Importance of Asphalt Binder Properties on Rut Resistance of Asphalt Mixture
Permanent deformation in asphalt mixture, also known as ?rutting?, is one of the most important distresses observed in asphalt pavements. Rutting in the asphalt mixture layer is the permanent deformation of the asphaltic layer near the surface. As the binder phase is responsible for the viscoelastic behavior of a mixture, it plays a dominant part in determining many of the aspects of pavement performance, such as resistance to permanent deformation. This phenomenon is more critical during the warm season of the earlier period of pavement life due to the asphalt binder lower stiffness. To strengthen asphalt pavements against damage, modifiers are added to the asphalt binder. The polymers that are used for binder modification can be divided into two broad categories, namely plastomers and elastomers. Plastomers increase the binder?s stiffness and toughness, while elastomers are known to increase the stiffness and elastic response. Elastic and viscous response of binders are important aspects in the behavior of polymer modified binders and are believed to have an important role in rut resistance of asphalt mixture,although the exact nature of their effect is not well understood. In this study, finite element simulation and image analysis were used to simulate the behavior of asphalt mixture and study the effect of elastic and viscous component of binder behavior on performance of asphalt mixture in rutting. Finite element method provides data that cannot be directly obtained from experiments and are not readily accessible, such as information regarding local stress and strain distribution of the components in the aggregate structure. A multi-scale approach was developed and used to investigate the performance of asphalt mixture across a number of scales. Binder viscoelastic properties used in the analysis were derived from analysis of rheological data derived using a Dynamic Shear Rheometer (DSR), and were consequently used to determine the viscoelastic properties of the mastic scale. Mastic scale response was used in the mortar scale. Finally the mortar scale response provided the input to the asphalt mixture scale. The results of this dissertation prove the hypothesis that binder elastic properties and viscous properties affect the rut resistance of hot mix asphalt. It is shown that the permanent deformation of the mixtures containing binders with higher viscous component (lower permanent deformation) is lower. It is also concluded that for the binders with same permanent deformation under the same step stress (same viscous component), higher elastic response of binder leads to lower permanent deformation of the asphalt mixture; however the effect of elastic component on rutting is minor in comparison with the effect of asphalt binder viscous component. In the case of aggregate gradation it is shown that a well-graded, intermediate gradation is capable of generating effective particle networks in micro- and macro-scale, thus reducing binder stress levels over all scales of the asphalt mixture. Overly fine aggregate gradation could prevent effective formation of aggregate skeletons at the macro scale. On the other hand use of overly coarse gradations leads to lower volume fractions of fine particles, impeding network formations and leading to softer mastic and mortar matrixes. It is shown that the structure of asphalt mixture changes continuously during the repeated creep loading test. It is believed that at the end of the secondary zone of rutting large deformations are initiated in the asphalt mixture resulting in degradation of the structure of the aggregates. This stage is the beginning of the tertiary zone of rutting. This study focuses on the effect of binder response elastic component and viscous component on rutting resistance of asphalt mixtures in primary and secondary zones where the structure of the mixture has not been degraded significantly due to initiation of large deformations. The results of this study clearly demonstrates, that mixtures containing binders with higher viscous component show lower permanent deformation. In such a context, the effect of viscous properties of the binder on rutting resistance of asphalt mixture is significantly higher than its elastic response.