Experimental Investigation into the Structural and Functional Performance of Graphene Nano-Platelet (GNP)-Doped Asphalt

Round 1

Reviewer 1 Report

The article concerns a very important topic which is experimental investigation of Graphene Nano Platelets Doped Asphalt. The title and abstract are appropriate; the subject fits with the journal’s aims and scope, but the review of literature is not adequate.

The authors should make major corrections and additions before publication. Please find below my detailed comments and recommendations for corrections.

Ø  The introduction is too short

Ø  Please describe better the standard used. This is not a standard test in any country. (e.g. General Specifications of National Highway Authority of Pakistan)

Ø  Figure 5 in not necessary

Ø  Conclusions section need to be revised to capture salient points and to emphasize the results obtained

Author Response

The observations of the reviewer have been addressed thoroughly in the attached “Response to Reviewer” document. The value additions from the study under review have been highlighted and explained point by point in the response. The motivation for the study has been emphasized addressing the comments of the reviewer. These key points have also been incorporated in the main manuscript for ready reference and perusal.

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper can be accepted for publication after major changes. Article on an exciting and recent research area: the use of nanoparticles to improve asphalt binder and mixture performance. The paper relates well to the topics of this Journal.

I didn’t find plagiarism or republication problems. Nevertheless, authors should evaluate in detail what is distinctly new in this submission, avoiding superposition with the previous study “ul Haq, M.F., Ahmad, N., Nasir, M.A., Jamal, Hafeez, M., Rafi, J., Zaidi, S.B.A., Haroon, W. Carbon nanotubes (CNTs) in asphalt binder: Homogeneous dispersion and performance enhancement (2018). Applied Sciences (Switzerland), 8 (12), art. no. 2651”. Both papers have a very similar approach and deal with very similar materials (only changing CNTs to GNPs). Several conclusions are the same, and this repetition of procedures reduces the innovative nature of this new submission (as it looks like the only development was changing CNTs to GNPs without any logical explanation for this change, particularly when CNTs have already presented excellent characteristics). This possible repetition is the central issue of this paper, and I encourage authors to cite the previous article and make some performance comparison to show the innovative nature of this new submission clearly.

Taking into account my previous comment, the approach presented in this paper does not seem to be sufficiently innovative. However, if authors can make the comparison with their last work and clearly state the strengths of this new paper, they may be able to catch the attention of future readers.

The organization of the article is right but can be improved. The description of some materials and methods is minimal, mainly because the methodology is very extensive and it is difficult to give more details on each test. Maybe some parts can be erased, and then it would be possible to provide more information in the other tests. Some experiments are evaluating the same property and can be deleted to simplify and detail the paper. E.g., storage stability can be assessed only with R&B, and PG results are not bringing additional information; rolling bottle test and bitumen-aggregate adhesion analysis are evaluating a similar property, and only one of them is necessary; the study of the polishing effect should be erased because it is not following a standard and mainly because WTT is not polishing the mixture.

The discussion of results must be improved, and further scientific additions to some results are welcome. Several conclusions can be justified on a simple basis taking into account that these materials are fillers that are stiffening the mixtures. Thus, authors should explain in detail the reason why these materials are different from fillers already used in all asphalt mixtures, with scientific hypothesis.

Authors should also explain if these solutions were easy to implement at industrial scale. Most asphalt plants are not equipped to carry out asphalt modification, especially if high shear mixing is required. Thus, explain your point of view about alternatives for the implementation of these type of solutions. Authors should also give much more attention to the different optimum asphalt binder contents obtained after adding GNPs. By increasing the binder amount the mixture is more expensive, and the durability may be more influenced by the increase of binder than by the use of GNPs. Authors should explain their point of view on this question.

Written English quality is good, but it can be improved (i.e., revision of a native speaker) to make the readability of the paper easier.

The objectives are clear, and the paper globally answers those objectives.

As previously mentioned, the methodology is extensive and is not explained in detail. The description of materials and methods can be improved (several examples will be presented later). Some results are significant and lead to interesting conclusions. Nevertheless, results must be analyzed in more detail, to give a better explanation and to demonstrate new findings of this paper.

