Mechanical Performance of a Hot Mix Asphalt Modified with Biochar Obtained from Oil Palm Mesocarp Fiber

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The study investigates the mechanical performance of hot-mix asphalt (HMA) modified with biochar derived from oil palm mesocarp fiber (BC-OPMF). Results indicate that a 10% BC-OPMF content significantly enhances the asphalt's stiffness, moisture resistance, and fatigue strength without altering the optimal asphalt binder content. The following problems needs to be addressed to improve the quality of the paper:

(1) The terms used in the manuscript need to be revised, for example, "asphalt cement" is suggested to be updated to "asphalt binder"

(2) A more detailed description of the experimental procedures is suggested, including the specific conditions under which the tests were conducted, This is particularly important for the asphalt binder tests and the asphalt mix design processes.

(3) Why 10% is the optimal content, it needs to be explained further.

(4) The test results showed that the modified HMA's stiffness, moisture resistance, and fatigue strength were improved by incorporating BC-OPMF, what is the mechanism behind the performance elevation;

(5) What are the disadvantages of using BC-OPMF as a HMA modifier?

Author Response

5-September, 2024

 

Dear Editor and Reviewers,

 

We would like to thank you and the reviewers for your valuable and insightful comments that have helped us to improve our manuscript. Please find also in the following paragraphs our answers to the comments. We have tried our best to clarify all the points raised. We hope that this new version of the manuscript is satisfactory for publication.

 

Reviewer comments

 

Reviewer #1: The study investigates the mechanical performance of hot-mix asphalt (HMA) modified with biochar derived from oil palm mesocarp fiber (BC-OPMF). Results indicate that a 10% BC-OPMF content significantly enhances the asphalt's stiffness, moisture resistance, and fatigue strength without altering the optimal asphalt binder content. The following problems needs to be addressed to improve the quality of the paper:

 

(1) The terms used in the manuscript need to be revised, for example, "asphalt cement" is suggested to be updated to "asphalt binder".

 

Answer: Thank you for your thoughtful and constructive feedback on our manuscript. Considering your comment, we changed the term 'asphalt cement' to 'asphalt binder'.

 

(2) A more detailed description of the experimental procedures is suggested, including the specific conditions under which the tests were conducted. This is particularly important for the asphalt binder tests and the asphalt mix design processes.

 

Answer: Thank you for your valuable comment. Considering your comment, additional information regarding the tests conducted on the binders is provided in lines 164 to 167, 171 to 175. Design criteria for the asphalt mixtures were also provided in lines 245 to 247.

 

(3) Why 10% is the optimal content, it needs to be explained further.

 

Answer: Thank you for your valuable comment. You are correct. For this reason, to better explain the selection of BC/AC=10%, the paragraph between lines 198 and 203 was rewritten.

 

(4) The test results showed that the modified HMA's stiffness, moisture resistance, and fatigue strength were improved by incorporating BC-OPMF, what is the mechanism behind the performance elevation.

 

Answer: Thank you for your valuable comment. This is a good question, as the reference literature is not clear in reporting the specific mechanisms affecting the improved performance of asphalt mixtures modified with BCs. Modifying binders and asphalt mixtures with BCs is a relatively new topic. Currently, we only have assumptions based on macro-level physical properties of the binders and mixtures. In our study, we were able to demonstrate that these macro properties are related and consistent. For example: i) stiffer and more viscous binders produced stiffer mixtures; ii) increases in ITS strength and resilient modulus tend to improve fatigue resistance when the loading mode is stress-controlled; iii) moisture damage tends to be lower when binders are more resistant to aging; iv) the performance of the mixture with our BC is similar to that reported in other studies with different BCs made from various biomasses; v) etc. All these aspects were noted in the manuscript. However, micro-level aspects were not studied. Specifically, the micro-level physical-chemical interactions and reactions were not analyzed. Therefore, the last paragraph of the conclusions makes recommendations for future studies (particularly to conduct chemical characterization) (lines 409 to 417). We appreciate your understanding in this regard.

 

(5) What are the disadvantages of using BC-OPMF as a HMA modifier?

 

Answer: Thank you for your valuable comment. This is another very good question. The results of our study show that the measured and evaluated properties improved when the binder was modified with BC. Furthermore, there is no need to increase the binder content or the manufacturing and compaction temperatures. However, the properties were measured to assess damage mechanisms (pavement distresses) occurring in asphalt pavements subjected to temperate climates (primarily fatigue cracking) or high temperatures (primarily rutting). According to the reference literature, a potential disadvantage could be using BC as a modifier for asphalt mixtures subjected to low service temperatures. However, this was not evaluated in our study because Colombia is a tropical country where such climatic conditions are not present. For this reason, the last paragraph of the conclusions suggests that future studies should evaluate the performance of mixtures subjected to low temperatures (lines 411 to 412). Another potential disadvantage could be the increased costs associated with purchasing BC and the binder modification process. However, this cost increase could be offset by a higher-performing material that is more durable in the pavement. For this reason, we also recommend that future research include full-scale testing, socioeconomic-environmental impact assessments, and LCCA and LCA modeling, among other analyses, at the end of the conclusions (lines 409 to 417).

