CALTRANS MOVES TO SUPERPAVE

The California Department of Transportation (Caltrans) is transitioning to Superior Performing Asphalt Pavement technology, or Superpave, for asphalt concrete mix design. With the increase in traffic loadings and volumes, change in axle configurations, depleting aggregate sources, and the inability to obtain, repair or calibrate current outdated equipment. Caltrans recognizes the need to change.

Superpave is a performance based standard of test procedures for asphalt concrete (AC) that was developed under the AASHTO Strategic Highway Research Program (SHRP). It is a comprehensive method of designing asphalt mixes tailored to specific unique performance requirements governed by the traffic, environment (climate), and layer location. It also facilitates selecting and combining asphalt binder, aggregate, and any necessary modifier to achieve the required level of pavement performance.

EQUIPMENT

Superpave utilizes the Superpave Gyratory Compactor (SGC) as a method of compacting test specimens during the mix design process and for Quality Control and Quality Acceptance testing during production.

The Superpave gyratory compactor has the ability to produce asphalt mix specimens to densities achieved under actual environmental and traffic loading conditions. Utilizing a 150 mm (5.90 inch) diameter mold, the SGC can accommodate large aggregate mixtures.

The SGC consists of a rigid reaction frame, loading system, and specimen height measurement. Depending on the model used it has the ability to compact asphalt mixture specimens at a variety of pressures. The mixture is compacted by a gyratory kneading action using a compaction angle of 1.25 degrees external and 1.16 degrees internal. The SGC typically gyrates at 30 RPM.

Superpave Gyratory Compactor (SGC) Model AFG2A, Distributed by Pine Instrument Co.		Troxler Gyratory Compactor Model 4140, Distributed by Troxler

In addition to the SGC, Caltrans will be utilizing the Hamburg Wheel Tracker (HWT). The HWT measures the rutting and surface moisture susceptibility of an asphalt paving mixture by rolling a steel wheel across the surface of an asphalt concrete slab that is immersed in 50°C (122°F) water. The HWT measures the combined effects of rutting and moisture damage by rolling a steel wheel across the surface of an HMA specimen that is immersed in 50°C (122°F) water. It measures the rutting rate (creep slope) of depth of rut verses number of passes of the steel wheel.

		Hamburg Wheel Tracker (HWT)
Hamburg Wheel Tracker (HWT) Inside View

Moisture susceptibility is measured as the intersection of the creep slope and the striping slope (Inflection point). The stripping slope is nothing more than the creep slope changes rutting rate.