Le Trong Nhan NGUYEN

Study on the aggregate gradation design for Superpave asphalt mixtures

Osamu TAKAHASHI

With the rapid increasing of airplane operation frequency in urban airports, particularly in international airports, some airport pavements have damaged in recent years before the service period is completed. Especially in asphalt surface course under severe loading conditions like the taxiways, traditional HMA mixtures have been pointed out that rutting resistance is inadequate.
Dense-graded mixes with 20 mm top-size, designed with Marshall Method, is solely applied in airport pavements in Japan. Rutting resistance can be improved using modified asphalt and carefully selected aggregate, but those are unacceptable in some situations due to economic reasons. Therefore, optimum aggregate gradation should be selected by a rational mix design procedure in order to obtain high rutting resistance mixture without cost increase.
Based on the background, this study discussed improvement of rutting resistance applying Superpave Design Method with an optimized aggregate gradation design procedure.
On the design process it is necessary to develop trial aggregate gradations based on the Superpave control points. These control points provide a wider range of options for gradation design. However, there are no specific guidelines on aggregate gradation selection to ensure that the mixture can obtain the volumetric requirements and satisfactory performance. Thus the desired gradation can only be obtained via trial and error.
The objective of this study is to reduce the steps of trial and error process in aggregate gradation design and to make engineers be able to adjust gradation easily without design skill. This study uses a systematic gradation design aids, called Bailey Method and Continuous Maximum Density (CMD), to evaluate gradation with the aggregate parameters.
In this study, totally 32 blends using two different locality aggregates groups have been examined to obtain the fundamental information on the effects of the aggregate parameters for Superpave requirements. The rutting resistance test was carried out to evaluate the performance of the mixtures.
The results showed that aggregate parameters of CMD-area and number of crossing points had high correlation with the VMA although aggregate resources were different. Adjusting the CMD-area is most effective to ensure the volumetric requirements for a Superpave mixture.
It is also shown that the mixture having smooth-shaped CMD plot and small amount of crossing point may produce good rutting performance for CALUW 70-90% gradations.

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