The use of high Recycled Binder Replacement Percentages (RBRs%) in asphalt surface mixtures is increasing. The asphalt binders in Reclaimed Asphalt Pavement (RAP) and Recycled Asphalt Shingles (RAS) are generally hardened and embrittled from oxidization and may not fully mobilize and blend with virgin materials. Consequently, high recycled content mixtures may be prone to cracking if appropriate measures to consider this effect are not taken during the mixture design process.
An in-depth study is needed to systematically evaluate the effects of asphalt content on the cracking and rutting performance of RAP and RAS mixtures to guide refinement of the NCDOT's mixture design procedure to achieve reliable performance. It is imperative that this study encompasses a broad range of recycled material sources because past research has demonstrated that RAP and RAS stockpiles across North Carolina vary greatly with respect to rheological properties. In addition, a review of asphalt plant operations for stockpiling, processing, and characterizing recycled materials is necessary to identify measures that can be imposed in the NCDOT's specifications to reduce the variability in RAP and RAS sources within the state.
Effect of RAP content on cost of asphalt mixtures (Zaumanis and Mallick 2016)
The objectives of the proposed research project are to:
- modify the NCDOT's procedures for the design of surface mixtures containing RAP and RAS to improve performance and
- modify the NCDOT's specifications to improve the consistency within and across RAP and RAS stockpiles within North Carolina.
These objectives will be accomplished by executing seven tasks:
- A literature review will be conducted to identify existing RAP and RAS mixture design procedures, the effects of recycled binder contribution assumptions on performance, and measures to reduce variability in RAP and RAS stockpiles.
- An operational review will be conducted to identify how contractors process, stockpile, characterize and use RAP and RAS under the current NCDOT guidelines.
- Six RAP and three RAP/RAS 'control' NCDOT mixture designs will be identified and acquired for evaluation within the project. Samples of the recycled material sources included in the mixture designs will be characterized prior to finalizing the selected list of mixtures to ensure that the study materials encompass a broad range of recycled material properties in terms of gradation, asphalt content, and extracted and recovered binder grade.
- Baseline virgin mixtures will be designed according to the NCDOT's current procedure using the same virgin aggregate as the corresponding control mixtures. For three of the control mixtures, designs will also be prepared at an additional recycled binder replacement percentage.
- Performance test specimens will be prepared for each mixture design from Tasks 3 and 4. In addition, for each of the recycled mixture, performance test specimens will be prepared at two additional asphalt contents. Rutting and fatigue performance testing will be carried out on the prepared specimens.
- Relationships between asphalt content and performance will be developed for each recycled mixture. These relationships will be used to identify the maximum virgin binder content allowable and to maximize cracking performance without having the rutting performance fall below a critical performance threshold for each mixture. The collective results will be used to identify appropriate revisions to the NCDOT's current recycled mixture design procedure to ensure reliable performance.
- A final report will be prepared that summarizes the findings of the previous tasks.
The research results will lead improved specifications that will facilitate the design of better-performing surface mixtures containing recycled materials. These specifications will improve the durability of NCDOT pavements and consequently decrease life-cycle costs.