Study of the Behavior of Square Concrete-Filled CFRP Steel Tubular under a Bending-Torsion Load

Round 1

Reviewer 1 Report

The paper compares the results of the experiment with numerical simulation data, determines the stress distributions in the samples at different loading stages, and analyzes the influence of the main parameters on the T-theta curve, such as the number of CFRP layers, steel strength, concrete strength, steel ratio and bending moment ratio. It is shown that the determining parameters are the steel yield strength of and the steel ratio.

The content of the work does not raise questions and it can be published after correcting several minor flaws:

Line 8 abbreviation CFRP should be explained in the abstract
Line 48 I think here is some mistake. The cases are equal: “no reinforcement in case of fire, no reinforcement in case of fire”
Line 52 FRP should be explained too
Line 89 and below. A space symbol should be between a number and a dimension
Line 98 the data values should be rounded correctly
Line 367 of concrete (a space is absent).

Author Response

Q1：Line 8 abbreviation CFRP should be explained in the abstract

A1：The authors thank the reviewer to present suggestions. CFRP is explained in the abstract. Please see the highlight text (in yellow colour) in abstract.

Q2：Line 48 I think here is some mistake. The cases are equal: “no reinforcement in case of fire, no reinforcement in case of fire”。

A2：The relevance of the text is revised in the Introduction. Please see the highlight text (in yellow colour) in Introduction.

Q3：Line 52 FRP should be explained too

A3：FRP is also explained in the abstract. Please see the highlight text (in green colour) in Introduction.

Q4：Line 89 and below. A space symbol should be between a number and a dimension

A4：The relevance of the text is revised. Please see the highlight text (in green colour) in Section 1.1

Q5： Line 98 the data values should be rounded correctly

A5：The data values is rounded correctly in Table 2.

Q6：Line 367 of concrete (a space is absent).

A6：The relevance of the text is revised.

Reviewer 2 Report

Study on Behavior of Square Concrete CFRP Filled Steel Tubular under Bending-Torsion Load – Q. Wang et al.

General comments: The current research presents an experimental approach to characterize CFRP tubes filled with reinforced concrete. The presented methodology encompasses a unique loading mode and is interesting to the readers. However, the paper is poorly written; extensive grammar editing is required before this paper can be recommended for publication. It was very difficult to follow the discussion and an attempt has been made to provide a few specific comments, which are provided below.  

Specific comments:

• Rewrite the abstract; break down the first sentence and use appropriate punctuation where necessary.
• How was the CFRP encapsulation performed? Provide a brief paragraph on the manufacturing of specimens after section 1.1.
• Describe what CFRP material was used as well as the layup sequence; comment on fiber and resin. Also provide the reference for the obtained CFRP properties.
• Rewrite Figure 2 sub-captions and caption
• Enlarge Figure 2(b) and clearly provide the specimen mounted onto the test rig
• Experimental “phenomenon” should read “procedure” and rewrite the section explaining clearly the failure modes. Otherwise, it is suggested to break this section down to discuss the failure phenomenon separately
• What is the loading rate adopted?
• How can the CFRP be homogenized as isotropic material in the analysis?
• Figure 9 shows the FE result following the test curve. What failure criterion was used to capture CFRP failure?
• Section 3.2.2: It says that the FE model shows crack initiation in the FE model. Please describe how is this obtained? If a cohesive of regularized FEM approach was used to predict crack initiation, please describe that in the previous section.
• The FE results provided in Figures 10-13 show reaction forces and how do they indicate crack initiation points?
• Were the FE strain fields compared against experimental results using DIC or strain gages?

 

Author Response

The authors wish to thank the reviewers for their time and efforts in reviewing our manuscript. We hope the changes summarised below have made the manuscript suitable for publication and we look forward to further response.

Note: Revisions based on Reviewer 's Comments are highlighted in red colour. To make the article more fluent, authors have revised some parts of the article, which are highlighted in colour.

Q1:Rewrite the abstract; break down the first sentence and use appropriate punctuation where necessary.

A1:Thanks for your suggestion. We rewrite the abstract, and check some problems in language and sentence patterns 

Q2:How was the CFRP encapsulation performed? Provide a brief paragraph on the manufacturing of specimens after section 1.1.

