Comparative Sustainability Assessment of Proprietary and Non-Proprietary Ultra-High Performance Concrete Mixtures

Round 1

Reviewer 1 Report

see attached

Comments for author File: Comments.pdf

Author Response

The authors would like to thank the editor and reviewers for their valuable comments.  The authors have incorporated their suggestions, which have improved our paper.  The authors’ replies to each comment are below in blue, and the newly updated texts are highlighted throughout the main text.

1. Review 1
2. Line 82 – change reference citation to [ ].

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2. Line 127 – Ordinary Portland Cement is already defined as OPC.

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3. Line 128 – SCM already defined.

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4. Line 352 – OPC already defined.

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5. Line 374 – SCM already defined.

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6. Line 405 – SCM already defined.

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7. Line 461 – SCM already defined.

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8. Line 467 – OPC already defined.

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9. Line 510 – UHPC already defined.

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10. Line 554 – UHPC already defined.

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11. Line 564 – OPC already defined.

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12. Line 565 – SCM.

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Reviewer 2 Report

Please refer to the attached document for the detailed review comments.

Comments for author File: Comments.pdf

Author Response

The authors would like to thank the editor and reviewers for their valuable comments.  The authors have incorporated their suggestions, which have improved our paper.  The authors’ replies to each comment are below in blue, and the newly updated texts are highlighted throughout the main text.

1. Review 2
2. Consider revising the title to highlight the comparative and sustainability assessment nature of the study. For example: Comparative Sustainability Assessment of Proprietary and Non-Proprietary Ultra-High Performance Concrete Mixtures.

We thank the reviewer for the valuable suggestion. As advised, we have revised the title to better reflect the comparative and sustainability assessment nature of the study. The updated title is:

“Comparative Sustainability Assessment of Proprietary and Non-Proprietary Ultra-High Performance Concrete Mixtures.”

2. Include key quantitative findings in the abstract to strengthen its impact. For instance, specify the percentage reduction in CO₂ emissions or cost when comparing non-proprietary to proprietary mixtures.

We thank the reviewer for this constructive suggestion. In response, we have revised the abstract to incorporate key quantitative findings that highlight the comparative sustainability advantages of non-proprietary UHPC mixtures. Specifically, we now report that non-proprietary mixtures such as UHPC-1 demonstrated up to a 65% reduction in energy consumption, 49% lower CO₂ emissions, and 80% lower cost compared to proprietary mixtures. This addition strengthens the abstract by directly presenting the major outcomes of our analysis. The added text is highlighted within the abstract.

3. Add keywords such as "life-cycle assessment", "low-carbon concrete", or "green construction materials" to enhance visibility and indexing relevance.

We thank the reviewer for the valuable suggestion. While our study does not perform a full life-cycle assessment (LCA), we agree that incorporating broader sustainability-related keywords will enhance visibility. Accordingly, we have added “low-carbon concrete” and “green construction materials” to the keywords section to better reflect the focus of this work. We have not included “life-cycle assessment” to avoid misrepresentation, since the scope of this study is limited to comparative analysis of energy, CO₂ emissions, and cost of UHPC mixtures rather than a full LCA.

4. The introduction would benefit from including more recent references (2022 – 2024), especially studies involving life-cycle assessment (LCA), AI-assisted UHPC optimization, or regional carbon emission baselines.

We appreciate this helpful suggestion. In the revised manuscript, we have expanded the Introduction to incorporate recent studies published between 2022–2024. Specifically, we have added references to: Recent LCAs of UHPC structural elements, highlighting the influence of curing methods, binder content, and functional units on environmental performance (Ji et al., 2022; Farahzadi et al., 2024). AI-assisted optimization of UHPC mixtures, which demonstrates the use of machine learning and generative algorithms to design low-carbon and cost-effective concretes (Le Nguyen et al., 2024).

5. Tables 1 and 2 explicitly cite the source for each value in the tables using inline references or footnotes. This improves transparency and reproducibility.

We thank the reviewer for this valuable suggestion. We have revised Table 1 (Adopted parameters) and Table 2 (Cost of constituent materials) to explicitly include the source of each parameter. Inline references have been added directly in the table footnotes and/or within the table cells (as appropriate) to ensure clear attribution of all energy, emission, and cost values. This revision enhances the transparency and reproducibility of the study, as each parameter can now be directly traced back to its source in the cited literature.

6. Some variables and units are not clearly defined in the text. Ensure all variables are introduced with consistent units and explanations.

