Phosphoric acid is used to increase the high temperature Performance Grading of hot mix asphalt to help resist the effects of traffic during summer temperatures. For quality control purposes, the amount of phosphoric acid present in a binder can be measured using X-ray fluorescence spectroscopy. However, x-ray fluorescence spectrometers can be cost prohibitive for some agencies.
The Turner-Fairbank Highway Research Center (TFHRC) has developed a very simple procedure that requires just a few simple chemicals and no specialized equipment. While the new procedure to detect phosphoric acid does not measure the amount of phosphoric acid in the binder, it clearly demonstrates whether phosphoric acid is or is not present, and can detect levels as low as 0.1 percent.
This qualitative test method can be used to identify the presence of phosphoric acid in asphalt. It is simple to do and requires no specialized equipment. If phosphoric acid is present in the binder, a blue color is developed in less than five minutes. The method will detect phosphoric acid at concentrations as low as 0.1 percent. The method is not quantitative. It does provide positive confirmation of the presence of phosphoric acid.
The presence of recycled engine oil bottoms (REOB) in the asphalt will interfere with the test. These residues, derived from the processing of waste engine oil, contain phosphorous residues from the oil additives. These will also give a blue color and may falsely indicate the presence of phosphoric acid. Other tests need to be conducted to differentiate between phosphoric acid and REOB. These involve the measurement of phosphorous, calcium and zinc levels by a technique such as X-ray fluorescence spectroscopy (XRF). For further assistance, please contact the TFHRC chemistry department at 202-493-3305.
While the test is very safe, users should observe appropriate chemical hygiene, operating, and disposal procedures. The Material Safety Data Sheets (MSDS) for the reagents should be studied before proceeding.
This method is adapted from ASTM test method D515-88 “Standard Test Methods for Phosphorus in Water.” Butanol is used to extract some of the acid present from the asphalt. The extracted acid is then transferred to the water phase. Its presence is detected by reaction with ammonium molybdate, potassium antimonyl tartrate, and ascorbic acid to form a blue color.
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Turner-Fairbank Highway Research Center
6300 Georgetown Pike
McLean, VA 22101-2296
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