Evaluation of Viscoelastic Poisson's Ratio and Anisotropy of Asphalt Mixtures
Thesis (M.S., Civil Engineering) -- University of Idaho, 2016 | The Poisson’s Ratio (PR) of asphalt mixtures is an important input parameter in the constitutive models for pavement design and performance analysis. The design and performance analysis of asphalt pavements depend upon measuring proper material properties such as complex modulus and PR. The PR is often assumed to be time independent or it is calculated as the negative ratio of transverse to axial strains in the time domain under uniaxial loading when the time dependency is considered. This study presented and used accurate methods to calculate the viscoelastic PR under various loading conditions and demonstrated the error associated with using inaccurate methods for calculating the PR. The results of this study demonstrated that the viscoelastic PR increased with time in uniaxial unconfined tension and compression tests where shear relaxation was generally faster than dilatational relaxation. While the viscoelastic PR decreased with time in uniaxial confined compression tests where the dilatational relaxation was faster than the shear relaxation. In addition, the results demonstrated that asphalt mixtures with coarse aggregate gradations had lower viscoelastic PR compared to asphalt mixtures with finer aggregate gradations. Also, the viscoelastic PR was found to be sensitive to the aging and moisture conditions. The viscoelastic PR decreased with aging and increased with the level and time of moisture conditioning of the asphalt mixtures. Finally, the error introduced by incorrectly calculating the PR as the negative ratio of transverse to axial strains in the time domain was found to be significant in some cases while it was insignificant in others.
In addition, this study explored the anisotropy of asphalt mixtures. A proper understanding of the anisotropic behavior of asphalt mixtures is needed for an accurate multiaxial characterization of asphalt mixtures under different conditions. Asphalt mixtures are subjected to various loading conditions while in service. This study tested asphalt mixture specimens subjected to hydrostatic pressure to evaluate the degree of anisotropy at various conditions. The results demonstrated that the degree of anisotropy is dependent on the mix design. Asphalt mixtures with coarse aggregate gradations were found to have higher level of anisotropy compared to mixtures with finer aggregate gradations. In addition, the degree of anisotropy increased with the increase of confining pressure and temperature. Finally, the porosity of asphalt mixture had relatively significant impact on the degree of anisotropy. The anisotropy increased with the increase of porosity. The analysis of the anisotropy using the X-ray CT and image analysis techniques confirmed that the degree of anisotropy decreases with the increase of density.