This project was aimed at exploring the feasibility of using materials derived from the guayule (pronounced, ‘why-YOO-lee’) plant in flexible pavement mixtures (FPMs). Work in the initial phase involved procuring test equipment and materials, literature review, extraction and recovery of petroleum-based binders from reclaimed asphalt pavement (RAP) and reclaimed asphalt shingles (RAS), development and implementation of methods to extract and recover guayule-based materials, and material characterization. Although several guayule-based materials were produced during the course of this project, one stood out from the rest as a viable recycling (softening) agent in high-RAP/RAS content FPMs: an acetone-extractable resin that is present as a residue in the commercially-produced, bulk guayule rubber. Binder-blending mixture experiments were performed that involved determining temperature-dependent stiffness parameters for various blends containing different proportions of reclaimed RAP and RAS binders, and a virgin binder which was either a commercially available, petroleum-based recycling agent (Cyclogen®L or simply CycL) or the guayule rubber resin (RR). Paired t-tests on ten different blends showed that the RR-RAP/RAS blends performed in the same way as proportionally-identical CycL-RAP/RAS blends in terms of high-temperature stiffness but were slightly less effective in terms of cold-temperature cracking resistance. The binder-blending mixture experiment results were also used to generate response surface models (RSMs) that were used for optimizing a blend that met select response parameter levels, and were, therefore, necessary in designing and producing FPMs with high proportions of RAP and/or RAS. Although the direct comparison of the RR to the CycL was not completely successful, a RR-RAP/RAS blend (estimated by the RSMs to result in a PG64-22 grade binder) was produced, and specification-verification testing (except flash point) was performed. The blend met all PG64-22 specifications for which testing was performed, except mass change (maximum allowable mass change is ±1.0%: the blend experienced a mass loss of 3.1%). Work in the final phase involved FPM design, production, and testing. Using the same PG64-22 binder blend developed and verified in the initial phase, a high-reclaimed-binder-content FPM was designed and produced that met gradation and most volumetric specifications for a Missouri DOT moderate-quality FPM. This FPM utilized only 5.5% (by total binder weight) virgin petroleum-based binder. The FPM was produced once using the RR and again using the CycL. The results of Hamburg wheel-track testing on these two FPMs showed that the RR-based FPM performed as well as the CycL-based FPM in terms of rutting and moisture-susceptibility resistance. In conclusion, the RR can, in a practical sense, be used as a recycling agent.

The contractor's final report is available.