Fatigue tests of bituminous materials performed on trapezoidal specimens induce a loss of stiffness, combined with a significant extension of the specimens at the beginning of the test when applying high strain levels. This paper presents the development of a viscoelastic constitutive law with unilateral damage for asphalt materials, based on the observation of tensile failure tests on asphalt binder lenses between two metal spheres. The unilateral damage is defined as the opening of the micro-cracks in the bitumen during extension and their closing during contraction. When put into a semi-analytical structural model of the trapezoidal specimen, this constitutive law leads to results close to those highlighted at the beginning of fatigue experiments for various experimental conditions (temperature, load frequency and strain amplitude level). According to the used modelling, the damage evolution law describes the fast initial homogeneous creation of the micro-cracks and then their slow development. The modelling is also able to explain a given percentage of the loss of stiffness. These results show mainly that the bituminous material extension is due to the dissymetric behaviour induced by the unilateral damage coupled with viscoelasticity. Therefore the asphalt mix extension is a way to quantify the level of the material damage. (c) 2004 RILEM. All rights reserved.
