Typically, bituminous mixtures are designed to have 6-7 % air voids soon after construction stage compaction, to facilitate binder movement without bleeding at high in-service temperature. The mixture is designed such that at least 2-4 % air voids will be maintained at the end of design life. During the initial service life, densification is expected and such densification can lead to the development of corrugations in the wheel path, traditionally called as rutting. The densification of asphalt layers along with the aging of the asphalt binder makes the bituminous mastic too stiff leading to the formation of interconnected cracks on the surface termed as fatigue damage. These issues may be tackled to a great extent if a suitable approach for the aggregate gradation design is adopted, which will help to minimize this densification and reorientation of the bituminous mixtures. However, the evolution of mixture density during the compaction stage as well as during the ser vice stage is not considered explicitly in the conventional mix design methods. Most of the traditional gradation design methodologies for bituminous mixtures adopt dense gradation following Fuller and Thompson (1907) method to achieve maximum density. As the focus of the traditional mix designs was towards achieving volumetric parameters by altering binder content, not much headway is made in terms of experimenting with different aggregate gradations. In this instance, one can appeal to particle packing based approaches, which provide a rational tool to design the packing of systems with particulate matter.

The aggregate gradation design appealing to particle packing has a cogent background to offer an interlocked aggregate gradation, which in turn minimizes these issues to a reasonable extent. This approach can account for the role of specific size ranges within a gradation range, in the aggregate structure formation of a bituminous mixture and hence an interlocked aggregate skeleton is expected to be formed. However, the holistic picture of aggregate packing will be obtained only by considering the effect of paste/mastic lubrication in the packing process due to the presence of the binder. While many dry particle packing based approaches exist due to Furnas (1928), Powers (1968), Lees (1970), Stoval et al. (1986), de Larrard (1994), and Kim (2006), only very few attempts are currently been carried out to incorporate them for the bituminous mixture design. The reason may be due to the constructability issues in terms of workability, compactability, and susceptibility to segrega tion. Also, there exist a lack of clarity in the selection of specification criteria and test protocols. The suitability of different specification criteria itself is a question of concern. The volumetric specifications are mostly empirical, based on the prevailing construction practices and hence more investigation is required to verify whether deviations in the volumetric specifications can be permitted. In addition, characterization of newer grade mixtures for design and distress is to be carried out. The sensitivity of different test protocols to capture the inherent variation of the bituminous mixtures owing to the variation in aggregate gradation is to be analyzed as it gives more information regarding the choice of suitable test measures. It is indeed challenging to quantify the effect of aggregate gradation in bituminous mixtures, as mostly the volume occupied by aggregates in different cases is almost the same and the major variation lies only in the distribution pattern o f aggregates.

In this seminar, I will trace back the evolution of various particle-packing concepts and will give some insights about the applicability of the same for the design of aggregate gradation of bituminous mixtures. I will also walk through the design and evaluation aspects of two aggregate gradations designed by me, appealing to particle packing theories, along with the control sample prepared based on regular dense graded bituminous mixtures. The methodology selected by me along with the interesting observations I make in identifying suitable test protocols for mechanical characterization as well as for quantification of permanent deformation will be discussed. The approach adopted to evaluate the associated material functions that can be sensitive to capture the influence of aggregate interlock will also be explored. As the next stage of my research I will be extend these aspects to another set of particle packing based approach for aggregate gradation design. I am planning t o converse about the possibility of linking material functions evaluated based on the mechanical characterization test, to their permanent deformation in the next seminar.