The Mechanical Behavior of High-Strength Concrete-Filled Steel Tubes: A Review
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe texts and markers in some figures are not clear, e.g., Figs. 1-6.
The mechanical behavior of HSC filled steel tubes are well discussed and reviewed in this paper, with many contributions. It is suggested to be accepted by addressing the following comments.
For Figs. 7-9, it is not necessary to keep the large size. They can be reduced to be identical to Fig. 10's size in the paper.
As a review paper on this commonly studied topic, the 32 references are quite insufficient. The authors need to discuss more and extend the review in depth. For example, when studying CFT columns, loading conditions are sensitive, i.e., the loading is applied on concrete core only or the whole sections. It can influence the loading conditions of the outer steel tubes. Please refer to "Confinement effectiveness of circular concrete-filled steel tubular columns under axial compression".
Another effect, that is suggested to be involved, is the material mix. Effect of aggregate size on stress-strain behavior of concrete confined by FRP or steel; as well as the problem of FRP confinement for coarse aggregate-free concretes (Some HSC is coarse aggregate-free) are suggested to be added.
Author Response
Comment 1: The texts and markers in some figures are not clear, e.g., Figs. 1-6.
Answer 1: The figure will be improved later. The given 10 days for the review did not permit to improve.
Comment 2: For Figs. 7-9, it is not necessary to keep the large size. They can be reduced to be identical to Fig. 10's size in the paper.
Answer 2: The size of figures were adjusted.
Comment 3: As a review paper on this commonly studied topic, the 32 references are quite insufficient. The authors need to discuss more and extend the review in depth. For example, when studying CFT columns, loading conditions are sensitive, i.e., the loading is applied on concrete core only or the whole sections. It can influence the loading conditions of the outer steel tubes. Please refer to "Confinement effectiveness of circular concrete-filled steel tubular columns under axial compression".7
Answer 3: Were include more references, like the proposed by the review. Problem of loading conditions was analyzed, based on steel jacket column, with full scale models. The proposed crack progression model permit better understand how friction is mobilized, specially in the D-Zones.
Comment 4: Another effect, that is suggested to be involved, is the material mix. Effect of aggregate size on stress-strain behavior of concrete confined by FRP or steel; as well as the problem of FRP confinement for coarse aggregate-free concretes (Some HSC is coarse aggregate-free) are suggested to be added.
Answer 4: Paper give some indications for this problem, because it can be only considered the aggregate presence of not, but the relationship of aggregate strength and the matrix. The general approach is that the higher or lower progression of cracks determines the effect of the tubes on the concrete and of concrete on the tube, the failure mode and failure loading. One of the references in the paper was based on aggregate-free concrete. However it was not clear the effect of the aggregate existence, suggesting further research based on the proposed approach.
Author Response File: 
Author Response.docx
Reviewer 2 Report
Comments and Suggestions for AuthorsThe authors researched concrete-filled steel tubes (CFST) for their structural benefits. They analyzed how steel tubes confine concrete to limit cracks and alter failure modes in high-strength materials.
The manuscript's scope is suitable for the journal; however, significant revisions are necessary in its current form. The comments are as follows:
1. The abstract requires enhancement as it is currently challenging to follow. It should include background information on the topic, objectives, methodology, key findings, and the significance of the study.
2. The introduction section lacks clarity and detail. It should provide more comprehensive coverage of CFST columns, design considerations, the significance of CFST, its applications, and should also incorporate a more extensive review of relevant literature. References may include: Structures, Elsevier, 31, pp. 428-461, DOI: 10.1016/j.istruc.2021.01.102 ;;; Journal of Constructional Steel Research, 211, DOI: 10.1016/j.jcsr.2023.108127.
(3) Novelty of the work must be clearly highlighted in the introduction section.
(4) Discuss the critical factors influencing the mechanical behavior of concrete-filled steel tubes (CFSTs), emphasizing the role of microcracks and micro-pillars in high-strength confined concrete. How do these factors compare with those affecting unconfined concrete?
(5) Analyze the transition in failure modes observed in high-strength concrete (HSC) when confined within steel tubes of CFSTs. Compare and contrast the formation of inclined planes with the typical failure patterns seen in other materials. What implications does this transition have for structural applications?
