Carbon Nanotubes (CNTs) in Asphalt Binder: Homogeneous Dispersion and Performance Enhancement
, Naveed Ahmad 1, Muhammad Ali Nasir 2, Jamal 3, Murryam Hafeez 1, Javaria Rafi 1, Syed Bilal Ahmed Zaidi 1 and Waqas Haroon 4Round 1
Reviewer 1 Report
The authors have developed an effective method for CNTs dispersion in bitumen and studied the improvement in the rheological, conventional and performance properties as well as the adhesion characteristics of asphalt binder with the addition of CNTs. Although some of the results are new, there are some major points that the authors should consider:
1.This article’s title use ‘Homogenous Dispersion’. It’s the most important point in the article but authors didn’t expound it clear. The method of characterizing the uniform dispersion of CNTs is too simple, only SEM.
2.In the second paragraph of introduction section, there are many applications about CNTs in cement, but there are no relevant articles and literatures in introduction.
3. The introduction section does not show the insufficiency of previous studies and the innovation of this study clearly.
4. Take care of the unit superscript and subscript of “2.1g/cm3” and “60m2/g” in table 2.
5. In line 112, it’s a wrong formulation of “aggregate material”.
6.In line 114, it would be better to indicate the kind of asphalt mixture, for example AC or SMA or other kinds.
7.In figure 2, 0.5mm or 5mm are rarely used as unit of gradation curve axis and it’s seldom to use 10 times to describe the gradation curve axis.
8.In table 3, three matters need to think over. Firstly, all these tests should notice in distinguishing coarse aggregates and fine aggregates. Secondly, check the spelling of “Un compacted”. Thirdly, “Elongation” does not meet the requirement.
9. The last sentence in 2.2 sample preparation section is lack of evidence and it should be the conclusion of this article.
10. In the mixer selection of first paragraph, the parameters of mixing rate and select mechanical stirrer are key points, why did the authors choose these parameters?
11. In the solvent selection for wet mixing, has anyone used these three types of solvents or what kinds of solvents did they use? Moreover, does methanol have any residual or affect the properties of asphalt? Although figure 6 has explained it, but it also needs to write the test method in this paragraph.
12. The magnifications of figures 5(a) and (b) are not the same, so it is not suitable to compare the dispersion of CNTs. It is recommended to compare the dispersion of CNTs with the same magnification.
13.What can we obtain from figure 3?
14.Figure 4 lacks of detailed description.
15. It needs to check the superscript (cm-1) in the axis of figure 6.
16. What is the test specification of FTIR and DMA?
17. In 2.4.4 moisture susceptibility analysis, in “Coating percentage of each sample is checked after 24 hours,48 hours and 72 hours”, whether the checked process is manual or image recognition
18. In line 267, two doses of 0.5% and 1.5% CNTs may be missed.
19. In line 269, the expression of “Marshal Mix” is inaccurate.
20. In line 272, in “OBC for unmodified sample was 4.38% while for 1% and 3% CNTs modified samples it was 4.42% & 4.47 % respectively”, the reasons for this change need to be analyzed.
21. The representation of the title in figure 12 is inappropriate.
22. The δ in the heading of figure 13 does not match the G* in ordinate of the picture.
23. In line 383, the more CNTs content, the better? The regularity of figure 15 is not very obvious, is there an optimal CNTs content?
24. It is suggested that the color of Figure 17 should be changed.
25. The conclusion should be refined, because the conclusion is not a repeat of the previous results.
Author Response
Response to Reviewer 1
The authors appreciate and acknowledge the reviewer’s effort and time spent for the review of this manuscript. Manuscript has been revised in the light of the reviewers’s comments and suggestions.
Point 1: This article’s title use ‘Homogenous Dispersion’. It’s the most important point in the article but authors didn’t expound it clear. The method of characterizing the uniform dispersion of CNTs is too simple, only SEM.
Response 1: Scanning Electron Microscopy (SEM) is considered as a reliable technique to check the dispersion of CNTs in bitumen. Most of the previous researchers [1–5] performed Scanning Electron Microscope to ensure dispersion of nanomaterials in substrates.
In order to ensure homogenous dispersion of CNTs in asphalt binder a three-step methodology was adopted for this research study (section 2.2 &2.3 of the main manuscript). In the first step one of the two available mixers (Mechanical Stirrer & High Shear Mixer) was selected. In the second step, selection of solvent for wet mixing was carried out. The third step involved selection of wet or dry mixing technique on the basis of storage stability test. Fourier Transform Infrared Spectroscopy (FTIR) was used to check complete removal of solvent from the bitumen. Therefore, homogenous dispersion was achieved through a step by step procedure where other tests such as FTIR and storage stability were also part of the methodology. Along with SEM, the storage stability test also helped characterizing uniform dispersion of CNTs in the binder.
Section 2.4 “Investigation into homogenous dispersion”, page no. 8, of the main manuscript has been beefed up accordingly.
2.4.3 Storage Stability Test
Storage Stability Test also ensured the homogenous dispersion of CNTs in the binder. The difference in softening point of the represented sample taken from the top and bottom of the storage test aluminum tube was within the permissible limit (2.2°C) for wet mixing technique. The results are shown in table below.
Table 4: Storage Stability test data for CNTs modified and unmodified binder
CNTs Dosage | 0% | 0.5% | 1% | 1.5% | 3% |
SP of Top Portion(°C) | 48.9 | 50.7 | 55.4 | 57 | 57.8 |
SP of Bottom Portion(°C) | 49.3 | 51.7 | 56.7 | 58.5 | 59.7 |
Difference(°C) | 0.4 | 1 | 1.3 | 1.5 | 1.9 |
Point 2: In the second paragraph of introduction section, there are many applications about CNTs in cement, but there are no relevant articles and literatures in introduction.
