Cutback bitumen is a range of bindersare produced by blending (mixing) penetration grade bitumen, such as paraffin or mineral turpentine.

Cutback Bitumen Description

Cutback Bitumen has Three types of solvents are used for the blending process:

• slow-curing
• medium-curing
• rapid-curing

The latter two are the most common in South Africa. The choice of the solvent determines the rate at which the bitumen will cure when exposed to air.

The viscosity of the cutback bitumen is determined by the proportion of solvent added: the higher the proportion of solvent, the lower the viscosity of the cutback.

Cutbacks differ from penetration grade bitumen in that they are more workable — in other words, they can be more easily reshaped. Less heat is required to liquefy cutback bitumen than penetration bitumen, making it easier to use at lower temperatures.

When the solvent has evaporated, the binder returns to its original penetration grade to tie the particles together.
Cutback bitumen gets its name from the solvent that is involved in the process, because the solvent “cuts back” or evaporates, leaving behind the binder to “get on with the job”. The solvent used in cutback bitumen is called the “cutter” or “flux“.

Definition of Types of Cutback Bitumen

slow-curing

Slow Curing or SC Cutback Bitumen are often used for dissolving or solvent asphalt cement. Slow curing Bitumen is also, can be used for occasionally for priming, graded aggregates .

medium-curing

medium curing of Cutback Bitumen is a blend several oil solvents such as kerosene in order to transform bitumen to soluble bitumen which is evaporated in oil solvent using process and the bituminous layer is remained to make cohesion, penetration and form a sealing layer.

rapid-curing

Rapid Curing Cutback Bitumen is a compound of light diluents of high volatility. It’s (RC) designed to react quickly primarily for spray applications, such as bond/tack coats, aggregate chips seals, sand seals, and similar surface treatments.

Types of Cutback Bitumen Based on Curing Time

Various solvents extracted from crude oil are used to prepare cutback bitumen.

There is 10 to 50% solvent in cutback bitumen, mostly kerosene, gasoline, diesel oil or fuel oil.

Cutback asphalt is classified based on the relative evaporation rate of the solvent compared to acetone, as listed below:

1- Rapid Curing (RC)

• Fast-evaporating solvents at ambient temperature are used to make rapid-curing cutback bitumen. This bitumen is called RC for short.
• The evaporation rate of the solvents used in this type of bitumen is 0.3 times more than the evaporation rate of acetone.
• Rapid curing grades are used in tack coats, chips seals and sand seals.

2- Medium-Curing (MC)

• Medium-curing bitumen is prepared by dissolving penetration bitumen in solvents with medium vaporability or volatility.
• Generally, the evaporation rate of these solvents is 0.8 to 0.3 times of acetone evaporation rate.
• Commonly, medium-curing Cutback asphalt are made with kerosene.
• MC does not evaporate fast when mixed with aggregates. For this reason, it is used as a binder in cold asphalt.

3- Slow Curing (SC)

• Slow Cutback asphalt is prepared by dissolving penetration bitumen with heavy distillate solvents.
• Their evaporation rate is 0.8 times lower than the evaporation rate of acetone.
• Typical solvents are gasoline or fuel oil.
• SC cutback bitumen is used in dust repression and cold mixed asphalt.

Grade of Cutback Bitumen

Rapid Curing:

• RC-30
• RC-70
• RC-250
• RC-800
• RC-3000

Medium Curing:

• MC-30
• MC-70
• MC-250
• MC-800
• MC-3000

Slow Curing:

• SC-70
• SC-250
• SC-800
• SC-3000

Process of Cutback Bitumen Production

Preparation of Bitumen and Solvent:

• The appropriate grade of bitumen (often penetration grade) is selected based on the desired properties of the final product.
• The solvent, typically kerosene or a similar petroleum-based product, is prepared and filtered to remove impurities.

