Laboratory Evaluation of Asphalt Binders and Mixtures Containing Polyphosphoric Acid

Four modified asphalt binders were investigated for performance grade, multiple stress creep and recovery (MSCR), mixture dynamic modulus, and mixture fatigue resistance: polyphosphoric acid (PPA) only, PPA plus Elvaloy, styrene–butadiene–styrene (SBS) only, and SBS plus PPA. MSCR data indicated that the binder modified with PPA only had the highest nonrecoverable compliance and lowest percentage of recovery, whereas the binders modified with PPA plus Elvaloy and with SBS plus PPA were best, with the lowest nonrecoverable compliance and highest percentage of recovery, depending on whether the extracted or laboratory binder was evaluated. The dynamic modulus test results illustrated a smaller difference between mixtures, except where the binder modified with PPA plus Elvaloy had a more desirable variation in stiffness (e.g., softer at high frequencies and low temperatures, and slightly stiffer at low frequencies and high temperatures). The fatigue life ranking was different before the data were normalized for controlled strain conditions with the use of viscoelastic continuum damage principles. Without normalization, data from the two SBS-modified mixtures (with and without PPA) had the highest average fatigue life; however, with normalization, the data for mixtures modified with PPA plus Elvaloy exhibited the highest average fatigue life. Implications of the results are that PPA modification strategies can provide adequate resistance to rutting and moisture damage and that modification with PPA only is not the same as (and is statistically less resistant to fatigue cracking than) modification with polymer or with polymer plus PPA. Also, comparable fatigue cracking resistance can be achieved with the use of SBS alone or SBS plus PPA, which uses less polymer in conjunction with PPA.