WHAT IS ADMIXTURES? CLASSIFICATION AND PURPOSE OF ADMIXTURES.

ADMIXTURES

Admixture is defined as a material other than the basic ingredient of concrete, cement, sand, aggregate and water. It is added to the concrete mix immediately before or during mixing, to add some specific property to fresh concrete. The properties commonly modified using admixtures are setting time, workability, air- entrainment, dispersion etc. The admixtures generally added in a relatively small quantity ranging from 0.005% to 2% by the weight of cement.

CLASSIFICATION OF ADMIXTURES

1. Accelerating admixtures (accelerators)
It increases initial setting time of concrete.

2. Retarding admixtures (retarders)
It decreases initial setting time of concrete and concrete sets quickly.

3. Water reducing admixtures (workability admixtures)
It reduces the water demand of cement for the heat of hydration activity. It is used in dry territories where water is very less acquired like gulf countries.

4. Air-entraining admixtures
It increases the present of air in concrete to make concrete light weight and permeable for specific applications.

5. Plasticizers 
It increases the workability of concrete with reducing water cement ratio.

6. Super-plasticizers
It is also increases the workability of concrete simultaneously decreasing the water to cement ratio. it also imparts strength to concrete.

Other type of admixtures are :
7. Pozzolana admixtures
8. Water proofing admixtures
9. Air-detraining admixtures
10. Bonding admixtures
11. Grouting admixtures
12. Corrosion  admixtures
13. Gas forming admixtures
14. Colouring admixtures
15. Alkali aggregate expansion inhibiting admixtures


PURPOSE OF USING ADMIXTURES

1. To increase the strength of concrete.
2. To accelerate the initial setting time of concrete.
3. To retard the initial setting time of concrete.
4. To improve the workability of concrete.
5. To increase the durability of concrete.
6. To reduce heat of hydration. 
7. To make light weight concrete.
8. To reduce permiability of concrete. 
9. To control the alkali aggregate expansion.
10. To increase the resistance to sulphate attack. 
11. To increase the bond between old and new concrete.
12. To reduce the segregation and bleeding of concrete.
13. To produce coloured concrete.
14. To control the corrosion of steel in concrete.

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