Adding mainly petroleum-type solvent to ordinary bitumen reduces its viscosity and is known as cutback bitumen. Cutback bitumen is used at low temperatures because its viscosity is lower than that of neat asphalt. The solvent evaporates away after a cutback and only Bitumen remains. The petroleum solvent in cutback bitumen vaporizes as it cures. In general, cutback bitumen is applied as a primer and tack coat.
Cutback bitumen is categorized into 3 groups, according to their curing time:
- Rapid Curing (RC) products are prepared with light, rapidly evaporating diluents such as naphtha or gasoline.
- Medium Curing (MC) is a blend of Bitumen with lighter hydrocarbons such as kerosene.
- Slow Curing (SC) ) often called "road oils" are usually a residual material produced from the fractional distillation of certain crude petroleum. 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.
There are several types of solvents that may be used to produce cutback asphalt, including petroleum-type solvents and coal types. A mixture of various solvents and an additive may also be used to improve final products (e.g. antistripping effect).
Liquid asphaltic materials come in six standard grades for each of these three types. MC stands for a medium cure, RC for a rapid cure. SC stands for a slow cure. Suffixes are numbers that indicate the consistency range of the material. There are six types of grades, grade 0 being the least liquid, and grade 5 being the most viscous, with intermediate grades ranging from less liquid to more viscous. As a rule of thumb, grade 0 materials are similar to heavy cream, and grade 5 materials are similar to heavy molasses in cold weather. Evaporation of the petroleum diluent is used to "cure" these products.
Cutback asphalt mixtures used to have an advantage due to their lower paving temperatures. However, nowadays cutback asphalt is used mostly for primer and/or tack coats.
Asphalts used for a prime coat, are spray applied to the top of untreated subgrade or base layers to fill voids, protect the subbase from moisture penetration, preserve fines and enhance the bond between the subbase and the subsequent pavement layers.
To improve bonding, hot mix asphalt pavement lifts are tacked with cutback asphalt. In order for the completed pavement structure to function as a single unit and provide adequate strength, this cutback promoted adequate bonding between construction lifts and especially between the existing road surface and overlay. When adjacent layers are not bonded, they behave as multiple, thin, independent layers, none of which are intended to withstand the bending forces imposed by traffic. Delamination (debonding) between layers can lead to longitudinal wheel path cracks, fatigue cracks, potholes, and other distress such as rutting, which reduces pavement life. To know more about this, contact the best bitumen suppliers in UAE.
Applications of Cutback Bitumen in Bituminous (asphalt) Pavement Construction and Maintenance:
-
Prime and Tack Coating
A primed aggregate base is usually unbound and is coated with a low viscosity binder. As a result, the top layer of the base of the road is intended to absorb this liquid, providing an easier surface for bituminous covering to adhere. This primer will allow traffic to carry on for a short period of time (although it is uncommon) and help control dust. Primers are typically applied between 0.5 and 1.4 liters per square meter. In addition, to use as primers, cutback bitumen is also used as tack coats, which are applied to underlying surfaces to improve the adhesion of the subsequent asphalt layer. In typical applications, 0.22 to 0.44 liters per square meter are applied.
-
Prime Sealing
It is possible to apply a primer seal for periods of 6 to 12 months if conditions are too cool for a proper primer process, or if traffic upsets primed surfaces before the final seal can be sprayed. Pre-mixed asphalt is made by mixing the cutback bitumen with pre-mixed bitumen that is suitable for primer sealing.
-
Spray Sealing
Cutting back bitumen is widely used for sprayed sealing applications, particularly in cooler weather where it provides improved initial stone retention due to its low viscosity. Bitumen cutbacks are typically sprayed onto the primed and aggregate-covered pavement in one application.
A short comparison between Bitumen, Cutback, and Emulsion
To reduce the viscosity of bitumen, it was necessary to heat it before applying.
A number of benefits are associated with cutback bitumen. The heating operation was abandoned following the production of the low viscosity bitumen. As a result of the volatile solvents in cutback bitumen, its viscosity was reduced, reducing the need for heating.
It was only the environmental impact of cutback bitumen that was of concern. Solvents that are volatile are poisonous to humans, nature, and the environment.
The industry was then introduced to emulsifiers and bitumen emulsion. Since only water evaporated, they were considered safe and non-toxic. Additionally, they were energy efficient.
