Abstract
The durability of asphalt pavements in tropical climates is often compromised by rutting, thermal cracking, and premature aging, particularly when penetration graded bitumen such as 60/70 is used. This study investigates the rheological properties of penetration graded bitumen modified with fly ash, an abundant industrial by product in Nigeria, with the dual aim of enhancing binder performance and promoting sustainable waste utilization. Fly ash samples collected from industrial sources in Kogi State were characterized through sieve analysis and chemical composition. Results confirmed that 97% of particles passed the 0.15 mm sieve and 83% passed the 0.075 mm sieve, while chemical analysis revealed high silica (48.92%) and alumina (24.22%) contents, classifying the material as Class F fly ash suitable for binder modification.Bitumen was blended with fly ash at dosages of 5%, 10%, 15%, and 20%, and tested using conventional methods (penetration, softening point, ductility, flash/fire point), Dynamic Shear Rheometer (DSR), Multiple Stress Creep Recovery (MSCR), rutting resistance, and fatigue life analysis. Conventional tests showed that penetration decreased from 62.7 mm (control) to 45.3 mm (20% FA), while softening point increased from 48.2 °C to 52.2 °C, confirming improved stiffness and thermal stability. MSCR results demonstrated that 15% FA achieved optimal elastic recovery (9.8% at 64 °C aged) and reduced Jnr values, while rutting resistance tests revealed that 15% FA passed the Superpave criterion at 70 °C (1132.90 Pa) and 20% FA passed at all temperatures up to 82 °C (6737.79 Pa at 82 °C). Fatigue resistance improved dramatically after aging, with 15% FA reaching 2.3 million cycles and 20% FA exceeding 9 million cycles, compared to 78,281 cycles for the aged control.The study concludes that fly ash significantly enhances the rheological and durability properties of penetration graded bitumen. 15% fly ash is recommended for balanced stiffness, elasticity, and rutting resistance, while 20% fly ash is optimal for long term heavy duty pavements requiring superior fatigue performance. These findings contribute new knowledge by establishing performance thresholds for fly ash modified bitumen in tropical climates and demonstrate the potential of locally sourced fly ash as a sustainable pavement material.

Key words: Keywords: Fly ash, bitumen modification, rheology, MSCR, rutting resistance, fatigue life.