Zinc Oxide as a Filler in a Hot-Mix Asphalt: Impact on Mechanical Properties

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study investigated the mechanical properties of hot mix asphalt mixtures with ZnO replacing natural fillers. The authors have carried out a good study and the test results can better support the conclusions of the tests. Specific comments are given below:

(1) Please explain how ZnO was obtained and whether purification process was required?

(2) Lin125, specific gravity missing units?

(3) How is the dispersion of ZnO in asphalt mixtures relative to natural fillers?

(4) The viscosity of asphalt mastic increases sharply after ZnO replaces natural filler. Please explain how asphalt mixture mixing and paving is considered with increased viscosity? Does it ensure adequate compaction of the mix?

 

 

Author Response

23-april, 2025

Dear Editor and Reviewers,

We would like to thank you for your valuable and insightful comments that have helped us to improve our manuscript. Please find also in the following paragraphs our answers to the comments. We have tried our best to clarify all the points raised. We hope that this new version of the manuscript is satisfactory for publication.

 

Reviewer 1

This study investigated the mechanical properties of hot mix asphalt mixtures with ZnO replacing natural fillers. The authors have carried out a good study and the test results can better support the conclusions of the tests. Specific comments are given below:

(1) Please explain how ZnO was obtained and whether purification process was required?

Answer: Thank you for your thoughtful and constructive feedback on our manuscript. Taking your comment into consideration, it is stated in lines 112 to 113 that the ZnO used is commercially available and sold over the counter. Furthermore, the supplier company is explicitly mentioned. Additionally, as indicated in lines 127 to 128, the ZnO was used as received, without undergoing any purification or treatment process.

 

(2) Lin125, specific gravity missing units?

Answer: Thank you very much for the comment. Yes, specific gravity is a dimensionless parameter, as it is determined as a ratio between two densities.

 

(3) How is the dispersion of ZnO in asphalt mixtures relative to natural fillers?

Answer: Thank you very much for your valuable comment. In this study, we did not conduct specific tests to evaluate this aspect. SEM imaging was performed on samples of the natural filler and ZnO; however, no observations were made on the asphalt mastic itself. Additionally, techniques such as digital image analysis, X-ray diffraction (XRD), or micro-computed tomography (Micro-CT) were not employed. In response to your comment, we have included a recommendation in lines 421–422 suggesting that future studies should assess the distribution and dispersion of ZnO within both the asphalt mastic and the asphalt mixture. We sincerely appreciate your understanding and thoughtful consideration of this matter.

 

(4) The viscosity of asphalt mastic increases sharply after ZnO replaces natural filler. Please explain how asphalt mixture mixing and paving is considered with increased viscosity? Does it ensure adequate compaction of the mix?

Answer: Thank you very much for the comment. Indeed, ZnO tends to increase the viscosity and stiffness of both the asphalt mastic and the asphalt mixture, as evidenced by the experimental results. This effect even led to an increase in the air void content of the mixtures when ZnO was used as a filler. A current challenge in mixture design lies in the fact that, in laboratory, the mixing and compaction temperatures are typically determined based on viscosity tests performed solely on the binder—without considering binder–aggregate interactions. This was also the approach adopted in our study, in order to adhere to standard procedures. Regarding the mixtures analyzed in this research, only the one in which the natural filler was fully replaced with ZnO (HMA-100) failed to meet volumetric composition specifications—specifically, due to a higher air void content—as established by national standards. Nevertheless, this mixture exhibited promising and noteworthy performance in the evaluated properties. Based on the results obtained so far, it can be concluded that adequate compaction and compliance with volumetric design criteria are ensured when up to 50% of the natural filler is replaced by ZnO. However, further research is needed to draw more definitive conclusions on this matter.

Reviewer 2 Report

Comments and Suggestions for Authors

Overall nice paper! I enjoyed reading it. Please consider the comments I made in the pdf version.

Comments for author File: Comments.pdf

Author Response

23-april, 2025

Dear Editor and Reviewers,

We would like to thank you for your valuable and insightful comments that have helped us to improve our manuscript. Please find also in the following paragraphs our answers to the comments. We have tried our best to clarify all the points raised. We hope that this new version of the manuscript is satisfactory for publication.

