Penetration Grade Bitumen is classified based on the depth (in tenths of a millimeter) that a standard needle penetrates the bitumen sample under controlled conditions (100 g load, 5 seconds, at 25°C), as per ASTM D5 / EN 1426.
This system reflects the relative hardness or softness of bitumen and directly influences its performance under traffic loads and temperature variations.
Lower penetration number → Harder bitumen
Higher penetration number → Softer bitumen
Grade Classification & Performance Logic
| Grade | Hardness | Typical Climate | Traffic Condition |
|---|---|---|---|
| 40/50 | Very Hard | Hot climates | Heavy & very heavy traffic |
| 60/70 | Medium-Hard | Moderate to hot climates | Standard highway traffic |
| 80/100 | Medium-Soft | Cool climates | Medium traffic |
| 120/150 | Soft | Cold climates | Light traffic / special use |
Detailed Grade-Wise Technical Description
1. Bitumen Grade 40/50 (Hardest Grade)
Low penetration results in very high stiffness and resistance to rutting and deformation.
Typical Uses: Expressways, high-temperature regions, heavy axle load roads, industrial pavements, airport runways.
Advantages: Excellent bleeding resistance, long pavement life under heavy traffic.
Limitations: Reduced flexibility in cold climates, requires controlled compaction temperature.
2. Bitumen Grade 60/70 (Most Common Grade)
Offers an optimal balance between hardness and flexibility, making it the global industry standard.
Typical Uses: National highways, urban roads, DBM layers, state highways.
Advantages: Good fatigue resistance, strong aggregate bonding, cost-effective.
3. Bitumen Grade 80/100 (Softer Grade)
Higher penetration provides improved flexibility and reduced cracking risk in cooler climates.
Typical Uses: Roads in cooler regions, surface dressing, low to medium traffic roads.
Limitations: Susceptible to rutting in hot climates and heavy traffic.
4. Bitumen Grade 120/150 (Softest Grade)
Very high penetration enables maximum flexibility and ease of application.
Typical Uses: Cold climate roads, spray applications, emulsions and cutback base material.
Limitations: Poor high-temperature stability, not suitable for heavy traffic.
Key Mechanical & Physical Properties
| Property | 40/50 | 60/70 | 80/100 | 120/150 |
|---|---|---|---|---|
| Penetration @25°C | 40–50 | 60–70 | 80–100 | 120–150 |
| Softening Point (°C) | 52–60 | 48–56 | 45–52 | 40–48 |
| Ductility (cm) | ≥75 | ≥75 | ≥100 | ≥100 |
| Viscosity @135°C | High | Medium | Medium–Low | Low |
| Flash Point (°C) | ≥220 | |||
| Solubility (TCE) | ≥99% | |||
Temperature Susceptibility & Climate Suitability
Hard grades resist deformation in hot climates, while softer grades prevent thermal cracking in cold regions.
Improper grade selection can lead to rutting (bitumen too soft) or cracking (bitumen too hard).
Binding & Adhesion Mechanism
Bitumen acts as a viscoelastic binder that coats aggregates, transfers load, resists moisture damage, and dissipates traffic-induced stress.
Harder grades enhance load transfer, while softer grades improve stress relaxation.
Refining & Grade Control Process
Vacuum distillation controls hardness by removing lighter fractions.
Air blowing (if required) adjusts penetration and softening point.
Asphaltene–Maltene balance determines final grade characteristics.
Standards & Specifications
ASTM D5 – Penetration
ASTM D36 – Softening Point
ASTM D113 – Ductility
IS 73 – Indian Standard
AASHTO M20
Storage, Handling & Transportation
Store between 150–180°C depending on grade.
Avoid prolonged overheating and water contamination.
Use insulated tankers and heated storage systems.
Quick Selection Guide
| Condition | Recommended Grade |
|---|---|
| Hot climate + heavy traffic | 40/50 |
| Moderate climate + standard traffic | 60/70 |
| Cool climate | 80/100 |
| Cold climate / spray works | 120/150 |