Superpave System
Pavements constructed of hot-mix asphalt are typically designed to last at least 20 years. Yet despite our best efforts, it is not uncommon to see severe rutting and cracking in asphalt pavements well before then, as environmental conditions and heavy traffic loadings take their toll. The result: rough rides, higher pavement maintenance and rehabilitation costs, and more work zones for motorists to negotiate.
The Superpave (SUperior PERforming Asphalt PAVEments) system was developed to give highway engineers and contractors the tools they need to design asphalt pavements that will perform better under extremes of temperature and heavy traffic loads.
Asphalt pavements account for more than 90 percent of all paved highways in the United States, and annual expenditures for asphalt pavements top $10 billion. If asphalt pavements can be designed to last longer we stand to reap the substantial benefits.
The Superpave system primarily addresses two pavement distresses: permanent deformation, which results from inadequate shear strength in the asphalt mix; and low temperature cracking, which is generated when an asphalt pavement shrinks and the tensile stress exceeds the tensile strength.
The Superpave system consists of three interrelated elements:
- Asphalt binder specification.
- Volumetric mix design and analysis system.
- Mix analysis tests and a performance prediction system that includes computer environmental and performance models.
Taking the Guesswork out of Binder Selection
Conventional viscosity and penetration tests for asphalt binders do not measure low-temperature properties. As a result, it can be difficult to select a binder that will work best under specific conditions. Two binders with the same viscosity grade might, for example, have the same consistency at the specified test temperature, but they might behave dramatically different at very low temperatures or at high pavement temperatures.
In contrast, the Superpave binder specification is a performance-based specification. It classifies binders into performance grades, based on a range of climates and pavement temperatures. The physical properties required for the binder are the same for all grades, but the temperature at which those properties must be attained is determined by the specific climatic conditions at the paving location. The specification applies to all unmodified binders and many modified binders.
Superpave binders are designated with a "PG" (performance grade) rating. The first number in the rating indicates the high-temperature grade; the second indicates the low-temperature grade. For example, a binder classified PG58-28 would meet the required physical properties at pavement temperatures as high as 58 degrees C and as low as -28 degrees C. The mix designer selects a Superpave binder based on the climate in which the pavement will serve and the traffic it will bear.
Three new tests are used to measure the physical properties of Superpave binders:
- Dynamic shear test, which measurer the binder's stiffness and phase angle at intermediate and high temperatures.
- Bending beam test, which measures the low-temperature stiffness of the binder.
- Direct tension test, which measures the low-temperature tensile and fracture properties.
The tests mimic actual environmental and traffic conditions at the project site. The results of the dynamic shear test indicate the binder's ability to withstand permanent deformation (which is often evidenced as rutting in the pavement) and fatigue cracking. The bending beam test results are used to predict low-temperature cracking problems. The results of the direct tension test provide additional information on how the hinder will perform at low temperatures.
