Thursday, 14 June 2018

Define Admixture and Additive ,Use ,Function , And Types of Admixture


Admixtures

                           Admixtures are the chemical compounds in concrete other than hydraulic cement (OPC), water& aggregate and mineral additives which are added to the concrete mix immediately before or during to modify. One or more specific properties of concrete in fresh or hardened state .

In other words the admixtures have formulated chemical composition and special chemical action and are used to modify certain properties of concrete.

The properties of concrete commonly modified by the admixtures are-
a)Rate of hydration.
b) Setting time. 
c) Workability. 
d) Dispersion.
e) Air-entrainment.

Additive 

              The additives are supplementary cementing materials. They have no formulated chemical composition and they do not have special chemical action distinct  from pozzolana. These materials do not have any binding property by themselves. 

But they react with calcium hydroxide liberated on hydration of cement to produce cementing compound with good binding properties. Additives are added in large quantities to improve performance of concrete and reduce the cost of construction .




Use Of Admixture

Admixtures arc used to achieve the following objectives:

a)To reduce the cost of concrete construction.

b) To modify the performance of hardened concrete.

c)To ensure the quality of concrete during mixing, transporting, placing, compacting & curing.

d) To overcome certain emergencies during concreting operations.
The effectiveness of an admixture depends on several factors including type and quantity of cement, water content, mixing time ,slump and temperatures of the concrete and air.

Functions of Admixtures 

                                     Admixtures are used for the following purpose:

a)To accelerate or retard the initial set of concrete.
b)To enhance the workability.
c)To improve the flowability & pumpabiliry of concrete.
d)To reduce the segregation in concrete.
e)To increase the strength of concrete by reducing the water content.
f)To increase the durability of concrete.
g.) To increase impermeability of concrete.
h) To control alkali-aggregate reaction
i)To reduce the corrosion of reinforcement in concrete.
j)To increase the resistance to chemical attack.
k)To reduce the heat of hydration.
l)To increase the bond between concrete & steel.
m)To increase the bond between old and new concrete surfaces
n)To produce cellular concrete, coloured concrete
o) To produce non-skid wearing surface.
p) To decrease the unit weight of concrete.
q) To produce concrete of fungicidal, germicidal and insecticidal properties.

Classification of Admixtures 


 The admixtures can be broadly classified into two category -
a) General category 
b) Specialty category

a) General category Admixtures: - The commonly used admixtures of this category are:

1) Accelerating admixtures: Example — Calcium Chloride (CaCl2)

2) Retarding admixtures: Example — Sugar, Soluble Starch.

3) Air-entraining admixtures: Example — Natural wood resin, Olive oil

4) Water reducing admixtures: Example — Plasticizers

b) Specialty category Admixtures: - The commonly used admixtures of this category are:

I) Grouting admixture: Example —Gels, Clays, Methyl cellulose.

2) Air-detraining admixtures: Example — Tributyl-phosphate.

3) Gas-forming admixtures: Example — Aluminium powder.

4) Corrosion inhibiting admixtures:Example —Sodium nitrate.

5) Shrinkage reducing admixtures: Example - Granulated iron & chemicals.

6) Water or damp proofing admixtures: Example — Natural wood resin, Olive oil, Silicate of soda.

7) Bonding admixtures: Example — Rubber, synthetic rubber.

8) Concrete surface hardening admixtures: Example — Silico fluorides.

9) Colouring admixtures: Example — Carbon black.

10) Fungicidal, germicidal & insecticidal admixtures: Example — Poly halogenated phenol.