Dosage-by-aging interactions in the stiffening of fly ash–modified bitumen: A compositional and rheological investigation

Fly ash, an abundant industrial by-product, was evaluated as a mineral filler modifier for 60/70 penetration-grade bitumen at 0, 5, 10, 15, and 20% by binder weight, combining particle and chemical characterization with conventional, rheological, and fatigue testing. The fly ash sourced from Kogi State, Nigeria, exhibited Class F-type chemical characteristics, although its SO₃ content exceeded the ASTM C618 limit for cementitious applications. Fly ash addition consistently stiffened the binder across all dosages. The central finding was a dosage-by-aging interaction: in the unaged condition, 15% fly ash passed the G*/sinδ rutting criterion one temperature increment higher than every other dosage, while 20% fly ash underperformed relative to 15% FA, consistent with the higher filler fraction disrupting binder continuity before oxidative hardening. After RTFOT aging this ranking reversed, with 20% fly ash remaining compliant across the entire 52–82°C range, attributed to its larger filler surface area providing more sites for interaction with oxidative aging products. Fatigue resistance showed a generally declining but non-monotonic trend with dosage when unaged, with a partial recovery at 15% FA, and improved substantially after aging at the higher dosages. These results demonstrate that fly ash is an effective bitumen modifier for hot-climate pavements, but optimum dosage is condition-dependent and should reflect the expected aging state and governing distress mechanism.