For the same strength or workability, concrete with a larger size aggregate will require a smaller amount of cement than concrete with a smaller size aggregate. The use of larger size aggregates would be useful because of the low consumption of cement in large scale concrete works. It will also reduce the heat of hydration and the associated thermal stress and shrinkage cracks. But in practice the size of aggregate cannot be increased to any extent due to limitation in mixing, handling and equipment keeping. Larger sized aggregates have lower wetting surface area per unit weight, and lower water-cement ratios leading to increased strength. On the other hand smaller sized aggregates have increased surface area leading to increased w/c ratio and lower strength gains. Normally 200kg/cm. up to strength2 Aggregates up to 40 mm long can be used and 300 kg/cm. can be used for strengths above2 In total up to 20 mm can be used.
Conditions Determining the 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 exceed one quarter of the minimum thickness of the member provided that the concrete can be laid without difficulty so that all reinforcement is well encircled and the corner of the form is filled.
- Plumps (large undressed stones embedded with others in concrete on larger works) above 160 mm and up to any appropriate size can be used in plain concrete works up to a maximum limit of 20 percent by volume of concrete.
- For heavy reinforced concrete members the nominal maximum size of aggregate should generally be limited to 6 mm less than the minimum apparent distance between the main bars or 5 mm less than the minimum cover of reinforcement, whichever is less.
- Where the reinforcement is widely spaced as in concrete slabs, the size limits of the aggregate may not be so significant and the nominal maximum size can sometimes be as large or as high as the maximum cover, except where porous aggregates are used. goes.
- Aggregates with a maximum size of 20 mm are generally considered satisfactory for reinforced concrete work.