2. What is cutback bitumen?
● Cutback bitumen is a liquid bitumen obtained by dissolving
penetration bitumen in a solvent. The solvents are obtained from the
oil distillation process in refineries.
● Depending on the drying time or, in other words, the rate of solvent
evaporation, cutback bitumen can be classified into three types
1. Slow-Curing (SC)
2. Rapid-Curing (RC)
3. Medium-Curing (MC)
3. BASED ON DRY TIME
● Cutback bitumen 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.(1)
● Rapid curing grades are used in tack coats, chips seals and sand
seals.
4. 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 bitumens 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.
5. 3- Slow Curing (SC)
Slow curing bitumen 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
6. BASED ON VISCOSITY RANGE
Each cutback bitumen type has different grades based on its viscosity
range, which is controlled by solvent amount.
The higher the amount of solvent, the lower the viscosity of cutback
bitumen. As a result, each type of cutback bitumen has grades with
different fluidity.
Due to the higher amount of solvent, some can flow easily at room
temperature. Also, there are grades with higher viscosity or lower
amount of solvent that must be heated to flow in order to use.
7. The viscosity or fluidity of a cutback bitumen is measured at 60 degrees
Celsius by ASTM D2170 standard method. Each grade belongs to one
viscosity range.
For example, the viscosity value of an MC cutback bitumen is 83 cSt.
According to the below table, it is in the range of 70 to 140 cSt. As a
result, the cutback bitumen is MC-70.
