Recycling of asphalt pavements is crucial to alleviating the growing demand for paving materials including both asphalt binder and aggregates. For this reason many states have adopted specifications for the use of reclaimed asphalt pavement (RAP) in hot mix asphalt (HMA). These specifications, however, are based on the percentage weight of RAP in the total mix. Because, RAP binder is usually much stiffer than virgin binders and contributes largely to the increased stiffness of the recycled mixture placing emphasis on the recycled binder content would be a more efficient way to optimize use of RAP. The state of North Carolina studied recycled materials with the objective of placing limits on percentage of binder contributed by RAP, instead of percentage by weight of mix replaced. As part of the research, one RAP material was selected and tested with different virgin binders and limits were derived. Since, recycled binders from different RAP sources exhibit different properties, the limits determined for binder from a single RAP source are not directly applicable to all RAP sources.
This research aimed to evaluate the effect of variability in the RAP binders on the recycled binder limits by examining extensively the rheological properties of recycled binders from different RAP sources. RAP stockpiles from different geographical regions were selected and the recycled binder extracted from RAP was characterized using the Dynamic Shear Rheometer and binder limits determined to identify differences among the stockpiles. Statistical inferences were derived from the rheological properties of the different RAP binders and the binder limits obtained for different stockpiles were used to develop draft specifications to select the optimum amount of recycled materials based on the variability between and within stockpiles.
The
recycled binder limits determined from this study take into consideration the
variability in the rheological properties of the RAP binders sampled from
various stockpiles. These limits are solely based on the expected blended
binder properties and do not account for the aggregate gradations of the RAP
material. Often times, there is a high proportion of fines in RAP and this
might be a limiting factor when high percentages of RAP are incorporated into
HMA. Therefore, the limits should be followed carefully by considering for how
the proportions defined by the limits affect the aggregate gradations of the
overall HMA mix. The limits obtained in this research study are based on some
underlying assumptions and if such assumptions stand invalid, the limits could
lead to detrimental results. Therefore, it is recommended to study the industry
practices on the selection criteria for RAP stockpiles in pavement projects and
adjust the limits accordingly. Additionally, the results could be tuned further
if blending was done with the RAP binders from the remaining stockpiles and
limits determined. Having the limits from all the nine stockpiles would help in
obtaining more robust generalized limits. Finally, this research assumed 100%
blending between virgin and RAP binders which may not be essentially true in
practice. In such cases, consideration should be given to how fractional
blending would affect the limits.
