Crumb Rubber (CR) and Low-Density Polyethylene (LDPE)-Modified Asphalt Pavement Assessment: A Mechanical, Environmental, and Life Cycle Cost Analysis Study

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1. The abstract is written chaotically and inconsistently. Abstract requires improvement. The aims, scope, main results (1-2 sentences), and conclusions should be clearly described in a single paragraph.
2. The authors should explain the terms “asphalt” and “bitumen” at the beginning of the article.
3. In general, the Introduction is quite voluminous and meaningful. However, there are some comments. If the authors claim that asphalt paving creates environmental problems (greenhouse gas emissions), then explaining why (citing references is not enough) is necessary. How are greenhouse gases formed from asphalt pavement? In my opinion, the environmental consequences of asphalt paving are insignificant. Are asphalt pavements a greater source of pollution than, for example, thermal or nuclear power plants?

Also, new additives to bitumen produced from natural raw materials, fossil fuels, wastes, and substances used partially to replace bitumen (lignin, biomaterials) must be considered more fully. The following works can be helpful in your discussion: https://doi.org/10.23939/chcht18.02.244

4. At the end of the Introduction, a single paragraph should summarize the existing research gaps and the purpose of the work arising from these gaps.
5. The novelty of the work should be clearly explained, as there is a very large number of works devoted to the modification of bitumen with polyethylene and/or shredded rubber (for example, see https://doi.org/10.23939/chcht17.03.674).
6. At the beginning of section 3, it is necessary to indicate the purpose of the modification: increasing the softening temperature, improving resistance to aging, obtaining a specific brand of polymer-modified bitumen, etc.
7. What results did the authors obtain regarding inhibiting bitumen aging?
8. Regarding the Conclusions. The first paragraph is very similar to an abstract. It is necessary to explain how the research relates to sustainable development. Indicate the limitations of applying your research.

Author Response

Comment 1: The abstract is written chaotically and inconsistently. Abstract requires improvement. The aims, scope, main results (1-2 sentences), and conclusions should be clearly described in a single paragraph.

Response 1: Thank you for the suggestion. We have rewritten the abstract to present the aim, scope, and key results. The revised abstract now better reflects the study's purpose and findings. Below is the new abstract:

“Abstract: Due to the growing consumption of plastic and rubber products, effective waste management solutions are crucial. This study evaluates the use of Crumb Rubber (CR), Low-Density Polyethylene (LDPE), and their combination (CR+LDPE), as asphalt binder modifiers for improving pavement performance and sustainability. The analyses covered two critical pavement layers: the wearing surface (WS) and the treated base (TB). The methodology included: (1) Binder Development and Testing, (2) Superpave Mix Design, (3) Mechanical Testing, including Indirect Tensile Strength Testing and Semi-Circular Bending Testing, (4) Life Cycle Cost Analysis, and (5) Carbon Footprint Analysis. Results revealed that CR+LDPE significantly enhanced fatigue resistance of the TB mixes with a fracture energy increase of 47%, and increased the flexibility index by 53% in the WS. CR increased the flexibility index by about 146% in the TB layer, while LDPE increased the fracture energy by 21% in the WS layer. The life cycle cost analysis demonstrated that using LDPE, CR, and CR+LDPE reduced the life cycle costs by about 16% in the WS layer. Meanwhile, the lifecycle carbon footprint analysis showed that using LDPE and CR+LDPE reduced the carbon footprint by about 87 % and 81% for the TB and WS layers, respectively. The study findings highlight the mechanical, economic, and environmental benefits of incorporating wastes into asphalt pavements.”

 

Comment 2: The authors should explain the terms “asphalt” and “bitumen” at the beginning of the article.

Response 2: We agree and have clarified the distinction in the introduction, as follows:

“It is worth noting that the terms bitumen and asphalt are often used interchangeably; however, bitumen refers to the binder, while asphalt refers to the composite of bitumen and aggregates (i.e., asphalt cement concrete mix). In the United States, the term asphalt refers to either the binder or the asphalt concrete mix.”