Response 2: Pavement materials modification through addition of polymers and other additives like fly ash and lime has been utilized a lot in the past[6]. Pavement engineers continuously look for innovative additives/modifiers in order to enhance the performance properties of road materials. Nanotechnology has recently gained popularity in the scientific world. Due to remarkable achievements of nanotechnology in other fields of engineering, researchers have started looking into its utilization in the field of civil engineering. Nanotechnology is being used a lot for the modification and enhancement in properties of cement concrete[7,8].Effect of nanomaterials on the properties of asphalt concrete is an emerging field and a lot is required to be explored.
The above-mentioned lines related to cement modification using CNTs and other nanomaterials has been added in the manuscript in line 64-70.
Point 3: The introduction section does not show the insufficiency of previous studies and the innovation of this study clearly.
Response 3: The introduction section has been modified as per the suggestion of the reviewer. The insufficiency of the previous studies and the innovation of this study has been discussed in detail (Page 2-3, line “90-121” of the main manuscript). The modified section is also provided below for ready reference.
Previously different researchers used different dosages of CNTs as a modifier to enhance the properties of asphalt binder. Gong et al, (2017) investigated the effect of CNTs on performance, chemical and structural properties of asphalt binder[9]. Amin et al. (2016) used MWCNT as an additive in bitumen and concluded that the modification improved both high and low temperature performance of bitumen[10]. Galooyak et al. (2015) added MWCNTs in asphalt binder and examined the effects on conventional and rheological properties and concluded that CNTs improved the rheological as well as conventional properties of bitumen [11]. In the previous research it was also observed that CNTs enhance the resistance of asphalt against ageing which resulted in the increase in the pavement life [10,12]. The Biggest challenge in developing CNTs reinforced material is the dispersion of CNTs in base binder. In order to homogenously disperse the CNTs in bitumen, long chains of CNTs produced by synthesis and agglomerates of CNTs produced by intermolecular van der Walls forces must be broken [13]. Inappropriate dispersion of CNTs may affect the mechanical properties of modified binder. CNTs are dispersed in bitumen by either using dry mixing technique or wet mixing technique. Most of the previous work has been carried out using dry mixing while limited work has been done using the wet mixing technique. Faramarzi et al, (2015) used both wet and dry mixing techniques and concluded that wet mixing technique has better ability to homogenously disperse CNTs as compared to the dry method. Kerosene was used as a solvent for this study [4]. The draw back with Faramazi’s study was that, kerosene didn’t fully evaporate from the bitumen after mixing which may influence the resulting binder’s properties. Ziari et al, (2012) compared different mixers in his study and concluded that with high shear mixer and ultrasonic mixer, homogenous dispersion of CNTs can be achieved [1]. It may be noted from the literature that though wet mixing is a better dispersion technique, the selection of appropriate solvent and its complete evaporation after achieving a homogenous dispersion matters the most. Therefore, a thorough study from this perspective was carried out in this research.
Basically, this study consists of two parts. The first part of this manuscript focuses on the dispersion of CNTs in asphalt binder while the second part studies the effect of CNTs on the performance properties of resulting asphalt binder. Dispersion of CNTs in bitumen is a complex phenomenon [1,4,9]. Although in previous researches different researchers used CNTs in asphalt binder to enhance the properties of asphalt binder but to authors best knowledge no comprehensive study was available for the dispersion of CNTs in bitumen. In this research, a detailed methodology was developed for the homogenous dispersion of CNTs in bitumen. Also, Previous work mostly studies the effect of CNTs on the rheological properties of bitumen but the effect of CNTs addition on bitumen-aggregate adhesion and moisture sensitivity is required to be explored.
Point 4: Take care of the unit superscript and subscript of “2.1g/cm3” and “60m2/g” in table 2.
Response 4: The required changes have been made in the manuscript, “Page no. 3, Table 2”
Point 5: In line 112, it’s a wrong formulation of “aggregate material”.
Response 5: The required correction has been made and Figure 2 “Gradation curve of aggregates used” (at Page 4 of the manuscript) has been modified accordingly.
Point 6: In line 114, it would be better to indicate the kind of asphalt mixture, for example AC or SMA or other kinds.
Response 6: The asphalt mixture type has been mentioned as suggested by the reviewer (Page no. 4, Line no. 140)
Point 7: In figure 2, 0.5mm or 5mm are rarely used as unit of gradation curve axis and it’s seldom to use 10 times to describe the gradation curve axis.
Response 7: The gradation curve axis has been modified as suggested by the reviewer. (Page no. 4, Figure 2)
Point 8.In table 3, three matters need to think over. Firstly, all these tests should notice in distinguishing coarse aggregates and fine aggregates. Secondly, check the spelling of “Un compacted”. Thirdly, “Elongation” does not meet the requirement.
Response 8: In table 3 all the tests have now been distinguished in coarse and fine aggregates. The spelling of Un-compacted has been corrected in the manuscript. The limiting value for flakiness index and elongation index is 15% as per the National Highway Authority (NHA) Pakistan specifications (NHA general specifications, 1998). The Table 3 has been corrected accordingly.
All the changes suggested by the reviewers have been made in the main manuscript (Table 3 ,Page no. 4).
