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	-	-	-	-