(6) Based on the experimental results, discuss the reasons behind the observed distortion in the strength index values of concrete-filled steel tubes (CFST) with varying compressive strengths. How does this distortion impact the understanding of CFST mechanical behavior and its implications for structural applications?
(7) Explain the relationship between confinement factor (λ) and the mechanical behavior of high-strength concrete (HSC) within CFST elements. Discuss how the confinement effect influences crack progression and failure modes, particularly the transition from longitudinal cracks to inclined planes under high confinement stresses.
Author Response
Comments 1: The abstract requires enhancement as it is currently challenging to follow. It should include background information on the topic, objectives, methodology, key findings, and the significance of the study.
Response 1: Abstract was changed focusing the approach presented in the paper to better understanding CFST behavior.
Comments 2: The introduction section lacks clarity and detail. It should provide more comprehensive coverage of CFST columns, design considerations, the significance of CFST, its applications, and should also incorporate a more extensive review of relevant literature. References may include: Structures, Elsevier, 31, pp. 428-461, DOI: 10.1016/j.istruc.2021.01.102 ;;; Journal of Constructional Steel Research, 211, DOI: 10.1016/j.jcsr.2023.108127.
Response 2: Were included more references, as suggested, an analysis of codes and identification of references in the introduction was improved.
Comments 3: Novelty of the work must be clearly highlighted in the introduction section.
Response 3: As a review paper, was reinforced the approach proposed to clarify some observed inconsistencies.
Comment 4: Discuss the critical factors influencing the mechanical behavior of concrete-filled steel tubes (CFSTs), emphasizing the role of microcracks and micro-pillars in high-strength confined concrete. How do these factors compare with those affecting unconfined concrete?
Response 4: The approach of paper means to understand behavior of concrete confined and unconfined in the same point of view, as a structural system after cracking. What differs in confined and unconfined condition is the crack pattern, crack size, distribution, etc…Effect of tube is to change this crack properties, from unconfined to confined condition.
Comment 5: Analyze the transition in failure modes observed in high-strength concrete (HSC) when confined within steel tubes of CFSTs. Compare and contrast the formation of inclined planes with the typical failure patterns seen in other materials. What implications does this transition have for structural applications?
Response 5: Transition in failure results from bracing effect of the tube that prevents failure of micro-pilars system. Failure of brittle materials is dependent from loading conditions, size, and other factors. Global understanding of this bracing effect and the failure mode is the contribution of the paper, not exactly focused in the structural applications. However, the failure mode transition which permits a less brittle failure is an advantage for mor ductile structures.
Comment 6: Based on the experimental results, discuss the reasons behind the observed distortion in the strength index values of concrete-filled steel tubes (CFST) with varying compressive strengths. How does this distortion impact the understanding of CFST mechanical behavior and its implications for structural applications?
Response 6: The reasons are discussed globally in the paper, corresponding to the change in failure mode from unconfined to confined concrete, by the tube effect. It seems that failure by inclined plans are unfavorable, concerning loading capacity, in high-strength concrete. More than structural applications, the understanding contribution is important for simulation models as FEM, which is a path for CFST development.
(7) Explain the relationship between confinement factor (λ) and the mechanical behavior of high-strength concrete (HSC) within CFST elements. Discuss how the confinement effect influences crack progression and failure modes, particularly the transition from longitudinal cracks to inclined planes under high confinement stresses.
Response 6: The confinement factor has influence in the progression of cracks. With higher confinement, crack progression is limited. Thus, higher confinement factor means more conditions for a population of small cracks, different from observations in unconfined conditions. Additionally, short cracks imply higher forces in the tube when instability occurs, as explained by a new added figure (Fig. 19). The influence of crack progression means a different structural system with different failure mode. The general approach of the paper and the analysis based on Rock Mechanics, permits clarify the described behavior.
Author Response File: Author Response.docx
Reviewer 3 Report
Comments and Suggestions for AuthorsDear Authors, the submission titled Review about mechanical behavior of high-strength concrete filled steel tubes, Manuscript ID: civileng-3049033, has some weaknesses that must be significantly improved before any further actions in the processing.
Please refer to the comments below:
1. The Abstract section must start with an introduction to the field of study. Further, the meaning of the review presented must be emphasized along the area of study.
