MnDOT attempts to limit asphalt pavement cracking by specifying Superpave performance-graded asphalt binders that have favorable properties at very low temperatures, the tech brief says. “However, more factors contribute to cracking than just the binder. The various aggregates that make up more than 90 percent (by weight) of a typical asphalt mix may each respond differently to low temperatures,” it says. “Crude oils from different geographic locations produce asphalt binders that interact differently with aggregates, and asphalt binder grades and contents can also vary from mix to mix.”

Complicating characterization of aggregates to fight thermal cracking is the increasing use of RAP and its residual asphalt binder as aggregate in mixes.

Challenge of RAP Aggregate

The use of RAP in recycled asphalt pavement is well accepted at the state, city and county levels. Almost all hot-mix asphalt (HMA) contains at least some RAP in many areas. But the RAP that can be added to hot-plant mix asphalt mixtures is limited to relatively low percentages and, in some areas, the use of RAP is prohibited in certain types of mixtures, such as surface courses. Typically, the maximum percentage of RAP allowed is anywhere from 15 to 30 percent by weight of HMA mixture.

Thermal crack intervals of 19 to 30 feet are typical, but may vary widely.

Yet, recent investigations show HMA materials with percentages in excess of 50 percent can be produced to perform to the same as “virgin” mixes. Agencies that are not currently allowing RAP into their HMA mixtures — and those that are only allowing small percentages of RAP — can safely increase the amount of RAP used without fear of shortening pavement life, provided that best practices are followed, according to a Federal Highway Administration (FHWA) Tech Brief titled High Reclaimed Asphalt Pavement Use.

As reported in Better Roads late last year, government agencies from coast to coast are trying to determine how to increase the percentage of RAP used in intermediate and friction (surface) courses (see “How Much RAP?,” November 2011, pp. 21-31).

With the new Asphalt Binder Cracking Device (ABCD), asphalt binder is poured around the rings, then cooled to a very low temperature in a controlled manner.

The aggregate in RAP should be considered as if it were just another stockpile of virgin aggregate, the Washington State DOT (WSDOT) says. As with virgin aggregate properties, RAP aggregate properties — foremost among them residual asphalt clinging to the aggregate particles — may limit the amount of RAP that can be used in a particular mixture.

RAP asphalt binder will blend with virgin asphalt binder in most any mix design, and the resulting properties of this blended asphalt binder must be understood, the state says. But the effect of RAP residual asphalt binder must be considered when using RAP in Superpave or any mix design, because RAP asphalt binder already is significantly aged because of its previous field life.

“This aged binder is generally stiffer than virgin asphalt binder and thus will cause the resultant binder blend to become more viscous (stiffer),” WSDOT says. “This, in turn, will cause the HMA to be more viscous (stiffer).”

As a result, successful RAP mix designs incorporating RAP above 15 percent by weight should analyze the stiffness of the existing residual binder in the RAP, and compare it to the stiffness of the virgin liquid binder, along with the proportions of each in the final product.

New Research

As the use of RAP in mixes isn’t going to decline, vigorous research now is under way to reconcile higher percentages of RAP with the danger of stiffness-induced low-temperature cracking.