Alternative Methods for Increasing the Durability of RAP Mixtures

It is commonly accepted that using reclaimed asphalt pavement (RAP) in asphalt mixtures can provide economic savings to contractors and highway agencies by reducing the demand of both the virgin binder and aggregates in asphalt mixtures. Recent increases in crude oil costs have caused escalations in both the cost of virgin asphalt binder and energy. This cost increase can be offset by using more RAP in asphalt mixtures. Recent reports have suggested that material costs can be decreased by 20 to 35 percent by using 25 to 50 percent RAP in asphalt mixtures. Although RAP may be used for the construction of a granular base course or shoulder material, the greatest economic and environmental benefits can be realized when RAP is used to replace virgin binder and aggregates in the production of asphalt mixtures. Before permitting high RAP percentages, agencies want assurance that high RAP mixes will provide satisfactory field performance. If high RAP mixtures cannot perform as well as virgin mixtures or even low content RAP mixtures, methods for improving the durability of these mixtures are necessary. One suggested method of increasing the durability of high RAP mixtures is to adjust the grade of the virgin binder. Current recommendations provided by AASHTO M 323 are based on levels of RAP percentages. Each level represents a RAP percentage by weight of the aggregate. When between 15 and 25 percent RAP is used in an asphalt mixture, current guidance suggests that mix designers should reduce both the high and low critical temperatures by one performance grade. When more than 25 percent RAP is in the mixture, blending charts should be used to determine the appropriate virgin binder grade; however, many state agencies want to minimize the use of solvents required for extracting and recovering the RAP binder. Additionally, some state agencies do not want to change the grade of binder more than one or two grades since incomplete mixing may result in soft areas in the pavement instigating early distresses. Additionally, softer binder grades may prove to be scarce or more expensive when purchased. Recent research has also suggested that the performance of RAP mixtures might also be related to the volume of the virgin binder in the mixture rather than the performance grade of the virgin binder (2). It has also been hypothesized that WMA might help improve high RAP mixture performance; however, little field data is available to strengthen this theory. This research plan was developed to assess whether increasing the volume of effective virgin binder, using a softer binder, or using a warm mix asphalt (WMA) technology aided in improving the durability of mixtures containing high percentages of RAP. The objective of this research was to evaluate how increasing the volume of virgin binder, using a softer grade of virgin asphalt binders affects the durability of RAP mixtures. In addition to changing the grade of the virgin binder, a WMA chemical additive was added to the control RAP mixtures to assess how using this WMA technology affected the mixture’s durability and rutting performance.