Microstructure and fracture morphology of carbon nano-fiber modified asphalt and hot mix asphalt mixt

$Page 1: Microstructure and fracture morphology of carbon nano-fiber modified asphalt and hot mix asphalt mixt$
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Materials and Structures ISSN 1359-5997 Mater StructDOI 10.1617/s11527-013-0035-3

Microstructure and fracture morphology ofcarbon nano-fiber modified asphalt and hotmix asphalt mixtures

Mohammad J. Khattak, Ahmed Khattab,Pengfei Zhang, Hashim R. Rizvi &Thomas Pesacreta

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ORIGINAL ARTICLE

Microstructure and fracture morphology of carbon nano-fiber modified asphalt and hot mix asphalt mixtures

Mohammad J. Khattak • Ahmed Khattab •

Pengfei Zhang • Hashim R. Rizvi •

Thomas Pesacreta

Received: 29 August 2012 / Accepted: 19 February 2013

� RILEM 2013

Abstract This paper focuses on the microstructure

and fracture surface morphology of neat and carbon

nanofibers (CNF) modified asphalts and hot mix

asphalt (HMA) mixtures using scanning electron

microscopy (SEM). Asphalt binder was modified with

1.5 % of CNF by weight of binder. The modified

asphalt was used to construct HMA mixtures at

various CNF dosages, mixed with aggregate, using

the Superpave Gyratory compactor. Small rectangular

specimens extracted from the center of large HMA

samples were tested under direct tension and the

fracture surface was examined under SEM. The SEM

analysis developed a fundamental understanding of

the role that the CNF modification plays in the

performance enhancement of asphalt and HMA mix-

tures. It was found that CNF not only possess good

adhesion characteristics but also exhibits high con-

nectivity and were evenly distribution throughout the

binder. The fracture surface morphology also revealed

that CNF exhibited crack bridging at micro/nano scale

which may enhance the resistance to cracking due to

repeated traffic loads.

Keywords Asphalt � Carbon-nanofibers � SEM �Micrographs � Fracture surface

1 Introduction and background

Asphalt is a historical material used in different

civilizations of the world and has been successfully

utilized by the modern pavement industry for the last

M. J. Khattak (&) � H. R. Rizvi

Department of Civil Engineering, University of Louisiana

at Lafayette, Lafayette, LA, USA

e-mail: [email protected]

H. R. Rizvi


A. Khattab

Department of Industrial Technology, University

of Louisiana at Lafayette, Lafayette, LA, USA


P. Zhang

Department of Mechanical Engineering, University

of Louisiana at Lafayette, Lafayette, LA, USA


T. Pesacreta

Department of Biology, University of Louisiana



T. Pesacreta

Diretor Microscopy Center, University of Louisiana


Materials and Structures

DOI 10.1617/s11527-013-0035-3

Author's personal copy

100 years [1]. Asphalt is a complex mixture of organic

molecules including n-heptane soluble resins (also

known as maltenes) and toluene soluble asphaltenes

(mw approx. 750–30,00 Da). In crude oil, the resins,

which are in some ways small molecular weight

versions of asphaltenes, stabilize the asphaltenes. If

resins are removed by processing, the asphaltenes can

form an extended nanocolloidal matrix or polyaro-

matic sheets, thus affecting mechanical properties

within mixtures. Not unexpectedly, asphaltenes can

impart high viscosity to crude oils during processing,

negatively impacting production. The same logic

applies to the production of asphalt during which

processing conditions can remove or alter resins.

Many studies have been conducted to modify and

improve asphalt binder’s strength and adhesion. Gen-

erally, different types of polymers and fibers have been

used to enhance asphalt binder rheological character-

istics. Such polymers and fibers are used in asphalt to

increase its visco-elastic, strength and stiffness prop-

erties, consequently improving the hot mix asphalt

mixtures (HMA) performance. Scanning electron

microscopy (SEM) analysis of asphalt is very difficult

as it is a liquid which flows as soon as heat is provided

to it. Electron microscopy is basically a bombardment

of electron to the sample and capturing the reflected

electrons or radiated electrons out of the sample. This

process raises the temperature of the sample, which

becomes worse in case of asphalt as it starts to flow.

Therefore, capturing asphalt images is very difficult.

