Review Reports - Durability and Flexural Response of RC Beams to Freeze–Thaw Cycles: Influence of Air Content

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Overall evaluation: This paper takes a 35MPa reinforced concrete beam as the research object, systematically exploring the influence of air content on the freeze-thaw durability of concrete and the flexural performance of the beam. Two methods of water freeze-thaw and air freeze-thaw were adopted for the experiments. The rule that low air content specimens have significant freeze-thaw damage and that specimens with an appropriate air content have the best anti-freeze performance was clarified. This supplemented the deficiency in the research on freeze damage at the concrete layer level of high-strength concrete components and provided experimental basis and data support for the anti-freeze design of concrete structures in cold regions. The experimental design of this paper was complete and the data was detailed. However, there was room for improvement in terms of mechanism analysis, data presentation, format standardization, and literature citation. After revision and improvement, it has the value for publication. The main review comments are as follows:

The paper uses the air freeze-thaw method (FTA) to test the RC beam, but only states that this method was proposed by the author without elaborating on the principle, applicable scenarios, and verification of its equivalence to the traditional water freeze-thaw method. It lacks scientific support for the method, and it is recommended to supplement the theoretical basis and method reliability verification content of the air freeze-thaw method.
In the conclusion section (page 16): The paragraph division is unreasonable, and the key conclusions are not presented in points. It is suggested to split the long paragraph, sort out the core conclusions in points, and improve the logical clarity.
Table 1 (page 3, concrete mix ratio table): The unit annotation in the table header is not clear, and some parameter abbreviations are not explained in the table notes. It is recommended to add full name annotations for all abbreviations and standardize the unit format.
Figure 1 (page 4, RC beam size diagram): The font size of the dimensions is small, the steel type and strain gauge position are blurry. It is recommended to enlarge the font size of the annotations and clearly label all component parameters.
Table 3 (page 4, specimen parameter table): The specimen number format is not uniform, and some cells are misaligned. It is suggested to standardize the numbering rules, adjust the table alignment, and improve readability.
Table 6 (page 13, yield load deflection table): The average value calculation results have not been verified, and some data have calculation errors. It is recommended to review all data to ensure the accuracy of the values.
The reference list in the paper (page 17): The literature format is not uniform. Some journal literature lacks volume number and page number, and DOI annotations are incomplete. It is recommended to standardize all literature formats according to the requirements of the journal.
Figure 10 (page 13, load-deflection curve diagram): The sub-figure numbering is chaotic, the color distinction of the curves is low. It is recommended to standardize the sub-figure numbering, optimize the color matching and line type of the curves.
The abstract section (page 1, abstract paragraph): The English abstract sentences are lengthy, and some professional terms are not expressed concisely. It is suggested to simplify the sentence structure, unify the professional vocabulary, and conform to the concise norms of the journal abstract.

Author Response

We would like to sincerely thank the reviewer for taking the time to carefully review our manuscript and for providing professional, constructive, and insightful comments. The reviewer’s valuable feedback greatly helped us improve the scientific rigor, clarity, and overall quality of the manuscript. In response to the comments, we have carefully revised the manuscript and believe that the quality and completeness of the paper have been significantly enhanced. We are grateful for the reviewer’s thoughtful evaluation and valuable contribution to improving our work.

Comments 1: The paper uses the air freeze-thaw method (FTA) to test the RC beam, but only states that this method was proposed by the author without elaborating on the principle, applicable scenarios, and verification of its equivalence to the traditional water freeze-thaw method. It lacks scientific support for the method, and it is recommended to supplement the theoretical basis and method reliability verification content of the air freeze-thaw method.

Response 1: Thank you for pointing this out. We agree with this comment. Therefore, we have revised the manuscript to more clearly explain the rationale for applying the FTA test and its relationship with the standardized FTW test. The revisions can be found in the revised manuscript, Lines 140–148 and Lines 154–156.

