Aging Resistance Evaluation of an Asphalt Mixture Modified with Zinc Oxide

Round 1

Reviewer 1 Report

Comments:

·       Paper is extraordinarily clear, well-written, and cites/reviews an extensive bibliography on the subject matter.  Statistics seem appropriate and complement the study. 

·       The RTFO pen of the binder at first appears to the reader to be considerably higher than the pen on the virgin binder.  Please use “pen retention”, “pen index”, or some term that reflects % change after RTFO?

·       Please cite a rough cost estimate for use of ZnO at recommended dosages.

·       My experience is that the commercial grade of ZnO described in this paper settles out of asphalt fairly quickly during storage unless it has a surface active agent or it is a more soluble form or Zn, such as Zn Octoate.  Do you run settlement tests to evaluate storage stability in tanks?  ZnO can serve as an H₂S scavenger during vulcanization of SBS polymers, so some work has been published in this regard. Possibly reference J.P. Planche.

·       Line 288:  Fine particles naturally stiffen asphalt. The term “crosslinked network structure” is appropriate to use for in-situ cross-linked polymer additives such as vulcanized SBS, but likely has little relevance for small particles which stiffen asphalt in a totally different way.  Although this is a citation from other authors, I would recommend deleting any reference to networks and cross-linking unless this has been proven to your satisfaction.  

·       Aging protocols for both binders and mixtures are well chosen and represent current best practice. However, binder aging indices such as PDI, SPI, and VAI represent best practice from two decades ago, but have been replaced by very different aging parameters developed to rank age-induced surface damage.  Changes in low temperature binder phase angle during aging are critical, so tools measuring binder or mixture modulus alone are insufficient. Newer binder testing protocols target embrittlement and require a Dynamic Shear Rheometer or Bending Beam rheometer, which deliver relatively new binder aging parameters (ΔTc, Glover-Rowe (G-R) parameter, R-value, cross-over modulus, etc.) which are generally determined at low to intermediate temperatures, not at pavement rutting temperatures. Primary authors for this new approach to rank binder damage accumulated during aging include: G. Rowe, M. Anderson, M. Elwardany, G. King, R. Moreas-Puchalski and many others. Mixture testing for aging has likewise changed, and now uses parameters such as Ideal-CT or I-Fit from SBC, as described by F. Zhou, N. Tran and others.  USAAT is a much more expensive but effective tool as well.  An excellent series of publications and final reports from NCHRP 9-58, 9-59, 9-60, 9-65 cover these subjects well.  A. Epps-Martin’s many publications using these new binder and mixture techniques to advance use of higher RAP concentrations in HMA mixes could be especially valuable.  Raquel Moreas will soon be publishing papers on NCAT’s evaluation of numerous asphalt additives submitted to FHWA as age-resistant modifiers. Her collective research work at NCAT offers an excellent approach to researchers studying age-resistant modifiers. This study is extensive and well done, but experienced binder formulators will be asking questions per my comments above before judging an additive’s viability for performance as an aging inhibitor or as a rheological modifier which counters the ill-effects of aging.  Given the high quality of the work with available tools and particularly strong writing skills, I strongly encourage the authors to seek funding to upgrade binder test equipment before continuing their research as mentioned in their last conclusion.

Author Response

18-april, 2024

Dear Editor and Reviewer,

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

Comments:

1. Paper is extraordinarily clear, well-written, and cites/reviews an extensive bibliography on the subject matter. Statistics seem appropriate and complement the study. 

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

2. The RTFO pen of the binder at first appears to the reader to be considerably higher than the pen on the virgin binder.  Please use “pen retention”, “pen index”, or some term that reflects % change after RTFO?

Answer: Thank you very much for the comment. You're right. For this reason, in Table 1 (line 132), "Penetration" was changed to "Percent penetration retained".

3. Please cite a rough cost estimate for use of ZnO at recommended dosages.

Answer: Thank you very much for the comment. The cost (US$/kg) of ZnO is shown on lines 135 to 136. The increase in the cost per m³ of HMA for using ZnO as a modifier is shown in lines 296 to 299.

