Comprehensive Review of Thermally Induced Self-Healing Behavior in Asphalt Mixtures and the Role of Steel Slag

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper is of interest content and sufficient structure. Some aspects require the authors’ further attention, before this paper is being further processed:

The methodology of selecting appropriate literature is missing in this review. Please include keywords that were included and excluded from your database search, etc.

While the focus of this review is on the self-healing processes, the importance and contribution of steel slag on pavements’ structural and functional performance when included into conventional asphalt mixtures needs to be better elaborated, together with findings and arguments about mix composition. You may see and cite https://doi.org/10.3390/buildings14061571

A comparative quantification of the steel slag mixture performance in terms of self-healing might need to be tabulated for better clarity. Please reconsider.

What types of roads and traffic volume could benefit more from the self-healing efficiency of steel slag mixtures? Please include some discussion points useful for practitioners and stakeholders.

Lines 635-642: Are there any current insights on the real-scale monitoring of steel slag mixtures in terms of micro-cracking development? Please elaborate. In addition, the term “integrating steel slag and self-sensing aggregates” does not make sense and requires rephrasing.

 

 

Author Response

Response to Reviewers’ Comments

Title: Comprehensive Review of Thermally Induced Self-Healing Behavior in Asphalt Mixtures and the Role of Steel Slag

Dear Editors/Reviewers:

Thank you for your letter and the reviewers’ comments concerning our manuscript. Those comments are all very valuable and helpful for revising and improving this paper. Some grammar and spelling errors has also been corrected. Furthermore, the relevant content has been added to the original manuscript according to the comments of reviewers and the major revised portions were marked in gray at the track changes version.

The responses are as followings:

Comments 1: The methodology of selecting appropriate literature is missing in this review. Please include keywords that were included and excluded from your database search, etc.

Response 1: Thank you very much for your guidance. We have supplemented the literature selection methodology in Lines 84–92 of the manuscript, with the following additions:

This review conducted a systematic literature search across the Scopus, Web of Science, and Engineering Village databases, focusing on publications from 2010 to 2024. The search strategy employed targeted keyword combinations, including "thermally induced" OR "heat-triggered" (to capture thermal activation mechanisms), "self-heal" OR "crack repair" (to address healing behaviors), "asphalt" OR "bitumen" (as binder-related terms), "steel slag" OR "industrial byproduct" (to identify functional aggregates), "electromagnetic induction" OR "microwave heating" (for thermal methods), and "energy efficiency" OR "heating uniformity" (to evaluate performance metrics). This approach ensured a balanced coverage of material properties, healing mechanisms, and sustainability considerations.

 

Comments 2: While the focus of this review is on the self-healing processes, the importance and contribution of steel slag on pavements’ structural and functional performance when included into conventional asphalt mixtures needs to be better elaborated, together with findings and arguments about mix composition. You may see and cite https://doi.org/10.3390/buildings14061571

Response 2: Thank you for your suggestions. The discussion on the impacts of incorporating steel slag and other waste materials into asphalt mixtures on pavement structures is indeed critical. The literature you recommended has been highly instructive for this analysis. In response, we have supplemented the relevant content in Lines 355–378 of the manuscript to address these aspects comprehensively. The specific content is summarized as follows:

The aforementioned physical and mechanical properties of steel slag enable it to positively influence pavement structures and road stability when used as an aggregate substitute, while also posing potential challenges[79]. The specific positive impacts in-clude:

1. Enhanced Mechanical Performance: When replacing coarse aggregates (par-ticularly BOFS and EAFS), steel slag significantly improves the anti-rutting, fatigue resistance, and permanent deformation resistance of asphalt mix-tures[80-82]. Its angular particles and rough surface enhance mechanical inter-lock among aggregates.
2. Superior Skid Resistance: Steel slag’s wear resistance, high polishing re-sistance, and irregular surface morphology substantially improve pavement skid resistance (e.g., higher British Pendulum Number (BPN) and Mean Tex-ture Depth (MTD) compared to conventional aggregates), making it suitable for high-traffic roads[83-85].
3. Temperature Adaptability: Steel slag mixtures exhibit higher dynamic mod-ulus across a broad temperature range (−10–54°C), extending their applica-bility to diverse climatic conditions[80].

