Phosphorous-31 NMR Studies of MnRoad Test Track Cells

Abstract

ABSTRACT Extensive research has been performed to determine the relationship between polyphosphoric acid (PPA) and hydrated lime in bitumen pavement mixes, but the fate of PPA in the paved road has not been determined. Perhaps as important, the mechanism of stiffening by PPA is not fully understood. Laboratory studies have shown that PPA and hydrated lime react in bitumen to form calcium phosphate salts; however, studies have not been performed on samples from an actual road mix. In this study, samples from Cells 33, 34, and 79 of the MnRoad Low Volume Pavement Test Track have been used as model pavements to determine the impact of hydrated lime when used as an antistrip additive in conjunction with PPA modified bitumen. The cells contain PPA, hydrated lime, a phosphate ester antistrip agent, and polymer. Samples of loose mix from the cells were extracted and separated into fractions of binder, fines, and bulk aggregate. Solid and liquid state 31P NMR measurements were then made on the separated fractions. Phosphorous NMR signals were present for all solid fractions of the extraction. Most of the signal was detected in the centrifuge fines as calcium phosphate, and little or no liquid state signal was detected in the extracted binder. These findings indicate that in the MnRoad cells, PPA reacted with hydrated lime to form mainly calcium phosphates. Also, a solid-state NMR signal that had a small chemical shift anisotropy was observed in the extracted binder and is attributed to the product from the reaction of Innovalt-W and hydrated lime.