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Effects of graphite on rheological and conventional properties of bituminous binders
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ScienceDirect International Journal of Pavement Research and Technology 10 (2017) 315–321 www.elsevier.com/locate/IJPRT
Effects of graphite on rheological and conventional properties of bituminous binders Yunus Erkusß 1, Baha Vural Ko¨k ⇑, Mehmet Yilmaz 1 Fırat University, Department of Civil Engineering, Elazıg˘, Turkey Received 1 March 2017; received in revised form 7 April 2017; accepted 8 April 2017 Available online 27 April 2017
Abstract In this study, the effects of graphite used for developing the rheological and conventional properties of bitumen were investigated using various bituminous binder tests. Penetration, softening point, rotational viscosity (RV), dynamic shear rheometer (DSR) and bending beam rheometer (BBR) tests were applied to bituminous binders modified with four different proportions of graphite by bitumen weight. The penetration values declined while softening point values increased with rising graphite content. While graphite induced 8 °C increases in mixing-compacting temperature by increasing the viscosity values, it also increased the rutting parameter. According to the BBR test, the deformation and stiffness values changed significantly with increasing graphite content, but the m-values did not change significantly. These results showed that graphite generally used for improving the thermal properties can improve to high temperature performance of mixtures. Ó 2017 Chinese Society of Pavement Engineering. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: Graphite; Bitumen; Conventional properties; Rheological properties
1. Introduction Due to viscoelastic behavior of asphalt binder, mechanical features of asphalt pavement vary significantly because of daily and seasonal temperature changes. Asphalt pavement surface temperatures can reach 70 °C in summer due to its high absorption coefficient to solar radiation [1]. This eventually degrades the durability of asphalt concrete. High temperatures will induce permanent deformations of asphalt concrete with the impact of traffic [2]. The high pavement temperature is usually induced by its high solar radiation absorption and can easily generate rut-
⇑ Corresponding author. Fax: +90 (424) 234 0114.
E-mail addresses: yerkus@firat.edu.tr (Y. Erkusß), bvural@firat.edu.tr (B.V. Ko¨k), mehmetyilmaz@firat.edu.tr (M. Yilmaz). 1 Fax: +90 (424) 234 0114. Peer review under responsibility of Chinese Society of Pavement Engineering.
ting problem [3–5]. Field results showed that thermalinduced strain is 1.4–2.0 times greater in cold seasons than in warm seasons following the same pavement temperature variations [6]. Viscoelasticity, modulus and shear strength of bitumen significantly influence the asphalt temperature [7]. Especially in a high-temperature zone, a small temperature increase will lead to a sharp decline in performance [8]. High temperatures can also accelerate thermal oxidation, which will result in degradation of the pavement performance. There are two main ways to prevent deterioration that occurred in the pavement due to high temperature. One of these is the use of polymers to improve the resistance of pavements to adverse effects of high temperature [9–11]. The other method is the use of thermal conductive materials to prevent excessive temperature increases in the pavement. Du and Wang showed that using graphite in asphalt mixture successfully transmits the temperature to the bottom layers. The temperature decreased 6.5 °C at 4 cm
http://dx.doi.org/10.1016/j.ijprt.2017.04.003 1996-6814/Ó 2017 Chinese Society of Pavement Engineering. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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depth from the surface, and this induced a 43.5% decrease in rutting using graphite as an additive [12]. It has been determined that it is necessary to add graphite alongside conductive fibers in order to achieve a reasonable heat transfer. There is an optimum value of conductive additive depending on the filler/bitumen ratio [13]. Liu et al. [14] determined that using 40% graphite with 0.3% carbon fiber induces 78% increase in dynamic modulus of the bituminous mixtures. The differences in the softening point values decrease after storage modulus test with the use of graphite instead of mineral filler. The complex modulus increases and aging properties improve with increasing graphite content [15,16]. Graphite can transform more free asphalt to structural asphalt depending on its high specific surface area. The softening point of bitumen increases from 45 ° C to 70 °C when graphite is used in the bitumen modification. The rutting parameter increases from 1.555 kPa to 3.745 kPa at 40 °C by 9% graphite [17]. Theoretically and experimentally, the materials containing graphite improves the thermal, electrical and mechanical properties more than the materials with other mineral powders. Although there are some studies about the properties of bituminous mixtures containing graphite, there are limited studies about the effect of graphite on rheological binder properties. In this study, the effects of different amounts of graphite on the properties of bitumen were investigated on a large scale. 2. Materials and methods Asphalt binder (B 50/70) was utilized as pure bitumen for preparation of modified binders. The graphite particle size is lower than 150 mm, and the carbon content is 84.5%. SEM images of graphite are shown in Fig. 1. Graphite was added into pure bitumen in 4 different ratios as 7%, 10%, 13% and 16% by weight of bitumen. They are represented as 0% (G0), 7% (G7), 10% (G10), 13% (G13) and 16% (G16) respectively. Asphalt binder (500 g ± 5 g)
Fig. 2. The relation between additive content and penetration.
