J. Cent. South Univ. Technol. (2008) 15(s1): 479-482

DOI: 10.1007/s11771-008-404-1

Influence of conductive additive on temperature susceptibility of asphalt binders

WU Shao-peng(吴少鹏), LI Bo(李 波), CHEN Zheng(陈 筝), HUANG Xu(黄 旭)

(Key Laboratory of Silicate Materials Science and Engineering of Ministry of Education,

Wuhan University of Technology, Wuhan 430070, China)

Abstract: The effects of graphite on temperature susceptibility of asphalt binders were investigated by penetration test, Ring & Ball softening point test and viscosity test. And penetration index (I_P), viscosity-temperature susceptibility (S_VT), and penetration-viscosity numbers (N_PV) were introduced to evaluate the effects. The results show that the penetration, softening point and viscosity of asphalt binder increase with the increase of content of graphite. This means that the addition of graphite makes asphalts stiffer. The results from I_P, N_PV and S_VT show that temperature susceptibility is reduced by the addition of graphite.

1 Introduction

Asphalt mixtures are widely applied in highways, airport runways, bridge decks and city roads. Simultaneously, scientists and engineers try to develop intelligent functions of asphalt mixtures. Electrical conductive asphalt mixtures are one of the currently interesting ones. It is reported that electrical conductive asphalt can produce an automatic snow-melting asphalt surface due to the excellent thermoelectric property^[1-2]. And it can also be applied as a functional asphalt layer for structure health monitoring^[3-4]. Since asphalt is a kind of insulated material with electrical resistivity of 10¹²-10¹⁴ Ω?m, conductive additives must be selected to provide low resistivity to asphalt mixtures. According to the previous studies, graphite is one of the ideal additives^[5]. The addition of graphite can greatly improve the electrical conductive property but also has effects on other properties.

Asphalt is a visco-elastic material and may exhibit either elastic or viscous behavior, or combination of these, depending on temperature and time over which the asphalt is observed. At sufficiently low temperatures and/or high frequencies, asphalt behaves as an elastic solid. As temperature increases and/or frequency reduces, the viscous property of asphalt becomes more obvious. As a result, the performance of asphalt pavements show severe temperature susceptibility such as high- temperature rutting, medium temperature fatigue and low temperature cracking damage^[6-7]. For this reason, the temperature susceptibility is the critical parameter for the rheological properties of asphalts^[8].

Previous researches are mainly focused on the thermal, mechanical and electrical properties of asphalt mixtures with different contents of graphite. But there are few researches referred to the effects of graphite on the rheological properties of asphalt.

A rheological investigation of asphalt binders with different contents of graphite was made by penetration test (PT), Ring & Ball softening point test (R&B) and viscosity test (VT) in this paper. Penetration index (I_P), viscosity-temperature susceptibility (S_VT) and penetration-viscosity numbers (N_PV) were calculated for temperature susceptibility analysis.

2 Experimental

2.1 Raw materials

A heavy-duty asphalt binder AH-90 was obtained from Panjin Northern Asphalt Ltd., Co. in Liaoning Province, China, with penetration of 85.9 (0.1 mm at  25 ℃, 100 g and 5 s), ductility of more than 120 cm at 15 ℃, and softening point of 41.1 ℃.

Microcrystal graphite powders were selected as conductive fillers. Graphite powders are with particle size of 150 μm, carbon content of 98.9%, ash content of 0.2%, iron content of 0.03%, electrical resistivity of  10^-4 Ω?m.

2.2 Specimens preparation

The graphite contents of 8%, 12%, 18% and 22% of

asphalt binders were selected according to the percolation model results of Ref.[2]. The asphalt binders containing different contents of graphite were prepared using a low shear mixer at (140±5) ℃ and at a speed of 125 r/min. The mixing time was 1 h to ensure a homogeneous dispersion of graphite in asphalt binders. For accurate evaluation of the effects caused by graphite, the base asphalts were also subjected to the same treatment as the graphite-asphalt blends.

2.3 Test methods

The penetration tests were conducted with 100 gm load for 5 s at 15, 25, and 35 ℃ according to ASTM D5 specification. The Ring & Ball softening point tests were performed followed by the ASTM D36 specification. Finally, the rotational viscosity tests were conducted by a Brookfield viscometer (model DV-II + Pro) according to ASTM D 2170 specification. The range of test temperatures is 60-170 ℃.

3 Results and discussion

3.1 Results of penetration tests

Penetration was measured in laboratory to indicate hardness. It is recognized that penetration values are the inverse of viscosity in that values of penetration indicate softness. The penetration index (I_P) can be calculated mathematically from the penetration values at two temperatures as given in Eqns.(1) and (2).

                           (1)

                              (2)

where  P is penetration, 0.1 mm; K is constant; T is temperature, ℃; A is coefficient.

The test results of penetration and calculated I_P are listed in Table 1. The results from Table 1 indicate that there is a general trend for a decrease in penetration to be associated with an increase content of graphite. This trend, as observed, appears to be the same at all three temperatures.

Table 1 Results of penetration and I_P

In order to furtherly evaluate the effects of graphite on the penetration of asphalt binders, ?P and ?I_P were introduced as given in Eqns.(3) and (4), respectively.

                           (3)

                         (4)

where  ?P is change of penetration, 0.1 mm; P₀ is penetration of neat asphalt; P_i is penetration of asphalt with graphite content of i%, i equals 8, 12, 18, 22; ?I_P is change of I_P;  is I_P of neat asphalt;  is I_P of asphalt with graphite content of i%, i=8, 12, 18, 22.

