Experimental Study on Bond Fatigue Between Carbon Fiber-Reinforced Polymer Bars and Seawater–Sea Sand Concrete Under Seawater Immersion and Dry–Wet Cycle Conditions

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1. In the section of the abstract, please provide some quantitative results about interface bonding performance under two different environments. In addition, the influence mechanism of the two environments on the bonding performances should be exposed. Please explain why higher stress levels enhance bond stiffness?

2. The introduction part needs to be further improved from the following aspects. 1) When considering the cost, gfrp and bfrp are generally used in concrete structures as reinforcing materials to replace steel bars. Please explain why this article select CFRP bars. 2) Some properties and advantages of different FRP materials should be compared and analyzed in terms of performance, price and potential application scenarios. 3) The durability performance of the FRP itself exposed to different service environments, is crucial for evaluating the bonding of the FRP-concrete interface. However, the information is missing in the present summary. 4) What is the application scenario for the fatigue behavior of FRP-concrete interface bonding? It is recommended to give a clear summary. Please consider the above comments and make necessary supplements by reviewing the relevant latest research work: Construction and Building Materials, 2024, 440:137470. https://doi.org/10.54113/j.sust.2024.000038. Composite Structures, 2022, 293, 115719. 

3. Please add the standard deviation of relevant data in Table 3. Please check other data and provide further relevant information.

4. It can be found that after 120 days of corrosion, the compressive strength of concrete has increased. Please provide some explanations for this abnormal phenomenon.

5. The table 4 gives some properties of the CFRP bars. It can be found that the tensile properties are relatively low. The tensile strength of CFRP for general engineering application should generally be above 2100 MPa. Please provide some relevant explanations, and imply information on the types of fibers in CFRP composites and the related fiber volume fractions.

6. For the fatigue bonding test of the CFRP-concrete interface, please imply the selection of anchorage system and anchoring process. The anchorage of CFRP is crucial and determines the success of the experiment. 

7. It can be found in Table 6 that the main failure modes are due to interface bonding failure and concrete splitting failure. However, the ultimate failure load is far lower than the ultimate tensile strength of CFRP itself. Does this mean that the CFRP performance can’t be fully exerted in concrete structures, resulting in the increase of materials and costs?

8. In Figure 7, it can be found that the three stress-slip curves show different trends. Please further analyze which parameters have a significant impact on the curve changes?

9. The influence mechanism of service environment, stress levels on fatigue life, bonding stress slip curve and fatigue stiffness of several samples should be analyzed and summarized. Furthermore, the relationship between fatigue life and stress level can be further drawn.

10. In figure 10, some data points are difficult to see clearly, so it is recommended to further expand the abscissa to display all data points.

11. The conclusion part can be condensed to 3 ~ 4 key points. In addition, it is suggested that some main research work of this paper should be briefly summarized at the beginning of the conclusion.

 

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

General Feedback

The manuscript presents a thorough experimental investigation into the fatigue bond behavior between CFRP bars and seawater sea sand concrete (SWSSC) under varying environmental conditions. The study is valuable for understanding marine infrastructure durability, particularly under fatigue loading. However, several areas require attention to ensure clarity, academic rigor, and broader applicability.

Section-by-Section Review

Abstract

• Strengths: The abstract clearly outlines the study's objectives, methodology, and key findings. The importance of the proposed constitutive model is emphasized.
• Weaknesses: The language in the abstract is dense and may be difficult for a general audience to understand. Simplifying complex terms or providing brief definitions could enhance readability.
• Recommendations: Revise sentences to be more concise and consider reorganizing to emphasize results and practical implications upfront.

Introduction

• Strengths: Provides a solid background on the issues of steel reinforcement in marine environments and the advantages of CFRP and SWSSC combinations. Relevant literature is well-cited.
• Weaknesses: The novelty of this study compared to cited works is not explicitly highlighted.
• Recommendations: Clearly articulate how this work differs from prior studies. For example, emphasize unique aspects of the experimental design or new insights provided by the fatigue bond-slip model.

