The Influence of Synthetic Reinforcing Fibers on Selected Properties of Asphalt Mixtures for Surface and Binder Layers

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript investigates the effect of aramid fiber on the pavement performance of asphalt mixtures. Base on the current manuscript, there are some points the authors need to consider for improving the paper, some detailed comments are shown as below:

1. The use of non-standard abbreviations, such as AC_surf 16 and AC_bin 16S, in the title is discouraged because it may lead to confusion.
2. The manuscript contains numerous grammatical errors and unclear modifier relationships. For example, the sentence “...One of the best for this purpose are aramid fibres...” is grammatically incorrect.
3. All abbreviations, including ITSR and IT-CY, should be defined in full upon first appearance. It is recommended to carefully review the manuscript to ensure consistent and proper introduction of all abbreviations.
4. “…The results of the crack propagation test are ambiguous…”, it is unclear what the author intends to convey.
5. The abstract needs to be polished. It should clearly include the research objective, research subject, methodology, results, and conclusions.
6. Section 2.1, the manuscript should directly specify the fiber type employed and the rationale for its selection. Microstructural images or molecular structures cited from other sources should not appear in the main text. Full details of all materials used, including their properties, must be provided.
7. Are ACO 16+ and ACL 16S dense-graded asphalt mixtures? Please use standard international terminology.
8. What type of polymer-modified bitumen is used? Is it SBS-modified bitumen? If so, is the mixing temperature 160 °C?
9. The experimental design should align with the research objectives. It is unclear why the 50/70 bitumen control group was included. Since the study focuses on the effect of aramid fiber on asphalt mixtures, a controlled-variable approach would be more appropriate.
10. How was the fiber dosage determined? How is the uniform distribution of aramid fiber in the asphalt mixture demonstrated?
11. Figure 3 and Figure 4, is the used gradation black? The gradation of ACO 16+ is outside the specified range.
12. It is recommended that the manuscript be structured into Introduction, Materials and Methods, Results and Analysis, and Conclusions. The experimental methodology should be fully described in one dedicated section.
13. There is no y-axis label in Figures 5 and 6. Is the dashed line representing the maximum limit?
14. There is no y-axis label in Figures 7 and 8.
15. Figure 9 and Figure 10，why does the fracture energy of the asphalt mixture decrease significantly after adding aramid fiber? Could it be due to uneven fiber dispersion?
16. Section 3.5, why are different test temperatures used for the 50/70 bitumen and the polymer-modified bitumen samples? Based on the current experimental design, such a comparison has little meaningful significance.
17. Lines 359-362，the air void content of Mix 6 is lower than that of Mix 4 and Mix 5. How was this conclusion determined?
18. The conclusions should be further polished to improve clarity and emphasize the key findings of the study.
19. There are few references from the past three years.

Comments on the Quality of English Language

The English could be improved to more clearly express the research.

Author Response

Reviewer N°1

Dear Reviewer, thank you for your insightful comments, for your time and energy spent on us. We have tried our best to improve the paper.

Comment n°1: The use of non-standard abbreviations, such as ACsurf 16 and ACbin 16S, in the title is discouraged because it may lead to confusion.

àAbbreviations were standard ones. For better clarity, we have changed them into international standard.

Comment n°2: The manuscript contains numerous grammatical errors and unclear modifier relationships. For example, the sentence “...One of the best for this purpose are aramid fibres...” is grammatically incorrect.

àWe have changed some text and did our best to correct most, if not all the errors.

Comment n°3: All abbreviations, including ITSR and IT-CY, should be defined in full upon first appearance. It is recommended to carefully review the manuscript to ensure consistent and proper introduction of all abbreviations.

àIt should be corrected now.

Comment n°4: “…The results of the crack propagation test are ambiguous…”, it is unclear what the author intends to convey.

àWe agree. New sentence: The crack propagation test showed no consistent improvement in fracture resistance, although some beneficial effects of fibre addition were observed under certain conditions.

Comment n°5: The abstract needs to be polished. It should clearly include the research objective, research subject, methodology, results, and conclusions.

àAbstract was rewritten.

Comment n°6: Section 2.1, the manuscript should directly specify the fiber type employed and the rationale for its selection. Microstructural images or molecular structures cited from other sources should not appear in the main text. Full details of all materials used, including their properties, must be provided.

àSome Figures were changed and details were added.

Comment n°7: Are ACO 16+ and ACL 16S dense-graded asphalt mixtures? Please use standard international terminology.

à For higher clarity, we have changed them into international standard.

