A thermodynamic viscoelastic model to capture the effects of confinement pressure on asphalt mixtures

The aim of this study is to develop a constitutive model capable of describing the effects of confinement pressure on the mechanical behaviour of asphalt mixtures. The model is developed within the context of a Gibbs-potential based thermodynamic framework. Functional forms are assumed for the Gibbs potential and the rate of dissipation, which is maximised to obtain the constitutive model. The Gibbs potential and the rate of dissipation are formulated to have separate components corresponding to the spherical and deviatoric parts of the stress tensor. Additionally, one of the model viscosity parameters is formulated to be pressure-dependent. The efficacy of the model is demonstrated with the aid of experimental measurements conducted at four different confinement pressures ($0\comma\; 70\comma\; 140\comma\; 380\, {\rm kPa}$0,70,140,380kPa) and two different temperatures (40 and $55<^>{\circ }{\rm C}$55 degrees C) (Rahmani et al. 2013, Effect of confinement pressure on the nonlinear-viscoelastic response of asphalt concrete at high temperatures, Construction and Building Materials, 47, 779-788; 2017, Constitutive modeling of coupled aging-viscoelastic response of asphalt concrete, Construction and Building Materials, 131, 1-15; Bazzaz et al. 2018, A straightforward procedure to characterize nonlinear viscoelastic response of asphalt concrete at high temperatures, Transportation Research Record, 2672 (28), 481-492). Some of the test results are used to calibrate the model, while the remaining data are used for model validation. It is found that the model reasonably predicts the mixture responses in the axial and radial directions at different confining pressures and temperatures.