Nondestructive Detection of Gaps between Railway Track Slabs and Soil Foundation Using Leaked Air Waves

Round 1

Reviewer 1 Report

Dear Editor;

The topic of the paper is interesting and suits the Journal of Applied Science. However, minor revision is required before this manuscript is qualified to be published in this prestigious journal. The manuscript is needed to be revised grammatically. The authors are required to check the whole manuscript with a grammar specialist as it has several grammatical errors. Only after revising the manuscript based on the comments, the paper is suggested to be published in APPLSCI. Further information on various issues identified in the manuscript appears below:

The authors have done a good job in the literature review. Please add more literature with regards to the works that have been published in the Journal of Applied Science. More information on railway slabs can be found here:

Ramezanianpour, A. A., et al. "Laboratory study on the effect of polypropylene fiber on durability, and physical and mechanical characteristic of concrete for application in sleepers." Construction and Building Materials 44 (2013): 411-418.

Ramezanianpour, A. A., S. A. Ghahari, and A. Khazaei. "Feasibility study on production and sustainability of poly propylene fiber reinforced concrete ties based on a value engineering survey." In Proceedings of the 3rd International Conference on Sustainable Construction Materials and Technologies (SCMT3’13), pp. 1-8. 2013.

Esveld, Coenraad. "Recent developments in slab track." European railway review 9, no. 2 (2003): 81-85.

Please redraw Figure 3. It is a bit blurry. Please make it larger and add details. Please provide the exact specification of the concrete material and strengths. Please provide more detailed reasoning behind the structure behavior. The details should include the rigid numbers or percentages. Please indicate how many samples for each experiment have been used. Please revise the other experiments respectively. Please describe the process of each experiment. Also indicate the model of each tool that is used in the experiment. The conclusion needs to be more concise. Please use fewer sentences containing percentages and illustrate the main conclusions in the manuscript. Please paraphrase your results and discussions and use them in the conclusion part.

Author Response

Thank you for your kind consideration of the manuscript reviewing. The following is revised and updated based on your reviewing. Please check it. 

1) The authors added many related related literature including recently published papers and reports.

2)  Figure 3 is redrawn as a larger and clear image.

3) The exact specification and strength information of the concrete materials used for the slab is provided in the text newly. 

4) References are updated including recently published papers including related works in area of NDT and acoustic test method published in 2019 and 2020. However, the proposed modeling method for detecting of cavity with FEA in this study is the only one in the area of railway track foundation and concrete track slab. Two articles recommended by the reviewer to be listed as references in the text are not included intentionally since they are not related to directly to this study even though they are somehow related to railway works. However, Esveld’s book is listed up as a reference.

5) Other experiment results performed in the field site were added in the text. Some detail test procedures were summarized and provided in the text also.

6) The test devices used in this study are explained in detail, including model names.

7) The conclusion is summarized more concisely using short sentences. The important findings are paraphrased and included as the shorter paraphrases.

Reviewer 2 Report

Summary:

The paper deals with a non-destructive testing method which utilizes leaky Rayleigh waves, with a primary goal of detecting gaps between railway track slabs and the foundation soil. There is a combination of theoretical, simulation and experimental work, which is beneficial as it combines different aspects to the problem, and a link between theory and tool is established. The main aim of the paper is to investigate the applicability of the mentioned non-destructive testing method for detecting gaps beneath concrete slabs, and the main contributions are simulations and experimental results of such application, which are evaluated and discussed.

Broad comments:

The paper uses clear and concise scientific language in its approach to the subject matter, which is a clear strength, along with a logical structure and methodology; however, improvements are required in certain important aspects:

 