Comment n°8: What type of polymer-modified bitumen is used? Is it SBS-modified bitumen? If so, is the mixing temperature 160 °C?

à Answer: We appreciate the reviewer’s observation. The polymer-modified binders used in this study (PMB 45/80-75 and PMB 25/55-60) are SBS-modified bitumen (styrene–butadiene–styrene). The selected mixing and compaction temperature of 160 °C falls within the manufacturer’s recommended range (155–165 °C) for SBS-modified binders, ensuring adequate workability and homogeneity without risking polymer degradation.

Comment n°9: The experimental design should align with the research objectives. It is unclear why the 50/70 bitumen control group was included. Since the study focuses on the effect of aramid fiber on asphalt mixtures, a controlled-variable approach would be more appropriate.

à Answer: We appreciate the reviewer’s valuable comment. The inclusion of the 50/70 paving-grade bitumen mixture served a specific purpose, to assess the potential of aramid fibres as an alternative or complementary modifier to polymer modification. This design allows comparison of fibre efficiency in two different binder systems: a polymer-modified binder (PMB 45/80-75) and a conventional bitumen (50/70). Such an approach reflects practical field conditions, where both binder types are commonly used in surface course mixtures. Although the main objective of the study was to evaluate the influence of aramid fibres, including both binder types provided a broader understanding of their interaction and effectiveness in different asphalt matrices. This rationale has been clarified in the revised manuscript (Section 2.1, lines

Comment n°10: How was the fiber dosage determined? How is the uniform distribution of aramid fiber in the asphalt mixture demonstrated?

à Answer: We thank the reviewer for this valuable comment. The selected fibre dosages of 0.02% and 0.04% by total mixture weight were chosen based on the manufacturer’s recommendations and previous research indicating that these values provide effective reinforcement without compromising mixture workability or homogeneity. Higher dosages were avoided to prevent clumping and uneven dispersion observed during preliminary laboratory trials.

Uniform fibre distribution was ensured by gradually introducing the fibres into the preheated aggregates during dry mixing at 150–160 °C, prior to binder addition. The mixing process was monitored visually to ensure homogeneous dispersion, and no fibre agglomeration was observed. This procedure has been described in the revised manuscript (Section 2.1, lines **).

Comment n°11: Figure 3 and Figure 4, is the used gradation black? The gradation of ACO 16+ is outside the specified range.

à Answer: We thank the reviewer for the observation. The gradation curve representing the tested mixtures was originally shown as a black solid line, while the standard envelope limits are indicated by red dashed lines.

Regarding the gradation of AC 16 surf (formerly ACO 16+), we acknowledge that the curve is slightly outside the upper boundary of the specified range at one sieve fraction. This small deviation results from aggregate availability and laboratory batching tolerances but remains within acceptable practical limits and does not affect the volumetric or mechanical compliance of the mixture according to ČSN 73 6121:2023.

Comment n°12: It is recommended that the manuscript be structured into Introduction, Materials and Methods, Results and Analysis, and Conclusions. The experimental methodology should be fully described in one dedicated section.

à Answer: We appreciate the reviewer’s valuable suggestion. The manuscript has been restructured to follow the recommended format with clearly separated sections: Introduction, Materials and Methods, Results and Analysis, and Conclusions. All experimental procedures, including mixture design, specimen preparation, and test methodologies (ITSR, stiffness modulus, crack propagation, and dynamic modulus), are now fully described in Section 2 (Materials and Methods). The Results and Analysis section focuses exclusively on the presentation and interpretation of experimental findings.

Comment n°13: There is no y-axis label in Figures 5 and 6. Is the dashed line representing the maximum limit?

à Answer: We thank the reviewer for noting this oversight. In the revised manuscript, Figures 5 and 6 have been updated to include a clear y-axis label “Air-void content [%]”. The figure legends have also been clarified: the dashed horizontal lines represent the type-testing limit criteria (upper and lower permissible limits), while the solid black lines indicate the control limits according to ČSN 73 6121:2023. These graphical and caption revisions have been implemented to improve readability and ensure unambiguous interpretation (Section 3.1, Figures 5 and 6).

Comment n°14: There is no y-axis label in Figures 7 and 8.

à Answer: We thank the reviewer for noting this omission. In the revised manuscript, Figures 7 and 8 have been updated to include the appropriate y-axis labels and improve readability.