Slow Curing Cutback Bitumen Specification (ASTM D2026)
| Slow Curing (SC) | SC-70 | SC-250 | SC-800 | SC-3000 | Test Methods | ||||
|---|---|---|---|---|---|---|---|---|---|
| Property | Min | Max | Min | Max | Min | Max | Min | Max | |
| Kinematic viscosity at 60°C, mm2/s | 70 | 140 | 250 | 500 | 800 | 1600 | 3000 | 6000 | ASTM D-2170 |
| Flash point (Cleveland open cup), °C | 66 | - | 79 | - | 93 | - | 107 | - | ASTM D-92 |
| Distillation test: | ASTM D-402 | ||||||||
| Total distillate to 360°C, volume % | 10 | 30 | 4 | 20 | 2 | 12 | - | 5 | |
| Solubility in trichloroethylene, % | 99.0 | - | 99.0 | - | 99.0 | - | 99.0 | - | ASTM D-2042 |
| Kinematic viscosity on distillation residue at 60°C, mm2/s | 400 | 7000 | 800 | 10000 | 2000 | 16000 | 4000 | 35000 | ASTM D-2170 |
| Asphalt residue: | ASTM D-243 | ||||||||
| Residue of 100 penetration, % | 50 | - | 60 | - | 70 | - | 80 | - | ASTM D-5 |
| Ductility of 100 penetration residue at 25°C, cm | 100 | - | 100 | - | 100 | - | 100 | - | ASTM D-113 |
| Water, % | - | 0.5 | - | 0.5 | - | 0.5 | - | 0.5 | ASTM D-95 |
Medium Curing Cutback Bitumen Specification (ASTM D2027 or AASHTO M82-75 2008)
| Medium Curing (MC) | MC-30 | MC-70 | MC-250 | MC-800 | MC-800 | Test Methods | |||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Property | Min | Max | Min | Max | Min | Max | Min | Max | Min | Max | |
| Kinematic viscosity at 60°C, mm2/s | 30 | 60 | 70 | 140 | 250 | 500 | 800 | 1600 | 3000 | 6000 | ASTM D-2170 |
| Flash point (Cleveland open cup), °C | 38 | - | 38 | - | 66 | - | 66 | - | 66 | - | ASTM D-92 |
| Distillation test: Distillate, volume percent of total distillate to 360°C: | ASTM D-402 | ||||||||||
| to 225°C | - | 35 | - | 25 | - | 20 | - | - | - | - | |
| to 260°C | 30 | 75 | 10 | 70 | 5 | 55 | - | 40 | - | 15 | |
| to 316°C | 75 | 95 | 65 | 93 | 60 | 90 | 45 | 85 | 15 | 75 | |
| Residue from distillation to 360°C, percent volume by difference | 50 | - | 55 | - | 67 | - | 75 | - | 80 | - | |
| Tests on residue from distillation: | |||||||||||
| Viscosity at 60°C, Pa | 30 | 120 | 30 | 120 | 30 | 120 | 30 | 120 | 30 | 120 | |
| Ductility at 25°C, cm | 100 | - | 100 | - | 100 | - | 100 | - | 100 | - | ASTM D-113 |
| Solubility in trichloroethylene, % | 99.0 | - | 99.0 | - | 99.0 | - | 99.0 | - | 99.0 | - | ASTM D-4 |
| Water, % | - | 0.2 | - | 0.2 | - | 0.2 | - | 0.2 | - | 0.2 | ASTM D-95 |
Rapid Curing Cutback Bitumen Specification (ASTM D2028 or AASHTO M81-92 2008)
| Rapid Curing (RC) | RC-70 | RC-250 | RC-800 | RC-3000 | Test Methods | ||||
|---|---|---|---|---|---|---|---|---|---|
| Property | Min | Max | Min | Max | Min | Max | Min | Max | |
| Kinematic viscosity at 60°C, mm2/s | 70 | 140 | 250 | 500 | 800 | 1600 | 3000 | 6000 | ASTM D-2170 |
| Flash point (Cleveland open cup), °C | - | - | 27 | - | 27 | - | 27 | - | ASTM D-92 |
| Distillation test: Distillate, volume percent of total distillate to 360°C: | ASTM D-402 | ||||||||
| to 190°C | 10 | - | - | - | - | - | - | - | |
| to 225°C | 50 | - | 35 | - | 15 | - | - | - | |
| to 260°C | 70 | - | 60 | - | 45 | - | 25 | - | |
| to 316°C | 85 | - | 80 | - | 75 | - | 70 | - | |
| Residue from distillation to 360°C, percent volume by difference | 55 | - | 65 | - | 75 | - | 80 | - | |
| Water, % | - | 0.5 | - | 0.5 | - | 0.5 | - | 0.5 | ASTM D-95 |
| Tests on residue from distillation: | |||||||||
| Viscosity at 60°C, Pa | 60 | 240 | 60 | 240 | 60 | 240 | 60 | 240 | |
| Ductility at 25°C, cm | 100 | - | 100 | - | 100 | - | 100 | - | ASTM D-113 |
| Solubility in trichloroethylene, % | 99.0 | - | 99.0 | - | 99.0 | - | 99.0 | - | ASTM D-4 |
| Water, % | - | 0.2 | - | 0.2 | - | 0.2 | - | 0.2 | ASTM D-95 |