Time Evolution of CO₂ Diffusivity of Carbonated Concrete

Comments

Answer

The paper presented interesting research. The language of the paper is readable and clear. The paper needs only minor revision.

Thank you for the comment. We will make revised paper as your comments.

Please clarify what was the real porosity distribution in your model. What was the concrete mixtures in this reference?

As suggested in this paper, porosity was calculated by HYMOSTRUC with a function of the degree of cement hydration under the conditions of mixing proportion properties as shown in Table 1, and the  table was omitted due to the author's mistake, and I am sorry for this.

The concrete mixing proportion used in all calculations are summarized in Table 1.

Please clarify that the porosity in your calculation means open pores, or total porosity (physically and not by the water open or closed pores)?

The concept is already well explained in Figure 3. The porosity means the volumetric fraction of empty space regardless of the moisture content, and the pore is divided into a space filled with moisture and a space that is not filled with moisture. The space filled with moisture is also a function of the degree of hydration, and the apparent diffusivity is calculated by ignoring the moisture content as shown in Figure 3 (a).

As shown in Figure 3 (b), the effective diffusivity is calculated by considering the amount of pores through which gas can actually diffuse, excluding the space filled with moisture, as the effective pore.

All of these are described again in details below the description of Figure 3.

Please correct the Figure 5 and Figure 7. The beginnings of the curves are strange. 

When X axis is 0, Y axis becomes 1. I think the picture looks a little strange because the time on the X axial is expressed in log scale, and time is expressed in log scale to show the trend over the entire time by compressing duration.

CEB 1990 Model Code is not up to date. Please check also the fib Model Code 2010. 

Thanks for this advice.

As a result of checking, there is no description on gas diffusivity in FIB model 2010. Thus, I referred to the existing CEB 1990.

Comments

Answer

This paper is an original paper about the time evolution of the CO2 diffusivity of carbonated and noncarbonated concrete.

This paper has a good organization, but it is not always very clear.

In my opinion, the authors do not create a new model. The authors only modify a comprehensive model with multi-functional factors (created by themselves previously) to estimate the CO₂ diffusivity.

Even so, I consider that, although short, the analysis made in chapter 3 is interesting. However, the explanation in Chapter 2 must improve considerably to make it clear how the results of Chapter 3 were obtained.

To improve the quality of the paper, some mistakes must be corrected, and a lot of changes must be made.

Thank you for the review. As you pointed out, this paper was revised, expressing the comprehensive explanation in more details. Thanks for the conceptual question and detailed reviewing.

General comments:

1. Introduction

- The Introduction is well written and easy to read. The authors give a good explanation of some fundamental concepts.

- Line 23: Remove the expression “by nature” and explain the real reasons.

- Line 46: Explain these listed factors better.

- The explanation of Figure 1 in the text is a little confusing and some of the information in Figure 1 is not understandable.

- Throughout the text there are different font sizes.

- Authors should describe and analyze what already exists in the literature on the subject of the paper.

- Authors should explain in a summarized but clear way what they want with this paper and what methods they are going to use.

-       - Thanks for this comment.

-       - Line 23: The expression was deleted and sentence “because the hydration reaction of C₃S (3CaO∙SiO₂) and C₂S (2CaO∙SiO₂) abundantly in cement” was added.

-       - Line 46: The influencing factors are classified into 4 categories and have already been explained in rough scale. There are detailed sub-factors for each the influencing factor for this, but there are too many, so only four representatives of a large category are described. Thus, we would like to ask for understanding of the reviewers.

-       - Reflecting the points, Figure 1 was explained with more details.

- Font size was controlled.

-       - The thesis topic was described and mentioned in the literature. As described in previous version, theoretical modeling or experiment on this topic are very rare. In very limited paper, the diffusivity was regarded as arbitrary constant and this is a first paper to define the diffusivity as a function of microstructural properties and hydration.

-       - The desirable schematic diagram was added as shown in Figure 2. This figure can explain the concept and outline of this paper.