Mixing:

• The bitumen and solvent are mixed together in a suitable mixing tank or unit.
• The ratio of bitumen to solvent determines the type and grade of cutback bitumen produced.
• Efficient mixing is crucial to ensure a homogeneous product. In some cases, colloidal mills may be used to break
• down bitumen into smaller droplets for better dispersion.

Cooling:

• After mixing, the cutback bitumen is cooled to achieve the desired viscosity. This is often done using a compressor or heat exchanger.

Applications of Cutback Bitumen

Cutback asphalt is primarily used in the road construction and maintenance industry due to its ease of application in colder temperatures. Its lower viscosity compared to pure bitumen makes it more workable.

Primary Applications:

• Surface Dressing: This involves spraying a thin layer of cutback bitumen on the road surface followed by the application of aggregate. This method is cost-effective for maintaining roads with medium to light traffic.
• Prime Coats: A thin layer of cutback bitumen applied to the base course before laying the asphalt layer. It helps to bind the base material and improve adhesion between the base and asphalt layers.
• Tack Coats: A thin, sticky layer applied between existing asphalt layers to ensure proper bonding and prevent slippage.
• Cold Mix Asphalt: Cutback asphalt can be used to produce cold mix asphalt, which is a mixture of aggregate, filler, and Cutback asphalt that can be used in colder weather conditions.
• Pipeline Coating: Cutback asphalt can be used as a protective coating for pipelines to prevent corrosion.
• Patch Repairs: Cutback asphalt can be used to repair potholes and other small defects in road surfaces.
• Roofing: In some cases, cutback bitumen can be used as a waterproofing material for roofs.

Applications of Cutback Bitumen in Bituminous (asphalt) Pavement Construction and Maintenance:

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 asphalt suitable for priming are also used for tack coats, which are applied to an underlying surface to help with the adhesion of subsequent asphalt layer. 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 asphalt suitable for primer sealing can also be used in the manufacture of pre-mix asphalt, which is used in patch repairs.

Spray Sealing

Cutback asphalt 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.

Advantage of 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 is more Bitumen left on the roadway after curing, for the same volume of binder applied. The advantages of cutback bitumen include:

• Ease of Application: Cutback asphalt has lower viscosity due to the added solvent, making it easier to handle and apply. This facilitates smoother application in various construction and maintenance projects.
• Versatility: Different grades of cutback bitumen are available, allowing for flexibility in choosing the appropriate type based on specific project requirements, weather conditions, and curing times.
• Quick Curing: Depending on the type of cutback bitumen used, it can offer rapid curing, allowing for faster completion of construction projects, especially in situations where quick setting and curing are crucial.
• Cost-Effectiveness: Cutback asphalt can be more cost-effective than other types of bitumen because of its lower viscosity and the ability to use less material for certain applications.
• Cold-Weather Application: Some types of cutback bitumen, like medium-curing and slow-curing grades, are suitable for cold-weather applications, providing a practical solution for construction in cooler climates.
• Adhesion Properties: Cutback asphalt exhibits good adhesion properties, ensuring proper bonding with various surfaces, which is essential for the effectiveness of asphalt mixes and waterproofing applications.

It’s important to note that while cutback bitumen offers these advantages, it also has some drawbacks, such as environmental concerns related to solvent emissions during application. Proper safety measures and environmental considerations should be taken into account when using cutback bitumen.

In Summary

Cutback bitumen is produced using three types of solvents: slow-curing, medium-curing, and rapid-curing, with medium and rapid-curing being the most common in South Africa. The term “cutback” refers to the solvent’s role in reducing the bitumen’s viscosity before it evaporates, allowing the binder to function efficiently. The solvent is also known as the “cutter” or “flux.”

Cutback bitumen is prepared by selecting the appropriate grade of bitumen and mixing it with a petroleum-based solvent. After mixing and cooling to the desired viscosity, it is used in road construction and maintenance for applications like surface dressing, prime and tack coats, cold mix asphalt, pipeline coating, patch repairs, and roofing.