Choosing Aggregate for Asphalt: Selection Criteria for Asphalt Mixes
Aggregate forms the main part of an asphalt mix. Learn how fractured faces, hardness, cleanliness and adhesion to bitumen shape asphalt performance.
Industrial applications
Mineral Filler in Asphalt: Role, Grading Limits and Dust-to-Binder Ratio
Mineral filler forms a mortar with bitumen that decides asphalt stability. Learn ASTM D242 limits, the dust-to-binder ratio and how filler type affects a mix.
Filler is a small share of an asphalt mix by weight, but its effect is far larger than its share. These very fine particles form a mortar with bitumen that binds the coarser particles together.
What filler does
- Fills the finest voids and raises the density of the mix
- Stiffens the bituminous mortar and reduces temperature sensitivity
- Helps reduce moisture sensitivity in some combinations
Grading limits
ASTM D242 sets these limits for mineral filler:
| Sieve | Percent passing |
|---|---|
| 600 µm | 100 |
| 300 µm | 95 to 100 |
| 75 µm | 70 to 100 |
The standard also limits the filler’s plasticity index to a maximum of 4. See the details on the stone powder page.
Dust-to-binder ratio
The amount of filler means little on its own and must be measured against the bitumen. In Superpave design, this ratio is usually controlled between 0.6 and 1.2. Too little leaves a soft mix prone to rutting; too much makes it brittle and hard to place.
Filler type
Limestone powder is the most common filler. Hydrated lime is also sometimes used to improve bitumen adhesion to the stone. Filler must be dry and free of lumps to disperse evenly in the mix. Clay in the filler increases moisture sensitivity.
Quality control
Filler grading is checked by sieve analysis with suitable washing, and plasticity by Atterberg limits. For the full picture of asphalt materials, see Choosing aggregate for asphalt.
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