Updated text in manuscript (Lines 140-148): Unlike the conventional freeze-thaw procedures specified in ASTM C666/C666M-15 (2015), which rely on water for the thawing process (Procedure A: freezing and thawing in water; Procedure B: freezing in air and thawing in water), the FTA test conducts both freezing and thawing in air. This modification was motivated by two factors: (1) the impracticality of using large volumes of water for freeze-thaw testing of full-scale structural members such as RC beams, and (2) the need to simulate dry winter conditions, as observed in regions like Seoul, where the freeze-thaw period coincides with very low humidity (Kim et al. 2021).

Updated text in manuscript (Lines 154-156): This comparative testing was performed to verify the reliability of the FTA test by establishing a correlation with the standardized FTW test in terms of material degradation characteristics.

Comment 2: In the conclusion section (page 16): The paragraph division is unreasonable, and the key conclusions are not presented in points. It is suggested to split the long paragraph, sort out the core conclusions in points, and improve the logical clarity.

Response 2: Thank you for pointing this out. We agree with this comment. Therefore, we have revised the Conclusion section by splitting the long paragraph and presenting the key conclusions in a point-by-point format to improve logical clarity. The revisions can be found in the revised manuscript, Conclusion section, Lines 398-426.

Updated text in manuscript (Lines 398-426): This study investigated the freeze-thaw resistance and flexural behavior of 35 MPa-grade concrete and reinforced concrete beams with different air contents. Based on the freeze-thaw tests on concrete specimens and the flexural tests on RC beams, the following conclusions can be drawn:

The concrete with moderate air content, C35-MA, exhibited the highest freeze-thaw resistance. This was confirmed by the relative dynamic modulus of elasticity, durability index, mass loss rate, and compressive strength results. In contrast, the low-air-content concrete, C35-LA, showed the most significant deterioration after freeze-thaw cycles.
The FTW and FTA test results showed similar trends in freeze-thaw resistance according to air content. However, the degree of deterioration was more pronounced in the FTW test than in the FTA test, indicating that water-based freeze-thaw exposure caused more severe material degradation than air-based freeze-thaw exposure.
In the flexural tests of RC beams, the low-air-content specimens showed reductions in yield load, yield deflection, and energy absorption capacity after freeze-thaw cycles. This indicates that insufficient air content can negatively affect the flexural performance and ductility of RC beams exposed to freeze-thaw conditions.
The RC beams with moderate and high air contents exhibited relatively small reductions in flexural performance after freeze-thaw cycles. These results suggest that an appropriate level of air content contributes to maintaining the structural performance of RC beams under freeze-thaw exposure.
Although freeze-thaw cycles caused noticeable degradation in the material properties of concrete, their influence on the flexural performance of RC beams was relatively limited. This is because the flexural behavior of RC beams is strongly affected by the tensile reinforcement and structural configuration. Therefore, the results should be interpreted as experimental trends, and further studies with a larger number of specimens are required to improve statistical reliability.

Comment 3: Table 1 (page 3, concrete mix ratio table): The unit annotation in the table header is not clear, and some parameter abbreviations are not explained in the table notes. It is recommended to add full name annotations for all abbreviations and standardize the unit format.

Response 3: Thank you for pointing this out. We agree with this comment. Therefore, we have revised Table 1 by standardizing the unit format in the table header and adding full-name annotations for all abbreviations in the table notes. The revisions are shown in the revised manuscript (Table 1).

Comment 4: Figure 1 (page 4, RC beam size diagram): The font size of the dimensions is small, the steel type and strain gauge position are blurry. It is recommended to enlarge the font size of the annotations and clearly label all component parameters.

Response 4: Thank you for pointing this out. We agree with this comment. Therefore, we have revised Figure 1 by enlarging the font size of the dimension annotations and clearly labeling the steel type, strain gauge positions, and other component parameters.

Comment 5: Table 3 (page 4, specimen parameter table): The specimen number format is not uniform, and some cells are misaligned. It is suggested to standardize the numbering rules, adjust the table alignment, and improve readability.

Response 5: Thank you for pointing this out. We agree with this comment. Therefore, we have revised Table 3 by standardizing the specimen numbering format, aligning the table, and improving overall readability.

Comment 6: Table 6 (page 13, yield load deflection table): The average value calculation results have not been verified, and some data have calculation errors. It is recommended to review all data to ensure the accuracy of the values.

