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.
Pavement recycling techniques, including full depth reclamation, cold in-place recycling, and cold central plant recycling, are effective at rehabilitating existing pavements and constructing new pavements while reducing construction costs, environmental impacts, and construction time. Despite these benefits, these techniques are not used as often as they could be, in part because agencies are unable to assess rapidly the quality of the recycled material or to determine when a recycled pavement has sufficient structural capacity to handle traffic or construction equipment for surfacing, or both, without causing damage, especially at early ages. To describe better the time-critical properties of recycled materials, a series of tests was conducted to assess the stiffness, penetration resistance, and shear and raveling properties of laboratory-produced recycled mixtures sampled from field projects. These tests were conducted to evaluate their potential to discriminate among various factors affecting the behavior of the materials in the laboratory. The study found that penetration resistance tests were most sensitive to the factors assessed, followed by stiffness and then torque values from shear and raveling tests. These tests will be further validated using additional mixture types in the laboratory, and their performance in the field will be studied.
Significant controversy has revolved around the Superpave gradation specifications, in particular the restricted zone, since the completion of the Strategic Highway Research Program (SHRP). Historically, dense-graded mixtures that encroach on the restricted zone had provided good performance prior to SHRP. However, current Superpave guidelines recommend that gradations passing through the restricted zone not be used. A synopsis of recent research related specifically to the impact of the Superpave restricted zone on performance of hotmix asphalt (HMA) is provided. The evolution and purpose of the Superpave restricted zone are presented, along with findings of both recently completed and ongoing research. Studies involving laboratory and full-scale accelerated performance tests of mixtures with gradations plotting above (ARZ), through (TRZ), and below the restricted zone were considered. The research reviewed clearly suggests that good performance can be achieved with fine-graded (ARZ and TRZ) mixtures and that no relationship exists between the Superpave restricted zone and HMA rutting or fatigue performance. It is suggested on the basis of this research that the restricted zone recommendation be eliminated from the Superpave volumetric mixture design specifications.
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