**Test 5. Viscosity Tests** on Bitumen Asphalt

**Test 5. Viscosity Tests**on Bitumen Asphalt

The viscosity test measures the viscosity of an asphalt. Both the viscosity test and the penetration test
measure the consistency of an asphalt at some specified temperatures and are used to designate grades
of asphalts. The advantage of using the viscosity test as compared with the penetration test is that the
viscosity test measures a fundamental physical property rather than an empirical value.
**Viscosity is defined as the ratio between the applied shear stress and induced shear rate of a fluid.**

When shear rate is expressed in units of 1/sec. and shear stress in units of Pascal, viscosity will be in
units of Pascal-seconds. One Pascal-second is equal to 10 Poises. The lower the viscosity of an asphalt,
the faster the asphalt will flow under the same stress.
**For a Newtonian fluid, the relationship between shear stress and shear rate is linear**, and thus **the
viscosity is constant at different shear rates or shear stress**. However,** for a non-Newtonian fluid, the
relationship between shear stress and shear rate is not linear**, and thus the **apparent viscosity will change
as the shear rate or shear stress changes**.

Asphalts tend to behave as slightly non-Newtonian fluids,
especially at lower temperatures. When different methods are used to measure the viscosity of an asphalt,
the test results might be significantly different, since the different methods might be measuring the
viscosity at different shear rates. *It is thus very important to indicate the test method used when viscosity
results are presented.*

The most commonly used viscosity test on asphalt cements is the Absolute Viscosity Test by Vacuum Capillary Viscometer (ASTM D2171).

The standard test temperature is 60 °C. The absolute viscosity test
measures the viscosity in units of **Poise**. The viscosity at 60 °C represents the viscosity of the asphalt at
the maximum temperature a pavement is likely to experience in most parts of the U.S.
When the viscosity of an asphalt at a higher temperature (such as 135 °C) is to be determined, the
most commonly-used test is the Kinematic Viscosity Test (ASTM D2170), which measures the kinematic
viscosity in units of Stokes or centi-Stokes.

Kinematic viscosity is defined as:
When viscosity is in units of Poise and density in units of g/cm, 3 the kinematic viscosity will be in
units of Stokes. To convert from kinematic viscosity (in units of Stokes) to absolute viscosity (in units
of Poises), one simply multiplies the number of Stokes by the density in units of g/cm^{3}.

* Read a detailed procedure of ***Bitumen Penetration Test** with Sample Data

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