Project Details

The usage of Reclaimed Asphalt Pavement (RAP) and Reclaimed Asphalt Shingles (RAS) in asphalt mixtures has increased significantly in recent years. While increasing recycled material usage has the potential to yield environmental and economic benefits, there is uncertainty in the validity of the current volumetric design and the associated performance of RAP and RAS mixtures. The mixture design procedure currently employed by the NCDOT assumes 100 percent binder contribution from recycled materials. However, it is currently poorly understood to what extent the recycled binder acts as "black rock" as opposed to actually blending with virgin asphalt. The erroneous assumption of complete blending will lead to an underestimation of required virgin asphalt binder content during mixture design and consequently yield mixtures with high cracking susceptibility. In addition, the recycled binder contribution is an important consideration when selecting the virgin binder grade for a mixture. Erroneous assumptions of blending may have negligible effects at low reclaimed binder contents but significant effects at higher contents. Therefore, given the increasing use of higher recycled material content mixtures in practice, an in-depth study is needed to develop an understanding of recycled binder contribution to improve the virgin binder grade selection and volumetric design of RAP and RAS mixtures.

To address this need, the proposed research will achieve three objectives: (1) elucidate binder contribution in RAP and RAS mixtures as a function of material and laboratory fabrication variables; (2) develop improved procedures for the virgin binder grade selection and volumetric design of RAP and RAS mixtures; and (3) develop a plan for long-term monitoring of field sections to validate the refined mixture design procedure. These objectives will be accomplished by executing seven tasks:

1. A literature review will be conducted to identify approaches to asphalt mixture design with high recycled binder content, factors affecting binder contribution, and RAP and RAS handling best practices, and laboratory fabrication procedures.
2. The effect of material factors (e.g., binder grade, recycled material content, etc.) on recycled binder contribution in RAP and RAS mixtures will be evaluated using a systematic study of laboratory fabricated samples. A novel tracer-based Energy Dispersive X-ray Spectroscopy (EDS) Scanning Electron Microscopy (SEM) method will be used to elucidate blending within produced mixtures.
3. A systematic study of laboratory mixing procedures complemented with performance testing will be conducted to inform improved handling of recycled materials in the laboratory and determine the significance of recycled binder contribution on performance.
4. If a willing contractor can be identified, plant mixtures will be produced using tracer doped virgin binder to determine the recycled binder contribution. A comparison of the blending levels in lab and plant fabricated mixtures will inform the refinement of the laboratory mixing procedure.
5. Proposed revisions to the NCDOT RAP and RAS volumetric mixture design and virgin binder grade selection procedures will be developed based on the expected percent binder contribution from the reclaimed material. A cost-benefit analysis will be conducted to evaluate the implications of implementing the proposed revisions.
6. A plan for long-term monitoring of field projects to validate the refined mixture design procedure will be developed for future use by the NCDOT.
7. A final report summarizing the findings of the research will be prepared.

The research will result in improved procedures for the virgin binder grade selection and volumetric design of mixtures with RAP and RAS. The research results are anticipated to improve the performance of RAP and RAS mixtures, consequently increasing pavement service life and reducing life cycle costs. The research products may also enable the design of satisfactory asphalt mixtures with higher RAP and RAS contents, which could reduce the use of virgin binder and aggregate.