Triaxial frequency sweep characterization for dense graded hot mix asphalt concrete mix design

Abstract

Asphalt concrete mix design methods, such as the Marshall method, have historically been based on physical and phenomenological material testing empirically correlated to observed field performance. Changing pavement field state conditions such as increased trucking, poorer quality aggregate resources, and the aged state of road infrastructure in Saskatchewan have resulted in recent pavement performance to be outside traditional empirical performance prediction inference. It has been recognized worldwide that a mechanistic based asphalt concrete mix design methodology that directly quantifies structural behaviour of pavement under diverse field state conditions could significantly assist pavement design engineers. However, SHRP Level II and III mechanistic asphalt concrete characterization has been shown not to be pragmatic for characterizing asphalt concrete mixes. The objective of this research was to investigate the use of mechanistic material properties obtained from triaxial frequency sweep characterization in the rapid triaxial tester (RaTT) in conjunction with SHRP gyratory compaction properties for designing asphalt concrete for different asphalt cement contents, traffic loads, traffic speeds, and temperatures. RaTT testing was more responsive to variation in asphalt cement content outside of acceptable ranges of volumetric properties relative to Marshall stability and flow. This demonstrated the importance of specifying acceptable volumetric properties of asphalt concrete mixes. Correlation of material properties with volumetric measurements validated triaxial frequency sweep characterization in the RaTT. Dynamic modulus, Poisson’s ratio, and phase angle results were in accordance with expected material behaviour, indicating that the RaTT provides reasonable asphalt concrete material properties. Also, the RaTT identified asphalt concrete to be a nonlinear viscoelastic material, as observed in the field. The RaTT was able to characterize SHRP gyratory compacted samples for the typical range of traction states, load frequencies, and temperatures that simulated a range of Saskatchewan field state conditions. Triaxial frequency sweep testing in the RaTT could significantly augment conventional volumetric mix analysis as well as the SHRP SuperpaveTM Level I asphalt concrete mix design system. RaTT testing was found to be cost effective, time efficient, and provided mechanistic material constitutive relations that can be employed for inelastic mechanistic mix design, road structural modelling, and asset management.