Research on the adhesion between warm mix recycled asphalt and aggregates at macro and micro scales

In view of the shifting focus of highway development in China from construction to maintenance, a substantial amount of Recycled Asphalt Pavement (RAP) materials is generated every year. Therefore, the recycling technology for asphalt pavements and methods to enhance the utilization rate of RAP materials have become focal points of academic research and engineering practice. The adhesion between asphalt and aggregates is crucial for the strength and durability of asphalt pavements. Given the complex composition of warm mix recycled asphalt and the unclear interactions with aggregates, it is of great practical significance to evaluate the applicability of warm mix recycled asphalt by exploring the adhesion between warm mix recycled asphalt and aggregate. To analyze the adhesion properties of warm mix recycled asphalt, this study employs Atomic Force Microscopy (AFM), contact angle tests, and pull-out tests to examine the effects of aged asphalt content, regeneration agents, warm mix agents, and various types of aggregates on the interfacial adhesion between asphalt and aggregates at both macro and micro scales. The results indicate that the incorporation of warm mix agents and regeneration agents can mitigate the negative impact of aged asphalt content on adhesion force. Specifically, when the aged asphalt proportion reaches 60 %, the adhesion force of warm mix recycled asphalt only decreases by 12.2 %. Moreover, the microstructural morphology of warm mix recycled asphalt is the closest to that of the original asphalt. The interfacial failure energy between asphalt and aggregates decreases with increasing proportions of aged asphalt. The adhesion of granite to asphalt is significantly affected by the aged asphalt content, while limestone exhibits lesser sensitivity to such changes. The addition of warm mix agents and regeneration agents contributes to an increase in the interfacial failure energy between asphalt and aggregates, with warm mix recycled asphalt displaying the highest interfacial failure energy. Aging not only weakens the cohesion work of asphalt but also reduces the adhesion work between asphalt and aggregates. As the aged asphalt proportion increases from 15 % to 60 %, the strongest adhesion work occurs under the synergistic action of warm mix agents and regeneration agents. Both contact angle tests and pull-out tests show that the adhesion performance of various aggregates with asphalt follows the order: limestone > basalt > granite. Correlation analysis reveals a robust relationship between macro and micro indicators, enabling an effective evaluation of the adhesion between warm mix recycled asphalt and aggregates. Overall, these experimental results demonstrate that warm mix recycling is an effective method that reduces the adverse effects of increasing aged asphalt content on adhesion performance, ensuring compatibility between asphalt and aggregates, and facilitating successful engineering implementation.