Reviewer 2

• Overall nice paper! I enjoyed reading it. Please consider the comments I made in the pdf version.

Answer: Thank you for your thoughtful and constructive feedback on our manuscript.

• Please provide a table in which you show the testing schedule of your work and how many specimens you produced for which test (e.g. 3 for ITSD, 3 for ITSC, 3 for CL etc.).

 Answer: Thank you very much for the comment. Taking your comment into consideration, we included a summary table (Table 3) in lines 215 to 219, showing the number of specimens prepared for each test and type of mixture.

• If possible, please add rpm for mixing.

Answer: Thank you very much for the comment. Taking your comment into consideration, in line 162, we included the mixing speed in rpm.

• Please add F0.05 = 7.71 in the table. You could also add an asterisk to the significant values and provide a short footnote at the bottom of the table.

Answer: Thank you very much for your comment. In response to your suggestion, we have marked the statistically significant values with an asterisk in all tables presenting the ANOVA results. Additionally, a brief explanatory footnote has been added to each relevant table (see lines 260-261, 318-319, 357-358, and 372-373). 

• Isn’t that confusing? why not say 1 is "good" performance and 3 is "bad" performance?

 Answer: Thank you very much for your comment. After careful consideration, we have chosen to retain the information as presented, as a value of 1 does not inherently imply good performance, nor does a value of 3 necessarily indicate poor performance. We kindly ask for your understanding and appreciate your thoughtful engagement with our work.

• Please add a statement on the influence of stiffness and binder-aggregate adhesion to low-temperature behaviour would.

Answer: hank you very much for your insightful comment. In this study, the properties of the mastic and asphalt mixtures at low temperatures were not evaluated; therefore, we are unable to draw conclusions regarding this aspect. Nonetheless, we have acknowledged its importance and included it as a recommendation for future research in the final paragraph of the conclusions “iii) evaluate low-temperature properties and resistance to aging” (line 419). We kindly ask for your understanding in this matter.

Reviewer 3 Report

Comments and Suggestions for Authors

This study investigates the replacement of natural filler  with  ZnO in hot-mix asphalt , evaluating its impact on mechanical properties and durability. The novelty lies in exploring ZnO as a filler rather than a binder modifier. The experimental approach is systematic, combining morphological, rheological, and mechanical performance tests. Results indicate ZnO enhances stiffness, moisture resistance, and fatigue life despite reduced binder content and higher porosity. Below are specific comments and recommendations for improvement.

1. While ZnO mixes exhibit higher porosity, the long-term durability risks are not discussed.
2. SEM images reveal ZnO’s tubular nanoparticles. Recommendation: Discuss how particle shape influences compaction, interlocking, and mechanical behavior.
3. Conventional tests (penetration, softening point) are used, but advanced rheological characterization is missing. Recommendation: Incorporate frequency sweeps or fatigue tests on mastics to better explain binder-filler interactions.
4. While ANOVA is applied, some trends (e.g., RM at 10 Hz for HMA-100 vs. Control) lack statistical significance but are presented as improvements.
5. Clarify why the HMA-100’s OAC (5.0%) was chosen despite failing volumetric criteria. How does this affect field applicability?

The manuscript presents a well-designed study with significant findings on ZnO’s viability as an HMA filler. The work aligns with infrastructure sustainability goals and merits publication after revisions.

Author Response

23-april, 2025

Dear Editor and Reviewers,

We would like to thank you for your valuable and insightful comments that have helped us to improve our manuscript. Please find also in the following paragraphs our answers to the comments. We have tried our best to clarify all the points raised. We hope that this new version of the manuscript is satisfactory for publication.

Reviewer 3

This study investigates the replacement of natural filler with ZnO in hot-mix asphalt, evaluating its impact on mechanical properties and durability. The novelty lies in exploring ZnO as a filler rather than a binder modifier. The experimental approach is systematic, combining morphological, rheological, and mechanical performance tests. Results indicate ZnO enhances stiffness, moisture resistance, and fatigue life despite reduced binder content and higher porosity. Below are specific comments and recommendations for improvement.

