The influence of aggregate packing characteristics on the compaction, volumetrics and rutting resistance of asphalt mixes

Abstract

Hot-Mix Asphalt (HMA) compaction is required to be undertaken during the laboratory design of asphalt mixes and asphalt pavements construction. Adequate HMA compaction is required to enable the asphalt mix to achieve stability, reduce water permeability, provide resistance against rutting and fatigue cracking, as well as to enhance the overall performance of the pavement structure. In HMA pavement layers, the aggregate structure is responsible for load transfer and providing resistance against pavement distresses. A strong aggregate structure relies on the optimal packing of the aggregates. Among other factors, the aggregate packing characteristics, binder volume and stiffness, and the air void distribution define the internal structure of the HMA, which in turn plays a significant role in the mechanical and volumetric properties of HMA. The internal structure of compacted HMA also depends on the compaction process or method.

With the above background, this thesis aimed to relate aggregate packing characteristics with HMA compactability, the resulting HMA volumetric properties, and the ability of the compacted asphalt mix to resist rutting. To ensure the achievement of the aim of the thesis, the study was divided into three specific objectives.

The first objective was geared towards understanding how the aggregate packing characteristics influence the compactability of HMA mixes. Six aggregate gradations were analysed to determine eight packing parameters, including two gradation parameters (shape factor and gravel to sand ratio), three traditional Bailey ratios, three rational Bailey ratios. It was established that the rational Bailey ratios provided a better description of the packing characteristics of the aggregate gradations. Subsequently, a gyratory compactor was used to compact HMA mixes that were prepared using each of the six gradations. The compaction data was analysed to determine HMA compactability parameters, namely: locking point, compaction energy index, compaction slope, traffic densification index and area under shear stress compaction curve. Subsequently, the compactability parameters were correlated to rational Bailey ratios. It was found that the traffic densification index, looking point, and compaction slope are related more logically to the aggregate packing.

The second objective investigated the air voids distribution in laboratory-compacted HMA samples and cores extracted from actual field road sections. The laboratory experiments were designed to also investigate the influence of sample height and compaction density. It was found that the smaller the sample height and the higher the compaction density, the higher the variation of air voids. The results also showed that the vertical distribution of air voids differs, with the middle part of the HMA samples found to exhibit higher compaction density than the bottom and top.

The third objective investigated the influence of the compaction method and compaction density on the HMA rutting resistance, using two different rutting tests: repeated simple shear test at a constant height and uniaxial repeated shear test. The two tests were used to compare the rutting resistance of samples compacting using gyratory and slab roller compactors. The results showed that the gyratory compacted samples had better rutting resistance than the slab roller-compacted samples, suggesting that the compaction method influenced the internal structure of the resulting samples and, consequently, the HMA rutting resistance. For the influence of density, the study found that HMA samples compacted to higher density had better rutting resistance than those compacted to lower density.