We thank the reviewer for this important observation. We have carefully reviewed the manuscript to ensure that all variables are consistently defined upon first use, with their associated units clearly stated. Specifically, we standardized the presentation of:

Energy values (reported in MJ/kg or MJ/m³ depending on context).

Emissions (reported in kg-CO₂/kg or kg-CO₂/m³).

Costs (reported in USD/kg or USD/m³).

Mechanical properties (e.g., compressive strength in MPa).

Mixture proportions (clearly stated in kg/m³).

7. For Figure 4, consider including trendlines and R² values to support the assertion of no clear correlation between compressive strength and sustainability indicators.

R² lines were added, and the discussion of Figure 4 is updated.

8. Proficiency Analysis: The scoring lacks objectivity. Introduce a normalized or weighted index method to quantitatively rank the mixtures by sustainability performance.

We appreciate the reviewer’s suggestion regarding the use of a normalized or weighted index method. In this study, we intentionally employed a straightforward scoring approach to keep the results transparent and directly interpretable, particularly for practitioners who may not be familiar with weighted sustainability indices. While we recognize that normalized indices can provide additional granularity, such methods often involve subjective weighting choices that may obscure practical comparisons. We therefore opted for a direct comparison of cost, energy, and emissions, which we believe provides a clear, practitioner-friendly assessment of the trade-offs between proprietary and non-proprietary UHPC mixtures. This choice ensures reproducibility and avoids introducing weighting factors that could bias the results. Future work may explore weighted or multi-criteria decision-making methods for ranking UHPC sustainability.

9. Sensitivity Analysis Although insightful, this section lacks visual impact. Adding bar charts or spider diagrams would better illustrate how variations in OPC and steel fiber content affect each sustainability parameter.

A bar graph and the following discussion were added: To complement the numerical results in Sections 4.1 to 4.4, Figure 5 shows a bar chart assessment of the sensitivity analysis conclusions. The bars show the relative changes in total energy, emissions, and cost under ± 20% variation of OPC and fiber content. The visualization obviously demonstrates that OPC variation exerts a much stronger influence on sustainability indicators than steel fibers, with increases in OPC leading to disproportionate rises in both energy demand and emissions. However, steel fiber variation yields slight shifts, principally in cost. This bar graph provides a summarizing visual confirmation of the numerical sensitivity results and highlights the robustness of the conclusions.

10. The paper notes regional variability but lacks quantitative comparisons. Consider including regional emission factors (e.g., US vs EU vs Asia) to demonstrate the practical implications of the findings in different locations.

We appreciate this insightful suggestion. We agree that regional emission factors (e.g., US vs. EU vs. Asia) can provide additional practical context for sustainability assessments. However, the present study is focused on a comparative evaluation of proprietary vs. non-proprietary UHPC mixtures under a consistent baseline. Introducing multiple regional emission datasets would require a separate comprehensive analysis that goes beyond the intended scope of this work. To ensure clarity, we have highlighted in the revised manuscript that our results are based on a uniform emissions baseline and are intended primarily for relative comparison of mixture performance, rather than region-specific benchmarking. This ensures the findings remain consistent, reproducible, and focused on the comparative objective of the study.

11. Limitations (Section 6): Add discussion of the following missing elements: Lack of transportation emissions analysis; Potential inaccuracies due to reliance on literature-based data.

We thank the reviewer for pointing out these important aspects. We have added a dedicated paragraph in Section 6 to acknowledge the exclusion of transportation-related emissions and the reliance on literature-based data. These limitations are now explicitly noted, along with a statement that future work can address them in greater detail.

12. Conclusions (Section 7): The conclusion reiterates earlier results but lacks synthesis. Revise to emphasize: Clear comparative summary of proprietary vs non-proprietary UHPC; Practical recommendations for material selection; Key takeaways for engineering practice and sustainability planning.

