Types of Bitumen
Cutback Bitumen
Bitumen blended with a petroleum solvent so it can be sprayed and worked cold — rapid, medium and slow curing grades.
Cutback bitumen is paving bitumen thinned with a volatile petroleum distillate. The solvent lowers viscosity enough for the binder to be applied at moderate temperature, then evaporates and leaves the bitumen to recover its original consistency in place. That evaporation is called curing, and how fast it happens defines the grade.
It is the practical answer where heating a binder to 160 °C is impossible — remote sites, small maintenance crews, priming a porous base that hot binder would not penetrate.
The three curing families
Rapid curing grades are cut with naphtha or gasoline. The solvent flashes off quickly, so the binder sets within hours. They are used for surface dressing, tack coats and crack sealing where the road has to reopen fast.
Medium curing grades use kerosene. Curing takes a day or two, which gives enough working time to mix and lay a cold-mix patch, and enough penetration to prime a granular base.
Slow curing grades, sometimes called road oils, use a heavy gasoil that barely evaporates at all. They stay workable for weeks and are used for dust control and for stabilising unsealed surfaces.
Grade designations
- RC-70, RC-250, RC-800, RC-3000 — rapid curing, by kinematic viscosity
- MC-30, MC-70, MC-250, MC-800, MC-3000 — medium curing
- SC-70, SC-250, SC-800, SC-3000 — slow curing
- The number is the minimum kinematic viscosity at 60 °C in centistokes
Applications
- Prime coats on granular bases before the asphalt layer (MC-30 and MC-70)
- Tack coats between pavement layers (RC grades)
- Surface dressing and chip sealing in cooler weather
- Cold-mix patching material that can be stockpiled and used as needed
- Dust palliative treatment on unsealed roads and haul routes (SC grades)
- Crack filling and joint sealing during maintenance
Handling and safety
Cutbacks contain flammable solvent and have a much lower flash point than straight bitumen, so storage temperature must stay well below it and ignition sources must be controlled. Environmental regulation in several markets now restricts solvent-based cutbacks in favour of emulsions, and we will advise where an emulsion is the better or the legally required choice.
Typical specification
| Kinematic viscosity @ 60 °C | ASTM D2170 — to grade |
|---|---|
| Flash point (tag open cup) | ASTM D3143 — 27 °C minimum for MC grades |
| Distillate to 360 °C | ASTM D402 — to grade, by volume |
| Residue penetration @ 25 °C | ASTM D5 — 120 to 250 |
| Water content | ASTM D95 — 0.2% 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.