was first heated to 165 ± 5 °C in a container. Then, graphite was added gingerly within 10 min, while the shear speed was kept at 1000 rpm. In sum, 5 different binders were evaluated. The effects of graphite on the binder’s penetration, softening point, rotational viscosity (RV), dynamic shear rheometer (DSR) rutting parameter and bending beam rheometer (BBR) tests were examined. 3. Results and discussion 3.1. Penetration test This test was created according to ASTM D5. In this test, a needle of specified dimensions is allowed to sink into the bitumen under a constant load (100 g) at 25 °C for 5 s. The distance of the needle sink (0.1 mm) is considered the penetration. The test is the basis upon which the penetration category of the asphalt binder is classified into standard penetration ranges. It is essential that the test methods are followed precisely because even a slight variation can cause large differences in the result. The penetration test data obtained from pure and modified binders are shown in Fig. 2. The results given here are the average of 6 measurements performed for a test sample. The increase in graphite content caused a steady decrease in penetration value. The G7, G10, G13 and G16 binders have lower penetration values than values of pure binder by 9.2%, 16.3%, 18.5% and 20.7%, respectively. 3.2. Softening point test The softening point value of the asphalt binders is defined with ring and ball according to ASTM D36. Mixtures prepared with high softening point bituminous binders could nicely resist deformation at high temperatures. The softening point of pure asphalt binder and 7%, 10%, 13%, and 16% graphite-modified bituminous binders were tested to evaluate the individual effects of the additives on softening characteristics. The results are shown in Fig. 3.
Fig. 1. SEM image of graphite.
Y. Erkusß et al. / International Journal of Pavement Research and Technology 10 (2017) 315–321
Fig. 3. The correlation between softening points and additive content.
No trend can be observed with increasing graphite ratio (Fig. 3); however, the addition of graphite induces an increase in the softening point. This could be resulting from the inhomogeneous dispersion of graphite particles. The maximum softening point was obtained for 16% graphitemodified binder, however this binder gave only 2 °C higher softening point compared to pure binder. These results suggest that graphite addition has little effect on the softening point of bitumen. The addition of graphite is not effective in changing the softening point values, but it is effective in the penetration values and is considered the reason for physical stiffening. 3.3. Penetration index The penetration index value represents a quantitative dimension of the response of asphalt binders to temperature changes. Knowing the penetration index of particular bitumen allows one to predict its behavior in an application. All bitumens display thermoplastic properties, i.e., they become softer when heated and harden when cooled. A few equations exist that describe the way that the viscosity (or consistency) varies with temperature. To determine the thermal sensitivity of bituminous binder, the penetration index (PI) is determined using the softening point and standard penetration test results of the binder. One of the best known is by Pfeiffer and Van Doormaal that states:
Fig. 4. The relation between additive content and PI.