Thank you for the valid comment. Our conclusion has been updated. The updated conclusion is as follows:

This study compared proprietary and non-proprietary UHPC mixtures in terms of energy demand, CO₂ emissions, and cost, with the following main findings:

• Comparative summary: While proprietary UHPC mixtures normally experienced higher compressive strength, they were also associated with greater energy, emissions, and cost. On the other hand, non-proprietary mixtures accomplished comparable mechanical performance with lower overall environmental and economic weights.
• Practical recommendations: From a sustainability standpoint, non-proprietary UHPC provides a more balanced option for structural applications where marginal strength gains do not justify considerably higher influences. Proprietary mixtures may remain desirable for projects requiring maximum strength-to-volume efficiency (e.g., prestressed bridge elements). However, for most traditional practices, non-proprietary substitutes offer a cost-effective and lower-carbon footprint.
• Key takeaways for practice: The findings highlight that material selection should not be based exclusively on mechanical properties. However, integrating sustainability metrics into engineering practice allows more informed decisions that align with both performance and environmental objectives. For planners and designers, adopting non-proprietary UHPC can help decrease emissions and costs, thus supporting broader green construction initiatives.
• Generally, this study demonstrates the value of comparative sustainability assessments in guiding UHPC mixture selection. Future research should expand the scope by including transportation impacts, regional emission baselines, and life-cycle assessments to deliver a more complete framework for sustainable material design.

13. Future Work (Section 8) Expand on promising directions such as: The use of industrial by-products (e.g., desulfurization gypsum, copper slag); AI-driven mix design optimization; Life-cycle performance monitoring of UHPC structures in real-world projects.

Section 8 (Recommendations for Future Work) already contains these suggestions.

14. References: Several references are inconsistently formatted. Ensure all entries include full publication information and are consistent with journal style. Include at least five new references from 2022 2024 to reflect the current state of research. Refs 2 and 15 are duplicated.

Updated.

15. Novelty Statement: The manuscript would benefit from a clearer statement of novelty. Specifically, explain how this work extends beyond previous sustainability studies on UPC and what new practical recommendations it enables.

We appreciate this suggestion. To clarify the contribution of our work, we added a dedicated novelty statement at the end of the Introduction. The new paragraph emphasizes that this is the first comparative sustainability assessment of proprietary versus non-proprietary UHPC mixtures across energy, emissions, and cost metrics. Unlike earlier studies, which often examined single mixtures or isolated sustainability factors, our study provides a holistic benchmark and translates the results into practical recommendations for engineering practice and sustainability planning.

Reviewer 3 Report

The authors conducted a study with the data from literature on the environmental impact of five proprietary ultra-high performance concrete (UHPC) and five non-proprietary UHPC mixes.  The study is interesting and is useful for researchers as well as project managers who are concerned about the carbon footprint of construction materials. The reviewer has the following comments.

Technical comments:

1. 1. In the Methodology section, the authors address both Energy and CO2 emissions of the 10 mixes. Are the CO2 emissions only during the production of each constituent material?  If so, how about the CO2 emissions that are linked to the energy used?   Please elaborate on this question.
   2. Please elaborate on the type of HRWR that is appropriate for UHPC. Are the energy and emission parameters for HRWRs in general, or for the kind most suitable for UHPC? 
   3. The selected five non-proprietary UHPCs were based on research done in two states, namely Arkansas and Montana. Throughout the U.S. several other states have developed non-proprietary UHPCs. Could you elaborate on why others were not selected, and why only two states have representation?
   4. Regarding Figure 4, if the two non-proprietary UHPC mixes without steel fibers are removed, there will be some level of correlation, especially for energy versus compressive strength.  Please elaborate on this comment.

Editorial Comments:

1. 1. In the Abstract, the authors refer to OPC, but do not define what it stands for. Please spell it out, followed by OPC in parentheses.
   2. The authors define UHPC, followed by the acronym in parentheses in multiple places. Only one time should be good enough.
   3. Lines 67-71: What was the first scenario?
   4. Line 161: Remove the negative sign in “ – 0.053 MJ/kg.”
   5. Section 2.1.6 needs to be revised to read better.
   6. Line 248: Change “20244” to “2024.”
   7. Lines 251-255: This information was given before.  Please delete or rewrite.
   8. Line 346: “226” was given in the previous sentence. Please be consistent.
   9. Table 5: Are the Energy, Emission, and Cost per cubic meter?  If so, please revise the column headings. 

Author Response

The authors would like to thank the editor and reviewers for their valuable comments.  The authors have incorporated their suggestions, which have improved our paper.  The authors’ replies to each comment are below in blue, and the newly updated texts are highlighted throughout the main text.

1. Review 3
2. In the Methodology section, the authors address both Energy and CO2 emissions of the 10 mixes. Are the CO₂ emissions only during the production of each constituent material? If so, how about the CO₂ emissions that are linked to the energy used?   Please elaborate on this question.