Property | Standard | Value | Limit |
Los Angeles Abrasion (Coarse) | ASTM C 131 | 15 | ≤15% |
Soundness (Course) | ASTM C 88 | 7.1 | ≤ 8% |
Soundness(Fine) | ASTM C 88 | 4.7 | ≤ 8% |
Water Absorption (Coarse) | ASTM C 127 | 1.02 | ≤ 2% |
Uncompacted Voids (Coarse) | ASTM C 1252 | 37.5 | ≥ 45% |
Flakiness (Coarse) | BS 812.108 | 5 | ≤ 15%* |
Elongation (Coarse) | BS 812.109 | 11 | ≤15%* |
Fractured Particles (Coarse) | ASTM D 5821 | 100 | ≥ 90% |
Sand Equivalent(Coarse) | ASTM D 2419 | 75 | ≥50% |
*[14]
Point 9: The last sentence in 2.2 sample preparation section is lack of evidence and it should be the conclusion of this article.
Response 9: The last sentence in 2.2 has been included in the conclusion of this article as suggested by the reviewer.
Different nanomaterials require different mixing conditions for their dispersion in bitumen. For some nanomaterials, dry mixing and mechanical stirrer is sufficient to disperse them in bitumen while some require High shear mixing and wet mixing for homogenous dispersion. So, a thorough study was required on uniform dispersion of CNTs in asphalt binder.
The above lines have been added in main manuscript at page 5 from line “152-156”.
Point 10: In the mixer selection of first paragraph, the parameters of mixing rate and select mechanical stirrer are key points, why did the authors choose these parameters.
Response 10: “Researchers have been utilizing different mixing techniques for dispersion of nanomaterials in the bitumen. The most commonly selected instrument is either mechanical stirrer or high shear mixer. Mechanical stirrer uses simple propeller/fan blade configuration for mixing while high shear mixer generates a vortex type movement within the material to ensure fast and aggressive mixing. This study evaluated both the types of mixers. One of the two mixers (mechanical stirrer and High Shear Mixer) was selected on the basis of dispersion of CNTs in asphalt binder. In many previous researches mixing rate of around 1500rpm with mechanical stirrer has been used for the dispersion of CNTs in bitumen[1,4,11] while a mixing speed of around 3000 rpm is generally selected for High Shear Mixer[1,15]. These mixing rates were selected for this study as well.”
The above discussion has also been accorporated into the main manuscript. (Page no. 5, Line no. 162-171).
Point 11: In the solvent selection for wet mixing, has anyone used these three types of solvents or what kinds of solvents did they use? Moreover, does methanol have any residual or affect the properties of asphalt? Although figure 6 has explained it, but it also needs to write the test method in this paragraph.
Response 11: Basically, very limited work been done on dispersion of CNTs in asphalt binder using wet mixing technique. Framarzi et al used kerosene oil for dispersion of CNTs in binder but he concluded that kerosene was not fully evaporated from the bitumen which effected the results[4].While Larisa used Toluene as a solvent in her research [16]. Any similar solvent that can be removed from the asphalt binder after dispersion of CNTs would be a good choice. Methanol does not have any residual effect on the properties of asphalt as it is evaporated from the mix. This was checked/confirmed with the help of FTIR analysis.
The relevant test method (ASTM E1552) has been mentioned in the paragraph as suggested by the reviewer at Page no. 8, Line no. 239-241 of main manuscript.
Point 12: The magnifications of figures 5(a) and (b) are not the same, so it is not suitable to compare the dispersion of CNTs. It is recommended to compare the dispersion of CNTs with the same magnification.
Response 12: Figure 5(b) has been replaced with a higher magnification image and the relevant paragraph has been modified accordingly. (Page no. 7, line no. 225-229)
Figure 5(a) shows an SEM image of CNTs modified asphalt binder prepared using mechanical stirrer while Figure 5(b & c) show the SEM images of CNTs modified asphalt binder prepared using high shear mixer. Figure 5(a) clearly shows CNTs agglomerate at a magnification of 4.05kx while Figure 5(b) shows evenly distributed CNTs even at a higher magnification of 8.88kx.
Point 13: What can we obtain from figure 3?
Response 13: Figure 3 demonstrates the dispersion and stability of CNTs in solvent(methanol). From figure 3(a) it can be seen that without sonication of solvent and CNTs, CNTs settled down at the bottom in the solvent while in Fig 3(b) it can be seen that after sonication and stirring of CNTs in solvent, agglomerates of CNTs were broken and CNTs did not settle down at the bottom making the sample storage stable. The Figure 3(a) has also been labeled for clarity.
Point 14: Figure 4 lacks detailed description.
Response 14: Figure 4 is showing the SEM image of CNTs in solvent prior to their addition in the asphalt binder.
The Figure 4 title has been modified accordingly. (Page no. 07)
Point 15: It needs to check the superscript (cm-1) in the axis of figure 6.
Response 15: Change has been made in Figure 6 according to reviewer’s suggestion at Page no. 8.
Point 16: What is the test specification of FTIR and DMA?
Response 16: Required changes have been made in line “239-241” and line “267-273” of the manuscript.
While FTIR was performed according to ASTM E1252. For FTIR, 6700 Nicolet(model) spectrometers was employed to get the asphalt spectra in the range of 400–4000 cm−1 wavenumber.
“DMA test was performed on Dynamic Shear Rheometer(DSR) according to AASHTO T 315. DSR Plates of 25mm dimeter with 1mm gap were used to test the samples at temperatures higher than 46°C while 8mm diameter plates with the gap of 2mm were used for testing the samples at temperatures less than 46°C. High temperature performance grading (PG) and frequency sweep tests were conducted. In frequency sweep test strain was kept constant as per superpave criteria (10% for unmodified sample and 2% for modified sample) and frequency ranged from 10-0.1 Hz at required temperatures.”
Point 17: In 2.5.4 moisture susceptibility analysis, in “Coating percentage of each sample is checked after 24 hours,48 hours and 72 hours”, whether the checked process is manual or image recognition.
Response 17: In 2.5.4 the moisture susceptibility analysis was carried out using rolling bottle technique as per (BS EN 12697-11) which involves a visual inspection of the samples after moisture conditioning.