3.4. Rotational viscosimeter test The rotational viscometer test can evaluate the workability during the mixing and compaction processes of a bituminous binder. The rotational viscosimeter values of the bituminous binders are determined according to AASHTO TP48 standard. In this study, a Brookfield DV-III rotational viscometer was used to determine the viscosity of pure and graphite-modified binders. The variation on viscosities of the pure and the modified binders at 135 °C and 165 °C (black bar) is given in Fig. 5. There is a high correlation between the graphite content and viscosities for both temperatures in Fig. 5. The increase in graphite content causes a steady increase in the viscosities at both temperatures. The viscosity of the pure binder increases by 34% and 44% with 16% graphite at 135 °C and 165 °C, respectively. The viscosity value (537.5 cP) obtained via the highest value (16%) of graphite is much lower than 3000 cP—the threshold value for workability of hot mix. Therefore, the viscosity increase due to the use of additive will not generate a negative impact in terms of workability. The mixing and compaction temperatures measured by viscosities are shown in Table 1; values meet the 170 and 280 cP. As shown here, the use of 16% graphite increases the mixing temperature by 9.6 °C and the compaction temperature by 7.8 °C. The variation in mixing and compaction temperatures with increasing graphite con-
A ¼ ðlog 800 log P 25 Þ=ðT SP 25Þ PI ¼ ð20 500AÞ=ð1 þ 50AÞ where PI = penetration ındex. P25 = penetration of asphalt binder at 25 °C. TSP = softening point temperature of asphalt binder. The PI values are determined using softening point and standard penetration test results of pure and modified binders (Fig. 4). The penetration index values steadily decrease except for the G16 binder that has an increase in graphite content. This indicates an increase in temperature susceptibility.
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Fig. 5. The variation on viscosities of the binder.
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Table 1 Mixing – compaction temperatures of bitumen. Binder type
G0 G7 G10 G13 G16
Temperature (°C) Mixing
Compaction
154.3 158.8 160.0 162.0 163.9
142.8 146.2 146.9 149.2 150.6
tent is shown in Fig. 6. The mixing and compaction temperatures increase linearly with increasing graphite content.
3.5. Dynamic shear rheometer test The dynamic shear rheometer test values of the bituminous binders were determined according to AASHTO TP5 standard. The essential viscoelastic parameters obtained from dynamic shear rheometer test are the magnitude of the complex shear modulus (G*) and the phase angle (d) [18]. In this study, the rheological tests were performed with a Bohlin DSRII rheometer under controlled-stress conditions at 52 °C, 58 °C, 64 °C, 70 °C, and 10 rad/s of frequency using a 25 mm diameter plate and a 1 mm gap opening. The variations on the rutting parameter (G*/sin d) values depend on the graphite content at different temperatures (Fig. 7). The rutting parameters of the graphitemodified binder increase perpetually with increasing additive content. The use of 16% graphite increases the rutting parameter of the pure binder by 21% according to the experimental results. When this improvement effect is evaluated in point of economic aspect, it is clear that the pavement with 21% higher rutting resistance will resist repeated traffic load for a longer period than the unmodified one providing less maintenance cost. It should be considered that the addition of graphite does not require an additional
cost on the construction of asphalt layer due to having approximately same price with the base bitumen. Graphite-modified binders have a better high temperature performance than pure binders. This is consistent with the viscosity results. The changes in the rutting parameters of binders with different graphite contents as a function of temperature are shown in Fig. 8. The rutting parameter (G*/sin d) values of all binders with increasing temperature were decreased in an exponential manner. The changes in the phase angle (d) values depend on the graphite content at different temperatures (Fig. 9). The phase angle values of G0, G7, G10, G13 and G16 binders were found as 86.56; 86.55; 86.03; 86.68 and 86.70 respectively. These results suggest that the use of graphite does not cause a positive or negative effect on the elastic features of the bituminous binder. Therefore, the use of graphite is likely to not affect the low temperature performance of mixtures. 3.6. Bending beam rheometer test The bending beam rheometer (BBR) test measures low temperature stiffness and relaxation properties of bituminous binders. These results give an index of capacity of bituminous binders to resist low temperature cracking [19]. The BBR value of the bituminous binders is determined according to AASHTO TP1 standard. Creep stiffness (S) and creep ratio (m) are calculated by measuring deflection during the test. Loading during the experiment represents thermal stresses exposed to very low temperatures of pavement. The bending beam rheometer results are given in Table 2. It should have low creep stiffness values; high creep ratio values to exhibit a high performance at low temperatures. Hence, the ‘‘l” value is obtained by dividing the creep stiffness to the creep ratio to make a more realistic assessment [20]. The small value of ‘‘l” is desirable for elastic behavior at low temperatures. The variations on the ‘‘l”
Fig. 6. The relation between additive content and mixing-compaction temperatures.
Y. Erkusß et al. / International Journal of Pavement Research and Technology 10 (2017) 315–321
Fig. 7. The relation between additive content and rutting parameter at different temperatures.
Fig. 8. The relation between temperature and rutting parameter at different graphite contents.
