Cutback Bitumen Description

Cutback Bitumen, or Liquid Bitumen, is a type of bitumen dissolved in petroleum solvents such as naphtha, gasoline, kerosene, or white spirit to reduce its viscosity. The method allows the bitumen to be used at a reduced temperature and under various environmental conditions. The type and percentage of solvent used directly influence the curing time and final viscosity of cutback bitumen.

Because cutback bitumen is liquid, it allows for more convenient application and penetration into aggregates, particularly useful in road construction and maintenance operations where temperature restrictions or rapid setting times are the issue. Furthermore, cutback bitumen will more frequently have a higher residual bitumen content than bitumen emulsions, with enhanced durability, greater adhesion of binders, and enhanced performance in the completed pavement.

Classification of Cutback Bitumen

Cutback bitumen is usually categorized into three classes based on the volatility of solvents:

Rapid-Curing (RC)

Uses very volatile solvents like gasoline or naphtha

Evaporates quickly

Medium-Curing (MC)

Comprises solvents like kerosene

Offers a compromise between curing time and application ease

Slow-Curing (SC)

Has fewer volatile oils (often called "road oils")

For slower-setting work and for lower temperatures

Each is also graded by a number indicating its kinematic viscosity.

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Applications of Cutback Bitumen

Prime and Tack Coating

Priming is the treatment with a low-viscosity binder of an unbound base course to provide adhesion for top layers of asphalt. Cutback bitumen is suited particularly for prime and tack coating and will typically be applied at the following rates:

Priming: 0.5–1.4 L/m²

Tack coating: 0.2–0.4 L/m²

Prime Sealing

In cold weather where there is inefficient priming, cutback bitumen can be used as primer sealing to seal the surface for 6–12 months. It is also suitable for application in pre-mix asphalt in patching repairs.

Spray Sealing

Cutback bitumen is widely used in spray sealing, particularly in cooler climates, due to its reduced viscosity and enhanced initial aggregate retention.

Cutback Bitumen Manufacturing

Cutback bitumen is prepared by blending penetration-bitumen (typically 70/100 or 160/220) with solvents like kerosene until the desired viscosity is achieved. In the United Kingdom, cutback bitumen is graded on the basis of flow time on a Standard Tar Viscometer (STV) into grades such as 50, 100, and 200 seconds.

For regular performance, cutback bitumen must meet some distillation and penetration specifications. Some products, like in the SHELPHALT series, include adhesion promoters to improve aggregate bonding and wet resistance.

Advantages of Cutback Bitumen Compared to Emulsions

In contrast with bitumen emulsions, cutback bitumen has a considerably greater residual bitumen content—typically over 80%, in contrast to emulsions with 40% to 65%. This results in:

Increased resistance and retention of binders upon curing

Enhanced reshaping ability compared to penetration grade bitumen

Greater applicability at lower temperatures as a result of lower heating requirements

Environmental Factors

While cutback bitumen offers certain practical benefits, its petroleum solvents are more energy resource-intensive in production and pose more environmental risks than the water-based bitumen emulsion formulation.

Conclusion

Cutback bitumen remains a significant material for road building and upkeep, especially where temperature restraints apply or where quick application is needed. Although there are environmental concerns, proper usage and blending allow for effective, long-lasting infrastructure solutions.

For more on related products, see our articles on Penetration Grade Bitumen and Bitumen Emulsions.

Grades & Properties of Cutback Bitumen