Response 6: Thank you for pointing this out. We carefully rechecked all values in Table 6, including the individual data and average values for yield load and yield deflection. As a result, no calculation errors were found, and therefore no numerical revisions were made to Table 6.

Comment 7: The reference list in the paper (page 17): The literature format is not uniform. Some journal literature lacks volume number and page number, and DOI annotations are incomplete. It is recommended to standardize all literature formats according to the requirements of the journal.

Response 7: Thank you for pointing this out. We agree with this comment. Therefore, we have revised the reference list by standardizing the literature format according to the journal requirements, including author names, journal titles, volume numbers, page or article numbers, and DOI information where available.

Comment 8: Figure 10 (page 13, load-deflection curve diagram): The sub-figure numbering is chaotic, the color distinction of the curves is low. It is recommended to standardize the sub-figure numbering, optimize the color matching and line type of the curves.

Response 8: Thank you for pointing this out. We agree with this comment. Therefore, we have revised Figure 10 by standardizing the sub-figure numbering and adding distinct symbols to the load-deflection curves to improve their visual distinction and readability.

Comment 9: The abstract section (page 1, abstract paragraph): The English abstract sentences are lengthy, and some professional terms are not expressed concisely. It is suggested to simplify the sentence structure, unify the professional vocabulary, and conform to the concise norms of the journal abstract.

Response 9: Thank you for pointing this out. We agree with this comment. Therefore, we have revised the Abstract by simplifying the sentence structure and unifying the technical terms to improve conciseness and readability.

Updated text in manuscript (Lines 16-20): RC beams were exposed to freeze-thaw cycles using the air freeze-thaw method and the ASTM C666/C666M-15 water freeze-thaw method. Their flexural behavior was evaluated through four-point bending tests. The results showed that low-air-content RC beams exhibited notable reductions in yield load and energy absorption capacity after freeze-thaw cycles, indicating decreased strength and ductility.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

In their study, the authors examine a topical issue from a safety and sustainable development perspective, the solution to which will reduce overall cement consumption by extending the service life of building structures. The paper experimentally validates the optimal degree of porosity of concrete in flexible reinforced concrete elements, ensuring maximum resistance to moisture freezing. A comparison of the traditional method and air freezing is of practical interest. The article has a logical and clear structure, a well-written presentation, and may be of great interest to readers.

However, there are several technical aspects that require clarification for the reviewer and correction:

Fig. 4. Indicate the make and model of dynamic modulus measuring equipment.

L185. Explain the physical meaning of the "durability index (D.I.)" in the text. Also, move equation (2) up the text.

L191. Fig. 6. The text refers to the "density index." Check for errors, is the "durability index" being used? If there is no error, provide the equation for determining the density index, explain its physical meaning in the text, and analyze the results.

L200 Check the correctness of the reference to Fig. 6. Perhaps this refers to Fig. 7? Decipher the values of the indicators included in equation (3).

Author Response

Comments 1: Fig. 4. Indicate the make and model of dynamic modulus measuring equipment.

Response 1: Thank you for pointing this out. We agree with this comment. Therefore, we have revised the manuscript by indicating the make and model of the dynamic modulus measuring equipment.

Updated text in manuscript (Lines 176-177): The relative dynamic modulus of elasticity was measured using a Digital Type Dynamic Young’s Modulus Meter (HJ-5350, Heungjin, Korea), as shown in Fig. 4.

Comments 2: L185. Explain the physical meaning of the "durability index (D.I.)" in the text. Also, move equation (2) up the text.

Response 2: Thank you for pointing this out. We agree with this comment. Therefore, we have revised the manuscript by explaining the physical meaning of the durability index (D.I.) and moving Equation (2) to an earlier position in the relevant section.

Updated text in manuscript (Lines 206-212): The durability index (D.I.) is a composite parameter that reflects the residual stiffness of concrete after repeated freeze-thaw cycles. Since the relative dynamic modulus of elasticity represents internal damage such as microcrack propagation and pore-structure degradation, a higher D.I. indicates that the concrete retains a greater proportion of its original elastic properties under freeze-thaw exposure. Therefore, the D.I. can be used to evaluate the overall freeze-thaw resistance of concrete, with lower values indicating more severe stiffness degradation and internal deterioration.

