Effect of carbon black on the dynamic moduli of asphalt mixtures and its master curves

Abstract

Modulus is one of the main parameters during the design of asphalt pavement structure, the newest specifications of China gives the dynamic moduli ranges of commonly used asphalt mixtures with base asphalt (BA) or styrene-butadiene-styrene modified asphalt (SBS MA), while the moduli ranges of mixtures with carbon black modified asphalt (CB MA) are not recommended. To investigate the CB effect on the dynamic moduli of CB MA mixtures, one commonly used asphalt mixture (AC-20) was designed with BA, SBS MA, and CB MA, respectively. Then, the uniaxial compression dynamic modulus tests were conducted at different temperatures and loading frequencies, the master curves of asphalt mixtures were analyzed based on the time-temperature equivalence principle. The results show that with increasing loading frequency, the temperature dependence of dynamic moduli of all asphalt mixtures tend to be less obvious. Both SBS and CB can decrease the temperature sensitivity of asphalt mixture, the SBS MA mixture has the lowest temperature sensitivity, followed by CB MA and BA mixture. In addition, CB and SBS can obviously improve the dynamic modulus of the BA mixture, enhance the anti-deformation performance of pavement structure, and the improvement effect of CB is almost the same with SBS.

Keywords

dynamic modulus / carbon black / master curve / modified asphalt / asphalt mixture

Introduction

In recent years, plenty of works were conducted to use the carbon black (CB) as a modifier for an asphalt binder [¹–⁴], the results indicated that CB had a good compatibility with asphalt binder [²²], and could significantly improve the performance of asphalt binder, such as improve the anti-aging performance (UV aging and thermo-oxidative aging) [²], the anti-rutting performance of asphalt mixtures [³]. However, there is little investigation of the CB effect on the modulus and its master curves of asphalt mixtures [⁴,⁵]. Asphalt is a typical viscoelastic material, the technical performance of which is greatly influenced by the temperature and load frequency [⁶,⁷]. The dynamic modulus of asphalt mixture can simulate the dynamic stress state of asphalt mixture, it is one of the main parameters in the design process of the asphalt pavement structure [⁸]. To design the structure more suitably, the uniaxial compression dynamic moduli of asphalt mixtures are always adopted in the design process of asphalt pavement in the world [⁹,¹⁰].

While, for the limitation of test instrument or time, it is difficult to test the dynamic modulus of asphalt mixture under extreme load conditions, such as extremely high and low frequency [¹¹]. Because of the time-temperature dependence characteristics of dynamic modulus of asphalt mixture, the dynamic modulus master curve of asphalt mixture can be established basing on the time-temperature equivalence principle, which can well describe this behavior and play an important role in pavement structure design and mechanical analysis of asphalt pavement [¹²,¹³]. Based on the mechanics-experience design guideline, the dynamic modulus master curve can be used to predict the dynamic modulus value over a full temperature and loading frequency range, so it can significantly decrease the experimental quantities and times.

In recently years, lots of work on the dynamic modulus of base asphalt (BA) and styrene-butadiene-styrene modified asphalt (SBS MA) mixture were conducted [¹⁴,¹⁵], and the suitable range of which in the condition of 20°C and 10 Hz was also recommended in the newest specifications for design of highway asphalt pavement of China. However, there are few studies on the CB MA. To improve the design parameters of the CB asphalt modified asphalt (CB MA) mixture, and study the effect of CB on the dynamic modulus of asphalt mixtures and its master curves, the commonly used asphalt mixture (AC-20) was produced with the CB MA, SBS MA, and BA with the penetration of 60/80 (BA), respectively. Then uniaxial compression dynamic modulus tests at five levels of temperatures (−10°C, 5°C, 20°C, 35°C, and 50°C) and six levels of loading frequencies (25, 10, 5, 1, 0.5, and 0.1 Hz) were conducted to study the dynamic moduli of these three asphalt mixtures, and the master curves of these three asphalt mixtures were analyzed based on the time-temperature equivalence principle.

Materials and experimental methods

Materials

Asphalt binders

Three kinds of asphalt binders, namely BA with the penetration of 60/80, SBS MA (BA+ 4 wt.% SBS, simply referred to as SBS MA) and CB MA (85% BA+ 15% CB, simply referred to as CB MA), were used. The temperature of the penetration test was 25°C, and temperature of the ductility tests for BA and SBS MA were 10°C and 5°C, respectively. The technical information of these three kinds of asphalt binders are listed in Table 1.

