Development of a Mechanistic-Based Approach to Evaluate Critical Conditions of Hot Mix Asphalt Mixtures
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The performance of an asphalt pavement is significantly impacted by the properties of the asphalt mixture, pavement structure, and the imposed environmental and traffic loading conditions. In particular, hot mix asphalt (HMA) mixtures are believed to have a critical combination of temperature and traffic loading rate which will result in excessive permanent deformation. Designing the appropriate mixture type and properties are significantly important tasks that pavement engineers make on a routine basis. For many years this key decision has been made relying upon empirical procedures that lack fundamental characterization that might not be representative to the specific project condition. In light of this limitation, there is a need to develop an advanced, yet realistically simplified approach to assess, based on closely simulated field conditions, the rutting susceptibility of HMA mixtures under a given set of traffic loads and environmental conditions that are applicable to the project. In the first phase of this study, dynamic mechanistic analysis with circular stress distribution was used to simulate field loading conditions. Extensive mechanistic analyses of three different asphalt pavement structures subjected to moving traffic loads at various speeds and under braking and non-braking conditions were conducted using the 3D-Move model. Predictive equations for estimating the anticipated deviator and confining stresses along with the equivalent deviator stress pulse duration as a function of pavement temperature, vehicle speed, and asphalt mixture's stiffness have been developed. This study presents a new mechanistic-based approach that consists of evaluating asphalt mixtures using the repeated load triaxial (RLT) test at field representative testing conditions to determine the critical temperature of the HMA beyond which the mixture becomes unstable. An HMA was considered appropriate for a specific project location if the determined critical temperature was greater than the effective asphalt pavement temperature for rutting determined using the Mechanistic-Empirical Pavement Design Guide (MEPDG) software. Predictive equations that account for the actual project characteristics such as climate conditions, material characteristics, operational speed, and traffic loading were developed in this study to estimate the effective asphalt pavement temperature. Nine HMA mixtures, each associated with a specific project that has performed well in rutting, were characterized and analyzed for rutting behavior. The critical temperature for each HMA was obtained using RLT results and actual field performance. Flow number criteria as a function of traffic level were also developed for the HMA mixtures. The proposed approach was validated using three additional mixtures from Nevada, two mixtures from the WesTrack accelerated test facility, three mixtures from the Minnesota Road Interstate test facility (MnROAD) and two additional mixtures from the MnRoad low volume closed loop test facility. Very good agreement between laboratory results and field performance from the 10 different HMA mixtures was achieved supporting the appropriateness of the proposed approach.Impact of the HMA mixture characteristics and its component interactions on the critical conditions was also studied. This study investigates the influence of aggregate characteristics using Aggregate Imaging Measurement Systems, the asphalt binder non-recoverable creep compliance using the Multiple Stress Creep Recovery test, and asphalt mixture air void content on the rutting performance of HMA mixtures. A comprehensive statistical model to predict the HMA critical temperature has been presented. The statistical model is able to effectively account for the influence of aggregate, binder and HMA mixture properties on rutting potential of asphalt mixtures.