Fatigue Behaviour of Concrete Using Siderurgical Aggregates

Round 1

Reviewer 1 Report

Dear Authors,

I must appreciate the quality of research in particular area. My comments/suggestions are:

1. The introduction and materials & methods section is well structured and provides a clear framework for the research. 2. It would be beneficial to provide a brief introduction to the experimental setup and limits before getting into the fatigue test to add novelty to the research. 3. Please consider generalizing or making more specific the repeated use of subheadings. 4. It would be useful to provide information on any numerical studies that have been done previously related to the resonance condition. 5. It would be beneficial to provide an explanation for the irregular spikes and declines of graphical data in Figure 9. 6. The research would be strengthened by including a comparison section to existing literature and benchmarks. 7. The overall work is done well and provides a clear basis for the research. 8. The focus on dynamic load scenarios is appropriate for the research. 9. The research could be further improved with the introduction of amendments to the traditional setup already stated in Figure 3.

Regards,

Author Response

Responses to the reviewers’ comments to the Manuscript Number: applsci-2182102

Fatigue behaviour of concrete using siderurgical aggregates

Dear editor,

We greatly appreciate the opportunity you give us to improve the paper with the valuable comments of the reviewers.

Below you can find the detailed comments and modifications.

Best regards,

The authors

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Reviewer 1 (Changes in blue)

 Dear Authors,

I must appreciate the quality of research in particular area. My comments/suggestions are:

1. The introduction and materials & methods section is well structured and provides a clear framework for the research.

The authors would like to thank the reviewer for considering that this research carried out is of quality. On the other hand, thank you for the recognition of the work done both in the introduction and in the methodology. The authors have always tried to reflect the current state of the art and to describe as simply as possible the methodology of the tests.

2. It would be beneficial to provide a brief introduction to the experimental setup and limits before getting into the fatigue test to add novelty to the research.

In accordance with the reviewer's recommendations, the authors have included as an introduction to the section dedicated to the experimental setup, a brief summary about it and about the existing limitations (lines 146-150):

“This methodology allows the determination of the fatigue limit using very few specimens in record time, taking measurements of the resonance frequency and strain of the specimens, in addition to the surface temperature of the specimens. The limitation is that the maximum capacity of the press is almost reached, having to use smaller specimens when the characteristic resistances of the concrete are higher than those of the SLC mix.”

3. Please consider generalizing or making more specific the repeated use of subheadings.

If the authors understand well, it is recommended that both in the methodology section and in the results section, the subtitles have the same name. This has been corrected and now the different sections are listed as shown:

1. Introduction
2. Methodology
   • Materials and mix proportions
   • Physical and mechanical characterization of concrete
   • Fatigue test
3. Results
   • Physical and mechanical properties of concrete
   • Fatigue test
4. Conclusions

4. It would be useful to provide information on any numerical studies that have been done previously related to the resonance condition.

In accordance with the reviewer's recommendation, and to complete the mechanical resonance condition from a theoretical point of view, information has been added in the introduction to complete this concept (Lines 81-84).

“Sainz-Aja et al. [25] recommend frequency range for concrete fatigue testing between 1 and 15 Hz. Low frequency (<1 Hz) reduces the fatigue limit because it increases the test time, creep damage. Comparing moderate frequency (10 Hz) and tests at very high frequency (90 Hz) it has been shown that temperature increases above 65 °C can significantly magnify creep damage.”

5. It would be beneficial to provide an explanation for the irregular spikes and declines of graphical data in Figure 9.

Following the reviewer's recommendation, an explanation of why irregular peaks have been generated in the strain of one of the specimens in the first load steps has been added to the text (lines 277-279):

“Irregular peaks generated by the strain of Specimen 1 of the LLC mix can be observed. This effect is due to noise in one of the strain gauges attached to the specimen during the first 3 load steps, after which it has disappeared”.

