Performance Evaluation of Stress-Absorbing Layer Mixtures Incorporating High-Content Oil-Rich RAP Fine Aggregate

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study investigates the performance of using high-content RAP fine aggregate in stress absorption layer (SAL) mixtures, demonstrating clear innovation and significant engineering and environmental value. The research design is systematic, data are comprehensive, and conclusions are generally reliable, providing valuable insights for the high-value recycling of RAP：

(1) While the introduction mentions the application background of O-RAP, it lacks a clear articulation of the study's innovative contributions relative to existing research. It is recommended to more explicitly state the unique contributions of this study at the conclusion of the introduction or within the abstract.

(2) In the Materials and Methods section, descriptions regarding the specific types of rejuvenators and high-elasticity modifiers, the basis for determining their dosages, and key parameters during mixture preparation are somewhat brief. It is advisable to supplement these details to enhance the reproducibility of the experiments.

(3) The paper effectively documents the trends in macro-properties with varying RAP content, but the explanation of underlying mechanisms is somewhat insufficient (e.g., attributing it solely to “aged asphalt”). It is recommended to include a discussion on the blending state between old asphalt, new asphalt, and the rejuvenator.

(4) Minor grammatical errors and awkward phrasing are present throughout the text. A comprehensive language review is advised to ensure the accuracy and fluency of the academic writing.

Minor Revision

Author Response

Thank you very much for your comments about our paper entitled "Enhancing Sustainability in Pavement Engineering: Performance Evaluation of Stress-Absorbing Layer Mixtures Incorporating High-Content Oil-rich RAP Fine Aggregate."

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This paper shows a very interesting set of experiments concerning the influence of the aspahlt mixtures with high content oil-rich RAP fine aggregate. This study provides a comprehensive evaluation of the rheological properties of recycled asphalt binder as well as the pavement performance of SAL mixtures via fatigue testing, moisture susceptibility evaluation, rutting resistance analysis, and overlay testing. The authors also conducted an environmental impact analysis with the goal of decarbonizing pavement construction.

The paper is well written. The autors refer to relevant references. The obtained results are logical and the explanations provided for those trends are detailed enough. The conclusions drawn are adequate and highlights the potentioal of oil-rich RAP fine aggregate reuse in the prodaction of asphalt mixuters contributing to sustainable waste menagement

Author Response

Reply: We sincerely appreciate you taking the time to review our manuscript and for your positive and constructive feedback on our research. Your affirmation of the paper's innovation, logical coherence, and potential environmental benefits is a tremendous encouragement to us. We have meticulously proofread and polished the entire text to ensure the manuscript is presented in its best possible form.

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript presents valuable insights into sustainable pavement engineering through the use of stress-absorbing layer mixtures with high-content oil-rich RAP.  Authors need to address the following comments

 

1. Title could be kept to the point, it is suggested to revise it to “ Performance Evaluation of Stress-Absorbing Layer Mixtures Incorporating High-Content Oil-rich RAP Fine Aggregate”
2. Introduction needs to be improved to provide better readability and rationale, especially from Line 84 to 103
3. Caption for Figure 1 should be revised to a more descriptive one
4. In order to improve the appearance, It is suggested to change color scheme used in Figure 1 to Blue and bright Red colors
5. Caption for Figure 3 should be revised to a more descriptive one
6. Line 289-292 provide literature support to these outcomes “This phenomenon is attributed to the restricted molecular mobility within the recycled asphalt system under  low-temperature stress, where the macromolecular chains are less capable of rearranging  spatially”
7. Section 3.3 “Correlation analysis” could be made more concise by providing the novel information only
8. The discussion of sustainability could be further strengthened by considering relevant research that addresses improvements in pavement performance through subgrade modification. For instance, the study by [Jaffar et al., 2022] entitled “Evaluation of conventional and sustainable modifiers to improve the stiffness behavior of weak sub-grade soil” provides useful evidence on how sustainable practices can enhance the performance of pavement systems at the foundation level, its inclusion would broaden the sustainability perspective of the current study and highlight synergies across different layers of pavement systems.
9. Conclusion seems more of a results section, please revise to make it concise and enlist the conclusions not the results.

 

 

 

 

Author Response

The manuscript presents valuable insights into sustainable pavement engineering through the use of stress-absorbing layer mixtures with high-content oil-rich RAP. Authors need to address the following comments.

(1) Title could be kept to the point, it is suggested to revise it to “Performance Evaluation of Stress-Absorbing Layer Mixtures Incorporating High-Content Oil-rich RAP Fine Aggregate”.

Reply: Thank you very much for this suggestion. We have revised the title of the paper in the manuscript.

(2) Introduction needs to be improved to provide better readability and rationale, especially from Line 84 to 103.

