Cutback Bitumen Description

Cutback Bitumen ( Liquid Bitumen ) is Bitumen that is dissolved in a solvent . Typical solvents include Naphtha, gasoline and kerosene, white spirit, etc. The type of solvent controls the curing time while the amount determines the viscosity of the Cutback Bitumen.

The advantage Cutbacks have over Emulsions is a much higher residual Bitumen percent, typically over 80% compares with over 40-65% for Bitumen emulsions. The result I The ability to reshape is a great advantage the cutbacks have over the common penetration grade bitumen. It also requires less heat to liquefy which makes cutback grades easier to use at lower temperatures more Bitumen left on the roadway after curing, for the same volume of binder applied.

The benefit Cutbacks offer compared to Emulsions, is a much higher residual Bitumen percentage, usually over 80% while the rate for emulsions is around 40-65%. As a result, for the same volume of binder applied, more Bitumen is left on the roadway after curing.

The petroleum solvents used require higher amounts of energy to manufacture and are expensive compared to the water and emulsifying agents used in emulsified asphalts.

Cutback Bitumen Production

Cutback bitumen is produced by blending 70/100 or 160/220 penetration bitumen with kerosene to a specified viscosity. It is classified in the UK by flow time through a standard tar viscometer (STV) into three grades: 50, 100, and 200 seconds. It is mainly used for surface dressing but is also used in the production of standard and deferred-set asphalts.

To ensure uniform in-service and evaporation properties, cutback bitumen must meet distillation and penetration requirements. The SHELPHALT series includes an "X" suffix, indicating the addition of a heat-stable adhesion agent that improves aggregate bonding and resists wet stripping.

AASHTO distinguishes between cutback bitumen in three categories based on the type of solvent: rapid curing (RC) with gasoline, medium curing (MC) with kerosene, and slow curing (SC) with diesel. Viscosity at 60°C by a capillary tube viscometer is the basis for measurement, and it is reported as centistokes. The British Standard, in contrast, requires three viscosity grades for surface dressing, measured at 40°C by a discharge viscometer.

Cutback Classification

Cutbacks are divided into two classifications, Rapid-Curing (RC) and Medium-Curing (MC), and slow-curing ( SC) depending on the solvent used. They are further defined by a number that indicates the minimum kinematic viscosity (fluidity) of the cutback.

•             Slow Curing often called "road oils," are usually a residual material produced from the fractional distillation of certain crude petroleums. Traditionally any kind of aromatic, naphthenic, and paraffinic oils are used. Slow Curing liquid bitumen materials can be prepared by blending bitumen with an oily petroleum fraction.

•             Medium Curing is a blend of Bitumen with lighter hydrocarbons such as kerosene.

•             Rapid Curing products are prepared with light, rapidly evaporating diluents such as naphtha or gasoline.

Cutback Bitumen Application

Prime and Tack Coating

The process of priming involves applying a low viscosity binder to a prepared but usually unbound aggregate base. It is intended to be absorbed by the top layers of the base and provide a surface more easily ‘wetted’ by a subsequent bituminous covering. The primer will be able to carry traffic for a short time (although this practice is uncommon) and help control dust. Generally, primers are applied at rates between 0.5 and 1.4 L/m2. Cutback bitumen suitable for priming are also used for tack coats, which are applied to an underlying surface to help with the adhesion of the subsequent asphalt layers. A typical application rate is between 0.2 and 0.4 L/m2.

Prime Sealing

Where temperatures are too cool for an effective priming operation, or where traffic is likely to upset a primed surface before the final seal can be sprayed, a primer seal can be used to give adequate protection of the pavement for periods of up to 6 to 12 months. Cutback bitumen suitable for primer sealing can also be used in the manufacture of pre-mix asphalt, which is used in patch repairs.

Spray Sealing

Cutback bitumen are used extensively in sprayed sealing applications, particularly in cooler weather where they provide improved initial stone retention due to their lower viscosity. Typically, a single application of the appropriate cutback bitumen is sprayed onto the primed pavement onto which aggregate is laid.

Advantages of Cutback Bitumen 

Cutback bitumen has several advantages when utilized in road construction and repair of pavements. One of its greatest advantages is that it is simple to apply because it is in liquid form at ambient temperatures, which allows for better workability and penetration into aggregates. This feature makes it particularly useful in cold or wet conditions where regular bitumen would be difficult to apply.

Additionally, cutback bitumen lessens the demand for high-temperature use, conserving energy and environmental damage. It is extensively used in prime coat, tack coat, and surface dressing operations, producing good adhesion and durable wear on road surfaces. However, because it includes volatile solvents, safe handling and protective measures must be undertaken when handling it.

Grades & Properties of Cutback Bitumen