Fig. 9. The relation between additive content and phase angle at different temperatures.
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Table 2 Bending beam rheometer results. Binder type
Deformation (mm)
St (MPa)
m-value
G0 G7 G10 G13 G16
0.2454 0.2330 0.2113 0.1482 0.1405
327.39 344.55 382.28 542.98 571.51
0.2747 0.2875 0.2855 0.3039 0.2714
the low temperature performance of the asphalt binder. In addition to these results the aging characteristics of graphite modified binders should be examined in further studies. Funding This study was funded by FUBAP (Firat University Scientific ResearchProjects Unit) under the project number MF 16.37. Conflict of interest The authors declare that they have no conflict of interest. Acknowledgement
Fig. 10. The relation between additive content and ‘‘l” value.
value depending on the graphite content are given in Fig. 10. The increases in graphite content caused a steady increase in the ‘‘l” value. The binder is much stiffer after 10% graphite content. 4. Conclusion In this study, the effects of graphite used to remedy the thermal features of bituminous mixtures on rheological and conventional features of bitumen were researched. Rheological and conventional tests were applied to modified bituminous binders at four different proportions of graphite by weight 7%, 10%, 13% and 16% of bitumen. The addition of graphite reduced the value of the penetration of the binders. The highest decrease was observed with 16% graphite modification compared to pure binder. The softening point values increase with the addition of graphite, but the largest increase occurs only 2 °C even in the highest content of graphite-modified bitumen. The addition of graphite improves the temperature susceptibility of binder compared to pure binder according to penetration index values. However graphite usage did not induce a lower value than 1 °C. This is accepted as a normal value in terms of temperature susceptibility. The addition of graphite increased the mixing and compaction temperatures and it leads to increases in the viscosity values. These values increased by approximately 8 °C when graphite was used in the highest rate. The use of 16% graphite increased the rutting parameter by 21% compared to pure binder without any change in the phase angle values. According to these results, graphite used to remedy the thermal features was determined to contribute to the high temperature performance. However, graphite worsened
This research was supported by FUBAP (Firat University Scientific ResearchProjects Unit) under the project number MF 16.37 for which the authors are greatly thankful. The findings and evaluations of the results of this study are not the official view of FUBAP. References [1] V. Bijsterveld, L.J. Houben, A. Scarpas, A.A. Molenaar, Using pavement as solar collector: effect on pavement temperature and structural response, Transp. Res. Rec. 1778 (1) (2001) 140–148. [2] P. Tongyan, L. Yang, W. Zhaoyang, Development of an atomisticbased chemophysical environment for modelling asphalt oxidation, Polym. Degrad. Stab. 97 (2012) 2331–2339. [3] Y.H. Huang, Pavement Analysis and Design, Prentice Hall Inc, New Jersey, 1993. [4] G.K. Chang, J.N. Meegoda, Micromechanical model for temperature effects of hot-mix asphalt concrete, Transp. Res. Rec. 1 (1999) 95–103. [5] D. Van Thanh, C.P. Feng, Study on Marshall and Rutting test of SMA at abnormally high temperature, Constr. Build. Mater. 47 (2013) 1337–1341. [6] S. Biswas, L. Hashemian, A. Bayat, Investigation on seasonal variation of thermal-induced strain in flexible pavements based on field and laboratory measurements, Int. J. Pavement Res. Technol. 9 (5) (2016) 354–362. [7] Q. Xue, L. Liu, Y. Zhao, Y.J. Chen, J.S. Li, Dynamic behavior of asphalt pavement structure under temperature-stress coupled loading, Appl. Therm. Eng. 53 (2013) 1–7. [8] Z. Suo, W.G. Wong, Nonlinear properties analysis on rutting behavior of bituminous materials with different air void contents, Constr. Build. Mater. 23 (12) (2009) 3492–3498. [9] Z. Krzysztof, Effect of styrene–butadiene–styrene content on the adhesion properties of bitumen before and after heat aging, Can. J. Civ. Eng. 35 (5) (2008) 454–460. [10] A. Adedeji, T. Grunfelder, F.S. Bates, C.W. Macosko, M. Stroup, D. E. Newcomb, Asphalt modified by SBS triblock copolymer, structures and properties, Polym. Eng. Sci. 36 (1996) 1707–1723. [11] X. Lu, U. Isacsson, Modification of road bitumens with thermoplastic polymers, Polym. Test. 20 (2000) 77–86. [12] Y. Du, S. Wang, Oriented heat release in asphalt pavement induced by high-thermal-conductivity roads, Appl. Therm. Eng. 90 (2015) 424–431. [13] A. Garcı´a, E. Schlangen, M. Ven, D. Vliet, Induction heating of mastic containing conductive fibers and fillers, Mater. Struct. 44 (2011) 499–508.
