Effect of polyphosphoric acid on fracture properties of asphalt binder and asphalt mixtures

This paper examines the effect of polyphosphoric acid (PPA) on asphalt's fracture properties. While PPA has been commonly used in the asphalt industry to increase the elasticity and stiffness of asphalt binder, its effects on fracture properties are not thoroughly understood. Fracture as relates to thermal cracking is often associated with asphalt binder rather than a stone skeleton. However, because PPA not only interacts with molecules of asphalt binder but also strongly interacts with stone aggregates due to its highly polar molecules, we hypothesize that the presence of PPA in asphalt will change both bulk and interface properties in asphalt composites, which consequently alter asphalt fracture properties. To test this hypothesis, various PPA dosages (0.5-1.5% wt.) were introduced to the original asphalt binder, and thermo-mechanical and fracture properties of asphalt binder and asphalt mixture were examined. The study results showed that introducing up to 1% PPA enhances the asphalt resistance to fracture; however, excessive PPA weakens the asphalt against cracking. Considering that polar molecules of bitumen supersede others to adsorb to stone aggregate surfaces, the observed weakening effect could be attributed to PPA's preferential adsorption to stone aggregate surfaces compared to those of asphalt binder. The study outcomes provide insights pertaining to PPA's role and its interplay with other components in asphalt pavements.