Comparative Assessment of Gyratory and Roller Slab Compaction Methods on HMA Rutting Resistance

Abstract: As part of the laboratory asphalt mix-design process, specimens are usually molded/compacted to evaluate the hot-mix asphalt (HMA) volumetric and mechanical properties, including rutting resistance – a critical attribute of flexible asphalt pavement performance. The choice of the laboratory compaction method depends on many factors, including the HMA mix-design method, specimen geometry (rectangular, prismatic, slab, cylindrical, or trapezoidal), equipment availability, etc. HMA compaction is a complex process involving aggregate reorientation and interlocking, affecting the internal structure of the compacted HMA. Consequently, different compaction methods may produce specimens with distinctive mechanical properties. The study presented in this paper comparatively investigated the influence of the gyratory and roller slab compactors on the rutting performance of two South African standard HMA mixes for the construction of flexible asphalt pavement wearing courses. The HMA mixes comprised of fine dense-graded sand-skeleton and coarse dense-graded stone-skeleton structures. Two sets of specimens for each HMA mix were molded and compacted using both methods to target 4.0% and 7.0% for mix-design and field-compaction air voids content, respectively, thus allowing for the comparative evaluation of the effects of compaction density on HMA rutting resistance. The rutting performance of the compacted specimens was subsequently assessed using the Uniaxial Repeated Shear Test (URST) and Repeated Simple Shear Test at Constant Height (RSST-CH) methods. The results showed that the compaction method significantly affects the HMA rutting performance. Overall, the outcomes of the study provide valuable insights into the rutting performance of HMA mixes compacted using different methods and the implications for existing rutting resistance specifications – as well as the need to consider (account for) the inherent “compaction method effect” when comparing HMA rutting performance.