Y. Erkusß et al. / International Journal of Pavement Research and Technology 10 (2017) 315–321 [14] X. Liu, W. Liu, S. Wu, C. Wang, Effect of carbon fillers on electrical and road properties of conductive asphalt materials, Constr. Build. Mater. 68 (2014) 301–306. [15] P. Pan, S. Wu, Y. Xiao, P. Wang, X. Liu, Influence of graphite on the thermal characteristics and anti-ageing properties of asphalt binder, Constr. Build. Mater. 68 (2014) 220–226. [16] H. Yao, Q. Dai, Z. You, M. Ye, Y.K. Yap, Rheological properties, low-temperature cracking resistance, and optical performance of exfoliated graphite nanoplatelets modified asphalt binder, Constr. Build. Mater. 113 (2016) 988–996. [17] X. Liu, S. Wu, Q. Ye, J. Qiu, B. Li, Properties evaluation of asphaltbased composites with graphite and mine powders, Constr. Build. Mater. 22 (2008) 121–126.
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[18] G.D. Airey, A.E. Hunter, B. Rahimzadeh, The influence of geometry and sample preparation on dynamic shear rheometer testing, in: 4th European Symp. on the Performance of Bituminous Hydraulic Material in Pavements, Transportation Research Board, Washington, D.C., 2002. [19] O. Mihai, X.L. Marasteanu, R.C. Timothy, R.V. Vaughan, H.T. David, E.N. David, Low Temperature Cracking of Asphalt Concrete Pavements, Minnesota Dept. of Transportation, Office of Research Services, 2004. [20] S. Liu, W. Cao, J. Fang, S. Shang, Variance analysis and performance evaluation of different crumb rubber modified (CRM) asphalt, Constr. Build. Mater. 23 (7) (2009) 2701–2708.
ScienceDirect International Journal of Pavement Research and Technology 10 (2017) 315–321 www.elsevier.com/locate/IJPRT
Effects of graphite on rheological and conventional properties of bituminous binders Yunus Erkusß 1, Baha Vural Ko¨k ⇑, Mehmet Yilmaz 1 Fırat University, Department of Civil Engineering, Elazıg˘, Turkey Received 1 March 2017; received in revised form 7 April 2017; accepted 8 April 2017 Available online 27 April 2017
Abstract In this study, the effects of graphite used for developing the rheological and conventional properties of bitumen were investigated using various bituminous binder tests. Penetration, softening point, rotational viscosity (RV), dynamic shear rheometer (DSR) and bending beam rheometer (BBR) tests were applied to bituminous binders modified with four different proportions of graphite by bitumen weight. The penetration values declined while softening point values increased with rising graphite content. While graphite induced 8 °C increases in mixing-compacting temperature by increasing the viscosity values, it also increased the rutting parameter. According to the BBR test, the deformation and stiffness values changed significantly with increasing graphite content, but the m-values did not change significantly. These results showed that graphite generally used for improving the thermal properties can improve to high temperature performance of mixtures. Ó 2017 Chinese Society of Pavement Engineering. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: Graphite; Bitumen; Conventional properties; Rheological properties
1. Introduction Due to viscoelastic behavior of asphalt binder, mechanical features of asphalt pavement vary significantly because of daily and seasonal temperature changes. Asphalt pavement surface temperatures can reach 70 °C in summer due to its high absorption coefficient to solar radiation [1]. This eventually degrades the durability of asphalt concrete. High temperatures will induce permanent deformations of asphalt concrete with the impact of traffic [2]. The high pavement temperature is usually induced by its high solar radiation absorption and can easily generate rut-
⇑ Corresponding author. Fax: +90 (424) 234 0114.
