Experimental Study on the Flexural Properties of Steel-Fibre-Reinforced Concrete Specimens with Different Heights

Round 1

Reviewer 1 Report

The submitted Essay with the Manuscript Number: " sustainability-2726735" and the title:" Experimental study on the flexural properties of steel fiber-reinforced concrete specimens with different heights" is an interesting experimental paper that evaluates the influence of specimen height, steel fiber type, and mixing method on the flexural properties of concrete specimens.

This experimental study aims to conduct three-point bending tests on concrete specimens with various heights, types of steel fibers, and mixing methods. This research aims to analyze steel fiber-reinforced concrete's flexural properties and characteristics. To conclude, the study found that adding steel fibers to concrete specimens improved their peak load and load-displacement curve, with the highest enhancement observed in the specimens with layered fiber distribution.

The study exhibits a comprehensive and well-articulated framework, showcasing adequate research. However, there are areas that require improvement to align with the necessary scientific standards. It is recommended to consider the following remarks and suggestions.

1. While the introduction offers a foundational background, the literature review seems somewhat constrained. To enhance the study's objectives, it is recommended to incorporate the following pertinent subjects:

(a) The latest advancement in the experimental exploration of mechanical characteristics of fiber-reinforced also involves investigating and monitoring damage using various Nondestructive Evaluation techniques.

(b) It is crucial to incorporate the fracture characteristics of the microstructure of cementitious specimens subsequent to mechanical testing, as this plays a significant role in comprehending the crack path during damage propagation on the fracture surface.

c)The application of nondestructive methods that consider the scattering of fracture energy within the microstructure is crucial for comprehending the trajectory of cracks during the propagation of damage on the fracture surface.

The subsequent articles, which are relevant in nature, can serve as exemplars of relative articles that have the potential to substantiate the aforementioned issues.

-"Acoustic emission for evaluating the reinforcement effectiveness in steel fiber reinforced concrete" Applied Sciences, 2021.

-"Cracking diagnosis in fiber-reinforced concrete with synthetic fibers using piezoelectric transducers," Fibers, 2022.

2. All abbreviations should be named at the beginning of usage.

3. Better English and syntaxis are needed in the manuscript

4. On Page 3, what is the C40 concrete standard? Some appropriate reference is needed.

5. What are the exact dimensions of the specimens produced? Also, a cross-section with the different layers could be helpful.

6. What is the reasoning for studying the specimen's height since it is known already that the static height of the specimens affects the stress distribution and flexural testing?

7. All the results should also be theoretically explained in more detail.

8. Since the main innovation of the study is the integrated layers by height, a better description of those layers' production is needed, together with a schematic representation and a theoretical explanation of the stress distribution at the interfacial zone among the layers

9. In Figure 1, clearer zoom Fotos are needed to understand the shape of each different type of steel fiber.

10. In Figure 2, a schematic representation could be helpful. What are the specimens' dimensions, and how were they preserved? What was their age at the time of testing?

11 In Figure 8, captions are needed with the code names of each line.

12. The titles are not presented appropriately for Table 2 and Figure 10.

 Extensive editing of English language required

Author Response

Please note that some figures can not be pasted here, please see the attached file.

1. While the introduction offers a foundational background, the literature review seems somewhat constrained. To enhance the study's objectives, it is recommended to incorporate the following pertinent subjects:

(a) The latest advancement in the experimental exploration of mechanical characteristics of fibre-reinforced also involves investigating and monitoring damage using various Nondestructive Evaluation techniques.

(b) It is crucial to incorporate the fracture characteristics of the microstructure of cementitious specimens subsequent to mechanical testing, as this plays a significant role in comprehending the crack path during damage propagation on the fracture surface.

1. c) The application of nondestructive methods that consider the scattering of fracture energy within the microstructure is crucial for comprehending the trajectory of cracks during the propagation of damage on the fracture surface. The subsequent articles, which are relevant in nature, can serve as exemplars of relative articles that have the potential to substantiate the aforementioned issues.

