Jonathan Perret
Undertaken in the field of design methods for flexible pavement, the present study focuses on the introduction of traffic loads, applied mainly to mechanistic empirical methods. In such research, strains due to traffic loads — calculated by pavement modeling based on linear elastic theory — are used for the evaluation of the life duration of the pavement. Real traffic loads on pavement vary significantly according to their intensity as well as their application modes. Our work aims to evaluate the relevance of the simplifying assumptions used in modeling to determine which parameters used in the description of traffic loads have more influence on the strains in flexible pavements. The study is chiefly based on experimental data. Strain measurements were made on several road structures, varying the main parameters of traffic loads, i.e. load intensity, inflation pressure of the tires and type of wheels. These measurements were carried out at various temperatures. Two structures featuring contrasted rigidities were modeled according to the French design method for flexible pavement. The horizontal strains exerted on these structures were studied at two depths : the bottom of the base course and the bottom of the wearing course. Then, experimental data were systematically compared with the modeling results. Strains at the bottom of the base course were both analyzed related to their maximal value — which corresponds to one of the design criteria used in mechanistic empirical methods — and as signals corresponding to the strain recordings in one given point during the passage of a traffic load. Analysis of maximal values allowed an evaluation of the influence of the studied load parameters and lead on to solutions to include them in calculations for equivalent traffic. The study of strain signals resulted in the definition of values allowing to express strain variations in function of time. The said values apply principally to notions like strain rates and the definition of frequencies that will best model strain variation in time with a sinusoidal curve. Such frequencies are obtained by decompositions in Fourier series. A relation between load speed and frequency has been defined. At the bottom of the wearing course, the study focused on strain signals only, with strange variations — which have been called "irregularities" — appearing in the horizontal strains at low depth. The scrutiny of these irregularities showed that bituminous materials located near to the road surface of flexible pavements are paradoxically more stressed horizontally than vertically. A situation that tends to link the behavior of bituminous layers in flexible pavements to the performance of a slam structure. Analysis of strain signals was supplemented by linear viscoelastic modeling based on Huet's model. If introducing the viscoelastic behavior of the bituminous layers showed no notable improvement in the quality of modeling, it nevertheless highlighted the necessity to model the pavement structure as a whole. Stress analysis underlined the effects of interactions between layers following different patterns of behavior and the importance of the interface conditions between these layers. Finally, the influence of the real vertical load pressure distribution on stress and strain in bituminous layers has been evaluated with elastic modeling. Variation of the loading conditions showed little effects on stress and strain distribution in the bituminous layers.
EPFL2003