Some specific problems or questions to be answered are related to:

- More details and discussion on materials properties in Section 2.1. Explain the consequences of using aggregates not meeting LA abrasion limit. Give more details in this part about the use of more than one fraction or dimension to produce the mixtures. Relate the properties of the GNPs with the finding of the paper presented later. Correct Table 1 (the first line in bold is not the heading of each column).

- PG is not obtained at 10 Hz (line 109). It is measured at 10 rad/s.

- Sometimes authors use Imperial units in this paper. It is better to use always SI units, as recommended in several papers, for a wider understanding of the readers.

- It is difficult to see the plate-like nature of GNPs in Figure 2, as well as the diameter and thickness identified in Table 1. Please give these details.

-  Something is wrong in Figure 3 (scale is 5 or 500 micron?). Moreover, the dispersion is difficult to see because this is a surface image, and it becomes difficult to believe that only those single spots can make such a difference in the performance of the binder. Thus, the analysis of this Figure should be presented differently.

- Explain the influence of Table 3 results in the cost of the mixtures, and also on the properties evaluated in the other parts of this paper. Different binder contents can make the evaluation of GNPS influence more difficult because some properties can be more influenced by the increase of binder.

- I don’t see the difference of 2.2 ºC mentioned in the text when I look at Table 4. Please correct or explain. The results of Table 5 are not necessary (the paper is too long).

- The study of rheology should present more details. Nothing is mentioned about the reference temperature (50 ºC) and the master curve construction process (later presented in the results). Also, different strain levels were used for bitumen and GNP binders, probably because the stress level was not adjusted. As GNP binders are stiffer than bitumen, they are being subjected to a lower strain level. However, nothing was said about the procedure to evaluate if these strain levels are in the linear region of both binder performance.

- Figure 4 was also used for Figure 2 and 3 (but it is was only presented later). Please reorganize the paper accordingly. Figure 5 is already known by all readers and can be erased.

- The reduction of ductility presented in Table 6 is huge, with only a slight increase of R&B for 2% GNP. This result is typical of a filler that stiffens the binder, and not of a modifier or doper. This solution can perform not very well at low temperatures, resulting in fatigue cracking. This binder may also have workability problems, due to the reduction of viscosity (with problems to pump the binder in the asphalt plant). Please address these questions.

- Figures 6, 7 and 8 should use the same colors and symbols (to make the paper easier to understand). This logic is also valid for other Figures in this paper.

- Figure 7 have values that are difficult to distinguish. Change the scale. Also, it looks like base binder has phase angle values near to 100º, which is impossible. Please check this question. Figure 8 is very similar to Figure 6 and can be erased. The results of the Superpave rutting factor can be explained only in the text.

- Concerning rutting resistance and stiffness modulus, it is difficult to understand the reason to present result at two temperatures. Taking into account the case of Pakistan, rutting only needs to be presented at 55 ºC, and probably this is also valid for stiffness (depending on the evaluation carried out by the authors). Also, authors should be more careful with these Figures, always checking the legends (see Figure 11), the marks in the axis for the units (see Figure 9 and 10) and the type of graphics used (there is no scale for frequency in Figures 11 and 12, neither linear or logarithmic). Correct all these issues. Also, the main WTT result (wheel tracking slope between 5000 and 10000 cycles is not presented and is not commented by authors (please improve this part of the paper).

- Connecting the results of Figures 13 and 14 is difficult. The amount of bitumen in Figure 14a does not seem to be as low as presented in Figure 13. I know this test is difficult to show in papers or reports, and thus I understand this issue. As the authors present an alternative way of evaluating binder-aggregate bond in section 3.6, I think it would be better to erase this part of the paper (since the conclusions are similar).

- To reduce the length of the paper, Figures 16 and 18 and Figures 17 and 19 can be joined together. Also, authors should explain the need for using SEM in this case to use such a low magnifying level (a simple microscope may have been enough). Please comment.

- The results of Table 8 after WTT should be erased. The use of WTT is not valid to simulate the polishing effect. Together with rutting, the binder exudation (movement to the surface) also occurs, and authors are evaluating something more related to rutting additional problems than with polishing. Also, authors should state that all other BPN values are only valid at a laboratory scale. Typically, BPN values are obtained in real roads only after some use of the road to study the mixture without the top layer of binder that is easily removed from the surface of the road after the first weeks of use. The BPN values presented in this paper are very dependent on that layer, and this should be stated.

