by reducing the penetration of aggressive liquids. Resistance to severe 

 weathering, particularly freezing and thawing, and to salts used for ice 

 removal is greatly improved by incorporation of a proper distribution of 

 entrained air. Entrained air should be used in all exposed concrete in 

 climates where freezing occurs. By using a suitable cement and a properly 

 proportioned mix, concrete will resist sulfates in soil, ground water, or 

 seawater, provided that concentrations are not in excess of 0.05 molar (7 

 grams) of Na 2 SO tt per liter of water. High-quality concrete will resist 

 mild acid attack, but no concrete has good resistance to strong acids; 

 special protection is necessary in this case. 



Sometimes concrete surfaces will wear away as the result of abrasive 

 action. In hydraulic structures, particles of sand or gravel in flowing 

 water can erode surfaces. The use of high-quality concrete and, in extreme 

 cases, a very hard aggregate may provide longer durability under these 

 exposures. More detailed discussion of the exposures that impact durability 

 of concrete and of the techniques to resist these impacts is discussed in 

 subsection 6. 



e. Density . For certain applications concrete may be used primarily 

 for its weight characteristic. To the extent possible, selection of con- 

 crete proportions should be based on test data or experience with the 

 materials actually to be used. Where such background is limited or not 

 available, estimates given herein may be employed. 



4. Components. 



Concrete is composed principally of cement, aggregates, and water. It 

 will contain some amount of entrapped air and may also contain purposely 

 entrained air obtained by use of an admixture or air-entrained cement. 

 Admixtures are also frequently used for other purposes such as to accel- 

 erate, retard, and improve workability, reduce mixing water requirement, 

 and increase strength, durability, density, and appearance. The required 

 characteristics are governed by the use to which the concrete will be put 

 and by conditions expected to be encountered at the time of placement. 

 These are often, but not always, reflected in specifications for the job. 



a. Mixing Water For Concrete . 



(1) General Requirements . Almost any natural water can be used as 

 mixing water for making concrete. Potable freshwater is usually acceptable 

 as satisfactory mixing water but should meet ASTM Standard C94. Water 

 suitable for making concrete, however, may not be fit for drinking. Water 

 high in chlorides should not be used in concrete containing steel reinforce- 

 ment. 



Water of questionable suitability can be used for making concrete if 

 mortar cubes made with it have 7- and 28-day strengths equal to at least 90 

 percent of comparison specimens made with tapwater. Mortar cubes should be 

 made and tested according to ASTM Standard C109. In addition, Vicat needle 

 tests (ASTM Standard C191) , should be made to ensure that impurities in the 

 mixing water do not adversely shorten or extend the setting time of the 

 cement. Excessive impurities in mixing water also may cause efflorescence, 

 staining, or corrosion of reinforcement. Therefore, certain optional 



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