Localized Damage Analysis of Cement Mortar Using X-ray Computed Tomography In Situ Compressive Loading and Digital Volume Correlation
Round 1
Reviewer 1 Report
I found the study to be well-structured and informative. The use of X-ray CT and DVC allowed for the analysis of the localized damage behavior of cement mortar in a precise and accurate manner. The findings suggest that unhydrated cement has a significant impact on the generation of localized damage, and that cracks in the material do not always expand with increasing load, which is a significant observation.
The study's conclusions are well-supported by the data obtained through the use of X-ray CT and DVC, and the findings are presented in a clear and concise manner. However, the author could further improve the study by providing a more in-depth discussion of the results and their implications. Additionally, the author could elaborate on the significance of the study's findings to the field of material science.
In conclusion, I recommend that this study be published after the suggested changes are made as below.
How did researchers make use of X-ray CT scans?
Please explain the role of digital volume correlation (DVC) in this research.
How did cement mortar play a role in this research, specifically?
In this research, what was picked up by the X-ray CT scanner?
Is there any correlation between the use of unhydrated cement and the occurrence of spot damage?
Does material cracks always grow bigger under pressure?
To what do we owe the bursting of preexisting fissures in the material?
What kinds of things can cause strain in a material to be distributed unevenly?
The greatest primary strains seen in the material caused what kind of damage?
How did you use the CT image's grey value and damage index to reflect damage within the specimen?
In this research, a new localization component was created to describe damage failure; please explain.
When did the internal material localization damage occur in relation to the external surface damage?
From what direction did the damage to the material initially occur, inside out or outside in?
What flaws, if any, did you see in the study's methodology?
What does this study mean for the future of cement mortar studies?
How does this research add to what has already been written about the subject of cement mortar?
What, if any, new methods or approaches were tried in this investigation?
How many cement mortar samples were analyzed, and what exactly did they consist of?
How might the results of this research be used in fields where cement mortar is routinely employed?
Author Response
We appreciate the reviewer’s advice.
1. How did researchers make use of X-ray CT scans?
Response: The authors conclude in a published review article:
Five analysis of concrete by CT as follows: Firstly, analyze the concrete damage rules by calculating the volume of each phase under different conditions. Secondly, study the relationship between pore and concrete damage by analyzing pore parameters, such as pore size, pore size distribution and different pore volume ratio, etc. Thirdly, study the variation rule of concrete fracture by analyzing the variation of crack width and volume. Fourthly, study the influence of fiber on the concrete properties by quantitatively evaluating the dispersion state and direction of the fibers in the concrete. Finally, build three-dimensional model based on CT images, obtain the elastic modulus and stress-strain distribution and compare with the test results.
This literature is cited in reference 6 in the manuscript.
2. Please explain the role of digital volume correlation (DVC) in this research.
Response: Thanks for the reviewer's comments. The role of DVC in this study is to obtain the full-field strain and the maximum principal strain of the computing unit of the specimen and to determine the damage distribution and development trend within the specimen through the strain.
3. How did cement mortar play a role in this research, specifically?
Response: Due to the limitations of micro-CT equipment, the specimen’s diameter is at most 10mm. If the concrete material is taken as the research object, because of the large size of the coarse aggregate, it is difficult to prepare the scanning specimen. Cement mortar material is a good choice, and the research results can provide a reference for the study of cement-based materials.
4. In this research, what was picked up by the X-ray CT scanner?
Response: The changes in the specimen in different stress states can be picked up through two-dimensional and three-dimensional images, which were obtained by X-ray CT scanning. The changes include the location and morphology of cracks, and so on.
5. Is there any correlation between the use of unhydrated cement and the occurrence of spot damage?
Response: The boundary of unhydrated cement is a weak area that is prone to damage under stress, but it also depends on its density. In the author’s other unpublished paper, it is found that the crack propagation path develops along the lower density area, so unhydrated cement and its boundary may not necessarily produce damage under stress.
6. Does material cracks always grow bigger under pressure?
Response: Cracks do not always grow larger under pressure. Figure. 8 in the manuscript shows that cracks inside the cement mortar did not always expand with the increase in load, and new cracks compressed the original cracks and reduced the width of individual cracks, resulting in no increase in the width of cracks with the increase in compressive stress.
7. To what do we owe the bursting of preexisting fissures in the material?
Response: Cracks form in the weak area of the matrix when the stress exceeds the tensile strength of the matrix, and the density of the cracks decreases. The area with low density is chosen as the crack propagation path, as mentioned in the answer to question 5. With the increase in stress, the crack area with low density expands preferentially and explosively.
