Admixture for Concrete

ADMIXTURES:
It is defined as a material other than water, aggregate and cement that is used as an ingredient of concrete to modify the properties of fresh or hardened concrete.

Function of Admixture:
-Placing and finishing qualities
-Workability
-Strength development
-Appearance (gets improved)
 -Re-uses of form work (facilitated)
 -Pumping quality.

Type of Admixture:
1. Water reducing admixtures     
(Plasticizers or Super
Plasticizers)
2. Retarding admixtures
3. Air-entraining agents (AEA)
4. Accelerating admixtures.

Plasticizer -
Plasticizers or any other type of
admixture are not substitutes for
badly  designed mixes or bad
practices.
Should only be used when there is
a valid reason to use it. 


Super-Plasticizers
   3 types of super Plasticizers
– Ligno Sulphate based
– Melamine sulphate based
– Nepthalene sulphate based
Dosages of Super Plasticizer-
If strength is the criteria
– Lowest w/c ratio and highest super Plasticizers
dose.
If workability is the criteria
– Highest w/c ratio for a particular strength & the
super Plasticizers equal to required workability.

Water Reducing  ADMIXTURES 
For full hydration w/c of 0.23 is sufficient but it
is generally kept 0.50 or above. 
 Excess water is used to overcome internal
friction between solid particles & to facilitate
mixing, placing, transportation & compaction.
Water reducing admixtures can reduce this
extra requirement of water.
Super-Plasticizers are better than conventional
Plasticizers. Over 25% water reduction is possible.

Retarding  ADMIXTURES 
Used where setting time & strength development
of concrete has to be delayed
The retarder delays only the hydration process
(but doesn’t affect the eventual process)
 Initial setting time can be delayed by more
than 3 hours.
Main uses – (1)  To eliminate the cold joints
     (2) To control set during long 
       transit periods and high temperature.  

Use of Retarder - 
To minimize the problem of over rapid
slump loss. 
 But how to minimize the slump loss
without reduced early strength gain ? 
 Use appropriate supplementary
cementing material & retarder be used
only as a last resort.

Air Entraining  Agents-
These are surface active agents that form stable
air bubbles. 
Function of air bubbles:
– To break capillary structure within the concrete
– To act as roller which enable the particles in the
mix to move freely thus providing plasticising
effect. 
Agents: Alkali salts of wood resin,Synthetic
detergents,Animal/ vegetable fats
Commercial makes:Vinsol resin,Teepol, Darex etc