CB

CB was provided by KIMKEY Environmental S&T Co.,LTD in Shanghai of China. The technical information of CB is listed in Table 2.

Aggregate

The crushed diabase produced in the Pingxiang city of China was used as the aggregate of asphalt mixtures. The technical information of aggregate is shown in Table 3. The limestone powder was used as the filler of asphalt mixture, there was no moisture and clumping phenomenon in limestone powder.

Asphalt mixtures

Mix ratio of asphalt mixtures

In this experiment, the gradation of mineral aggregate and optimum oil aggregate ratio were designated according to Marshall method. The upper and lower limits of aggregate grading were determined by the standard of “Technical Specifications for Construction of Highway Asphalt Pavements” (JTG F40-2004) of China, the aggregate grading of AC-20 asphalt mixture is shown in Fig. 1, the oil aggregate ratios of these three kinds of asphalt mixtures are shown in Table 4.

Volume parameters of asphalt mixtures

The volume parameters of asphalt mixtures are shown in Table 5. From Table 5, the volume parameters of all asphalt mixtures meet technical requirements of dense-graded asphalt mixtures.

Dynamic modulus test

Test equipment

The uniaxial compression dynamic modulus test was conducted according to the criterion of ASTM D 3497 and AASHTO TP 62-03 of the United States. The equipment used in the dynamic modulus test of asphalt mixture was the Superpave Basic Performance Tester (SPT) manufactured by IPC company in Australia.

Test frequency

Driving speed is one of the main factors affecting the loading frequency. For instance, the load frequency of 0.1 Hz represents the situation that the driving speed is very slow, which is similar to the driving status of vehicles of special sections such as urban intersections and toll stations. When the load frequency is 5 Hz, the corresponding driving speed is about 30–40 km/h, which is equivalent to the running speed of highway of Grade II and below. The corresponding driving speed for the load frequency of 10Hz is about 60–65 km/h, and the load frequency at 25Hz is equivalent to the driving speed above 120km/h. Therefore, the load frequency in the range of 0.1, 0.5, 1, 5, 10, and 25 Hz can be selected to include the actual driving speed on the road.

Test temperature

Test temperatures of 5°C, 25°C, and 40°C are recommended for the uniaxial compression dynamic modulus test in the criterion of ASTM D3497, and AASHTO TP62-03 recommends that the test temperature are −10°C, 4.4°C, 21.1°C, 37.8°C, and 54.4°C, respectively. In this paper, the dynamic moduli of three different asphalt mixtures were investigated in a wide range of temperatures (−10°C to 50°C), namely −10°C, 5°C, 20°C, 35°C, and 50°C.

Results and discussion

Dynamic modulus

Five parallel tests were performed at every temperature and loading frequency, the average of the five trials was taken as the final test result. A total of 150 trials were conducted. The test results of the dynamic moduli of BA mixture, SBS MA mixture and CB mixtures are shown in Figs. 2–4, respectively.

It can be observed from Fig. 1 that the dynamic modulus results of BA mixture decreases with increasing temperature, while increases with the increase of loading frequency. For instance, when the temperature increases form −10°C to 50°C, the dynamic modulus results of BA mixture decreases by 94.1%, 95.6%, 96.4%, 97.7%, 98.1%, and 98.6% at the loading frequency of 25 Hz, 10 Hz, 5 Hz, 1 Hz, 0.5 Hz, and 0.1 Hz, respectively. In addition, with the increase of the loading frequency, the temperature effect on dynamic modulus of BA mixture tends to be less obvious; with the increase of temperature, the loading frequency dependence of dynamic modulus of BA mixture tends to be less obvious. From Figs. 2–3, the temperature and loading frequency effects on the dynamic modulus of SBS MA mixture and CB MA mixture are almost the same as the BA mixture.

