Carbonation Process of Reinforced Concrete Beams under the Combined Effects of Fatigue Damage and Environmental Factors

Round 1

Reviewer 1 Report

Remark 1. In Figure 2 description “8@50” requires correction.

Remark 2. Figure 3a is illegible.

Remark 3. Table 4 and line 187: 1, the Authors use d, but it is not described.

Remark 4. Line 207, 208: “The average width of the main cracks was approximately 80-120 mm in the second and third stages” Is this correct information? This is an incredibly large crack width, if we consider the dimensions of the beam.

Remark 5. Line 232, 233, 234, 235: the Authors use n and N but it is not described.

Remark 6. Line 261, 262: “Fig. 9 shows that the porosity of the fatigue-damaged concrete specimens B2, B3, B4, and A2 …” In Figure 9, the results for beams B2, B3, B4, and A2 have not been marked.

Remark 7. Figure 9 requires clarification, e.g., there is no description of x, y, R and fatigue damage ratio.

Remark 8. Figure 16, 17, 18, 19, the Authors use D, but it is not described.

Remark 9. Editing and format correctness should be checked throughout the article.

Author Response

1. 1. The reviewer’s comment: In Figure 2 description “8@50” requires correction.

   The authors’ answer: The “8@50” has been revised (Page 4, Figure 2).

   2. The reviewer’s comment: Figure 3a is illegible

   The authors’ answer: The Figure 3a has been revised (Page 5, Figure 3).

   3. The reviewer’s comment: Table 4 and line 187: 1, the Authors use d, but it is not described.

   The authors’ answer: Table 4 and line 187 have been revised. (Page6, Table 4 and line 187).

   4. The reviewer’s comment: Line 207, 208: “The average width of the main cracks was approximately 80-120 mm in the second and third stages” Is this correct information? This is an incredibly large crack width, if we consider the dimensions of the beam.

   The authors’ answer: The sentence has been revised (Page7).

   5. The reviewer’s comment: Line 232, 233, 234, 235: the Authors use n and N but it is not described.

   The authors’ answer: n and N have been described (Page8).

   6. The reviewer’s comment: Line 261, 262: “Fig. 9 shows that the porosity of the fatigue-damaged concrete specimens B2, B3, B4, and A2 …” In Figure 9, the results for beams B2, B3, B4, and A2 have not been marked.

   The authors’ answer: In Figure 9, the results for beams B2, B3, B4, and A2 have not been marked.  (Page10, Figure 9).

   7. The reviewer’s comment: Figure 9 requires clarification, e.g., there is no description of x, y, R and fatigue damage ratio.

   The authors’ answer: The description of x, y, R and fatigue damage ratio have been added (Page10, Figure 9).

   8. The reviewer’s comment: Editing and format correctness should be checked throughout the article.

   The authors’ answer: Editing and format correctness are checked throughout the article.

Reviewer 2 Report

Title:  Carbonation Process of Reinforced Concrete Beams Under the Combined Effects of Fatigue Damage and Environmental Factors  

 

In this paper, the carbonation process of reinforced concrete (RC) beams considering the combined effect of fatigue load and environmental factors was investigated experimentally in an environmental simulation chamber based on meteorological environmental data.  It is found that fatigue damage did not have a significant effect on the most probable pore radius of the concrete, the total pore volume of the most probable pore notably increased.  The topic is quite interesting and authors should consider following comments for revision.

 

What is the relationship and difference between the artificial acceleration simulation carbonation environment and real service carbonation environment? How can the results from the accelerated test correlate to the real service condition?

 

The formatting and English writing are poor. Please further check the formatting and improve the English significantly.

 

There are a lot of materials and methods adopted to strength or retrofit the reinforced concrete structures after carbonation or deterioration. More literature and references on this perspective should be referred and included to give a more comprehensive evaluation and comparison in terms of pros and cons.

 

It is stated that “The results showed that the combined effects of fatigue damage and environmental factors accelerated the carbonation process of RC beams”. This is common sense. What is the novel findings from this study and how can the novel findings from current research work benefit the research and engineering practice of reinforced concrete? A clear evaluation is needed.

Author Response

1. The reviewer’s comment: What is the relationship and difference between the artificial acceleration simulation carbonation environment and real service carbonation environment? How can the results from the accelerated test correlate to the real service condition?

