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Superpave Mix Design

The Superpave gyratory compactor is a transportable device whose primary function is to fabricate test specimens by stimulating the effect of traffic on an asphalt pavement. The specimens fabricated with the gyratory compactor are used to determine the volumetric properties (air voids, voids in the mineral aggregate, and voids filled with asphalt) of Superpave mixes. Those properties, measured in the laboratory, indicate how well the mix will perform in the field. The gyratory compactor is also well-suited for quality control/quality assurance, as it can be set up at the job site to verify that the delivered asphalt mix meets the job mix volumetric specifications.

By kneading mixes to simulate construction compaction and traffic loads, the Superpave gyratory compactor provides specimens that are much more representative of actual in-service pavements. The level or amount of compaction is dependent on the environmental conditions and traffic levels expected at the job site.

To create a mix with a high degree of internal friction and thus a high shear strength, the Superpave mix design procedures include requirements for aggregate angularity and gradation. The design goal: a strong stone skeleton that will resist rutting, yet include enough asphalt and voids to improve the durability of the mix.

Predicting Pavement Performance

The Superpave system includes mix analysis procedures that predict how well a mix will perform in the field. These procedures are intended to provide additional information on asphalt mixes that will be placed in pavements with very high traffic volumes and loads. Two new, sophisticated pieces of laboratory equipment-the Superpave shear tester and the indirect tensile tester-are used to measure specific engineering properties of the laboratory-compacted asphalt mix. The test results are then entered into software models that predict how many equivalent single-axle loads the pavement will carry, or how much time will elapse, before a certain level of rutting, fatigue cracking, or low-temperature cracking develops.

The test procedures for the Superpave shear tester and the indirect tensile tester are currently being refined to ensure that the procedures are sound and that results are repeatable. The performance prediction models are also undergoing evaluation and validation and will be refined as necessary.

Making it Happen - Partners in Implementation

Switching to the all-new Superpave system is a big task, necessitating careful planning and coordination among all the partners in the highway industry.

To ensure that the switch to the Superpave system would be well planned and coordinated, the Federal Highway Administration (FHWA) established a national Asphalt Technical Working Group (TWG). Composed of representatives from highway agencies, suppliers, contractors, academia, and FHWA, the Asphalt TWG provides advice on how best to encourage the adoption of the Superpave system within the highway industry. Assisting the Asphalt TWG are several expert task groups composed of specialists in pertinent subject areas, such as binders, asphalt mixes, and pavement modeling.

The regional asphalt user-producer groups also play a key role in the Superpave implementation effort. Made up of highway agencies and companies that use and produce asphalt binders, the user-producer groups have outlined a sensible, planned strategy for adopting the Superpave system on a regional basis. They also provide a forum for discussing common concerns and arriving at workable solutions.

To bring the Superpave system and equipment closer to the users, five Superpave regional centers have been established. Under an FHWA program they will conduct a thorough and coordinated shakedown of the procedures used with the Superpave shear test and indirect tensile test. The centers, operated jointly by universities and State departments of transportation, will also provided training on a regional basis.

Getting Up To Speed

To help State highway agencies and contractors become proficient with the new Superpave procedures and devices, FHWA sponsors training courses in the use of the binder test equipment and mix equipment. The hands-on training is conducted at the National Asphalt Training Center, which FHWA established at the Asphalt Institute in Lexington, Kentucky.

During the first 18 months of Superpave training, the center held 30 courses in the Superpave binder and mix procedures and trained approximately 600 engineers and technicians. FHWA recently selected the Asphalt Institute to conduct the second phase of Superpave training. This phase will continue for the more advanced mix analysis equipment. Training and assistance will be taken directly to the users and the regional centers.

In addition, under an FHWA contract, the University of Maryland and its project team are evaluating, validating, and refining the software and pavement performance models that form the core of the Superpave mix analysis and performance prediction procedures. Once the Superpave software is ready for release, the university will provide technical assistance and training workshops.

Not all training is conducted in the laboratory. FHWA's mobile asphalt laboratory brings technical assistance and training in the Superpave system directly to highway agencies and contractors at job sites around the country.

Timeline for SUPERPAVE

The Asphalt TWG has set a target date of 1997 for nationwide implementation of the Superpave binder specification and 2000 for nationwide adoption of the Superpave volumetric mix design procedures. Most State highway agencies and many contractors already have the necessary binder test equipment and the Superpave gyratory compactor. Some States are already building Superpave pavements, and other projects are in the pipeline.

Better Roads Ahead

The Superpave system will mean major changes in the way we design asphalt mixes. It requires significant investments in equipment and training. But the extensive testing and validation conducted to date on the Superpave system indicate that Superpave pavements will last longer. Longer lasting pavements will mean lower maintenance costs and fewer highway work zones. And that means better roads ahead.