CUTBACK BITUMEN is a range of binders that are produced by adding a Hydro Carbon Solvent such
as Paraffin or Mineral Turpentine to the penetration grade bitumen and mixing them. As the solvent
evaporates, the binder returns to its original form (penetration grade) to stick the particles with
one another. Cutback bitumen gets the name from the solvent which is used in the process, because the
solvent cuts back or evaporates, leaving the binder on the surface to do the rest of the work.
Three types of solvents, based on the time of evaporation are used in the mixture:
SLOW CURING (SC), MEDIUM CURING (MC) and RAPID CURING (RC) SOLVENTS.
A rapid curing (RC) solvent will disappear faster than a medium curing
(MC) solvent and the same relation works between MC and SC. Evaporation
rate of the solvent which effects the setting time of the bitumen,
determines the curing. The viscosity of the cutback however,
is dependent on the proportion and amount of solvent added:
the higher the amount, the lower the viscosity of the cutback.
The ability to reshape is a great advantage the cutbacks have
over the common penetration grade bitumen. It also requires
less heat to liquify which makes cutback grades easier to use at lower temperatures.
Widely used cutback bitumen grades are MC 30 and RC 250.
The letters refer to the curing speed of the solvent, and the number determines the viscosity of the product.
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.
Environmental regulations. Cutback asphalts contain volatile chemicals that evaporate into the air.
Emulsified asphalts evaporate water into the atmosphere and can be considered an eco-friendly product.
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.
MANUFACTURING OF CUTBACK BITUMEN
Cutback bitumen are manufactured by blending either 70/100 penetration grade or
the 160/220 with kerosene to comply with the desired viscosity specification.
The main application of cutback bitumen is in the surface dressing but a notable
amount is also used in Manufacturing of both deferred and standard set asphalts.
In addition to STV tests and solubility, cutback bitumen grades have to be in compliance
with a distillation specification I621 and should also fulfil the penetration requirements
on the residual bitumen. This ensures that during application and in service, the solvent
(diluent) will evaporate at a predictable rate with consistency and that the residual bitumen
will have the appropriate properties in the project. The suffix X on the SHELPHALT range of
cutback bitumen, demonstrates that they have been doped with a heat-stable passive adhesion agent
special formulation. This additive contributes in the wetting of the aggregates and builds up a
resistance stripping of the binder from the aggregates in the presence of water.
AASHTO categorize the cutback bitumen into 3 types: rapid curing (RC) where the solvents used are petrol/gasoline, medium curing (MC) made with kerosene and slow curing (SC) which contains diesel oil. A capillary tube viscometer is used to measure the viscosity at 60C. The flow in the viscometer is highly affected by the gravity, and the vacuum is not needed. The viscosity here uses centistokes as the unit whereas, for pure bitumen measuring unit is poise. The units of strokes and poise are related to one another through the density of the tested material.
The British Standard demonstrates specifications for three viscosity grades of cutback bitumen intended surface dressings applications. Measurement is using a discharge viscometer, and viscosity is defined with the time in seconds for 50millilitres of the binder to flow through a standard orifice at 40C.
SPECIFICATIONS OF CUTBACK BITUMEN
Cutback bitumen is a mixture of penetration grade bitumen and petroleum solvents. Type and characteristics of the solvent used determines the speed (rate) at which the bitumen will cure or set up when exposed to air. A rapid-curing (RC) solvent will evaporate faster than a medium-curing (MC) solvent. The viscosity of the cutback bitumen is determined by the proportion of solvent added – the higher the proportion of solvent, the lower is the viscosity of the cutback.
ADVANTAGE OF CUTBACK BITUMEN
It can be easily applied at lower temperatures than penetration grades because of its lower Bitumen Solvents viscosity.
A disadvantage is that cutback bitumen consumes non-renewable energy resources which are eventually lost during evaporation.
APPLICATIONS OF CUTBACK BITUMEN IN BITUMINOUS (ASPHALT) ROAD CONSTRUCTION, PAVING AND MAINTENANCE:
PRIME & TACK COATING
The process of priming includes a low viscosity binder to a prepared (usually unbound) aggregate base.
It is intended to be blended by the top layers of the base and to be absorbed by it in order to provide
a surface which can easily be wetted by another bituminous covering afterwards. The primer will be able
to carry traffic for a short time (though it’s uncommon in practice) and helps in controlling the dust.
Usually, primers are applied at a rate between 0.5 and 1.4 L/m2. Cutback bitumen grades are an excellent
choice for priming and are also used for tack coating, being used to an underlying surface to help in the
adhesion built between the first layer and the subsequent asphalt layer.
In locations with cold temperatures that interrupt an effective priming operation, or where traffic is likely to damage a
primed surface before spraying the final seal layer, a primer seal can be applied to give enough protection of the pavement
surface for periods of up to 6 to 12 months. These cutback grades can also be used in the process of manufacturing the
pre-mix asphalt, which is used in patch repairs.
Cutback bitumen types are widely used in spraying sealing applications, particularly in low
temperatures where they offer a much better initial stone retention because of their lower viscosity.
Commonly, a single application of the suitable cutback bitumen is sprayed onto the primed pavement where aggregates lay.
CUTBACK BITUMEN STANDARD
ASTM D 2026, D 2027and D 2028for SC, MC and RC.
AASHTO M 81, M 82 for RC and MC.
EN 15522 Cut back and Fluxed Bituminous Binder.