Point 18: In line 267, two doses of 0.5% and 1.5% CNTs may be missed
Response 18: To check the effect of CNTs on the asphalt mixtures Wheel Tracker test was performed. The only purpose of wheel tracker was to check the effect of CNTs on rutting performance pf asphalt mixtures. So only three dosages of 1% and 3% of CNTs were taken to perform Wheel Tracker and study its effect.
Point 19: In line 269, the expression of “Marshal Mix” is inaccurate.
Response 19: The mentioned expression has been corrected in the main manuscript Page no 8 line “311-314” of manuscript and same is reproduced below.
“First Marshall Method of Mix design was carried out to determine the optimum binder content of all 3 samples. Marshall Method of Mix design was performed as per ASTM D1559 using hammer weight to 4.5lb, hammer drop height of 18in and application of 75 blows on each side (for heavy traffic) of the specimen.
Point 20: In line 272, in “OBC for unmodified sample was 4.38% while for 1% and 3% CNTs modified samples it was 4.42% & 4.47 % respectively”, the reasons for this change need to be analyzed.
Response 20: The reason for the change in OBC with CNT dosage has been discussed in the main manuscript (Page no. 10, Line no. 297-304) and the same is reproduced below.
“However, researchers have experienced both increase and decrease in the optimum binder content requirements while utilizing Nano materials[17][18]. Here in this study, an increase in optimum binder content was experienced with the increase in CNTs dosage. Same trend of increases in OBC by adding nanomaterial was observed by Chelovian and Shafabakhsh [19].This increase in OBC could be attributed to large surface area of CNTs used in this study. This Can also be due to the increase in viscosity of the asphalt binder with the addition of addition of the nano material which made binder stiff. A higher viscosity leads to a thicker binder film of the modified bitumen in the mix thus increasing the binder volume in the mix.[20,21].”
Point 21: The representation of the title in figure 12 is inappropriate.
Response 21: Title of the figure 12 has been modified as suggested by the reviewer (Page no. 13).
Point 22: The δ in the heading of figure 13 does not match the G* in ordinate of the picture.
Response 22: Figure 13 Y-axis title has been corrected. (Page no. 14).
Point 23: In line 383, the more CNTs content, the better? The regularity of figure 15 is not very obvious, is there an optimal CNTs content?
Response 1: Black diagrams can be used to differentiate between thermosrheologically simple and complex materials. The base binder Black diagram in Fig. 15 shows a very smooth curve, which indicates time–temperature equivalency for this binder (thermorheologically simple). The return of the Black diagrams toward lower phase angles for all CNTs modified sample shows breakdown of Time-Temperature-Superposition-Principle (TTSP) and presence of CNTs modification for these binders (thermorheologically complex) [22,23]. Black diagram curves simultaneously provide information about the elastic and stiffness properties of the binders however, as modified binders do not provide smooth curves, it is difficult to decide optimum CNTs content on the basis of these curves.
Point 24: It is suggested that the color of Figure 17 should be changed.
Response 24: The colors of the Fig 17 at “Page 16” as suggested by the reviewer.
Point 25: The conclusion should be refined, because the conclusion is not a repeat of the previous results.
Response 25: The conclusion section of the main manuscript has been revised as suggested by reviewer. the same is reproduced below for ready reference (Page 17-18, Line 463-490)
· Wet mixing techniques better helps in achieving homogenous dispersion of CNTs in bitumen as compared to dry mixing.
· Sonication and magnetic stirring is necessary to improve the stability of CNTs in solvent.
· Introduction of CNTs in asphalt binder resulted in reduction in Penetration value and ductility value while it increased the softening point value of the bitumen which means stiffness of the bitumen increased with the addition of CNTs.
· With the addition of CNTs the PI value of bitumen increases which means the bitumen temperature sensitivity decreased and thermal susceptibility was also decreased.
· Complex shear modulus(G*) value increased and phase angle of the bitumen tend to decrease with increase in CNTs which means the stiffness and elastic behavior of bitumen improved with the addition of CNTs in asphalt binder. While rut factor value also improved which indicates the increase in resistance against permanent deformation.
· High Performance Grade(PG) of the bitumen increased with an increase in the CNTs dosage.PG 70 is recommended for most of the regions of the country. This study aimed for achieving a PG 76 after a great bump in the required PG 70 in order to accommodate for the overloading on the highways of the country. The required PG of 76oC was achieved with 3% addition of CNTs in bitumen. Asphalt samples modified with 3% CNTs showed improved results for all the parameters examined in this study. Hence 3% CNTs dosage was selected as the required optimum dosage.
· Bitumen Bond Strength value improved by adding CNTs in bitumen in both 24 hours dry and 24 hours moist conditions while moisture susceptibility of bitumen decreased with addition of CNTs.
· From wheel tracker test results, it is concluded that the addition of 1 % and 3% CNTs in bitumen reduced the rut depth by 25% and 37% respectively when tested at 40oC.This is an indication of an increase in the resistance against permanent deformation at higher temperature for CNTs modified binder.
· Although wet mixing is a complicated process but is preferred over the dry mixing technique because it ensures homogenous dispersion of CNTs in the binder.
References: -
1. Hasan, Z.; Kamran, R.; Mohammad, F.; Ahmad, G.; Hosein, F. Evaluation of Different Conditions on The Mixing Bitumen and Carbon Nano-Tubes. Int. J. Civ. Environ. Eng. IJCEE-IJENS 2012, 12, 53–59.
2. Ziari, H.; Hosein, F.; Goli, A. Using the Statistical Analysis of Carbon Nano-tubes Dispersion in Bitumen Employing Software MINITAB. Int. J. Transp. Eng. 2013, 1, 125–136.
