ASPHALT BINDERS AND ASPHALT MIXTURES - Materials for Civil and Construction Engineers - page 331

9.6 Characterization of Asphalt

Many tests are available to characterize asphalt cement. Some tests are com-
monly used by highway agencies, while others are used for research. Since
the properties of the asphalt are highly sensitive to temperature, all asphalt
tests must be conducted at a specified temperature within very tight toler-
ances (The Asphalt Institute 1989).
Before the SHRP research, the asphalt cement specifications typically
were based on measurements of viscosity, penetration, ductility, and soften-
ing point temperature. These measurements are not sufficient to properly de-
scribe the viscoelastic and failure properties of asphalt cement that are needed
to relate asphalt binder properties to mixture properties and to pavement per-
formance. The new Performance Grade binder specifications were designed to
provide performance-related properties that can be related in a rational man-
ner to pavement performance (McGennis 1994).

■ Performance Grade Characterization Approach

The Performance Grade tests used to characterize the asphalt binder are per-
formed at pavement temperatures to represent the upper, middle, and lower
range of service temperatures. The measurements are obtained at tempera-
tures in keeping with the distress mechanisms. Therefore, unlike previous
specifications that require performing the test at a fixed temperature and vary-
ing the requirements for different grades of asphalt, the Performance Grade
specifications require performing the test at the critical pavement temperature
and fixing the criteria for all asphalt grades. Thus, the Performance Grade phi-
losophy ensures that the asphalt properties meet the specification criteria at
the critical pavement temperature.
Three pavement design temperatures are required for the binder specifi-
cations: a maximum, an intermediate, and a minimum temperature. The
maximum and minimum pavement temperatures for a given geographical
location in the United States can be generated using algorithms contained
within the SHRP software, based on weather information from 7500 weather
stations. The maximum pavement design temperature is selected as the highest
successive seven-day average maximum pavement temperature. The mini-
mum pavement design temperature is the minimum pavement temperature
expected over the life of the pavement. The intermediate pavement design
temperature is the average of the maximum and minimum pavement design
temperatures plus 4°C.
Laboratory tests that evaluate rutting potential use the maximum pave-
ment design temperature, whereas tests that evaluate fatigue potential use
the intermediate pavement design temperature. Thermal-cracking tests use
the minimum pavement design temperature plus 10°C (18°F). The minimum
pavement design temperature is increased by 10°C to reduce the testing

9.6.1