SEM analysis played very important role in under-

standing the mixture behavior and examining the

microchemistry of asphalt binder and its mixtures [2–

7]. Electron microscopy analysis of asphaltenes has

indicated that, at the molecular level, they consist of

stacked aromatic rings that are laterally attached to

alkyl chains that can sometimes disrupt the stacking

[8]. Other structural studies have shown that, depend-

ing upon the molecules that surround the asphaltenes,

they can form agglomerate particles, porous structures

and smooth surfaces [9]. SEM-energy dispersive

spectroscopy (SEM-EDS) has shown that several

types of heteroatoms including the elements S, V,

and Si are associated with asphaltenes [10].

Understanding the microstructure of fibers and the

role that they play in the asphalt can be accomplished

using SEM imaging and analysis. The fiber diameters

and their specific surface area play a key role in

absorption and adhesion with asphalt binder [11, 12]

has shown that fibers provide bridging between

conductive clusters within the asphalt binder to

complete and enhance conductivity of the material.

SEM analysis of the mixture also proves that organic

fibers are torn into pieces due to mixing and they are

able to increase their surface area almost ten times

than the polymer and mineral fibers [13]. It was

observed that fibers make 3 dimensional networks

within the asphalt binder and perform better at high

temperature mixing [13, 14]. Kim et al. [14] used SEM

images to understand the healing process of asphalt

binder. They found that in sand-asphalt mixtures the

fracture healing occurs during the rest period of

loading, which is a basic difference between the

laboratory and field experimentation. The healing rate

was dependent upon healing time and binder viscosity,

which was confirmed by the SEM images. SEM

analysis has also made it possible to study the

differences between the phase distributions of mal-

eated polypropylene and isotactic polypropylene [15].

Smart materials are receiving significant popularity

in recent years due to their excellent mechanical,

thermal, and electrical properties as well as their

sensing capabilities. These materials not only can

sense excessive loading but also detect degradation,

and damage due to environmental effects. Similar

concept is finding its way in highway and pavement

design. Developing a smart HMA mixture that can

sense the load response and deliver the information

will enhance the efficiency of maintaining the high-

way and pavement network. A smart HMA mixture

can be developed using conductive carbon nanofibers

(CNF) as a modifier that can allow the conventional

HMA mixtures to exhibit a piezoresistive effect. CNF

modified asphalts and HMA mixtures are also believed

to exhibit better mechanical performance due to

increased stiffness and enhanced fracture resistance

attributed to high fiber aspect ratio and micro crack

bridging mechanism [16].

2 Objective of the study

The main objective of the study is to conduct

comprehensive analysis of SEM micrograph of asphalt

binders and HMA mixtures. The microstructure and

morphology of asphalt binders with and without CNF

was analyzed. Furthermore the fracture surface of

HMA mixture tested under direct tension mode was



investigated. Such analysis yielded a fundamental

understanding of the role that CNF play in enhancing

the electrical and mechanical characteristics of HMA

mixtures.

3 Test materials and methods

3.1 Materials

The viscosity graded asphalt cement AC5 (PG52-22),

was obtained from a vendor in Atlanta, GA. Crushed

limestone was supplied by a local HMA contractor

located in Lafayette, LA. 19 mm nominal maximum

size aggregates were used to construct HMA samples.

Vapor-grown CNF (Polygraf III) produced by Applied

Sciences was used for asphalt and HMA 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 fiber

has a high performance per cost ratio and a good

interfacial bonding with materials. The CNF are also

known to have high electric conductivity. Commer-

cially available kerosene and acetone were used as

solvents to disperse the CNF, ultimately mixing it with

asphalt and/or aggregate for constructing HMA

mixtures.

3.2 HMA sample preparation

Khattak and Khattab [16] have reported a detailed

procedure of mixing CNF in HMA mixture. The

mixing procedure is briefly discussed below.

1. Kerosene–CNF mixture was prepared by adding

1.5 % of CNF by weight of asphalt binder into

kerosene and thoroughly mixed using sonication

and high shear mixing. The kerosene–CNF mix-

ture was then mixed with asphalt melted at 60 �C.

The oil bath temperature was slowly raised to

150 �C during mixing using the shear mixer for

170–175 min. The kerosene was observed to

completely evaporate while leaving the CNF in

the asphalt binder. This procedure produced

homogenous CNF modified asphalt blend.

2. The acetone-CNF mixture was also prepared

using the sonication and shear mixing technique.

The mixture was placed in a tray and the acetone

was allowed to evaporate at room temperature

using a small pedestal fan. After 3–4 h of

continuous drying at room temperature the CNF

was placed in an oven at 60 �C for 6 h to achieve

completely dry CNF that were free of clumps.

