(2) Choice of Maximum Size of Aggregate . Large maximum sizes of 

 well-graded aggregates have less voids than smaller sizes. Hence, concretes 

 with the larger sized aggregates require less mortar per unit volume of 

 concrete. Generally, the maximum size of aggregate should be the largest 

 that is economically available and consistent with dimensions of the struc- 

 ture. In no event should the maximum size exceed one-fifth of the narrowest 

 dimension between sides of forms, one-third the depth of slabs, nor three- 

 fourths of the minimum clear spacing between individual reinforcing bars, 

 bundles of bars, or pretensioning strands. 



(3) Estimation of Mixing Water and Air Content . The quantity of 

 water per unit volume of concrete required to produce a given slump depends 

 on the maximum size, particle shape, and grading of the aggregates, and on 

 the amount of entrained air. It is not greatly affected by the quantity of 

 cement. Table 11 provides estimates of required mixing water for concretes 

 made with various maximum sizes of aggregate, with and without air entrain- 

 ment. Depending on aggregate texture and shape, mixing water requirements 

 may be somewhat above or below the tabulated values, but they are suf- 

 ficiently accurate for the first estimate. Such differences in water 

 demand are not necessarily reflected in strength. 



Table 11 indicates the approximate amount of entrapped air to be 

 expected in nonair-entrained concrete in the right side of the table and 

 shows the recommended average air content for air-entrained concrete in 

 the left side of the table. The use of normal amounts of air entrainment 

 in concrete with a specified strength near or about 34 megapascals (5 000 

 pounds per square inch) may not be possible due to the fact that each added 

 percent of air lowers the maximum strength obtainable with a given combina- 

 tion of materials (Tuthill, 1960). 



When trial batches are used to establish strength relationships or 

 verify strength-producing capability of a mixture, the least favorable 

 combination of mixing water and air content should be used. This is, the 

 air content should be the maximum permitted or likely to occur, and the 

 concrete should be gaged to the highest permissible slump. This will 

 avoid developing an overoptimistic estimate of strength on the assumption 

 that average rather than extreme conditions will prevail in the field. 



(4) Selection of Water-Cement Ratio . The required water-cement 

 ratio is determined not only by strength requirements but also by factors 

 such as durability and finishing properties. The average strength selected 

 must, of course, exceed the specified strength by a sufficient margin to 

 keep the number of low tests within specified limits (Table 12). For 

 severe conditions of exposure, the water-cement ratio should be kept low 

 even though strength requirements may be met with a higher value. Table 



13 gives limiting values. 



(5) Calculation of Cement Content . The amount of cement per unit 

 volume of concrete is fixed by the determinations made above. The required 

 cement is equal to the estimated mixing water content divided by the 

 water-cement ratio. If, however, the specification includes a separate 

 minimum limit on cement in addition to requirements for strength and 

 durability, the mixture must be based on whichever criterion leads to the 

 larger amount of cement. The use of pozzolanic or chemical admixtures 

 will affect properties of both the fresh and hardened concrete. 



96 



