Seismic Fragility Analysis of the Aging RC Columns under the Combined Action of Freeze–Thaw Cycles and Chloride-Induced Corrosion
Round 1
Reviewer 1 Report
Overall, the present study offers new insight regarding the influence of freeze-thaw cycling and chloride-induced corrosion on the degradation of the seismic performance of reinforced concrete (RC) structures which shall complement previous studies which mostly only accounted for the individual phenomenon. The adopted methodology is sound and systematically described. The proposed model can be helpful for other researchers who are planning to conduct detailed analyses in this field. Besides, the results obtained from the investigated case study are eye-opening. They can provide insights to practicing engineers and government agencies when dealing with future bridge rehabilitation projects, particularly those in extreme environments. The paper is also well-written.
There are only two main concerns from the reviewer’s point of view: 1) the validity of the proposed model since it was fitted based on minimal data (if possible, it would be great if authors could try to compile results from other available studies then revalidate the model using these new datasets); 2) the information provided for the adopted numerical modeling strategy is somewhat limited, some crucial information like the modeling of bond and its degradation and the chosen plastic hinge length of the RC columns have not been explicitly described in the draft.
Detailed remarks:
1. In the “Introduction” section, the reviewer suggests that the authors add figures showing typical deteriorated RC structures (either taken from the actual structures or the laboratory specimens) caused by freeze-thaw cycling, chloride-induced corrosion, and both phenomena. These figures would be helpful for general audiences to understand why these phenomena are critical and must be considered during the design and assessment of existing structures in extreme environments;
2. Figure 2 and Eq. 6, the reviewer suggests checking the validity of the expression against the test results of other available studies (literature). The number of specimens tested from the present study is relatively small hence questioning the appropriateness of the curve fitting.
3. Figures 3-5, similarly to the previous comment, the reviewer suggests the authors also collect and compile additional data from other available tests to increase the reliability of the fitted concrete parameters.
4. The present study does not account for the degradation of a bond between the concrete and steel, which may also affect the seismic behavior. Can the authors add a paragraph to justify why this aspect is not considered and why it is still justifiable to neglect the bond degradation?
5. The numerical study does not explain the adopted plastic hinge length to convert the column’s curvature to rotation. Could the authors add a paragraph to describe it? Please also comment on whether there is a time-dependent influence on the plastic hinge length of these RC columns.
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Dear authors,
Your manuscript is interesting concerning the methodology used for investigation of the reinforced concrete columns subjected to combined actions of freeze-thaw cycling and chloride-induced corrosion. Moreover, the time-dependent seismic fragility analysis was made for a study case – a bridge.
The manuscript matches with the topic of the Buildings journal since the paper refers to the research on reinforced concrete columns and theoretical analysis for a bridge regarding the seismic fragility.
The paper cannot be accepted for publication in this form because some data is missing and the paper should be re-organized much better so that readers can understand more easily the research. The authors should take into account the recommendations and suggestions shown below, in order to improve their manuscript.
General recommendations:
1.The authors must change the text “composite actions” with “combined actions” in title of the manuscript. The word “cycles” is also more appropriate than “cycling”. Moreover, the title could be improved and changed to refer also to fragility analysis of the bridge (just if this important for the authors and for the research).
2. The authors must give some important results achieved (numerical or percent results) in the end of the abstract. The results suggested in the abstract are only general, they do not even indicate in percentages the degradation of the properties. Readers' interest increases when some significant results are given.
3.The authors should reorganize the structure of the paper because usually a manuscript referring to an experimental research, should contains the following important sections: 1. Introductions; 2. Materials and work method; 3. Results; 4. Conclusions. In this context, I would like to recommend to re-organize the text of the paper in the following sections: 1.Introduction; 2. Reinforced concrete structures investigated and work method (2.1. Reinforced concrete samples tested; 2.2. Work method; 2.3. Time-dependent seismic fragility analysis of the bridge); 3. Results and discussions (3.1. Effects of the combined actions of the freeze-thaw cycles and chloride-in-Corrosion; 3.2. Results on the time-dependent seismic fragility); 4. Conclusions. The text related to the equations used can be interated as sub-sections of the section “2.2. Work method”. The authors are free to decide how they want to organize the text in the sections suggested or they can make some changes.
Other recommendations and suggestions:
4. The authors must give more details about the C40 concrete regarding the standard used for codification and the content of the constituents expressed in %wt.
5.Fig. 1: The caption of Fig. 1(b) should be: “Number of the freeze-thaw cycles”. The caption of Fig. 1(d) should be: “compression test”.
6. How many specimens were contained in every set of specimens tested? For example: How many specimens were subjected to 100 freeze-thaw cycles and then, tested in compression? The question is the same for permeability test.
7.The authors must give details about the freeze-thaw cycle. What was the freezing time? What was the freezing time? How long was the thaw?
8. What procedure was used for freezing? What was the cooling rate?
9. The authors must give details about all equipment used for: manufacturing of the reinforced concrete samples; freezing; thaw; compression test; permeability test and so on.
10. The authors should add the standard deviation for each property given in Table 1.
11. The authors should check and indicate the references used for all equations given in the manuscript.
12. In title of the section 4.3, the text “analysis” must be corrected.
13. The authors must give some interpretations and explanations of the results regarding the degradation of the mechanical properties by considering the microscopic analysis of the failure areas. The effects of NaCl solutions must be analysed by using the microscopic analysis. In my opinion the microscopic analysis of the degraded samples is mandatory in the context of the analysis of the combined effects of the freeze-thaw cycles and NaCl solutions. The authors should improve their research.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Dear authors,
I read again carefully the revised version of your manuscript. I checked all changes and improvements according to my recommendations and suggestions.
You made a lot of improvements indeed.
I would like to suggest you just the following minor improvement.
In the revised version of your manuscript, you added the text regarding the equipment used, but you should give also the name of the manufacturer (and country for the location) for each equipment used.
After you will make the above minor improvement, I would like to recommend the publishing of your paper in Buildings journal.
Author Response
Please see the attachment.
Author Response File: 
Author Response.docx