8. What kinds of things can cause strain in a material to be distributed unevenly?
Response: Due to the heterogeneous characteristics of cement-based materials, the materials’ response degree to the stress is different, resulting in uneven stress distribution. The results of this study also show that the bias phenomenon exists in the uniaxial compression loading process, and the bias can also lead to uneven stress distribution in the material.
9. The greatest primary strains seen in the material caused what kind of damage?
Response: Because all of the values for the maximum principal strain are positive, the maximum principal strain inside the specimen is tensile strain. The specimen will eventually develop macroscopic cracks along the large tensile deformation when under stress.
10.How did you use the CT image's grey value and damage index to reflect damage within the specimen?
Response: The gray value of the CT image will vary with the change in substance density. When cracks occur in the specimen under stress, the density of the specimen decreases, and the gray value of the CT image decreases accordingly, indicating that the small cracks forming will cause damage. The damage index takes the relationship between the amount of damage and the maximum principal strain inside the specimen as the evaluation index of the internal damage. The larger the damage index is, the greater the damage degree of the specimen is.
11.In this research, a new localization component was created to describe damage failure; please explain.
Response: The internal damage of the specimen is ultimately caused by the large maximum principal strain. First, the distribution range of the maximum principal strain of all units is analyzed by the probability density function. Then the high-amplitude strain units under different stress states were determined, and the coordinate position of high-amplitude strain units was analyzed. The correlation coefficient was introduced to determine the localization factor. The larger the localization factor is, the more dispersed the space distribution of the high-amplitude strain units is, indicating that the damage of cement mortar is relatively small; otherwise, the smaller the localization factor is, the more concentrated the space distribution of the strain units is, implying greater damage.
12.When did the internal material localization damage occur in relation to the external surface damage?
Response: Unfortunately, DVC calculations were not performed on all the scanned data. The strain cloud image of the two groups of data could not accurately determine when the internal material localization damage occurred in relation to the external surface damage. This is a very interesting question, and we will adopt this suggestion in the subsequent research.
13.From what direction did the damage to the material initially occur, inside out or outside in?
Response: When the force applied to the specimen was less than 3000 N, it was discovered through the analysis of two-dimensional and three-dimensional CT images that the specimen's surface did not change significantly, but that internal cracks had developed. Since the initiation load could not be determined, it was concluded that the damage developed from the inside out based on CT images, which were also verified by DVC results.
14.What flaws, if any, did you see in the study's methodology?
Response: Firstly, when the platform is stopped, part of the force is reduced by relaxation; the force can’t maintain a constant value after stopping loading, which can cause errors in the test results. Secondly, after DVC calculation, it is found that bias occurs in the loading process, and the direction of damage development is uncertain.
15.What does this study mean for the future of cement mortar studies?
Response: Through X-ray CT in-situ loading scanning and DVC calculation, the damage distribution inside the material can be defined, and the direction of damage development can be determined, which can provide a reference for the study of the macroscopic mechanical behavior of cement mortar materials.
16.How does this research add to what has already been written about the subject of cement mortar?
Response: Firstly, the use of the in-situ loading scanning method enables the real-time capture of cement mortar changes under various stress states. Secondly, the maximum principal strain distribution, which results from DVC calculation, is what leads to cracking. Thirdly, the high-amplitude strain units are used to introduce the correlation coefficient, and the localized factor is defined to describe the degree of material damage.
17.What, if any, new methods or approaches were tried in this investigation?
Response: Firstly, an in-situ compression scanning test was used to study the real-time change of cement mortar. Secondly, DVC is used to calculate the strain of the sample and analyze the influence of the maximum principal strain on the sample damage. Thirdly, the localization factor is defined to describe the distribution of damage inside the sample.
18.How many cement mortar samples were analyzed, and what exactly did they consist of?
Response: In general, three samples are selected as a group for testing to observe the influence on the test results caused by sample differences. However, due to the substantial amount of CT image data and the expensive cost, only one sample was tested in this experiment. The sample is composed of hydrated cement, unhydrated cement, sand, pores, and microcracks. However, since it is difficult to distinguish cement base and sand by threshold value, sand is not labeled in Figure 4, and sand and cement are unified into the cement matrix.
19.How might the results of this research be used in fields where cement mortar is routinely employed?
Response: 1. Due to the influence of unhydrated cement on the cracking behavior of mortar, during the use of cement mortar, special attention should be paid to sufficient cement hydration. 2. The localized damage of cement mortar may develop from the inside out. In the application of cement mortar, surface damage should not only be taken as the failure criterion, but also the damage inside the material should be paid attention to.