E-mail addresses: yerkus@firat.edu.tr (Y. Erkusß), bvural@firat.edu.tr (B.V. Ko¨k), mehmetyilmaz@firat.edu.tr (M. Yilmaz). 1 Fax: +90 (424) 234 0114. Peer review under responsibility of Chinese Society of Pavement Engineering.
ting problem [3–5]. Field results showed that thermalinduced strain is 1.4–2.0 times greater in cold seasons than in warm seasons following the same pavement temperature variations [6]. Viscoelasticity, modulus and shear strength of bitumen significantly influence the asphalt temperature [7]. Especially in a high-temperature zone, a small temperature increase will lead to a sharp decline in performance [8]. High temperatures can also accelerate thermal oxidation, which will result in degradation of the pavement performance. There are two main ways to prevent deterioration that occurred in the pavement due to high temperature. One of these is the use of polymers to improve the resistance of pavements to adverse effects of high temperature [9–11]. The other method is the use of thermal conductive materials to prevent excessive temperature increases in the pavement. Du and Wang showed that using graphite in asphalt mixture successfully transmits the temperature to the bottom layers. The temperature decreased 6.5 °C at 4 cm
http://dx.doi.org/10.1016/j.ijprt.2017.04.003 1996-6814/Ó 2017 Chinese Society of Pavement Engineering. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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Y. Erkusß et al. / International Journal of Pavement Research and Technology 10 (2017) 315–321
depth from the surface, and this induced a 43.5% decrease in rutting using graphite as an additive [12]. It has been determined that it is necessary to add graphite alongside conductive fibers in order to achieve a reasonable heat transfer. There is an optimum value of conductive additive depending on the filler/bitumen ratio [13]. Liu et al. [14] determined that using 40% graphite with 0.3% carbon fiber induces 78% increase in dynamic modulus of the bituminous mixtures. The differences in the softening point values decrease after storage modulus test with the use of graphite instead of mineral filler. The complex modulus increases and aging properties improve with increasing graphite content [15,16]. Graphite can transform more free asphalt to structural asphalt depending on its high specific surface area. The softening point of bitumen increases from 45 ° C to 70 °C when graphite is used in the bitumen modification. The rutting parameter increases from 1.555 kPa to 3.745 kPa at 40 °C by 9% graphite [17]. Theoretically and experimentally, the materials containing graphite improves the thermal, electrical and mechanical properties more than the materials with other mineral powders. Although there are some studies about the properties of bituminous mixtures containing graphite, there are limited studies about the effect of graphite on rheological binder properties. In this study, the effects of different amounts of graphite on the properties of bitumen were investigated on a large scale. 2. Materials and methods Asphalt binder (B 50/70) was utilized as pure bitumen for preparation of modified binders. The graphite particle size is lower than 150 mm, and the carbon content is 84.5%. SEM images of graphite are shown in Fig. 1. Graphite was added into pure bitumen in 4 different ratios as 7%, 10%, 13% and 16% by weight of bitumen. They are represented as 0% (G0), 7% (G7), 10% (G10), 13% (G13) and 16% (G16) respectively. Asphalt binder (500 g ± 5 g)
Fig. 2. The relation between additive content and penetration.
was first heated to 165 ± 5 °C in a container. Then, graphite was added gingerly within 10 min, while the shear speed was kept at 1000 rpm. In sum, 5 different binders were evaluated. The effects of graphite on the binder’s penetration, softening point, rotational viscosity (RV), dynamic shear rheometer (DSR) rutting parameter and bending beam rheometer (BBR) tests were examined. 3. Results and discussion 3.1. Penetration test This test was created according to ASTM D5. In this test, a needle of specified dimensions is allowed to sink into the bitumen under a constant load (100 g) at 25 °C for 5 s. The distance of the needle sink (0.1 mm) is considered the penetration. The test is the basis upon which the penetration category of the asphalt binder is classified into standard penetration ranges. It is essential that the test methods are followed precisely because even a slight variation can cause large differences in the result. The penetration test data obtained from pure and modified binders are shown in Fig. 2. The results given here are the average of 6 measurements performed for a test sample. The increase in graphite content caused a steady decrease in penetration value. The G7, G10, G13 and G16 binders have lower penetration values than values of pure binder by 9.2%, 16.3%, 18.5% and 20.7%, respectively. 3.2. Softening point test The softening point value of the asphalt binders is defined with ring and ball according to ASTM D36. Mixtures prepared with high softening point bituminous binders could nicely resist deformation at high temperatures. The softening point of pure asphalt binder and 7%, 10%, 13%, and 16% graphite-modified bituminous binders were tested to evaluate the individual effects of the additives on softening characteristics. The results are shown in Fig. 3.
