# Evaluation of Low-Temperature Performance of Recycled
## 4 Conclusions and Recommendations  
In this project, thermal oxidative aged RAP (THRAP) and ultraviolet aged RAP (UVRAP) were prepared in a laboratory to simulate RAPs with varying thermal histories. The asphalt content in the RAPs was determined using the combustion furnace method, and aged asphalt was extracted through an extraction experiment. By analyzing three major indicators and the changes in the PI value, the content of the rejuvenator was determined. Subsequently, one original asphalt mixture and six types of RAMs with varying contents and thermal histories of RAP were prepared. The objective was to evaluate the suitability of RAPs from various sources under cold winter conditions in seasonally frozen regions.  
Figure 15: Residual absolute value and standardized residual.  
1. The RAP-wrapped asphalt lacks a distinct'sandwich' structure, indicating that there are no clear boundaries between the old and new asphalt. Additionally, the microscopic morphology of the asphalt on the surface of the new aggregate in RAMs appears rougher than that of the original asphalt mixture, displaying a significant number of micropores and microcracks. This rougher surface morphology may potentially lead to a decrease in the LTP of mixtures.
2. Regardless of the thermal history of the RAP, the LTP of an RAM tends to decrease as the RAP content increases. For RAMs with differing thermal histories but identical RAP content, when the ambient temperature is above the transition temperature, the LTP of an RAM with UVRAP is superior to that of mixtures with THRAP. Conversely, the opposite trend is observed when the ambient temperature falls below the transition temperature.
3. Different thermal histories of RAPs exert varying effects on the low-temperature viscoelastic behavior of an RAM. The UVRAP accelerates the transition of the mixture from viscoelasticity to elasticity within a narrow temperature drop range and extends the temperature range during which the mixture remains in the elastic stage before failure. Conversely, the addition of the THRAP produces the opposite effect.
4. The prediction of cracking temperature using the Burgers model, along with an analysis of the model's accuracy through residuals indicated that the model exhibited low accuracy in predicting the LTP of an RAM with THRAP. However, it effectively predicts the LTP for an RAM mixed with RAPs from the same source or with similar thermal histories.  
In summary, the results of this study indicate that RAPs with varying thermal histories exert different effects on the LTP of RAMs. This finding offers valuable guidance for the application of such mixtures in the regions of seasonal freezing. However, this research has certain limitations. The use of RAPs generated indoors can't simulate the complex thermal history conditions encountered during actual road surface usage. Addressing this complexity should be a focus of future research. We recommend that subsequent studies utilize the RAPs sourced from different projects for thermal histories comparisons, or conduct indoor simulations that encompass a broader range of aging conditions. This approach will facilitate a more comprehensive evaluation of the impacts of various thermal histories RAPs on the performance of RAMs at low temperatures.  
Data curation, C.J. and Q.A.; formal analysis, C.J. and Q.A.; funding acquisition, Y.C.; methodology, C.J. and Y.C.; supervision, Y.C.; validation, C.J., Y.C. and Q.A.; writing--original draft, C.J. and Q.A.; writing--review and editing, C.J. and Y.C. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation Project of China (Grant No. 52168063); the Basic Scientific Research Expenses Program of Universities directly under Inner Mongolia Autonomous Region (JY20220297); and the Natural Science Foundation of Inner Mongolia in China (2022 MS05022, 2023 MS05027).  
The data presented in this study are available on request from the authors.  
The authors declare no conflicts of interest.