Asphalt Cement (AC) is a viscosity-graded paving bitumen historically used in the USA, Canada, and several international markets before the introduction of Performance Grade (PG) specifications.
AC grades classify bitumen based on its absolute viscosity measured at 60°C, which closely correlates with pavement stiffness, rutting resistance, and load-bearing performance.
Higher AC number = higher viscosity = higher stiffness & rut resistance
AC Grade Classification
| AC Grade | Viscosity @60°C (Poise) | Relative Hardness | Performance Level |
|---|---|---|---|
| AC-10 | ~1000 | Soft | Light–medium traffic |
| AC-20 | ~2000 | Medium | Medium traffic |
| AC-30 | ~3000 | Hard | Heavy traffic |
| AC-40 | ~4000 | Very Hard | Very heavy traffic |
Detailed Grade-Wise Performance Explanation
1. AC-10 Bitumen
Low viscosity provides excellent flexibility and workability at lower temperatures.
Applications: Cold and temperate climates, low to medium traffic roads, flexible surface courses.
Advantages: Reduced thermal cracking, lower production temperatures, easy compaction.
Limitations: Lower rut resistance in hot climates.
2. AC-20 Bitumen
Balanced stiffness and flexibility ensures stable performance across moderate temperatures.
Applications: Urban roads, medium-traffic highways, binder and wearing courses.
Advantages: Improved fatigue resistance, moderate deformation control.
3. AC-30 Bitumen
High viscosity and stiffness deliver excellent load-bearing performance.
Applications: National highways, dense bituminous mixes, high-traffic urban roads.
Advantages: Strong rutting resistance, long pavement service life.
Limitations: Requires higher mixing and compaction temperatures.
4. AC-40 Bitumen
Very high viscosity provides maximum resistance to permanent deformation.
Applications: Expressways, industrial roads, bus lanes, intersections, heavy-duty pavements.
Advantages: Superior stability under heavy axle loads, excellent hot-climate performance.
Limitations: Reduced flexibility at low temperatures; not suitable for cold regions without modification.
Typical Mechanical & Physical Properties
| Property | AC-10 | AC-20 | AC-30 | AC-40 |
|---|---|---|---|---|
| Viscosity @60°C (Poise) | ~1000 | ~2000 | ~3000 | ~4000 |
| Penetration @25°C (dmm) | 80–100 | 60–80 | 50–70 | 40–60 |
| Softening Point (°C) | 40–47 | 45–52 | 47–55 | 50–60 |
| Ductility @25°C (cm) | ≥75 | ≥50 | ≥40 | ≥25 |
| Flash Point (°C) | ≥220 | |||
| Solubility in TCE (%) | ≥99 | |||
Climate & Traffic Suitability Guide
| Climate / Traffic Condition | Recommended AC Grade |
|---|---|
| Cold climates | AC-10 |
| Moderate climates | AC-20 |
| Hot climates | AC-30 |
| Extremely hot + heavy traffic | AC-40 |
Load-Bearing & Rutting Resistance Mechanism
AC bitumen behaves as a viscoelastic material. The viscous component dissipates stress, while the elastic component enables recovery after load removal.
Higher AC grades increase stiffness modulus, reduce shear strain, and improve rut resistance at elevated pavement temperatures.
Refining & Production Process
Vacuum distillation separates asphaltic residue from crude oil.
Controlled blending and air blowing adjust viscosity.
Asphaltenes provide stiffness, while maltenes ensure flexibility and workability.
AC vs VG vs PG Systems
| Aspect | AC | VG | PG |
|---|---|---|---|
| Basis | Viscosity @60°C | Viscosity @60°C & 135°C | Pavement temperature |
| Temperature Sensitivity | Moderate | Better | Best |
| Current Usage | Limited | Widely used | Advanced projects |
Storage, Handling & Safety
Storage temperature: 150–180°C.
Avoid overheating to prevent oxidation.
Use insulated tanks and moisture-free handling systems.
Why AC Grades Still Matter
✔ Simple viscosity-based classification
✔ Proven historical performance
✔ Suitable for non-Superpave projects
✔ Cost-effective for traditional pavement designs