We appreciate the reviewer’s question. In this study, both embodied energy (MJ/kg) and CO₂ emissions (kg CO₂/kg) for each constituent material were obtained from the literature. The reported CO₂ emission values already include the impacts associated with producing 1 kg of the respective material, which encompasses both process emissions (e.g., calcination in cement production) and energy-related emissions where available in the data sources. These unit values were then scaled according to the mixture proportions to obtain the total energy and emission values for each UHPC mixture. We maintained energy and CO₂ as two separate indicators to allow independent comparison. We have updated the end of the Mythology Section and before starting the subsections. 

2. Please elaborate on the type of HRWR that is appropriate for UHPC. Are the energy and emission parameters for HRWRs in general, or for the kind most suitable for UHPC?

We thank the reviewer for this comment. We have now clarified in the manuscript (in both the Energy and Emissions subsections of the Methodology) that the values used for high-range water-reducing admixtures (HRWRs) are derived from general literature sources. While the HRWR type most suitable for UHPC is polycarboxylate ether (PCE)-based, our dataset reflects generic HRWR values. This note has been added to the text to ensure transparency. Please note that the contribution of HRWR to total energy and emissions is minimal.

3. The selected five non-proprietary UHPCs were based on research done in two states, namely Arkansas and Montana. Throughout the U.S., several other states have developed non-proprietary UHPCs. Could you elaborate on why others were not selected, and why only two states have representation?

We appreciate the reviewer’s observation. To clarify, all five non-proprietary UHPC mixtures analyzed in this paper are from our previous research conducted in Arkansas. These mixtures were selected to ensure consistency in data sources and comparability of results, since they were developed and tested under the same research framework. The Montana study was not included in the analysis; it was only cited to demonstrate that additional non-proprietary UHPC mixtures have been developed across the U.S. Our intention was not to provide an exhaustive survey of all regional UHPCs, but rather to build upon our validated dataset for a controlled comparative assessment.

4. Regarding Figure 4, if the two non-proprietary UHPC mixes without steel fibers are removed, there will be some level of correlation, especially for energy versus compressive strength. Please elaborate on this comment.

We thank the reviewer for this insightful observation. Indeed, the inclusion of the two fiber-free non-proprietary UHPC mixtures reduces the apparent correlation between compressive strength and sustainability indicators. When these two mixtures are excluded, a slightly stronger trend can be observed, particularly in the relationship between compressive strength and embodied energy. However, since the scope of this study was to evaluate all 10 mixtures as reported, we retained the complete dataset in Figure 4 to provide a holistic comparison. To acknowledge the reviewer’s point, we have added a note in the Discussion clarifying the influence of fiber content on the observed trends. Also, we updated Figure 4 to show the trending line with R² values to demonstrate the correlation. The following discussion is added for Figure 4.

As shown in Figure 4, regression trendlines were added to assess probable correlations between compressive strength and sustainability metrics. The obtained R² values were 0.18 (energy-based proficiency), 0.05 (CO₂-based proficiency), and 0.14 (cost-based proficiency). All three values are well below 0.20, confirming very weak explanatory power. This specifies that compressive strength accounts for only a small part of the variability in sustainability performance. Consequently, enhancements in compressive strength alone do not necessarily translate into improved ecological or economic results, and mixture composition (binder selection, SCMs, and curing methods) remains the main factor affecting sustainability. It should be noted that the two non-proprietary mixes without steel fibers contribute to the observed scatter. Excluding them results in a modest improvement in correlation (particularly for energy); nevertheless, the full dataset was taken to ensure reliability and reasonable comparison across all mixtures.

5. In the Abstract, the authors refer to OPC, but do not define what it stands for. Please spell it out, followed by OPC in parentheses.

Updated.

6. The authors define UHPC, followed by the acronym in parentheses in multiple places. Only one time should be good enough.

Updated.

7. Lines 67-71: What was the first scenario?

The paragraph was updated and highlighted with the main text.

8. Line 161: Remove the negative sign in “ – 0.053 MJ/kg.

Updated.

9. Section 2.1.6 needs to be revised to read better.

revised.

10. Line 248: Change “20244” to “2024.”

Updated.

11. Lines 251-255: This information was given before. Please delete or rewrite.

The repeated information was deleted.

12. Line 346: “226” was given in the previous sentence. Please be consistent.

Updated.

13. Table 5: Are the Energy, Emission, and Cost per cubic meter? If so, please revise the column headings.

Updated.