The authors’ answer: The test methods of concrete structure durability mainly include real test method and simulation test method. The simulation test method is to adopt various artificial methods to accelerate the degradation of the durability of concrete structure. When the required degree of degradation is reached, the durability test can be carried out. The advantages of this method are high controllability, short test period, low test cost and high test repeatability. The disadvantages of this method are that if the method is not properly selected, it may lead to a great difference between the deterioration development mechanism of concrete structure in the simulated test conditions and the real use environment, and there is a similar relationship between the simulated environment and the actual environment. Most of the existing carbonation simulation tests adopt the standard carbonation chamber test, that is, the carbonation of concrete under the conditions of constant temperature and humidity and carbon dioxide concentration. This kind of test method basically ignores the similarity between the simulated environment and the actual environment. The artificial climate environment simulation test method adopted in this paper refers to a test method that simulates the natural climate environment (sunshine, rain, temperature, humidity, CO₂, etc.) by artificial methods in the artificial climate laboratory, and at the same time strengthens the role of one or several control factors to accelerate the degradation of the durability of the test piece. It is a test method between the real test method and the carbonation chamber test method. It has the advantages of real results, good correlation and acceleration. It is also the focus, difficulty and development direction of the durability test of concrete structure. The results from the accelerated test correlate to the real service condition by a similar relationship formula, which has been published in other papers.

2. The reviewer’s comment: The formatting and English writing are poor. Please further check the formatting and improve the English significantly.

The authors’ answer: Editing and format correctness are checked throughout the article. The manuscript listed below was edited for proper English language, grammar, punctuation, spelling, and overall style by one or more of the highly qualified native English speaking editors at American Journal Experts (Certificate Verification Key: E965-C267-1318-A173-DEFP).

 

3. The reviewer’s comment: There are a lot of materials and methods adopted to strength or retrofit the reinforced concrete structures after carbonation or deterioration. More literature and references on this perspective should be referred and included to give a more comprehensive evaluation and comparison in terms of pros and cons.

The authors’ answer: The authors agree with the reviewer’s comment. A large amount of experimental and theoretical research has been performed to study the effects of material properties, pore size distribution, visible cracks, temperature, moisture content and static loads on concrete carbonation. But few researchers have attempted to characterize the combined effects of carbon dioxide concentration and fatigue damage on concrete carbonation. Experimentally investigating the carbonation process of RC beams under the combined effects of carbonation and fatigue damage in an environmental simulation chamber is more scarce. In this paper, the carbonation process of RC beams considering the combined effects of fatigue load and environmental factors was investigated experimentally in an environmental simulation chamber based on meteorological environmental data. The literatures and references on this perspective are is very few. The paper tries to give a more comprehensive evaluation and comparison in terms of pros and cons.

4. The reviewer’s comment: It is stated that “The results showed that the combined effects of fatigue damage and environmental factors accelerated the carbonation process of RC beams”. This is common sense. What is the novel findings from this study and how can the novel findings from current research work benefit the research and engineering practice of reinforced concrete? A clear evaluation is needed.

The authors’ answer: The sentence has been revised (Page1, abstract).

Reviewer 3 Report

This work is interesting and I appreciate a huge number of experiments which have been carried out. However, It’s not well written, language should be improved. The structure of paper should also rearrange in a better way. Few things below the authors should address before publication:

1. Table 2: should describe how to determine chemical and mineralogical compositions, and what is CSH2, is that gypsum?
2. MIP measurement: you have to detail how you prepare the samples because it would significantly affect the results. Fig. 3 gives no added values, should delete.
3. Carbonation test: The age of sample, initial conditions (saturation, RH) should be clearly described.
4. Fig. 8: what is pore distribution area?
5. Fig. 10: It seems that  the carbonation depth does not follow square root of time law, and different between 2 methods, i.e. phenolphthalein or pH, the authors should give clear explanations.
6. Modelling part: there are still a lot of issues here, the authors should consider the following items carefully:

• Why do you need eqs. 1-7, in the end you ignore the advective and electrical terms?
• Eq. 12: the diffusivity through the cracks also depends on the depth of cracks, not only width and length.
• Line 415: how do you know Dagg?
• Eq. 21 is controversial, as the carbonation depends mainly on the hydrated products, not the clinkers, so it depends on the hydration degree.
• Eq. 23: you should use a comprehensive water retention curve, as this would significantly affect the results.
• Eq. 28: are you should that relationship would work for your system?
• How do you account for the changes in porosity and saturation degree during carbonation?    

Author Response

1. The reviewer’s comment: Table 2: should describe how to determine chemical and mineralogical compositions, and what is CSH2, is that gypsum?

The authors’ answer: The chemical and mineralogical compositions are determined according to the actual bridge structure concrete. The CSH2 is an abbreviation of gypsum.

2. The reviewer’s comment: MIP measurement: you have to detail how you prepare the samples because it would significantly affect the results. Fig. 3 gives no added values, should delete.

The authors’ answer: The preparation detail of samples has been added (Page5).

3. The reviewer’s comment: Carbonation test: The age of sample, initial conditions (saturation, RH) should be clearly described.

