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Effects of Rayleigh Damping Parameters on the Subgrade’s Apparent Non-Linearity

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The effect of the subgrade’s apparent non-linearity captured during Falling Weight Deflectometer (FWD) testing has been a major challenge for pavement engineers in trying to backcalculate reliable layer moduli. The current state of the practice backcalculation software account for the subgrade’s apparent non-linearity by using a non-linear subgrade model or introducing an artificial rigid layer at a certain depth to obtain more realistic moduli. While these models tend to provide relatively acceptable results for typical pavements in many cases, they appear to be ineffective in backcalculating reliable moduli for rigid or thick and stiff flexible pavement structures.  Dynamic models, which provide a more realistic approximation, tend to focus on the viscoelastic behavior of the asphalt concrete layer and the non-linear behavior of the unbound layers.  This paper focuses on identifying the shortcomings of the current models and developing a simple and robust model that approximates the overall behavior of the entire pavement structure.  A two-dimensional (2D) axisymmetric finite element (FE) model was developed addressing the limitations of the existing models, and properly incorporating the subgrade’s damping behavior. The model was used for the backcalculation of layer moduli for flexible and rigid pavement structures built over the same subgrade at the Federal Aviation Administration (FAA) National Airport Pavement Test Facility (NAPTF). The backcalculation with the new model produced reasonable layer moduli that are consistent with the type of material in each layer, and more importantly, it produced layer moduli that are almost equal for the same subgrade under the different pavement structures and types.


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