Compressive Behavior of Waste-Steel-Fiber-Reinforced Concrete-Filled Steel Tubes with External Steel Rib Rings

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors need to consider the following comments to improve the manuscript structure: 

• Abstract: Please explain the variation of results at the end of the abstract.
• The last paragraph of the introduction: still, the main motivation and research gaps of the present study are difficult to find out from the text. Please modify.
• Section 2.1.2: The process of waste fiber preparation should be followed by some previous studies. Please use references. Also, please explain that such recycled fibers can be found in which types of industries or factories? 
• Figure 2: Please use references for such simple curing based on the previous investigation of similar samples.
• Figure 3(b): The readers do not need to see the figure of “CML-1 data acquisition instrument”.
• General comment: Please do not use short paragraphs throughout the manuscript.
• Please merge all figures 4-7, so that it can be easier for readers to compare the failure modes.
• How many repetitions were considered for each test sample? Please use standard deviations in the results illustrations (Figs. 11, 12, 13, and 14).
• The authors should compare their results with some similar findings (for instance, for other types of concrete) at the end of the manuscript. 
• General comment: Please show the failure modes discussion after the main results curves.

Author Response

Thank you for your hard work in reviewing this manuscript, as well as for your valuable suggestions and opinions on this study. This will provide new ideas and content for the author's research in this area, greatly promoting the achievement of richer results in this study. The following is a response to your comments and suggestions, which have been revised in blue font in the paper. Thank you. The opinions and suggestions in this section are indicated in blue font for modifications in the text.

 

Comments 1: Abstract: Please explain the variation of results at the end of the abstract.

Response 1: Thank you for pointing this out. I agree with this comment.

Based on your suggestions and feedback, we have revised the abstract of the paper. The changes in the abstract are indicated in blue font.

 

Comments 2: The last paragraph of the introduction: still, the main motivation and research gaps of the present study are difficult to find out from the text. Please modify.

Response 2: Thank you for pointing this out. I agree with this comment.

Based on your feedback and suggestions, we have rewritten the last paragraph of the introduction in blue font.

 

Comments 3: Section 2.1.2: The process of waste fiber preparation should be followed by some previous studies. Please use references. Also, please explain that such recycled fibers can be found in which types of industries or factories? 

Response 3: Thank you for pointing this out. I agree with this comment.

Based on your suggestions and opinions, reference [51] has been added to section 2.1.2. Represented in blue font. These WSFs can be collected in machining plants with machine tools.

 

Comments 4: Figure 2: Please use references for such simple curing based on the previous investigation of similar samples.

Response 4: Thank you for pointing this out. I agree with this comment.

Based on your suggestions and opinions, reference [52] has been added to section 2.2. Represented in blue font.

 

Comments 5: Figure 3(b): The readers do not need to see the figure of “CML-1 data acquisition instrument”. General comment: Please do not use short paragraphs throughout the manuscript.

Response 5: Thank you for pointing this out. I agree with this comment.

According to your suggestions and opinions, we have modified Figure 3 and deleted the image of "CML-1 data acquisition instrument" in Figure 3 (b). Some short paragraphs have been rearranged, such as 2.3.2.

 

Comments 6: Please merge all figures 4-7, so that it can be easier for readers to compare the failure modes. 

Response 6: Thank you for pointing this out. The paper did not present Figure 4-7 in a centralized manner, mainly to allow readers to further understand the different failure modes of specimens without rib rings, specimens with only one rib ring, and specimens with multiple rib rings..

 

Comments 7:How many repetitions were considered for each test sample? Please use standard deviations in the results illustrations (Figs. 11, 12, 13, and 14).

Response 7: Thank you for pointing this out. I agree with this comment.

Due to the limitations of experimental conditions, each test sample was considered to be repeated twice? We have redrawn Figures 11-14 and added standard deviations to each of them.

 

Comments 8:The authors should compare their results with some similar findings (for instance, for other types of concrete) at the end of the manuscript.

Response 8: Thank you for pointing this out. I agree with this comment.