-"Acoustic emission for evaluating the reinforcement effectiveness in steel fibre reinforced concrete" Applied Sciences, 2021.

-"Cracking diagnosis in fibre-reinforced concrete with synthetic fibres using piezoelectric transducers," Fibres, 2022.

Response: Thank you for your suggestion, these aspects are really should strengthen in the introduction part, has been strengthened in the manuscript, and cited the above literature.

For example, Anastasios et al. [18] tested the SFRC beam during the bending process, and carried out acoustic emission monitoring (AE). It was found that the AE behavior at low load level can well explain the amount of steel bars, thus indicating the final mechanical properties. Maristella et al. [19] studied the standard 150 mm cube made of FRC and pasted a small lead zirconate titanate (PZT) patch network on the surface of the sample. It was found that the mechanical impedance of FRC cracking was changed due to repeated loading and various compressive stress levels, resulting in a corresponding change in the signal of each PZT.

2. All abbreviations should be named at the beginning of usage.

Response: Thank you for your suggestion, has been modified in the manuscript, in the first time when the abbreviation is given.

3. Better English and syntaxis are needed in the manuscript

Response: Thank you for your suggestion. It has been modified in the manuscript to improve the language problem, and got the help of professional polishing agency.

4. On Page 3, what is the C40 concrete standard? Some appropriate reference is needed.

Response: C40 is designed according to Specification for mix proportion design of ordinary concrete, which is supplemented in this manuscript.

5. What are the exact dimensions of the specimens produced? Also, a cross-section with the different layers could be helpful.

Response: The specific dimensions were length*width = 550*150, and the height is variable. It was designed to be 30 mm, 60 mm, 90 mm, 120 mm and 150 mm, respectively.

Fig. 1 Size of the specimen

6. What is the reasoning for studying the specimen's height since it is known already that the static height of the specimens affects the stress distribution and flexural testing?

Response: Because there are beams with different sizes and heights in practical engineering, most of the indoor tests are concentrated in fixed sizes. The study of sample height can provide reference for the design of beams with different sizes and heights.

7. All the results should also be theoretically explained in more detail.

Response: Thank you for your suggestion, and explained the experimental phenomena from a theoretical point of view. The statistical size effect theory holds that the unit of nonuniform strength in the specimen is the root cause of the effect of size, and the probability of a unit of low strength is proportional to the effect of size. Therefore, as the specimen height increases, the effect of the size of the concrete specimen in each working case increases. This is highly consistent with previous conclusions [45].

8. Since the main innovation of the study is the integrated layers by height, a better description of those layers' production is needed, together with a schematic representation and a theoretical explanation of the stress distribution at the interfacial zone among the layers.

Response: Thank you for your suggestion, added a schematic diagram in the manuscript, and explained the experimental phenomena from a theoretical point of view.

As shown in Fig. 11, the steel fibre acts as a bridge during the crack propagation process of the specimen, connecting the two ends of the force and improving the stress distribution. During the loading process, the lower end of the specimen is mainly subjected to tension at both ends, and TFZ is formed.

Fig. 11 The stress failure diagram of the specimen

Author Response File: Author Response.pdf

Reviewer 2 Report

In this study, the effects of specimen height, steel fibre mixing method and steel fibre type on the peak load, effect of size, section characteristics, strain characteristics and characteristics of the load–displacement curve of concrete specimens were investigated by performing three-point bending tests on different types (specimen height, steel fibre type, admixture) of steel fibre-reinforced concrete and the damage characteristics of the specimens were also analysed. A number of conclusions were drawn. The reviewers give the following recommendations:

1.Whether the upward trend of B-M-150 in the load-displacement curves of the milling type in Fig. 7(b) of this paper at the residual stage (CD stage) is due to the fibre characteristics of the type or some other reason, it is suggested that the authors should provide further explanation.

