The impact of carbon nano-fiber modification on asphalt binder rheology

Construction and Building Materials 30 (2012) 257–264

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The impact of carbon nano-fiber modification on asphalt binder rheology Mohammad Jamal Khattak a,⇑, Ahmed Khattab b,1, Hashim R. Rizvi a,2, Pengfei Zhang c,1 a

Department of Civil Engineering, University of Louisiana at Lafayette, 254J- Madison Hall, Lafayette, LA 70504, United States Department of Industrial Technology, College of Engineering, University of Louisiana at Lafayette, Lafayette, LA 70504-2972, United States c Department of Mechanical Engineering, University of Louisiana at Lafayette, Lafayette, LA 70504, United States b

a r t i c l e

i n f o

Article history: Received 26 November 2010 Received in revised form 1 December 2011 Accepted 4 December 2011 Available online 2 January 2012 Keywords: Asphalt binder Viscoelastic Carbon nanofiber Fatigue life Master curve

a b s t r a c t Nano-reinforced materials hold the potential to redefine the field of transportation materials both in terms of cost effectiveness and long term pavement performance. This study focuses on the exploratory analysis of the mixing procedure of carbon nanofibers (CNFs) with asphalt cement (AC) and discusses the visco-elastic and fatigue characteristics of neat and CNF-modified AC binders. Three types of AC were modified with varying percentages of CNF. Two CNF-asphalt mixing procedures, dry process and wet process, were investigated. The dynamic shear rheometer was utilized to determine complex shear modulus (G⁄), phase angle (d), and fatigue characteristics of neat and CNF-modified AC for a range of temperatures and loading frequencies. The G⁄/sin d values and rotational viscosity analyses revealed that the CNF-modified AC exhibited improved visco-elastic response and resistance to rutting. Furthermore, substantial increases in fatigue life were observed with CNF modification. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Nanoreinforced materials hold the potential to redefine the field of traditional materials both in terms of performance and potential applications [1–5]. Hussain et al. [1] showed that the biggest challenge in developing nanocomposite is the dispersion of nanoparticles or chemical compatibility with matrix materials. They identified that improving the carbon nanofibers (CNFs)/matrix interfacial adhesion issue and complete dispersion must be resolved before achieving the full potential of nanoreinforced composite materials. Dispersion of nano-fibers has been one of the largest challenges due to the aggregation of the nanofibers. Improvement of material properties can be achieved by a proper dispersion technique. Improper dispersion leads to nanofiber damage and size break down which deteriorate the material properties. Khattab et al. [6] developed a dispersion technique of CNF by combining sonication and high shear mixing techniques to achieve the highest degree of dispersion. In their study, several sonication periods with several power rates and several mixing speeds were investigated in order to optimize the dispersion process. Nanoclay modification improves some characteristics of asphalt binders and asphalt mixtures such as rutting. However, it did not mitigate the fatigue problem and hence more research is required

before it can be applied on a large scale [7]. Research has shown that nanocalcium carbonate (nano-CaCO3) modified asphalt [8,9] can enhance asphalt’s rutting resistance and improves its low -temperature toughness. Nanoclays such as sodium montmorillonite and organophilic montmorillonite have shown improvements in viscosity, complex shear modulus and phase angles of styrene–butadiene–styrene (SBS) copolymer modified asphalt [10]. CNF have shown significant improvements in the mechanical properties of polymer composites [11–14] due to their high modulus, tensile strength, and high aspect ratio. In this study, CNF are used to modify the asphalt binder. It is believed that CNF modification will produce a good network of fibers in the asphalt that will enhance the mechanical properties. CNF are known to have high aspect ratio up to 1700. The fiber network may bridge across micro-cracks developed due to loading and environmental effects causing a hindrance to their growth and consequently increasing the strength and fracture properties of the mixture. This paper focuses on the mixing procedure of CNF with asphalt and the effect of various dosages of CNF on the visco-elastic and fatigue characteristics of the asphalt cement (AC). 2. Objectives The objectives of this study are to:

⇑ Corresponding author. Tel.: +1 337 482 5356; fax: +1 337 482 6688. E-mail addresses: [email protected] (M.J. Khattak), [email protected] (A. Khattab), [email protected] (H.R. Rizvi), [email protected] (P. Zhang). 1 Tel.: +1 337 482 6166; fax: +1 337 482 6661. 2 Tel.: +1 337 482 5356; fax: +1 337 482 6688. 0950-0618/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2011.12.022

 Develop and compare various mixing procedures of CNF and AC binder.  Evaluate the visco-elastic and fatigue characteristics of neat and CNF-modified AC.

