How to grade bitumen

Bitumen used for paving grades are categorized according to viscosity (degree of fluidity) gradings. The higher the grade, the stiffer the bitumen gets.

Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms (and for fluids only), viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity. Put simply, the less viscous the fluid is, the greater its ease of movement (fluidity).


The test procedures used for paving grades of bitumen are as follows:

• Penetration Test;  used to determine the consistency of bitumen by measuring the distance that a standard needle will penetrate vertically into a sample
• Viscosity Test; is a more scientific measure of consistency than Penetration. Various tests are used to measure the resistance to flow of bitumen and to thereby define its consistency.
• Flash Point: used to measure the temperature to which a sample of bitumen may be safely heated by establishing the temperature at which a small flame causes the vapour above the sample to ignite or flash.
• Ductility: gives an indication of the extent to which a sample of the material can be stretched before breaking. A standard briquette of bitumen, placed in a mould in a water bath heated to 15°C, is pulled apart, usually at a speed of 5 cm per minute. The length of the thread of bitumen at the moment when it breaks, expressed in centimetres, is the ductility of the sample.
• Solubility and the Presence of Insolubles: indicates the degree of contamination of the bitumen by other matter and therefore the presence of pure bitumen. The Australian test measures the percentage of matter that is insoluble in toluene.
• Effect of Heat and Air: is determined to simulate the conditions obtained when the bitumen is used to manufacture hot-mix. In the Rolling Thin Film Oven Test a moving film of bitumen is heated in an oven at 163°C for 60 minutes. The viscosity is measured before and after treatment.
• Softening Point: a measurement of the temperature at which a sample of bitumen held in a ring in a water bath allows a steel ball of specified weight to fall to a point at a specified distance below it.

The chart below shows typical applications for paving grade of BP bitumens:

Applications for Bp class
Bitumen class	Sprayed Sealing	Asphalt
		Light	Medium	Heavy	Extra Heavy
Class 170	ok	ok
Class 320		ok	ok	ok
Class 600				ok	ok

AASHTO M 226 and ASTM D 3381 Viscosity Grades

Standard	Grading based on Original Asphalt (AC)						Grading based on Aged Residue (AR)
AASHTO M 226	AC-2.5	AC-5	AC-10	AC-20	AC-30	AC-40	AR-10	AR-20	AR-40	AR-80	AR-160
ASTM D 3381	AC-2.5	AC-5	AC-10	AC-20	AC-30	AC-40	AR-1000	AR-2000	AR-4000	AR-8000	AR-16000

Following table is the specification for performance grade bitumen

PROPERTY	GRADES				STANDARD
	PG 76-10		PG 82-10
	Min	Max	Min	Max
Flash Point, COC, °C	230	-	230	-	T48
Viscosity @ 135°C, Pa·s	135°C	3.0	135°C	-	ASTM D4402
Dynamic Shear @ 70°C, G*/sin d, kPa	1.00	-	-	-	-
After RTFO	-	-	-	-	-
Dynamic Shear @ 70°C, G*/sin d, kPa	2.20	-	-	-	-
Mass Loss, %	-	1%	-	1%	-
After PAV @ 100°C	-	-	-	-	-
Dynamic Shear @ 28°C, G*·sin d, kPa	-	5000	-	-	-
Creep Stiffness @ -12°C, S, MPa	-	300	-	-	-
Creep Stiffness @ -12°C, M-value	0.300	-	-	-	-

Bitumen test for Viscosity

Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms (and for fluids only), viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity. Put simply, the less viscous the fluid is, the greater its ease of movement (fluidity).

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/cm3.