3. Faramarzi, M.; Arabani, M.; Haghi, A.K.; Motaghitalab, V. A Study on the Effects of CNT’s on Hot Mix Asphalt Marshal-Parameters. In 7thSASTech 2013, Iran, Bandar-Abbas; 2013; pp. 1–9.
4. Faramarzi, M.; Arabani, M.; Haghi, A.K.; Mottaghitalab, V. Carbon nanotubes-modified asphalt binder: Preparation and characterization. Int. J. Pavement Res. Technol. 2015, 8, 29–37, doi:10.6135/ijprt.org.tw/2015.8(1).29.
5. Li, R.; Xiao, F.; Amirkhanian, S.; You, Z.; Huang, J. Developments of nano materials and technologies on asphalt materials – A review. Constr. Build. Mater. 2017, 143, 633–648, doi:10.1016/j.conbuildmat.2017.03.158.
6. Golewski, G.L. Generalized fracture toughness and compressive strength of sustainable concrete including low calcium fly ash. Materials (Basel). 2017, 10, 1–22, doi:10.3390/ma10121393.
7. Raki, L.; Beaudoin, J.; Alizadeh, R.; Makar, J.; Sato, T. Cement and concrete nanoscience and nanotechnology. Materials (Basel). 2010, 3, 918–942, doi:10.3390/ma3020918.
8. Sanchez, F.; Sobolev, K. Nanotechnology in concrete - A review. Constr. Build. Mater. 2010, 24, 2060–2071, doi:10.1016/j.conbuildmat.2010.03.014.
9. Gong, M.; Yang, J.; Yao, H.; Wang, M.; Niu, X.; Haddock, J.E. Investigating the performance, chemical, and microstructure properties of carbon nanotube-modified asphalt binder. Road Mater. Pavement Des. 2017, 0629, 1–24, doi:10.1080/14680629.2017.1323661.
10. Amin, I.; El-Badawy, S.M.; Breakah, T.; Ibrahim, M.H.Z. Laboratory evaluation of asphalt binder modified with carbon nanotubes for Egyptian climate. Constr. Build. Mater. 2016, 121, 361–372, doi:10.1016/j.conbuildmat.2016.05.168.
11. Galooyak, S.S.; Palassi, M.; Farahani, H.Z.; Goli, A. Effect of carbon nanotube on the rheological properties of bitumen. Pet. Coal 2015, 57, 556–564.
12. Omar, H.A.; Yusoff, N.I.M.; Sajuri, Z.; Ceylan, H.; Jakarni, F.M.; Ismail, A. Determining the effects of aging on halloysite nano-tube modified binders through the pull-off test method. Constr. Build. Mater. 2016, 126, 245–252, doi:10.1016/j.conbuildmat.2016.09.038.
13. Santagata, E.; Baglieri, O.; Tsantilis, L.; Chiappinelli, G. Fatigue properties of bituminous binders reinforced with carbon nanotubes. Int. J. Pavement Eng. 2015, 16, 80–90, doi:10.1080/10298436.2014.923099.
14. National Highway Authority Pakstan,General Specifications; 1998;
15. Amin, I.; El-Badawy, S.M.; Breakah, T.; Ibrahim, M.H.Z. Effect of Functionalization and Mixing Process on the Rheological Properties of Asphalt Modified with Carbon Nanotubes. Am. J. Civ. Eng. Archit. 2016, 4, 90–97, doi:10.12691/ajcea-4-3-4.
16. Larisa, U.; Nikolay, S.; Aleksandr, S.; Natalya, S.; Irina, S. Improving the quality of asphalt coating with carbon nanomodifiers. IOP Conf. Ser. Mater. Sci. Eng. 2015, 87, doi:10.1088/1757-899X/87/1/012051.
17. Ameri, M.; Vamegh, M.; Rooholamini, H.; Haddadi, F. Investigating Effects of Nano/SBR Polymer on Rutting Performance of Binder and Asphalt Mixture. Adv. Mater. Sci. Eng. 2018, 2018, doi:10.1155/2018/5891963.
18. Qasim, Z.I.; Qasim, Z.I. Effect of Filler Content on Properties of Asphaltic Mixtures for Marshall and Superpave Gyratory Compactor. Al-Nahrain J. Eng. Sci. 2017, 20, 183–193.
19. Chelovian, A.; Shafabakhsh, G. Laboratory evaluation of Nano Al2O3effect on dynamic performance of stone mastic asphalt. Int. J. Pavement Res. Technol. 2017, 10, 131–138, doi:10.1016/j.ijprt.2016.11.004.
20. Hussain, M.H.M.; Ghaly, N.F.; Ibrahim, I.M. Modified Hot Mix Asphalt for Road Maintenance. World Appl. Sci. J. 2008, 5, 236–245.
21. Rafi, J.; Kamal, M.; Ahmad, N.; Hafeez, M.; Faizan ul Haq, M.; Aamara Asif, S.; Shabbir, F.; Bilal Ahmed Zaidi, S. Performance Evaluation of Carbon Black Nano-Particle Reinforced Asphalt Mixture. Appl. Sci. 2018, 8, 1114, doi:10.3390/app8071114.
22. Airey, G.D. Use of Black Diagrams to Identify Inconsistencies in Rheological Data. Road Mater. Pavement Des. 2011, 4, 403–424, doi:10.1080/14680629.2002.9689933.
