Evaluation of compaction sensitivity of Saskatchewan asphalt mixes

Saskatchewan Ministry of Highway and Infrastructure (SMHI) currently use the Marshall compaction method for the preparation of hot-mix asphalt laboratory samples. Due to increases in commercial truck traffic on most provincial highways, there has been an observed increasing trend in the occurrence of permanent deformation within the hot-mix asphalt concrete (HMAC) layer. One of the most important material properties found to influence the resistance of HMAC to structural permanent deformation is volumetric air voids within the mix. End product air voids within a hot mix asphalt concrete pavement in the field is simulated by the method of compaction used during the laboratory design process. Based on findings of the Strategic Highway Research Program (SHRP), the gyratory compactor is believed to better simulate field compaction of asphalt mixes at the time of construction, as well as better predict mix consolidation over the field performance period. However, the SuperpaveTM sample preparation protocol specifies a fixed angle of gyratory compaction, which may not be the optimal parameters to evaluate Saskatchewan hot-mix asphalt concrete mixes during the laboratory mix design phase. The primary objective of this research was to investigate the relationship between laboratory characterization and field evaluation of Saskatchewan SPS-9A asphalt mixes across alternate laboratory compaction protocols. A second objective of this research was to quantify the effect of gyratory and Marshall compaction energy on the physical and mechanical properties of Saskatchewan SPS-9A asphalt mixes in the laboratory. The third objective of this research was to compare field ground penetrating radar dielectric permittivity profiles and rutting performance across Saskatchewan SPS-9A test sections. The hypothesis of this research is that gyratory laboratory compaction will provide improved sensitivity in the characterization of physical asphaltic mix properties. It is also hypothesized that varied volumetric properties of HMAC mixes influence the mechanistic triaxial frequency sweep material properties of both conventional Saskatchewan and SuperpaveTM dense graded HMAC mixes. The laboratory portion of this research included volumetric and mechanical properties of the seven Saskatchewan SPS-9A asphaltic mixes. The scope of this research included an investigation of the Saskatchewan Specific Pavement Study-9A (SPS-9A) asphalt mixes constructed in Radisson Saskatchewan in 1996. Physical volumetric properties as well as mechanistic triaxial frequency sweep properties were characterized across all seven Radisson SPS-9A mixes. Rutting after ten years of performance in the field was quantified as well as in situ ground penetrating radar dielectric permittivities of the Radisson SPS-9A test sections. Based on the findings of the study, there was a significant reduction in VTM with an increase in Marshall compaction energy from 50 to 75 blows. Marshall stability was observed to be higher at 75 blow compared to 50 blows across the test sections. Similarly, with regards to gyratory sample preparation, there was an observed reduction in VTM with an increase in gyratory compaction energy. VTM of SuperpaveTM mixes were higher than VTM SMHI Marshall mixes. VTM of the SuperpaveTM mixes were above acceptable SMHI limits at all angles of gyration at Ndesign. SuperpaveTM gyratory compactor accurately predicted field air voids of the Radisson SPS-9A asphalt after ten years of traffic loading at 2.00° angle of gyration. In general, this research showed significant sensitivity of volumetric material properties across both Marshall and gyratory compaction energy. This research also demonstrated that there was an improvement in the triaxial mechanistic material properties of the Radisson SPS-9A HMAC mixes with an increase in gyratory compaction energy. Dynamic moduli across all test section mixes increased with an increase in gyratory compaction energy. Similarly, it was shown that Poisson’s ratio generally increased with an increase in compaction energy across all test sections. Phase angle also increased with an increase in gyratory compaction energy. Radial microstrain (RMS) displayed the most significant sensitivity to increased gyratory compaction energy. This research concluded that compaction energy in the laboratory can significantly influence the volumetric and mechanistic properties of hot-mix asphalt concrete mixes. As indicated by the field performance of the Radisson SPS-9A test sections, it is known that both volumetric and mechanistic properties can influence field performance. Mechanical material properties of HMAC may be improved by increasing compaction energy, as long as volumetric properties are adhered to. The use of rapid triaxial frequency sweep testing demonstrated the ability to characterize mechanistic material properties as a function of varied compaction energy. Based on the findings of this research, it is recommended that Saskatchewan asphalt mixes, both Marshall and SuperpaveTM types, be characterized using gyratory compaction with 2.00° angle of gyration and the SHRP specified number of gyrations. Further, the gyratory compacted samples provide the ability to characterize the mechanistic material constitutive properties of asphaltic mixes for mechanistic based road structural design purposes. Future research should evaluate the relationship of laboratory material properties to the field performance of various Saskatchewan asphalt mixes across various field state conditions