Performance evaluation of vacuum gas oil as a recycling agent for aged asphalt: rheological performance and softening-dominated mechanism

Vacuum gas oil (VGO) was evaluated as petroleum-derived recycling agent for laboratory-aged asphalt. Rheological performance was characterised by frequency and temperature sweeps, multiple stress creep and recovery, linear amplitude sweep, and bending beam rheometer tests. Continuous performance grades and ΔTc were determined, while thermogravimetric analysis, thin-film oven mass loss, and additional pressure aging vessel aging were used to assess thermal stability and re-aging behaviour. VGO effectively softened aged asphalt and improved low-temperature flexibility and relaxation, but progressively reduced high-temperature rutting resistance. An 8% dosage provided the best overall balance, with continuous high- and low-temperature grades of 69.98 and −27.34 °C and a ΔTc of −3.01 °C. Spectroscopic and microscopic analyses indicated a mechanism dominated by physical dilution, compositional redistribution, and surface-microstructure reorganisation. Overall, VGO is best characterised as a softening-dominated recycling agent. These findings support potential application of VGO in asphalt recycling and its value-added utilisation.