6. The research would be strengthened by including a comparison section to existing literature and benchmarks.

The authors consider it very appropriate to make a comparison with recycled concrete in terms of fatigue limit. The literature in this regard is very scarce, since the method used is novel in concrete. However, a comparison has been made in terms of fatigue limit with recycled concrete from aggregates from ballast and railway sleepers. The comparison has been included in the text (Lines 298-301):

“Making a comparison with the high-frequency fatigue limit values obtained by Sainz-Aja et al. [26], it is observed that the behavior of the SLC mix is similar to that of the RC-M mix (ballast aggregates with a small replacement of recycled sleeper aggregate) for either of the two criteria used.”

7. The overall work is done well and provides a clear basis for the research.

The authors appreciate that the reviewer recognizes the good work done by the group, which encourages the authors in future research that tries to elucidate the fatigue behavior of these concretes.

8. The focus on dynamic load scenarios is appropriate for the research.

The authors appreciate and agree this thought. The following sentence has been included in order to improve this issue (Lines 95-98)

“The most common scenarios in which today it can be detected fatigue failures are slender structures. It is increasingly common to reduce thicknesses, due to the increase in the resistance of materials, as is the case of skyscrapers, railway sleepers and bridges. In these cases, it is considered convenient to take into account possible fatigue damage.”

9. The research could be further improved with the introduction of amendments to the traditional setup already stated in Figure 3.

First of all and to improve clarity, we have changed the text and typography of the parts that make up the experimental setup of the fatigue test (Figure 3). Regarding the modifications of the equipment to adapt them to the test, the following considerations have been included in the text (Lines 178-182).

“It is worth mentioning that in order to carry out the test, the traditional setup of the servo-hydraulic press had to be modified. This machine usually works in tension (E.g. tests on steel cords and bars) so two rigid mechanized steel bases have been bolted to the upper and lower jaws and a ball joint has been placed between the upper plate and the specimen.”

Figure 3. Preparation of the specimens and experimental setup used.

Author Response File: Author Response.docx

Reviewer 2 Report

This paper compares the high-frequency fatigue behavior of siderurgical aggregate concrete with limestone concrete through the Locati approach. The result shows that concrete with siderurgical aggregates has a higher fatigue limit in absolute terms both for the criterion of reduction of the resonance frequency and for the criterion of 80% of the stress range (11% and 13% higher, respectively). Thus, siderurgical aggregate concrete should be taken into consideration for more application areas, such as foundations or superstructure components for railroads. Overall, the manuscript is well-illustrated and logical and has a certain significance in the field. Some revision is recommended and the details are presented as follows.

 1. In page 3, line 98, “The development of the research is based on the comparison of a concrete with siderurgical aggregates (SA) with a conventional concrete with limestone aggregates (NA)”. Generally speaking, NA represents the term “natural aggregate” in most studies. To avoid misunderstanding, it is advisable to standardize the terminology. 

2. In Figure 1, Figure 4~10, as a suggestion, different line styles could be used and different textures could be added to the bar charts to ensure that black and white printing is still legible.

3. In Figure 2, To make the figure clearer to read, it is suggested that the format of the Y axis label be modified.

4. In page 7, line 203, “The SA s an improved ITZ, favoured by the shape of the siderurgical aggregates”. The sentence reads weirdly, and it appears that several words are missing. Please consider rechecking and revising it.

5. In page 11, line 298, “however there is an evident relationship between strain and temperature”. The temperature and average strain trends of SLC and LLC are comparable in Figure 10. The conclusion is less convincing since SAs have a greater coefficient of thermal conductivity. To guarantee that this phenomenon is not a unique outcome of the experiment, is there relevant literature that likewise demonstrates the correlation between these two factors? 

6. The author may consider comparing the fatigue performance of siderurgical aggregate concrete tested in this paper with that of recycled concrete in different application fields in the existing literature, and discuss whether it is suitable to the current engineering practice, such as: https://doi.org/10.1007/978-3-662-53987-3 ;   doi: 10.1111/j.1460-2695.2012.01673.x

Author Response

Responses to the reviewers’ comments to the Manuscript Number: applsci-2182102

Fatigue behaviour of concrete using siderurgical aggregates

Dear editor,

We greatly appreciate the opportunity you give us to improve the paper with the valuable comments of the reviewers.

Below you can find the detailed comments and modifications.

