Viscosity Test of Bitumen: Methods, Grades and Limits

A viscosity test of bitumen puts a number on how thick the binder is at a chosen temperature. Viscosity replaced the older penetration test as the basis for grading in many places because it is a true physical property measured in real units. It tells a plant how hot to run, tells a designer how the binder will hold up on a summer afternoon, and is the basis of the VG grades used in India. Here is how the main methods work and what the common limits are.

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Quick facts

Absolute (dynamic) viscosity: at 60 degrees C, vacuum capillary, ASTM D2171 / AASHTO T 202. Reported in poise.
Kinematic viscosity: at 135 degrees C, gravity capillary, ASTM D2170 / AASHTO T 201. Reported in centistokes (cSt).
Rotational viscosity: usually at 135 degrees C, spindle viscometer, AASHTO T 316 / ASTM D4402. Reported in Pa·s.
Units: 1 Pa·s = 10 poise = 1,000 centipoise.

Why two temperatures?

60 degrees C (140 degrees F) is close to the hottest temperature a dark pavement reaches in summer, so viscosity there is a stand-in for rutting resistance. 135 degrees C (275 degrees F) is in the range where binder is pumped, sprayed and mixed, so viscosity there predicts handling. A binder can be stiff enough at 60 and still too runny or too thick at 135, which is why grading systems check both.

Absolute viscosity at 60 degrees C

At 60 degrees C bitumen is too thick to flow through a narrow tube under its own weight, so the absolute viscosity test pulls it through with a controlled partial vacuum. The binder sits in a calibrated glass capillary viscometer inside a bath. The operator times how long the binder front takes to pass between marks, then multiplies by the tube's calibration factor to get viscosity in poise. This was the basis of the older AC grades in the United States. AC-20, for example, was specified at 2,000 ± 400 poise in AASHTO M 226.

Kinematic viscosity at 135 degrees C

At mixing temperature the binder is fluid enough to flow by gravity. The kinematic test times that flow through a different capillary design and reports the result in centistokes. Because gravity is doing the work, the result depends on the binder's density as well as its stickiness. Divide dynamic viscosity by density and you get kinematic viscosity.

Rotational viscometer (Superpave)

The rotational test, often called the Brookfield test after a common instrument maker, places a small sample of hot binder in a heated chamber and spins a cylindrical spindle in it at a set speed. The torque needed to keep it turning is converted to viscosity. It is faster than capillary methods and works on modified binders that can plug a glass tube. AASHTO M 320 limits unaged PG binder to 3 Pa·s at 135 degrees C, a pumpability check rather than a performance one.

VG grades in IS 73

India grades paving bitumen by viscosity at 60 degrees C:

VG-10: 800 to 1,200 poise; kinematic at 135 degrees C at least 250 cSt.
VG-20: 1,600 to 2,400 poise; at least 300 cSt.
VG-30: 2,400 to 3,600 poise; at least 350 cSt.
VG-40: 3,200 to 4,800 poise; at least 400 cSt.

The same standard also checks viscosity after short-term aging in the rolling thin film oven. A big jump in viscosity after aging points to a binder that hardens fast.

Picking mixing and compaction temperatures

Labs plot viscosity against temperature on a special chart and read off two bands. For unmodified binders, the Asphalt Institute guidance sets lab mixing where viscosity is 0.17 ± 0.02 Pa·s and lab compaction where it is 0.28 ± 0.03 Pa·s, and recommends not mixing above 177 degrees C (350 degrees F). Polymer-modified binders do not follow that chart well, so suppliers give their own temperatures.

Viscosity vs penetration

The penetration test measures consistency at 25 degrees C with a needle, an empirical result in tenths of a millimeter. Viscosity is measured in real units at the temperatures that matter for rutting and handling. Two binders with the same penetration can behave quite differently at 60 degrees C, which is the main reason agencies moved to viscosity grading and later to PG grading built on the dynamic shear rheometer.

Questions people ask

What does a viscosity test of bitumen measure?

It measures how strongly the binder resists flow at a set temperature. Tests at 60 degrees C describe how the binder behaves on a hot pavement, and tests at 135 degrees C describe how easily it pumps and mixes at the plant.

What are the main viscosity tests?

Absolute viscosity at 60 degrees C with a vacuum capillary tube (ASTM D2171), kinematic viscosity at 135 degrees C with a gravity-flow capillary (ASTM D2170), and rotational viscosity with a spindle viscometer (AASHTO T 316, ASTM D4402), which is the method used in the Superpave PG system.

What is the viscosity of VG-30 bitumen?

Indian standard IS 73 sets the absolute viscosity of VG-30 at 60 degrees C between 2,400 and 3,600 poise, with a minimum kinematic viscosity of 350 cSt at 135 degrees C.

What is the 3 Pa·s limit?

AASHTO M 320, the PG binder specification, caps the rotational viscosity of unaged binder at 3 Pa·s at 135 degrees C so it can be pumped and handled. The limit can be waived if the supplier warrants the binder can be pumped and mixed at safe temperatures.

How are mixing and compaction temperatures picked from viscosity?

For unmodified binders, the Asphalt Institute method uses the temperature range where viscosity is 0.17 plus or minus 0.02 Pa·s for lab mixing and 0.28 plus or minus 0.03 Pa·s for lab compaction. Modified binders follow supplier guidance instead.

What is the difference between poise and Pa·s?

They are both units of dynamic viscosity. One Pa·s equals 10 poise, so 2,000 poise is 200 Pa·s. Kinematic viscosity, reported in centistokes (cSt), is dynamic viscosity divided by density.

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