Rheological properties and modification mechanism of polyphosphoric acid-modified asphalt

In this study, rheological properties and modification mechanism of polyphosphoric acid (PPA)-modified asphalt were investigated by engineering and chemical tests. Different asphalt source, PPA level, and PPA dosage were applied to produce binders. Rheological behaviour of binders was described by experimental and theoretical methods including conventional tests, temperature sweep test and Burgers model based on the bending beam rheometer test. Chemical characterisation was explored through SARAs, functional groups and molecular size using thin-layer chromatography-flame ionisation detector, nuclear magnetic resonance, and gel permeation chromatography. Relationship between chemical and physical properties of binders was established. The results showed that PPA dosage and asphalt source had a significant effect on penetration, softening point and ductility. PPA level could have little impact. Rutting deformation resistance was enhanced by adding PPA. Burgers model fitting parameters showed that PPA-modified asphalt may have a better creep behaviour at low temperature, and stress relaxation was accelerated as well. PPA addition transferred colloidal system of binders from sol type to gel type. Chemical structures were altered where oxhydryl groups in PPA chemical structures reacted with polar groups in asphalt, possibly producing aethers or esters. The cyclisation and grafting between carbon chains may happen and further make asphalt more complex and large-weight molecular structures. Colloidal index (CI) and percent of large molecular size could be used to indicate the change in physical properties and predict modification degree of binders.