 

Comment 3.1: In general, the Introduction is quite voluminous and meaningful. However, there are some comments. If the authors claim that asphalt paving creates environmental problems (greenhouse gas emissions), then explaining why (citing references is not enough) is necessary. How are greenhouse gases formed from asphalt pavement? In my opinion, the environmental consequences of asphalt paving are insignificant. Are asphalt pavements a greater source of pollution than, for example, thermal or nuclear power plants?

Response 3.1: We appreciate this comment. While asphalt pavements are not a key GHG source, compared to thermal and nuclear power plants, their environmental impact can not be overlooked due to their energy-intensive production, transportation, mixing, construction, and rehabilitation, taking into consideration the scale and frequency of road construction on a global level.  

The following part was amended accordingly:

“Despite asphalt pavements' utility and effectiveness in providing durable and reliable infrastructure, asphalt pavement poses significant environmental challenges throughout its lifecycle, due to the energy-intensive production of bitumen, high temperatures required for mixing, fuel use during transportation and construction, and emissions from maintenance and end-of-life processes [1,2]. Taking into consideration the scale and frequency of road construction on a global level, their environmental impact cannot be overlooked[3].”

 

Also, new additives to bitumen produced from natural raw materials, fossil fuels, wastes, and substances used partially to replace bitumen (lignin, biomaterials) must be considered more fully. The following works can be helpful in your discussion: https://doi.org/10.23939/chcht18.02.244.

Response 3.2: Thank you for the recommendation. The following part was added to the literature review.

“In consequence, studies have explored the incorporation of sustainable alternatives to traditional asphalt modifiers due to their environmental merits and support for circular economy solutions. Among these are lignin, bio-oils, and other waste-based materials[4]”

Comment 4: At the end of the Introduction, a single paragraph should summarize the existing research gaps and the purpose of the work arising from these gaps.

Response 4: Thank you for your review. The following part was added.

“Despite previous efforts to promote sustainability in the infrastructure industry, such as incorporating waste materials like CR and LDPE in asphalt mixes, key knowledge gaps remain regarding their understanding of performance and interactions of wastes with mix components. In addition, previous work lacks standardized methodologies, comprehensive mechanical testing, and integrated sustainability assessments. This study addresses these gaps by evaluating CR and LDPE-modified binders through rheological, mechanical, economic, and environmental analyses to support sustainable pavement design.”

Comment 5: The novelty of the work should be clearly explained, as there is a very large number of works devoted to the modification of bitumen with polyethylene and/or shredded rubber (for example, see https://doi.org/10.23939/chcht17.03.674).

Response 5: We appreciate your comment. The novelty of our study lies in the integrated comparative approach, with mechanical, environmental, and economic analyses of CR, LDPE, and combined CR and LDPE-modified asphalt binders. This comprehensive approach, especially under varying mechanical testing including moisture conditions, complemented by cost and environmental analysis, adds significant new insight. This has been clarified in the Introduction and Conclusion.

The provided study was used to strengthen the literature review.

 

Comment 6: At the beginning of section 3, it is necessary to indicate the purpose of the modification: increasing the softening temperature, improving resistance to aging, obtaining a specific brand of polymer-modified bitumen, etc.

Response 6: Thank you for your feedback. At the beginning of section 3, the following sentence was added:                         

“In this section, the study evaluates the performance of the binders in terms of their enhanced thermal stability, fracture properties, crack resistance, and sustainability.”

Comment 7: What results did the authors obtain regarding inhibiting bitumen aging?

Response 7: Thank you for your question. While the study demonstrates improvements in high-temperature performance and cracking resistance, it does not provide direct evidence or specific aging resistance test results. As such, the following point was added/mentioned for future work, as follows:

 the study focused on short term aging which should be complemented by long-term investigations to assess the aging behavior”.

Comment 8: Regarding the Conclusions. The first paragraph is very similar to an abstract. It is necessary to explain how the research relates to sustainable development. Indicate the limitations of applying your research.

Response 8: The conclusion has been amended.