Comments 3: L191. Fig. 6. The text refers to the "density index." Check for errors, is the "durability index" being used? If there is no error, provide the equation for determining the density index, explain its physical meaning in the text, and analyze the results.

Response 3: Thank you for pointing this out. We agree with this comment. Therefore, we have corrected the y-axis label in Figure 6 from “density index” to “durability index,” as the intended parameter was the durability index.

Comments 4: L200 Check the correctness of the reference to Fig. 6. Perhaps this refers to Fig. 7? Decipher the values of the indicators included in equation (3).

Response 4: Thank you for pointing this out. We agree with this comment. Therefore, we have corrected the figure reference from Fig. 6 to Fig. 7 and revised the manuscript by adding explanations for the parameters included in Equation (3).

Updated text in manuscript (Lines 225-227): The mass loss rates of concrete subjected to different freeze-thaw methods are presented in Fig. 7. The mass loss rate was calculated using Equation (3), where ΔWₙ is the mass loss rate after n freeze-thaw cycles (%), W₀ is the initial mass of the specimen before freeze-thaw cycles, and Wₙ is the mass of the specimen after n freeze-thaw cycles.

Reviewer 3 Report

Comments and Suggestions for Authors

1.The introduction states that high-strength concrete requires special attention, but the experiment uses concrete with a strength of 35 MPa which is not high-strength concrete, but ordinary concrete. This is misleading.
2. The authors use two different methods, FTW and FTA, but provide no statistical justification for the comparison. The number of specimens (12 beams, 2 per group) is insufficient for reliable conclusions. Statistical analysis (standard deviations, confidence intervals) is lacking.
3. Table 1 indicates that the water-cement ratio for C35-LA is 0.60, while for C35-HA it is 0.45. This means that the compressive strength was not strictly controlled, despite the stated same value (~38 MPa). Different W/C ratios and different superplasticizer contents fundamentally alter the structure of concrete, and the effect on frost resistance may be related not only to air content but also to porosity and capillary structure. This distorts the interpretation.
4. Calculated values for the cracking moment and theoretical yield strength are not provided for four-point bending. A comparison of loads for beams with different air contents (22.92 vs. 24.27 kN) may be within the margin of error, but the authors draw categorical conclusions.
5. The authors claim that optimal air content (5.3%) provides the best frost resistance. However, in terms of relative dynamic modulus, FTW C35-MA is 89.91%, while C35-HA is 85.55%. A difference is only 4.36% and without statistical significance, this conclusion is incorrect. In terms of mass loss, FTW C35-MA (0.47%) is better than C35-HA (0.92%), but in FTA C35-HA is better (0.48% vs. 0.53%). Therefore, the effect is ambiguous.
6. The conclusion about the "optimal air content" (5.3%) is unfounded. Just three measurement points with a large air content step (4 percentage points) do not allow constructing a response function and finding the global maximum frost resistance. The authors' statement should be reformulated as "of the three levels tested, concrete with an air content of 5.3% demonstrated the best frost resistance." Any claims of optimality require either more detailed parameter variation or reference to a well-known theory supported by numerous independent studies. Furthermore, varying the water/cement ratio and superplasticizer dosage between mixtures introduces a distorting factor, preventing the improvement from being attributed solely to the air content.
7. The conclusion that the reduction in concrete properties under cycling does not significantly affect the performance of RC beams due to reinforcement is trivial and widely known. It does not require a dedicated study with such small sample size.

8. The authors excessively cite their previous works (11, 12, 13, 15, 16) – 5 out of 36 references, exceeding reasonable limits for self-citation. However, these works describe the same FTA method, and this article is essentially a minor extension of it.
9. Line 200: Incorrect figure citation. It should be Fig. 7 instead of Fig. 6.

Author Response

Comment 1: The introduction states that high-strength concrete requires special attention, but the experiment uses concrete with a strength of 35 MPa which is not high-strength concrete, but ordinary concrete. This is misleading.