M.J. Khattak et al. / Construction and Building Materials 30 (2012) 257–264

3. Test materials

cmax ¼

Three types of viscosity graded asphalts were used: AC5 (PG52-22), AC30 (PG64-22) and polymer modified PAC30 (PG70-28). Vapor-grown CNF PR-24XTPS (Polygraf III) produced by Applied Sciences was used for asphalt modification. This functionalized CNF has a diameter of 60–150 nm, length of 30–100 lm, tensile modulus of 600 GPa and tensile strength of 7 GPa. The CNF has surface area of 45 m2/g, dispersive surface energy of 85 mJ/m2, moisture content <5 with an Iron and Polyaromatic Hydrocarbons content of <1400 ppm and <1 mg PAH/g, respectively. The fiber has a high performance per cost ratio and a good interfacial bonding with matrix materials. Commercially available kerosene was used as a solvent to disperse the CNF and ultimately mixing it with the AC binders. The specific gravity and boiling point of the kerosene are 0.75 and 155 °C, respectively. Due to various processing techniques used in this study, the AC binders were classified as follows:     

Neat AC – asphalts with no processing and no CNF modifications. Wet processed AC – asphalts processed with kerosene only. Wet CNF modified AC – asphalts processed with CNF-kerosene mixture. Dry CNF modified AC – asphalt processed with dry CNF. Dry processed AC – asphalt processed with same mixing time and temperature as that of dry CNF modified AC but without adding dry CNF.

4. Testing program 4.1. Sonication and particle size distribution The Omni Sonic Ruptor, with 300 watts and 20 kHz, and Omni Mixer Homogenizer, with 600 watts and up to 18000 rpm, were used to develop the dispersion procedure. The control parameters involved in this study are: weight of CNF, sonication time, and shear mixing time. During sonication, the holding tray was adjusted to make sure the processing tip was placed at the center of the cup and inserted 40 mm deep into the CNF-dispersant mixture. A state of the art system, called laser light diffraction analyzer (LDA) system for nano-particle size analysis provided extended CNF size measurement distribution by fully utilizing the data provided by the three solid-state lasers (two blue and one red) and two detector arrays. The size distribution provided information about CNF aggregation and also size break down. This system has advantages over other laser particle size analyzers because it measures light scattered over nearly the entire angular spectrum. Thus, the multi-laser system provided direct and continuous measurements without curve fitting or blending of two different measurement technologies or wavelengths. These features resulted in improved resolution and consistency. 4.2. Complex shear modulus (G⁄) Bohlin’s Dynamic Shear Rheometer (DSR) was used to conduct frequency sweep tests on the triplicates samples of AC binders. The test procedure is similar to AASHTO TP5. The AC binder was sandwiched between lower fixed plate and an upper oscillation plate. Torque (T) was applied and deflection angle (h) was measured. The gap between the two plates is maintained at 1 mm for higher temperature testing (>46 °C) and for low temperature testing (<46 °C) a gap of 2 mm was maintained. The constant shear stress was such that the resulting strain remained within 10–12% for temperature testing of 46 °C or higher. For temperatures lower than 46 °C the shear stress was corresponding to the shear strain of 2%. All tests were conducted at temperatures of 1, 10, 20, 25, 34, 46, 54, 60, and 64 °C for a range of frequencies from 1 Hz to 60 Hz at logarithmic increments. This frequency sweep test was conducted to determine the complex shear modulus (G⁄) and phase angle (d) of the AC binders. The d represents the time lag between the applied shear stress and shear strain. The following equations were used.

smax ¼

2T

p  r3

ð1Þ

G ¼

hr h

ð2Þ

smax cmax

ð3Þ

where T = maximum applied torque, r = radius of the binder specimen (12.5 mm for high temperature testing and 4 mm for low temperature testing), h = deflection (rotation) angle, and h = thickness of specimen (1 or 2 mm). The main advantage of frequency sweep test at various temperature is to construct the master curve for G⁄ and d at reference temperatures using the time–temperature supper positioning [15]. Horizontal shifting is performed by plotting the frequency and modulus data on a log–log scale (Fig. 1) and then shifting the resulting curves along the frequency (horizontal) axis by a constant factor, as shown in the following equation:

fref ¼ aðTÞ  f

ð4Þ

where fref is the shifted or reduced frequency to a reference temperature, f is the original frequency and a(T) is the shift factor as a function of temperature. For precision, a second order polynomial relationship between the logarithm of the shift factor i.e. log a(Ti) and the temperature is used (Fig. 2). The relationship can be expressed as follows: 2