Fig. 1. SEM image of graphite.
Y. Erkusß et al. / International Journal of Pavement Research and Technology 10 (2017) 315–321
Fig. 3. The correlation between softening points and additive content.
No trend can be observed with increasing graphite ratio (Fig. 3); however, the addition of graphite induces an increase in the softening point. This could be resulting from the inhomogeneous dispersion of graphite particles. The maximum softening point was obtained for 16% graphitemodified binder, however this binder gave only 2 °C higher softening point compared to pure binder. These results suggest that graphite addition has little effect on the softening point of bitumen. The addition of graphite is not effective in changing the softening point values, but it is effective in the penetration values and is considered the reason for physical stiffening. 3.3. Penetration index The penetration index value represents a quantitative dimension of the response of asphalt binders to temperature changes. Knowing the penetration index of particular bitumen allows one to predict its behavior in an application. All bitumens display thermoplastic properties, i.e., they become softer when heated and harden when cooled. A few equations exist that describe the way that the viscosity (or consistency) varies with temperature. To determine the thermal sensitivity of bituminous binder, the penetration index (PI) is determined using the softening point and standard penetration test results of the binder. One of the best known is by Pfeiffer and Van Doormaal that states:
Fig. 4. The relation between additive content and PI.
3.4. Rotational viscosimeter test The rotational viscometer test can evaluate the workability during the mixing and compaction processes of a bituminous binder. The rotational viscosimeter values of the bituminous binders are determined according to AASHTO TP48 standard. In this study, a Brookfield DV-III rotational viscometer was used to determine the viscosity of pure and graphite-modified binders. The variation on viscosities of the pure and the modified binders at 135 °C and 165 °C (black bar) is given in Fig. 5. There is a high correlation between the graphite content and viscosities for both temperatures in Fig. 5. The increase in graphite content causes a steady increase in the viscosities at both temperatures. The viscosity of the pure binder increases by 34% and 44% with 16% graphite at 135 °C and 165 °C, respectively. The viscosity value (537.5 cP) obtained via the highest value (16%) of graphite is much lower than 3000 cP—the threshold value for workability of hot mix. Therefore, the viscosity increase due to the use of additive will not generate a negative impact in terms of workability. The mixing and compaction temperatures measured by viscosities are shown in Table 1; values meet the 170 and 280 cP. As shown here, the use of 16% graphite increases the mixing temperature by 9.6 °C and the compaction temperature by 7.8 °C. The variation in mixing and compaction temperatures with increasing graphite con-
A ¼ ðlog 800 log P 25 Þ=ðT SP 25Þ PI ¼ ð20 500AÞ=ð1 þ 50AÞ where PI = penetration ındex. P25 = penetration of asphalt binder at 25 °C. TSP = softening point temperature of asphalt binder. The PI values are determined using softening point and standard penetration test results of pure and modified binders (Fig. 4). The penetration index values steadily decrease except for the G16 binder that has an increase in graphite content. This indicates an increase in temperature susceptibility.
317
Fig. 5. The variation on viscosities of the binder.
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Table 1 Mixing – compaction temperatures of bitumen. Binder type
G0 G7 G10 G13 G16
Temperature (°C) Mixing
Compaction
154.3 158.8 160.0 162.0 163.9
142.8 146.2 146.9 149.2 150.6
tent is shown in Fig. 6. The mixing and compaction temperatures increase linearly with increasing graphite content.