Key Challenges in Application:

1. Dosage Sensitivity: Coarse aggregate replacement yields optimal results, but substituting fine aggregates or fillers increases moisture susceptibility[85]. High dosages (>30%) may elevate bulk density and air voids, necessitating adjust-ments in binder content and raising costs[87].
2. Volumetric Stability Risks:Free calcium/magnesium oxides in steel slag pose hydration-induced expansion risks, requiring pretreatment (e.g., carbonation) to mitigate swelling[88].

 

Comments 3: A comparative quantification of the steel slag mixture performance in terms of self-healing might need to be tabulated for better clarity. Please reconsider.

Response 3: Thank you for your suggestions. The performance differences between steel slag and other materials in self-healing applications are indeed critical. In Section 3.2 of the manuscript, we systematically elaborate on these distinctions through a point-by-point comparison of steel slag with limestone and basalt, focusing on the following parameters: thermal conductivity and diffusivity, heat storage capacity, dielectric properties, magnetic permeability, and particle size and content. This multidimensional analysis clarifies how steel slag's unique properties enhance its efficacy as a functional aggregate in thermally activated self-healing systems.

 

Comments 4: What types of roads and traffic volume could benefit more from the self-healing efficiency of steel slag mixtures? Please include some discussion points useful for practitioners and stakeholders.

Response 4: Thank you very much for your guidance. Current research on the application variability of self-healing technologies in steel slag-asphalt mixtures remains underdeveloped, resulting in limited data availability. Nevertheless, priority application scenarios for these mixtures can be predicted based on the synergies between self-healing mechanisms and steel slag’s inherent properties (e.g., thermal conductivity, mechanical reinforcement). We have provided a brief elaboration on these aspects in Lines 402–409. The specific details are as follows:

Steel slag-asphalt mixtures, leveraging these properties (thermal conductivity, mechanical strength, and self-healing capabilities), exhibit particularly superior performance under specific road types and traffic conditions. Examples include urban roads, highways, sun-exposed parking lots, and bridge pavements in tropi-cal or subtropical regions. This is attributed to the region’s relatively higher am-bient temperatures, which enable faster pavement temperature elevation com-pared to colder regions, thereby enhancing the efficiency of self-healing mecha-nisms.

 

Comments 5:Lines 635-642: Are there any current insights on the real-scale monitoring of steel slag mixtures in terms of micro-cracking development? Please elaborate. In addition, the term “integrating steel slag and self-sensing aggregates” does not make sense and requires rephrasing.

Response 5: Thank you for your suggestions. Current research on crack initiation and propagation detection technologies for asphalt pavements includes the implementation of Fiber Bragg Grating (FBG).Fiber Bragg Grating (FBG) utilizes optical fiber sensors embedded in the pavement to monitor strain distribution in real-time, locating crack initiation points.However, there is limited research on integrating crack detection with automated heating control, and this research direction holds potential for future studies. Additionally, we apologize for the terminology inconsistencies and have revised the problematic phrasing in Line 680 of the original text to enhance clarity and accuracy.

Reviewer 2 Report

Comments and Suggestions for Authors

1- Could you elaborate on the differences in aggregate gradation and binder content among AC-13, PAC-13, and SMA-13 mixtures that may explain the noticeably lower volume expansion in SMA-13?

2- Based on the observed expansion behaviour, what is your recommended upper limit of steel slag content for safe use in each mixture type?

3- Was any pretreatment (e.g., natural ageing, carbonation, chemical treatment) applied to the steel slag in the expansion tests shown in Figure 6? If yes, please specify the method; if no, how would pretreatment alter these trends?

4- Could the choice of binder (e.g., modified asphalt) potentially reduce expansion in mixtures like AC-13 that showed higher instability?

5- Are there any long-term or field performance data that validate these expansion behaviours under real environmental exposure (e.g., rainfall, freeze-thaw)?

6- Given the chemical heterogeneity of steel slag from different regions (Table 2), how consistent are the self-healing efficiencies across slag sources?

7- Have any tests been conducted to assess heavy metal leaching from steel slag in asphalt, or are lifecycle environmental assessments (LCA) available

8- What is the experimental basis for the 2 mm upper limit for effective healing? Do you have recommendations for addressing larger crack scenarios?

9- Based on your review, what are the optimal thermal activation parameters (temperature, duration, and power) that maximize healing while avoiding binder aging?

Author Response

Response to Reviewers’ Comments

Title: Comprehensive Review of Thermally Induced Self-Healing Behavior in Asphalt Mixtures and the Role of Steel Slag

Dear Editors/Reviewers:

Thank you for your letter and the reviewers’ comments concerning our manuscript. Those comments are all very valuable and helpful for revising and improving this paper. Some grammar and spelling errors has also been corrected. Furthermore, the relevant content has been added to the original manuscript according to the comments of reviewers and the major revised portions were marked in gray at the track changes version.