2.From the strain curve in Fig. 8 of this paper, it can be seen that the destruction of the control specimen occurred in about 150s, while the destruction of the specimen doped with steel fibre occurred in 100s, which is a big difference between the two destruction times, the reasonableness of the test is open to question, and it is recommended that the authors make further explanations.

3. In section 4.2 of this paper, the authors gave three typical interface features, but did not correspond each feature to the experimental results, so it is suggested that the authors increase this part of the work to enrich the content of the article.

4. Other parts of the article's formatting and presentation also need to be revised, for example:

(1) In the introduction, the author cites only one article about "the effect of size on steel fibre concrete", which is difficult to reflect the necessity of this study, and suggests that the author should increase the number of related literature to improve the scientific nature of the article.

(2) Other parts of the article's formatting and presentation also need to be revised, for example:

The data for Table 1 in this paper are missing and the authors need to add them.

(3) Eq (2) in the text uses γ to characterise the degree of dimensional influence, and its subscript form should be parametric

(4) In section 3.3 of this paper, "A-M-150 is slightly larger than those of specimens A-S-150 and A-E-150." is misstated.

(5) There are two inconsistent table headers in Table 2; the picture is not centred in Fig. 1; Eq (1) font formatting and no annotations; the figure name of Fig. 7 is incorrect;The axes of Figures 6 and 7 have the same scale as far as possible for comparison; Figure 8 has an incorrect vertical scale; Fig. 10 does not have a figure name and there is inconsistency in the numbering of the control groups.

Comments for author File: Comments.pdf

Descriptions are in the active voice whenever possible.

Author Response

Please note that some figures can not be pasted here, please see the atacched file.

1. Whether the upward trend of B-M-150 in the load-displacement curves of the milling type in Fig. 7(b) of this manuscript at the residual stage (CD stage) is due to the fibre characteristics of the type or some other reason, it is suggested that the authors should provide further explanation.

Response: It is worth noting that the CD stage of B-M-150 has a slight rise, which is mainly due to the wavy surface of the milling fibre. Therefore, when the fibre is pulled out, there may be a climbing effect at its peak position, and the curve rises briefly.

2. From the strain curve in Fig. 8 of this manuscript, it can be seen that the destruction of the control specimen occurred in about 150s, while the destruction of the specimen doped with steel fibre occurred in 100s, which is a big difference between the two destruction times, the reasonableness of the test is open to question, and it is recommended that the authors make further explanations.

Response: Thank you for your suggestion. The data have been verified and modified. The reason may be that the strain has been recorded at the time of preloading. However, this manuscript mainly studies the strain and does not pay attention to the time, so it has no great influence on it.

3. In section 4.2 of this manuscript, the authors gave three typical interface features, but did not correspond each feature to the experimental results, so it is suggested that the authors increase this part of the work to enrich the content of the article.

Response: Thank you for your suggestion. According to your suggestion, the typical section characteristics are modified.

Table 3

Typical section characteristics

As shown in Fig. 11, the steel fibre acts as a bridge during the crack propagation process of the specimen, connecting the two ends of the force and improving the stress distribution. During the loading process, the lower end of the specimen is mainly subjected to tension at both ends, and TFZ is formed.

Fig. 11 The stress failure diagram of the specimen

4. Other parts of the article's formatting and presentation also need to be revised, for example:

(1) In the introduction, the author cites only one article about "the effect of size on steel fibre concrete", which is difficult to reflect the necessity of this study, and suggests that the author should increase the number of related literature to improve the scientific nature of the article.

Response: Thank you for your suggestion. The literature on size effect has been increased to 4 to ensure the rationality of the manuscript. [42] Bajaj V, Singh S P, Singh A P, et al. Flexural fatigue analysis of hybrid fibre-reinforced concrete, Magazine of concrete research, 2012, 64(4): 361-373.

