DETERMINATION OF ABSOLUTE VISCOSITY
In all bituminous works bitumen is required to be heated up to a definite temperature, so that the specific grade of bitumen may reach at its proper viscosity label, so that it can be mixed correctly with the aggregates to get a homogenous mix.
Definition of Absolute or Dynamic Viscosity of a Newtonian Liquid
It is defined as an internal friction, such that if a tangential force of one dyne (0.00001 N) acting on planes of unit area separated by unit distance of the liquid produces unit tangential velocity, the CGS unit for the viscosity of the liquid is 1 Poise (Dyne.Sec/Cm2).
Definition of Newtonian Liquid
A liquid in which the shear stress is directly proportional to the rate of shear strain. The constant ratio of shear stress to the rate of shear strain is defined as the coefficient of viscosity of the liquid. If this ratio is not constant then the liquid is called as a non-Newtonian liquid.
To determine the properties that may contributed towards the viscosity grade of bitumen.
(i) Cannon-Manning Vacuums Viscometer
(iii) Timing Devise (Stop watch)
|Cannon – Manning vacuum capillary Viscometer|
Heat the sample to a temperature less than 90°C for bitumen above their individual approximate softening point temperature awaiting it has become sufficiently fluid to pour. Pour about 20 ml into a suitable container and maintain it to a constant temperature of 135 ± 5.5°C stirring intermittently to stop local overheating and permit the entrapped air to escape.
Charging of the viscometer is done by pouring the prepared sample to within ± 2 mm of fill line E. Keep the charged viscometer in an oven or bath at a maintained temperature of 135 ± 5.5°C for a period of 10 ± 2 min to allow large air bubbles to flow out.
Uphold the bath at the test temperature within ± 0.1 °C. Place the charged viscometer vertically in the water bath with the help of a holder so that the uppermost timing mark is at least 2 cm below the surface of the bath liquid. Create a vacuum of 30 ± 0.05 cm of mercury in the vacuum system and connect it to the viscometer with the valve closed. After the viscometer has continued in the bath for 30 ± 5 min open the valve and allow the bitumen to flow into the viscometer. Measure to within ± 0.5 sec the time required for the foremost edge of the meniscus to pass between successive pairs of timing marks. After completion of the test, take out the viscometer from the bath and place it in an upturned position in an oven maintained at 135 ± 5°C until bitumen is exhausted off thoroughly from the viscometer. Clean the viscometer thoroughly by washing several times with an appropriate solvent fully. The tube should be dried by passing a slow stream of filtered dry air through the capillary for 2 minutes. Occasionally the instrument shall clean with chromic acid to remove organic deposits if any. Finally wash thoroughly with distilled water and acetone and dry in clean air.
Result and Reporting
Determine and report the absolute viscosity to three significant figures ,by the following equation: Viscosity Poises = Kt
Where, K= selected calibration factor, in poise per second
t = flow time, in seconds.
Always report the test temperature and vacuum with the viscosity test results. For example, viscosity at 60°C, 30 cm Hg Vacuum in poises.
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