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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
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.
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