What is significance of specifying the
maximum size of aggregates?
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
& richness, the water-cement ratio can be lowered with a consequent
increase in strength.
of the largest possible maximum size, consistent with placing
sometimes recommended in order to minimize the amount of cement
required and to
minimize shrinkage. However, experimental results show that above the 38.1mm
size (1 ½ inch) maximum size the gain in strength due to reduced
requirement is offset by the detrimental effects of lower bond area.
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
the point of view of strength there is no advantage in using aggregate
maximum size greater than about 25 to 40mm (i.e. 1 to 1 ½
inches). Whereas, in
case of lean concrete (containing say 165kg of cement per cum of
use of 6 inch (i.e 150mm) aggregate is advantageous.
maximum size of aggregate that can be used is also determined by the
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,
the depth of slabs, or three-fourths of the minimum clear spacing