Understanding the Activation–Aging Coupling Effect: How Initial Aging Levels and RAP Content Influence RAP Binder Performance

Preheating reclaimed asphalt pavement (RAP) is a crucial step in recycling processes because it simultaneously activates the RAP binder and deepens its aging levels, leading to complex coupling effects (CE) on mixture performance. In practical construction, the initial aging state of the RAP binder and RAP content exhibit considerable variations, leading to intricate CE outcomes. It is crucial to gain a deeper understanding of these effects and determine the optimal preheating conditions to enhance the utilization of the RAP binder. This study builds on previously published coupling effect evaluation methods and models to further explore the impact of two key factors: initial aging levels of RAP binder and RAP content. Specifically, for a quantitative evaluation, activation and aging indices were obtained through activation and gel permeation chromatography (GPC) tests. The CE was evaluated using the coupling effect model and validated through semi-circular bending (SCB) tests. Additionally, enhanced activation and aging models were developed, incorporating preheating temperature and initial aging levels as variables. The results indicated that higher initial aging levels required higher preheating temperatures for activation, with 170°C typically sufficient for most aging levels. The aging index was temperature-dependent and slightly affected by the initial aging state. In addition, optimal preheating temperatures were determined for RAPs with varying aging levels based on the CE index. For highly aged RAP, a higher preheating temperature was recommended, prioritizing activation over aging. The impact of CE became more pronounced with higher RAP content and a greater initial aging level but was negligible when the RAP content was 15% or lower.