4. My experience is that the commercial grade of ZnO described in this paper settles out of asphalt fairly quickly during storage unless it has a surface active agent or it is a more soluble form or Zn, such as Zn Octoate.  Do you run settlement tests to evaluate storage stability in tanks?  ZnO can serve as an H2S scavenger during vulcanization of SBS polymers, so some work has been published in this regard. Possibly reference J.P. Planche.

Answer: Thank you very much for the comment. You're right. In lines 140 to 150 we report something about it, and in line 150 the high density of ZnO is shown. The above could induce particle settling problems during storage. Unfortunately, we do not evaluate storage stability in tanks. To take your comment into account, in lines 398 to 399 we recommend carrying out this evaluation in future works. We hope for your kind understanding.

5. Line 288:  Fine particles naturally stiffen asphalt. The term “crosslinked network structure” is appropriate to use for in-situ cross-linked polymer additives such as vulcanized SBS, but likely has little relevance for small particles which stiffen asphalt in a totally different way.  Although this is a citation from other authors, I would recommend deleting any reference to networks and cross-linking unless this has been proven to your satisfaction.  

Answer: Thank you very much for the comment. We removed the term "crosslinked network" on line 291.

6. Aging protocols for both binders and mixtures are well chosen and represent current best practice. However, binder aging indices such as PDI, SPI, and VAI represent best practice from two decades ago, but have been replaced by very different aging parameters developed to rank age-induced surface damage.  Changes in low temperature binder phase angle during aging are critical, so tools measuring binder or mixture modulus alone are insufficient. Newer binder testing protocols target embrittlement and require a Dynamic Shear Rheometer or Bending Beam rheometer, which deliver relatively new binder aging parameters (ΔTc, Glover-Rowe (G-R) parameter, R-value, cross-over modulus, etc.) which are generally determined at low to intermediate temperatures, not at pavement rutting temperatures. Primary authors for this new approach to rank binder damage accumulated during aging include: G. Rowe, M. Anderson, M. Elwardany, G. King, R. Moreas-Puchalski and many others. Mixture testing for aging has likewise changed, and now uses parameters such as Ideal-CT or I-Fit from SBC, as described by F. Zhou, N. Tran and others.  USAAT is a much more expensive but effective tool as well.  An excellent series of publications and final reports from NCHRP 9-58, 9-59, 9-60, 9-65 cover these subjects well.  A. Epps-Martin’s many publications using these new binder and mixture techniques to advance use of higher RAP concentrations in HMA mixes could be especially valuable.  Raquel Moreas will soon be publishing papers on NCAT’s evaluation of numerous asphalt additives submitted to FHWA as age-resistant modifiers. Her collective research work at NCAT offers an excellent approach to researchers studying age-resistant modifiers. This study is extensive and well done, but experienced binder formulators will be asking questions per my comments above before judging an additive’s viability for performance as an aging inhibitor or as a rheological modifier which counters the ill-effects of aging.  Given the high quality of the work with available tools and particularly strong writing skills, I strongly encourage the authors to seek funding to upgrade binder test equipment before continuing their research as mentioned in their last conclusion.

Answer: Thank you for your thoughtful and constructive feedback on our manuscript. It is very important for us to take this observation into account in future phases of the research project. For this reason, taking into account your comment, in lines 399 to 402 a recommendation was written to consider evaluating in future works other aging indexes measured at low and intermediate temperatures.

Author Response File: Author Response.docx

Reviewer 2 Report

The paper presents a detailed comprehensive investigation on aging resistance of asphalt mixture with zinc oxide. This paper contains some useful aspects and the authors have identified an area that needs further work.

1. The purpose of the paper should be reflected in the abstract and introduction. More quantitative conclusions should be provided in the abstract.

2. The most important problem of inorganic powder modified asphalt is its compatibility and dispersion. Prolonging the mixing time can improve its modification effect, but when testing the softening point increment (in Fig. 5), there will still be sedimentation and segregation. How does the author consider it?

3. In Fig. 7, four AC contents are used to determine the optimal asphalt content, rather than the conventional five-point method, why?

