SUMMARYNowadays, the problem of rutting of flexible pavements linked to permanent deformations occurring in the unbound layers is taken into account only by mechanistic empirical formulas. Finite element modelling of realistic boundary value problems with incremental rheological models will lead to unrealistic calculation time for large cycle numbers. The objective of the authors is to present a simplified model which can be used to model the flexible pavements rutting with the finite elements framework. This method is based on the shakedown theory developed by Zarka which is usually associated to materials like steels. It has been adapted for granular materials by introducing a yield surface taking into account the mean stress influence on the mechanical behaviour and a dependency of the hardening modulus with the stress state. The Drucker-Prager yield surface has been used with a non-associated flow rule. Comparisons with repeated load triaxial tests carried out on a subgrade soil have been done. These comparisons underline the capabilities of the model to take into account the cyclic behaviour of unbound materials for roads. Finally, a discussion, dealing with the use of the simplified method within a finite element modelling of a full-scale experiment, is presented.
Based on previous works on polymers and regular bitumens, a new method for determining the molecular weight distribution (MWD) of asphaltic paving binders, using the phase angle (δ) of the complex modulus measured in the linear viscoelastic domain, has been developed in this paper. MWDs of pure petroleum bitumens, natural bitumens, modified bitumens and artificially aged bitumens are calculated by this method and compared with MWDs obtained by gel permeation chromatography. The two methods give comparable results, but the δ-method developed here is more sensitive to the molecular weight of species. This method is very simple to implement, and seems a powerful tool in the determination of MWD of bitumens, in the visualisation and repartition of modifiers in the bitumen matrix and in the ageing monitoring and quantification.
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