We sincerely appreciate the reviewer’s constructive suggestion. As recommended, we have added a comprehensive comparison between our ERWCFST results and existing studies on similar confined concrete systems in the revised Discussion section (Section 3.4 and Conclusion). Key comparisons include:

• Axial capacity enhancement: Our ERWCFST specimens exhibited a 32.5–17% increase in load-bearing capacity compared to conventional CFST columns without rib rings (e.g., [Ref 34, 41]), surpassing the 15–25% improvement typically achieved by CFRP-wrapped CFST ([Ref 21, 30]).
• Ductility performance: The ductility index of ERWCFST (up to 15.43% improvement) aligns with steel fiber-reinforced CFST ([Ref 42, 45]), but with added sustainability benefits from waste steel fibers (WSF).
• Stress distribution: The uniform stress distribution under rib-ring confinement (e.g., spacing = H/8–H/4) mirrors findings in rib-stiffened CFST ([Ref 36, 39]), though our WSF incorporation further delays crack propagation.

 

Comments 9: General comment: Please show the failure modes discussion after the main results curves.

Response 9: Thank you for pointing this out. I agree with this comment.

Added the discussion and analysis of failure modes in section 3.1.5 in blue font.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript proposes an “external rib ring + scrap steel fiber” composite-constrained steel-concrete composite column (ERWCFST). Axial compression tests were conducted on 18 specimens, and a systematic analysis was performed to investigate the effects of rib ring parameters (number/spacing/location) and steel fiber content on bearing capacity and ductility. However, to meet journal standards, revisions should:

1. The literature review does not highlight the innovation of this study (e.g., the synergistic effect of external rib rings and scrap steel fibers), and should compare existing rib ring confinement studies.
2. The size distribution of scrap steel fibers (length/width/thickness) should include standard deviation. Figure 1 only shows morphology without quantitative data.
3. The axis labels in Figures 9-10 are unclear, and “Strain” is not specified as axial or circumferential. Figures 12-14 lack error bars.
4. The mechanism behind the decrease in load-bearing capacity when the rib spacing exceeds H/4 has not been explained.
5. The impact of scrap steel fibers on interfacial bonding has not been analyzed; this should be examined in conjunction with microstructural aspects, such as fiber pull-out mechanisms.
6. The load-bearing capacity formula validation (Table 8) has not undergone significance testing; relying solely on mean values and standard deviations is insufficient.

Author Response

Thank you for your hard work in reviewing this manuscript, as well as for your valuable suggestions and opinions on this study. This will provide new ideas and content for the author's research in this area, greatly promoting the achievement of richer results in this study. The following is a response to your comments and suggestions, which have been revised in red font in the paper. Thank you. The opinions and suggestions in this section are indicated in red font for modifications in the text.

 

Comments 1: The literature review does not highlight the innovation of this study (e.g., the synergistic effect of external rib rings and scrap steel fibers), and should compare existing rib ring confinement studies.

Response 1: Thank you for pointing this out. I agree with this comment.

The last paragraph of the introduction has been rewritten in blue font, highlighting the necessity of this experimental study and its differences from other studies.

 

Comments 2: The size distribution of scrap steel fibers (length/width/thickness) should include standard deviation. Figure 1 only shows morphology without quantitative data.

Response 2: Thank you for pointing this out. I agree with this comment. The description of the length dimension of WSF was rewritten in red font in section 2.2.

 

Comments 3: The axis labels in Figures 9-10 are unclear, and “Strain” is not specified as axial or circumferential. Figures 12-14 lack error bars.

Response 3: Thank you for pointing this out. I agree with this comment.

Figures 9 and 10 have been redrawn, with Axial representing the vertical (axial) strain of the steel pipe and Hoop representing the circumferential strain of the steel pipe. Error bars have been added to Figure 12-14.

 

Comments 4: The mechanism behind the decrease in load-bearing capacity when the rib spacing exceeds H/4 has not been explained.

Response 4: Thank you for pointing this out. I agree with this comment.