3. In order to construct CNF modified HMA

mixtures, desired amount of dry CNF was mixed

with the aggregate in a rotary mixer at room

temperature for 15 min. After rotary mixing, the

CNF modified asphalt binder along with aggre-

gate-CNF mixture was placed in the oven for

1.5 h at mixing temperature of 150 �C. A

thorough blend of CNF, aggregate and CNF

modified asphalt was prepared in a steel bowl

using a low-speed mixing spindle at mixing

temperature of 150 �C. The CNF modified HMA

mixture was then placed in an oven for 2 h at

compaction temperature of 135 �C. Finally, the

HMA mixture was compacted at 135 �C in a

150 mm diameter steel mold using the Super-

pave gyratory compactor.

Superpave mixture design was conducted and an

optimum asphalt content of 4 % was obtained for

target air voids of 4 %. A cylindrical sample of HMA

of about 115 mm height and 150 mm diameter was

compacted using the Superpave� gyratory compactor

and sliced into three specimens of 35 ± 2 mm thick-

ness using a water cooled diamond saw.

3.3 SEM

The Microscopy Center at University of Louisiana at

Lafayette conducted various tests to characterize the

basic morphology and microstructure of the CNF, neat

asphalt binders, CNF modified asphalt binder, and

fracture surface morphology of HMA mixtures. In

order to better visualize the structural components of

the asphalt and their inter-relationships, the asphalt

samples were extracted with n-heptane that removes

the maltenes but not the asphaltenes or CNF. High-

resolution SEM was used to study the binder mor-

phology and fracture surface morphology and the

failure mechanisms. The samples used for the SEM

fracture surface morphology analysis were

38 9 15 9 3 mm2 sections of HMA cut from the

center of 35 mm thick and 150 mm diameter asphalt

concrete specimen using a diamond blade saw

(Fig. 1). The specimens were fractured under direct

tension at a rate of 5 mm/min. The surface was



examine under 15 kV of energy at various magnifica-

tion factors such as 2000, 6000, 14000, 18000 and

22,000, to understand the morphology and micro-

structure of CNF modified HMA mixtures. Specimens

were tested with and without gold coating. After

specimens were placed on a small aluminum stub, a

gold sputter thin coating of 0.6 nm was applied at

room temperature using the electron microscopy

sciences system. The gold coating reduces the SEM

beam damages, reduces specimen charging, and

improve secondary electron emission along with edge

resolution.

Fig. 1 HMA specimen for fracture morphology analysis using SEM. a cross-section, b plan view, c longitudinal section, d fractured

specimen under direct tension, e fracture surface for SEM analysis

Fig. 2 a SEM images of neat carbon nanofibers at two different locations and b SEM image of dispersed CNF in solvent



4 Results and analysis

4.1 CNF morphology

The CNF and the degree of dispersion in asphalt binder

and HMA mixtures were characterized by SEM.

Figure 2a shows the neat carbon nanofibers, as provided

by the vendor, without any dispersion techniques. Under

a high magnification factor of 2,000, relatively large

CNF agglomerates were observed as a result of

entanglement and aggregation between individual

nanofibers. Characterization of the dispersed CNF in

solvent under SEM is shown in Fig. 2b. The figure

indicates that the nanofibers were much dispersed and

loosely placed than those without dispersion techniques.

4.2 Morphology of neat asphalt

Initial attempts to examine the samples without a gold

coating in the environmental mode showed that the

surface morphology of the ‘‘neat’’ samples was

quickly altered by the energy of the electron beam,

Fig. 3a. The area in the center of this image is where

the sample had been previously scanned at a higher

magnification. The area to the left shows the original,

smooth surface morphology of the sample. The area to

the right is the aluminum stub. Figure 3b shows the

zoomed portion of Fig. 3a illustrating the altered

morphology, which appears to consist of a series of

snake-like structures, all with smooth surfaces. Similar

snake like structure has been reported by Shin

et al. [7].

The surface morphology of asphalt binder mixed

with kerosene at 150 �C using a shear mixer for

170–175 min was similar to that of the ‘‘neat’’

asphalt. These samples also began to charge (i.e.

randomly emit electrons) quickly because they were

non-conductive and therefore unable to dissipate the

energy of the beam. This made extended observation

impossible. In contrast, the surface of samples that

had been modified by the addition of CNF was

covered with linear structures, Fig. 4. The linear

CNF were irregularly distributed across the surface.

In keeping with the conductivity measurements, the

surface morphology of the CNF-asphalt was not

altered and snake-like structure was not formed

during SEM observation. Additionally, the samples

did not get charged during the experimentation. This

indicates that the CNF-asphalt was significantly more

conductive than the ‘‘neat’’ asphalt and therefore was

able to provide a path to ground the electron beam

energy.

