Weathering Mechanisms in Bitumen Modified with Polyphosphoric Acid

This study examines the effect of weathering on bituminous composites containing polyphosphoric acid (PPA). PPA has been commonly used in bitumen to increase bitumen's stiffness and elastic properties. However, the effect of PPA on bitumen's properties is highly affected by the interplay between PPA and other bitumen modifiers such as liquid and solid additives. Wax-based additives generally reduce PPA's efficacy, and a mineral additive such as montmorillonite (MMT) clay increases PPA's efficacy. MMT has been typically used to enhance the resistance of a bituminous matrix to aging. This paper uses laboratory experiments to evaluate changes in the properties of bitumen containing PPA and MMT when exposed to water. Samples were conditioned in water at 60°C for up to 100 h, and their thermomechanical properties were evaluated at different intervals. The study results showed that the presence of PPA in bitumen alleviates the swelling of MMT when montmorillonite-doped bitumen is exposed to water. However, extended water exposure reduces the elastic properties of bitumen. This is attributed to PPA's losing its branches as water promotes the separation of PPA's branches from its backbone, leading to a reduction in binding energy between bitumen and PPA. The latter was reflected in a rapid loss of bitumen's elasticity, healing capacity, and shear thinning, as water conditioning continued.