The results of ?P and ?I_P are shown in Fig.1.

Fig.1 Effects of graphite on ?P and ?I_P

It can be seen from Fig.1 that ?P increases with an increase of content of graphite at a constant temperature. For example, ?P increases from 21% to 40% when the content of graphite ranges from 8% up to 22%. It means that the effects of graphite on hardness of asphalt are more obvious under a higher amount. It can also be observed that ?P presents an ascending trend with the increase of temperature. For example, ?P increases from 40% at 15 ℃ up to 55.6% at 35 ℃ at the same graphite content of 22%. It means that the effects of graphite on hardness of asphalt are more obvious at higher temperatures. The results of ?I_P show that the increase trend of temperature susceptibility is more prominent under a higher graphite amount.

3.2 Results of softening point tests

Softening point was measured to indicate the high temperature properties of asphalt. It is recognized that softening point values reflect the high temperature viscosity in that higher values of softening point indicate better rutting resistance of asphalt. There is also a strong relationship between penetration and softening point. The first presumptive application of the original formulation is to assume that the temperature at which softening point can be measured corresponds to a penetration of 800, as described in Eqn.(5):

                            (5)

where  T₈₀₀ is equivalent weight softening point, ℃.

The results of test softening point T_R&B and T₈₀₀ are shown in Fig.2. Fig.2 shows that with an increasing amount of graphite, both T_R&B and T₈₀₀ show dramatical increase tend. T_R&B increases from 44.1 ℃ up to 56.0 ℃ when graphite content varies from 0% to 22%. It increases approximately 12 ℃. This result indicates that the addition of graphite can greatly improve the high temperature properties of asphalt. It means that the conductive asphalt can obtain better rutting resistance.

Fig.2 Results of T_R&B and T₈₀₀

3.3 Results of viscosity tests

Viscosity of asphalt appears obvious temperature- dependency property. Saal empirical equation is the most widely used expression to describe the relationship between temperature and viscosity, which is given in Eqn.(6):

              (6)

where  η is viscosity, Pa·s; n and m are coefficients; T is temperature, ℃.

The viscosity-temperature curves are shown in Fig.3.

It can be observed from Fig.3 that at the same temperature, the viscosity of asphalt binders increases with an increasing amount of graphite. And under the same graphite amount, the viscosity of asphalt binders

Fig.3 Viscosity-temperature curves of conductive asphalt

decreases while the temperature increases. For example, the viscosity increases from 355 up to 2 330 mPa·s at 135 ℃ when the contents of graphite range from 0 to 22%. It means that the addition of graphite can make asphalts stiffer, which is good for the high temperature property.

Normally, the evaluation of rutting resistance of asphalt mixtures is conducted at 60 ℃. For this reason, the viscosity at 60 ℃ is very important. Higher value of viscosity results in excellent rutting resistance. As shown in Fig.3, the viscosity of asphalt binders increases in magnitude when graphite contents range from 0 up to 22%. It implies that the addition of graphite can reduce the fluidity deformation of asphalts, which leads excellent rutting resistance.

3.4 Temperature susceptibility analysis

There are three widely used approaches to characterize the temperature susceptibility of asphalt: penetration index (I_P), viscosity-temperature susceptibility (S_VT) and penetration-viscosity number (N_PV).

Temperature susceptibility is defined as the rate at which the consistency of bitumen changes with a change in temperature.  As a result, viscosity- temperature susceptibility (S_VT) is derived from viscosity-temperature curves, which is described as Eqn.(7):

            (7)

where  η₁ is viscosity at T₁, Pa·s; η₂ is viscosity at T₂, Pa·s. Herein, T₁=60 ℃, T₂=135 ℃.

N_PV is based on penetration at 25 ℃ and viscosity at either 135 ℃ or 60 ℃, which can be calculated by Eqn.(8):

      (8)

where  P₂₅ is penetration at 25 ℃, the unit is 0.1 mm; η₆₀ is viscosity at 60 ℃, Pa·s.

The comparison of I_P, S_VT and N_PV is illustrated in Fig.4.

Fig.4 Comparison of I_P(a), S_VT(b) and N_PV(c)

It can be seen that all three I_P, S_VT and N_PV present similar increasing trends with an increasing amount of graphite. For example, I_P varies from -1.537 up to -0.338 when graphite content increases from 0% to 22%, while S_VT varies from -3.486 up to -2.932. It obviously reflects that the addition of graphite can effectively make the asphalt less temperature susceptibility.

I_P between +1 and -1 is considered to be reasonable to ensure good performance for asphalt. It can be inferred that the addition of graphite can ensure good performance for AH-90 paving asphalt.

Fig.4 also shows that there is no further change in temperature susceptibility when the content of graphite exceeds 18%. For example, when the content of graphite increases from 18% to 22%, I_P only increases from -0.354 to -0.338 while S_VT varies from -2.986 to -2.932. It means that there is a proper amount for graphite.

4 Conclusions

1) The addition of graphite can obviously increase the softening point and viscosity of asphalt binders, which can ensure good high temperature properties for asphalt.

2) The addition of graphite can increase I_P, S_VT and N_PV, which indicates that conductive asphalt is less temperature susceptibility, especially at high graphite content.

3) There is no further effect on temperature susceptibility when graphite content exceeds the proper value.

References

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(Edited by CHEN Can-hua)

Foundation item: Project(NCET-05-0656) supported by Education Ministry for the New Century Excellent Talents, China

Received date: 2008-06-25; Accepted date: 2008-08-05

Corresponding author: LI Bo, Master; Tel: +86-27-87162595; E-mail: lip@whut.edu.cn