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 38.1mm
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 40mm (i.e. 1 to 1 ½
inches). Whereas, in
case of lean concrete (containing say 165kg of cement per cum of
concrete), the
use of 6 inch (i.e 150mm) aggregate is advantageous.
The
nominal
maximum size of aggregate that can be used is also determined by the
structural limitationslike
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.
