This paper deals with the evolutive behaviour of bituminous mixture on steel orthotropic plate, such as the Millau viaduct. This is of great importance when dealing with durability : fatigue cracks in the asphalt layer at right angles of the orthotropic plate stiffeners and shear cracks between steel and pavement. Therefore, a five-point bending fatigue test was developed by the French Laboratoire des Ponts et Chaussées (LCPC) in the 70's. It has been improved since 2003 at the ENTPE laboratory, to test bituminous mix which will be able to support large strains on this kind of steel bridge. Test temperatures are -10°C and +10°C. The sample (580 x 200mm) is constituted by a 12 or 14mm thick steel plate reinforced at the center, a 3 mm thick sealing sheet and specific bituminous mix. This test is characterized by a sine load (e.g. : -320 to -3 200daN for a steel deck of 12mm) over several million cycles. Bituminous mix is good if there is no damage at -10°C after 1 million cycles, and at 10°C after 2 million cycles. Measurements on samples and in situ observations on real steel deck are very consistent. The aim of this study is to detect cracks through the asphalt but also to obtain a continuous follow-up of the mechanical changes in the material properties during the test. Consequently, two displacement sensors (Linear Variation of Differential Tension) are positioned on the upper side of the sample where cracks are likely to appear. It is shown that significant strain increases and decreases are observed when cracks are created and propagated in the thickness of the asphalt pavement. Moreover, a non-destructive test with ultrasonic wave transducers will be presented. P-wave (compression) and S-wave (shear) are used in transmission in a homogeneous area with an adequate excitation frequency. This original method allows us to calculate Young's modulus and Poisson's ratio in the asphalt layer by inverse analysis with the assumption of linear viscoelasticity. P-wave and S-wave velocities increase and their amplitude decrease clearly as the number of load cycles grows. Then, signals stabilize before significant amplitude decrease. These measurements help damage evolution curve plotting as a function of the material. Finally, results of displacement sensors and wave transducers are shown to be very consistent and complementary. Nevertheless, the interest is that ultrasonic waves address to the mechanical evolutive behaviour of the material.
Abstract