The Prospect of Microwave Heating: Towards a Faster and Deeper Crack Healing in Asphalt Pavement
Round 1
Reviewer 1 Report
This paper analyses the healing performance by microwave method. It is well explained, in general, and properly designed, although in my humble opinion it does not provide novel information to the state of the art.
Some comments should be considered in order to improve the understanding and clarity of the manuscript.
- Materials are not well explained: What was the type of bitumen used? Was it a 50/70 penetration grade? 70/100? The flow of the binder could have some impact in the healing.
- Why were those percentages of bitumen selected? Why are they different for each PA mixture? I assume some test were performed in order to achieve a suitable design of PA mixtures. The quantity of binder has also an impact in the mixture healing.
- The structural properties are not presented, at least the percentage of voids of each mixture should be included in order to compare properly their behaviour. Do they have a similar percentage of voids? Did the different quantity of steel wood change this parameter? The percentage of voids influences the healing ratio but also the heat transmission.
- In relation to Figure 5.a I think there is a mistake with the key. I would try to make the figures bigger in order to see the axles well.
- The comparison of index healing using different methods (Figure 7) is an interesting result, but it is not well explained: was the same type of bitumen used? The particle size distribution was the same? I mean, a minimum data to compare the different methods in a suitable way is missing.
- I would not include references in the conclusion, I think the comment and the reference should be included in the discussion of results.
Author Response
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Author Response File: 
Author Response.pdf
Reviewer 2 Report
In this work, the authors studied the microwave healing effect on the porous asphalt concrete with different content of Steel wool fibers for improved microwave heating efficiency. The study showed this microwave healing system can achieve fast heating and effective crack healing in asphalt concrete but has negative effects such as uneven heating for low healing less efficient compared to induction healing system. The overall aspect of the paper is good; the paper is well-written. This paper can be accepted in its present form, See my comments below.
- Are there any external forces applied to the sample to maintain the surface contact during healing?
- For the practical point of view, any other sources can be considered to provide microwave heating besides microwave oven?
Author Response
General comment by authors to the editor and reviewers:
Thank you very much for your constructive comments on our manuscript. We have addressed all of concerns and we hope that the manuscript can now be accepted for publication. Summary of changes and our response to reviewers’ comments are given below.
In this work, the authors studied the microwave healing effect on the porous asphalt concrete with different content of Steel wool fibers for improved microwave heating efficiency. The study showed this microwave healing system can achieve fast heating and effective crack healing in asphalt concrete but has negative effects such as uneven heating for low healing less efficient compared to induction healing system. The overall aspect of the paper is good; the paper is well-written. This paper can be accepted in its present form, See my comments below.
Are there any external forces applied to the sample to maintain the surface contact during healing?
For the practical point of view, any other sources can be considered to provide microwave heating besides microwave oven?
In response to the reviewer’s first question, the fracture faces of the bent specimen were slightly closed before microwave heating and no external forces were applied during healing.
In response to the reviewer’s second question, in practice, since the microwave is dangerous to the nearby creatures and environment, the authors would recommend to carefully design and develop a special and safe microwave heating system to focus the heating only at the damage site.
Reviewer 3 Report
The authors present microwave heating as an efficient method to heal asphalt damage, as long as the asphalt composition includes steel wool fibres to increase its absorption capacity. While the authors repeat all throughout the manuscript that the microwave heating is an effective method to heal the asphalt, presented results show that healing performance is worse than other techniques. The election is then not justified, and the possibility to improve the results is only suggested, but without any evidence. Also, the novelty of the study is not clear, according to what it is presented by the authors in the introduction. Which would be then the main result of this paper? Maybe that steel fibers addition is verified as an effective means to improve the microwave absorption capacity of the material, which is, on the one hand, quite straightforward, and on the other hand, already assessed and quantified in previous studies mentioned by the authors.
Introduction. The authors present a literature survey where the possibility to perform asphalt healing with microwaves seems to be already verified as a possibility, including the use of steel fibers. Thus, the main novelty of this study would be the application to porous asphalt? It is not clear up to which extent this is a sufficient step ahead beyond the state of the art.