Reviewer 2 Report

Comments and Suggestions for Authors

The paper investigates the use of biochar derived from oil palm mesocarp fiber (BC-OPMF) as a modifier for hot-mix asphalt (HMA). The topic is relevant and contributes to the growing body of research on sustainable materials in pavement engineering. However, the following points could be furtherly addressed and improved.

(1) The style of this manuscript is weird. It seems section 4 is missed. Besides, section 3 only presents the results, and is there any discussion?

(2) A detailed particle size analysis, as particle size can significantly affect the dispersion of biochar in the asphalt binder and the resulting mechanical properties.

(3) The SEM images provide some insight into the surface morphology of BC-OPMF, but quantitative data on the surface area and porosity would enhance the understanding of how BC-OPMF contributes to the mechanical properties of the modified asphalt.

(4) While the study focuses on high-temperature performance (e.g., rutting resistance), it is also essential to assess the low-temperature performance, especially for regions with wide temperature fluctuations.

(5) The current conclusions are too long, and should be concise. It is also suggested that the authors refrain from making definitive claims about the material's potential without acknowledging the study's limitations.

Comments on the Quality of English Language

Engish writing is fine.

Author Response

5-September, 2024

 

Dear Editor and Reviewers,

 

We would like to thank you and the reviewers for your valuable and insightful comments that have helped us to improve our manuscript. Please find also in the following paragraphs our answers to the comments. We have tried our best to clarify all the points raised. We hope that this new version of the manuscript is satisfactory for publication.

 

Reviewer comments

 

Reviewer #2: The paper investigates the use of biochar derived from oil palm mesocarp fiber (BC-OPMF) as a modifier for hot-mix asphalt (HMA). The topic is relevant and contributes to the growing body of research on sustainable materials in pavement engineering. However, the following points could be furtherly addressed and improved.

 

(1) The style of this manuscript is weird. It seems section 4 is missed. Besides, section 3 only presents the results, and is there any discussion?

 

Answer: Thank you for your thoughtful and constructive feedback on our manuscript. The manuscript style may seem somewhat "weird" because some results are presented in Section 2 (e.g., results from the experimental phase conducted on the binders) to facilitate the understanding of the paper. On the other hand, the journal template mentions that the Conclusions section is not mandatory, but we consider this section relevant due to the omission of the suggested Discussion section (Section 4) in the template. This is because we preferred to present the analysis immediately alongside the results. Indeed, Section 3 presents the results obtained from the experimental phase conducted. However, it also includes the statistical analysis, the percentage differences of the measured properties, and displays the possible causes that lead to changes in these properties when BC is used as a modifier. In addition, the results are correlated with those reviewed in the reference literature. Therefore, we believe that Section 3 could be named "Results and Analysis" (line 283). We appreciate your kind understanding in this regard.

 

(2) A detailed particle size analysis, as particle size can significantly affect the dispersion of biochar in the asphalt binder and the resulting mechanical properties.

 

Answer: Thank you for your valuable comment. All BC particles were smaller than 0.075 mm. Indeed, particle size could influence the results. However, in our study, despite having amorphous particles of different sizes, the entire BC dispersed homogeneously in the binder at high temperature (lines 203 to 212). Future studies should be conducted to evaluate the effect of particle size on the performance of the binder and the modified HMA. Based on your observation, a recommendation regarding this was added in line 414.

 

(3) The SEM images provide some insight into the surface morphology of BC-OPMF, but quantitative data on the surface area and porosity would enhance the understanding of how BC-OPMF contributes to the mechanical properties of the modified asphalt.

 

Answer: Thank you for your valuable comment. Information on maximum particle size and porosity, along with the elemental chemical composition, is presented in lines 143 to 149. Surface area was not measured. However, we recommended considering the surface area in future studies (line 414). We appreciate your understanding regarding this.

 

(4) While the study focuses on high-temperature performance (e.g., rutting resistance), it is also essential to assess the low-temperature performance, especially for regions with wide temperature fluctuations.

 

Answer: Thank you for your valuable comment. In our country (Colombia), the climate is tropical. Most regions have a warm climate, but cities such as Bogotá have a temperate climate. For this reason, in our study, we did not evaluate properties at low temperatures (we assessed performance at intermediate and high temperatures). Indeed, evaluating performance at low temperatures is important; however, in this initial phase of the study, we were more interested in understanding performances that are more consistent with Colombia's climate. According to the reference literature, a potential disadvantage of using BC as an asphalt mixture modifier is the possible deterioration of performance at low service temperatures, as the mixtures tend to become brittle. For this reason, in the final paragraph of the conclusions (lines 411 to 412), we recommend conducting future research to evaluate the performance of BC-modified mixtures at low temperatures.

 

(5) The current conclusions are too long, and should be concise. It is also suggested that the authors refrain from making definitive claims about the material's potential without acknowledging the study's limitations.

 

Answer: Thank you for your valuable comment. Based on your suggestion, we have shortened the conclusions (lines 378 to 417). On the other hand, lines 409 to 417 mention that the conclusions cannot be considered definitive. In the same paragraph, considering the study's limitations, recommendations for future work are provided.

 

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The reviewer is satisfied with the authors' responses.