In addition, we add a conclusion at the end of the revised manuscript as follows:
Through X-ray CT in-situ loading scanning and DVC calculation, the damage distribution inside the material can be defined, and the direction of damage development can be determined, which can provide a reference for the study of the macroscopic mechanical behavior of cement mortar materials. During the use of cement mortar, we should not only take surface damage as the criterion of failure but also pay attention to the damage inside the material.
It is appropriate to refer to lines 723-728 of the revised manuscript.
Meanwhile, the English writing has undergone significant revision from the English major teacher. Please read the revised manuscript since there have been many changes.
Author Response File: 
Author Response.docx
Reviewer 2 Report
This manuscript factually hardly extracts any exciting outcome from the highly potential modern tools. This lacks thorough analysis or any significant research content.
The manuscript also requires a major overhauling in terms of English technical writing. It contains several grammatical errors and incorrect/long complex sentences.
The manuscript also need to address the following technical issues ;
How many samples were tested to arrive at a meaningful conclusion taking into account the statistical variations ?
The compressive loading was obviously not very symmetric as the results show. Hence, the interpretation that damage initiated deeper internally and not from surface is not justified.
Loading as expected was applied from zero initial condition to a maximum 5000N in steps of 2000N, 2500, 3000, ..and so on. But, observations and results & discussions concentrated only on 3600N & 5000N. It would have been more exciting and useful to know at what load/stress actually damage initiated internally as detected using these highly potential modern tools.
Author Response
- This manuscript factually hardly extracts any exciting outcome from the highly potential modern tools. This lacks thorough analysis or any significant research content.
Response: We agree with the reviewer’s opinion. During the study, we realized that in-situ CT and DVC are modern tools with great potential. At the same time, we also came to a sobering realization about the size and complexity of the in-situ CT and DVC data. In order to produce more intriguing outcomes, the capacity for data processing and efficient information extraction needs to be increased.
- The manuscript also requires a major overhauling in terms of English technical writing. It contains several grammatical errors and incorrect/long complex sentences.
Response: We appreciate the reviewer’s advice. The English writing has undergone significant revision from the English major teacher. Please read the revised manuscript since there have been many changes.
- The manuscript also need to address the following technical issue:
（1）How many samples were tested to arrive at a meaningful conclusion taking into account the statistical variations?
Response: In general, three samples are selected as a group for testing to observe the influence on the test results caused by sample differences. However, due to the substantial amount of CT image data and the expensive cost, only one sample was tested in this experiment. However, in order to capture the variation of the sample under pressure, seven scans were conducted on one sample. The author is aware of this issue and will make an effort to choose as many samples as possible for testing in subsequent research.
（2）The compressive loading was obviously not very symmetric as the results show. Hence, the interpretation that damage initiated deeper internally and not from surface is not justified.
Response: Thanks for your insightful suggestions. The damage manifested itself from the inside out, as evidenced by the 2D and 3D CT images, and the DVC results supported this finding. The DVC results, however, also indicated that bias was present during the loading procedure. When we came to the conclusion that the damage began from within, we neglected to account for the impact of bias. Yet, our most recent CT in-situ test revealed that the damage in the concrete sample also occurred from the inside out. The contents of damage development in the paper following DVC analysis are described as follows: The damage develops from the inside out, which may be caused by the phenomenon of compression load or bias pressure. This is because the development of damage in this experiment may be influenced by bias pressure. In order to reach a reliable conclusion, more research is required.
It is appropriate to refer to lines 29–31 and lines 694–697 of the revised manuscript.
（3）Loading as expected was applied from zero initial condition to a maximum 5000N in steps of 2000N, 2500, 3000, ..and so on. But, observations and results &discussions concentrated only on 3600N & 5000N. It would have been more exciting and useful to know at what load/stress actually damage initiated internally as detected using these highly potential modern tools.
Response: This is wise counsel. For DVC computation, we only chose the two groups of CT scans that clearly changed. First of all, the force difference between the two nearby scan spots was minimal in order to record the sample damage change during CT scanning, and the CT images did not vary much. Second, there are a lot of scanning intervals. The amount of DVC data will be enormous if all scanning data is used to compute. Nonetheless, it could be possible to determine how the damage occurs if all of the scanning data is calculated. We'll use this recommendation in our further research.
Author Response File: Author Response.docx
Reviewer 3 Report
Dear Author,
Please refer to the comments attached in the file.
Kind regards
Reviever
Comments for author File: 
Comments.pdf
Author Response
Please see the attachment
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
After careful review, the revision is deemed suitable for publication.
Reviewer 3 Report
After reviewing the authors' statements and the corrections that have been addressed, I suggest that the article should be published in its current form.
