Flakiness Index and Elongation Index of Aggregate

The flakiness index measures how much of a stone supply is made of thin, slab-shaped pieces. Its companion, the elongation index, measures the long, sliver-shaped ones. Good road and concrete stone is roughly cubical: it locks together, compacts well and resists breaking. Too many flaky or elongated pieces and the material gets harder to compact and weaker in service. Both tests use simple slotted gauges and a balance, so they are among the fastest checks a quarry or site lab can run.

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Quick facts

Standard: IS 2386 (Part 1) in India; BS 812 and EN 933-3 use similar ideas.
Flaky particle: thickness less than 0.6 times the mean sieve size of its fraction.
Elongated particle: length more than 1.8 times the mean sieve size.
Sample: enough stone to give at least 200 pieces in each fraction tested.
Not used on particles smaller than 6.3 mm.

What "mean sieve size" means

Each fraction is defined by two sieves, the one it passed and the one it stopped on. The mean size is the average of the two. Stone passing 20 mm and retained on 16 mm has a mean size of 18 mm. Its thickness gauge slot is 0.6 × 18 = 10.8 mm wide and its length gauge gap is 1.8 × 18 = 32.4 mm. The gauges have one slot or gap for each fraction, so every piece is judged against its own size group.

How the flakiness index test works

Sieve the dried sample into the standard fractions, from 63 mm down to 6.3 mm.
Gauge each piece. Try to pass every particle of a fraction through the thickness slot made for that fraction. Pieces that slip through are flaky.
Weigh all the flaky material from all fractions together.
Calculate: flakiness index = (weight of flaky pieces ÷ weight of all pieces gauged) × 100.

How the elongation index test works

The length gauge is a row of pins. For each fraction, every particle is held against the gap for its size. A piece too long to fit between the pins is elongated. Weigh those pieces and divide by the weight of the sample gauged, times 100.

The combined index

Many specifications use a combined value. Run the flakiness test first and set the flaky pieces aside. Run the elongation test only on the non-flaky stone that remains. Add the two percentages together. Doing it in that order stops a thin and long piece from being counted twice.

Worked example (made-up numbers)

Total stone gauged: 5,000 g. Flaky pieces: 600 g. Flakiness index = 600 ÷ 5,000 × 100 = 12%.
Non-flaky stone left: 4,400 g. Elongated pieces in it: 660 g. Elongation index = 660 ÷ 4,400 × 100 = 15%.
Combined index = 12 + 15 = 27%.

Typical limits

IS 383 (aggregate for concrete): combined flakiness and elongation index of 40% maximum, which the engineer may relax based on test results.
Road work: highway specifications set their own shape limits by layer, and asphalt surface courses get the strictest ones. Use the table in your contract.

Why shape matters for asphalt and base

Flaky stone tends to snap under rollers, which changes the gradation right where you need it to hold. It packs with more voids and less interlock, so a mix can be harder to get to density and more prone to rutting. In unbound base, slabby stone shingles and slides instead of locking. Shape also feeds into the strength tests: a supply with a high flakiness index often posts a worse crushing value and impact value than the same rock crushed to a better shape.

How quarries reduce flakiness

Particle shape depends a lot on crushing. Cone and impact crushers set up for a good reduction ratio, a choke-fed crusher chamber and screening out the worst sizes all tend to give more cubical stone. That is why the same rock can test well from one plant and poorly from another.

Flakiness index vs ASTM D4791

US specifications use flat and elongated particles per ASTM D4791, which compares each particle's longest dimension to its thickness with a proportional caliper at a set ratio. It answers the same question in a different way, so do not swap results between the two systems.

Questions people ask

What is flakiness index?

It is the share, by weight, of coarse aggregate pieces with a thickness is less than 0.6 times their mean sieve size. A high flakiness index means a lot of thin, plate-like pieces.

What is elongation index?

It is the share, by weight, of pieces with a length more than 1.8 times their mean sieve size. These are the long, needle-like pieces.

How do you calculate flakiness index?

Sieve the sample into size fractions, pass each piece through the matching slot on a thickness gauge, and weigh everything that slips through. Flakiness index = (total weight passing the gauges / total weight gauged) x 100.

What is the combined flakiness and elongation index?

Run the flakiness test first, set the flaky pieces aside, then run the elongation test on what is left. Add the two percentages. IS 383 caps the combined value at 40% for concrete aggregate, and road specifications usually set tighter limits for asphalt layers.

Why are flaky aggregates bad?

Thin and long pieces break more easily under rollers and traffic, pack poorly, lower workability and are harder to compact to target density. They also tend to lie flat and create weak planes in a mix or base.

Is flakiness index the same as ASTM flat and elongated particles?

No. IS 2386 and the British method use fixed ratios to the mean sieve size. ASTM D4791 compares a particle's own longest and shortest dimensions at a ratio the specification chooses, such as 3:1 or 5:1, so the numbers are not interchangeable.

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