The short answer
A particle is flaky when its thickness is less than 0.6 of its mean size, and elongated when its length is more than 1.8 times its mean size. MoRTH limits the combined flakiness and elongation index to 35 per cent for wet mix macadam and for the bituminous courses. The result depends on the rock and on how the crusher is run: a choke-fed cone at the right setting, or a VSI shaping stage, brings it down.
Aggregate can pass every sieve in the stack and still be the wrong shape. A sieve measures the middle dimension of a stone. It says nothing about whether the stone is a cube, a chip or a splinter, and a road cares a great deal about the difference.
01What the two words mean
Each size fraction is defined by the two sieves it lies between, and its mean size is the average of those two openings. A stone from the 20 to 16 mm fraction has a mean size of 18 mm. It is flaky if it slips through a slot 0.6 times that wide, which is 10.8 mm. It is elongated if it is longer than 1.8 times the mean, which is 32.4 mm. The test is carried out with a thickness gauge and a length gauge under IS 2386 Part 1, and it is not applied to material finer than 6.3 mm.
02How the combined index is worked out
MoRTH adds the two indices, with one rule that is easy to miss. The flaky particles are taken out first. Elongation is then measured only on what is left, and expressed as a share of that non-flaky portion.
- Sample weight
- 2,000 g
- Passing the thickness gauge (flaky)
- 360 g
- Flakiness index: 360 / 2,000
- 18.0%
- Non-flaky remainder
- 1,640 g
- Retained on the length gauge (elongated)
- 230 g
- Elongation index: 230 / 1,640
- 14.0%
- Combined index
- 32.0%
That sample passes a 35 per cent limit with three points to spare. Calculating elongation on the full 2,000 g would have given 29.5 per cent and a flattering result. Laboratories do make that mistake, so check the working when a result looks too good.
03The limits
| Use | Limit | Reference |
|---|---|---|
| Granular sub-base | None stated | MoRTH Clause 401 |
| Wet mix macadam | 35% | MoRTH Table 400-12 |
| Dense bituminous macadam | 35% | MoRTH Table 500-8 |
| Bituminous concrete | 35% | MoRTH Table 500-16 |
| Concrete aggregate | 40% | IS 383:2016 |
A contract may set a tighter figure. The earlier revision of the MoRTH specification used 30 per cent, and some schedules still carry it.
04Why shape matters
- Flat stones lie flat. Under a roller they stack like tiles and leave planes along which the layer can shear.
- Thin stones break. A flake that snaps during compaction has changed the grading after it was approved.
- Poorly shaped stone packs loosely, so a bituminous mix needs more binder to fill the voids.
- Long pieces bridge across gaps and hold the particles around them apart.
05What sets the result
The rock
Some stone wants to break into slabs. Rocks with bedding or a grain, and some fine-grained basalts, fracture along flat planes however carefully they are crushed. Massive granite tends to break cubical. This is the part no plant setting can alter, and the reason a new quarry should be tested for shape before a contract is signed on it.
The crusher
A jaw makes flaky stone, which is acceptable because nothing it produces goes straight to the product. A cone makes good shape when its chamber is kept full, so that stone breaks against stone, and poor shape when it is trickle-fed. An impactor makes the best shape of all, because it breaks particles along their weakest dimension first.
The setting
Stone that comes out close to the size of the crusher's closed side setting is the most cubical. Stone much smaller than the setting is the flakiest, since it was made by slivers shearing off larger pieces. A cone set at 25 mm will make better 20 mm than a cone set at 40 mm from which the 20 mm is merely screened out. Worn liners matter for the same reason: they open the setting and change the chamber profile.
06Bringing it down
- Choke-feed the cone. Fit a level sensor or a surge bin if the feed is uneven.
- Match the closed side setting to the product that has to pass the limit.
- Change liners on wear, not on failure.
- Lower the reduction ratio asked of each stage. Three stages make better shape than two.
- Add a VSI as the final stage for the 20 mm and 10 mm fractions.
- Screen out the slot-shaped fraction. A slotted deck removes flakes that a square mesh passes.
The stages themselves are described in jaw, cone and VSI, and the other limits the same stone has to meet are in tests on aggregate for road work.