[43] Yoo D Y, Banthia N, Kang S T, et al. Size effect in ultra-high-performance concrete beams, Engineering Fracture Mechanics, 2016, 157: 86-106. [45] SU J. The research on the size effect of concrete behavior in compression and tension, Changsha: Hunan University, 2013: 29-34. (In Chinese)

(2) Other parts of the article's formatting and presentation also need to be revised, for example:

The data for Table 1 in this manuscript are missing and the authors need to add them.

Response: Thank you for your suggestion, has been added in the manuscript.

Table 1

The flexural strength of specimens in cases A and B

(3) Eq (2) in the text uses γ to characterise the degree of dimensional influence, and its subscript form should be parametric

Response: Thank you for your suggestion. The γ in equation 2 is calculated by using the specific situation, because there is a corresponding relationship between γ and f, and the subscript of γ represents its number.

                                                                    (2)

In the equation, f₃₀ and f₆₀ are the flexural strengths of the specimens with heights of 30 mm and 60 mm, respectively, and the unit is MPa. The degree of the effect of size in the other case A can be obtained by the same method.

(4) In section 3.3 of this manuscript, "A-M-150 is slightly larger than those of specimens A-S-150 and A-E-150." is misstated.

Response: Thank you for your suggestion, which has been modified in the original text. When the shear steel fibre is added, the peak load of specimen A-S-150 is slightly larger than those of specimens A-M-150 and A-E-150.

(5) There are two inconsistent table headers in Table 2; the picture is not centred in Fig. 1; Eq (1) font formatting and no annotations; the figure name of Fig. 7 is incorrect;The axes of Figures 6 and 7 have the same scale as far as possible for comparison; Figure 8 has an incorrect vertical scale; Fig. 10 does not have a figure name and there is inconsistency in the numbering of the control groups.

Response: Thank you for your advice, in the text in accordance with the recommendations were modified.

Author Response File: Author Response.pdf

Reviewer 3 Report

recommend 

1.       Abstract:

1)      The most important findings should be numerically added.

2.       Introduction:

2)      Findings of references 25-27 should be added in the text.

3)      The introduction should include recent publications. The reviewer recommend “Experimental and Analytical Investigation of Fracture Characteristics of Steel Fiber-Reinforced Recycled Aggregate Concrete”

4)       There are many sentences in the text that have errors in grammar and should be corrected. The authors suggest doing a proof of English reading and editing a manuscript to correct all grammar errors.

3.       Specimen Preparation

5)      On what basis do the authors consider the range or level of each of the experimental variables? Would you please elaborate on the manuscript to better understand the readers?

6)      The tested specimens were a fraction of the real-life size of beams. Can the authors include a discussion on the eminent "scale effects"?

4.       Bending Test

7)      Method of loading (displacement or forced control), installation of strain gauges and a schematic diagram for the test-setup should be added in the text.

8)      A table including the miture of the concrete should be added

9)      All standards used in tests should be mentioned.

10)  To which design code the specimens were designed?

11)  How can the results of this paper help to improve the construction industry?

12)  The findings of this research should be compared with previous results.

13)  How can the authors ensure homogenous distribution of fibers inside the mixture?

14)  All units of Equations should be added

5.       Conclusion:

15)  The conclusion section should be re-written to include specific general points. The conclusion in its current form is considers as a discussion.

Minor revision

Author Response

Please note that some figures can not be pasted here, please see the attached file.

1. Abstract:

1) The most important findings should be numerically added.

Response: Thank you for your suggestion, which has increased the number of important conclusions in the abstract of the manuscript. The peak load of the specimen increases by adding three kinds of steel fibres via two methods, and the increase is closely related to the height of the specimen.

2. Introduction:

2) Findings of references 25-27 should be added in the text.