4. ZnO can improve the thermal oxidative aging resistance of asphalt. In addition to its alkalinity, it is also related to its microstructure. The effect of microscopic analysis of ZnO on performance of asphalt should be strengthened.

5 The ordinate scale of Fig.7(b) should be adjusted.

Author Response

18-april, 2024

Dear Editor and Reviewer,

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

Comments:

The paper presents a detailed comprehensive investigation on aging resistance of asphalt mixture with zinc oxide. This paper contains some useful aspects and the authors have identified an area that needs further work.

1. The purpose of the paper should be reflected in the abstract and introduction. More quantitative conclusions should be provided in the abstract.

Answer: Thank you for your thoughtful and constructive feedback on our manuscript. In the Abstract (lines 18 to 19), taking into account your comment, we provide a sentence reporting the purpose of the study. In the Introduction, the purpose of the study is mentioned in lines 111 to 114. The quantitative results are many and we have a word limit in the Abstract that we have already exceeded. For this reason, we believe that it is better to write general conclusions. We hope for your understanding in this regard.

2. The most important problem of inorganic powder modified asphalt is its compatibility and dispersion. Prolonging the mixing time can improve its modification effect, but when testing the softening point increment (in Fig. 5), there will still be sedimentation and segregation. How does the author consider it?

Answer: Thank you very much for the comment. You're right. In lines 140 to 150 we report something about it, and in line 150 the high density of ZnO is shown. The above could induce particle settling problems during storage. Unfortunately, we do not evaluate storage stability tests. To take your comment into account, in lines 398 to 399 we recommend carrying out this evaluation in future works. We hope for your kind understanding.

3. In Fig. 7, four AC contents are used to determine the optimal asphalt content, rather than the conventional five-point method, why?

Answer: Thank you very much for the comment. The aggregate and asphalt binder (AC 60-70) of the HMA in this study have been used in multiple researches by us. Also, the type of HMA. We know in advance that the range of OAC varies between 5.0 and 5.5%. Additionally, we know that AC contents below 4.5% and above 6.0% generate HMAs with very low mechanical performance. For this reason, we work with asphalt binder contents between 4.5% and 6.0%. We hope for your kind understanding.

4. ZnO can improve the thermal oxidative aging resistance of asphalt. In addition to its alkalinity, it is also related to its microstructure. The effect of microscopic analysis of ZnO on performance of asphalt should be strengthened.

Answer: Thank you very much for the comment. In SEM observations, we visualize and measure the size and shape of the nanoparticles. We also evaluate the elemental chemical composition (lines 137 to 149). The above, to verify the purity of ZnO, size and morphology reported by other researchers. That is, the observations in the SEM helped support some conclusions reported in the reference literature, which are described in some sections of the manuscript (e.g., lines 239 to 244; 291 to 292; 335 to 338; 354 to 357).

5. The ordinate scale of Fig.7(b) should be adjusted.

Answer: You're right, the scale was wrong. Thank you very much for detecting the error. We made the respective change in Fig. 7b (line 301).

Author Response File: Author Response.docx

Reviewer 3 Report

This manuscript investigates the changes in the base properties of nano-ZnO modified asphalt after thermo-oxidative aging. And relevant tests were conducted on asphalt mixtures. The study is complete and informative, and the results show that nano-ZnO can promote the resistance of asphalt to thermo-oxidative aging. The whole manuscript is rich in content and fluent in language, but there are still some problems.

1. RTFOT and PAV are conventional aging tests, which do not need to be introduced at great length in the introduction.

2. The authors spend a large part of the abstract and introduction to introduce the effects of UV aging on this modified asphalt, UV aging tests can be considered as a control.

3. The tests used to evaluate the degree of aging of asphalt are too basic and not persuasive enough.

4. what is the rational for the aging index defined by the authors in the text?

5. The language of the text can be further condensed.

6. the conclusions are too concise and conclusions 1 and 4 could be combined.

7. after the appearance of abbreviations in the abstract, the full name does not need to be used in the text, and can be replaced by an abbreviation.

