Influence of Recycling Processes on Properties of Fine Recycled Concrete Aggregates (FRCA): An Overview

Round 1

Reviewer 1 Report

Thank you for submitting this article, I enjoyed reading about your work.  I appreciate the effort it took to incorporate the data from so many papers into a comprehensive review.  

Although this paper is technically sound, the quality of English writing needs to be improved prior to publication. An English Editor is highly suggested.  I offer a few examples of some issues with grammar, verb tense, and spelling below, but I simply stopped taking note after a few pages. 

From a technical standpoint, I think the manuscript needs to acknowledge that in the vast majority of situations, FRA is considered a waste product.  The primary goal of most, if not all, crushing operations is the production of coarse RCA (CRA in your paper).  Most folks are trying to minimize the fine fraction of material produced, since it is difficult to use in many applications, and the limitations on most specifications on fine material makes it problematic to produce.  That being said, we still need to understand the properties of FRA and its performance so that beneficial reuses can be found.  Please acknowledge somewhere early in the paper that FRA is often a secondary product, not the primary product (CRA).

Abstract - a few suggestions 

line 14 - replace "to obtain" with "that produce"

line 15 - to perform a

granulometry is not a commonly used term in some areas, including my own.  In fact I have not heard that term.  Perhaps use "Particle size analysis" or "sieve analysis"?

line 20 - jaw crusher and impact crusher (singular)

Introduction - the first sentence is a run-on sentence and should be clarified.

line 35 - damage, not damages.  alternation of

line 40 - delete the

line 46 - replace usual with typical

line 56 - "transformation of CDW into FRCA with an adequate recycling process."  here is a good point to acknowledge that the primary product is typically coarse RCA.  Fine RCA is always a co-product, and is the subject of this study.  

Also, it would be good to either use FRA or FRCA, but not both.  Be consistent

line 63 - research aimed to

line 79 - comminution is not a commonly used word

line 80 - release the phases of CDW - do you mean separate out different components?

From this point in the manuscript on, I did not make note of issues with English - please retain the services of an English Editor to improve this.

Mechanical process - some diagrams or photos would help enhance this.  Also, Snyder et al. 2018 provides some details on this process, including strengths and challenges of each, that could be used to enhance this portion.  https://intrans.iastate.edu/app/uploads/2018/09/RCA_practioner_guide_w_cvr.pdf

Section 5.1 - line 238 - 0 to 4 (units?)

Figure 1 - I assume the black line bands on these diagrams are ASTM C33 limits?  Please label on the figures.

For Figures 2, 3, and 5 - when a range is not provided, does this mean that the average value reported is actually a single value?  Please clarify somehow the number of values used to produce the range (or non-range, just average) of each bar in these graphs.  provide n=? for each bar.

Throughout the manuscript, pieces of equipment and other things that are not proper nouns are capitalized - Rotor Crusher and Ball Mill should just be rotor crusher and ball mill.

My comments on the English are listed above.  Please retain the services of an English Editor.  The technical content is sound, but the English quality detracts from the technical quality.

Author Response

We are grateful for the reviewers' effort in analysing and understanding the manuscript. We truly believe the reviewer’s comments helped improve the manuscript, especially the text writing. The authors apologise for the errors, and it is grateful for the text corrections.

The individual Reviewers’ comments are addressed below, where we respond to each of them. The line numbers refer to the final revised manuscript, and we have also attached an annotated manuscript to highlight the changes we have made to the original manuscript.

______________________________________________________________________________

COMMENTS AND SUGGESTIONS FOR AUTHORS – REVIEWER 1

Suggestion 1:

Although this paper is technically sound, the quality of English writing needs to be improved before publication. An English Editor is highly suggested. I offer a few examples of some issues with grammar, verb tense, and spelling below, but I simply stopped taking notes after a few pages.

Response 1:

Thank you for the suggestion. A general revision of the English writing was carried out to improve the quality and comprehension of the text.

Suggestion 2:

From a technical standpoint, I think the manuscript needs to acknowledge that in the vast majority of situations, FRA is considered a waste product. The primary goal of most, if not all, crushing operations is the production of coarse RCA (CRA in your paper). Most folks are trying to minimize the fine fraction of material produced since it is difficult to use in many applications, and the limitations on most specifications on fine material make it problematic to produce. That being said, we still need to understand the properties of FRA and its performance so that beneficial reuses can be found. Please acknowledge somewhere early in the paper that FRA is often a secondary product, not the primary product (CRA).