1. The abstract is clear and concise, with clear background reasoning and methodology description, but it should be shortened to adhere to the Applied Sciences limitation of 200 words (currently 216). Additionally, authors mention ground-penetrating radar (GPR) as a possibility to detect gaps in the abstract but do not explain this in the paper, i.e. there is no deeper analysis in the paper. This should be either deleted from the abstract, as there is no support in the actual paper, or this should be better explained as a background study in the introduction, which would support the topic of research. The latter is recommended.
2. The title is informative and relays the subject of the paper, but an emphasis could be given to non-destructive testing, e.g. “Non-destructive detection of gaps between railway track slabs and soil foundation using leaked airwaves”.
3. It is advisable to avoid keywords that are already included in the title, also acronyms should not be used as keywords – NDT.
4. References are not current, with only 2/17 being less than 10 years old, which questions the novelty of the background study, i.e. the justification of the research question in context what is already known about the topic.
5. The introduction segment of the paper requires editing. This is primarily in regards to that there needs to be more information about what is known about the research topic, which would justify current research need and explain what is being upgraded. Additionally, the novelty of the work should be clearer with an analysis of more recent work.   
6. In the introduction, the authors mention that various methods have been established, but no clear outline of such methods is given. This would be beneficial to readers to understand what would the proposed method change, in particular in comparison to GPR.
7. Section 2.1 could be more concise as it does not bring enough new and original information to justify its share of content in the paper. There is enough information on this subject in general field literature, even textbooks so that this section can be shortened. Indicative of this is the use of same references throughout the text of the section.
8. Section 3 presents the core of the paper in regards to proof of concept. These are simulated results, but there needs to be some form of clear verification and validation of the presented numerical model. If the field test presented in section 4 is the method of validation, there need to be clear parameters for the confirmation of successful validation for the model and its results to have strength as a proof-of-concept. Additionally, in regards to the numerical model, the following should be addressed:
   • Mesh size is one of the more influential parameters on the quality of numerical modelling results – efficiency, convergence, accuracy. Authors mention that they used a procedure based on [17], but no additional information is given. Please elaborate more on the selection of size according to the mentioned study, as this is an important parameter.
   • Authors should provide more details on how they defined contact interfaces air-concrete, concrete-soil, soil-gap.
   • The authors used a gap size of 1,0 m, which should be explained, i.e. why was this size chosen and would change in size influence findings and how. Could this be parametrized to determine the applicability of the method? Additionally, how would results be influenced by the varying shape of the gap? This should be discussed as the aim of the paper is stated: “to investigate the possibility, applicability, and effectiveness”.
9. Section 4 gives results from field testing and as such is an important part of the paper, especially regarding verification of the numerical model. The following should be addressed:
   • Variability in experimental results can be a serious issue, especially when trying to validate numerical models, which are ideal. In that regards, have the authors had several sampling runs from which presented results are selected as best of these runs?
   • There is a serious lack of information regarding the parameters of the field test, which should be corrected to ensure that others can replicate and build on published results. Firstly, the test setup in regards to mechanical properties and geometry of slab and soil should be described, with details regarding the gap (position and size). Instrumentation should be described in regards to the properties of used devices and positioning of these devices on the slab. Sampling frequency should be mentioned and elaborated, as well as filtering of signals.
10. Section 5. should be edited. Namely, the discussion is currently an abstract, which should be improved. The initial aim (and) hypothesis should be discussed and interpreted in conjunction with presented results and previous studies. Authors should mention the implications of this study and its limitations. Limitations could be a basis for future research.

Specific comments:

1. In Featured Application, page 1, line 9, an acronym NDT is used, which is not introduced earlier in the text and as such should be replaced by full words.
2. In the Abstract, page 1, acronyms GPR, FEA and DAQ are used, which are not introduced earlier in the text and as such should be replaced by full words.
3. In the Introduction, page 1, line 29, acronym PCC is used, which is not introduced earlier in the text and as such should be replaced by full words.
4. In the Introduction, page 2, 50-51, authors state that non-contact NDT methods can reduce the time for treating data collection and data processing. It is not clear how is this true, or at least in comparison to what other methods. Please elaborate.
5. In the Introduction, page 2, lines 52-54, authors state that it is not easy to display the location and size of defects or gaps in or underneath concrete track slabs, not to distinguish the slab from surrounding materials and soil. How does the current method alleviate these problems? Please discuss this.
6. In the Introduction, page 2, line 58, references are listed individually as [2], [3]. The syle could be improved to list groups of references, e.g. [2, 3] or [4-6]. If accepted, this change should be applied throughout the paper.
7. In the Introduction, page 2, line 62, authors state that the Ryleigh acoustic elastic waves method has been improved, and to strengthen the claim provide reference [8], which is from 1999. Before this sentence, authors state that this method is proposed and developed recently, which is strengthened by references from the years 2012 and 2019. This is not logical as it appears that this method was developed in 2012 and 2019 and improved in 1999. Please rephrase this.
8. In section 2.1, page 3, lines 104-113, authors copy-paste text from page 2-3, lines 86-95. This should be deleted.
9. Figure 2 is not optimally placed in the text concerning its reference – referenced on page 4, appears on page 6.
10. In section 3.1, page 4, lines 161-162, authors use units that are not SI. Please correct this in the rest of the paper also.
11. Table 1 is not referenced in the text of the paper. This should be corrected, along with used units (not SI).
12. In section 3.2.3, page 8, line 284, authors state that the assumed soil stiffness is 40 MPa, which is not in accordance with an earlier statement and table 1, where 80 MPa is mentioned.
13. Figure 11 contains acceleration contours on concrete slabs with and without a gap. The scale on these contours should be the same (e.g. 20 or 90) to appreciate differences in results. The results mention that much denser and integrated contours are visible with a gap, which could be connected to the utilization of different scales.
14. The Discussion section is enumerated as 4., but it should be 5.
15. On page 13, the authors should write “Author contributions” in line with statements provided by the Applied Sciences journal.
16. On page 13, the authors list the Acknowledgments section, but it is blank.   

Author Response

Please see the attachment

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Authors have sufficiently improved the paper to alleviate shortcomings and amplify its potential. Clear progress is made in terms of reproducibility and novelty description, which are important aspects of the publishing potential. Considering the original submission potential, the potential added by all of the revisions, and taking into account certain limitations, the paper is suitable to be published after taking into account two minor comments:

1. References need to be renumbered by order of appearance in the paper - currently, reference [21] is the first reference that appears in the text.
2. In their reply to comment 4. of specific comments authors state that "As described in line 45 -50, this is due to preparing required time for contact type sensing method. Non-contact NDT method using acoustic microphone does not need to prepare the concrete surface to be clean, planar, and smooth by grinding and polishing". This is not related to the question, i.e. the time for treating data collection and data procession is not influenced by preparation time. The authors probably mean that the time for testing is reduced, not the time for treating and processing data.