logaðTiÞ ¼ a  Ti þ b  Ti þ c

ð5Þ

where a(Ti) = shift factor as a function of temperature Ti, Ti = temperature of interest, °C a, b and c = coefficients of the second order polynomial. 4.3. Dynamic shear fatigue characteristics DSR was used to conduct dynamic fatigue test at 20 °C on the triplicate samples of AC binders. The test step up and protocol was similar to AASHTP TP5 test procedure. The AC sample measuring 8 mm in diameter and 2 mm thick sandwiched between lower (fixed) and upper plate (oscillating) of DSR was subjected to repeated sinusoidal oscillation under constant shear stress. The test was run at frequency of 10 Hz until complete failure of sample occurred or until the number of cycles (oscillation) reached 10,000. The shear stress level for AC5 binder was maintained at 20 kPa while for AC30 and PAC30 the stress level was kept at 90 kPa. These stress levels corresponds to the viscoelastic shear stress levels for each binder type. The linear visco-elasticity holds if the modulus does not drop more than 10% from its initial value [16]. The G⁄ and d values were obtained for each cycle. The G⁄ values

100,000,000

Complex Shear Modulus, G* (Pa)

258

10,000,000 o

1 C

1,000,000 100,000

Sigmoidal Function

10,000

Master Curve Data

o

20 C o

34 C

1,000

o

52 C

100 10 1 0.001

0.01

0.1

1

10

100

1000

10000

Reduced Frequency, f (Hz) Fig. 1. Typical frequency sweep test data at various temperatures and master curve at 20 °C.

M.J. Khattak et al. / Construction and Building Materials 30 (2012) 257–264

was used in this study mainly due to the fact that it was petroleum based product, cheap and easily available. In general, the CNF-solvent was mixed with AC using a low shear mixer at medium to high temperature until sufficient amount of the solvent is evaporated. The following two elements were investigated to determine the mixing time:

8.0 6.0

Shift Factor, a(T)

4.0 y = 0.0011x2 - 0.2751x + 4.8303 R 2 = 0.996

2.0

259

0.0

1. The mixing time at which the mass of the solvent-asphalt mix became the same as that of the neat AC binder. This was the function of rate of evaporation of solvent. 2. The mixing time at which the G⁄/sin d parameter of solventasphalt binder became the same as that of the neat AC binder.

-2.0 -4.0 -6.0 -8.0 0

10

20

30

40

50

60

70

Temperature, (oC) Fig. 2. Typical shift factor as a function of temperature.

were then plotted against the number of cycles and fatigue life was determined as the number of cycles at which the G⁄ reached 50% of its initial value. 4.4. Rotational viscosity (g) A Brookfield rotational viscometer (DVR III) was used to determine the viscosity of the binders at 135 and 165 °C. This method of measuring viscosity is detailed in AASHTO TP48 test procedure. About 10.5 ml of heated asphalt binder was poured into the cylinder. The spindle was lowered into the binder and the assembly was allowed to reach an equilibrium test temperature. The test was run at 20 rmp and the viscosity readings in Pa s were recorded at 60 s. Two samples were tested and if the difference between the two exceeded 2% the third test was conducted. The viscosity at 60 °C was obtained using the DSR test conducted at a frequency of 1.59 Hz. 5. Procedures 5.1. Mixing procedures of CNF with asphalt The following two mixing procedures were adopted for mixing CNF with asphalt: 1. Wet process – the CNF was dispersed in the solvent by sonication and high shear mixing and then the CNF-solvent mixture was mixed with asphalt at high temperature using a mechanical mixer. 2. Dry process – the CNF was initially dispersed in the solvent, oven dried to completely evaporate the solvent and mixed with asphalt using a mechanical mixer at high temperature. In order to mix the CNF with the AC, the fundamental concern was a uniform and even dispersion of the CNF in the AC. This could be achieved by evenly dispersing it in a solvent and finally mixing it with AC. The solvent chosen should have the ability to dissolve in AC at low to medium temperatures without significantly affecting the mechanical properties of AC binder upon sufficient evaporation. The solvent should also have low viscosity at room temperature in order to disperse the CNF uniformly. Furthermore, it should have a low evaporation rate at room temperature (stability) and high evaporation rates at medium to high temperatures. Various solvents are available that can be dissolved in AC at room temperature and significantly decrease its viscosity. Kerosene, varsol, turpentine, acetone, and others fall under such category. Kerosene

