Viscosity test of bitumen

Viscosity is the property of a fluid that resists flow. For bituminous binders it determines the temperatures at which the material is suitable for spraying, mixing and compaction, so it is a measure of the flow property, or consistency.

Methods

1. Orifice (efflux) viscometers - Saybolt Furol and the Standard Tar Viscometer (STV), which measure the time taken for a fixed volume of binder to pass through a standard orifice (used for cutbacks, tars and emulsions).
2. Capillary viscometers - kinematic viscosity at 135 degrees C and absolute viscosity at 60 degrees C (vacuum capillary viscometer, used for VG grading in IS:73).
3. Rotational viscometer (Brookfield) - for modified binders.

Standard Tar Viscometer (road tar viscometer) test (IS:1206)

Apparatus: A brass cup (about 100 mm in diameter) with an orifice of 10 mm (4 mm for the more fluid grades) at the bottom, closed by a ball valve; the cup sits in a water jacket with a heater, thermometer, stirrer, a graduated receiver (100 ml) and a stopwatch.

     thermometer
        |   __ __ 
     |  v  |    | cup (binder)
     |_____|    |
     | water jacket (heated) |
        \ _ /   <- orifice 10 mm
          |
        [receiver, 25-75 ml marks]

Procedure:

1. Select the orifice size and the test temperature (for example 25, 40 or 60 degrees C for cutbacks and tars).
2. Heat the water bath to the test temperature and bring the binder in the cup to that temperature (stirring for uniform temperature), with the orifice closed by the valve.
3. Fill the cup to the mark with the binder.
4. Place the receiver under the orifice and lift the valve so that the binder flows.
5. Start the stopwatch when the binder in the receiver reaches the 25 ml mark and stop it when it reaches the 75 ml mark.
6. The time in seconds for this 50 ml to flow is the viscosity value (STV seconds; Saybolt Furol seconds in the Saybolt test). Repeat the test and take the average.

Significance

• Gives the proper grade of binder (for example, the VG-10 to VG-40 classification by viscosity) and shows whether it is suitable for the climate.
• Gives the mixing and compaction temperatures (for example, the temperatures at which the viscosity is 170 +/- 20 cSt for mixing and 280 +/- 30 cSt for compaction).
• Shows the suitability of a cutback or emulsion for spraying, and is used for quality control.