Master curves of dynamic modulus

Three dynamic modulus master curves of these three kinds of asphalt mixtures were established according to the time-temperature equivalence principle. The general form of the master curvilinear equation [¹⁹,²⁰] is shown in Eq. (1), the test frequency is converted to the reference frequency according to the Arrhenius Eq. (2), then the final form Eq. (3) of the master curve can be obtained by substituting the Eq. (2) into Eq. (1). The shift factor for the each test temperature to convert to the reference temperature can be calculated from Eq. (4) [²¹]. The maximum modulus of the mixture is calculated according to Eq. (5). P_c is a parameter of Eq. (5), which is calculated according to Eq. (6), where the voids filled with asphalt of asphalt mixture and the voids in mineral aggregate of asphalt mixture can be measured.

(1)Log\left|E^{*}\right|=\log (min)+{\displaystyle \frac{(\log (max)-\log (min))}{\mathrm{1}+e^{\beta +\gamma \log f_r}}},

(2)\mathrm{L}\mathrm{o}\mathrm{g}{f}_r=\mathrm{l}\mathrm{o}\mathrm{g}f+{\displaystyle \frac{\mathrm{\Delta }{E}_a}{\mathrm{19.14714}}}\left({\displaystyle \frac{\mathrm{1}}{T}}-{\displaystyle \frac{\mathrm{1}}{T_r}}\right),

(3)\mathrm{L}\mathrm{o}\mathrm{g}\left|E^{*}\right|=\log \left(min\right)+{\displaystyle \frac{(\log (max)-\log (min\left)\right)}{\mathrm{1}+{\mathrm{e}}^{\beta +\gamma (\log f+\frac{\mathrm{\Delta }{E}_a}{\mathrm{19.14714}}\left[\frac{\mathrm{1}}{T}-\frac{\mathrm{1}}{T_r}\right])}}},

(4)\mathrm{L}\mathrm{o}\mathrm{g}\left[a\right(T\left)\right]={\displaystyle \frac{\mathrm{\Delta }{E}_a}{\mathrm{19.14714}}}\left({\displaystyle \frac{\mathrm{1}}{T}}-{\displaystyle \frac{\mathrm{1}}{T_r}}\right),

|E^{*}{|}_{\max }=P_c[\mathrm{4200000}(\mathrm{1}-{\displaystyle \frac{VMA}{\mathrm{100}}})

+\mathrm{435000}\left({\displaystyle \frac{VFA*VMA}{\mathrm{10000}}}\right)

(5)+{\displaystyle \frac{\mathrm{1}-P_c}{{\displaystyle \frac{\mathrm{1}-{\displaystyle \frac{VMA}{\mathrm{100}}}}{\mathrm{4200000}}}+{\displaystyle \frac{VMA}{\mathrm{435000}\left(VFA\right)}}}}

(6)P_c={\displaystyle \frac{{\left(\mathrm{20}+{\displaystyle \frac{\mathrm{435000}\left(VFA\right)}{VMA}}\right)}^{\mathrm{0.58}}}{{\mathrm{650}+({\displaystyle \frac{\mathrm{435000}\left(VFA\right)}{VMA}})}^{\mathrm{0.58}}}},

where, |E*| is dynamic modulus of asphalt mixture, ksi; min is minimum limit of dynamic modulus, ksi; max is maximum limit of dynamic modulus, ksi; b and g are regression coefficients; f_r is reduced frequency at reference temperature, Hz; f is loading frequency at test temperature, Hz; DE_a is regression coefficient of activation energy; T is test temperature, K; T_r is reference temperature, K; A(T) is shift factor of temperature; |E*|_max is maximum dynamic modulus of mixture, ksi; VMA is voids in mineral aggregate of asphalt mixture; VFA is voids filled with asphalt of asphalt mixture.

The parameters in the Arrhenius equation were fitted according to the Mastersolver Version 2.2 from NCHRP09-29. The parameters of the Arrhenius equation for these three asphalt mixtures at a reference temperature of 20°C are obtained by constraining the variance (R²) to be higher than 0.99 and S_e/S_y lower than 0.05, the shift factor, the reduced frequency and the regression coefficients of the master curve of BA mixture, SBS MA mixture and CB mixtures are shown in Table 6.

