Backcalculation Method to Determine Effective Asphalt Binder Properties of Recycled Asphalt Pavement Mixtures

State departments of transportation are beginning to require the use of higher percentages of recycled asphalt pavement (RAP) in hot-mix asphalt. To help implement higher RAP contents, better procedures are needed for evaluating the binder properties of higher RAP content asphalt mixtures without chemical extraction. In particular, new and innovative methods are required to determine how the addition of RAP influences mechanical properties of the composite (RAP + virgin) asphalt mixtures. A method is summarized for determining what is referred to as the effective asphalt binder stiffness properties of asphalt mixtures blended with RAP. The proposed extraction-less procedure uses the Hirsch model to backcalculate the asphalt binder stiffness (G*) from asphalt mixture stiffness (E*). In addition, an analytical procedure to determine the phase angle (delta) is discussed. Test data comparing predicted versus measured asphalt binder properties of plant-produced asphalt mixtures with 0% RAP are presented. Additional validation data are presented for plant-produced asphalt mixtures of varying RAP percentages. Examples of implementing the backcalculated binder properties in the Mechanistic-Empirical Pavement Design Guide, as well as comparing the backcalculated asphalt binder properties with mixture fatigue test results, are also shown and discussed.