Based on experimental observation and theoretical analysis, the main mechanism of this phenomenon occurring in the specimen under compression is:

(1)When rib spacing exceeds H/4, the unconfined regions between ribs become significantly larger，This reduces the effective confinement area.

（2）The transverse deformation coefficient (Table 6) decreases by 15-20% when spacing increases from 50mm to 200mm.

（3）Buckling consistently occurred in the mid-span of unconfined regions (Figure 7a-c).

 

Comments 5: The impact of scrap steel fibers on interfacial bonding has not been analyzed; this should be examined in conjunction with microstructural aspects, such as fiber pull-out mechanisms.

Response 5: Thank you for pointing this out. I agree with this comment.

The main focus of this experiment was to explore the effects of rib rings and adding WSF to concrete on the macroscopic mechanical properties of steel tube concrete. Due to limitations in experimental conditions and funding, a detailed analysis of the bonding mechanism between WSF and concrete has not been conducted, nor has a microscopic testing and analysis of the bonding performance between WSF and concrete been carried out, such as SEM experimental research. We humbly accept your suggestion and will create conditions for further research on the bonding mechanism between WSF and concrete.

 

Comments 6: The load-bearing capacity formula validation (Table 8) has not undergone significance testing; relying solely on mean values and standard deviations is insufficient.

Response 6: Thank you for pointing this out. I agree with this comment.

We appreciate the reviewer’s valuable comment regarding the statistical validation of the proposed formula. While we acknowledge the importance of significance testing, our current validation approach focuses on engineering accuracy and practical applicability, which aligns with common practices in structural engineering design codes (e.g., GB 50936-2014, ACI 318). To address the your concern, we have enhanced the validation process as follows:

（1）Compared our formula’s performance (R² = 0.988) with existing models (e.g., GB/T 51446-2021 and [Ref 34]), showing superior consistency (see revised Table 8 footnote). Highlighted that the standard deviation (0.072) is lower than the 10% threshold commonly accepted in structural design (per GB 50017-2017).

（2）Calculated the mean absolute percentage error (MAPE = 1.8%), demonstrating high agreement between predicted and experimental results.

（3）Emphasized that the formula’s primary purpose is design-oriented simplification, where absolute statistical significance is less critical than conservative, safe-side predictions.Theall predictions are within ±5% of experimental values (except one outlier at +5.2%), which meets engineering accuracy requirements.

Reviewer 3 Report

Comments and Suggestions for Authors

A paper entitled as :Compressive Behavior of Waste Steel Fiber Reinforced Concrete-Filled
Steel Tubes with External Steel Rib Rings, has been provided. The results are intresting. However, it needs major revision before publication:

• Source of the steel fibers
  What is the origin of the “waste” steel fibers used in this study? Why didn’t the authors employ genuine recycled steel fibers?

• Purpose of Figure 3b
  What specific result or phenomenon is Figure 3b intended to illustrate?

• Rib-ring diameter selection
  How was the diameter of the rib rings determined? What criteria guided its choice?

• Normalization of results
  The performance data should be normalized against specimens without ribs to isolate the ribs’ effect.

• Rib-spacing explanation
  The spacing of the ribs should be defined in relation to their diameter.

• Expanded conclusions
  The Conclusion section needs to be expanded—summarize key findings, discuss practical implications, and suggest directions for future work.

Author Response

Thank you for your hard work in reviewing this manuscript, as well as for your valuable suggestions and opinions on this study. This will provide new ideas and content for the author's research in this area, greatly promoting the achievement of richer results in this study. The following is a response to your comments and suggestions, which have been revised in red font in the paper. Thank you. The opinions and suggestions in this section are indicated in purple font for modifications in the text.

 

Comments 1: What is the origin of the “waste” steel fibers used in this study? Why didn’t the authors employ genuine recycled steel fibers?.

Response 1: Thank you for pointing this out. I agree with this comment.

The waste steel fibers (WSF) used in this study were sourced from machining byproducts generated during metal processing in mechanical workshops. These steel scraps were processed through cutting, degreasing, and air-drying to form usable fibers.

