A mix design for concrete consists of specifying following:
1) as stiff mixes as practical under the proposed concrete mixing and casting conditions;
2) maximum permissible size of coarse aggregate; and
3) adequately sized and properly proportioned fine aggregate and coarse aggregate.
In this post I shall discuss maximum aggregate size that can be used in design. Large maximum sizes of aggregates produce less voids than smaller sizes. Hence, concretes with the larger-sized aggregates require less mortar per unit volume of concrete, and of coarse it is the mortar which contains the most expensive ingredient, cement. Thus the ACI method is based on the principle that the maximum size of aggregate should be the largest available so long it is consistent with the dimensions of the structure.
In practice the dimensions of the forms or the spacing of the rebars controls the maximum coarse aggregate size. ACI 211.1 states that the maximum coarse aggregate size should not exceed:
1) one-fifth of the narrowest dimension between sides of forms,
2) one-third the depth of slabs,
3) 3/4-ths of the minimum clear spacing between individual reinforcing bars, bundles of bars, or pre-tensioning strands.
Special Case: When high strength concrete is desired, best results may be obtained with reduced maximum sizes of aggregate since these produce higher strengths at a given w/c ratio.