Materials and Methods

• Strengths: Detailed descriptions of materials, specimen preparation, and testing protocols enhance reproducibility.
• Weaknesses: The methodology section is overly detailed, making it challenging to focus on key experimental innovations.
• Recommendations: Summarize routine steps and focus on novel aspects of the methodology. For instance, describe why specific stress levels were chosen or how the bond-slip model is an improvement over existing models.

Results

• Strengths: Comprehensive data presentation with figures and tables that effectively illustrate trends and key findings.
• Weaknesses: Some figures (e.g., fatigue bond-slip curves) are not adequately explained in the text, leaving the reader to interpret their significance independently.
• Recommendations: Provide more discussion on why certain trends (e.g., degradation patterns under dry-wet cycling) occur. Relate findings back to practical applications, such as marine structure design.

Discussion

• Strengths: The discussion includes an interpretation of experimental results and comparisons with existing literature.
• Weaknesses: The manuscript lacks critical analysis of potential limitations in the experimental setup and assumptions made in the constitutive model.
• Recommendations: Add a subsection discussing limitations, such as the scalability of laboratory results to real-world conditions. Include future directions for research.

Conclusion

• Strengths: Summarizes the key findings and emphasizes the practical relevance of the proposed model.
• Weaknesses: Conclusions are descriptive rather than evaluative and do not fully address the broader implications for marine engineering.
• Recommendations: Highlight specific recommendations for engineers and researchers based on your findings. For example, propose design modifications or testing standards for CFRP-SWSSC systems.

References

• Strengths: Comprehensive and up-to-date.
• Weaknesses: There is an over-reliance on certain sources, and not all key references in the field are included.
• Recommendations: Integrate more diverse and recent studies to strengthen the manuscript's credibility.

English Language and Style

• Strengths: Technical terms are accurately used, and the manuscript demonstrates familiarity with the field's conventions.
• Weaknesses: There are grammatical errors, overly long sentences, and inconsistent verb tenses.
• Recommendations: Seek professional editing to improve grammar, sentence structure, and flow. Ensure consistent use of technical terminology.

Summary of Recommendations

1. Add a dedicated "Discussion" section to compare findings with the literature and discuss limitations.
2. Improve English language quality for clarity and precision.
3. Simplify technical sections to make the manuscript more accessible to a broader audience.
4. Highlight the study's novelty and contributions in both the introduction and conclusion.
5. Update references to include recent studies and address gaps in cited literature.

Overall Assessment

• Decision: Minor Revision
• Rationale: The study is well-designed and significant in scope but requires improvements in clarity, organization, and critical discussion. The requested changes are not fundamental but will significantly enhance the manuscript's quality and impact.

Comments on the Quality of English Language

The English could be improved to more clearly express the research.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript is well written, and is recommended for publication with minimum variation and addressing of the following questions:

 

Reviewer Comments:

This study delves into the durability of CFRP bars in marine environments, particularly under the challenging conditions of seawater immersion and dry-wet cycles. By examining the bond performance through static and fatigue pull-out tests, we aim to enhance the understanding of how these materials behave in real-world applications.

Performance and Bond Strength

1. Despite the challenges posed by seawater immersion and dry-wet cycling, the bond strength variation between CFRP bars and seawater sea sand concrete remains below 5%. What could be the reasons for this minimal variation in bond strength even when the materials themselves degrade over time?
2. It appears that fatigue loading has a more significant impact on bond strength than exposure to seawater. Why is this the case? What specific aspects of fatigue loading make it the dominant factor affecting bond performance?

Fatigue Loading and Bond-Slip Behavior

3. Under seawater immersion, the bond-slip curve exhibits a four-stage behavior. What factors contribute to this specific trend, and how does this compare with standard environments or dry-wet cycling conditions?
4. In standard environments, micro-slip stiffness decreases by 12.31%. How does this stiffness degradation compare to the rates observed under seawater immersion and dry-wet cycling, and what factors might explain these differences?
5. After 2 million fatigue cycles, bond strength reduces due to surface abrasion of CFRP bars and the development of microcracks in the concrete. Can you elaborate on the role these two phenomena play in weakening the bond?
6. Stress concentration at crack tips appears to accelerate crack growth during fatigue loading. How does this mechanism work, and how does it contribute to the overall reduction in bond performance?