Line 76-77. The authors state that “Sun et al [23] compared the effect of steel fibres and steel slags and reported that the asphalt mixture with steel slag showed a better performance of healing”. Why the authors chose steel fibers then? Please justify. Which is the impact of using steel fibers instead of slag in the price of asphalt? Is it still affordable?
Table 1. Please explain the meaning of the first column and why it is in mm.
Figures. Please increase the font size of legends and axes-labels all throughout the paper.
From the thermal images (Figure 4) it seems that the material is very prone to thermal runaway in those zones showing much more absorption capacity than their surroundings. Which would be the effect of overheating in these zones? Would it be still acceptable for real application? Which is the temperature limit of this material?
From Figure 7, it is clear that microwave healing efficiency is lower than in the rest of techniques. However, the authors state that “it is believed that, if the microwave is guided to focus on the damaged site, the microwave healing system can be an effective supplement for induction healing to achieve a deeper damage repairing in asphalt.” The authors did not show any evidence of this. Many doubts arise at this point:
- Maybe there is a relation between the healing efficiency and the optimum temperature that has not been correctly set in this study. Which was the healing temperature with the other techniques?
- The microwave heating showed to be very uneven (thermal images). Maybe the fact that the authors set the optimum temperature as the maximum temperature achieved locally, it meant that the bulk of the sample did not reach enough temperature around the cracks.
- If the sample was placed inside a multimode microwave oven, I guess a turntable was employed for uniformity. Please clarify. If not, this would promote even more local runaway effect in some points of the sample.
Author Response
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Author Response File: Author Response.pdf
Reviewer 4 Report
Please refer to:
Page4, l42: The authors should show the information on the size of the microwave applicator and sample size. This information is required to evaluate the concentration of microwave electromagnetic field.
page4, l. 152: The bending test usually conducted using a bar-shaped specimen. Why is the SCB sample this shape?
Figure 1: The authors should show the sample size with a scale bar.
page6, 3.3: What is the distribution of elements in samples after microwave heating? Does the elemental distribution change by multiple microwave irradiation?
Figure 5 (a): What is the T1, T2, and B.
page 6, 235: The authors indicated that bitumen overflowed out of the surface might be due to heat concentration inside of the asphalt mixture.
1) The authors should show cross-sectional views by SEM or other means and discuss from the inside of the sample.
2) Can you think of any other cause, such as plasma was generated on the surface?
Figure 6 (a): The authors should show the sample size with a scale bar. In addition, cross-sectional view of the sample is required to discuss the mechanism of bitumen overflow.
Page 7, l.251: 1) Uniform heating could be achieved in induction heating? If so, the authors should show the data.
2) As you said, microwave heating resulted nonuniform heating, especially, inside of the sample became higher. If so, the authors should conduct induction heating with the estimated internal temperature. Why have you conducted induction heating at the same surface maximum temperature?
Figure 7: As you can see from Fig. 5., the maximum stress obtained from bending test in this study exhibited a very large uncertainty. The authors should show the bar graph and an uncertainty in induction heating at each SCB bending and healing cycles and discuss the difference between microwave heating and induction heating.
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
I have no further comments.
Thanks
Author Response
Thank you very much for your constructive comments on our manuscript.
Reviewer 3 Report
The authors answered and clarified some of the points, but not all (some of the questions made about the contents of lines 76-77 were not answered). After these clarifications, the assessment made after the first revision have not changed, because the novelty beyond the state of the art is still not enough, and the main conclusion extracted from the paper (combination of induction and microwave heating as the best solution for asphalt healing) is not supported by measurements. Other presented results, such as the improvement of asphalt absorption with the addition of more steel fibres , or higher penetration depth respect to induction heating respond to well-known characteristics of microwave heating, could be inferred from previous studies and do not justify enough novelty to be published.
Author Response
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Author Response File: Author Response.pdf
Reviewer 4 Report
It has been appropriately revised and is considered acceptable for publication.
Author Response
Thank you very much for your constructive comments on our manuscript.
