Types of admixtures to use in construction

Admixtures  are  sometimes  used  in  concrete  mixtures to  improve  certain  qualities,  such  as  workability, strength,   

durability,   water tightness,   and   wear resistance. 

They   may   also   be   added   to   reduce segregation, reduce
  the  heat  of  hydration,  entrain  air, and accelerate or retard  
setting and hardening. We should note that the same results 
can often be obtained  by  changing  the  mix  proportions 
 or  by selecting  other  suitable  materials  without  

resorting  to the  use  of  admixtures 
 (except  air-entraining  admixtures when  necessary).

Whenever  possible,  comparison should be made 
between these alternatives to determine which  is 
 more  economical  or  convenient. Any admixture
 should be  added  according  to  current specifications 
and under the direction of the crew leader.

Workability Agents

           Materials, such as hydrated lime and bentonite,  are  used  to  improve  workability.  These  materials 
increase   the   fines   in   a   concrete   mix   when  
 an aggregate  is  tested  deficient  in  fines 
 (that  is,  lacks sufficient  fine  material).

Air-Entraining Agents

            The   deliberate   adding   of   millions   of  
 minute disconnected air bubbles to cement paste, 
if evenly diffused,  changes  the  basic  concrete 
 mix  and increases durability, workability, and strength. 

The acceptable  amount  of  entrained  air  in  
a  concrete  mix, by volume, is 3 to 7 percent. 
Air-entraining agents, used with types I, II,
 or III cement, are derivatives of natural  
wood  resins,  animal  or  vegetable  fats, 
 oils, alkali   salts   of   sulfated   organic   

compounds,   and water-soluble soaps. 
Most air-entraining agents are in liquid 
form for use in the mixing water. 

Accelerator 

                 The  only  accepted  accelerator 
 for  general concrete work is calcium chloride 
with not more than2 percent by weight of 

the cement being used. This accelerator is 
added as a solution to the mix water and is
 used to speed up the strength gain. 

Although the final strength is not affected,
 the strength gain for the first 7 days is greatly
 affected. The strength gain for the first 7 days

 can be as high as 1,000 pounds per square 
 inch  (psi)  over  that  of  normal  concrete 
 mixes.

Retarders

             The accepted use for retarders is to reduce
 the rate hydration.This   permits   the   placement 
  and consolidation  of  concrete  before  initial  set.
  Agentsnormally used are fatty acids, sugar, and 
starches