5.1.1. Dispersion of CNF in solvent In this study, several cases of CNF-kerosene mixture were prepared by sonication and high shear mixing as shown in Table 1. Nine different combinations of sonication and shear mixing parameters were investigated (Table 1). Qualitative and quantitative comparisons were conducted for all the mixtures. Samples were compared based on the volume and level of sedimentation observed after the samples were allowed to settle for an extended period of time. The particle size analyses were conducted using the laser light diffraction analyzer (LDA) system. Table 2 shows the mean particles size at 10%, 16%, 30%, 50%, 60%, 84% finer for the neat CNF, as provided by the vendor, and CNF-kerosene mixtures (cases 1, 3, 4, 5, and 9). These cases showed the least amount of CNF sedimentation without any lumps based on a qualitative analysis. Fig. 3 shows the typical particles size distributions of the neat CNF and case 1 as obtained from the LDA system. It can be seen from the figure that 70% of neat CNF particles are less than 100 lm. Fig. 3 also shows the effect of a 24 min sonication sequence. The sonication energy dropped the size of the particles within an order of magnitude where 70% of the particles are about 10 lm or less. The data in Fig. 4 reveals that increasing the sonication time from 8 min to 24 min helped to move the curve of the size distribution towards left, indicating a smaller particle size distribution. Fig. 5 illustrates the effect of CNF concentration and high shear mixing. It can be observed from the figure that for the same level of dispersion energy, a lower concentration of CNF in kerosene produces a homogenous CNF size distribution (case 9). Adding 2 min of high shear mixing at 3000 rpm to case 1 provided enough energy for a better degree of dispersion with the majority of CNF at 600 nm and about 70% of the particles about 3 lm or less (case 9). The qualitative and quantitative comparison for the 9 cases studied showed that the optimum CNF dispersion with a homogenous distribution was obtained by adopting the following procedure. Approximately 290 g (400 ml) of kerosene was placed in a stainless steel jar. The desired mass of CNF was mixed and sonicated. During sonication, the sonication horn was placed at the center of the jar and inserted 40 mm deep into the CNF- kerosene mixture. The CNF were dispersed by three cycles of sonication with a power of 240 watts at 90% pulse rate and sonication time of 8 min each with an idle (cool-down) time of 25 min between cycles followed by 2 min of high shear mixing at 3000 rpm. This procedure provides dispersion energy just enough to break the CNF aggregation without damaging the fibers. However, continually increasing dispersion energy would cause CNF re-agglomeration and immense size break down. 5.1.2. Wet mixing process 5.1.2.1. Mixing time of solvent-asphalt mixture. One hundred and 50 g (150 g) of AC was heated at 135 °C for 45 min in an oven and then mixed with 250 g of kerosene in a glass beaker. The beaker containing the kerosene-asphalt mixture was placed in a

260

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Table 1 CNF dispersion data summary.

a b c

Case no.

Weight of CNF (g)

Weight of kerosene (g)

1 2 3 4 5 6 7 8 9

4.3 6.9 6.9 6.9 6.9 6.9 6.9 6.9 4.3

290 290 290 290 290 290 290 290 290

Sonication parameters

High shear mixer

Power (watts)

Time (min)

240 240 240 240 240 240 240 240 240

24 16 8 16 24 24 8 16 24

(8–25)a (8–25)b (8–25)b (8–25)a (8–25)a (8–25)b (8–25)a

Pulse rate (%)

Speed (rpm)

Time (min)

90 90 90 90 90 90 90 90 90

– – 3000 3000 3000 3000/18,000c – – 3000

– – 2 2 2 4/2c – – 2

Three cycles sonication time of 8 min each with an idle time of 25 min in between cycles. Two cycles sonication time of 8 min each with an idle time of 25 min in between cycles. High shear mixing at 3000 rpm for 2 min after 16 and 24 min of sonication, then 2 min at 18,000 rpm at the end.

Table 2 Summary of carbon nanofibers size distribution. Mean particle size (lm)

Case no.

Neat CNF 1 3 4 5 9

% Finer

112.6 4.07 92.76 65.04 67 3.24

10

16

30

50

60

84

2.276 0.213 0.701 0.392 0.308 0.178

6.56 0.339 1.389 0.538 0.481 0.247

41.59 0.792 12.36 2.009 1.388 0.443

88.15 2.639 70.15 29.8 28.87 0.917

117.7 4.09 97.65 60.68 60.4 1.349

220.6 7.59 196.2 148.4 162.5 4.72

100

10

10

Case 1: 4.3g-CNF, 400 ml-Kerosence, 24 min-Sonication, No Shear Mixing CNF Neat -% Finer

80

Case 1-% Finer

6

60

4

40

20

2

Particle Size Distribution, (%)

8

9

% Finer

Particle Size Distribution, (%)

CNF Neat

Case 3: 6.9g-CNF, 400ml-Kerosence, 8 min-Sonication, Shear Mixing 2 min @ 3000 rpm Case 4: 6.9g-CNF, 400ml-Kerosence, 16 min-Sonication, Shear Mixing 2 min @ 3000 rpm Case 5: 6.9g-CNF, 400ml-Kerosence, 24 min-Sonication, Shear Mixing 2 min @ 3000 rpm

8 7 6 5 4 3 2 1

0 0.001

0.01

0.1

1

10

100

1000

0 10000

Particle Size, μm

0 0.001

0.01

0.1

1

10

100

1000

10000

Particle Size, mm

Fig. 3. CNF particle size distribution of neat CNF and CNF in kerosene after 24 min of sonication.