Stress Levels and Bond Stiffness

7. Higher stress levels seem to weaken bond stiffness but also increase slip and hysteresis loop areas. What might explain this dual effect of stress levels on the bond performance?
8. The study reports specific rates of stiffness degradation (e.g., 11.55%, 10.53%, and 12.22%) at 50%, 60%, and 70% stress levels in standard environments. What mechanisms could explain these rates, and why are they different for each stress level?
9. Prolonged seawater immersion results in a higher stiffness degradation rate (14.81%) compared to standard conditions. What could be the reasons for this intensified effect under seawater exposure?
10. Dry-wet cycling leads to the highest stiffness degradation (16.04%). Why does this process have a more severe impact on stiffness compared to prolonged seawater immersion?

Slip and Residual Slip

11. High-stress fatigue loading in seawater immersion and dry-wet cycling significantly increases both slip and residual slip. What are the underlying causes of this phenomenon?
12. Slip increments at different stress levels (50%, 60%, and 70%) are not proportional to the increase in stress. Why is this the case? Are there non-linear behaviors or threshold effects at play?
13. Dry-wet cycling has a greater impact on slip and residual slip than seawater immersion alone. What might explain this difference? Does the alternating exposure to moisture and air exacerbate damage?

Corrosion and Bond Stiffness

14. Corrosion caused by seawater immersion impairs bond strength and increases initial slip displacement. How does the corrosion process affect the bond at a microstructural level?
15. Interestingly, higher stress levels during prolonged immersion and dry-wet cycling improve bond stiffness in some cases. Why does this happen, and how does it align with the general expectation of stiffness degradation under stress?
16. Specimens subjected to 120 days of immersion and dry-wet cycling show more pronounced bond stiffness increases at high-stress levels. What factors could explain this surprising behavior, and are there long-term implications?

Constitutive Modeling

17. The study developed a fatigue bond-slip constitutive model for CFRP bars and seawater sea sand concrete. What experimental data and parameters were used to construct this model?
18. How does this new model differ from existing models designed for steel bars and conventional concrete? Are there unique features or adaptations specific to CFRP and seawater sea sand concrete?
19. Can the proposed constitutive model predict failure mechanisms at excessive stress levels? If so, how are these mechanisms incorporated into the model’s framework?

 

General questions:

1. Are the research objectives clearly defined, and do they align with the study's findings?
2. Is the literature review comprehensive, and does it adequately support the need for this study?
3. Are there any sections that repeat information unnecessarily? If so, which ones?
4. Is the methodology section detailed enough to allow for replication of the study?
5. Are the experimental design and setup adequately described, including the rationale for chosen methods?
6. Is the sample size justified, and does it meet the standards for statistical significance?
7. Were appropriate control conditions established in the experiments? How were they implemented?
8. Are the data collection methods clearly described? Are there any potential biases in data collection?
9. Is the statistical analysis appropriate for the data type? Are the methods clearly explained?
10. Are the results presented in a clear and logical manner? Are tables and figures effectively used?
11. Does the discussion adequately interpret the results, and does it connect back to the research questions?
12. Are the limitations of the study clearly stated? How might they impact the findings?
13. Do the conclusions drawn in the paper logically follow from the results presented?
14. Are the practical implications of the findings discussed? How can they be applied in practice?
15. Is the terminology used throughout the paper consistent and appropriate for the audience?
16. Are all figures and tables of high quality, clearly labeled, and referenced in the text?
17. Are the references current and relevant to the study? Are there any key studies missing?
18. Are there any noticeable technical errors or inconsistencies in the data or analysis?
19. What is the overall contribution of this paper to the field? Does it provide new insights or advancements?

 

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Accept in present form

Author Response

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Author Response File: Author Response.docx