Response: Thank you for your suggestion, which has been supplemented in the original text. For example, Qiang et al. [25] carried out an experimental study of steel fibre-reinforced concrete in acidic environments, and it is found that when the volume ratio of steel fibre is 1.5 %, the corrosion resistance of steel fibre reinforced concrete is the best. Li et al. [26] carried out a flexural toughness test and inversion study of the thermal conductivity of steel fibre-reinforced concrete members after fires, and it is verified that the incorporation of steel fibre into concrete helps to reduce its internal thermal stress difference and improve the crack resistance and fire resistance of the concrete. Li et al. [27] analysed the flexural behaviour of ultrahigh-performance hybrid fibre-reinforced concrete under environmental and high-temperature conditions. The hybrid combination of polyethylene ( PE ) and steel fibre can effectively improve the proportional limit, fracture modulus, toughness and toughness index of UHPFRC.

3) The introduction should include recent publications. The reviewer recommend “Experimental and Analytical Investigation of Fracture Characteristics of Steel Fibre-Reinforced Recycled Aggregate Concrete”

Response: Thank you for your recommendation, strengthened the introduction, and added this reference.

4) There are many sentences in the text that have errors in grammar and should be corrected. The authors suggest doing a proof of English reading and editing a manuscript to correct all grammar errors.

Response: Thank you for your suggestion. The grammatical errors in the manuscript have been rechecked, and got the help of professional polishing agency.

3. Specimen Preparation

5) On what basis do the authors consider the range or level of each of the experimental variables? Would you please elaborate on the manuscript to better understand the readers?

Response: Thank you for your suggestion, this part has been added in the manuscript. In concrete, steel fibre was divided based on two kinds of mixing methods. The influence of different distribution forms of steel fibre on the flexural strength and failure characteristics can be analyzed by Case A, Case B and samples with different sample heights. The height is divided into 5 equal parts, which increases the research scope of sample height and ensures the credibility of the research.

6) The tested specimens were a fraction of the real-life size of beams. Can the authors include a discussion on the eminent "scale effects"?

Response: There are various components of different sizes in concrete structures, from small to large, including beams, columns, plates, walls, etc. Components of different sizes will show different characteristics in terms of stress and deformation. In engineering design and construction, it is necessary to design and construct concrete members of different sizes according to specific uses and bearing requirements. Understanding the size effect helps to better predict and control structural performance. By studying the size effect of concrete, the mechanical properties of components with different sizes, including strength, stiffness and deformation, can be predicted more accurately, which is helpful for the rational design of structures. It is helpful to optimize the design of concrete structure, improve the bearing capacity and seismic performance of the structure, reduce the use of materials, and achieve the goal of economical and efficient design. It is helpful to evaluate the safety of different size components in the process of stress, and provide a reliable basis for the safe operation of the structure. It can promote the sustainable development of concrete structures, reduce resource consumption, and improve the life and maintainability of structures.

4. Bending Test

7) Method of loading (displacement or forced control), installation of strain gauges and a schematic diagram for the test-setup should be added in the text.

Response: Thank you for your suggestion. A schematic diagram of the test method has been added to the manuscript. The strain gauge is directly connected to the strain box.

Fig. 1 Size of the specimen

Fig. 3 Photograph of specimen loading

8) A table including the miture of the concrete should be added

Response: Thank you for your suggestion, increased the table of concrete mix proportion

Table 1

Mixing ratio of the specimen

9) All standards used in tests should be mentioned.

Response: Thank you for your suggestion. It has been modified according to the suggestion. The criteria used are added in Section 2.1. (Ministry of Housing and Urban Rural Development of the People's Republic of China. Specification for mix proportion design of ordinary concrete concrete JGJ 55-2011. Beijing: China Construction Industry Press, 2011)

10) To which design code the specimens were designed?

Response: Thank you for your suggestion. It has been modified according to the suggestion. The standard used is added in Section 2.1. Specification for mix proportion design of ordinary concrete.

11) How can the results of this manuscript help to improve the construction industry?

Response: The study of size effect can promote the more effective use of building materials. By optimizing the size of the beam, the demand for materials can be reduced and the engineering cost can be reduced on the premise of ensuring structural stability. In addition, it can help engineer better design and optimize the building structure.