Minor editing of English language required

Author Response

18-april, 2024

Dear Editor and Reviewer,

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

Comments:

This manuscript investigates the changes in the base properties of nano-ZnO modified asphalt after thermo-oxidative aging. And relevant tests were conducted on asphalt mixtures. The study is complete and informative, and the results show that nano-ZnO can promote the resistance of asphalt to thermo-oxidative aging. The whole manuscript is rich in content and fluent in language, but there are still some problems.

1. RTFOT and PAV are conventional aging tests, which do not need to be introduced at great length in the introduction.

Answer: Thank you for your thoughtful and constructive feedback on our manuscript. You are right, both tests are widely known by the academic and scientific community. However, in other articles that we have published, some reviewers have asked us to describe these tests in the “Materials and Methods” section. We wanted to report them in the Introduction because we were describing the phenomenon of aging in pavements and this information would help us not to report it in the “Materials and Methods” section. We hope for your understanding in this regard.

2. The authors spend a large part of the abstract and introduction to introduce the effects of UV aging on this modified asphalt, UV aging tests can be considered as a control.

Answer: Thank you for your thoughtful and constructive feedback on our manuscript. The effect of UV aging is shown extensively in the Abstact and Introduction in order to support the novelty of the study. Due to its physical-chemical properties, ZnO has traditionally been studied as a modifier of asphalt binders to improve resistance to UV aging. However, to evaluate thermo-oxidative resistance, few studies have been carried out. For this reason, we make a detailed description so that the reader understands the novelty. We hope for your kind understanding in this regard.

3. The tests used to evaluate the degree of aging of asphalt are too basic and not persuasive enough.

Answer: Thank you very much for the comment. The aging indexes that we use are those traditionally used in multiple studies. Indeed, there are other newer indexes such as ΔTc, Glover-Rowe (G-R) parameter, R-value, cross-over modulus, Ideal-CT or I-Fit from SBC, etc., generally obtained at low and intermediate temperatures of service. In this first phase of the study, we evaluate the traditional indexes. However, we consider that other indexes should be measured and evaluated in future works. It is very important for us to take this observation into account in future phases of the research project. For this reason and taking into account your comment, a recommendation in this regard was reported in lines 399 to 402.

4. what is the rational for the aging index defined by the authors in the text?

Answer: Thank you very much for the comment. We chose the aging indexes most used in the reference literature. The PDI, SPI and VAI are the most used to evaluate the aging resistance of asphalt binders. On the other hand, the LTOA/STOA ratio of the resilient modulus (stiffness under cyclic loading) is perhaps the most used index in the case of HMAs. The LTOA/STOA ratio of the S/F parameter of the Marshall test was chosen because S/F is an indirect indicator of stiffness under monotonic loading. The LTOA/STOA ratio of the ITS test parameters (ITSD and ITSC) was also used because these undergo a direct correlation with stiffness.

5. The language of the text can be further condensed.

Answer: Thank you very much for the comment. We don't know how to condense the language in the manuscript. We tried to shorten the writing as much as we could. We hope for your kind understanding in this regard.

6. the conclusions are too concise and conclusions 1 and 4 could be combined.

Answer: Thank you very much for the comment. We combine conclusions 1 and 4 (see lines 371 to 375).

7. after the appearance of abbreviations in the abstract, the full name does not need to be used in the text, and can be replaced by an abbreviation.

Answer: Thank you very much for the comment. For us, according to APA, as a general rule, all non-standard acronyms/abbreviations in research papers should be written out in full on first use (in both the abstract and the paper itself), followed by the abbreviated form in parentheses. We will ask about it in the Journal and we will make the respective change. Thank you very much again for your comment.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

There are some improvements in Results and Discussion section according to my comments, and the answers of the authors are exhaustive. The manuscript has been revised carefully. The organization and logic have also been strengthened. I don’t have any other questions and suggest having this manuscript to be published after further proofreading.

Author Response

Thank you for your thoughtful and constructive feedback on our manuscript. We reviewed the entire manuscript again and made some minor changes based on comments from the Editor to reduce the duplicate rate.