The authors’ answer: The age of sample, initial conditions have been added (Page5).

4. The reviewer’s comment: Fig. 8: what is pore distribution area?

The authors’ answer: The pore distribution area has been revised (Page9, Figure8).

5. The reviewer’s comment: Fig. 10: It seems that the carbonation depth does not follow square root of time law, and different between 2 methods, i.e. phenolphthalein or pH, the authors should give clear explanations.

The authors’ answer: The different between two methods has been described (Page10). In the paper, the carbonation depth of the concrete was measured by the phenolphthalein reagent and the pH value method. The carbonation depth measured by the pH meter and phenolphthalein reagent titration is not exactly the same, but the overall development trend is similar. Like the existing literature, the carbonation depth of fatigue-damaged concrete still follows square root of time law by the phenolphthalein reagent titration method. The carbonation depth measured by phenolphthalein reagent titration is larger than that measured by the pH meter, and the ratio of the two methods is approximately 0.8-1.0. The graphs show that the carbonation process exhibits a two-stage trend.

6. The reviewer’s comment: Why do you need eqs.1-7, in the end you ignore the advective and electrical terms?

The authors’ answer: Based on the law of mass conservation and the law of mass transfer, the transfer process of CO₂ can be expressed as a general carbonization equation considering the combination of various driving mechanisms (diffusion, convection, binding and electromigration) by Eqs.1-7.

7. The reviewer’s comment: Eq. 12: the diffusivity through the cracks also depends on the depth of cracks, not only width and length.

The authors’ answer: The authors agree with the reviewer’s comment. It is assumed that the effective diffusion coefficient of carbon dioxide within the thickness of protective layer concrete is the same along the direction of crack depth. The main research object of this paper is the carbonation of concrete in the protective layer of RC structure. The test results shown that as the number of fatigue loading cycles increases, the fatigue damage of RC beams generally exhibits a three-stage rule. Fatigue loading will cause an increase in concrete porosity and cracking in the pure bending zone of RC beams. The maximum crack width in the first two stages of fatigue damage is between 0 and 0.25 mm, and the spacing of the main cracks is relatively stable. Based on the model of crack distribution in an RC beam and the theory of a two-hole medium, a new model is proposed to describe the carbon dioxide transport process in damaged, cracked RC beams, as shown in Fig. 13.

8. The reviewer’s comment: Line 415: how do you know D_agg?

The authors’ answer: We set the diffusion coefficient of carbon dioxide in aggregate D_agg=0 in the paper.

9. The reviewer’s comment: Eq. 21 is controversial, as the carbonation depends mainly on the hydrated products, not the clinkers, so it depends on the hydration degree.

The authors’ answer: The authors agree with the reviewer’s comment that the carbonation depends mainly on the hydrated products. The mineral composition of ordinary portland cement mainly includes clinkers of C₃S, C₂S, C₃A, and C₄AF. The hydration degree of four kinds of clinker is related to its hydration reaction rate, temperature and humidity, etc. The products of carbonation reaction in concrete after hydration mainly include CH, CSH, and un-hydrated of C₃S, C₂S. The four products of carbonation reaction are considered in the total carbonation reaction rate of concrete in the Eq.21from references.

10. The reviewer’s comment: Eq. 23: you should use a comprehensive water retention curve, as this would significantly affect the results.

The authors’ answer: The bilinear model of water retention curve is used in the paper.

11. The reviewer’s comment: Eq. 28: are you should that relationship would work for your system?

The authors’ answer: The reference [27] obtained the relationship between the concentration of CO₂ gas and the pH value of pore solution according to the ionization and dissolution equilibrium process of CO₂ in pore solution. Through this relationship, the pH value of pore solution can be estimated directly from the concentration distribution of CO₂, simplifying the calculation steps of pH by Eq.28.

12. The reviewer’s comment: How do you account for the changes in porosity and saturation degree during carbonation?    

The authors’ answer: In the carbonation process, carbon dioxide gas dissolves in pore solution to form carbonic acid, which reacts with hydrated product calcium hydroxide and hydrated calcium silicate to form calcium carbonate. The volume of the product is larger than the reactant, resulting in the decrease of concrete porosity, carbonation rate and transmission capacity. But compared with carbonation, the pore structure changes caused by fatigue damage are more obvious, which is also the focus of this paper.

Round 2

Reviewer 2 Report

The authors need to give a direct answer towards the novelty and new findings of this research work.

Author Response

The reviewer’s comment:The authors need to give a direct answer towards the novelty and new findings of this research work.

The authors’ answer: The novelty and new findings of this research work are as following:

(1)The carbonation process of reinforced concrete (RC) beams considering the combined effect of fatigue load and environmental factors was investigated experimentally in an environmental simulation chamber based on meteorological environmental data. The existing carbonization test mainly adopts the standard rapid carbonization test method without considering the environmental similarity.

