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Analytical Procedures for Determining Phosphorus Content in Asphalt Binders and Impact of Aggregate on Quantitative Recovery of Phosphorus from Asphalt Binders

Polyphosphoric acid has been increasingly used as a means of producing modified binders for the past 10-15 years in North America. While there have been reports of isolated use of phosphoric and polyphosphoric acids prior to the advent of Superpave Performance Graded binders, the increased demand for high performance binders resulting from the adoption of Performance Graded binders stimulated more widespread research into the means by which polyphosphoric acid could effectively and economically enable binder suppliers meet these demands. Consequently asphalt suppliers in all regions of the United States and Canada turned to polyphosphoric acid to meet the new specifications. It was found that polyphosphoric acid when used at levels as low as 0.5% by weight of binder could increase the high temperature PG grade of some binders by one full grade. Most binders required approximately 0.8% to 1.2% polyphosphoric acid by weight of binder and some required considerably more; sometimes more than 2%. Still other asphalt suppliers found that the addition of low levels, typically less than 0.5% by weight, of polyphosphoric acid to polymer modified binders enabled them to reduce polymer loading without negatively impacting mixture performance and in some reported cases enhancing mixture performance. The growing realization by user agencies that they were or might be receiving binders containing polyphosphoric acid led to several reactions. Some agencies banned the use of the material, although there was no standardized procedure for measuring the acid content in binder. Other agencies wanted to be able to determine the amount, if any, of acid present in their binders. Still others who had used polyphosphoric acid modified binders for several years with no obvious ill effects paid the entire issue little heed. FHWA implemented a wide ranging, multi-year investigation into the impact of polyphosphoric acid levels, ranging from 0.2% to 4%, on the physical and moisture sensitivity properties of binders as well as the performance characteristics of mixtures produced using those acid containing binders. At MTE a decision was made to develop a rapid, reliable technique for determining the amount of phosphorus in asphalt binders. This task was undertaken to fill an obvious need in the industry. In some areas polyphosphoric acid was being blamed for project failures with no analytical evidence or supplier acknowledgement that the acid had been used in the binder. Any projects which did use acid and which exhibited problems would certainly be worthwhile evaluating to determine the amount of acid used in the binder and the extent to which that acid content caused the problems. Further motivation came in 2008 when world crude prices spiraled out of control and the supply of butadiene needed to produce SBS polymers decreased precipitously. Under these circumstances some agencies came to the realization that a modest amount of polyphosphoric acid could help to stretch the SBS supply. With a reliable tool for determining the amount of phosphorus in the binder it would be possible to establish limits on the amount of polyphosphoric acid that would be acceptable to the agency. Still more recently some researchers have suggested that pavement problems attributed to polyphosphoric acid may actually be due to the use of the bottoms from the re-refining of engine oils, so called REOB or Reclaimed Engine Oil Bottoms. Lube oils contain fairly high quantities of zinc dithiophosphate and the phosphorus that shows up in some asphalt binders may actually come from REOB and not polyphosphoric acid. It is against this complex background of factors that the work reported herein has been conducted.

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  • English

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  • Accession Number: 01328026
  • Record Type: Publication
  • Files: TRIS
  • Created Date: Jan 20 2011 11:22AM