Response 2:

Thank you for the comment. Paragraph 3, line 46, was modified to clarify this point.

“The recycling plants primarily aim to produce Coarse Recycled Concrete Aggregates (CRCA) which are easier to reintroduce into the market as raw material for concrete manufacturing. As regards the Fine Recycled Concrete Aggregates (FRCA), they are typically generated involuntarily in recycling plants [6], and their use is more prevalent in countries that are pioneers in recycling, wherefore few studies focus on the production of the fine fraction when compared to the production of the coarse fraction.

Suggestion 3: line 14 - replace "to obtain" with "that produce"

Response 3: The word was replaced.

Suggestion 4: line 15 - to perform a.

Response 4: The word was replaced.

Suggestion 5: granulometry is not a commonly used term in some areas, including my own. I have not heard that term. Perhaps use "Particle size analysis" or "sieve analysis"?

Response 5: Thanks for the suggestion. Since this term is not widely used, we replaced it with the term “particle size distribution (PSD)” from EN 933-1.

Suggestion 6: line 20 - jaw crusher and impact crusher (singular)

Response 6: The words were replaced.

Suggestion 7: Introduction - the first sentence is run-on and should be clarified.

Response 7: The first paragraph was rewritten to clarify the idea we want to convey.

“Construction is one of the largest contributors to global greenhouse gas (GHG) emissions. The demand for raw materials to produce building materials is expected to double by 2060 leading to an increase of GHG [1]. Being concrete the most used building material, rethinking its production is nowadays a priority to lower the environmental impacts. To this end, the choice of raw materials plays an essential role.”

Suggestion 8: line 35 - damage, not damages. alternation of.

Response 8: The words were replaced.

Suggestion 9: line 40 - delete the

Response 9: The word was deleted.

Suggestion 10: line 46 - replace usual with typical

Response 10: The word was replaced.

Suggestion 11: line 56 - "transformation of CDW into FRCA with an adequate recycling process."  here is a good point to acknowledge that the primary product is typically coarse RCA.  Fine RCA is always a co-product and is the subject of this study     

Response 11: This point was addressed in paragraph 3, according to the second comment.

Suggestion 12: Also, it would be good to either use FRA or FRCA, but not both. Be consistent

Response 12: The abbreviation was replaced.

Suggestion 13: research aimed to

Response 13: The sentence was corrected.

Suggestion 14: line 79 - comminution is not a commonly used word

Response 14: Thank you for the suggestion. In this sentence, the most correct word is recycling.

Suggestion 15: line 80 - release the phases of CDW - do you mean separate different components?

Response 15: No, I meant reducing particle size. I modified the sentence to make it clearer.

Suggestion 16: Mechanical process - some diagrams or photos would help enhance this.  Also, Snyder et al. 2018 provide some details on this process, including the strengths and challenges of each, that could be used to enhance this portion.

https://intrans.iastate.edu/app/uploads/2018/09/RCA_practioner_guide_w_cvr.pdf

Response 16: We agree that diagrams and photos better illustrate the technologies. However, we chose not to include it because the idea of this topic is just to present the reader with an overview of the technologies. We focus on the article's main objective, which is to correlate the acquisition processes with the properties of the aggregates.

Suggestion 17: Section 5.1 - line 238 - 0 to 4 (units?)

Response 17: Thank you for the suggestion. The unit is mm. The sentence was corrected.

Suggestion 18: Figure 1 - I assume the black line bands on these diagrams are ASTM C33 limits?  Please label the figures.

Response 18: Yes, you are correct, they are the limits of ASTM C33. It is indicated in the legend, first item (Low/Upper limit ASTM C33)

Suggestion 19: For Figures 2, 3, and 5 - when a range is not provided, does this mean that the average value reported is a single value?  Please clarify somehow the number of values used to produce the range (or non-range, just average) of each bar in these graphs. provide n=? for each bar.

Response 19: Yes, thank you for paying attention to this, it was corrected, it is just a value and not the average. I didn't put the max and min values ​​so the figure wouldn't be polluted. However, it is possible to identify these extremes with the grid lines on the graph. I entered the value of n next to each bar,

Suggestion 20: Throughout the manuscript, pieces of equipment and other things that are not proper nouns are capitalized - Rotor Crusher and Ball Mill should just be rotor crusher and ball mill.