Response 1: Thank you for your valuable comment. We agree with the reviewer’s concern that the previous Introduction may have caused confusion by referring to high-strength concrete, even though the concrete used in this study was 35 MPa-grade. Accordingly, we revised the first paragraph of the Introduction to clearly state that the study focuses on 35 MPa-grade concrete and removed the misleading description related to high-strength concrete.

Updated text in manuscript (Lines 28-39): With recent advances in construction technology, concrete with relatively high design compressive strength has been increasingly used in various reinforced concrete structures (Hung et al. 2021; Hui 2025). In particular, 35 MPa-grade concrete is widely applied in general structural members because it provides adequate mechanical performance, durability, and structural stability(Oesman & Harry, 2025). According to ASTM C94/C94M-23 (2023), the recommended air content may be reduced by up to 1% for concrete with a compressive strength of 5000 psi, approximately 35 MPa, or higher. Although concrete in this strength range generally has a relatively dense matrix and improved resistance to water penetration, its freeze-thaw resistance may still be influenced by air content under repeated freezing and thawing conditions(Shah et al., 2021; Kia, 2023; Zhe et al., 2021). Therefore, it is necessary to evaluate the effect of air content on the freeze-thaw durability and flexural performance of 35 MPa-grade reinforced concrete members.

Comment 2: The authors use two different methods, FTW and FTA, but provide no statistical justification for the comparison. The number of specimens (12 beams, 2 per group) is insufficient for reliable conclusions. Statistical analysis (standard deviations, confidence intervals) is lacking.

Response 2: Thank you for pointing this out. We agree with this comment. Therefore, we have revised Table 6 by adding the standard deviation values together with the mean values for the yield load and yield deflection results.

Comment 3: Table 1 indicates that the water-cement ratio for C35-LA is 0.60, while for C35-HA it is 0.45. This means that the compressive strength was not strictly controlled, despite the stated same value (~38 MPa). Different W/C ratios and different superplasticizer contents fundamentally alter the structure of concrete, and the effect on frost resistance may be related not only to air content but also to porosity and capillary structure. This distorts the interpretation.

Response 3: Thank you for pointing this out. We agree with this comment. Therefore, we have revised the manuscript to clarify that the water-cement ratio was adjusted to obtain comparable compressive strength among mixtures with different air contents, and that the superplasticizer dosage was adjusted to secure adequate workability as the water-cement ratio decreased. We also added a statement that changes in the water-cement ratio and superplasticizer dosage may affect the pore and capillary structures of concrete; therefore, the results should be interpreted as the combined effect of mixture adjustment and air content under a comparable compressive strength level.

Updated text in manuscript (Lines 89-96): In general, an increase in air content can reduce the compressive strength of concrete; therefore, the water-cement ratio was adjusted to achieve comparable compressive strengths across the mixtures. The superplasticizer dosage was also adjusted to secure adequate workability as the water-cement ratio decreased. However, because changes in the water-cement ratio and superplasticizer dosage may also affect the pore and capillary structures of concrete, the results of this study should be interpreted as the combined effect of mixture adjustment and air content under a comparable compressive strength level.

Comment 4: Calculated values for the cracking moment and theoretical yield strength are not provided for four-point bending. A comparison of loads for beams with different air contents (22.92 vs. 24.27 kN) may be within the margin of error, but the authors draw categorical conclusions.

Response 4: Thank you for pointing this out. We sincerely appreciate this professional and important comment. In this study, the cracking moment could not be reported because the initial cracking load was not separately identified during the flexural tests. We apologize for this limitation. Therefore, we have added the theoretical yield strength calculated under the four-point bending condition to the manuscript. In addition, to address the reviewer’s concern that the differences in yield load among beams with different air contents may fall within the experimental margin of error, we have added standard deviation values together with the mean values in Table 6.

Updated text in manuscript (Lines 119-121): The theoretical yield load of the RC beam was calculated as approximately 18.3 kN under the four-point bending condition.

Comment 5: The authors claim that optimal air content (5.3%) provides the best frost resistance. However, in terms of relative dynamic modulus, FTW C35-MA is 89.91%, while C35-HA is 85.55%. A difference is only 4.36% and without statistical significance, this conclusion is incorrect. In terms of mass loss, FTW C35-MA (0.47%) is better than C35-HA (0.92%), but in FTA C35-HA is better (0.48% vs. 0.53%). Therefore, the effect is ambiguous.

