For the same strength or workability, concrete with large size aggregate will require lesser quantity of cement than concrete with a smaller size aggregate. In a mass concrete work the use of larger size aggregate will be useful due to the lesser consumption of cement. This will also reduce the heat of hydration and the corresponding thermal stresses and shrinkage cracks.
But in practice the size of aggregate cannot be increased to any limit on account of the limitation in the mixing, handling and placing equipment. In large size aggregates surface area to be wetted per unit weight is less, and the water cement ratio is less which increases the strength.
On the other hand in smaller size aggregates the surface area is increased which increases w/c ratio and lower strength is achieved. In general for strength upto 200 kg/cm2 aggregates upto 40 mm may be used and for strength above 300 kg/cm2 aggregate upto 20 mm may be used.
CONDITIONS THAT DECIDE MAXIMUM SIZE OF COARSE AGGREGATE TO BE USED IN CONCRETE
The following conditions decide the maximum size of coarse aggregate to be used in concrete
- It should not be more than one fourth of the minimum thickness of the member provided that the concrete can be placed without difficulty so as to aurround all reinforcement thoroughly and fill the corner of the form.
- Plump (large undressed stone embedded with others in concrete on large work) above 160 mm and upto any reasonable size may be used in plain concrete work upto a maximum limit of 20 percent by volume of concrete.
- For heavily reinforced concrete members the nominal maximum size of the aggregate should be usually restricted to 6 mm less than the minimum clear distance between the main bar or 5 mm less than the minimum cover to the reinforcement, whichever is less.
- Where the reinforcement is widely spaced as in solid slabs, limitations of the size of the aggregate may not be so important and the nominal maximum size may sometime be as great as or greater than the maximum cover except where porous aggregate are used.
- For reinforced concrete work aggregates having a maximum size of 20 mm are generally considered satisfactory.