A new part is added to explain the research contribution to sustainable development, as follows:

“This research promotes sustainable development by using recycled waste into asphalt mixtures, reducing landfill waste, improving pavement durability, lowering maintenance costs, and reducing carbon emissions.”

In addition, the following limitations and future work statements were added to the Conclusion.

“Finally, the study was conducted under controlled laboratory conditions, which may not fully replicate field performance influenced by traffic and environmental factors. The study recommends that future research include field validation to confirm the same performance under service conditions. In addition, the study focused on short term aging which should be complemented by long-term experiments to assess the long-term aging behavior”.

Reviewer 2 Report

Comments and Suggestions for Authors

Asphalt pavement is the basis for safe road traffic. Its strength and cost characteristics as well as its impact on environmental components are important. The production of asphalt obviously involves the consumption of large quantities of non-renewable primary resources. The manuscript submitted for review presents the results of research on the use of secondary raw materials - waste - for modification of asphalt pavements. The aim of the paper can be labelled as a study to promote sustainable and environmentally friendly pavement design. The work addresses the factors outlined above, therefore it is included in the Scope of the journal.
The methodology of the work includes five steps, and is well structured. The authors propose commonly used wastes (CR, LDPE and their mixture) as asphalt modifiers, thus offering a solution to the current problems of waste management and resource conservation, which makes the study highly significant. The authors declare cost reduction in pavement production (up to 16 %) and reduction of carbon footprint
The paper presents detailed test approaches including different moisture conditions, the experimental data are well visualised and the conclusions seem logical and consistent.

Comments and suggestions to the manuscript:
1. The choice of 7 % CR and 5 % LDPE is based on previous studies but is not detailed, the authors should briefly explain why these ratios were chosen or refer to the specific material properties that determined this choice.
2. Table 2 shows that CR does not improve high temperature performance. In the reviewer's opinion, the reasons for this should be further discussed in the paper.
3. The paper uses a discount rate of 28 %, which is quite high and may negatively affect the results of the business case. The choice of discount rate should be explained.

Author Response

Asphalt pavement is the basis for safe road traffic. Its strength and cost characteristics as well as its impact on environmental components are important. The production of asphalt obviously involves the consumption of large quantities of non-renewable primary resources. The manuscript submitted for review presents the results of research on the use of secondary raw materials - waste - for modification of asphalt pavements. The aim of the paper can be labelled as a study to promote sustainable and environmentally friendly pavement design. The work addresses the factors outlined above, therefore it is included in the Scope of the journal.
The methodology of the work includes five steps, and is well structured. The authors propose commonly used wastes (CR, LDPE and their mixture) as asphalt modifiers, thus offering a solution to the current problems of waste management and resource conservation, which makes the study highly significant. The authors declare cost reduction in pavement production (up to 16 %) and reduction of carbon footprint
The paper presents detailed test approaches including different moisture conditions, the experimental data are well visualised and the conclusions seem logical and consistent.

Comments and suggestions to the manuscript:
1. The choice of 7 % CR and 5 % LDPE is based on previous studies but is not detailed, the authors should briefly explain why these ratios were chosen or refer to the specific material properties that determined this choice.

Response 1: Thank you so much, we are honored with your feedback. Thank you for raising this point. The following paragraph was added to clarify the percentage choice point:

“Such percentages were selected based on previous laboratory trials with waste dosages (3%, 5%, 7%, and 10% )[5–7]. The optimum content was determined using rotational viscosity testing results and performance grade (PG) evaluations to ensure compliance with viscosity specifications to achieve optimal binder performance. All prior studies were conducted using the same source materials and tested in the same laboratory setting [5–7].”

2. 
   Table 2 shows that CR does not improve high temperature performance. In the reviewer's opinion, the reasons for this should be further discussed in the paper.

Response 2: Thank you for your feedback. We have expanded the discussion on CR's performance and the following sentence was added:

“It is worth mentioning that CR alone may not significantly enhance rutting resistance when its elasticity is insufficient at elevated service temperatures. This might be attributed to the fact that rubber paricles soften when heated, which comprmise their structural integrity. In contrast, other additives such as polymers are specifcally engineered to maintain stiffness and enahnce rutting resistance under high-temperature conditions”

3. 
   The paper uses a discount rate of 28 %, which is quite high and may negatively affect the results of the business case. The choice of discount rate should be explained.

