Types of Bitumen
Viscosity Grade
Binders classified by absolute viscosity at 60 °C — the AC and AR grading system used across North America and South Asia.
Viscosity grading measures the binder at 60 °C, the temperature a pavement surface actually reaches on a hot day. That makes it a more direct predictor of high-temperature performance than penetration, which is measured at 25 °C and tells you little about summer rutting.
Two families exist. AC grades are tested on the original binder as it leaves the refinery. AR grades are tested after rolling thin film oven ageing, which simulates the hardening that occurs during mixing and laying, so the number describes the binder in something closer to its in-service condition.
AC grades — original binder
- AC-2.5 — very soft, cold regions and specialist cold mixes
- AC-5 — soft grade for cold to temperate climates
- AC-10 — medium-soft, moderate climates and light traffic
- AC-20 — the most widely used grade, warm climates and normal traffic
- AC-30 — hard grade for hot climates and heavy traffic
- AC-40 — very hard, extreme heat and very heavy axle loading
AR grades — aged residue
- AR-1000 — soft residue grade
- AR-2000 — medium residue grade
- AR-4000 — standard paving residue grade
- AR-8000 — hard residue grade for hot climates
- AR-16000 — very hard residue grade for extreme service
Why it is specified
Viscosity is a fundamental engineering property with a defined unit, whereas penetration is an empirical index. That means viscosity values can be used directly in mix design calculations and in predicting how the binder will behave in the mixing plant and under the roller.
The system also grades on a property measured at service temperature rather than at laboratory ambient, which correlates better with rutting resistance in hot climates.
Typical specification
| Absolute viscosity @ 60 °C | ASTM D2171 — to grade, in poise |
|---|---|
| Kinematic viscosity @ 135 °C | ASTM D2170 — 210 cSt minimum for AC-20 |
| Penetration @ 25 °C | ASTM D5 — 40 minimum for AC-20 |
| Flash point | ASTM D92 — 232 °C minimum |
| Solubility | ASTM D2042 — 99% minimum |
| Thin film oven test | ASTM D1754 — viscosity ratio 4.0 maximum |
Values are indicative and given for guidance. Contract specification is agreed in writing and certified per batch on the certificate of analysis issued with each shipment.
