The Use of Phosphoric Acid to Stiffen Hot Mix Asphalt Binders

CHAPTER 4. AGING

DOES PHOSPHORIC ACID CATALYZE ASPHALT OXIDATION IN HOT MIX ASPHALT BINDERS?

Phosphoric acid is used as a blowing catalyst in producing asphalt for roofing; consequently, fears have been expressed that its use as a binder modifier will cause premature aging of paving asphalts. To test this concern, samples were modified with 105- and 115-percent phosphoric acid and aged at 100 °C in the PAV under air pressure. Because phosphorus pentoxide has been used as a blowing catalyst, it was included in the study.

Samples were stored in open ¼-pint cans. The level of acid used was 1 percent (calculated as 100-percent acid). The stiffness was measured as |G*|/Sinδ at 64 °C. The results are shown in figure 29 to figure 32 , and the numerical data are shown in the appendix in table 36 through table 39 .

To determine whether any stiffness change was due to oxidation or simple the result of exposure to 100 °C temperature for 300 hours, the tests were repeated under nitrogen pressure instead of air. Figure 33 to figure 36 show the data graphically; the numerical results are in table 40 through table 43 .

Figure 29. Chart. PAV aging of asphalt AAD-1 modified with 1-percent phosphoric acid at 100 °C under air.

Figure 30. Chart. PAV aging of asphalt AAK-1 modified with 1-percent phosphoric acid at 100 °C under air.

Figure 31. Chart. PAV aging of asphalt AAM-1 modified with 1-percent phosphoric acid at 100 °C under air.

Figure 32. Chart. PAV aging of asphalt ABM-1 modified with 1-percent phosphoric acid at 100 °C under air.

Figure 33. Chart. PAV aging of asphalt AAD-1 modified with 1-percent phosphoric acid 100 °C under nitrogen.

Figure 34. Chart. PAV aging of asphalt AAK-1 modified with 1-percent phosphoric acid at 100 °C under nitrogen.

Figure 35. Chart. PAV aging of asphalt AAM-1 modified with 1-percent phosphoric acid at 100 °C under nitrogen.

Figure 36. Chart. PAV aging of asphalt ABM-1 modified with 1-percent phosphoric acid at 100 °C under nitrogen.

CONCLUSIONS

• Except for binder AAK-1, the phosphoric acid modified binders aged less than the unmodified controls.
• With the exception of AAK-1, phosphorus pentoxide did not cause increased stiffness compared to acids.
• The increase in stiffness of the samples aged under nitrogen was much less than those aged under air pressure. This suggests that any increase in stiffness is due to asphalt oxidation rather than chemical reaction of the binders with phosphoric acid.