Response 3: We appreciate this important point. The 28% rate reflects local economic conditions and inflation risk in developing countries like Egypt. This clarification was added to the paper. The following part was amended:

“The discounted rate used in the study was 28 percent, as per the Central Bank of Egypt, reflecting the high cost of capital and economic conditions specific to the region”.

Reviewer 3 Report

Comments and Suggestions for Authors

This paper studies the "Crumb Rubber (CR) and Low-Density Polyethylene (LDPE) Modified Asphalt Pavement Assessment: A Mechanical, Environmental, and Life Cycle Cost Analysis Study" which is an interesting topic, while there are still some problems that need to be addressed as follows:
1. The TSR values ​​of the surface mix were below the 80% threshold. While acknowledging this, the paper should have discussed the mitigation strategies (e.g. hydrated lime, anti-stripping agents) in more depth, especially in the conclusion section.
2. The contradictory trends between FI and FE (e.g., LDPE increases FE but decreases FI) should be discussed more clearly in the Discussion section. This will help deepen the understanding of the brittleness and toughness of the material.
3. The choice of CR (7%), LDPE (5%), and CR+LDPE (3% each) was based on past work, but the paper should briefly explain why these specific ratios were chosen over others.
4. The introduction is insufficient, the logic is good but some recent study is needed like "Performance of rubber modified asphalt mixture with tire-derived aggregate subgrade"
5. The 28% discount rate is unusually high for an infrastructure project. The authors should justify this discount rate in more detail and compare it with international norms to provide a clearer understanding.
6. Figures 8-11 (fracture energy and flexibility index) would benefit from error bars or statistical indicators to reflect variability and support the conclusions
7. The manuscript is mostly clear, but occasionally contains inappropriate wording. Please check the grammar errors.
8. Ensure consistency in the use of acronyms (e.g., use "WC" and "TB" after they are defined)

Author Response

This paper studies the "Crumb Rubber (CR) and Low-Density Polyethylene (LDPE) Modified Asphalt Pavement Assessment: A Mechanical, Environmental, and Life Cycle Cost Analysis Study" which is an interesting topic, while there are still some problems that need to be addressed as follows:
1. The TSR values ​​of the surface mix were below the 80% threshold. While acknowledging this, the paper should have discussed the mitigation strategies (e.g. hydrated lime, anti-stripping agents) in more depth, especially in the conclusion section.

Response 1: This is a remarkable comment and we have already considered it in our paper in section 3.3 that reads: “However, the insignificant effect of the modification on the TB layermay be attributed to the coarser aggregate configuration, which relatively facilitates water drainage, reducing the moisture damage risk compared to the WS layer with a denser and finer aggregate structure.”

Another paragrph was added in the conclusion to explain the mitigation stratigies as follows:

“ In response to the low TSR values, a possible solution to improve the moisture resistance of the WS layer is to use liquid anti-stripping agents or hydrated lime. Hydrated lime, when incorporated into the asphalt binder, increase adhesion between the binder and the aggregates, thereby reducing the stripping potential [70]. Furthermore, the modification of the binder using hydrated lime can also reduce the oxidative aging of the binder, resulting in enhanced durability. Some of the liquid anti-stripping agents, such as the amine or silane-based, also increase the adhesion, especially when used with moisture-sensitive aggregates [71]. Hence, future stuides should consider the above-mentioned additives in future studies to account for the high potential of stripping action in the modified asphalt.”

2. 
   The contradictory trends between FI and FE (e.g., LDPE increases FE but decreases FI) should be discussed more clearly in the Discussion section. This will help deepen the understanding of the brittleness and toughness of the material.

 

Response 2: An excellent comment to deepen the understanding of the material's behavior. This was taken into consideration as follows: “LDPE mix absorbs a lot of energy before cracking, but once it starts cracking, it fails rapidly like a tough but brittle material (e.g., glass).” & “The CR mix is not very tough, but it fails gradually, not suddenly, and it flexes well despite not absorbing a lot of energy before cracking.”

