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