1. While ZnO mixes exhibit higher porosity, the long-term durability risks are not discussed.

Answer: Thank you for your thoughtful and constructive feedback on our manuscript. You are absolutely right. Indeed, long-term performance was not discussed in depth, as no specific experimental phase was conducted for that purpose (e.g., laboratory testing under accelerated aging, predictive modeling, full-scale testing, or field monitoring). However, parameters related to durability, such as the Tensile Strength Ratio (TSR) and the Cantabro Loss (CL), suggest a promising long-term performance of ZnO when used as a filler. In response to your comment, a paragraph was added in Section 3.6 (lines 386–390) highlighting the potential risks that ZnO-modified mixtures could face due to their higher porosity. Additionally, a recommendation was included for future works to evaluate the influence of ZnO on the long-term mechanical performance of asphalt mixtures (lines 414–416).

2. SEM images reveal ZnO’s tubular nanoparticles. Recommendation: Discuss how particle shape influences compaction, interlocking, and mechanical behavior.

Answer: Thank you very much for the comment. In response to your comment, information and discussion regarding the agglomerations of tubular ZnO nanoparticles observed under SEM have been included (lines 121–125, 273–277, 341–347).

3. Conventional tests (penetration, softening point) are used, but advanced rheological characterization is missing. Recommendation: Incorporate frequency sweeps or fatigue tests on mastics to better explain binder-filler interactions.

Answer: Thank you very much for your thoughtful comment and recommendation. Regrettably, we do not have access to a Dynamic Shear Rheometer (DSR) to perform the proposed rheological tests, nor do we currently have the necessary resources or financial support to conduct them in external laboratories. Consequently, we have included a recommendation in line 418 suggesting that future studies address this important aspect. We kindly ask for and sincerely appreciate your understanding in this matter.

4. While ANOVA is applied, some trends (e.g., RM at 10 Hz for HMA-100 vs. Control) lack statistical significance but are presented as improvements.

Answer: Thank you very much for your valuable comment. In Section 3.4 of the Results, it is noted that the general trend for RM in the HMA-100 is to exhibit higher values compared to the Control HMA. However, it is also mentioned that this increase was statistically significant for only one loading frequency. We kindly note that there is no explicit statement claiming an overall improvement in RM. We sincerely appreciate your understanding on this matter.

5. Clarify why the HMA-100’s OAC (5.0%) was chosen despite failing volumetric criteria. How does this affect field applicability?

Answer: Thank you for your insightful comment. The selection of 5.0% as the Optimum Asphalt Content (OAC) for HMA-100 was based primarily on the mechanical performance observed in the Marshall test. Although this binder content does not fully satisfy the volumetric criteria—particularly in terms of air voids, indicating a more porous structure—it resulted in significantly higher Marshall stability and a favorable stability-to-flow ratio compared to mixtures with higher asphalt contents. Increasing the binder content to meet volumetric requirements (e.g., 5.7%) was found to considerably reduce the mixture’s mechanical strength and would also lead to higher material costs due to the greater amount of asphalt binder required. Thus, a balance was sought between volumetric compliance and structural performance. Regarding field applicability, while the increased porosity may raise concerns about durability or moisture susceptibility, the improved resistance to plastic deformation suggests that HMA-100 could still perform well under traffic loading. However, further investigations—such as moisture susceptibility and long-term durability tests—would be necessary to fully assess its suitability for field application. To clarify the selection of the 5.0% OAC for HMA-100, the wording of the paragraph in lines 244 to 255 has been revised and improved.

6. The manuscript presents a well-designed study with significant findings on ZnO’s viability as an HMA filler. The work aligns with infrastructure sustainability goals and merits publication after revisions.

Answer: Once again, thank you for your thoughtful and constructive feedback on our manuscript.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The author addressed my concerns very well. The manuscript is ready for publication.

Reviewer 3 Report

Comments and Suggestions for Authors

The revised version substantially strengthens the scientific validity and clarity of your work, reflecting your commitment to addressing the reviewers’ feedback comprehensively.