Ahmed Hassan <br />As an abstract: Huet-Sayegh model (1963) gives a set of constitutive equations of a visco-elastic material which accounts well for the behaviour of asphalt pavement layers, especially regarding thermal effects. This model allows rather good predictions of experimental data. The French pavement design method consists in a pavement mechanistic analysis based on the Burmister multilayer elastic model (1943) -LCPC software ALIZE (1982)-. In that model, the Huet-Sayegh behaviour is taken into account with its equivalen elastic modulus at the 15°C French average temperature and a 10 Hz frequency. That frequency value is assumed to be equivalent to the standard 72 km/h French vehicle speed. Such semi-analytical calculations provide relatively good stress and strain fields for heavy traffic pavements but it is less satisfactory for flexible pavements with low traffic, for high temperature gradients and for the analysis of damages under slow heavy loads. Therefore the complete visco-elastic behaviour of each asphalt pavement layers has to be taken into account. The aim of this paper is to present a thermo-visco-elastic multi-layer model using the Huet-Sayegh behaviour. By means of the Fast Fourier transform method, the equations of the model are solved in the coordinate system of the moving load. Results are successfully compared with an analytical solution, finite element results and accelerated pavement testing data. A software called Visco Route (i.e. Visco Road) based on this modelling has been developed.<br />➢ Mechanical model: The proposed method consists in using the asphalt mixes Huet-Sayegh behaviour and a double Fourier transform to solve analytically the problem along the depth in the space of horizontal wavenumbers. The solution in the moving load reference system is got by a double inverse Fourier transform. Then one can calculate the displacements and the stresses in different positions in the road according to the intensity of the load, the temperature or the speed of the vehicles.<br />➢ Visco-elastic constitutive relation: The road structure is a multilayer half-space made of elastic or visco-elastic layers with a thermovisco-elastic behaviour described by the Huet and Sayegh (1963) constitutive relation. This model consists in two parallel branches. The first is made of a spring and two parabolic dampers giving the instantaneous and retarded elasticity of asphalt. The second is made of a spring leading to the static or long-term elasticity of asphalt second is made of a spring leading to the static or long-term elasticity of asphalt