With the new Superpave mix design system in use today, more and more coarse graded mix designs are being used. With these coarse graded mixes, there are more interconnected voids in the pavement layers, causing higher permeability. These permeable pavements allow water to pass through them and cause premature failures. Thus, it is important that gradations be developed for surface course mixtures, which are coarser in nature with fewer interconnected voids so that their performance is not affected by the moisture damage.
The Bailey method is proposed to be used for analyzing the packing characteristics of a blend of aggregates. The Bailey method of Gradation Analysis presents the foundation for a comprehensive gradation evaluation and modification procedure. It outlines a method to combine aggregates that provide interlock as the backbone for the aggregate skeleton. Aggregate ratios, which are based on particle packing principles, are used to analyze the particle packing of the overall aggregate structure.
This study will provide a sound methodology of modifying surface course mixture gradations to yield low permeability and superior performance. A band of gradations will be recommended meeting the criteria of low permeability and superior performance. In order to ensure quality performance of pavements, the mixtures made with modified gradations and having low permeability will be evaluated in terms of their resistance to rutting and fatigue cracking using the Simple Shear Tester and the Asphalt Pavement Analyzer.
The primary objectives of this study are to:
- Select several 12.5 mm and 9.5 mm mixtures with gradations on the coarser side and review their void structures and permeability characteristics.
- Modify the gradations of the above mixes, using the Bailey Method, to arrive at the desired void structure.
- Evaluate the developed gradations in terms of permeability and other performance characteristics, e.g. rutting and fatigue characteristics.
- Evaluate the effect of air voids on permeability characteristics at 4% and 7% air voids.
- Recommend a gradation band that will have low permeability as well as high resistance to rutting and fatigue cracking.
