Study on Eccentric Compression Mechanical Characteristics of Basalt Fiber-Reinforced Recycled Aggregate Concrete-Filled Circular Steel Tubular Column

Comment (1): Some explanations for Figure 3c.

Response: Thanks for the reviewer's careful review. This question has been revised in the manuscript. The revised content is as follows:

The actual diagram of internal BFRAC crack was described by Fig.3 (b), and the schematic diagram of internal BFRAC crack was described by Fig.3 (c). Please refer to Line 200-202 on page 7

Comment (2): Considerable stiffness (line 443)?

Response: Thanks for the reviewer's careful review. This question has been revised in the manuscript. The revised content is as follows:

The cover plate was configured as a rigid material that has large stiffness, as well the material property was configured to be elastic. Please refer to Line 446 on page 19.

Comment (3): elastic 444 modulus is 1000 times (line 445)?

Response: Thanks for the reviewer's careful review. This question has been revised in the manuscript. The revised content is as follows:

The cover plate's elastic modulus was 1000 times of the steel utilized, and the Poisson's ratio was set to 0.3. Please refer to Line 447 on page 19.

Comment (4): Some practical applications in conclusion?

Response: Thanks for the reviewer's careful review. This question has been revised in the manuscript. The revised content is as follows:

Based on the existing calculation equation of CFST, the calculation equation of stable compressive bearing capacity for C-BFRRACFST column was presented, as well as the N/N_u-M/M_u correlation curve equation was modified. The calculation results demonstrated close alignment with the finite element simulation results, as well provided reference for the related design and application in engineering construction. Please refer to Line 587-591, on page 25.

Comment (1): The number of variables is much more than the number of specimens. How only eight column specimens can reflect several values of RCA, BF, L/D, and e?

Response: Thanks for the reviewer's careful review. This question has been revised in the manuscript. The revised content is as follows:

When exploring the mechanical properties of C-BFRRACFST columns with different replacement ratio of RCA under eccentric compression, other parameters such as BF content of 2 kg/m³, L/D of 8, as well as e of 35 mm were maintained. When exploring the mechanical properties of C-BFRRACFST columns with different BF content under eccentric compression, other parameters such as replacement ratio of 50%, L/D of 8, as well as e of 35 mm were maintained. When exploring the mechanical properties of C-BFRRACFST columns with different L/D under eccentric compression, other parameters such as replacement ratio of 50%, BF content of 2 kg/m³, as well as e of 35 mm were maintained. When exploring the mechanical properties of C-BFRRACFST columns with different e under eccentric compression, other parameters such as replacement ratio of 50%, BF content of 2 kg/m³, as well as L/D of 8 were maintained.

Comment (2): What are the quantities appeared in columns 5 and 6 in Table 3? (Alpha and xsi)

Response: Thanks for the reviewer's careful review. This question has been revised in the manuscript. The revised content is as follows:

Table 3 Related parameters and measured strength of specimens under eccentric compression

Note: a represents the steel ratio, a=A_s/A_c, A_s, A_c represent the cross-sectional area of steel tube and core concrete, respectively; x represents the constraint effect coefficient, x=af_y/f_c; N_u represents the specimen's peak load; the naming method for specimen takes CE-50-2-8-35 as an example. CE represents the eccentrically compressed specimen, 50 represents the replacement ratio of RCA, 2 represents BF content, 8 represents length-diameter ratio, as well as 35 represents eccentricity.

Please refer to Line 158, Table 3.

Comment (3): How the friction between steel tubes and concrete inside is evaluated? And, how this affects the results? And how this can be related to the type of concrete used here (i.e., with the special types of fibers and aggregates)?

Response: Thanks for the reviewer's careful review. This question has been revised in the manuscript. The revised content is as follows:

When the shear stress transmitted parallel to the interface reaches the critical value, relative sliding occurs between the interfaces. Therefore, the tangential contact model adopts the Coulomb friction model. The interface friction coefficient of steel tube and concrete is set to μ = 0.6, which can meet the requirements of model convergence.

Comment (4): Line 194: “Finally, severe plastic deformation developed locally during the middle for specimen, as well as the test is terminated.” So, the final failure mode is due to buckling issue or not?

