Superpave Gyratory Compactor

Starting in 2014, the California Department of Transportation (Caltrans) will transition to a new performance-based standard for testing hot-mix asphalt (HMA). The new standard, expected to be implemented state-wide July 1, 2014, is called Superior Performing Asphalt Pavement, or “Superpave.” Superpave is the product of a $150 million research program that was established in 1987 to improve the performance of roadways throughout the United States. The new standard involves a comprehensive method of designing HMA mixes that are specifically oriented to performance requirements, factoring in anticipated traffic patterns, climate, and layer locations.

California now joins most of the United States and Europe in the use of Superpave. Pilot projects are currently underway at several sites in California. According to Caltrans, the decision to transition to Superpave technology was spurred by a number of circumstances, including increases in traffic loadings and volumes, changes in axle configurations, dwindling sources of aggregate, and difficulties repairing and calibrating outdated testing equipment. Under current Caltrans specifications, pavements typically last 10 to 20 years and usually require maintenance or repair during that time frame. With the new Superpave standard, the goal is pavement with less deterioration issues such as rutting, low temperature cracking, and fatigue cracking, resulting in nearly maintenance-free pavement with a longer lifespan of 20 to 30 years.

New Test Methods and Equipment
Superpave utilizes a gyratory compactor to compact test specimens as part of the mix design process, and to provide quality control/quality assurance testing during construction. Components of the compactor include a rigid reaction frame, a loading system, and a device to measure the specimen height. The mixture is compacted by applying a constant load and then subjecting the sample to a gyratory kneading action. The gyratory compactor is able to compact the specimens using motions similar to those that occur during the field placement operations. The process allows the engineer to select appropriate binders, aggregate gradations, and modifiers to achieve the desired level of performance. Specimens fabricated with the gyratory  compactor are used to determine the volumetric properties (density, 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. In addition to the gyratory compactor, a Hamburg Wheel Tracker is used to measure the rutting and moisture susceptibility of the gyratory-compacted HMA samples. A steel wheel weighing approximately 158 pounds is rolled across the surface of compacted HMA specimens that are immersed in a water bath maintained at a temperature of approximately 122 to 144 degrees Fahrenheit, depending upon the grade of the asphalt binder. The Hamburg Wheel Tracker measures the combined effects of rutting and moisture damage under the worst conditions expected for new pavement, such as heavy loads, hot weather, and water.

Evaluation of Rehabilitation Techniques
As the Hamburg Wheel Tracker simulates the actual conditions that traffic applies to HMA roadways, it can also be used to assist local cities and other governing agencies in assessing the use of new rehabilitation techniques and procedures for existing pavement. Cores or slabs of asphalt can be removed from the roadway after a renovation test strip is completed. These samples can then be tested to compare the performance of the test strip to that of new pavement or, in some cases, the existing pavement. With the resulting information, the client can make a better value assessment as to which technique or procedure or both would produce the most cost- effective solution for their particular circumstances.

Impact on HMA Suppliers
To create a mix with a high degree of internal friction and good shear strength, Superpave mix design procedures include refined requirements for aggregate angularity and gradation. The design goal: a strong HMA mix that resists rutting, yet includes enough asphalt and voids to improve durability. What does this mean for contractors working on Caltrans projects? Only higher quality aggregate will be able to meet the more stringent requirements. Fewer sources of high quality aggregate will likely translate to higher aggregate costs for contractors working on Caltrans projects. Lesser quality aggregate will be limited to use in pavement constructed by private developers or projects under the jurisdiction of local agencies that do not utilize Caltrans funds.

Earth Systems—Caltrans Certified for Superpave Testing
Earth Systems now offers testing of HMA to the new Superpave standard. A gyratory compactor and Hamburg Wheel Tracker are currently available through our Palmdale office, which is certified by Caltrans to perform the Hamburg Wheel Tracker and gyratory tests. Samples can be submitted at any of our California locations for testing in Palmdale; additional equipment will soon be available in other Earth Systems’ locations. For more information regarding testing using the gyratory compactor and Hamburg Wheel Tracker for Caltrans projects, or to assist in value assessment of pavements, please contact Robert Down, PE at 805.544.3276 or