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What is significance of specifying the maximum size of aggregates?

The maximum size of coarse aggregate used in concrete has an effect upon surface area and economy. As the aggregate particle size increases, the surface area to be wetted per unit weight decreases. (Please refer: Why the surface area increases as the particle size decreases?). Thus for a specified workability & richness, the water-cement ratio can be lowered with a consequent increase in strength.

Use of the largest possible maximum size, consistent with placing requirements, is sometimes recommended in order to minimize the amount of cement required and to minimize shrinkage. However, experimental results show that above the 38.1 mm size (1 ½ inch) maximum size the gain in strength due to reduced water requirement is offset by the detrimental effects of lower bond area.

It may be noted that the best maximum size of aggregate from the consideration of strength is a function of the richness of the mix. For usual concretes, from the point of view of strength there is no advantage in using aggregate with a maximum size greater than about 25 to 40 mm (i.e. 1 to 1 ½ inches). Whereas, in case of lean concrete (containing say 165 kg of cement per cum of concrete), the use of 6 inch (i.e 150 mm) aggregate is advantageous.

The nominal maximum size of aggregate that can be used is also determined by the structural limitations like the size and shape of the concrete member and by the clear spacing between reinforcing bars. In general, it should not be more than one-fifth to one-fourth of the thickness of concrete section, one-third the depth of slabs, or three-fourths of the minimum clear spacing between reinforcing bars.