• For Figure 7, the left vertical axis is now labeled “Indirect tensile strength, ITS [MPa]”, and the right vertical axis is labeled “Indirect tensile strength ratio, ITSR [%]”.
• For Figure 8, the y-axis is labeled “Stiffness modulus, IT-CY [MPa]”.

All revised figures now contain complete axis titles and consistent formatting in accordance with MDPI graphical standards.

Comment n°15: Figure 9 and Figure 10，why does the fracture energy of the asphalt mixture decrease significantly after adding aramid fiber? Could it be due to uneven fiber dispersion?

à Answer: We appreciate the reviewer’s insightful observation. The decrease in fracture energy (Gf) after adding aramid fibres, particularly at 0 °C, is primarily attributed to the increased stiffness and reduced ductility of the asphalt matrix caused by the incorporation of high-modulus aramid fibres within a polymer-modified binder. Under low-temperature loading, this stiffening limits plastic deformation prior to cracking, thereby reducing the measured fracture energy. At 15 °C, where the binder is more compliant, the fibres contribute more effectively to crack bridging, and Gf partially increases.

Uneven fibre dispersion was considered; however, the fibres were added gradually during dry mixing at 150–160 °C and visually inspected for homogeneity. No significant clumping or segregation was observed, suggesting that the reduction in Gf results mainly from the thermomechanical interaction between the stiff aramid fibres and the polymer-modified binder rather than from poor fibre distribution.

Comment n°16: Section 3.5, why are different test temperatures used for the 50/70 bitumen and the polymer-modified bitumen samples? Based on the current experimental design, such a comparison has little meaningful significance.

à Answer: We thank the reviewer for this valuable comment. The different test temperatures were selected in accordance with the relevant standards ČSN EN 12697-22 and ČSN 73 6121:2023, which specify different reference temperatures depending on the binder type and pavement layer. Mixtures containing conventional 50/70 bitumen were tested at 50 °C, while those with polymer-modified bitumen (PMB) were tested at 60 °C, reflecting the higher softening point and service temperature of PMBs. Binder-course mixtures (AC 16 bin) were tested at 15 °C, as required for base layers with lower in-service temperatures. Accordingly, the wheel-tracking test results are intended not for direct cross-comparison across binder types, but to evaluate each mixture’s compliance and performance under its appropriate service condition.

Comment n°17: Lines 359-362，the air void content of Mix 6 is lower than that of Mix 4 and Mix 5. How was this conclusion determined?

à Answer: We thank the reviewer for this observation. We re-checked the density/compaction data and agree that Mix 6 does not exhibit higher air-void content; its compaction degree is 100.0% compared with 99.2% for Mix 4 and Mix 5, indicating comparable or slightly lower air-voids. We have removed the statement linking Mix 6’s rutting to higher air-voids and revised the text to reflect a more plausible explanation based on mastic rheology (higher local viscosity and reduced stress relaxation at 15 °C with 0.04% fibres) rather than air-voids.

Comment n°18: The conclusions should be further polished to improve clarity and emphasize the key findings of the study.

à Answer: Conclusions were rewritten.

Comment n°19: There are few references from the past three years.

à Answer: New references were added.

 

 

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript titled “The Influence of Aramid Reinforcing Fibers on Selected Properties of Asphalt Mixtures Type AC_surf 16 and AC_bin 16S” was received. This study systematically investigated the influence of aramid fibres at varying dosages on the service performance of two typical asphalt mixtures. This was achieved through void content determination, indirect tensile strength (ITS) testing, moisture susceptibility (ITSR) assessment, stiffness modulus testing, crack propagation behaviour analysis, rutting tests, and dynamic modulus master curve construction. The paper employs a controlled variable approach, combining different binder types to conduct an in-depth analysis of the fibre reinforcement mechanism. The research demonstrates a degree of innovation and practical engineering value, with a systematically comprehensive experimental design and a clear overall logical framework. Prior to publication, certain sections require modification and further elaboration, summarised as follows:

1. The macro-level context mentioned in the abstract's opening, such as “The growth of world population”, lacks sufficient direct relevance to aramid fibres. It is recommended to supplement with more specific industry background, such as the current high incidence of asphalt pavement defects, to sharpen the focus of the context.
2. The introduction notes that comparative studies on aramid fibres in ACO 16+ and ACL 16S asphalt concrete remain scarce. It is recommended to explicitly state how this research addresses this gap, thereby highlighting the significance of undertaking this study.
3. The introduction mentions contributions to “pavement sustainability” but remains somewhat vague. It is recommended that specific data be added to either the introduction or discussion section, detailing how aramid fibres extend pavement lifespan and reduce maintenance costs. This should quantify their environmental benefits and highlight their economic and societal significance.
4. While numerous studies have explored the application of aramid fibres in asphalt, the paper fails to cite relevant literature. It is recommended that such references be supplemented to substantiate the findings. Furthermore, as the paper draws heavily upon literature predating 2010, it is advisable to incorporate recent studies (within the last 5-10 years) concerning aramid fibres and asphalt mixtures to enhance the paper's timeliness.
5. The terms “ACO 16+, ACL 16S”appearing in the main text are inconsistent with “ACsurf 16, ACbin 16S” used in the title. Similarly, the terms “bitumen” and “asphalt” are used inconsistently. It is recommended that the entire text be reviewed for consistency.
6. Section numbers for “2.1. Aramid Fibres” and “2.1. Bitumen Mixtures” are duplicated; verification is recommended.
7. The article mentions incorporating aramid fibres at concentrations equivalent to 0.02% and 0.04% of the mixture's weight, alongside polymer-modified bitumen at dosages of 4.7% and 5.4%. The rationale for selecting these specific dosages should be clarified.
8. The method of fibre dispersion is crucial, yet the article merely mentions “surface treatment” without detailing the specific procedure. It is recommended that the steps be supplemented.
9. “2. Materials and Methods” only mentions aramid fibres and asphalt mixtures without detailing the methodology. The test procedures described in Section 3.4 (SCB test), Section 3.5 (wheel tracking test), and Section 3.6 (4-point bending test) should be relocated to “2. Materials and Methods” to optimise the article's structure.
10. ITS testingsimultaneously employs both the Czech national standard ČSN EN 12697-12 and the AASHTO T283 protocol. However, the rationale for selecting between these two methods and the correlation of their results remain unspecified. It is recommended that this information be supplemented.
11. Section 3.5 mentions that the ACL16S mixture exhibits poor resistance to permanent deformation, yet this explanation merely addresses the increased void content in the asphalt mixture superficially. It fails to delve into the characteristics of the fibre material, such as the smoothness of the fibre surface or issues at the fibre-binder interface. The results section should not merely describe experimental findings; it must incorporate discussion on the mechanisms underlying these results. The discussion section should include comparisons with similar studies and highlight the engineering application value of this research.
12. The results section predominantly employs descriptive language (such as “slight increase”), which constitutes subjective characterisation lacking statistical testing. It is recommended that all data be presented with quantitative descriptions and supplemented with inferential statistics.
13. The conclusion statesthat “The results did not confirm the expected trend in this respect. However, from the point of view of the experience of the FCE CTU Prague...”, the author should focus on presenting the unique findings of this study rather than dwelling excessively on the experiences of previous research.

Author Response

Reviewer N°2

Dear Reviewer, thank you for your insightful comments, for your time and energy spent on us. We have tried our best to improve the paper.

Comment n°1: The macro-level context mentioned in the abstract's opening, such as “The growth of world population”, lacks sufficient direct relevance to aramid fibres. It is recommended to supplement with more specific industry background, such as the current high incidence of asphalt pavement defects, to sharpen the focus of the context.

àAnswer: We appreciate the reviewer’s constructive suggestion. The opening of the abstract has been revised to provide a more industry-specific context focused on asphalt pavement deterioration and the need for advanced reinforcement technologies.

You will find the final abstract according to the three reviewers at the end.

Comment n°2: The introduction notes that comparative studies on aramid fibres in ACO 16+ and ACL 16S asphalt concrete remain scarce. It is recommended to explicitly state how this research addresses this gap, thereby highlighting the significance of undertaking this study.

àAnswer: We thank the reviewer for this valuable suggestion. The Introduction section has been revised to explicitly explain how the present study addresses the identified research gap

Comment n°3: The introduction mentions contributions to “pavement sustainability” but remains somewhat vague. It is recommended that specific data be added to either the introduction or discussion section, detailing how aramid fibres extend pavement lifespan and reduce maintenance costs. This should quantify their environmental benefits and highlight their economic and societal significance.

àAnswer: We appreciate the reviewer’s valuable recommendation. The manuscript has been revised to include quantitative information from relevant studies demonstrating the potential environmental and economic benefits of using aramid fibres in asphalt mixtures (Line **).

Comment n°4: While numerous studies have explored the application of aramid fibres in asphalt, the paper fails to cite relevant literature. It is recommended that such references be supplemented to substantiate the findings. Furthermore, as the paper draws heavily upon literature predating 2010, it is advisable to incorporate recent studies (within the last 5-10 years) concerning aramid fibres and asphalt mixtures to enhance the paper's timeliness.

