Development and validation of a viscosity-equivalence framework for rejuvenator dosage optimization in RAP binders using waste engine oil and waste transformer oil

Optimizing the rejuvenator dosage is critical for restoring the workability and performance of reclaimed asphalt pavement (RAP) binders. However, existing dosage selection approaches are generally based on individual engineering properties or viscosity measured at one or a few temperatures, resulting in property-specific optimum dosages and overlooking viscosity behavior across the practical mixing and compaction temperature range. This study proposes and validates a viscosity-equivalence-based framework to determine the optimum dosage of waste engine oil (WEO) and waste transformer oil (WTO) for RAP binders. The framework employs viscosity-ratio (VR) analysis to establish viscosity equivalence between rejuvenated RAP binders and the control binder over the practical mixing and compaction temperature range. The framework identified 12.5% WEO and 8.5% WTO as the optimum dosages achieving strict viscosity equivalence (VR ≈ 1.0), while 7.5% WTO was retained alongside 12.5% WEO as a conservative evaluation baseline. Downstream validation confirmed that both target formulations restored penetration, softening point, and continuous high PG to levels statistically equivalent to the virgin control binder (p ≥ 0.23). Spectral deconvolution verified partial chemical de-aging, diluting IC=O (Carbonyl Index) and IS=O (Sulfoxide Index) indices by 51–73% (p < 0.001) while remaining above control binder levels. Rheological testing on unaged blends demonstrated enhanced rutting resistance with 21.4–26.2% lower non-recoverable compliance (J nr,3.2 ) than the control binder (p < 0.001), meeting AASHTO M 332 Standard traffic criteria. Concurrently, intermediate fatigue life (N f at 25 °C) expanded by 4.57 × (WTO) and 15.0 × (WEO) over the control binder (p < 0.001), with WEO exhibiting 3.29 × higher endurance than WTO (p < 0.001). The proposed framework provides a systematic, reproducible methodology for optimizing rejuvenator dosage and can be extended to other rejuvenator systems following appropriate experimental validation.