6th RILEM Symposium PTEBM'03, Zurich, 2003 380 PREDICTION OF THE INTRINSIC DAMAGE DURING BITUMINOUS MIXES FATIGUE TESTS Didier Bodin*, Chantal de La Roche*, Jean-Michel Piau* and Gilles Pijaudier-Cabot ** * Laboratoire Central des Ponts et Chaussées Route de bouaye BP 4129, F-44341 Bouguenais cedex, France ** R&DO, Laboratoire de Génie Civil de Nantes Saint-Nazaire Ecole Centrale de Nantes, BP 92101, F-44321 Nantes cedex 3, France Abstract The cyclic character of pavement loads induces material micro-structural changes inside the pavement layers leading to macro-cracks. Fatigue resistance of asphalt concrete is assessed using different constant strain or stress amplitude cyclic laboratory tests. Two major phenomenological aspects cause the stiffness decrease rate: microcracking which provides mechanical damage and viscoelastic thermal dissipation which affects the binder stiffness. A uncoupled thermomechanical and damage approach is presented for the interpretation of laboratory fatigue tests and applied here to displacement controlled tests. This fatigue interpretation allows a more intrinsic damage evaluation corrected from the thermal effects that do not occur in the field. 1. Introduction Fatigue performances of asphalt pavement materials are generally assessed using laboratory fatigue tests involving cyclic loading. For bituminous materials, these tests can be performed on different sample geometries, submitted to sinusoidal loading which can be imposed force or displacement. During these tests, the global stiffness, calculated by the ratio of force to displacement amplitudes, decreases, following a three stage evolution process (fig. 1): Phase I shows a rapid evolution of the stiffness. It is followed by Phase II which corresponds to a more regular stiffness decrease. Fig. 1 : three stages stiffness evolution scheme during fatigue tests The specimen fracture then occurs in Phase III, due to strain localisation and macro crack propagation. During the first two phases of the fatigue tests, more deeply investigated in this paper, the stiffness decrease is then due to two major phenomena: the mechanical damage and the temperature increase due to viscous dissipation. The approach presented in this paper aims at simulating the load of stiffness during the fatigue loading history using a damage model,