What is Admixture?
Admixtures are the chemical compounds in concrete other than hydraulic cement (O.P.C), water & aggregate and mineral additives which are added to the concrete mix immediately before or during modification. 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.
Use Of Admixture
Admixtures are 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
The following are the functions of admixtures:
a) To accelerate or retard the initial set of concrete.
b) To enhance the workability.
c) To improve the flow-ability & pump-ability 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 the 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 and coloured concrete.
o) To produce a 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 categories – a) General category b) Speciality 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) Speciality category Admixtures
The commonly used admixtures of this category are:
1) Grouting admixture: Example —Gels, Clays, Methylcellulose.
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 — Polyhalogenated phenol.
What is 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 the hydration of cement to produce a cementing compound with good binding properties. Additives are added in large quantities to improve the performance of concrete and reduce the cost of construction.