Fig. 4. The effect of sonication time on the particle size distribution for CNFkerosene.

pre-heated oil bath at 120 °C and mixed thoroughly using the low shear mixer for 30 min. While the mixing was in progress, the temperature of the bath was gradually raised to 160 °C and the mixing continued for additional 150 min. The mass of the kerosene-asphalt mixture was recorded initially at every 30 min and then every 10 min during the last 30 min of mixing. In addition, samples were extracted to determine the G⁄ and d of the kerosene-asphalt mixtures at various time intervals. Fig. 6 shows G⁄/sin d and percentage of kerosene mass remaining during the mixing procedure. The data in the figure indicates that G⁄/sin d of kerosene-asphalt of AC5 increases with the increase of mixing time. This is because the increase in viscosity of AC binder due to the evaporation of the kerosene. It was observed that at the mixing time between 160 and 165 min the G⁄/sin d of AC became almost equal to the G⁄/sin d of the neat AC. Further mixing of AC significantly increased the G⁄ values thus indicating short

term aging of AC due to processing at high temperatures and shear mixing. In this study, all such ACs were referred as wet processed AC. It was also observed that G⁄/sin d of the kerosene-asphalt was a function of percentage of kerosene remaining in AC as shown in Fig. 6. It was found that the G⁄/sin d values increased with the decrease of percentage of kerosene in AC. Approximately, at 1.5% of kerosene remaining in AC the G⁄/sin d of kerosene-asphalt equals G⁄/sin d of neat AC. Additional mixing to reduce the kerosene content significantly increased G⁄/sin d values relative to the neat AC. For example, to achieve 1% kerosene content in AC the G⁄/sin d increased by 60% thus, exhibiting significant short term aging. Therefore, based on the aforementioned discussion the mixing time corresponding to 1.5% of kerosene remaining in AC was selected for this study. Two additional tests were conducted to verify the kerosene content as shown in Fig. 6. It was found that the G⁄/sin d

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M.J. Khattak et al. / Construction and Building Materials 30 (2012) 257–264 10 Case 1: 4.3g-CNF, 400ml-Kerosence, 24 min-Sonication, No Shear Mixing Case 5: 6.9g-CNF, 400ml-Kerosence, 24 min-Sonication, Shear Mixing 2 min @ 3000 rpm Case 9: 4.3g-CNF, 400ml-Kerosence, 24 min-Sonication, Shear Mixing 2 min @ 3000 rpm

Particle Size Distribution, (%)

9 8 7 6 5 4 3 2 1 0 0.001

0.01

0.1

1

10

100

1000

10000

Particle Size, mm Fig. 5. The effect of high shear mixing and CNF concentration on particle size distribution.

100

600

the viscosity values of the neat, wet processed and CNF modified ACs at 60 °C. However, at temperatures greater than 60 °C, the CNF modified AC exhibited significant higher values relative to neat and processed ACs. Similar results were found for AC5 neat and CNF modified binders using the dry process (Fig. 7). Fig. 8 shows the comparison of viscosities at temperature of 135 °C of the different types of AC binders prepared with the different mixing procedures used in this study. It is obvious that regardless of AC type and mixing procedure, the viscosity values at 135 °C increase with the increase in CNF content. However, the rate of increase is a function of AC Type (AC5, AC30 and PAC30) and mixing procedure. The lower viscosity asphalts (AC5) exhibited higher percentage increase relative to higher viscosity asphalts (AC30). Furthermore, the dry process of CNF mixing yielded a higher rate of increase in viscosities relative to wet process of mixing. In general, the CNF modified AC showed 2–7 times higher viscosity than the processed AC. Mostly, CNF modified ACs have viscosity values higher than the Strategic Highway Research Program (SHRP) viscosity limit of 3 Pa s at 135 °C. 6.2. Effect of processing on asphalt binder

G*/sin (δ)

80

450

G*/sin (δ) of AC Neat

60

300 40

G*/sin( δ), kPa

Kerosene Remaining, (%)

% Kerosene Remaining

In this study master curves for G⁄/sin d were generated at 20 °C by using the G⁄ and d values obtained at each temperature for a range of frequencies. A typical G⁄/sin d master curve for neat AC5 for a range of reduced frequencies along with the shift factor is shown in Fig. 9. It was found that the G⁄/sin d master curves can be modeled using the sigmoidal function as shown below.