2. The Introduction section is small and does not review the current state of knowledge in an area of study performed. I suggest adding some historical results and advertising them against current methods, like:
(1) https://doi.org/10.1016/j.jestch.2019.01.003
(2) https://doi.org/10.1016/j.engfailanal.2023.107541
(3) https://doi.org/10.1016/j.conbuildmat.2011.12.083
The 32 literature sources are not convincing.
3. The motivation of the work is hidden from a good understanding of the reader, especially from the Introduction section. Usually, the motivation is highlighted at the end of this initial section.
4. The values from formulas (1) and (2) were received from previous studies, as concluded by the Authors but, respectively, what about the equations? It looks like they are not newly proposed so they should be referenced to the primary sources.
5. What is the main conclusion from Figures 1 and 2? How does it influence the final conclusions of the submission?
6. What are the normal condition, mentioned in line 248?
7. The critical discussion in subsections 3.1, 3.2 and 3.3 does not exist or is negligible. From that issue, any limitations of the review presented are unknown.
8. The Conclusions are long but not properly presented. Firstly, it should be divided into separated but numbered gaps.
9. Still with the last section, the main advantage of the review included must be emphasized. Further, any limitations should be addressed, if exist from the review presented.
10. An additional Nomenclature section is required.
Generally, the proposed manuscript is interesting but, respectively, must be improved significantly.
Author Response
Comment 1: The Abstract section must start with an introduction to the field of study. Further, the meaning of the review presented must be emphasized along the area of study.7
Answer 1: Abstract was changed focusing more the proposed approach of the paper for the CFST problems.
Comment 2: The Introduction section is small and does not review the current state of knowledge in an area of study performed. I suggest adding some historical results and advertising them against current methods, like:
(1) https://doi.org/10.1016/j.jestch.2019.01.003
(2) https://doi.org/10.1016/j.engfailanal.2023.107541
(3) https://doi.org/10.1016/j.conbuildmat.2011.12.083
The 32 literature sources are not convincing.
Answer 2: Introduction was improved focusing some limitations of current knowledge, making an analysis of design codes and questions arising from that. Were included more references.
Comment 3. The motivation of the work is hidden from a good understanding of the reader, especially from the Introduction section. Usually, the motivation is highlighted at the end of this initial section.
Answer 2: Motivation of the work was reinforced, for example, from some limitations FEM models, as wall non-coherency of experimental results with the common idea of confinement.
Comment 4. The values from formulas (1) and (2) were received from previous studies, as concluded by the Authors but, respectively, what about the equations? It looks like they are not newly proposed so they should be referenced to the primary sources.
Answer 4: Equations seem to be presented in the paper.
Comment 5. What is the main conclusion from Figures 1 and 2? How does it influence the final conclusions of the submission?
Answer 5: Figures 1 and 2, show that is difficult to find a trend for the relationship between the considered parameters. It means that is not possible do relate the confinment factor with the strength index, or the strength index with the compressive strength, without taking into consideration other aspects. The proposed approach of a mechanistic understanding of the internal destruction process of concrete and the interaction between concrete and tube aims to create a perspective to clarify the dispersion of values and to have stronger relation between the different parameters. Inclusion in the analysis the internal destruction processes and the affecting factor seems to give a path for next devellopments
Comment 6. What are the normal condition, mentioned in line 248?
Answer 6: New text:
The effect of steel tube in the filling concrete does not correspond to active confinement, unless some external transversal prestress is used or expansive concrete. The steel tube has a passive effect, which is generated by the internal destruction processes of concrete corresponding to crack initiation or/and progression.
Comment 7. The critical discussion in subsections 3.1, 3.2 and 3.3 does not exist or is negligible. From that issue, any limitations of the review presented are unknown.
Answer 7: It was included a critical discussion of chapter 3.
Comment 8. The Conclusions are long but not properly presented. Firstly, it should be divided into separated but numbered gaps.
Answer 8: Was changed for a new structure.
- Still with the last section, the main advantage of the review included must be emphasized. Further, any limitations should be addressed, if exist from the review presented.
Some limitations were included, as they exist, but with a connection of the proposed approach for the paper.
Comment 10. An additional Nomenclature section is required.
Was included a nomenclature section
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsThis reviewer is satisfied with the authors responses, and therefore recommend the revised version of the manuscript for publicaiton.
Reviewer 3 Report
Comments and Suggestions for AuthorsThe submission can be recommended for publication in its current, revised form.