12) The findings of this research should be compared with previous results.

Response: Thank you for your suggestions have been added to the manuscript with a comparison of previous research results. The statistical size effect theory holds that the unit of nonuniform strength in the specimen is the root cause of the effect of size, and the probability of a unit of low strength is proportional to the effect of size. Therefore, as the specimen height increases, the effect of the size of the concrete specimen in each working case increases. This is highly consistent with previous conclusions [45].

13) How can the authors ensure homogenous distribution of fibres inside the mixture?

Response: By stirring in batches if the fibre, and appropriately increase the stirring time.

14) All units of Equations should be added

Response: Thank you for your suggestion. The specific units are given in equations 1 and 2. F-peak load (N); L-distance between supports, L=450 mm ; b-specimen width b=150 mm ; h-specimen height (mm). In the formula, f₃₀ and f₆₀ are the flexural strengths of the specimens with heights of 30 mm and 60 mm, respectively, and the unit is MPa. The degree of the effect of size in the other case A can be obtained by the same method.

5. Conclusion:

15) The conclusion section should be re-written to include specific general points. The conclusion in its current form is considers as a discussion.

Response: Thank you for your suggestion. The conclusion has been rewritten.

In this manuscript, through a flexural test of steel fibre-reinforced concrete in different specimen heights, the influence of the steel fibre type and mixing method on the peak load, section fibre number, characteristics of the load-displacement curve, section characteristics and strain characteristics of the specimen is analyzed. The following conclusions are obtained.

(1) The addition of three types of steel fibres enhances the peak load of concrete specimens in both Case A and Case B. In Case A, the peak load increases proportionally with the specimen height, exhibiting a linear relationship in the control group. The peak load of specimens with added steel fibres surpasses that of the control group at equivalent specimen heights. The impact of concrete specimen size on flexural strength becomes more pronounced as the specimen height increases in both working cases, highlighting the close correlation between peak load and specimen height, as well as the method of fibre addition.

(2) In Case A, the quantity of cross-sectional fibres exhibits a linear increase with the rise in specimen height. Notably, the milling type demonstrates the smallest slope in the linear relationship between the number of cross-sectional fibres and specimen height, followed by the shear type, and the end hook type exhibits the largest slope. Similarly, in Case B, the number of cross-sectional fibres also experiences a linear growth with increasing specimen height, and this linear characteristic is more pronounced than in Case A. Furthermore, the three types of steel fibres share a consistent slope in the linear relationship between the number of cross-sectional fibres and specimen height.

(3) The load‒displacement curve of the specimen can be divided into a slow growth stage (OA stage), a nearly linear growth stage (AB stage), a sharp drop stage (BC stage) and a residual stage (CD stage). The load‒displacement curves of the three types of steel fibres at different specimen heights show differences in the CD stage of the load‒displacement curve, which basically shows that as the height of the specimen increases, the fluctuation increases. In addition, the load on the CD section in case A first decreases sharply and then gradually slows, while the load on the CD section in case B first decreases sharply and then gradually stabilizes.

(4) The control group exhibits distinct brittle failure characteristics, characterized by a flatter failed section and reduced fluctuation as the specimen height increases. In contrast, the specimens with three types of steel fibres in both methodological cases demonstrate a more tortuous development of cracks compared to those without steel fibres. In concrete flexural specimens with lower heights, macroscopic cracks initially emerge in the tensile zone at the bottom, gradually extending to the loading surface with increasing load. Tensile failure areas, densely populated with fibres, may still experience secondary cracks or debris shedding.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The answers at the comments and the modifications of the revised manuscript are accurate and reliable. Consequently, the revised paper is suitable for publishing.

Reviewer 2 Report

The author's manuscript is well revised and agreed to be accepted.

Moderate improvements in English.

Reviewer 3 Report

The paper can be accepted