The Use of Phosphoric Acid to Stiffen Hot Mix Asphalt Binders

CHAPTER 7. SUMMARY OF CONCLUSIONS

The following summarizes the conclusions of this study:

• The phosphoric acid content of binders can be readily measured using XRF, and from the phosphorus content, a presumed level of phosphoric acid can be calculated.
• The addition of phosphoric acid to asphalt binders causes an increase in the heptane insoluble fraction, which is not necessarily accompanied by a corresponding increase in binder stiffness. The phosphorus from the acid all ends up in the heptane insoluble phase.
• The stiffening effect of phosphoric acid is asphalt dependent.
• Any grade of phosphoric acid can be used.
• Phosphoric acid does not appear to cause increased binder aging except in the binder AAK-1 used in the study.
• Phosphoric acid in asphalt binders reacts with lime (calcium hydroxide).
• Phosphoric acid in asphalt binders does not readily react with limestone aggregates.
• Phosphoric acid is not readily leached from HMA mixes although some consideration might be given to the possible environmental impact of very high levels of modification.
• According to the Hamburg test results, phosphoric acid can negatively affect the stripping of asphalt mixes. It may also negate the effect of some liquid amine antistrip additives. It works very well with lime-treated aggregates.