Reply: Thank you very much for this comment. We have enhanced the readability and logical flow of the introduction section in the manuscript.

In section 1: "The rational application of O-RAP can reduce the demand for virgin asphalt, thereby lowering resource consumption and carbon emissions associated with asphalt pavement construction, and improving overall resource utilization efficiency [23]. Furthermore, the use of a high percentage of O-RAP has been shown to effectively re-duce road maintenance costs and enhance material efficiency [24]. With appropriate rejuvenation treatment, the adhesion characteristics of O-RAP can be improved, con-tributing to a more stable interfacial structure within the Stone Mastic Asphalt (SAL) mixture. Given that the production of virgin asphalt and the mining of aggregates are energy-intensive processes—and major contributors to carbon emissions in the con-struction sector—the incorporation of recycled materials such as RAP represents not only an effective waste management strategy but also a crucial pathway toward de-carbonizing infrastructure development.

To address the challenges of limited incorporation rates and unclear performance relationships associated with O-RAP, this study systematically investigates its application in SAL mixtures based on advanced separation technology. While previous research has largely focused on conventional RAP in asphalt layers, this work specifically targets the high-value utilization of O-RAP—a challenging material due to its high aged asphalt content—in SAL mixtures, which inherently require high binder content. Consequently, this research pushes the boundaries by targeting an unprecedented O-RAP incorporation rate of 70%.The key innovations of this research include: (i) determining the maximum feasible O-RAP incorporation rate (70%) in SAL while satisfying performance specifications; (ii) clarifying the comprehensive performance trade-offs and correlations through a multi-scale experimental methodology; and (iii) quantifying the substantial environmental benefits achieved through this high-recycling strategy."

(3) Caption for Figure 1 should be revised to a more descriptive one.

Reply: Thank you very much for this suggestion. We have already revised the title of Figure 1“Schematic flowchart of the overall research methodology” in the manuscript.

(4) In order to improve the appearance, It is suggested to change color scheme used in Figure 1 to Blue and bright Red colors.

Reply: Thank you very much for this suggestion. We have already modified the color scheme for Figure 1 in the manuscript.

(5) Caption for Figure 3 should be revised to a more descriptive one.

Reply: Thank you very much for this suggestion. We have already revised the title of Figure 3“The physical photograph of overlay test setup.” in the manuscript.

(6) Line 289-292 provide literature support to these outcomes “This phenomenon is attributed to the restricted molecular mobility within the recycled asphalt system under low-temperature stress, where the macromolecular chains are less capable of rearranging spatially”.

Reply: Thank you very much for this suggestion. We have incorporated the corresponding supporting evidence for the conclusions into the manuscript.

In section 1: " …low-temperature stress, where the macromolecular chains are less capable of rearranging spatially [29]."

29. Li, K.; Yan, X.; Wang, Y.; Ai, T.; Niu, Y.; Chen, Y. Design and modification mechanism of a novel high-viscosity thermoplastic polyurethane modified asphalt. Constr Build Mater 2024, 433, https://doi.org/10.1016/j.conbuildmat.2024.136585.

(7) Section 3.3 “Correlation analysis” could be made more concise by providing the novel information only.

Reply: Thank you very much for this suggestion. We have comprehensively streamlined Section 3.3 “Correlation Analysis” by removing the introductory explanation of Pearson's correlation coefficient fundamentals and eliminating redundant or self-evident explanations of mechanisms. The revised discussion now focuses on the most novel key finding of this study: the potential negative impact of high fatigue life on interlaminar crack resistance and high-temperature stability. The modified paragraphs are more concise and highlight the core insights of the research.

In section 3.3: " The Pearson correlation analysis (Figure 12) revealed that the N_f of the recycled asphalt binder is strongly positively correlated with the water stability, low-temperature crack resistance, and fatigue performance of the SAL mixtures. This confirms that binders with superior fatigue life enhance the overall durability of the mixture [36].

The most critical finding, however, lies in the strong negative correlations observed between the binder's N_f and two key performance metrics of the SAL: its anti-reflective cracking resistance and high-temperature stability. This indicates a significant performance trade-off: while a high- N_f binder improves durability, it may simultaneously compromise the structural integrity of the interlayer. The lower creep stiffness associated with a higher N_f likely increases susceptibility to deformation under traffic and thermal loading. This softening can lead to stress concentrations at the interface, reducing its ability to resist reflective cracking and rutting [37]. Therefore, optimizing the O-RAP content and binder formulation must carefully balance these competing effects to ensure satisfactory performance across all critical parameters."

