Flat and Elongated Particles: Test, Ratios and Limits
Flat and elongated particles are pieces of crushed stone that look more like slivers or chips than cubes. A few in a load do no harm. Too many make asphalt harder to compact, can snap apart under the roller and can leave a mix short on room for binder. That is why most asphalt specs limit them, and why aggregate labs spend time sorting stones with a special caliper.
Crushed stone calculator →The short version
- The standard: ASTM D4791.
- The tool: a proportional caliper set to a ratio such as 3:1, 4:1 or 5:1.
- Superpave rule: 5:1 ratio, maximum 10 percent for most traffic levels.
- Why it matters: compaction effort, particle breakage and binder room in the mix.
The three shapes
Every stone has a length (longest side), a width (middle) and a thickness (shortest). The test sorts particles by comparing those:
- Flat: width compared with thickness is over the ratio.
- Elongated: length compared with width is over the ratio.
- Flat and elongated: length compared with thickness is over the ratio.
The ratio is set by the spec. Specs have used ratios anywhere from 2:1 up to 5:1. A 5:1 particle is very slab-like, so a 5:1 limit only catches the worst shapes. A 3:1 check is much stricter.
Why shape matters in asphalt
Compaction. Cube-like stones roll and slide into a tight pack as the roller works. Flat or long stones catch on their neighbors and resist moving, so the mix needs more passes to hit density, or never quite gets there.
Breakage. Thin pieces tend to snap across their narrow side under rollers and traffic. Each break turns one coarse particle into smaller ones, which shifts the gradation finer than the design. Those extra fines fill space between the stones, leaving less room for binder. A mix in that state can become sensitive to small changes in binder content and may rut or shove. The fresh broken faces are also bare of binder, which can hurt durability.
For gap-graded mixes such as stone mastic asphalt, where coarse stones bear directly on each other, shape matters even more, and specs often check a stricter ratio.
How the test is run
- Sieve the sample. Split out the coarse sizes being tested, usually particles caught on the 3/8 inch sieve (9.5 mm) or the No. 4 sieve (4.75 mm), using a sieve analysis.
- Reduce each size to about 100 particles.
- Check each particle. For the flat and elongated check, set the caliper's large opening to the particle's length. If the particle, turned to its thinnest side, slips through the small opening, it counts. Flat-only and elongated-only checks work the same way using width and thickness, or length and width.
- Count or weigh. Total up the particles that fail and those that pass, by count or by oven-dry mass as the spec requires.
By count, the test takes about half an hour. By mass, the sample must be oven-dry first, which adds an overnight drying step.
The math
Percent = failing particles ÷ (failing + passing particles) × 100
Example (made-up numbers): of 100 particles in the 1/2 to 3/8 inch fraction, 9 slip through the 5:1 caliper. That fraction is 9 percent flat and elongated.
Many specs want a weighted result across sizes. Example (made-up numbers): the 3/4 to 1/2 inch fraction makes up 40 percent of the coarse sample and tests at 6 percent. The 1/2 to 3/8 inch fraction makes up 60 percent and tests at 9 percent. Weighted result = 0.40 × 6 + 0.60 × 9 = 2.4 + 5.4 = 7.8 percent, which would pass a 10 percent limit.
Superpave limits
| 20-year design ESALs (millions) | Max flat and elongated (5:1) |
|---|---|
| Under 0.3 | No requirement |
| 0.3 to under 3 | 10% |
| 3 to under 10 | 10% |
| 10 to under 30 | 10% |
| 30 and up | 10% |
The ESAL figure is the expected traffic in the design lane over 20 years, no matter how long the pavement is actually designed to last. Shape is one of the Superpave "consensus" aggregate properties, along with crushed faces, fine aggregate angularity and clay content. Agencies outside Superpave may use other ratios and limits, so check your spec.
Shape and other aggregate tests
Particle shape is separate from toughness. A hard rock can still crush into slivers, and a cube-shaped stone can still be soft. That is why specs pair the shape check with wear tests such as the Los Angeles abrasion test. Crushing method plays a big part in shape: crusher type, the gap setting and how full the crusher is kept can all change how cubical the product comes out.
Does shape matter for a gravel driveway?
Somewhat. Angular, roughly cubical stone such as a good #57 stone or crusher run locks together and stays put. Lots of thin, flaky pieces tend to shift, break down and work into the soil faster. For ordering, the crushed stone calculator converts area and depth into tons.
Questions people ask
What are flat and elongated particles?
A flat particle is much wider than it is thick. An elongated particle is much longer than it is wide. A flat and elongated particle is much longer than it is thick. 'Much' is set by a ratio such as 3:1 or 5:1.
Which test method is used?
ASTM D4791. Each particle is checked with a proportional caliper set to the ratio in the spec. Results are reported as a percent by count or by mass.
What is the Superpave limit?
Superpave checks the length to thickness ratio at 5:1 and allows a maximum of 10 percent flat and elongated particles for projects with 0.3 million design ESALs or more.
Why are flat and elongated particles a problem in asphalt?
They tend to lock up instead of sliding into a dense arrangement, so the mix takes more effort to compact. They can also snap during rolling, which makes the gradation finer, cuts the room left for binder and exposes uncoated faces.
How many particles are tested?
About 100 particles from each size fraction being checked, usually the material caught on the 3/8 inch sieve (9.5 mm) or the No. 4 sieve (4.75 mm).
How do quarries reduce flat and elongated stone?
Crusher type and setup matter most. Many producers use cone or impact crushers, adjust feed rates and add later crushing stages to make more cube-shaped stone. Results vary a lot with the rock itself.
Related guides
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