Comparative performance of rubberized hot mix asphalt under ALF loading

In response to the Intermodal Surface Transportation Efficiency Act (ISTEA) of 1991, Louisiana constructed five projects to evaluate several methods of using discarded tire rubber in highway pavements. The field performance was quite variable. To achieve the agreement among state agencies on the engineering benefits of using crumb rubber modifier (CRM) in HMA pavement and to define the circumstances where the LaDOTD can use asphalt rubber materials in the most cost effective way, a full-scale research program has been conducted at the Louisiana Transportation Research Center (LTRC) to evaluate the performance of CRM-HMA asphalt pavement under Accelerated Loading Facilities (ALF). Three ALF test lanes were constructed at the Louisiana Pavement Research Facility (LPRF), one with conventional mixtures, one with a CRM-HMA wearing course and one with a CRM-HMA base course. The observed field data were used as the basis for the performance comparisons among the test lanes. In this study, the 2-D finite element analytic model called FLEXPASS was used to predict the performance of the ALF test lanes. Laboratory test data was collected to develop appropriate material modeling parameters that are used to predict the performance of the ALF test lanes. The predicted results of performance derived from these numerical simulations of the test lanes have been evaluated and compared with the field data to determine how well the numerical model predicted performance. The purposes of this study includes evaluating the overall performance of hot mix asphalt mixtures containing CRM as compared with similar mixes with conventional HMA under ALF loading, and identifying the optimal location in the pavement structure that the LaDOTD can use asphalt rubber materials in a cost-efficient manner. Based on the results from this study, it is observed that there is good agreement between FLEXPASS predictions and measured field performance and that FLEXPASS can be used to successfully model Louisiana flexible pavements. Based on the results of this study, it is found that even though CRM asphalt mixtures and conventional asphalt mixtures behaved very similarly in the laboratory characterization, ALF test lane contained CRM-HMA base course exhibited significantly smaller rut depth than the other two test lanes. Test lane contained CRM-HMA wearing course exhibited similar rut depth as the lane with conventional mixes. Predictions of serviceability for the lane with CRM-HMA Type 5A base course were higher than the lanes with all conventional materials or with the CRM-HMA Type 8 wearing course. The overall performance of CRM-HMA base course was better than the performance of CRM-HMA in the surface course. From the results of this study, the author concluded that: (a) the DOTD should consider extending the use of modified binders in all flexible pavement layers in the light of the superior performance of the AR Type 5A base section; and (b) the DOTD should consider adding asphalt rubber hot mix to its list of available base course materials