Response 20: This word was reviewed throughout the entire manuscript.

Author Response File: Author Response.docx

Reviewer 2 Report

1 In Table 2, the author compared the jaw crusher, impact crusher and cone crusher respectively in respect of efficiency, aggregate quality, fines content, production cost, energy consumption, and wearing, but did not indicate the basis of comparison.

2 Line 207: In "Recycling Processes", the advantages of the environmental impact of FRCA's production are not clearly expressed, and the author is requested to collect detailed data for further clarification.

3 In "Granulometry", Please further explain the difference between LAB and CW particle sizes in equivalent recycling processes.

4 Line 360: Please explain further why "For the LAB FRCA, water absorption decreased as the number of processes increased, however, for the CW FRCA is inverse".

5 In Fig. 2, the water absorption of CW FRCA processed by the rotary drum mill can reach as low as 2.5%, which is lower than that of all other recycling processed FRCA, please explain further the reason.

6 "For FRCA LAB in the 0.063 mm to 4 mm range, oven-dry density values increase as the number of Jaw Crusher crushing repetitions is raised", but in Fig. 3 the oven-dry density value after Jaw Crusher 4x processing is lower than Crusher 3x, please explain.

7 In Figure 4, the R² values of the fitted equations for the water absorption and oven-dry density of FRCA are too low to indicate a correlation between the water absorption and density of FRCA. Similarly, in Fig. 6, the value of R² is too low to indicate a correlation between water absorption and adhered paste of FRCA.

8 Please further refine the conclusions and suggest prospects for FRCA applications.

Minor editing of English language required

Author Response

We are grateful for the reviewers' efforts in analysing and understanding the manuscript. We truly believe the reviewer’s comments helped improve the manuscript, especially the text writing. The authors apologise for the errors, and it is grateful for the text corrections.

The individual reviewers’ comments are addressed below, and we respond to each of them. The line numbers refer to the final revised manuscript, and we have also attached an annotated manuscript to highlight the changes we have made to the original manuscript.

______________________________________________________________________________

COMMENTS AND SUGGESTIONS FOR AUTHORS – REVIEWER 2

Suggestion 1:

In Table 2, the author compared the jaw crusher, impact crusher and cone crusher respectively in respect of efficiency, aggregate quality, fines content, production cost, energy consumption, and wearing, but did not indicate the basis of comparison.

Response 1: Thank you for your attention to this issue. The original table does not contain these criteria, but the reference was consulted again, and a column was added with the basis of comparison for each characteristic.

Reference: 83. Alaejos, P.; de Juan, M.S.; Rueda, J.; Drummond, R.; Valero, I. (2013). Quality Assurance of Recycled Aggregates. In: Vázquez, E. (eds) Progress of Recycling in the Built Environment. RILEM State-of-the-Art Reports, Volume 8, p. 229-273. Springer, Dordrecht. Doi: https://doi.org/10.1007/978-94-007-4908-5_6.

Suggestion 2:

Line 207: In "Recycling Processes", the advantages of the environmental impact of FRCA's production are not clearly expressed, and the author is requested to collect detailed data for further clarification.

Response 2: Thank you for the suggestion. This topic was not treated in more depth because we did not want to escape the article's objective, which is to perform an evaluation focused on the influence of processes on the properties of aggregates. However, we agree that the environmental impacts of FRCA production should be evaluated globally because they are related to aspects such as the reduction of GHG emissions and fuel consumption due to the transportation of natural raw materials and also the costs due to the management and disposal of CDW in large urban centres. We then add a paragraph dealing with this subject, which begins at line 225.

“When assessing the environmental impacts related to the production of FRCA it is important to consider the study area and the indirect costs with the disposal of CDW throughout the entire lifecycle. Studies indicate that crushing natural or recycled aggregates costs about the same (U$D 2-3/t) [6]. However, the use of CDW is advantageous because it can generate incoming between U$D 25-30/t for acceptance and disposal of these materials, while the costs for limestone/granite are around U$D 1-1.5/t [6]. In regions where the availability of natural raw materials is scarce, such as in large urban centres or very isolated regions, the distance to the nearest deposit can make transportation very ex-pensive and logistically difficult and the use of CDW aggregates can be more sustainable.”