(8) The discussion of sustainability could be further strengthened by considering relevant research that addresses improvements in pavement performance through subgrade modification. For instance, the study by [Jaffar et al., 2022] entitled “Evaluation of conventional and sustainable modifiers to improve the stiffness behavior of weak sub-grade soil” provides useful evidence on how sustainable practices can enhance the performance of pavement systems at the foundation level, its inclusion would broaden the sustainability perspective of the current study and highlight synergies across different layers of pavement systems.

Reply: Thank you very much for this suggestion. We have expanded the discussion of sustainability to encompass the entire pavement system, including subgrade improvement, which significantly broadens the scope of this study. The discussion of environmental impacts in Section 3.4 cites the important study by [Jaffar et al., 2022]. Based on this, we elaborate on the synergistic effects achieved by combining high-performance recycled surface course material (O-RAP in SAL) with sustainable subgrade improvement techniques. This provides a more comprehensive demonstration of the overall sustainability potential of the pavement system.

In section 3.4: " …This high-value recycling strategy in the surface layer can be synergistically combined with sustainable practices in other pavement layers to maximize system-wide benefits. For instance, research by Jaffar et al. [39] demonstrated that sustainable modifiers could effectively improve the stiffness behavior of weak subgrade soil, thereby enhancing the foundational support for the entire pavement structure. This integrated approach not only conserves virgin materials and reduces carbon emissions across multiple pavement layers but also potentially extends the service life of the entire system, offering a comprehensive pathway toward decarbonizing pavement construction."

39. Jaffar, ; Abid, M.; Khan, S.; Jafri, T.; Rehman, Z.; Tariq, M.; Ng, A. Evaluation of conventional and sustainable modifiers to improve the stiffness behavior of weak sub-grade soil. Sustainability, 2022, 14(5), 2493. https://doi.org/10.3390/su14052493.

(9) Conclusion seems more of a results section, please revise to make it concise and enlist the conclusions not the results.

Reply: Thank you very much for this suggestion. We have thoroughly rewritten the entire conclusion section, removing detailed results data and process descriptions in favor of emphasizing the final findings of this study, the trade-offs it reveals, and their implications for industry practice.

In section 4: " …This study comprehensively evaluated the viability of incorporating high-content O-RAP into SAL mixtures, leading to the following principal conclusions:

(1) Binder Performance Modification: The introduction of O-RAP effectively modifies the rheological properties of the asphalt binder, shifting its behavior toward greater rigidity and elasticity. Critically, this enhancement in high-temperature performance is achieved without adversely affecting the low-temperature performance grade, confirming the binder's suitability for use in varied climates.

(2) Mixture Performance and Trade-offs: The use of O-RAP in SAL mixtures presents a definable performance trade-off. While it significantly enhances high-temperature stability and rutting resistance, it concurrently increases mixture brittleness, leading to reductions in low-temperature crack resistance, moisture stability, and fatigue life. Nonetheless, a viable incorporation threshold of up to 70% O-RAP has been established, as mixtures at this level continue to meet standard performance specifications, successfully balancing recycling goals with functional requirements.

(3) System-Level Correlation: The strong correlation between binder N_f and key mixture properties underscores that binder-level performance is a reliable predictor of mixture behavior. The identified negative correlation between N_f and the mixture's rutting resistance reveals a critical performance contradiction that must be actively managed during the material design phase to optimize overall pavement durability.

(4) Demonstrated Environmental Efficacy: The integration of O-RAP provides substantial and quantifiable environmental benefits. The 70% O-RAP mixture achieves an approximate 70% reduction in virgin material consumption and a net carbon emission reduction of 26.29 kg CO₂-eq per ton, establishing high-content O-RAP recycling as a powerful strategy for decarbonizing pavement construction.

In summary, this research validates the technical feasibility and environmental necessity of utilizing high-content O-RAP in SAL applications. The findings provide a scientific basis and practical guidance for industry to adopt this technology, enabling a transition towards more resource-efficient and low-carbon pavement infrastructure without compromising structural performance."

Reviewer 4 Report

Comments and Suggestions for Authors

Excellent work with a clear contribution. Minor revisions: 

1. Has there been any previous attempt to use more than 70% RAP content in SAL? A comparative comment on this in the introduction would strengthen the novelty claim.
2. The rejuvenator’s chemical composition is vaguely described. Provide a commercial name or detailed chemical breakdown for it,
3. Reference [25] is used to support Table 2; please clarify if these values were taken directly or validated in this study.
4. How was the critical strain (e.g., 2.5%, 5%) selected for Nf determination in fatigue analysis?
5. What test standards followed for LAS and BBR?
6. In Table 4, the magnitude difference (e.g., 10^12 vs. 10^9) suggests a need for discussion on how practically significant such reductions are.

Author Response

Excellent work with a clear contribution. Minor revisions:

(1) Has there been any previous attempt to use more than 70% RAP content in SAL? A comparative comment on this in the introduction would strengthen the novelty claim.