The dynamic modulus master curves of BA mixture, SBS MA mixture and CB mixtures are shown is shown in Fig. 5. In general, the rutting of asphalt pavement is more serious in high-temperature areas (or low speed sections), which is the same as the distributing law of dynamic modulus. When the frequency is high (temperature is low), the dynamic modulus of all asphalt mixtures are small; and vice versa. In the range of low frequency (or high temperature), with the decrease of the loading frequency (or the increase of the temperature), the dynamic modulus of the three mixtures decrease sharply, the deformation resistance of asphalt mixtures decrease significantly. On the contrary, under the conditions of low temperature and high frequency, the dynamic modulus increases, as a result, the pavement is easier to crack.

It can be observed from Fig. 5 that the master curves of CB MA and SBS MA mixtures are always above the master curves of AC-20 mixture with BA over the test frequency range, the CB and SBS have a significant effect on improving the dynamic modulus of the BA mixture, as a result, the anti-deformation performance of CB MA and SBS MA mixtures are much better than that of BA mixture. With the increase of loading frequency, the dynamic modulus of BA mixture increase more sharply than that of CB MA and SBS MA mixtures. With the increase of loading frequency (and temperature), the dynamic modulus of BA mixture increases more sharply than that of CB MA and SBS MA mixtures, which shows that the loading frequency (and temperature) dependence of BA mixture is more significant than that of CB MA and SBS MA mixtures.

There is an intersection between the dynamic modulus master curve of AC-20 with CB MA and SBS MA nearly 1.0 Hz. In the range of low frequency (smaller than 1.0 Hz), the dynamic modulus of SBS MA mixture is higher than that of CB mixture, which indicates that SBS has a more significant improvement effect on the high-temperature performance of BA mixture than that of CB at low loading frequency (or high temperature). While, in the range of 1.0–8.0 Hz, the difference between the moduli of CB MA and SBS MA mixtures are very small. In the right part of the curve (higher than 8.0 Hz), the dynamic modulus of CB MA mixture is higher than that of SBS MA mixture.

The relationship between the shift factors of the BA mixture, SBS MA mixture and CB mixtures and test temperature are shown in Fig. 6. Combined with the Fig. 5, it can fully reflect the relationship between the dynamic modulus of asphalt mixture and temperature (or frequency), and be used to analyze and predict the anti-deformation performance of pavement structure.

From Fig. 6, the relationship of the shift factors of asphalt mixture versus test temperature can reflect the sensitivity of the mixture performance to the temperature. The smaller the absolute value of shift factor is, the more significant is the temperature sensitivity of the mixture performance. In the temperature range from −10°C to 50°C, the order of the absolute value of shift factor is SBS MA mixture>CB MA mixture>BA mixture, which shows that the SBS MA mixture has the best temperature sensitivity, the loading frequency (and temperature) dependence of BA mixture is more significant than that of CB MA and SBS MA mixtures, SBS and CB can decrease the sensitivity of the mixture performance to the temperature, and improve the anti-deformation performance of pavement structure. In addition, it can be observed form Fig. 6 that the absolute value of shift factor of CB MA mixture is almost the same as SBS MA mixture over the whole range of temperature, which shows that the improvement effect on the anti-deformation performance of pavement structure of CB is almost the same with SBS.

Conclusions

The dynamic moduli of AC-20 asphalt mixtures with the CB MA, SBS MA and BA were investigated at five levels of temperatures and six levels of loading frequencies, in addition, the master curves of these three asphalt mixtures were analyzed according to the time-temperature equivalence principle. The following conclusions are obtained:

1) The dynamic moduli of BA, CB MA, and SBS MA mixtures decrease with increasing temperature or decreasing loading frequency. While, with the increase of the loading frequency (and temperature), the temperature (and loading frequency) dependence of dynamic modulus tends to be less obvious.

2) CB and SBS can obviously improve the dynamic modulus of the BA mixture, as a result, the anti-deformation performance of CB MA and SBS MA mixtures are much better than that of BA mixture, and decrease the loading frequency (and temperature) dependence of BA mixture.

3) The absolute value of shift factors of SBS MA and CB MA mixtures are higher than that of BA mixture, which shows the SBS MA mixture has the lowest temperature sensitivity, followed by CB MA mixture and BA mixture;

4) Both SBS and CB can decrease the temperature sensitivity of asphalt mixture, and the improvement effect on the anti-deformation performance of pavement structure of CB is almost the same with SBS.

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