Suggestion 3: In "Granulometry", Please further explain the difference between LAB and CW particle sizes in equivalent recycling processes.

Response 3: The explanation was made in section 5.1.

" Figures 1 (a) e (b) show a wide array of processes with varying stages, repetitions, and techniques. The variation in particle size distributions is more pronounced for aggregates originating from concrete waste (CW) compared to those from the laboratory (LAB), which is reasonable as CW aggregates are more heterogeneous. "

Suggestion 4: Line 360: Please explain further why "For the LAB FRCA, water absorption decreased as the number of processes increased, however, for the CW FRCA is inverse".

Response 4: Line 325 - The text has been rewritten to clarify the idea.

“For the LAB FRCA, water absorption decreased as the number of processes increased. However, for the CW FRCA, this behaviour is contrary, probably because the water ab-sorption value is more dependent on the source material, which in this case is more heterogeneous, than on the number of steps and characteristics of the process. For the CW FRCAs in the 0.063 mm to 4 mm exhibited higher absorption values. This is because, de-spite being made from concrete waste, their compositions are more heterogeneous. Some CW FRCAs in the 0 mm to 4 mm exhibited absorption values similar to LAB FRCAs. However, in general also exhibited higher absorption values. For these FRCA types, the use of rotor crusher and ball mill techniques had the opposite effect compared to LAB FRCA; they increased water absorption. The primary factor responsible for this is the het-erogeneity of these materials.”

Suggestion 5: In Fig. 2, the water absorption of CW FRCA processed by the rotary drum mill can reach as low as 2.5%, which is lower than that of all other recycling processed FRCA, please explain further the reason.

Response 5: It is difficult to explain the reason for this value because the CW aggregates are very heterogeneous, and there is no way to know if the low absorption value is due to the equipment or due to the source materials of the CW.

Suggestion 6: "For FRCA LAB in the 0.063 mm to 4 mm range, oven-dry density values increase as the number of Jaw Crusher crushing repetitions is raised", but in Fig. 3 the oven-dry density value after Jaw Crusher 4x processing is lower than Crusher 3x, please explain.

Response 6: The difference between the oven-dry density values is small, around 1%. This difference may be due to test errors. When this is considered, the values after the 3x crusher stabilise, and the process no longer influences the oven-dry density.

Suggestion 7: In Figure 4, the R2 values of the fitted equations for the water absorption and oven-dry density of FRCA are too low to indicate a correlation between the water absorption and density of FRCA. Similarly, in Fig. 6, the value of R2 is too low to indicate a correlation between water absorption and adhered paste of FRCA.

Response 7: Thank you for the observation. Line 434 - In Figure 4, the equations and lines were removed, and the text was rewritten to clarify the analysis of the figure.

“The number of data points correlating water absorption and oven dry density for FRCA for different processing conditions can, in some cases, be questioned whether the data are representative, because of the limited amount of results. Nevertheless, some trends are visible: i) increasing oven-dry density with the decrease of water absorption; ii) higher water absorption when fines below 0.063 mm are included, for the same range of oven-dry density. For LAB FRCAs, there is a concentration of values around 2400 kg/m³, ranging from 1940 kg/m³ to 2520 kg/m³ (Sd = 122,5), for all processes, while for CW FRCAs, the values are more dispersed, ranging from 1960 to 2660 (Sd = 164,8).”

Although the correlation in Figure 6 is low, the graph shows a trend observed in some research: the more adhered mortar in the recycled aggregates, the greater the water absorption due to the pores of the hardened paste.

Suggestion 8: Please further refine the conclusions and suggest prospects for FRCA applications

Response 8: The text was revised and rewritten to clarify the conclusions and prospects for FRCA applications.

Round 2

Reviewer 1 Report

Thank you for addressing my comments.  There are still English grammatical issues in this paper, but hopefully you are able to work with the editorial staff to make these corrections.

This still needs editorial improvement in order to be published.  The corrections have English grammatical issues.

Author Response

Thank you.

A revision was made to the article to accomplish your comment.

Best regards,

The authors

Reviewer 2 Report

Thank you for the author's response. This manuscript can be considered for publication.

Author Response

Thank you,

The authors.