Reply: Thank you very much for this suggestion. We have already demonstrated in the introduction section of the manuscript that direct comparison with previous studies more effectively highlights the innovative nature of the 70% blending target in this research.

In section 1: " …adversely affecting interlayer cohesion and fatigue resistance [22]. To date, research on high-RAP content in SAL remains scarce, with most previous studies focusing on in-corporation rates typically below 30% for conventional RAP. The application of specially processed O-RAP at very high levels (e.g., exceeding 50%) represents a significant challenge and an under-explored area. This lack of knowledge regarding the maximum feasible incorporation rate and the comprehensive performance trade-offs at high O-RAP con-tent constitutes a critical research gap...

… which inherently require high binder content. Consequently, this research pushes the boundaries by targeting an unprecedented O-RAP incorporation rate of 70%..."

(2) The rejuvenator’s chemical composition is vaguely described. Provide a commercial name or detailed chemical breakdown for it.

Reply: Thank you very much for this suggestion. We have described the detailed composition of the regenerating agent in the manuscript.

In section 2.1: " …The rejuvenator, typically composed of maltenes and oil-based compounds, aims to re-store the chemical balance of the aged asphalt by replenishing lost lighter oils, thereby improving its flexibility and adhesion. Detailed chemical composition analysis can be found in our previous research [7, 25]. ..."

(3) Reference [25] is used to support Table 2; please clarify if these values were taken directly or validated in this study.

Reply: Thank you very much for this comment. Reference [25] is a prior study published by our team, which details the separation technique and fundamental properties of O-RAP. In this study, the materials used are identical to those in Reference [25]. These values were directly measured and verified through experimentation.

(4) How was the critical strain (e.g., 2.5%, 5%) selected for N_f determination in fatigue analysis?

Reply: Thank you very much for this comment. We selected the critical strain value primarily based on the classical definition of fatigue failure in asphalt binder and widely accepted industry standards to ensure the comparability and scientific rigor of the research results. The specific rationale is as follows: Adherence to classical theory and universal standards: In dynamic shear rheometer fatigue testing of asphalt binder, fatigue life is defined according to the universal guideline established by the U.S. SHRP program and widely adopted in standards such as AASHTO T315. This guideline is recognized as effectively characterizing the critical point where material stiffness degrades to “failure” under cyclic loading. Adopting this recognized failure criterion enables direct comparison of the fatigue life data from this study with extensive published literature data, thereby more clearly positioning the fatigue performance level of O-RAP modified asphalt.

Therefore, our determination of the N_f value is not based on arbitrarily selecting a fixed strain value, but rather on identifying the failure point through the material's stiffness decay response during cyclic loading. This method is widely recognized as one of the most reliable and reproducible approaches for evaluating the fatigue performance of asphalt binder.

(5) What test standards followed for LAS and BBR?

Reply: Thank you very much for this suggestion. We have already specified the testing standards followed by LAS and BBR in the manuscript.

In section 2.3.1.3: " …To characterize the low-temperature rheological behavior of the recycled asphalt binder, BBR testing (Cannon TE-BBR) was conducted. Test standards refer to AASHTO T313. ..."

In section 2.3.2.1: " The experimental procedure was conducted in strict compliance with the TEX-248-F specification from Texas, USA. For the SAL mixtures, ..."

(6) In Table 4, the magnitude difference (e.g., 10^12 vs. 10^9) suggests a need for discussion on how practically significant such reductions are.

Reply: Thank you very much for this suggestion. We fully agree that assessing the magnitude of fatigue life degradation is of paramount importance in practical engineering applications. We have incorporated an in-depth discussion on this matter into the manuscript.

In section 3.1.2: " …. Nevertheless, the recycled asphalt binders still maintain a relatively high level of fatigue resistance.

Notably, the N_f exhibits a reduction of several orders of magnitude with increasing RAP content, particularly at the 5% strain level. While this indicates a significant decline in the material's resistance to damage under high-strain conditions, the practical implications of this reduction must be interpreted within the context of realistic pavement service environments. In actual pavement structures, the strain levels experienced by the asphalt binder layer are typically much lower, often below 0.1%. At these lower, more representative strain levels, the fatigue life of all mixtures, including the 70% RAP variant, would be substantially higher and likely sufficient for many design life requirements. Therefore, the drastic reduction observed here primarily defines the performance boundary of high-RAP mixtures under extreme loading. It under-scores the importance of limiting the maximum strain in pavement design when utilizing high-RAP content binders, rather than precluding their use altogether. The feasibility of high-RAP mixtures is thus confirmed for applications where traffic-induced strains are controlled, aligning with the performance-based specification philosophy."

 

Author Response File: Author Response.docx