What is SHRP ?
History
Changing regulations and standards within the industry have created a shift away from penetration/viscosity based testing and towards performance grading. Performance grading correlates required physical properties of asphalt to temperature profiles for the planned area of usage.
Performance Grading uses a testing schedule for hot mix asphalt pavements called Superpave which was developed by the Strategic Highway Research Program (SHRP). Performance Graded Asphalt Cement or PGAC is evaluated at three critical stages of its lifecycle via special conditioning procedures. The first stage represents the transporting and handling of original asphalt binder before mixing. The second stage represents the short-term aging following HMA production and construction. The third stage considers the continued long-term aging during the pavement service life. The performance required of the binder in each analysis remains constant for all PG grades, but the temperature at which the properties must be achieved changes in consideration of the asphalt grade required for the project site, climate, traffic loading, and speeds anticipated.
Understanding PG grades
Each grade is represented by two numeric values representing the temperature rating of the binder in degrees Celsius. For example, a PG 70-22 is rated for a climate having an average 7-day maximum design temperature of 70 °C and a minimum pavement design temperature of -22 °C. The upper temperature rating is based upon the highest 7-day running average temperature of the pavement, 5 cm below the pavement surface. Tests and specifications are also established for modified asphalt binders.
Click here to view the PG-Chart used to identify a binder.
Advantages
Some advantages realized through SHRP testing include:
- Better characterization of the low - mid temperature binder performance leading to less fatigue failure and thermally induced cracking.
- More reproducible and precise test results often eliminate the need for re-testing, saving both time and money.
- Improved resistance to pavement rutting and shoving is realized through rheological testing which simulates actual loading cycles experienced by in service pavements.
- At upper service temperatures, as experienced in plant processes where pumping and mixing of hot liquid asphalt occurs, capillary viscosity is not reflective of actual liquid binder behavior. These properties are more accurately characterized by the Superpave testing schedule.
- Environmental and service conditions alter the performance of asphalt pavement over time. Superpave analysis evaluates pavement response to these factors.