# Absolute Viscosity

Viscosity is simply a measure of a fluid’s resistance to flow and is described by the following equation:

- Canonical URL: https://pavementinteractive.org/reference-desk/testing/asphalt-tests/absolute-viscosity/
- Last updated: 2026-11-01
- Review: reviewed by licensed professional engineers on the previous Pavement Interactive site, where reviewers were not recorded by name
- Standards: AASHTO T 202, ASTM D2171

Viscosity is simply a measure of a fluid’s resistance to flow and is described by the following equation:

$$\mu = \frac{\tau}{\gamma}$$

where:

- $$\mu$$ = viscosity (in cgs units of poise). poise = dyne-sec/cm² = g/cm-sec (the SI unit of viscosity is the Pa-sec = N-sec/m² = 10 poise)
- $$\tau$$ = shear stress
- $$\gamma$$ = shear rate

[Asphalt binder](https://pavementinteractive.org/glossary/asphalt-binder/) viscosity is typically measured at 60° C (140° F) because it approximates the maximum HMA pavement surface temperature during summer in the U.S.

The basic absolute viscosity test measures the time it takes for a fixed volume of asphalt binder to be drawn up through a capillary tube by means of vacuum, under closely controlled conditions of vacuum and temperature (ASTM, 2003 [^1]). Although absolute viscosity is an improvement over the [penetration test](https://pavementinteractive.org/reference-desk/testing/binder-tests/penetration-test/), it still only measures viscosity at one temperature and thus does not fully characterize an asphalt binder’s consistency over the expected range of construction and service conditions.

## Standard Test Methods

- AASHTO T 202 and ASTM D 2171: Viscosity of Asphalts by Vacuum Capillary Viscometer

## Sources

[^1]: American Society for Testing and Materials (ASTM) (2003). Annual Book of ASTM Standards, Volume 04.03, Road and Paving Materials; Vehicle-Pavement Systems. ASTM International. West Conshohocken, PA.