150

log jG = sin dj ¼ a þ

20

165 min 0 0

50

100

150

200

0 250

Mixing Time, (min) Fig. 6. Typical % of kerosene mass remaining and G⁄/sin d at 20 °C as a function of mixing time.

values of the additional samples were within ±5% of the first sample. 5.1.2.2. Mixing of CNF-kerosene mixture with asphalt. The required amount of CNF was initially dispersed in kerosene using the sonication and shear mixing techniques discussed earlier. The CNF-kerosene mixture of 250 g was obtained in a 600 ml glass beaker. The neat AC was heated at 135 °C for 45 min in an oven and about 150 g of it was gently poured into the CNF-kerosene mixture. The mixture was then placed in a pre-heated oil bath and mixed thoroughly using the low shear mixer as stated above.

b 1 1 þ expdþeðlog fR Þ

ð6Þ

where G⁄ = complex shear modulus; d = phase angle; fR = frequency of loading at reference temperature; a and b = fitting parameters for a given set of data; a represents the minimum value of G⁄/sin d and a + b represents the maximum value of G⁄/sin d; and d and e = parameters describing the shape of the function. A similar G⁄/sin d master curve was generated for wet processed asphalt and compared with the neat one to evaluate the effect of wet processing (Fig. 10). It can be seen that the G⁄/sin d of wet processed AC exhibits slightly lower values at higher frequencies (or lower temperatures) and higher values at lower frequencies (or higher temperatures). On average the differences between the two are about 18%. Recall, the mixing procedure was based on the mixing time at which G⁄/sin d of neat AC equaled the wet processed AC at temperature of 20 °C and frequency of 1.59 Hz. At this mixing time about 1.5% kerosene was remaining in the AC. It seems

10000 PAC30 Neat PAC30 1% Wet CNF AC5 Neat AC5 1%Dry CNF

1000

PAC30 Wet Processed PAC30 2.5% Wet CNF AC5 Dry Processed AC5 2%Dry CNF

100

Viscosity, Pa-s

5.1.3. Dry mixing process The CNF-kerosene mixture was poured on a thin aluminum pan and placed in an oven at 150 °C for complete drying. The required amount of dried CNF was then mixed with 150 g of AC at 135 °C using the low shear mixer for 30 min. In order to evaluate the effect of processing, the AC was mixed for 30 min using the low shear mixer without the CNF addition. In this study, all such AC binders were referred as dry processed AC.

SHRP Viscosity Limit o of 3 Pa-s @135 C

10

1 Compaction Viscosity Range (0.31-0.25 Pa-s)

6. Results and analysis 6.1. Rotational viscosity (g)

Mixing Viscosity Range (0.19-0.15 Pa-s)

0.1

0.01 45

Typical viscosities of PAC30 and AC5 neat, wet processed and wet CNF modified AC at various temperatures are shown in Fig. 7. The data in the figure illustrates that there are minimal differences in

60

75

90

105

120

135

150

165

180

195

210

Temperature, oC Fig. 7. Typical temperature-viscosity curve for neat, processed and CNF modified AC binders.

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Rotaional Viscosity, Pa-s @135C

10

8

6.6

6.2

6

5.4 4.5 3.9

4

3.4

SHRP (3 Pa-s) 2.5

2.1

1.9

2

1.4 0.3

1.0

0.6

1.0

0 Neat Proc

1%

2%

Neat Proc

AC5-Dry Process

2%

Neat Proc 2.5%

AC30-Dry Process

Neat Proc

AC30-Wet Process

1% 2.5%

PAC30-Wet Process

Fig. 8. Comparison of viscosities at 135 °C of AC binders prepared different mixing procedures.

100000000 6.0

Shift Factor

3.0

10000000

y = 0.0008x2 - 0.2491x + 4.4387 R2 = 0.9970

0.0 -3.0 -6.0

1000000

-9.0

G*/Sin ( δ), Pa.

0

20

40

60

1C

80

Temperature (oC)

10 C

100000

20 C 25 C

10000

34 C 46 C 52 C

1000

60 C 64 C

100

Mater Curve Sigmoidal Curve

10 0.0001

0.001

0.01

0.1

1

10

100

1000

10000

Reduced Frequency (Hz) Fig. 9. Typical G⁄ master curve at 20 °C and shift factor for neat asphalt, AC5.

100,000 1000000

10000

PAC30 Wet Processed PAC30 1%Wet CNF PAC30 2.5% Wet CNF

AC5 Neat AC5 Wet Processed (165 min)

10,000

G*/Sin(δ), kPa.

G*/Sin ( δ), kPa.

100000

1000 100 10 1 0.1 0.001

1,000

100

0.01

0.1

1

10

100

1000

10000

Reduced Frequency, (Hz) Fig. 10. Effect of wet processing on the G⁄ master curve at 20 °C.

that this amount of kerosene was sufficient to affect the visco-elastic behavior of asphalt. The kerosene kept the asphalt softer at higher frequencies but had no effect at lower frequencies. Another possible explanation for such a behavior of wet processed AC was

10 0.001

0.01

0.1

1

10

100

1000

10000

Reduced Frequency, (Hz) Fig. 11. Effect of CNF on the G⁄ master curve at 20 °C.

that the kerosene may have degraded the asphalt molecular structure due to mixing at higher temperatures for longer periods of

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M.J. Khattak et al. / Construction and Building Materials 30 (2012) 257–264 15,000

12,500

G*/sin( δ ), Pa.