23. Asgharzadeh, S.M. Modified Asphalt Binders Evaluation Using Rheological Master Curves and Black Diagrams Mixing Conditions. In Proceedings of 5th IASTEM International Conference; 2015; pp. 5–11.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript presents very valuable results related to the modification of property of asphalt with carbon nanoyubes (CNTs). The presented methodology is clear and related to the current state of the art in this subject. To improve the quality of the paper it needs a revision before final publication. The suggestions are as follows:
(1) First of all, it is necessary to change the title of the manuscript. In the current version, the title only gives an abbreviation of the expression Carbon Nanotubes, i.e. CNTs. Properly, the full name should already be given in the title and the abbreviation in brackets. Therefore, the title of the article should be as follows: “Carbon Nanotubes (CNTs) in Asphalt Binder: Homogeonus Dispersion and Performance Enhancement”.
(2) The article is in the field of engineering of building materials. It concerns their modification with very fine active additives which characterized large surface area. This topic has already been the subject of publication in the journals from MDPI Database. It is therefore required that the authors comment on the results of previous articles. In the introduction section, the following article should be discussed and cited:
“Generalized fracture toughness and compressive strength of sustainable concrete including low calcium fly ash”, Materials, 2017.
(3) Please provide more details on SEM AND TEM microscopes used in the studies.
(4) In what units the dimensions in Figure 7 are given. Please complete this.
(5) Please show the results on figures 16 and 17 in different colors and not in shades of gray.
(6) Please explain it precisely, why exactly 4 CNTs were selected, i.e.: 0.5, 1.0, 1.5 and 3.0%. It's known how will be a tendency in the obtained results. Would not it be better to choose 3 options: 0.5, 1.5 and 3.0.
(7) CNTs is quite expensive material. Will the presented method of asphalt modification be profitable if the method is used in industrial conditions ? Please discuss this issue.
Author Response
Response to Reviewer 2 comments
The authors appreciate and acknowledge the reviewer’s effort and time spent for the review of this manuscript. Manuscript has been revised in the light of the reviewers’s comments and suggestions.
The introduction section has been improved following the suggestions from the reviewer. The research design has been further elaborated by explaining the research motivation and improving the problem statement. The results section has also been modified accordingly. Conclusions have further been beefed up with reference to results and supporting arguments.
Point 1: First of all, it is necessary to change the title of the manuscript. In the current version, the title only gives an abbreviation of the expression Carbon Nanotubes, i.e. CNTs. Properly, the full name should already be given in the title and the abbreviation in brackets. Therefore, the title of the article should be as follows: “Carbon Nanotubes (CNTs) in Asphalt Binder: Homogenous Dispersion and Performance Enhancement”.
Response 1: Title of the manuscript has been changed as suggested.
Point 2: The article is in the field of engineering of building materials. It concerns their modification with very fine active additives which characterized large surface area. This topic has already been the subject of publication in the journals from MDPI Database. It is therefore required that the authors comment on the results of previous articles. In the introduction section, the following article should be discussed and cited:
“Generalized fracture toughness and compressive strength of sustainable concrete including low calcium fly ash”, Materials, 2017.
Response 3:
The suggested modification has been incorporated into the manuscript at page 2, line “58-59” and is reproduced below for ready reference.
“Civil engineering materials modification through addition of polymers and other additives like fly ash and lime has been utilized a lot in the past[1]”.
Point 3: Please provide more details on SEM and TEM microscopes used in the studies.
Response 3: More details about SEM and TEM have been included into the manuscript (Page 4, Line 132-134) and ( (page 5 and lines “173-177”) and the same is reproduced below for ready reference.
SEM is based on scattered electrons while TEM is based on transmitted electrons. SEM focuses on the sample’s surface and its composition whereas TEM provides the details about internal composition. TEM can show many characteristics of the sample, such as morphology[2].
On the other hand, SEM also shows the morphology of samples. In Scanning Electron Microscopy (SEM) specimen is examined point by point directly in a moving electron beam. Electrons reflected by the specimen are used to form a magnified, three-dimensional image on a television screen[3].
Point 4: In what units, the dimensions in Figure 7 are given. Please complete this.
Response 4: In Figure 7 all the dimensions are in millimeter (mm). The unit has also been mentioned in the figure title.
Point 5: Please show the results on figures 16 and 17 in different colors and not in shades of gray.
Response 5: Required changes have been made in the manuscript at Page no. 16. The modified figures are copied below for ready reference.
Point 6: Please explain it precisely, why exactly 4 CNTs were selected, i.e.: 0.5, 1.0, 1.5 and 3.0%. It's known how will be a tendency in the obtained results. Would not it be better to choose 3 options: 0.5, 1.5 and 3.0.
Response 6: As given in section 3.2 our main objective of the research was to improve the high temperature performance of the asphalt binder. It is given in section 3.2 that the required PG for in this study for Pakistan is PG 76. So, main objective of this research was to obtain higher failure temperature, i.e., more than 76°C. First CNTs were added with the interval of 0.5% (0.5%,1%,1.5%,2%,2.5% and 3%). At 3% of CNTs PG grade of 76°C was achieved as shown in figure 11. Once the required PG of 76 was achieved, then for the conventional testing the dosage interval was increased from 0.5% to 1.5%.
Dosages of 0.5%, 1%, 1.5% and 3% were selected to compare the effect of different CNTs content on the performance properties of asphalt binder. For Mixture testing we further increased the dosage gap and used only two dosages of 1% and 3% CNTs.
(Figure 11, Page no. 13)
Point 7: CNTs is quite expensive material. Will the presented method of asphalt modification be profitable if the method is used in industrial conditions? Please discuss this issue.
Response 7:
1 No doubt CNTs are expensive material. However, due to the achievement of exceptional results from CNTs in different fields of science in the recent past, they are now being produced in large quantities using different techniques and their cost is decreasing progressively.
