A Rational Framework for the Mix Design of Bituminous Mixtures

The definition of the optimum recipe for a bituminous mixture is a fundamental step in the construction of durable and long lasting pavements. Depending on the type of binder and aggregates that are being used different volumetric proportions can be chosen, and each of them has an important effect on the final performance of the mix. The techniques used to define the composition of the mix may range from the traditional Marshall method to rational approches based upon the fatigue and rutting behaviour of the composite materials as proposed in the advanced levels of SHRP mix design. However, even recent research has stressed the importance of volumetric mix design, considered as the basis of more complicated design approaches that require expensive equipment and time consuming procedures. The research activities that are summarized in this paper deal with the volumetric design of bituminous mixtures for the Italian environment. Following previous research findings of the Belgian Road Research Institute, a rational framework for mix desing has been defined and experimentally validated. Based upon a theoretical modelization of the volumetric disposition of different sized aggregates and of the binder, the proposed method enables the pavement engineer to obtain mixes characterized by a preselected air void level. Part of the experimental work has also focused on the mechanical properties of the mixes as measured by the Nottingham Asphalt Tester (NAT). A relationship between volumetric and mechanical properties has been observed and is discussed in the paper. Finally, evidence is given of the possibility of implementing the method in Italy through an adequate harmonization with the existing specification.