10,000

7,500

5,000

2,500

0 Neat Proc

1%

2%

AC5-Dry Process

Neat Proc

2%

Neat Proc 2.5%

AC30-Dry Process

AC30-Wet Process

Neat Proc

1% 2.5%

PAC30-Wet Process

Fig. 12. Comparison of G⁄/sin d of various CNF modified asphalt binders at 60 °C.

time. Such degradation could have affected the visco-elastic response of the asphalt. As previously stated, the dry CNF was mixed with the AC5 for 30 min using the mechanical mixture at temperature of 135 °C. This mixing might have aged the AC due to the evaporation of lower molecular weight oils. To evaluate the effect of short-term aging due to dry processing, the AC was subjected to same mixing time and temperature as that of CNF modified AC but without adding CNF. The AC samples were extracted and G⁄/sin d at higher temperature were measured and compared with the neat AC. The data indicated that, regardless of the test temperature and test frequencies, the increases in the G⁄ values at higher temperatures due to mixing process were in the ranges of 30–40%. This implies that short-term aging of the AC was caused due to the mixing processes and hence all comparisons between the various ACs must be in referenced to the processed binders to evaluate the effects of CNF modifications.

At frequency levels of 1 Hz and lower (25–64 °C), the average increase is 35% for 2.5% CNF contents in AC. On the other hand, such increase is only 15% at frequency levels higher than 1 Hz (1–20 °C). It was also observed that there was no significant difference between the processed and 1% CNF modified AC. Fig. 12 illustrates the effect of CNF for various types of AC and processing conditions at 60 °C. The CNF modified AC5 exhibited improvements up to 22% with respect to dry processed AC. On the other hand, the AC30 showed only 6% increases in the G⁄/sin d values relative to dry processed AC binders. The AC30 and PAC30 binders modified with 2.5% CNF using the wet process depicted improvements of 23% and 42%, respectively over the wet processed AC binders. These improvements indicate high resistance to rutting for CNF modified asphalt binders. The higher improvement using the wet processes was mainly due to the even dispersion of CNF using the sonication techniques followed by the higher shear mixing.

7. Dynamic shear fatigue characteristics

6.3. Effect of CNF modification In order to evaluate the effect of CNF modification, G⁄/sin d master curves were generated and modeled using the sigmoidal function for processed and CNF modified AC (Fig. 11). The comparison of the master curves revealed that the addition of CNF improved the G⁄/sin d of AC at low frequencies and at higher temperatures.

The ACs were subjected to repeated shear stress under sinusoidal wave form of 1.59 Hz at 20 °C to evaluate the fatigue behavior of neat, processed and CNF modified AC. Fig. 13 depicts a typical plot of G⁄ as a function of number of load cycles. It can be seen from the figure that as the number of load cycles increases the G⁄ of the

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Fig. 14. Comparison of normalized fatigue life of CNF modified AC at 20 °C.

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AC decreases. However, the rate of decrease of G⁄ is a function of AC type. The ACs were assumed to have complete fatigue failure when the G⁄ reached 50% of its initial value as shown in Fig. 13. The number of load cycles corresponding to 50% G⁄ values was then referred to as fatigue life. For example, it took 2450 cycles for PAC30 wet processed AC to reach fatigue failure however the 2.5% CNF modified PAC30 exhibited a higher fatigue life of 6530 cycles. In order to compare the fatigue life of the various ACs used in this study, the fatigue life of ACs were normalized with respect to the processed ones as shown in Fig. 14. Regardless of mixing process the fatigue life of CNF modified binders were 2–3 times the processed ones. It is interesting to note that the improvements in G⁄ were only between 22% and 42% and no significant changes in d values were observed but the increases in fatigue life were substantial. Such behavior can be attributed to the CNF reinforcement and micro-crack bridging mechanism. It is believed that CNF modification has produced a good network of fibers in the asphalt due to their high aspect ratio up to 1700. The fibers network may have bridged across the micro-cracks developed due to dynamic shear loading thus causing hindrance in their growth and consequently increasing the fatigue characteristics of the CNF modified AC binders. 8. Conclusions and recommendations This study provides exploratory analyses for mixing procedures and various characteristics of CNF modified AC binders. The developed laboratory procedure has presented the proof of the concept indicating that the homogenous dispersion of CNF produces good visco-elastic and fatigue characteristics of CNF modified AC binders. However, the mixing procedure has some limitations and challenges such as using a large quantity of solvent to disperse a small amount of CNF as well as the scalability for industrial applications. It is recommended to further explore other mixing techniques to add higher percentages of CNF, with a minimum amount of solvent, without a significant increase in the modified binder viscosity at mixing and compaction temperatures of hot mix asphalt mixtures (HMA). Furthermore, the chemical impact of solvent on the AC properties and the effect of CNF modification on the mechanistic characteristic of HMA need to be investigated. Based on the laboratory test results, the following conclusions can be drawn:  Regardless of mixing process, the fatigue life of CNF modified AC was 2–3 times the processed ones. Such behavior can be attributed to the increase in G⁄ values and mainly to crack bridging mechanism by CNF. The CNF network may have bridged across the micro-cracks developed due to dynamic shear loading thus causing hindrance in their growth and consequently increasing the fatigue life of the CNF modified AC.  The wet processed AC showed a slightly lower G⁄/sin d at higher frequencies and higher values at lower frequency levels the neat AC. While, the dry mixing process increased the G⁄/sin d up to 47%.

