It’s defined as the fluid property of the material as resistance to flow
To determine the viscosity of bitumen by tar viscometer as per IS: 1206 (Part 2)
Tar viscometer, cup, valve, receiver, thermometer etc.
1- Preparation ofthe Sample – Heat the sample to a temperature not more than 60°C for the tars and pitches and not more than 90°C for bitumens above their respective approximate softening point temperature respect ively until it has become sufficiently fluid to pour. Transfer about 20 ml into a suitable container and maintain it to a temperature of 135 ± 5.5°C stirring occasionally to prevent local overheating and allow the entrapped air to escape.
2- Charge the viscometer by pouring the prepared sample to within ± 2 mm of flll line E. Place the charged viscometer in an oven or bath maintained at 135 ± 5.5°C for a period of 10 ± 2 min to allow large air
bubbles to escape.
3- Testing – Maintain the bath at the te st temperature within ± O.I°C. Place the charged viscometer vertically in the waterbath with the help of a holder so that the uppermost timing mark is at least 2 embelow the surface of the bath liquid.
4- Establish a vacuum of 30 ± 0.05 em of mercury in the vacuum system and connect it to the viscometer with the valve closed. After the viscometer has remained in the bath for 30 ± 5 min open the valve and allow the asphalt to flow into the viscometer. Measure to within ± 0.5 s the time required for the leading edge of the meniscus to pass between successive pairs of timing marks.
5- Upon completion of the test, remove the viscometer from the bath and place it in an inverted position in an oven maintained at 135 ± 5°C until asphalt is drained off thoroughly from the viscometer.
6- Clean the viscometer thoroughly by rinsing several times with an appropriate solvent completely. Dry the tube by passing a slow stream of filtered dry air through the capillary for 2 minutes.
7- Periodically clean the instrument with chromic acid to remove organic deposits. Rinse thoroughly with distilled water and acetone and dry with clean air.
Calculate and report the absolute viscosity to three significant figures, by the following equation:
Viscosity Poises = Kt
K= selected calibration factor, in poise per second; and
t = flow time, in seconds.
The duplicate test results should not differ by more than the following:
1- Repeatability 7 percent of their mean.
2- Reproducibility 10 percent of their mean
Significance of viscosity test
1- Too viscous bitumen would result in inadequate and non-uniform coating of the aggregates.
2- Very low viscosity would again result in inadequate coating as the bitumen will tend to bleed.
3- Therefore viscosity at 135oC is a true reflection of the quality of bond that is likely to be formed with the aggregate.