This paper presents findings from an ongoing research study at the University of Illinois that aims to develop and calibrate improved models for unbound aggregate rutting through laboratory characterization of aggregate materials used for unbound base and subbase applications in the state of North Carolina. Extensive triaxial laboratory testing was performed to establish a robust link between the number of load applications, stress levels, shear stress and sheer strength ratios, and permanent deformation responses. A framework was established for considering the strong correlation that commonly exists between permanent deformation and shear strength characteristics, as opposed to resilient modulus properties, in the laboratory characterization of the permanent deformation behavior of various types of aggregate materials. Trends of permanent strain accumulations from repeated load triaxial tests were adequately captured in a new rutting model whose development took into account the shear stresses applied at 25%, 50%, and 75% of the shear strength properties of these materials under similar field loading confinement conditions. The research shows that this model is an improvement on the rutting damage model for unbound aggregate currently used in AASHTO's mechanistic–empirical pavement design approach because it offers better material characterization and rutting prediction of the unbound base or subbase layer.
This paper presents findings from an ongoing research study at the University of Illinois focused on developing and calibrating an improved permanent deformation model for unbound aggregate materials through laboratory testing and characterization. The project scope included testing sixteen aggregate materials, commonly used in the state of North Carolina for pavement base courses, in the laboratory through monotonic and repeated load triaxial testing. This paper primarily focuses on quantifying effects of aggregate gradation on permanent deformation behavior. To accomplish this, four materials were tested at both: (1) "source gradations," i.e. original gradations from quarry, and (2) an "engineered gradation," i.e., standard reference gradation at which aggregate specimens were prepared for testing. Predictive rutting models were developed to consider the influences of shear strength and applied stress states on permanent deformation accumulation. Rutting model parameters obtained from testing aggregate specimen at one gradation could be used to reasonably predict the permanent deformation accumulation in another sample given the shear strength properties did not show notable differences. For specimens corresponding to significantly different amounts and/or plastic nature of fines, the permanent strain levels predicted using one set of model parameters differed significantly from those predicted using another set of model parameters developed for another gradation. Moreover, the effects of gradation on permanent strain accumulation were significantly more pronounced at the higher shear stress ratios (e.g. 0.75), compared to lower shear stress ratios; which is defined as the ratio between the shear stress applied to a specimen during repeated load triaxial testing compared to the corresponding shear strength under the same confinement.
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