The primary failure mechanism of unbound aggregate materials in flexible pavements is rutting or accumulation of permanent deformation. Basically, permanent deformation of unbound materials is influenced by many factors including stress level, numbers of loading, principal stress axis rotation under moving wheel loading, shear strength, moisture content and so on. Many models, both purely mechanics-based and mechanistic-empirical, have been developed to predict permanent strain accumulation. Purely mechanics-based models are hard to implement in pavement design because they are often complicated and time-consuming in prediction. Mechanistic-empirical models are widely used in pavement design due to fast computation and acceptable accuracy of prediction. The recently developed UIUC model shows good applicability for permanent deformation prediction based on repeated load triaxial tests results often conducted at constant confining pressure. Consequently, the effect of principal stress axis rotation on permanent deformation is not considered. This drawback limits the use of UIUC model in pavement design because the actual principal stress axis rotation due to moving wheels greatly increases permanent deformation. In this study, a modified UIUC model has been proposed based on multi-ring shear tests results, which could simulate principal stress axis rotation in the pavement structure. A new parameter, (Rs)ave, is added to capture the effect of principal stress axis rotation on the permanent strain. This modified UIUC model shows good applicability to predict permanent axial strains of crusher-run materials studied in laboratory testing for the effects of principal stress axis rotation and moisture.
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