Response 5: Thank you for pointing this out. We agree with this comment. Therefore, we have revised the manuscript to clearly state that the relative dynamic modulus values reported in this study were obtained as the average of five repeated measurements for each specimen. In addition, for the mass loss results, we moderated the statement that “C35-MA clearly showed the best performance” and revised the discussion to explain that the effect of air content was relatively clear in the FTW test, whereas the differences among mixtures were small in the FTA test because the external moisture supply was limited under air-based freeze-thaw conditions.

Updated text in manuscript (Lines 190-191): The relative dynamic modulus values reported in this study were obtained as the average of five repeated measurements for each specimen.

Updated text in manuscript (Lines 245-249): In the FTW test, differences in mass loss rate with air content were clear, and among the three mixtures tested, C35-MA showed the lowest mass loss rate, indicating a tendency toward improved resistance to mass loss. In contrast, in the FTA test, the differences in mass loss rate among the mixtures were relatively small because the external supply of moisture was limited under air freeze-thaw conditions.

Comment 6: The conclusion about the "optimal air content" (5.3%) is unfounded. Just three measurement points with a large air content step (4 percentage points) do not allow constructing a response function and finding the global maximum frost resistance. The authors' statement should be reformulated as "of the three levels tested, concrete with an air content of 5.3% demonstrated the best frost resistance." Any claims of optimality require either more detailed parameter variation or reference to a well-known theory supported by numerous independent studies. Furthermore, varying the water/cement ratio and superplasticizer dosage between mixtures introduces a distorting factor, preventing the improvement from being attributed solely to the air content.

Response 6: Thank you for pointing this out. We agree with this comment. Therefore, we have revised the Conclusion section by replacing the expression “optimal air content” with “Among the three air content levels tested in this study, the concrete with moderate air content, C35-MA, exhibited the highest freeze-thaw resistance”

Updated text in manuscript (Lines 402-403): Among the three air content levels tested in this study, the concrete with moderate air content, C35-MA, exhibited the highest freeze-thaw resistance.

Comment 7: The conclusion that the reduction in concrete properties under cycling does not significantly affect the performance of RC beams due to reinforcement is trivial and widely known. It does not require a dedicated study with such small sample size.

Response 7: Thank you for pointing this out. We agree with this comment. Therefore, we have revised the Conclusion section to emphasize that the novelty of this study lies not in the general role of reinforcement, but in experimentally evaluating the relationship between material-level freeze-thaw deterioration and member-level flexural performance of RC beams under the air-based freeze-thaw condition.

Comment 8: The authors excessively cite their previous works (11, 12, 13, 15, 16) – 5 out of 36 references, exceeding reasonable limits for self-citation. However, these works describe the same FTA method, and this article is essentially a minor extension of it.

Response 8: Thank you for pointing this out. We agree with this comment. Therefore, we have reduced the number of self-citations to two and revised the manuscript by adding a more detailed explanation of the freeze-thaw test method used in this study.

Updated text in manuscript (Lines 139-148): The FTA test is an experimental method proposed by the authors and subsequently utilized by other researchers (Kim et al. 2021; Rustamov et al. 2021; Kim 2023). Unlike the conventional freeze-thaw procedures specified in ASTM C666/C666M-15 (2015), which rely on water for the thawing process (Procedure A: freezing and thawing in water; Procedure B: freezing in air and thawing in water), the FTA test conducts both freezing and thawing in air. This modification was motivated by two factors: (1) the impracticality of using large volumes of water for freeze-thaw testing of full-scale structural members such as RC beams, and (2) the need to simulate dry winter conditions, as observed in regions like Seoul, where the freeze-thaw period coincides with very low humidity (Kim et al. 2021).

Comment 9: Line 200: Incorrect figure citation. It should be Fig. 7 instead of Fig. 6.

Response 9: Thank you for pointing this out. We agree with this comment. Therefore, we have corrected the figure citation from Fig. 6 to Fig. 7.

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

All reviewer's comments have been addressed.