 

3. 
   The choice of CR (7%), LDPE (5%), and CR+LDPE (3% each) was based on past work, but the paper should briefly explain why these specific ratios were chosen over others.

Response 3: Thank you for your valuable comment. The following paragraph was added to the paper:

“Such percentages were selected based on previous laboratory trials with waste dosages (3%, 5%, 7%, and 10% [5–7]. The optimum content was determined using rotational viscosity testing results and performance grade (PG) evaluations to ensure compliance with viscosity specifications to achieve optimal binder performance. All prior studies were conducted using the same source materials and tested in the same laboratory setting [5–7].”

 

4. The introduction is insufficient, the logic is good but some recent study is needed like "Performance of rubber modified asphalt mixture with tire-derived aggregate subgrade"

 

Response 4: We appreciate this recommendation. Three more recent studies, including the suggested valuable study, were added to the literature review, namely:

“Pyshye S, Lypko Y, Demchuk Y, Kukhar O, Korchak B, Pochapska I, et al. Characteristics and Applications of Waste Tire Pyrolysis Products: A Review. Chemistry & Chemical Technology 2024;18:244–57. https://doi.org/10.23939/chcht18.02.244.

Jin D, Xin K, Yin L, Mohammadi S, Cetin B, You Z. Performance of rubber modified asphalt mixture with tire-derived aggregate subgrade. Constr Build Mater 2024;449:138261. https://doi.org/10.1016/j.conbuildmat.2024.138261.

Nagurskyy A, Grynyshyn O, Khlibyshyn Y, Korchak B. Use of Rubber Crumb Obtained from Waste Car Tires for the Production of Road Bitumen and Roofing Materials from Residues of Ukrainian Oil Processing. Chemistry & Chemical Technology 2023;17:674–80. https://doi.org/10.23939/chcht17.03.674.”

5. 
   The 28% discount rate is unusually high for an infrastructure project. The authors should justify this discount rate in more detail and compare it with international norms to provide a clearer understanding.

Response 5: We appreciate this important point. The 28% rate reflects local economic conditions and inflation risk in developing countries like Egypt. This clarification was added to the paper. The following part was amended:

“The discounted rate used in the study was 28 percent, as per the Central Bank of Egypt, reflecting the high cost of capital and economic conditions specific to the region”.

 

6. Figures 8-11 (fracture energy and flexibility index) would benefit from error bars or statistical indicators to reflect variability and support the conclusions

Response 6: Thank you for the advice. Error bars were added in the revised manuscript as shown below.

7. The manuscript is mostly clear, but occasionally contains inappropriate wording. Please check the grammar errors.

Response 7: Thank you for your comment. We went over the wording and the grammar. The whole paper was thoroughly checked and mistakes were corrected in the revised manuscript.

8. 
   Ensure consistency in the use of acronyms (e.g., use "WC" and "TB" after they are defined)

Response 8: Thank you for your comment. We have checked the consistency of acronyms. All were amended in the revised manuscript.


To check the figures attached with response #6, please check the attached file

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

Two suggestions for paper revision before its publication: 1. check the English language for accuracy and clarity, ensuring each word is correct. 2. Include error bars for each result in the figures to enhance the clarity and reliability of the data representation if you have conducted independent cases.

Comments on the Quality of English Language

The English in the paper must be improved and carefully checked word by word.

Author Response

Two suggestions for paper revision before its publication:

1. check the English language for accuracy and clarity, ensuring each word is correct.

Response 1: Thank you for your feedback. The paper was thoroughly revised for publication.

 

2. Include error bars for each result in the figures to enhance the clarity and reliability of the data representation if you have conducted independent cases.

Response 2: Error bars were added to the graphs in the revised manuscript and also as were shown in the response for comment 6 of reviewer 3.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript has been significantly improved.

Reviewer 3 Report

Comments and Suggestions for Authors

The author has been revised based on my comments.