3.5. Dynamic shear rheometer test The dynamic shear rheometer test values of the bituminous binders were determined according to AASHTO TP5 standard. The essential viscoelastic parameters obtained from dynamic shear rheometer test are the magnitude of the complex shear modulus (G*) and the phase angle (d) [18]. In this study, the rheological tests were performed with a Bohlin DSRII rheometer under controlled-stress conditions at 52 °C, 58 °C, 64 °C, 70 °C, and 10 rad/s of frequency using a 25 mm diameter plate and a 1 mm gap opening. The variations on the rutting parameter (G*/sin d) values depend on the graphite content at different temperatures (Fig. 7). The rutting parameters of the graphitemodified binder increase perpetually with increasing additive content. The use of 16% graphite increases the rutting parameter of the pure binder by 21% according to the experimental results. When this improvement effect is evaluated in point of economic aspect, it is clear that the pavement with 21% higher rutting resistance will resist repeated traffic load for a longer period than the unmodified one providing less maintenance cost. It should be considered that the addition of graphite does not require an additional
cost on the construction of asphalt layer due to having approximately same price with the base bitumen. Graphite-modified binders have a better high temperature performance than pure binders. This is consistent with the viscosity results. The changes in the rutting parameters of binders with different graphite contents as a function of temperature are shown in Fig. 8. The rutting parameter (G*/sin d) values of all binders with increasing temperature were decreased in an exponential manner. The changes in the phase angle (d) values depend on the graphite content at different temperatures (Fig. 9). The phase angle values of G0, G7, G10, G13 and G16 binders were found as 86.56; 86.55; 86.03; 86.68 and 86.70 respectively. These results suggest that the use of graphite does not cause a positive or negative effect on the elastic features of the bituminous binder. Therefore, the use of graphite is likely to not affect the low temperature performance of mixtures. 3.6. Bending beam rheometer test The bending beam rheometer (BBR) test measures low temperature stiffness and relaxation properties of bituminous binders. These results give an index of capacity of bituminous binders to resist low temperature cracking [19]. The BBR value of the bituminous binders is determined according to AASHTO TP1 standard. Creep stiffness (S) and creep ratio (m) are calculated by measuring deflection during the test. Loading during the experiment represents thermal stresses exposed to very low temperatures of pavement. The bending beam rheometer results are given in Table 2. It should have low creep stiffness values; high creep ratio values to exhibit a high performance at low temperatures. Hence, the ‘‘l” value is obtained by dividing the creep stiffness to the creep ratio to make a more realistic assessment [20]. The small value of ‘‘l” is desirable for elastic behavior at low temperatures. The variations on the ‘‘l”
Fig. 6. The relation between additive content and mixing-compaction temperatures.
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Fig. 7. The relation between additive content and rutting parameter at different temperatures.
Fig. 8. The relation between temperature and rutting parameter at different graphite contents.
Fig. 9. The relation between additive content and phase angle at different temperatures.
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Table 2 Bending beam rheometer results. Binder type
Deformation (mm)
St (MPa)
m-value
G0 G7 G10 G13 G16
0.2454 0.2330 0.2113 0.1482 0.1405
327.39 344.55 382.28 542.98 571.51
0.2747 0.2875 0.2855 0.3039 0.2714
the low temperature performance of the asphalt binder. In addition to these results the aging characteristics of graphite modified binders should be examined in further studies. Funding This study was funded by FUBAP (Firat University Scientific ResearchProjects Unit) under the project number MF 16.37. Conflict of interest The authors declare that they have no conflict of interest. Acknowledgement
Fig. 10. The relation between additive content and ‘‘l” value.
value depending on the graphite content are given in Fig. 10. The increases in graphite content caused a steady increase in the ‘‘l” value. The binder is much stiffer after 10% graphite content. 4. Conclusion In this study, the effects of graphite used to remedy the thermal features of bituminous mixtures on rheological and conventional features of bitumen were researched. Rheological and conventional tests were applied to modified bituminous binders at four different proportions of graphite by weight 7%, 10%, 13% and 16% of bitumen. The addition of graphite reduced the value of the penetration of the binders. The highest decrease was observed with 16% graphite modification compared to pure binder. The softening point values increase with the addition of graphite, but the largest increase occurs only 2 °C even in the highest content of graphite-modified bitumen. The addition of graphite improves the temperature susceptibility of binder compared to pure binder according to penetration index values. However graphite usage did not induce a lower value than 1 °C. This is accepted as a normal value in terms of temperature susceptibility. The addition of graphite increased the mixing and compaction temperatures and it leads to increases in the viscosity values. These values increased by approximately 8 °C when graphite was used in the highest rate. The use of 16% graphite increased the rutting parameter by 21% compared to pure binder without any change in the phase angle values. According to these results, graphite used to remedy the thermal features was determined to contribute to the high temperature performance. However, graphite worsened
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