Best regards,

The authors

----

Reviewer 2 (Changes in brown)

This paper compares the high-frequency fatigue behavior of siderurgical aggregate concrete with limestone concrete through the Locati approach. The result shows that concrete with siderurgical aggregates has a higher fatigue limit in absolute terms both for the criterion of reduction of the resonance frequency and for the criterion of 80% of the stress range (11% and 13% higher, respectively). Thus, siderurgical aggregate concrete should be taken into consideration for more application areas, such as foundations or superstructure components for railroads. Overall, the manuscript is well-illustrated and logical and has a certain significance in the field. Some revision is recommended and the details are presented as follows.

The authors thank reviewer 2 for the positive analysis of this work, both of its organization and its significance in the field that concerns us. The following are the changes made based on the reviewer's recommendations:

1. In page 3, line 98, “The development of the research is based on the comparison of a concrete with siderurgical aggregates (SA) with a conventional concrete with limestone aggregates (NA)”. Generally speaking, NA represents the term “natural aggregate” in most studies. To avoid misunderstanding, it is advisable to standardize the terminology.

In accordance with the reviewer's recommendation, and to avoid any confusion, it has been specified that the term NA corresponds to “natural aggregates”, specifying that in this case they are limestone aggregates (lines 99-101):

“The development of the research is based on the comparison of a concrete with siderurgical aggregates (SA) with a conventional concrete with natural aggregates (NA), which are limestone aggregates, to verify the goodness of siderurgical aggregates.”

2. In Figure 1, Figure 4~10, as a suggestion, different line styles could be used and different textures could be added to the bar charts to ensure that black and white printing is still legible.

The authors agree with the reviewer's suggestions, and have changed the pattern of the lines in Figure 1 to improve their readability when the document is printed in black and white, for example (Line 121):

Figure 1. Grading distribution of the designed mixes.

3. In Figure 2, To make the figure clearer to read, it is suggested that the format of the Y axis label be modified.

The authors apologize for the typo that caused the Y axis label to not display properly. Fixed this bug, improving the readability of the graph (Line 157):

Figure 2. Locati test scheme.

4. In page 7, line 203, “The SA s an improved ITZ, favoured by the shape of the siderurgical aggregates”. The sentence reads weirdly, and it appears that several words are missing. Please consider rechecking and revising it.

Indeed, for some reason the sentence is unfinished because there are missing words. This error in the manuscript has been corrected and now the sentence has remained as shown (Lines 209-210):

“The SA concretes show an improved ITZ, favoured by the shape of the siderurgical aggregates, which makes the aggregate interlock with the paste and increases the friction between the two [32].”

5. In page 11, line 298, “however there is an evident relationship between strain and temperature”. The temperature and average strain trends of SLC and LLC are comparable in Figure 10. The conclusion is less convincing since SAs have a greater coefficient of thermal conductivity. To guarantee that this phenomenon is not a unique outcome of the experiment, is there relevant literature that likewise demonstrates the correlation between these two factors?

The authors consider that there is an evident relationship between both factors because the results demonstrate it. However, it is true that siderurgical aggregates have a higher coefficient of thermal conductivity, due to the ferrous nature of the aggregates. This can favor the aforementioned increase in temperature, as already expressed in the same paragraph. Quantifying both effects has been impossible and after reviewing the literature no correlation has been found in this regard, perhaps because there are no experiences similar to the one developed.

6. The author may consider comparing the fatigue performance of siderurgical aggregate concrete tested in this paper with that of recycled concrete in different application fields in the existing literature, and discuss whether it is suitable to the current engineering practice, such as: https://doi.org/10.1007/978-3-662-53987-3 ; doi: 10.1111/j.1460-2695.2012.01673.x

The authors consider it very appropriate to make a comparison with recycled concrete in terms of fatigue limit. The literature in this regard is very scarce, since the method used is novel in concrete. However, a comparison has been made in terms of fatigue limit with recycled concrete from aggregates from ballast and railway sleepers. The comparison has been included in the text (Lines 298-301):

“Making a comparison with the high-frequency fatigue limit values obtained by Sainz-Aja et al. [26], it is observed that the behavior of the SLC mix is similar to that of the RC-M mix (ballast aggregates with a small replacement of recycled sleeper aggregate) for either of the two criteria used.”

Author Response File: Author Response.docx