Response: Thanks for the reviewer's careful review. This question has been revised in the manuscript. The revised content is as follows:

For the specimens with L/D of 8 and 11, the final failure mode was bending failure caused by the global buckling of the steel tube; for the specimens with L/D of 5, the final failure mode was bending failure caused by the the interaction between global buckling and local buckling. Please refer to Line 224-227, on page 7-8.

Comment (5): Fig 2: when eccentricity and the slenderness are the same, it seems there is no difference regarding the fiber/aggregate contents in terms of axial capacity. If so, how the novelty of the work can be explained, and what would be the objective and benefits of using BF and RCA?

Response: Thanks for the reviewer's careful review. This question has been revised in the manuscript. The revised content is as follows:

The RCA was used to alleviate the environmental problems caused by over-exploitation of natural coarse aggregate (NCA). BF was used to make up for the performance weakness of recycled aggregate concrete (RAC), improve the ductility of specimens and expand the application range of RAC. This study proposes basalt fiber-reinforced recycled aggregate concrete-filled circular steel-tubular (C-BFRRACFST) column that combines two modification methods of steel tube and fiber, which may greatly enhance the mechanical properties of RAC.

Comment (6): The fibers essentially enhance the tensile strength, postponing the initiation of concrete cracks. This will probably postpone the confinement action, as well. Then, if the column is subjected to buckling, the steel tube may be less effective compared to the case that there is no fiber in the concrete. Please comment on this.

Response: Thanks for the reviewer's careful review. This question has been revised in the manuscript. The revised content is as follows:

As the reviewer said, the fiber postponing the initiation of concrete cracks and probably postpone the confinement action. However, the addition of BF to recycled aggregate concrete-filled circular steel-tube did not significantly improve the bearing capacity, but significantly improved the ductility of the specimen, with the increase of 20.1%.

Comment (7):  It is advised to clearly list the failure modes to show if buckling happened or not? Please explain how BF and RCA can improve stability of the column specimens?

Response: Thanks for the reviewer's careful review. This question has been revised in the manuscript. The revised content is as follows:

For the specimens with L/D of 8 and 11, the final failure mode was bending failure caused by the global buckling of the steel tube; for the specimens with L/D of 5, the final failure mode was bending failure caused by the the interaction between global buckling and local buckling. Please refer to Line 224-227, on page 7-8.

BF can obviously improve the ductility coefficient of C-BFRRACFST column under eccentric load. In contrast, RCA has an adverse effect on the bearing capacity and ductility for specimen. The RCA was used to alleviate the environmental problems caused by over-exploitation of natural coarse aggregate (NCA). BF was used to make up for the performance weakness of recycled aggregate concrete (RAC), improve the ductility of specimens and expand the application range of RAC.

Comment (8): A reference to the other models developed for FRP-confined concrete is needed in Section 4.3.3. It is suggested to cite the following paper in this regard: https://doi.org/10.1007/s00521-016-2658-0     

Response: Thanks for the reviewer's careful review. This question has been revised in the manuscript. The revised content is as follows:

Currently, there are relevant reports on the calculation model of bearing capacity of confined concrete [32]. Combined with the research object of this study, the typical N/N_u−M/M_u strength curve of concrete-filled steel tubular (CFST) flexural members was illustrated via Fig.14. Please refer to Line 505-506, on page 21.

Comment (9): Conclusion 5: Can the ductility improvement by BF be expected when there is no eccentricity (or at small values of eccentricity)?

Response: Thanks for the reviewer's careful review. This question has been revised in the manuscript. The revised content is as follows:

According to the previous research of the research group, it was found that the ductility coefficient of C-BFRRACFST was significantly improved when the BF content was from 0 to 4 kg/m³ under axial load, and the increase was 17.04%. (Zhang, X.G.; Luo, C. Y.; Wang, J. B.; Kuang, X. M.; Huang, Y. J. The Axial Compression Behavior of Basalt Fiber-Reinforced Recycled Aggregate Concrete-Filled Circular Steel-Tubular Column. Sustainability 2023, 15, 14351. https://doi.org/10.3390/su151914351 )

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