A Limit to RAP Content?

Use of larger quantities of RAP in highways might be attractive today due to the obvious savings in material costs, but that might lead to trouble “down the road,” say José P. Aguiar-Moya and Jorge A. Prozzi, Ph.D., University of Texas-Austin, and Feng Hong, Ph.D., P.E., Texas DOT, in their January 2011 TRB paper, RAP: Save Today, Pay Later?

In Texas, the DOT allows up to 30-percent RAP in base mixtures and up to 20-percent RAP in surface mixtures. “There are many advantages that are associated with the use of RAP, including economic benefits due to the reduction in virgin asphalt binder and new aggregates required, environmental benefits associated with the use of a recycled material, significant energy savings, and short-term performance benefits due to increased rutting resistance,” the authors say. “However, field observations have raised some concerns in terms of the long-term performance of mixtures containing RAP compared to those of virgin mixes.”

In order to address these concerns, the authors used data from FHWA’s Long-Term Pavement Performance (LTPP) project’s SPS-5 experiment in Texas to quantify and compare the field performance of pavement sections containing RAP to those of those that don’t.

Based on the SPS-5 data, simple performance models were developed for rutting and cracking of the pavement structure. The models were then used to statistically quantify the effect of RAP on each type of distress and to estimate the expected pavement life of a given overlay, with and without RAP.

“As expected, the results indicate that there is a significant gain in rutting resistance when using RAP,” the authors say. “However, pavements containing RAP develop cracking earlier, and at a faster rate, so short-term savings may be offset by additional overlays later in the life of the pavement. This raised the following concern: Are we saving today to pay later?”

The authors conclude that RAP may not be always the most economical solution, and that lifecycle cost analysis is imperative to assess the real benefits and costs of the various alternatives. “The interim results indicate that, under particular scenarios, the use of RAP might not be the most economic choice,” the authors say. “Where and how much RAP should be used should be determined through a case-by-case analysis.”

The authors don’t want to discourage higher amounts of RAP in mixes, but emphasize that pavement designers need to be cautious with the use of RAP and to take into consideration that pavement structures with RAP might deteriorate faster in the long run, mainly in cases where RAP is used in thin overlays.

“Increasing RAP percentages is not always the solution,” they write. “Consequently, it is important that proper deterioration models be developed and calibrated for the different regions where RAP is used so that proper economic analysis is applied for determining whether or not to use RAP in each specific project. Pavement managers should consider that using RAP today may result in initial construction savings, but the long-term maintenance and rehabilitation costs might overshadow these initial benefits.”

ETG Guides RAP Use

Also guiding the effort toward higher percentages of RAP is the Reclaimed Asphalt Pavement Expert Task Group, or RAP ETG. Created in 2007 by the FHWA, the RAP ETG is composed of materials specialists from FHWA, state DOTs, AASHTO, NAPA, NCAT at Auburn University in Alabama, and other stakeholders.

Its mission is to advance the use of RAP in asphalt paving applications by providing information emphasizing the production of high-quality, high-RAP mixtures, the performance of asphalt mixtures containing RAP, technical guidance on high-RAP projects and RAP research activities.

In September 2011, activity of the RAP ETG was the subject of an FHWA Tech Brief titled High Reclaimed Asphalt Pavement Use (the ETG defines high RAP as 25-percent or more RAP in an asphalt mixture by weight of the total mix).