The responses are as followings:

Comments 1: Could you elaborate on the differences in aggregate gradation and binder content among AC-13, PAC-13, and SMA-13 mixtures that may explain the noticeably lower volume expansion in SMA-13?

Response 1: Thank you very much for your guidance. Through a thorough review of the literature, it was identified that the three types of mixtures exhibit significant differences in gradation, composition, and steel slag treatment methods. Due to these inherent disparities, the comparability among the mixtures is substantially limited. Consequently, we have decided to remove the relevant figure to avoid potential misinterpretations and ensure the integrity of the analysis.

 

Comments 2: Based on the observed expansion behaviour, what is your recommended upper limit of steel slag content for safe use in each mixture type?

Response 2: Thank you for your suggestions. Regarding the maximum allowable dosage of steel slag in different types of road construction mixtures, we posit that distinct physicochemical reactions may occur between steel slag and specific mixture compositions. Therefore, regulatory limits on steel slag content should be established based on national standards for volumetric expansion rate control in road materials. As mentioned in Section 3.2 of the manuscript regarding the electromagnetic properties of steel slag, "the optimal steel slag content for balanced heating efficiency and workability is 30–50%, beyond which agglomeration may reduce uniformity." Therefore, the upper limits for steel slag content in different types of asphalt mixtures should be determined by integrating national standards with self-healing efficiency requirements.

 

Comments 3: Was any pretreatment (e.g., natural ageing, carbonation, chemical treatment) applied to the steel slag in the expansion tests shown in Figure 6? If yes, please specify the method; if no, how would pretreatment alter these trends?

Response 3: Thank you very much for your guidance. Through a thorough review of the literature, it was identified that the three types of mixtures exhibit significant differences in gradation, composition, and steel slag treatment methods. Due to these inherent disparities, the comparability among the mixtures is substantially limited. Consequently, we have decided to remove the relevant figure to avoid potential misinterpretations and ensure the integrity of the analysis.

Comments 4: Could the choice of binder (e.g., modified asphalt) potentially reduce expansion in mixtures like AC-13 that showed higher instability?

Response 4: Thank you for your question. The selection of modified asphalt can reduce the expansion rate of asphalt mixtures containing steel slag through multiple mechanisms. Specifically, polymer-modified asphalt improves the interfacial bonding between steel slag aggregates and the asphalt binder, thereby reducing water infiltration. Additionally, amine-based additives or hydrated lime in modified asphalt decrease the moisture susceptibility of the mixture, inhibiting the hydration reactions of steel slag.

 

Comments 5: Are there any long-term or field performance data that validate these expansion behaviours under real environmental exposure (e.g., rainfall, freeze-thaw)?

Response 5: Thank you very much for your guidance. In Chapter 5, when discussing the challenges of self-healing technologies for steel slag asphalt mixtures, it is noted that the current lack of long-term monitoring data for carbonation-pretreated steel slag hinders the evaluation of this technology’s durability. This gap also results in insufficient data on the long-term performance of the self-healing mechanisms. Addressing this limitation will constitute a critical focus of future research to validate the sustainability and reliability of such systems under real-world conditions.

 

Comments 6: Given the chemical heterogeneity of steel slag from different regions (Table 2), how consistent are the self-healing efficiencies across slag sources?

Response 6: Thank you for your question. Indeed, regional variations in the chemical composition of steel slag can significantly affect its self-healing efficiency. The reasons are as follows:

1. Compositional Differences: Variations in calcium/magnesium oxide (CaO/MgO) content may alter the specific heat capacity of the slag. For example, high-CaO slag can reduce the effective heat storage capacity due to endothermic hydration reactions, thereby shortening the duration of optimal self-healing temperatures.
2. Magnetic Permeability Dependence: The magnetic permeability of steel slag relies on its ferromagnetic components (e.g., Fe₃O₄). Slag with higher Fe₃O₄ content enables induction heating via eddy current effects.

 

Comments 7: Have any tests been conducted to assess heavy metal leaching from steel slag in asphalt, or are lifecycle environmental assessments (LCA) available

Response 7: Thank you for raising this concern. The potential heavy metal leaching risks associated with steel slag, due to its inherent heavy metal content, are indeed a valid and critical point. Current research demonstrates that when steel slag is encapsulated within asphalt mixtures, its leaching potential is significantly reduced, with heavy metal concentrations remaining below safety thresholds. We have revised the relevant section (Lines 321–326) to clarify this mechanism and added supplementary references to support the argument.

