxylic acids and salts. Except for sugar and carbohydrates, they have water reducing properties as well. Sugar in small quantities up to a maximum of 0.05% by weight of cement is the most commonly used retarder.
Water Reducing Admixtures
These are used to reduce the water content while maintaining the workability of concrete. These are classified into the following two main groups :
a.) Lignosulphonic acid and their salts,
b.) Hydroxylated carboxylic acid and their salts.
These admixtures increase the setting time by 2 to 6 hours the normal setting time or an accelerated set can be achieved by incorporating a suitable accelerator in the admixture.
These are used to entrain air in the form of very small disconnected air bubbles in concrete usually to increase workability and resistance to freezing and thawing.The commonly used air-entraining agents are,
a.) Animal and vegetable oils and fats,
b.) Natural wood resins and their sodium salts such as vinsol resin,
c.) Alkali salts of sulphated and sulphonated organic compounds.
Functions of Admixtures :
1. To accelerate the initial set of concrete i.e. to speed up the rate of development of strength at an early age.
2. To retard the initial set i.e. to keep concrete workable for a longer time placement.
3. To enhance workability.
4. To improve the penetration and pump ability of concrete.
5. To reduce the segregation in grout and concrete mixtures.
6. To increase the resistance to chemical attack.
7. To reduce the heat of hydration.
8. To produce cellular concrete.
9. To produce non-skid wearing surface
10. To decrease the weight of concrete per cubic meter.
11. To inhibit the corrosion or reinforcement in concrete.
12. To enhance the bond of concrete to the steel reinforcement.
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