Warm-mix asphalt (WMA) is much like hot-mix asphalt (HMA), but it is produced at lower plant temperatures than conventional HMA. Key benefits of the reduced WMA production temperature include the reduction of fuel consumption and of emissions. Granite Construction performed two WMA paving demonstration projects from its Indio, California, facility in early 2008. Both projects were paved with WMA produced with the free water method (Astec Double Barrel Green). The objectives of these demonstrations were ( a) to demonstrate that WMA with reclaimed asphalt pavement could be produced and placed at lower temperatures while yielding mix properties and field compaction similar to those of conventional HMA and ( b) to construct field test sections so that WMA and HMA performance could be compared side by side. These objectives were accomplished by producing and placing the WMA and by completing an in-depth sampling and testing program to compare the WMA and HMA paved on similar test sections and produced with similar methods (the only exception was production temperatures). The initial field performance of the WMA and HMA has been similar, and the long-term performance will be monitored. The WMA demonstration objectives were achieved, with the WMA exhibiting mix properties and field compaction similar to those of the HMA, with slightly lower initial stiffness, as expected. The potential rutting concern with WMA has not been an issue in this arid Southern California climate, and the sections placed on the haul road into and out of the Indio plant have been exposed to significant truck traffic.
When developed in the early 1990s the Superpave method of asphalt design was structured similar to the Marshall method of asphalt design. In the Marshall method asphalt was designed with 3 to 5% air voids (typically 4%). Compaction specifications typically left the mixture with 8% air voids on the road. It was reasoned that traffic would compact the asphalt mixtures and achieve an ultimate density of 96%, the same as the design. Superpave calls for a design air void content of 4.0% (no range) and compaction specifications allow the mixture to have 7 to 8% air voids. Subsequent research in the early 2000s showed that after traffic densification air voids stabilize at about 6%. This paper discusses proposed changes Superpave asphalt design method to design at 5% air voids. Target air voids after compaction would be the same, 5%. To achieve compaction of 95% (5% air voids) requires changes to the laboratory compactive effort in the Superpave design method. The concept of designing and compacting asphalt at 5% air voids is inspired by the LCPC method of asphalt design used in France. The perceived benefit to Superpave is an improvement in mixture durability expected from lower air permeability leading to a reduced rate of oxidative hardening. This paper discusses the concept of setting air void levels in asphalt design, research to identify changes to the Superpave method and recommendations for design compactive effort to be used.
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