2 Under certain environmental/loading conditions sometimes the pavements constructed using unmodified binders fail prematurely. With CNTs modified asphalt the failure of pavement can be delayed or in other words the pavements can be made to last longer hence substantially decreasing the cost of maintenance and rehabilitation, which is far greater than the cost of the CNTs.
3 On the other hand, due to the increase of the asphalt strength with addition of CNTs, the thickness of pavement layers can be decreased consequently compensating the cost of CNTs.
Reference: -
1. Golewski, G.L. Generalized fracture toughness and compressive strength of sustainable concrete including low calcium fly ash. Materials (Basel). 2017, 10, 1–22, doi:10.3390/ma10121393.
2. Stadtländer, C.T.K.-H. Scanning electron microscopy and transmission electron microscopy of mollicutes: challenges and opportunities. Mod. Res. Educ. Top. Microsc. Clemson Univ. Clemson, SC 29634, USA 2007, 122–131.
3. Collett, B.M. Scanning Electron Microscopy: A Review And Report Of Research In Wood Science. Wood Fiber Sci. 1967, 2, 113–132.
Author Response File: Author Response.pdf
Reviewer 3 Report
Please explain about the novelty of the paper. What does it add to the current literature? The effects of CNT on the rheological properties of asphalt binder has been evaluated in the previous research. In your research, your main goal is stated as finding a good method for the dispersion of CNT in asphalt binders. However, only a short part of the paper compares the difference between dry mixing method and wet mixing method. The big portion of your paper discusses the effects of CNT on the properties of asphalt binders and asphalt mixtures, which is done in the previous paper.
Line 26: latter
Lines 42-43: this sentence should be modified. Do you mean asphalt mixture/asphalt concrete?
Line 44: it is actually common and expected for pavements to fail before reaching the end of their service life. Due to the premature failures in asphalt pavement, several maintenance cycles are considered throughout during the service life of the pavement.
Lines 44-46: this statement is incorrect. Many types of modified asphalt binders (e.g., SBS-modified binder or rubber-modified binder) are categorized as tradition pavement materials, however, they perform acceptable in the field. Researchers always make attempts to improve the properties of the pavement materials, but it does not mean that the current materials fall short in meeting the practical demands for present and future pavements.
Line 50: it would be suggested to add the most recent review paper in here.
Behnood, A., Modiri Gharehveran, M. Morphology, rheology, and physical properties of polymer-modified asphalt binders, European Polymer Journal, 2018. https://doi.org/10.1016/j.eurpolymj.2018.10.049.
Line 68: Please change to “ … Multi-walled CNTs (MWCNTs)…” since you are using MWCNT in line 72 but you did not define before line 72.
Line 73: Please add the reference (i.e., [7]) for this statement at the end of the sentence.
In Table 2, please have the full form of “SSA”. In addition, please have “2” in the form of superscript.
It would be suggested to add the vertical grid lines in Figure 2.
Line 127: What do you mean by “high frequencies”? do you mean using high shearing effort?
Line 145: was carried out
Line 177: please remove the second “the solution was”
Caption of figure 5 should be revised.
Writing style suddenly changes after line 256!!
In section 2.4.2, it has been stated that DSR tests were conducted at the temperatures below 46C. However, I could not find the results of those tests. Please also clear on what type of binder you conducted the DSR tests. Were those aged or unaged?
Author Response
Response to Reviewer 3 comments:
The authors appreciate and acknowledge the reviewer’s efforts and time, they spent on the review of this manuscript. Manuscript has been revised in the light of the reviewers’s comments and suggestions.
Please explain about the novelty of the paper. What does it add to the current literature? The effects of CNT on the rheological properties of asphalt binder has been evaluated in the previous research. In your research, your main goal is stated as finding a good method for the dispersion of CNT in asphalt binders. However, only a short part of the paper compares the difference between dry mixing method and wet mixing method. The big portion of your paper discusses the effects of CNT on the properties of asphalt binders and asphalt mixtures, which is done in the previous paper.
Novelty of the paper is explained below and has been incorporated in the main manuscript at page 3 line “113-121” for ready reference
Basically, this study consists of two parts. The first part of this manuscript focuses on the dispersion of CNTs in asphalt binder while the second part studies the effect of CNTs on the performance properties of resulting asphalt binder. Dispersion of CNTs in bitumen is a complex phenomenon [1–3]. Although in previous researches different researchers used CNTs in asphalt binder to enhance the properties of asphalt binder but to authors best knowledge no comprehensive study was available for the dispersion of CNTs in bitumen. In this research, a detailed methodology was developed for the homogenous dispersion of CNTs in bitumen. Also, Previous work mostly studies the effect of CNTs on the rheological properties of bitumen but the effect of CNTs addition on bitumen-aggregate adhesion and moisture sensitivity is required to be explored.
Point 1: Line 26 : Latter
Response 1: Suggested correction has been manuscript at line 26 of the main manuscript.
Point 2: Lines 42-43: this sentence should be modified. Do you mean asphalt mixture/asphalt concrete?
Response 2: Sentence has been modified at line 43 of the main manuscript.
Point 3: Line 44: it is actually common and expected for pavements to fail before reaching the end of their service life. Due to the premature failures in asphalt pavement, several maintenance cycles are considered throughout during the service life of the pavement.
Response 3: The relevant section of the manuscript has been further elaborated, and is reproduced below for reference. (Page 2, Line no. 44-54)
“Due to a rapid increase in the traffic loading and volume, pavements in Pakistan fail prematurely and the revival of their serviceability normally requires a lot of resources and finance. Pavements constructed utilizing conventional materials (especially virgin bitumen with softening points far less than the temperatures to which the pavements are exposed) fail many times within the design life putting a lot of burden on the annual maintenance budget and ultimately increasing the life cycle cost of the pavement. If the bitumen is made to last/survive longer, the pavement failures can be delayed which will minimize the life cycle cost of the pavements. Regular wear and tear is expected but total failure before the end of the design life, as it happens a lot in Pakistan, is not desirable. It not only effects the maintenance costs but causes a lot of nuisance for the road users in terms of delays, accidents, and vehicle operating costs. This study aims to address this issue”.
