Types of admixtures

Admixtures  are  sometimes  used  in  concrete  mixturesto  improve  certain  qualities,  such  as  workability,strength,   durability,   watertightness,   and   wearresistance.They   may   also   be   added   to   reducesegregation,  reduce  the  heat  of  hydration,  entrain  air,and accelerate or retard setting and hardening.We should note that the same results can often beobtained  by  changing  the  mix  proportions  or  byselecting  other  suitable  materials  without  resorting  tothe  use  of  admixtures  (except  air-entraining  admixtureswhen  necessary).Whenever  possible,  comparisonshould be made between these alternatives to determinewhich  is  more  economical  or  convenient.Anyadmixture  should  be  added  according  to  currentspecifications and under the direction of the crew leader.Workability AgentsMaterials, such as hydrated lime and bentonite,are  used  to  improve  workability.  These  materialsincrease   the   fines   in   a   concrete   mix   when   anaggregate  is  tested  deficient  in  fines  (that  is,  lackssufficient  fine  material).Air-Entraining AgentsThe   deliberate   adding   of   millions   of   minutedisconnected air bubbles to cement paste, if evenlydiffused,  changes  the  basic  concrete  mix  andincreases durability, workability, and strength. Theacceptable  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 ofnatural  wood  resins,  animal  or  vegetable  fats,  oils,alkali   salts   of   sulfated   organic   compounds,   andwater-soluble soaps. Most air-entraining agents are inliquid form for use in the mixing water.AcceleratorThe  only  accepted  accelerator  for  generalconcrete work is calcium chloride with not more than2 percent by weight of the cement being used. Thisaccelerator is added as a solution to the mix water andis used to speed up the strength gain. Although thefinal strength is not affected, the strength gain for thefirst 7 days is greatly affected. The strength gain forthe first 7 days can be as high as 1,000 pounds persquare  inch  (psi)  over  that  of  normal  concrete  mixes.Retarderso fThe accepted use for retarders is to reduce the ratehydration.This   permits   the   placement   and