US System Specification
In the United States, Congress established the Strategic Highway Research Program (SHRP) in 1987 as a US $150 million research program to improve the performance and durability of US roads. US $50 million was used to develop performance-based specifications.
This SHRP (Sharp) research resulted in the SuperPave (Superior Performing Asphalt Pavements) specification system, now widely applied in North America and beyond.
Superpave examines rutting, low temperature cracking and fatigue cracking for:
- Binder specification
- Mixture design
- Analysis system
This specification system evaluates bitumen binders and gives results on a Performance Grade (PG) scale.
Kraton Polymers
The use of Kraton polymer modification can help to extend the temperature range in the PG system. This will enable the formulation of binders for extreme climates, for heavy and/or slow moving traffic conditions. The choice of base bitumen hardness will determine the level of minimum and maximum temperatures.
Measurements
The following test methods are applied:
High temperature properties
- For the high temperature properties relevant for permanent deformation the Dynamic Shear Rheometer is used to determine complex modulus and phase angle shift.
Low temperature properties
- For low temperature performance relevant for low temperature cracking, a combination of the Bending Beam Rheometer (BBR) and the Direct Tensile Test (DTT) is used to determine a critical temperature.
Short term aging properties
- For short term aging conditioning of samples, the Rolling Thin Film Oven Test (RTFOT) is applied to simulate the aging in processing.
Long term aging properties
- For simulation of long term aging to condition the sample, the Pressure Aging Vessel (PAV) test is used to mimic the effect of aging over the pavement's lifetime.
Grading
The grades are divided according to their lowest temperature and highest temperature for use, with adjustments made for heavy and or slow moving traffic. This results in the PG grades (Performance Grades) which have now become common place in the US.
