Recyclability of Polyphosphoric Acid–Modified Asphalt: Laboratory Study

Over the past 15 years, the use of polyphosphoric acid (PPA) in asphalt pavements has been increasing. Given this general time frame, many of these original asphalt pavements containing PPA will soon be rehabilitated and thereby produce recycled asphalt pavement (RAP) material containing PPA. With an industry driven by reducing material costs and promoting sustainability, the use of higher percentages of RAP is being pursued. This trend raises the question of the general recyclability of PPA-modified asphalt and whether it performs as well as conventional polymer-modified RAP when used in hot-mix asphalt. PPA-modified asphalt has been extensively studied in recent years, from its fundamental mechanisms to general field performance. However, research concerning the recyclability of PPA-modified asphalt has yet to be conducted. The present study evaluated the performance of asphalt mixtures modified with various percentages of laboratory-produced RAP. Laboratory aging procedures were used to produce RAP similar to RAP sampled from the field. Three asphalt binders were evaluated: styrene–butadiene–styrene (SBS) only, PPA-SBS, and PPA only. All three asphalt binders graded out to a PG 76-22. Test results indicated that at RAP percentages as high as 45%, all three RAP types resulted in similar performance with respect to permanent deformation, fatigue cracking, stiffness, and moisture damage susceptibility. The research effort concluded that RAP containing PPA is not detrimental to hot-mix asphalt and should perform in a similar manner to RAP containing SBS-modified asphalt.