Characterization of the three-dimensional linear viscoelastic behavior of asphalt concrete mixtures

Mechanistic-empirical pavement design methods typically require the identification of the complex Young's modulus and Poisson's ratio for the characterization of bituminous layers. However, since in small deformation the frequency-dependent shear and bulk responses of an isotropic body are decoupled, the complex shear and bulk moduli are generally considered fundamental response functions. The objective of this study was to perform the experimental characterization of the three-dimensional response of asphalt mixtures in the linear viscoelastic domain, through the simultaneous measurement of the complex moduli E∗, K∗ and G∗ and the complex Poisson's ratio ν∗. The testing program consisted of cyclic compression and cyclic tension-compression uniaxial tests on cylindrical specimen, with the measurement of both axial and transverse strain. In particular, frequency sweeps were carried out at temperatures between 0, and 40 °C and at axial strain levels between 15 and 60 με. Experimental results highlighted that, for the tested mixture, the time-temperature superposition principle was applicable to both the bulk and shear response, and consequently to the axial response. E∗ and G∗ showed very similar trends in terms of both stiffness moduli and loss angle, whereas K∗ values highlighted smaller frequency dependence. The time-temperature superposition principle was also applicable to ν∗ whose master curves can be qualitatively described using the local approximation to the Kramers-Kronig relations. Results suggest that the simultaneous assessment of bulk and shear response may be a useful tool for the performance characterization of asphalt mixtures.