Asphalt binders specifications for paving applications generally rely on a limited number of properties. This set of properties is essentially linked to the consistency of the product as a function of temperature. Consistency is now replaced by a more scientific word, “rheology” (literally, the science of flow) and everybody now talks about the rheological properties of asphalt binders.

Still, the way these properties are quantified has evolved over time. In the absence of simple test procedures, early evaluation of bitumen rheology for specification purposes was based on tactile observations or even chewing, until Bowen proposed to quantify it by the needle penetration test in 1888 (see [1] for a historical review - the current verison of test is described below).

Now, essentially two ways to evaluate the rheological properties of asphalt binders coexist. The first one, based on penetration grades, is the official one in Europe and some other geographies, and relies on empirical tests such as penetration and ring and ball softening temperature. The second one, also known as Performance Grading (PG), is based on more intrinsic properties such as the complex modulus and is used for paving specifications in the USA.