 

Comments 8: What is the experimental basis for the 2 mm upper limit for effective healing? Do you have recommendations for addressing larger crack scenarios?

Response 8: Thank you for your input. The conclusion that self-healing technology is suitable for micro-cracks smaller than 2 mm is derived from the fact that Section 2.1.2 indicates capillary flow theory—one of the theoretical foundations adopted by the self-healing technology—has an optimal application range for cracks under 2 mm. For larger cracks, repair methods such as crack sealing/pouring, grooving and sealing, or preventive treatments like fog seals and micro-surfacing can be employed.

 

Comments 9: Based on your review, what are the optimal thermal activation parameters (temperature, duration, and power) that maximize healing while avoiding binder aging?

Response 9: Thank you very much for your guidance. Based on the reviewed studies, optimizing thermal activation parameters requires balancing healing efficiency with asphalt aging risks. The optimal temperature range is 80–100°C temperatures. At optimal healing temperatures (80–100°C), the increased kinetic energy of binder molecules enhances interdiffusion, with small molecular weight maltenes (e.g., saturates and aromatics) migrating faster than asphaltenes, leading to a gradual recovery of mechanical properties.

For electromagnetic induction heating, the recommended duration is 5–10 minutes, as steel slag with an Fe₃O₄ content exceeding 20% can heat a 5 cm depth to 70°C within 5 minutes. Simultaneously, the induction power should be maintained at 2–4 kW/m² to ensure efficient heating without localized overheating.

For microwave heating, due to its higher heating efficiency compared to electromagnetic induction heating, the recommended duration is 2–5 minutes, with a power range of 0.5–1.5 W/g.

 

Reviewer 3 Report

Comments and Suggestions for Authors

This paper presents an interesting review. In general, the document is easy to follow.

Structure and content are fairly well organized. Maybe the paper is a bit extensive and some of the first sections could be shortened to be more concise as part of the information is further explored later. The authors are free to consider if to make (or not) that fine tuning of the document.

My major questions are:

From your paper I would expect to find a clear guidance about how much of slag is needed to properly explore the self healing benefits. But I didn’t find any suggestion from the authors. Could you share your views on this?

The limitations of self-healing methods (as described in 2.3) are important. How these limitations can be overcome? And to which extent they don’t limit the interest in this application?

 

Other comments:

Authors should check if the references and citations are correct. Possibly their is some error in the numbering or the citation rational is strange. For instance, the 1st sentence of the introduction: “Asphalt pavements play a critical role in contemporary transportation systems, forming the essential foundation for roads, highways, and airport runways across the globe [1,2].” Is supported by the references 1 and 2. The references 1 and 2 are:

1. Yao, Y.; Chen, X.; Li, J.; Hu, H.; Vizzari, D.; Peng, Y. Towards sustainable and efficient inductive charging pavement systems: current progress and future directions. Construction and Building Materials 2024, 449, 138532.
2. Li, J.; Qin, Y.; Zhang, X.; Shan, B.; Liu, C. Emission characteristics, environmental impacts, and health risks of volatile organic compounds from asphalt materials: a state-of-the-art review. Energy & Fuels 2024, 38, 4787-4802.

These references don’t seem very aligned with the content of the sentence.

The same for reference 3 and respective citation and others through the introduction.

 

Figure 4 is not easy to understand. For instance, the density is indicated in %. It is % of what?

In this context, one property that is also important is the affinity asphalt binder-aggregate. Here, I can recommend you a reference: https://doi.org/10.3390/app13095716

In this reference the affinity asphalt-aggregate of steel slag is positioned as better than greywacke, basalt and granite aggregates, and worse than limestone. I find this property more logically integrated in Figure 4 rather than density.

Author Response

Response to Reviewers’ Comments

Title: Comprehensive Review of Thermally Induced Self-Healing Behavior in Asphalt Mixtures and the Role of Steel Slag

Dear Editors/Reviewers:

Thank you for your letter and the reviewers’ comments concerning our manuscript. Those comments are all very valuable and helpful for revising and improving this paper. Some grammar and spelling errors has also been corrected. Furthermore, the relevant content has been added to the original manuscript according to the comments of reviewers and the major revised portions were marked in gray at the track changes version.

