VOC Emission Reduction and Rheological Optimization of Recycled Asphalt with USP Warm Mix Additive

To mitigate high-temperature VOC emissions and reduce construction temperatures in recycled asphalt, a USP warm mix additive was introduced into waste soybean oil (WSO) recycled asphalt. This study systematically investigates the effects of USP content (1%, 3%, 5%, and 7%) and construction temperature (140 °C and 160 °C) on VOCs emission characteristics, inhibition mechanisms, and rheological performance. The results show that USP reduces total VOC emissions, with the key inhibition effect achieved at 5% USP. At 160 °C, the VOCs inhibition rate reached 39.5% at 5% USP, while at 140 °C it increased to 80.8%; the findings suggest that lower temperatures enhance the inhibitory effect. The results show USP cuts emissions via physical mechanisms. However, the significant FTIR and DSC analyses suggest that VOC reduction appears to be primarily governed by physical mechanisms, including phase-change cooling, physical encapsulation, and migration retardation, without altering the chemical structure of asphalt. Moreover, the important rheological results indicate that although USP slightly decreases the high-temperature complex modulus and rutting factor, the evidence demonstrates that it improves the percent recovery (R0.1 from 53% to 66%; R3.2 from 21% to 40%) and low-temperature crack resistance through decreased stiffness S and increased m-value. In light of these significant findings, the study demonstrates that USP exhibits a multi-performance balance, appearing to reduce VOC emissions and improve workability while moderately weakening high-temperature deformation resistance. Notwithstanding the reduced high-temperature resistance, the key evidence could demonstrate that USP enhances elastic recovery and low-temperature performance in the results. The optimal USP content appears to be 5%, providing the critical compromise between emission reduction and pavement performance for WSO recycled asphalt.