The Influence of Polyphosphoric Acid Dosage on the Physical, Rheological, Ageing, and Microstructural Characteristics of Bitumen

Polyphosphoric acid (PPA) is an effective chemical modifier for improving the performance characteristics of bitumen. This study investigated the influence of PPA dosage on the physical, rheological, ageing, and microstructural characteristics of 60/70 penetration grade bitumen. PPA with 115% H3PO4 equivalent concentration was incorporated at 0.5%, 1.0%, 1.5%, and 2.0% by weight of bitumen. The binders were evaluated through conventional physical tests, ageing indices, dynamic shear rheology, master curves, Black and Cole–Cole diagrams, field emission scanning electron microscopy (FESEM), and Fourier transform infrared spectroscopy (FTIR). Increasing PPA concentration reduced penetration from 62 to 24 dmm and increased the softening point from 46.5 to 68 °C, while viscosity at 135 °C increased from 0.38 to 1.83 Pa·s. The Penetration Index (PI) and Penetration Viscosity Number (PVN) results also indicated reduced temperature susceptibility. Rheological evaluation showed a pronounced response at approximately 1.0% PPA, where G*/sinδ increased from 1.01 to 12.35 kPa and the high temperature grade increased from Performance Grade (PG)64 to PG 82. Master curves showed a stronger PPA influence at lower reduced frequencies, while Black and Cole–Cole analyses indicated an increased relative elastic contribution. These rheological characteristics remained evident following Rolling Thin Film Over (RTFO) ageing, although the ageing response varied with PPA concentration and the evaluated property. FESEM showed homogeneous PPA incorporation without visible phase separation, while FTIR revealed progressive changes in the phosphate-related spectral region. Among the investigated dosages, 1.0% PPA provided the most balanced performance based on the physical properties, high temperature rheological response, short-term ageing behaviour, and microstructural characteristics evaluated in this study. However, further evaluation of low temperature and long-term ageing performance is required before establishing an overall optimum PPA dosage.