Penetration grade bitumen is a classification system for bitumen based on its hardness or consistency, determined through the standardized penetration test. Since bitumen behaves as a semi-solid, temperature-sensitive material, this test provides a simple, repeatable way to measure how "hard" or "soft" a given bitumen is — a property that directly affects how it performs once mixed into asphalt and laid on a road.
The Penetration Test
The test involves loading a standard needle (100 g) onto the surface of a bitumen sample maintained at 25°C, and measuring how far the needle sinks into the sample over exactly 5 seconds. The resulting penetration depth is measured in tenths of a millimeter — so a needle that sinks 6.5 mm records a penetration value of 65.
| Test | Unit | Method | 30-40 | 40-50 | 60-70 | 85-100 | 100-120 | 200-300 |
|---|---|---|---|---|---|---|---|---|
| Specific Gravity @25°C | g/cm³ | ASTM D70 | 1.01-1.06 | 1.01-1.06 | 1.01-1.06 | 1.01-1.05 | 1.01-1.04 | 0.99-1.04 |
| Penetration @25°C | mm/10 | ASTM D5 | 30-40 | 40-50 | 60-70 | 85-100 | 100-120 | 200-300 |
| Softening Point | °C | ASTM D-36 | 55-63 | 52-60 | 49-56 | 45-52 | 42-49 | 32 min |
| Ductility @25°C | cm | ASTM D-113 | 100 min | 100 min | 100 min | 100 min | 100 min | 100 min |
| Flash Point | °C | ASTM D-92 | 250 min | 250 min | 250 min | 230 min | 250 min | 177 min |
| Solubility in Trichloroethylene | wt% | ASTM D-2042 | 99.5 min | 99.5 min | 99.5 min | 99.5 min | 99.5 min | 99 min |
| Spot Test | - | A.A.S.H.T.T 102 | Negative | Negative | Negative | Negative | Negative | Negative |
| Drop in Penetration After Heating | % | ASTM-D5 & D6 | 20 max | 20 max | 20 max | 20 max | 20 max | 37 max |
| Loss on Heating | wt% | ASTM-D6 | 0.2 max | 0.2 max | 0.2 max | 0.2 max | 0.2 max | 1 max |
How Grades Are Named?
Penetration grade bitumen is labeled with a range (e.g., 40/50, 60/70, 80/100, 85/100), where the two numbers represent the minimum and maximum acceptable penetration values in tenths of a millimeter under standard test conditions. For example:
- Bitumen 60/70 means the needle penetrates between 60 and 70 tenths of a millimeter (6.0–7.0 mm) at 25°C.
- Bitumen 40/50 is harder — the needle penetrates only 40–50 tenths of a millimeter.
Common commercial grades span roughly from 30/40 (very hard) up to 180/200 (very soft), covering the range typically used in paving, roofing, and industrial applications.
Why It Matters: Climate-Based Selection?
Penetration depth correlates — empirically, though only approximately — with how the binder will perform once placed in service. This gives penetration grading direct practical relevance to road design:
- High-penetration ("soft") bitumen, like 85/100, remains more flexible at low temperatures, resisting the brittleness and thermal cracking that can occur in cold climates.
- Low-penetration ("hard") bitumen, like 40/50, resists deformation and rutting under heavy traffic loads and high pavement temperatures, making it suitable for hot climates.
This is why bitumen suppliers typically offer a range of penetration grades, and why selecting the right one depends heavily on the expected service temperature range of the region where the pavement will be built — not just on cost or availability.
