(2) Water-Reducing and Retarding Admixtures . Water-reducing 

 admixtures are used to improve the quality of concrete, to obtain specified 

 strength at lower cement content, or to increase the slump of a given 

 mixture without increase in water content. They also may improve the 

 properties of concrete containing aggregates that are harsh or poorly 

 graded, or both, or may be used in concrete that must be placed under 

 difficult conditions. Set-retarding admixtures delay the onset of harden- 

 ing, prolonging the period when the concrete is workable. Both types of 

 admixtures are useful when placing concrete by means of a pump or when 

 using a tremie process. The materials that are generally available for use 

 as water-reducing admixtures and set-retarding admixtures fall into four 

 general classes: 



(a) Lignosulfonic acids and their salts; 



(b) modifications and derivatives of lignosulfonic acids and 

 their salts; 



(c) hydroxylated carboxylic acids and their salts; and 



(d) modifications and derivatives of hydroxylated carboxylic 

 acids and their salts. 



Hydroxylated carboxylic acid salts act as water-reducing, nonair- 

 entraining retarders. Lignosulfonates are available as the calcium, 

 sodium or ammonium salts. Admixtures of classes (a) and (c) can be used 

 either alone or combined with other organic or inorganic, active or essen- 

 tially inert substances. They are water-reducing, set-retarding admixtures. 



Admixtures of classes (b) and (d) are water-reducing admixtures offered 

 as combinations of substances designed either to have no substantial effect 

 on rate of hardening or to achieve varying degrees of acceleration or 

 retardation in rate of hardening of concrete; these admixtures may include 

 an air-entraining agent. 



The composition of the Portland cement affects the air-entraining 

 properties of lignosulfonate admixtures in concrete. Concrete containing 

 a lignosulfonate retarder generally requires 5 to 10 percent less water 

 than comparable concrete without the admixture. Compressive strengths at 

 2 or 3 days are usually equal to or higher than those of corresponding 

 concrete without the admixture and the strength at 28 days or later may be 

 1Q to 20 percent higher. 



Lignosulfonic acid salts, carboxylic acid salts, or modifications or 

 derivatives thereof can be mixed or reacted with other chemicals that 

 entrain air, modify setting time, or affect the strength development of 

 concrete. Calcium chloride, neutralized wood resins, alkyl aryl sulfo- 

 nates, and triethanolamine are examples of additives that have been used. 

 The use of compounded or modified water reducers usually causes a water 

 reduction of 5 to 10 percent at equal air content. Compressive strengths 

 at ages greater than 2 days are usually from 10 to 20 percent higher than 

 those of similar concretes without admixture. 



(3) Accelerating Admixtures. Accelerating admixtures are added to 

 concrete either (a) to increase the rate of early strength development, (b) 

 to shorten the time of setting, or (c) for both purposes. Chemicals which 



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