(2) Through mercury intrusion test, the influence of fatigue loading on the change of concrete pore structure is tested and analyzed. The result shows that the porosity of concrete notably increases with increasing fatigue damage. The distribution of pore size will change with the development of fatigue damage, among which the harmful mesopores and macropores exhibit the most notable changes. The fatigue damage has no obvious effect on the peak pore diameter of concrete; however, the fatigue damage will substantially increase the peak strength. Compared with the macroscopic index in the existing literatures, such as beam stiffness and material residual strain, the change in concrete pore structure can more directly and accurately characterize the effect of fatigue damage on the transport characteristics of concrete materials.

(3) The carbonation numerical model (CNM) considering medium transport and fatigue damage characteristics of RC beam was proposed to simulate the carbonation process of RC beams. Based on the mechanism of carbon dioxide transport, the carbonation model is scarce considering the effect of pore structure change and crack caused by fatigue loading on carbon dioxide transport

(4)The results showed that both the carbonation depth and fatigue damage exhibit a three-stage development law. The depth and rate of carbonation are related to concrete pores and macroscopic cracks. In the carbonation analysis of fatigue-damaged RC beams, the changes in both the pore structure and fatigue cracks caused by repeated fatigue loading on carbonation should be considered.

Reviewer 3 Report

The authors have only addressed half of the reviewer’s comments. The left comment numbers 1, 6, 10, 12  should be reconsidered. In addition, following issues should be addressed:

1. Lines 153-156. Sentences are repeated, and how are you sure that the samples are completely dried after 21 days of drying?
2. Lines 304-305: if you want to conclude that the carbonation depth evolution still follows square root of time, then you should change the X-axis of Figs. 9, 10, 16.

Author Response

1.The reviewer’s comment: The authors have only addressed half of the reviewer’s comments. The left comment numbers 1, 6, 10, 12  should be reconsidered.

(1). The reviewer’s comment: Table 2: should describe how to determine chemical and mineralogical compositions, and what is CSH2, is that gypsum?

The authors’ answer: The chemical and mineralogical compositions of concrete are determined according to the Wen Tanhe Bridge in china. The CSH2 is an abbreviation of gypsum.

(6). The reviewer’s comment: Why do you need eqs.1-7, in the end you ignore the advective and electrical terms?

The authors’ answer: The author wants to deduce the general mass transfer equation considering the combination of various driving mechanisms (diffusion, convection, binding and electromigration) by Eqs.1-7 based on the law of mass conservation and the law of mass transfer. Then, the combination item of various driving mechanisms is selected according to the demand and the different media transmission. In the paper, the advective and electrical terms is ignored in the process of CO₂ transport in concrete.

(10). The reviewer’s comment: Eq. 23: you should use a comprehensive water retention curve, as this would significantly affect the results.

The authors’ answer: the authors agreed with the reviewer’s comment that the water retention curve would affect the process of CO₂ transport in concrete. In the paper, the bilinear model of water retention curve is used in the existing literatures.

(12). The reviewer’s comment: How do you account for the changes in porosity and saturation degree during carbonation?    

The authors’ answer: In the carbonation process, carbon dioxide gas dissolves in pore solution to form carbonic acid, which reacts with hydrated product calcium hydroxide and hydrated calcium silicate to form calcium carbonate. The volume of the product is larger than the reactant, resulting in the decrease of concrete porosity, carbonation rate and transmission capacity. But compared with carbonation, the pore structure changes caused by fatigue damage are more obvious, which is also the focus of this paper. The change of saturation degree in concrete is related to the external and internal environment of concrete and carbonation reaction. Carbonation process is a relatively slow process, the changes in saturation degree during carbonation is ignored.

2.The reviewer’s comment: Lines 153-156. Sentences are repeated, and how are you sure that the samples are completely dried after 21 days of drying?

The authors’ answer: Sentences (Lines 153-156) have been repeated.

3.The reviewer’s comment: Lines 304-305: if you want to conclude that the carbonation depth evolution still follows square root of time, then you should change the X-axis of Figs. 9, 10, 16.

The authors’ answer: The phenolphthalein reagent titration method roughly estimates the depth of complete carbonation, while pH meter method may determine the complete carbonation area, partial carbonation area and non carbonation area in the concrete, revealing the carbonation process of concrete. The results by the phenolphthalein reagent titration method were just a correction for the results by pH meter method. The results analysis is mainly based on the results by pH meter method. The carbonation depth measured by the pH meter and phenolphthalein reagent titration is not exactly the same. The carbonation depth evolution by the pH mete do not full follow square root of time. Then the X-axis of Figs. 9, 10, 16 shows carbonation times.