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Concrete admixtures are added to change the properties of concrete to make it function as required. Admixtures are used to modify properties of both fresh and hardened concrete as discussed below:

Functions of admixtures to modify fresh concrete properties:

a) To increase workability without increasing water content or to decrease the water content at the same workability.

b) To retard or accelerate both initial and final setting times.

c) To reduce or prevent settlement.

d) To create slight expansion in concrete and mortar.

e) To modify the rate or capacity for bleeding or both.

f) To reduce segregation of concrete, mortars and grouts.

g) To improve penetration and or pumpability of concrete, mortars and grouts.

h) To reduce rate of slump loss.

concrete-admixture-functions

Functions of admixtures to modify hardened concrete properties:

a) To retard or reduce heat generation during early hardening.

b) To accelerate the rate of strength development.

c) To increase the strength of concrete or mortar (Compressive, tensile or flexural).

d) To increase the durability or resistance to severe conditions of exposure including the application de-icing salts.

e) To decrease the capillary flow of water.

f) To decrease the permeability to liquids.

g) To control the expansion caused by the reaction of alkalis with certain aggregate constituents.

h) To produce cellular concrete.

i) To increase the bond of concrete to steel reinforcement.

j) To increase the bond between old and new concrete.

k) To improve impact resistance and abrasion resistance.

l) To inhibit the corrosion of embedded metal.

m) To produce coloured concrete or mortar.

While modifying any particular property, care should be taken to ensure that other properties of concrete are not affected adversely.

Read More on Concrete Admixtures and Admixtures Guide