Recent development of polyphosphoric acid composite modified asphalt in pavement

Polyphosphoric acid (PPA) had a significant benefit as a supplemental modifier to improve the performance of polymer modified asphalt and bio-asphalt due to the presence of network structure formed through chemical modification. To promote the development of PPA combined with other modifiers, this paper provided a systematic review of PPA composite modified asphalt and its mixture. Firstly, the chemistry and modification mechanism of PPA were discussed. Subsequently, the properties of PPA composite modified asphalt and its mixture were comprehensively evaluated. Moreover, composite modifications of PPA with other modifiers such as crumb rubber and polyethylene (PE) were reviewed, which presented satisfying performance and environment advantage. Long term performance evaluation, environmental assessment and economic analysis were also discussed. Various microscopic techniques could confirm the improvement of high temperature performance, which stands out as its most exceptional property. Besides, PPA composite modified asphalt processed excellent anti-aging property and storage stability, but the poor low temperature property and undesirable adhesion property were potential defects that restricted its application. PPA composite modified asphalt mixture presented analogous properties to the corresponding asphalt. Notably, low temperature performance of asphalt mixture was questionable due to inconsistent results from different test. At present, despite that there is insufficient long-term performance observation on utilizing PPA composite modified asphalt into pavement, it still presents promising application prospect due to outstanding property and economic benefit.