Characterization of unbound materials for flexible pavement design

Abstract

A flexible pavement's response to traffic loads is a critical issue that is directly related to the pavement design procedures and its overall expected life. In general flexible pavements consist of two layers, the bound asphalt upper layers, and the underlying unbound aggregate layers, and both of these layers are expected to the support loads imposed upon them from traffic under the conditions expected during their design. For the current research, the focus is on the characterization of the unbound aggregate materials in the base layer for flexible pavement design purposes through laboratory and model-based investigations for pavement design procedures. Current pavement design procedures, for unbound layers, are often empirically based on previous research and/or based on in-situ experience. However, it is known that these unbound materials behave in a much more complex manner and that their behavior can have a significant impact during pavement design procedures. Over the years knowledge concerning the influences related to the behavior of these materials has been researched and discussed. These materials are known to behave in a non-linear manner dependent on a range of factors including materials based and condition-based factors. Despite these known influencing factors pavement design procedures still most often continue to consider their behavior as either static and/or linear in nature most often for simplicity of calculation purposes. For materials, testing procedures have been developed over the years that can define this known nonlinear behavior in relationship to the conditions the unbound materials are expected to experience within the underlying pavement layers. With the development of laboratory testing procedures, the development of constitutive based regression models commenced with multiple models aiming to most accurately define the behavior of these unbound materials based on mechanical or physical properties. Many constitutive models were developed over the years, however, their full potential for incorporation into pavement design procedures was not effective and feasible until the development of computer-based modeling programs that can more rapidly assess the predicted responses within pavement structures. Despite the potential for their inclusion into computer-based Finite Element Modeling (FEM) most often this relatively new arena of FEM based pavement design analysis still limits and overly simplifies unbound material behavior in modeling and analysis procedures. The majority of current research into these FEM analysis procedures limit their design of unbound pavement materials to a linear-isotropic based behavior as research has been focused more on the in-depth investigation of the upper bound pavement layers. This oversimplification can potentially have a significant effect of the pavements structures overall responses and is necessary to be included within FEM based pavement analysis procedures in order to more accurately predict pavement responses. This imbalance and lack of knowledge on the impact that the nonlinear anisotropic laboratory-based FEM modeling of unbound base materials may have on overall pavement responses is the focus of the current dissertation. With this knowledge gap in mind, the current research focuses on investigating the pavement responses through FEM analysis, incorporating nonlinear anisotropic laboratory-based modeling of the unbound base layer and the impact this characterization has on a wide array pavement structures and on their predicted pavement responses at critical locations.