Selection of Maximum size of aggregate for concrete

he nominal maximum size of coarse aggregate should be as large as possible within the limits specified but in no case greater than one-fourth of the maximum thickness of the member, provided that the concrete can be placed without difficulty so as to surround all reinforcement thoroughly and fill the corners of the form. For most work, 20 mm aggregate is suitable. Where there is no restriction to the flow of concrete into sections, 40 mm or larger size may be permitted. In concrete elements with thin sections, closely spaced reinforcement or less cover, consideration should be given to the use of 10mm nominal maximum size. 
Plums above 160 mm and up to any reasonable size may be used in plain concrete work up to a maximum limit of 20 percent by volume of concrete when specifically permitted. The plums shall be distributed evenly and shall be not closer than 150mm from the surface. 
For heavily reinforced concrete members as in the case of ribs of main beams, the nominal maximum size of the aggregate should usually be restricted to 5 mm less than the minimum clear distance between the main bars or 5 mm less than the minimum cover to the reinforcement which ever is less.