Past and on-going NCDOT research projects have provided insights on critical aspects of the performance of asphalt mixtures containing recycled asphalt materials (RAM). NCDOT RP 2019-21 found that agglomerations of RAM particles exist in asphalt mixtures. These agglomerations act as 'black rocks' and prohibit complete recycled binder availability. However, all experiments conducted in NCDOT RP 2019-21 employed a laboratory bucket mixer to prepare asphalt mixture samples, which may not reflect mixing in an asphalt plant.
Furthermore, the NCDOT does not currently specify how to preheat virgin aggregate and RAM when producing asphalt mixtures in the laboratory, which may affect recycled binder contribution. NCDOT RP 2019-21 identified three adjustments to mixture design to account for recycled binder availability, termed availability adjusted mix design (AAMD).
One of the proposed changes was to use only the available recycled binder to calculate the 'effective' rather than total recycled binder replacement percentage (RBR%). The effective RBR% is lower than the total RBR% for a given mixture. Consequently, the reliance on the total RBR% may result in an effective binder system that is softer than what was expected. Shifting to specifications based on the effective RBR% would increase the amount of RAM that can be used in a mixture without adjusting to a PG 58-28 virgin binder and increase the maximum RAM that can be incorporated into a mixture without exceeding maximum limits. However, the performance implications of specifying the effective rather than the total RBR% have not been assessed.
Laboratory Mixers
Correspondingly, the objectives of the proposed research are to: (1) verify or adjust recycled binder availability findings of past and ongoing NCDOT funded research by studying asphalt mixtures produced under conditions that reflect those in an asphalt plant; (2) identify an appropriate laboratory virgin aggregate and RAM preheating procedure for standardization; (3) evaluate the implications of using the effective RBR% for specifying virgin binder grades and limiting maximum RAM contents. The objectives will be accomplished by executing the following five tasks:
- A literature and operational review will be conducted to identify the state of the knowledge and practice on recycled binder availability considerations in RBR% specifications and both laboratory and plant mixing procedures.
- A temperature-controlled laboratory pugmill mixer will be used to evaluate the effects of the following plant conditions that may differ from conventional laboratory mixing on recycled binder contribution: (1) the use of superheated virgin aggregate combined with ambient temperature RAM, (2) the use of wet rather than pre-dried RAM, and (3) mechanical aggregate degradation from mixing. Comparative experiments will be conducted in a conventional laboratory bucket mixer. The results will inform whether or not the findings of NCDOT RP 2019-21 are likely extensible to plant-produced mixtures or require adjustment. The findings will also be used to identify a material preheating procedure for standardization.
- The performance and cost implications of applying the current NCDOT RBR% limits for virgin binder grades and maximum RAM contents to the effective RBR% will be assessed for six asphalt mixtures under both current NCDOT and AAMD mixture design methods.
- The results from the previous tasks will be used to recommend specification changes for determining or assuming recycled binder availability values, laboratory material preheating, and recommendations to either use of effective or total RBR% for virgin binder grade and maximum RAM content limits.
- A final report will be prepared based on the results of the previous tasks.
The research results are expected to yield specification changes that will improve the overall usage, consistency, and performance of RAP/RAS mixtures in North Carolina. Increasing permissible RAM contents through effective RBR% specification could potentially reduce costs and waste while better preserving the environment.