 The CNF modified AC5 using the dry process exhibited G⁄/sin d improvement up to 22% at 60 °C. On the hand, the wet processed AC30 and PAC30 binders modified with CNF depicted an improvement of 42%, which indicates high resistance to rutting.  The increase in G⁄/sin d of CNF modified AC is a function of frequency and temperature. At frequency levels of 1 Hz or less (temperature range from 25 to 64 °C), the average increase is 35% for PAC30 and 2.5% CNF wet processed modified AC. However, the average increase is only 15% at frequency levels higher than 1 Hz (temperature range from 20 to 1 °C).

Acknowledgments The authors wish to express their sincere thanks to the University of Louisiana at Lafayette and the Louisiana Transportation Research Center (LTRC)-Transportation Innovation for Research Exploration (TIRE) program for their financial support. References [1] Hussain F et al. Review article: polymer–matrix nanocomposites, processing, manufacturing, and application: an overview. J Compos Mater 2006;40(17):1511–75. [2] Lee WJ et al. The mechanical properties of MWNT/PMMA nanocomposites fabricated by modified injection molding. Compos Struct 2006;76:406–10. [3] Lin JC et al. Mechanical behavior of various nanoparticle filled composites at low-velocity impact. Compos Struct 2006;76:30–6. [4] Kumar S et al. Processing and characterization of carbon nanofiber/ syndiotactic polystyrene composites in the absence and presence of liquid crystalline polymer. Compos Part A 2007;38:1304–17. [5] Fiedler B et al. Fundamental aspects of nano-reinforced composites. Compos Sci Technol 2006;66:3115–25. [6] Khattab A, et al. Preliminary process investigation of manufacturing high temperature polymer nanocomposites. In: SAMPE technical conference proceedings. new materials and processes for a new economy, Seattle, WA, Society for the Advancement of Material and Process Engineering; 2010, p. 9. [7] Ghile DB. Effects of nanoclay modification on rheology of bitumen and on performance of asphalt mixtures. M.S. thesis. Delft University of Technology, Delft, Netherlands; 2006. [8] Liu D-L, Bao S-Y. Research of improvement of SBS modified asphalt pavement performance by organic montmorillonite. J. Build. Mater., China 2007;10(4):500–4. [9] Ma F, Zhang C, Fu Z. Performance and modification mechanism of nano-CaCO3 modified asphalt. Wuhan Ligong Daxue Xuebao (Jiaotong Kexue Yu Gongcheng Ban). J Wuhan Univ Technol 2007;31(1):88–91 [Transportation Science and Engineering]. [10] Yu J et al. Effect of montmorillonite on properties of styrene–butadiene– styrene copolymer modified bitumen. Polym Eng Sci 2007;47(9):1289–95. [11] Tandon GP, Ran Y. Influence of vapor-grown carbon nanofibers on thermomechanical properties of graphite-epoxy composites. In: Proceedings for the 17th annual technical conference ASC. [12] Glasgow DG, Tibbetts GG. Surface treatment of carbon nanofibers for improved composite mechanical properties. In: SAMPE technical conference proceedings; 2004, Long Beach, CA. [13] Finegan C et al. Surface treatments for improving the mechanical properties of carbon nanofiber/thermoplastic composites. J Mater Sci 2003;38:3485–90. [14] Gibson T, Rice B, Ragland W. Formulation and evaluation of carbon nanofiber based conductive adhesives. In: SAMPE technical conference proceedings; 2005, Long Beach, CA. [15] Ferry JD. Viscoelastic properties of polymers. John Wiley & Sons, Inc.; 1961. [16] Kim YR, Little DN. Linear viscoelastic analysis of asphalt mastic. J Mater Civil Eng 2004;16:122–32.