Point 4: Lines 44-46: this statement is incorrect. Many types of modified asphalt binders (e.g., SBS-modified binder or rubber-modified binder) are categorized as tradition pavement materials, however, they perform acceptable in the field. Researchers always make attempts to improve the properties of the pavement materials, but it does not mean that the current materials fall short in meeting the practical demands for present and future pavements.
Response 4: The relevant section of the manuscript has been modified in line with the above discussion. (Page 2, line no. 54)
In these sentences, the authors were focusing on the locally available pavement materials in Pakistan. In Pakistan, premature pavement failures are quite common when locally produced binders are used without modification. Researchers in the country have been looking into different materials like SBS and rubber modified binders but no single material has gained complete acceptance in the industry. This study is an effort to explore other suitable materials that could survive the local harsh traffic and environmental conditions.
Point 5: Line 50: it would be suggested to add the most recent review paper in here.
Behnood, A., Modiri Gharehveran, M. Morphology, rheology, and physical properties of polymer-modified asphalt binders, European Polymer Journal, 2018. https://doi.org/10.1016/j.eurpolymj.2018.10.049.
Response 5: The suggested article has been added in the manuscript at Line no. 60 of the main manuscript.
Point 6: Line 68: Please change to “ … Multi-walled CNTs (MWCNTs)…” since you are using MWCNT in line 72 but you did not define before line 72.
Response 6: Suggested change has been made in Line no. 86.
Point 7: Line 73: Please add the reference (i.e., [7]) for this statement at the end of the sentence.
Response 7: Reference has been added at the end of the sentence according to the reviewer’s suggestion at Line no. 94 of main manuscript.
Point 8: In Table 2, please have the full form of “SSA”. In addition, please have “2” in the form of superscript.
Response 8: Suggested changes have been made in Table 2 at Page no. 3.
Point 9: It would be suggested to add the vertical grid lines in Figure 2.
Response 9: Vertical grid lines have been added in Figure 2, Page no. 4.
Point 10: Line 127: What do you mean by “high frequencies”? do you mean using high shearing effort?
Response 10: Here higher frequencies mean higher number of Rotations per Minutes (RPM).
Point 11: Line 145: was carried out
Response 11: The required correction has been made at line 180 as suggested by the reviewer.
Point 12: Line 177: please remove the second “the solution was”
Response 12: “the solution was” has been removed at Page 6 , Line no. 210 .
Point 13: Caption of figure 5 should be revised.
Response 13: Caption of Figure 5 has been revised at Page 07 in the main manuscript.
Point 14: Writing style suddenly changed after line 256.
Response 14: Overall manuscript writing style has been improved.
Point 15: In section 2.4.2, it has been stated that DSR tests were conducted at the temperatures below 46C. However, I could not find the results of those tests. Please also clear on what type of binder you conducted the DSR tests. Were those aged or unaged?
Response 15: The binder in this research was unaged. The main motivation for this study was to improve the high temperature performance of the asphalt mixtures that are being used by the local pavement industry. As mentioned in the introduction of the manuscript, poor flexible pavement performance at high temperatures is the major problem that is faced by the pavement industry in the country (Pakistan). The asphalt pavements in the country fail prematurely because of mix rutting. Mix rutting is observed only in the high temperature areas of the country. The lower softening point values of the locally produced unmodified binders make them susceptible to rutting during the summers.
Performance grading (PG) is based on the concept that properties of asphalt binder should be related to the conditions under which it is used. According to temperature zoning done by Mirza et al. [4], PG 70-10 would be sufficient for most parts of the country. Base asphalt binder used in the country has PG 58-22 which is softer than the required PG 70-10. As mentioned earlier, the problematic areas are only the high temperature areas of the country where, even in winters, temperatures hardly fall below 0°C. Therefore, only high PG values were of concern and studied during this research. The PG grade was bumped by one value to accommodate for the overloading of heavy vehicles in the country. Therefore, the main aim was to use a dosage of nanomaterial that would help achieve the 76°C threshold
References
1. Hasan, Z.; Kamran, R.; Mohammad, F.; Ahmad, G.; Hosein, F. Evaluation of Different Conditions on The Mixing Bitumen and Carbon Nano-Tubes. Int. J. Civ. Environ. Eng. IJCEE-IJENS 2012, 12, 53–59.
2. Faramarzi, M.; Arabani, M.; Haghi, A.K.; Mottaghitalab, V. Carbon nanotubes-modified asphalt binder: Preparation and characterization. Int. J. Pavement Res. Technol. 2015, 8, 29–37, doi:10.6135/ijprt.org.tw/2015.8(1).29.
3. Gong, M.; Yang, J.; Yao, H.; Wang, M.; Niu, X.; Haddock, J.E. Investigating the performance, chemical, and microstructure properties of carbon nanotube-modified asphalt binder. Road Mater. Pavement Des. 2017, 0629, 1–24, doi:10.1080/14680629.2017.1323661.
4. Mirza, M.W.; Abbas, Z.; Rizvi, M.A. Temperature Zoning of Pakistan for Asphalt Mix Design. Pakistan J. Eng. Appl. Sci. 2011, 8, 49–60.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
I am glad to see that the authors have considered my comments and revised the manuscript very carefully. In my opinion, this manuscript can be accepted for publication in Journal of Applied Sciences.
Reviewer 2 Report
No comments.
Reviewer 3 Report
I have no further comments. Thanks!
