Message from the Director: Not a Fan of Recycled Binder Availability Factors

The ongoing research and application of recycled binder availability factors deeply concern me. The discourse surrounding RAP/RAS binder availability started decades ago, with some claiming that RAP is merely black rock, suggesting the aged binder within RAP/RAS isn’t active in the mixture as a binding agent. I’ve never believed in the black rock theory. My early career involved designing mixes with RAP based on viscosity blending charts, which assumed complete blending of RAP binder with virgin binder to form a composite binder with properties roughly approximating a weighted average of both constituents. However, this approach seems flawed upon reflection. Thus, enters the partial binder availability concept.   

I’m not opposed to the idea that a portion of recycled binder may remain inactive; however, I’m skeptical of applying a single number (percentage) to all recycled binders and situations due to various influencing factors affecting activation. One powerful combination of factors is time and temperature. Several studies have demonstrated that the diffusion rate of a RAP/RAS binder and virgin binder depends on time and temperature during production and transportation to the paving site. Moreover, diffusion will continue, albeit at a slower rate, as the mix cools and may even continue at ambient temperatures. Thus, “activation” isn’t a single number but rather a process increasing over time. However, the time-temperature combination differs from mix to mix, even within a given day of mix production, so activation is not a constant.    

The properties and compatibility of the recycled and virgin binders also play a crucial role in blending. Not all RAP is the same, and not all virgin binders are the same. Some RAP is much more aged than other RAP depending on how old the pavement was at the time of milling, the depth of milling, the type of mixes in the pavement, and whether or not a pavement preservation treatment has been applied to the pavement at some point in its life. Similarly, RAS properties can vary considerably depending on material type, age, and source.   

Another limitation of recycled binder availability factors is their failure to fully capture the potential of recycling agents (a.k.a. mix rejuvenators) to restore the physical and chemical properties of recycled binders, thereby enhancing the field performance of high-recycled-content mixtures. Growing evidence from well-documented field trials show bio-based rejuvenators can help achieve long-lasting field performance and reduce a pavement’s carbon footprint.   

While recycled binder availability is an approach that may improve the cracking resistance of high RAP/RAS mixtures by adding more virgin binder, it has limitations, especially in specifying a universal binder availability factor across diverse materials and mix designs. A more effective approach lies in specifying mixture performance tests for various distresses, such as cracking, rutting, and moisture damage, as part of mix design and quality assurance. This is where the promise of balanced mix design shines— allowing the results of mix performance tests to guide decisions, acknowledging the intricate interactions of materials that defy simple predictions.  

Randy C. West, Ph.D., P.E. | Director and Research Professor