àAnswer: New references were added.

Comment n°5: The terms “ACO 16+, ACL 16S” appearing in the main text are inconsistent with “ACsurf 16, ACbin 16S” used in the title. Similarly, the terms “bitumen” and “asphalt” are used inconsistently. It is recommended that the entire text be reviewed for consistency.

àAnswer:

For higher clarity, we have changed them into international standard.

Comment n°6: Section numbers for “2.1. Aramid Fibres” and “2.1. Bitumen Mixtures” are duplicated; verification is recommended.

àAnswer: Corrected.

Comment n°7: The article mentions incorporating aramid fibres at concentrations equivalent to 0.02% and 0.04% of the mixture's weight, alongside polymer-modified bitumen at dosages of 4.7% and 5.4%. The rationale for selecting these specific dosages should be clarified.

àAnswer: We appreciate the reviewer’s valuable recommendation. The manuscript has been revised.

Comment n°8: The method of fibre dispersion is crucial, yet the article merely mentions “surface treatment” without detailing the specific procedure. It is recommended that the steps be supplemented.

àAnswer: Description added.

Comment n°9: 2. Materials and Methods” only mentions aramid fibres and asphalt mixtures without detailing the methodology. The test procedures described in Section 3.4 (SCB test), Section 3.5 (wheel tracking test), and Section 3.6 (4-point bending test) should be relocated to “2. Materials and Methods” to optimise the article's structure.

àAnswer: Article’s structure was reorganized.

Comment n°10: ITS testing simultaneously employs both the Czech national standard ČSN EN 12697-12 and the AASHTO T283 protocol. However, the rationale for selecting between these two methods and the correlation of their results remain unspecified. It is recommended that this information be supplemented.

àAnswer:  Description added.

Comment n°11: Section 3.5 mentions that the ACL16S mixture exhibits poor resistance to permanent deformation, yet this explanation merely addresses the increased void content in the asphalt mixture superficially. It fails to delve into the characteristics of the fibre material, such as the smoothness of the fibre surface or issues at the fibre-binder interface. The results section should not merely describe experimental findings; it must incorporate discussion on the mechanisms underlying these results. The discussion section should include comparisons with similar studies and highlight the engineering application value of this research.

àAnswer:  Section was rewritten.

Comment n°12: The results section predominantly employs descriptive language (such as “slight increase”), which constitutes subjective characterisation lacking statistical testing. It is recommended that all data be presented with quantitative descriptions and supplemented with inferential statistics.

àAnswer:  We understand that and some sentences were rewritten.

Comment n°13:  The conclusion states that “The results did not confirm the expected trend in this respect. However, from the point of view of the experience of the FCE CTU Prague...”, the author should focus on presenting the unique findings of this study rather than dwelling excessively on the experiences of previous research.

àAnswer:  Conclusions section was rewritten.

 

Reviewer 3 Report

Comments and Suggestions for Authors

Actual and useful research work. It represents results of natural experiments of asphalt mixtures type ACsurf 16 and ACbin 16S with aramid reinforcing fibers.

The introduction provides sufficient background and includes all relevant references. This chapter is logically built and well structured.

Research design is appropriate - it presents all necessary data, methodology and results with analysis.

 Methods and results are clearly presented - mainly in graphical form and with a few tables. All figures and tables are clear and well-presented.

Conclusions are supported by the results. 

Very useful and serious work!

In my opinion the value of this work will be improved significant if the conclusion chapter is rearranged. Present conclusions are direct results from each case study as natural test in previous chapter. The final conclusions could be more common and consistent, e.g. this parameter leads to that effect, other parameter does not affect, etc. Also, it is appropriate to mention necessity of exact next investigation if some questions are not solved here.

In the same manner the abstract could be improved by consistent conclusions.   

Author Response

In my opinion the value of this work will be improved significant if the conclusion chapter is rearranged. Present conclusions are direct results from each case study as natural test in previous chapter. The final conclusions could be more common and consistent, e.g. this parameter leads to that effect, other parameter does not affect, etc. Also, it is appropriate to mention necessity of exact next investigation if some questions are not solved here.

In the same manner the abstract could be improved by consistent conclusions. 

 

Reply: Dear Reviewer, thank you for your insightful comments, for your time and energy spent on us. We have tried our best to improve the paper.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript is revised according to my comments, and the revision is satisfactory and suitable for the publication.