A2: The authors agree with the reviewer’s comments in this point. Authors add a paragraph on the manufacturing of specimens after section 1.1. Please see the highlight text (in green colour) in Section 1.1.

Q3:Describe what CFRP material was used as well as the layup sequence; comment on fiber and resin. Also provide the reference for the obtained CFRP properties.

A3: CFRP is a derivative of carbon fiber, which is similar to cloth and used to strengthen building structures. The layup sequence of CFRP is also added in Section 1.1. A reference is added, and properties of building structure adhesive used between CFRP is added in Section 1.2. Please see the highlight text (in green colour) in Section 1.2, and Table 5.

Q4:Rewrite Figure 2 sub-captions and caption

A4:Figure 2 sub-captions and caption are revised.

Q5:Enlarge Figure 2(b) and clearly provide the specimen mounted onto the test rig

A5:The authors agree with the reviewer’s comment in this point. Figure 2(b) are enlarged.

Q6: Experimental “phenomenon” should read “procedure” and rewrite the section explaining clearly the failure modes. Otherwise, it is suggested to break this section down to discuss the failure phenomenon separately

A6: Thanks for your suggestion. We renamed Section 1.4 as Failure mode of materials and modified most of the content. Please see the highlight text (in green colour) in Section 1.4.

Q7:What is the loading rate adopted?

A7: The graded loading system is adopted in the test. Within the elastic range, each level of loading is 1/10 of the estimated ultimate bearing capacity, and the next level of loading is carried out after holding the load for 2~3min. When the tension reaches about 70% of the estimated ultimate bearing capacity, the loading of each stage is reduced to 1/15 ~ 1/20 of the estimated ultimate bearing capacity. After yielding, the specimens shall be loaded slowly and continuously until the jack reaches the maximum range, and the test shall be stopped. Please see the highlight text (in green colour) in Section 1.3.

Q8:How can the CFRP be homogenized as isotropic material in the analysis?

A8:Because the CFRP is only sustain tensile load along the fiber direction, and the stresses in other directions are assumed to be 0.001MPa (a very small value close to zero), the CFRP is not homogenized as isotropic material in the analysis.

Q9:Figure 9 shows the FE result following the test curve. What failure criterion was used to capture CFRP failure?

A9: Figure 9 shows the comparison results between FE and test. The main purpose is to verify whether the finite element modeling method is reasonable in the macro (displacement and load change of specimen under loading), and this curve will not reflect the fracture of CFRP. We show the fracture mode comparison of CFRP in Figure 10.

Q10:Section 3.2.2: It says that the FE model shows crack initiation in the FE model. Please describe how is this obtained? If a cohesive of regularized FEM approach was used to predict crack initiation, please describe that in the previous section.

A10:ABAQUS does not shows crack initiation in the FE model. Therefore, in Section 3.2.2, we use the stress direction and value to describe the failure modes of various materials. For steel tube, we use almost the same torsional deformation to compare. For concrete, we use the maximum stress direction of concrete to express the direction of cracks. For CFRP, we use the stress value to compare. In the finite element simulation, when some stress arrows disappear, it indicates that the CFRP has broken, and compare the CFRP at this part with the test.

Q11:The FE results provided in Figures 10-13 show reaction forces and how do they indicate crack initiation points?

A11:As described in the previous answer, we have made a connection between the fracture of CFRP and the stress. When CFRP breaks, it means that the stress here is 0, and vice versa. For a clearer explanation, we have made a corresponding supplementary explanation in section 3.2.2. Please see the highlight text (in green colour) in Section 3.2.2.

Q12:Were the FE strain fields compared against experimental results using DIC or strain gages?

A12:Thanks for your suggestion. We refer to the relevant literature. Most of the finite element verification is carried out by comparing the load displacement curve and failure mode. Secondly, considering the length of the article, we do not compare the stress-strain relationship. If the reviewer thinks this needs to be added, we will add it in the subsequent revision.

Round 2

Reviewer 2 Report

The manuscript may be accepted in its current revised form.