During the 2007 hot mix asphalt (HMA) paving season, several newly placed HMA pavements in New York State showed signs of pavement flushing. Initial investigations of the flushing showed it was not attributable to the factors associated with conventional flushing. While conventional flushing is typically attributable to mix factors such as excess fines, excess asphalt binder, low air voids or to construction factors, such as contamination or allowing traffic on the pavement before the HMA has had time to set up, NYSDOT defined the condition as “Atypical” flushing. “Atypical” flushing describes a pavement that has flushed without any known reason. Field investigations conducted by NYSDOT showed that there were several key characteristics of the “Atypical” flushing:

• Flushing began to appear a couple of weeks after placement;
• The pavement continued to be soft and pliable weeks after placement; and
• The compacted mix moved, rutted, and/or shoved under traffic.

The flushed pavements fitting this “Atypical category were limited to Central New York (Syracuse, Utica, Watertown, and the surrounding areas), and were placed from May to August 2007. Additional facts regarding the “Atypical” flushed pavements were:

• Different contractors constructed the pavements;
• Different HMA producers made the HMA mixtures;
• Different HMA mix designs were affected;
• All HMA mixtures were made with limestone aggregates;
• All HMA mixtures were made with PG64-28 binder that were modified with polyphosphoric acid (PPA);
• The PG64-28 binder was supplied by three (3) different Primary Sources;
• NYSDOT unsure on the concentration of PPA in the binders used;
• All retained binders were verified as a PG64 high temperature. Meanwhile, binder recovered from field cores tested 1 to 2 grades lower.
• Gradations of cores taken in the wheelpath, where the “Atypical” flushing occurred, showed gradations finer than the gradations performed during production. Meanwhile, gradations from cores taken outside the wheelpath, where flushing did not occur, showed to be relatively comparable to gradations performed during production.

After the initial recognition of the flushing problem, NYSDOT directed the asphalt supplier to stop using PPA modified PG64-28 and substitute it with a “neat” PG64-22 asphalt binder without making adjustments to the mix design. The “Atypical” flushing did not appear in any of the pavements placed after the substitution. Unfortunately, the root cause of the “Atypical” flushing has yet to be determined.

Research Obejectives

The objectives of this research are to determine the causes behind the “Atypical” HMA pavement flushing experienced in NYS during the 2007 paving season, and to determine if PPA modified PG Binders use should be restricted in New York.