Matrix-assisted laser desorption ionization time-offlight mass spectrometry (MALDI-TOF MS) is widely

Implementation of the Superpave mix design method has encouraged the use of coarser hot-mix asphalt (HMA) mixtures, which require tight control of both the overall gradation and the percent passing the 0.075-mm (No. 200) screen. There is some concern that use of reclaimed asphalt pavement (RAP) in Superpave mixtures may be seriously limited because stockpiles of RAP may have widely variable gradations as well as high percentages of minus 0.075-mm material. The possibility of splitting RAP stockpiles by using the coarser RAP fraction in a typical 12.5 mm below-the-restricted-zone Superpave gradation has been evaluated. The finer RAP fraction was used in an above-the-restricted zone 12.5-mm Superpave gradation. Two sources of RAP (Georgia and Minnesota) were used so that a wide range of asphalt and aggregate properties would be represented. Screening the RAP allowed up to 40 percent of the coarse RAP fraction to be used and still meet below-the-restricted zone Superpave gradation requirements. This was mainly due to the significant reduction in the finer aggregate fractions, especially the minus 0.075-mm material. The use of RAP in these mixtures resulted in a savings of between 18 and 25 percent in the required neat asphalt. A noticeable increase in mixture stiffness with as little as 15 percent RAP was observed. This change in mixture properties suggested that a softer grade of neat binder might be needed. A maximum of 15 percent of the fine RAP fraction was used to produce an acceptable above-the-restricted-zone Superpave gradation. The net savings in neat asphalt content was 25 percent. Little change was observed in tensile strengths because of the addition of this RAP fraction. However, there was a substantial increase in mixture stiffness at intermediate to warm temperatures. This increase was also observed as a 20 percent reduction in the asphalt pavement analyzer rut depth when RAP was used. The indirect tensile creep compliance decreased when RAP was added. Although there was little difference in compliance at -20°C, there were decreases of about 30 and 50 percent at -10°C and 0°C test temperatures, respectively.

Perpetual asphalt pavements have been designed with single threshold values for the horizontal strains at the bottom of the asphalt concrete layer and for vertical strains on top of the subgrade to prevent the occurrence of bottom-up fatigue cracking and subgrade rutting. Several thresholds have been utilized for design based on laboratory and field test results. Limiting strain distributions were recently proposed for perpetual pavement design instead of single threshold values for controlling the horizontal and vertical strains. These design criteria were developed with data collected from test sections at the National Center for Asphalt Technology pavement test track. The objective of this study was to conduct additional analyses of eight perpetual pavement sections located in different climatic regions, to support the proposed limiting strain criteria. These sections were simulated in the PerRoad perpetual pavement design software to determine the horizontal strains at the bottom of the asphalt concrete layer and the vertical strains on top of the subgrade. The strains were then analyzed to evaluate the proposed limiting strain criteria. Based on the simulations, the limiting horizontal strain distribution above the 60th percentile can effectively differentiate the calculated strain distributions of the eight perpetual pavement sections from those of the test sections that failed due to bottom-up fatigue cracking on the test track. In addition, the calculated vertical strains on top of the subgrade at the 50th percentile were lower than the proposed 200-microstrain limit. These results provide additional support for utilizing the proposed strain criteria in perpetual asphalt pavement design.

Pavement macrotexture is considered the primary factor in skid resistance at speeds over 65 km/h (40 mph). A total of 18 projects (9 Superpave and 9 Marshall mix designs) were evaluated to determine if the macrotexture of the pavement surface was influenced by gradation changes associated with the move to Superpave mix designs. The Federal Highway Administration ROSAN high-frequency laser system was used to measure macrotexture. Changes in microtexture, also a factor in skid resistance, were evaluated using a British Pendulum (BP) tester to determine the BP numbers of laboratory-compacted samples. Results indicated that the macrotexture did not change as a result of changes in mix design practices. The nominal maximum size of aggregate seemed to be the key factor in change in pavement surface macrotexture. Mixes, either Superpave or Marshall, with a nominal maximum size 9.5 mm or 12.5 mm have macrotextures of less than 0.5 mm. Based on information in the literature, the skid resistance for these Alabama pavements was estimated at 40 or higher.

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