The responses are as followings:

This paper presents an interesting review. In general, the document is easy to follow.

Comments 1: Structure and content are fairly well organized. Maybe the paper is a bit extensive and some of the first sections could be shortened to be more concise as part of the information is further explored later. The authors are free to consider if to make (or not) that fine tuning of the document.

Response 1: Thank you for your feedback. We acknowledge that the introduction section may indeed contain excessive content. After careful consideration, we have streamlined it by removing a portion of the material.

 

Comments 2: From your paper I would expect to find a clear guidance about how much of slag is needed to properly explore the self healing benefits. But I didn’t find any suggestion from the authors. Could you share your views on this?

Response 2: Thank you for your question. This paper aims to review the research progress in asphalt self-healing technologies, with a specific focus on steel slag-asphalt mixtures. While current studies on the influence of steel slag dosage in asphalt on self-healing performance remain limited, findings in Section 3.2 suggest that the optimal steel slag content ranges between 30% and 50%.

 

Comments 3: The limitations of self-healing methods (as described in 2.3) are important. How these limitations can be overcome? And to which extent they don’t limit the interest in this application?

Response 3: Thank you very much for your guidance. To mitigate asphalt aging caused by excessive temperatures, the use of modified bitumen and antioxidants can significantly reduce such effects. Additionally, temperature control technologies should be applied during heating to maintain temperatures below 100°C. For moisture interference, pre-drying with microwaves prior to the primary heating phase is recommended to lower the moisture content within cracks, thereby minimizing steam pressure effects during the repair process.

 

Comments 4: Authors should check if the references and citations are correct. Possibly their is some error in the numbering or the citation rational is strange. For instance, the 1st sentence of the introduction: “Asphalt pavements play a critical role in contemporary transportation systems, forming the essential foundation for roads, highways, and airport runways across the globe [1,2].” Is supported by the references 1 and 2. The references 1 and 2 are:

    Yao, Y.; Chen, X.; Li, J.; Hu, H.; Vizzari, D.; Peng, Y. Towards sustainable and efficient inductive charging pavement systems: current progress and future directions. Construction and Building Materials 2024, 449, 138532.

    Li, J.; Qin, Y.; Zhang, X.; Shan, B.; Liu, C. Emission characteristics, environmental impacts, and health risks of volatile organic compounds from asphalt materials: a state-of-the-art review. Energy & Fuels 2024, 38, 4787-4802.

These references don’t seem very aligned with the content of the sentence.

Response 4: Thank you for your careful reading. We sincerely apologize for this oversight. Upon re-evaluation, we have removed the cited reference to ensure strict consistency between the literature citations and their corresponding contextual relevance in the text.

 

Comments 5: The same for reference 3 and respective citation and others through the introduction.

Response 5: Thank you for highlighting this concern. We sincerely apologize for this oversight. Upon re-evaluation, we have removed the cited reference to ensure strict consistency between the literature citations and their corresponding contextual relevance in the text.

 

Comments 6: Figure 4 is not easy to understand. For instance, the density is indicated in %. It is % of what?

Response 6: Thank you very much for your guidance. Upon thorough review, we confirm that the unit of density in Figure 4 was indeed labeled incorrectly. We sincerely apologize for this oversight and have rectified the error in the revised manuscript.

 

Comments 7: In this context, one property that is also important is the affinity asphalt binder-aggregate. Here, I can recommend you a reference: https://doi.org/10.3390/app13095716. In this reference the affinity asphalt-aggregate of steel slag is positioned as better than greywacke, basalt and granite aggregates, and worse than limestone. I find this property more logically integrated in Figure 4 rather than density.

Response 7: Thank you for recommending the reference, which highlights the differences in asphalt affinity between steel slag and other materials. These differences indeed significantly affect the application of steel slag in asphalt mixtures. In response, we have incorporated this discussion in Lines 309–313 of the revised manuscript and added citations to the recommended reference. Additionally, we have added a citation in Lines 45–48.

Neves, J.; Crucho, J. Performance Evaluation of Steel Slag Asphalt Mixtures for Sustainable Road Pavement Rehabilitation. 2023, 13, 5716.

Crucho, J.M.L.; de Picado-Santos, L.G.; das Neves, J.M.C.; Capitão, S.D.J.T.E. The TEAGE ageing method for asphalt mixtures. 2020, 2, 100030.

 

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors revised their paper according to the reviewer's suggestions. Thank you.