Guest Blog Post: Upping the RAP – Putting Research into Practice
All too often, a research project is completed and the final report just sits on a shelf. But, if an
agency invests a little effort in exploring how the results can be used, there may be big payoffs for the agency, contractors and the taxpaying, traveling public. (By the way, researchers are not off the hook – they have a responsibility to help with implementation.)
An excellent example of this follow-through leading to significant savings is recent work by the Indiana Department of Transportation (DOT) to increase the allowable reclaimed asphalt pavement (RAP) contentin their specifications.
Research at the North Central Superpave Center at Purdue University, sponsored by the Federal Highway Administration (FHWA)investigated the behavior of five sets of plant-produced mixes containing up to 40 percent RAP.
The results of mixture testing suggested that the current American Association of State Highway and Transportation Officials (AASHTO) guidelines for changing the virgin binder grade as the RAP content increases, which Indiana had adopted, were too conservative for those materials and those plants. Instead of changing the virgin binder grade above 15 percent RAP, up to 25 percent could be used before the mix properties were significantly affected. RAP contents over 25 percent up to 40 percent could be accommodated by decreasing the virgin binder by one grade.
Aware of the direction the study was leading, the Indiana DOT proactively assessed the properties of RAP statewide. They sampled RAP stockpiles at 33 plants –about a third of the certified plants in the state. They recovered binders from these RAP sources and characterized their PG grades. The results showed that the average RAP binder grade was PG 90-11 with standard deviations of about 5 on the high temperature and 3 on the low temperature. INDOT went on to explore the average true grade of virgin binders based on their QA acceptance tests. Then they looked at how much RAP they could use with those binders using the calculation in AASHTO M323.
The results confirmed that INDOT could allow up to 25 percent RAP with a PGxx-22 grade virgin binder without changing the virgin binder grade and could allow up to 40 percent RAP by lowering the high and low temperature PG grades by one grade.
Implementing this specification change allows contractors to use higher RAP contents with a more economical virgin binder while maintaining desirable mixture properties and performance – saving money and being environmentally friendly at the same time.
About the authors: Matt Beeson, P.E., is an asphalt engineer for the Indiana Department of Transportation. Rebecca S. McDaniel, P.E., Ph.D.., is technical director for the North Central Superpave Center at Purdue University.