Influence of laboratory aggregate compaction method on the particle packing of stone mastic asphalt

The type of aggregates and their packing characteristics under compaction are key factors for the design of asphalt mixtures with improved performance, namely, with respect to resistance to permanent deformation. A good example is Stone Mastic Asphalt (SMA), known by its stone-on-stone structure. In the U. S.A., the aggregate particles packing characteristics in a SMA, specially the stone-on-stone effect, are normally assessed using the "manually dry-rodded" method. However, this method may not be representative of field aggregate particle packing conditions, which may compromise the SMA performance. This article presents new findings regarding aggregate laboratory compaction methods to optimise the coarse aggregate structure in a SMA. Particle breakage, bulk density, air voids (compacted & uncompacted skeleton) in the aggregate / coarse aggregate were assessed for existing methods as well as for new methods using existing compactors, but with different procedures and/or specific devices, e.g. Proctor hammer. The assessed methods were: (1) "non-compaction"; (2) "manually dry-rodded" method; (3) established Proctor compaction; (4) modified Proctor compaction (light and heavy compaction) and (5) steel roller compaction. The 2 latter "new methods" aimed at mechanically simulating the dry-rodded method and the effect of field compactors, respectively. The results highlight that the new laboratory compaction methods developed with Proctor and steel roller compactor, provide a particle packing that is more representative of the field conditions, comparatively to other aggregate compaction methods.