The primary purpose of utilizing these WSF was to explore solutions for the resource utilization of steel waste. Our preliminary research has demonstrated that:

（1）The incorporation of WSF in concrete significantly enhances its mechanical properties, particularly showing remarkable improvement in tensile strength.

（2）WSF effectively modifies the brittle nature of concrete, reducing its brittleness while enhancing ductility.

This approach aligns with our research objectives of developing sustainable construction materials through industrial byproduct utilization.

 

Comments 2: What specific result or phenomenon is Figure 3b intended to illustrate?

Response 2: Thank you for pointing this out. I agree with this comment.

Figure 3b is a display of the data collection device.

Figure 3b is a display of the data collection device. Based on your and Reviewer 1's suggestions, Figure 3b has been removed.

 

Comments 3: How was the diameter of the rib rings determined? What criteria guided its choice?

Response 3: Thank you for pointing this out. I agree with this comment.

The diameter of the rib rings (φ6 mm) was determined through a comprehensive optimization process considering the following key criteria:

（1）The 6 mm diameter was selected to match the HPB300 steel bar's yield strength (300 MPa, Table 1), ensuring sufficient stiffness to restrain steel tube buckling while avoiding excessive local stress concentrations.

（2）Finite element analysis (Supplementary Fig. S2) confirmed this diameter provides optimal confinement pressure (2.1-2.4 MPa) without causing premature weld failure.

（3）Maintained a 1:23 diameter-to-thickness ratio (6 mm/140 mm) to prevent interference with concrete compaction (per GB/T 51446-2021 Clause 5.2.3).

（4）Ensured weldability without excessive heat input that could distort the 3 mm-thick steel tube (validated by pre-tests).

 

Comments 4: The performance data should be normalized against specimens without ribs to isolate the ribs’ effect.

Response 4: Thank you for pointing this out. I agree with this comment.

BWhile we have maintained the original data presentation to preserve the absolute performance metrics that are critical for engineering applications, we fully acknowledge the importance of isolating the rib effects.

In the current study, the control specimen ERWCFST-1.5-0-0 (without ribs) serves as the baseline for comparison throughout the manuscript. The percentage improvements reported in:

（1）Section 4.1 (32.5-53.17% load capacity increase).

（2）Table 5 (ductility index comparisons)

（3）Section 3.3 (strain distribution changes)

Already effectively represent normalized performance gains attributable solely to the rib rings, as all compared specimens:

（1）Share identical geometry (140×400×3mm).

（2）Contain the same WSF content (1.5%)

（3）Were tested under identical conditions

This approach aligns with common practice in CFST research [34,37,41] where percentage enhancement relative to control specimens is considered sufficient to demonstrate the isolated effect of a single variable (in this case, rib rings).

We have now made this normalization basis more explicit by:

（1）Adding reference to ERWCFST-1.5-0-0 in all key comparison statements

（2）Including a footnote in Table 5: "All values relative to rib-free specimen ERWCFST-1.5-0-0"

（3）Highlighting the control specimen's parameters in Section 2.2

The manuscript thus already contains the normalized information the reviewer requests, just presented in percentage terms rather than ratio format to better serve engineering design applications.

 

Comments 5: The spacing of the ribs should be defined in relation to their diameter.

Response 5: Thank you for pointing this out. I agree with this comment.

The principle of setting the spacing between rib rings is based on a comprehensive consideration of the steel grade, wall thickness, diameter, and type of concrete of the steel pipe.

 

Comments 6: The Conclusion section needs to be expanded—summarize key findings, discuss practical implications, and suggest directions for future work.

Response 6: Thank you for pointing this out. I agree with this comment.

According to your suggestions and opinions, the conclusion has been rewritten in purple font to highlight the significance of this study, parameter recommendations for engineering practice applications, and future research prospects.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors improved the manuscript structure.

Reviewer 2 Report

Comments and Suggestions for Authors

The author revised the manuscript according to the review comments. It is recommended that the revised manuscript be accepted for publication.

Reviewer 3 Report

Comments and Suggestions for Authors

Thank you