- Reevaluate all conclusions of the paper after the revision carried out according to my previous comments. Then, evaluate if the conclusions answer the objectives of the paper and show innovative findings of this work.

Author Response

The observations of the reviewer have been addressed thoroughly in the attached “Response to Reviewer” document. The value additions from the study under review have been highlighted and explained point by point in the response. The motivation for the study has been emphasized addressing the comments of the reviewer. These key points have also been incorporated in the main manuscript for ready reference and perusal.

Author Response File: Author Response.pdf

Reviewer 3 Report

This manuscript presents a valuable study that investigates experimental research into the structural and functional performance of graphene nano platelets (GNPs) doped asphalt. This article is very interesting and has various test results. But there are some observations to make the article more complete.

Line 86: Authors explained various benefits on GNPs as additives in asphalt. In the industry, economic view is also major issue. I recommend that the unit price ($/gram) is added into the table. It should be helpful to use GNPs.

Line 106: In order to understand the volumetric characteristic, please add the volume % with weight % by binder.

Line 120: The viscosity during mixing of graphite or carbon based additives is different on additives amount and types. Is there any problem when mixing? Also, sinking phenomenon may be happen when mixing with GNPs at high temperature (over 150° C). That may make non-homogeneous that the bottom part has more GNPs. Please explain if the authors have any method to prevent this problem.

Line 282: The authors mentioned GNPs in asphalt contribute the rutting resistance. Based on the reference (Rew, Y., Baranikumar, A., Tamashausky, A. V., El-Tawil, S., & Park, P. (2017). Electrical and mechanical properties of asphaltic composites containing carbon based fillers. Construction and Building Materials, 135, 394-404.), this research also mentioned this point. Also, indirect tension test showed the optimum amount of carbon based fillers to have the improved mechanical strength. Is there any observation like this phenomenon?

Line 317: 4% GNPs should be added in the legend. Please add it.

Author Response

The observations of the reviewer have been addressed thoroughly in the attached “Response to Reviewer” document. The value additions from the study under review have been highlighted and explained point by point in the response. The motivation for the study has been emphasized addressing the comments of the reviewer. These key points have also been incorporated in the main manuscript for ready reference and perusal.

Author Response File: Author Response.pdf

Reviewer 4 Report

This is an interesting well organized laboratory research. But, in my opinion is rather long, as if two studies are described in one. Some contents need to be reduced or even be deleted.

The subject is covered by the journal’s scope. No plagiarism has been found.

The references are all new because the subject treated has been focused the last decade, but their number could be well greater, in order to help the readers.

In addition to laboratory tests, it is necessary to perform field experiments on the construction of GNP-reinforced asphalt pavements

Specification given in reference 9, is not known worldwide; some explanation is needed

The colors in figures are not consistent

In figure 3, line 129 the title give a scale of 5μm.

The axes in figures 11 and 12 are confusing (scale, values, tick position, labels)

In figure 11, the legend for the green data line is missing.

In line 436 (GNP) instead of ( GNP )

In line 439 Civil ad must be rewritten as Civil and

In line 440 after thesis only comma is needed

In line 441 Evolution: instead of Evolution :

Some minor corrections should include the uniformity of units and symbols in different parts of the paper. For instance SI units should be used everywhere, no spaces are required between the temperature and degrees Centigrade (150° C, 150 °C, 150°C). Table 1 has to appear in the same page and the first line must not be underlined. Instead of beginning temperature, initial temperature should be used (lines 109, 110). In line 56, 102, the brackets [2 - 3] [7- 8].

Comments for author File: Comments.pdf

Author Response

The observations of the reviewer have been addressed thoroughly in the attached “Response to Reviewer” document. The value additions from the study under review have been highlighted and explained point by point in the response. The motivation for the study has been emphasized addressing the comments of the reviewer. These key points have also been incorporated in the main manuscript for ready reference and perusal.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

In my opinion, this paper should be accepted for publication taking into account that:

-    A considerable effort has been made to improve the overall quality of the article considering the suggestions of the reviewers;

-    In my previous revision, I mentioned that this paper should be accepted if authors are willing to make the suggested changes to the manuscript, and they tried to carry out all those changes.

The authors presented a very comprehensive and effective answer to the questions raised, making changes or justifying their decisions whenever those changes are not feasible. Thus, by looking at the current version of the paper, I believe it could be accepted as it is.