Blue metal — also called jelly or coarse aggregate — is crushed granite graded by size. Using the right size gives denser, stronger concrete; using the wrong one leaves honeycombing or wastes cement.
Sizes and typical uses
| Size | Typical use |
|---|---|
| 6 mm | Precast items, paver blocks, thin toppings |
| 12 mm | Thin slabs, precast elements, congested reinforcement |
| 20 mm | Most RCC work — slabs, beams, columns, footings |
| 40 mm | PCC, mass concrete, foundations, road base |
The rule for maximum size
IS 456 says the nominal maximum size of coarse aggregate should be as large as possible, but not more than one-quarter of the minimum thickness of the member. It must also pass easily between reinforcement bars and into the cover. That is why 20 mm is the standard for most building RCC.
Mixing sizes
For RCC, a blend of 20 mm and 12 mm is common — the smaller stones fill the gaps between the larger ones and make the mix denser. Your engineer or ready mix supplier will set the proportion.
What good aggregate looks like
- Cubical, angular pieces — not flat or elongated.
- Clean — no dust coating, clay or organic matter.
- Hard and sound — should not break easily with a hammer.
- Meets IS 383 for grading.