1. Material Modeling of CO2 Diffusivity for Concrete

- Line 84 says that concrete is a composite material composed of two phases. Wouldn't it make sense to consider three phases and analyze ITZ separately? I later saw that the author also suggests this consideration as future work. It is something that I think should have been done by now.

- What was it based on to define that those are the 4 main factors? This choice must be explained in detail.

- What do the different parameters of Equations 9 and 10 mean? What are the references of these Equations?

- Just reading Chapter 3, I realized that “Pore blocking due to moisture” will give rise to two diffusivities. Authors should explain this in Chapter 2.3.3.

- What values ​​(volumes, concentrations, etc.) were considered to obtain that “the CO₂ diffusivity of aggregate was regarded as a constant to be 1×10^-11cm²/s”?

- The results obtained in Chapter 3 cannot appear without any explanation. There are many values that we do not know how they were chosen. The equations in Chapter 2 alone are not enough to obtain the values in Chapter 3!

- Durability analysis considering ITZ is being carried out within a limited range. The effect of ITZ on the diffusivity of the entire concrete can be simply reflected in terms of dimensional concept, but it is still a very difficult topic. The topic on ITZ effect can be a full story of paper. That is why ITZ is outside scope of the study.

- The four factors consist of factors that affect the inflow of fluid and factors that affect the diffusivity of CO₂gas. The main factors were determined based on the knowledge about general diffusion theory below and author's research experience on durability for 20 years.(adsorption and diffusion in nanoporous materials, R.M.. Roque, CRC Press, 2006; Diffusion in solids, M.E. Glicksman, John wiley & sons, 2000) Besides these, we considered a lot of theoretical concepts on diffusivity of porous medium. Furthermore, the chloride diffusivity model obtained from the author's previous work, which is the basis of this study, was compared with experiment to prove its accuracy. In this paper, the concept was applied to CO₂ gas.

- The sources of Equations 9 and 10 are specified.

-The diffusivity of aggregate was taken from literature. According to the theory of diffusive transportation, diffusivity is an intrinsic value of a material that is not affected by volume and concentration. If reactivity between gas and concrete is ignored, the diffusivity is influenced by only microstructure of material.

- As pointed out, the definitions of apparent and effective diffusivities are mentioned again in Chapter 2.3.3.

- The results of Chapter 3 were completely obtained from the Chapter 2 and information of mixing properties of concrete as shown in Table 1. Because Chapter 2 was based on the author's previous work, all theoretical recipes was briefly summarized. However, the schematic diagram of the overall study was revised and presented in Figure 2 conceptually.

1. Results and Discussion

- This is by far the most interesting chapter in this paper. I like the content and the clear way it is presented. I only regret that it is not very extensive.

- I do not consider it is sufficient to say that “The result was similar to experiment of Martin et al.”. It is necessary to put more information about the mentioned papers.

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-        - As pointed out, this study is designed to obtain the CO₂ diffusivity, based on the previous work of the author, and that is why you think it is not extensive. But we would like to emphasize that this is a first paper to deal with the CO₂ diffusivity with time.

-        - Following your advice, more information about the study of Martin et al. was elaborated in the paper in terms of RH vs. diffusivity.

1. Conclusions

- This chapter presents few conclusions. On the one hand, this demonstrates the lack of abundance of innovative material present in this paper.

Even so, I believe that the conclusions must be improved.

- The authors humbly take this comment. However, as described in 1. Introduction, we would like to mention that there are very few papers dealing with the CO₂diffusivity, which is difficult to determine even by experiment. Thus, we think it is also meaningful to apply author’s previous work for estimating the CO₂diffusivity.

- As you pointed out, the content of the conclusion has been refined.

Specific comments / small mistakes:

- Line 33: The expression “By the way” is in a different font size.

- Line 40: “reinforced concrete” instead of “concrete”.

- Line 90 and Line 95: F(Do(S,T) or F(Do(T))?

- Line 113: F (Stmicro) instead of F(Do).

All comments was reflected.