Requirements for the water-cement ratio and air entrainment are described 

 in detail below. 



(d) Water-Cement Ratio. Frost-resistant regular weight 

 concrete should have a water-cement ratio not to exceed the values given in 

 Table 13. Because the determination of the rate of absorption for light- 

 weight aggregates is uncertain, it is impracticable to calculate the water- 

 cement ratio of concretes containing such aggregates. For these concretes, 

 a specified 28-day compressive strength of 27.6 megapascals (4 000 pounds 

 per square inch) is recommended. For severe exposures, some have found it 

 also desirable to specify a minimum cement content of 3.28 kilonewtons per 

 cubic meter (564 pounds per cubic yard) , and only that amount of water 

 necessary to achieve the desired consistency. 



(e) Entrained Air. Too little entrained air will not protect 

 cement paste against cyclic freezing. Too much air will unduly penalize 

 the strength. About 7 percent of air in the mortar for severe exposure, and 

 about 5 percent for moderate exposure are reasonable. Frequent determina- 

 tions of the air content of the concrete should be made. For regular 

 weight concrete, the following test methods may be used: volumetric 

 method (ASTM Standard C173), pressure method (ASTM Standard C231), or the 

 unit weight test (ASTM Standard C138). An air meter may be used to provide 

 an approximate indication of air content. For lightweight concrete, the 

 volumetric method is recommended. 



The air content and other characteristics of the air void system in 

 hardened concrete may be determined microscopically (ASTM Standard C457) . 

 ASTM Standard C672 is often used to assess the resistance of concrete to 

 deicer scaling. 



d. Summary. Table 16 summarizes various techniques and practices that 

 may be employed to obtain maximum durability. 



7 . Reinforcing-Prestressing Materials . 



a. Steel Reinforcing . Steel reinforcing should conform to the 

 following ASTM requirements: 



(1) Bar reinforcement, ASTM Standard A615 Grade 40 or 60; 



(2) cold drawn wire, ASTM Standard A82; and 



(3) welded wire fabric, ASTM Standard A185, when zinc coated 

 (galvanized), not less than 2.39 newtons per square meter (0.8 

 ounce per square foot) of Grade 5 "Prime Western" conforming to 

 ASTM Standard B6 . 



b. Steel, Prestressed. High tensile steel is almost the universal 

 material for producing prestress and supplying the tensile force in pre- 

 stressed concrete. Such steel can take any of three forms: wires, strands, 

 or bars. The most widely used at present are the strands, which are 

 grouped, in parallel, into cables. Strands are fabricated in the factory 

 by twisting wires together, thus decreasing the number of units to be 

 handled in the tensioning operations. Steel bars of high strength have 



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