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.
Industrial applications
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.
In an asphalt mix, bitumen is the binder and aggregate is the load-bearing skeleton. Aggregate makes up most of the mix by weight, so asphalt’s stability under load, heat and moisture depends heavily on the properties of this material.
Key properties of asphalt aggregate
Fractured faces and particle shape
Crushed particles with several faces interlock and increase the mix’s resistance to rutting. The percentage of fractured faces is measured by ASTM D5821. Flat and elongated particles break during compaction and must be limited.
Hardness and durability
Aggregate must resist crushing under rollers and abrasion under traffic. The Los Angeles abrasion and sulfate soundness tests are the usual measures of these properties.
Cleanliness
A coating of dust on particles prevents bitumen from adhering. The sand equivalent test measures the cleanliness of the fine fraction and has an important place in asphalt mix specifications.
Adhesion to bitumen
Some rocks, siliceous rocks in particular, have a greater affinity for water, and bitumen may separate from their surface when moisture is present. This is called stripping and should be checked during mix design.
The role of each grading fraction
| Fraction | Typical material | Role in the mix |
|---|---|---|
| Coarse aggregate | Crushed gravel | Load-bearing skeleton and interlock |
| Fine aggregate | Crushed sand | Filling voids and stability |
| Filler | Stone powder | Stiffening the bituminous mortar |
Mineral filler forms a mortar with bitumen that binds the particles together. Filler grading limits are defined in ASTM D242. Too much makes the mix brittle, so the filler-to-bitumen ratio is controlled in the mix design.
The combined grading curve
Asphalt materials usually come from several separate stockpiles combined in set proportions. The accuracy of that blend depends on the uniformity of each stockpile. If the grading of one stockpile changes, the final curve moves outside the design band and the optimum bitumen content changes too.
Summary
Asphalt material specifications in Iran are set by Publication 234 and the project’s technical specification. For the requirements of this field, see Road construction.
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