(a)

(b)

Fig. 3 a Surface morphology of neat asphalt altered due to

electron beam exposure. b zoomed portion of Fig. 3a

Fig. 4 Surface morphology of CNF modified asphalt showing

linear structure



4.3 Morphology of extracted asphalt

In order to better visualize the structural components

of the asphalt and their inter-relationships, the samples

were extracted with n-heptane that removes the

maltenes but not the asphaltenes or CNF. After the

initial one-week extraction, the ‘‘neat’’ sample showed

a rough, irregular surface structure, Fig. 5a. Some

smooth, plate-like areas were present and there was a

wide variety of differently sized particles present as

well. After a two-month extraction with n-heptane, the

plate-like structures were no longer observed, Fig. 5b,

and a uniformly sized assemblage of small particles

was present. Figure 5c shows these particles, which

represented the asphaltene component of the original

asphalt at higher magnification, with diameters in the

range of 200–300 nm. Rather than being discrete,

individual particles were linked to each other in an

irregular series. The surface of the particles was

smooth, possibly as a result of the gold coating. The

structure of the kerosene-treated asphalt was similar to

that of the ‘‘neat’’ sample.

The initial extraction of CNF-asphalt revealed

numerous and lengthy CNF throughout the sample,

Fig. 6a–c. As was observed with the ‘‘neat’’ sample,

the asphalt that surrounded the CNF was non-uniform

in its structure with numerous smooth and rough areas.

Further extraction showed that the CNF were still

tightly intermixed with the asphalt but the surrounding

asphaltene areas had become uniformly particulate,

just as had been observed with the ‘‘neat’’ sample,

Fig. 6b. These samples, like the unextracted CNF-

asphalt, did not ‘‘charge’’ when they were observed

with the electron beam, indicating that even after the

extraction of the maltenes, the asphaltene-CNF mix-

ture was sufficiently conductive to electrically ground

the samples. Additionally, crack bridging by CNF can

be observed in Fig. 6c.

4.4 EDS analysis of neat asphalt

Asphaltene is a complex and incompletely understood

mixture of chemical components. The extensively

extracted ‘‘neat’’ asphaltene showed a relatively

regular particulate array. So, understanding the ele-

mental composition of this highly organized fraction

became an interesting issue. The elemental composi-

tion was very simple, consisting only of C, O, and S as

shown Fig. 7. Repeated attempts were made to find

other elemental signatures but the only possible

candidates were present in concentrations that were

at least a decade below the nominal detection limit of

0.1 % and thus had to be considered as being due to

Fig. 5 Surface morphology of neat asphalt treated with n-heptane

for a 1 week, b 2 months, c 2 month at high magnification



background noise. Some Al was detected but this was

presumably due to the composition of the SEM stub

that the samples were mounted on.

The above analyses reveal that the structure of

asphalt and asphaltenes can be examined with our

existing suite of microscopes. It will be an interesting

possibility to use atomic force microscopy in the

future to more precisely examine the structure of these

materials at very high magnification without the need

for gold coating. The tendency of ‘‘neat’’ asphalt to

form semi-regular snake-like arrays during SEM

observation is interesting and may indicate some

underlying asphaltene macro-structure that was

degraded during the succeeding chemical extraction

with n-heptane.

The CNF-asphalt and CNF-asphaltene was excep-

tionally conductive. The fibers were tightly bound

within the asphaltene matrix yet were still sufficiently

interconnected even after extraction of the maltenes so

that they could continue to provide a continuous

conductive path. The CNF-asphalt and the CNF

asphaltene should be considered as a cheap and

effective alternative to the graphite paste that is

commonly used to ground SEM samples. Elemental

analysis indicated that only a single heteroatom

element was present in the highly extracted ‘‘neat’’

sample. The simplicity of this composition suggests

that the lengthy extraction method used here can be

used to reduce the chemical complexity of asphaltenes

while still retaining their macro-structural character-

istics. In the future, this may enable a better under-

standing of the essential components of asphaltene

structure.

4.5 HMA fracture morphology

The fracture surfaces of HMA specimens shown in

Fig. 1e were examined using the SEM at various

magnification factors of 2000, 6000, 14000, 18000 and

22,000. Figure 8 shows the fracture surface morphol-

ogy of processed HMA mixtures. The fracture surface

seems rough and irregular along with nano- to micro-

scale air voids. Such voids can be responsible for crack

initiation. A closer look at a portion of Fig. 8a at a

magnification factor of 6,000 reveals a popping out of

fine aggregate that might have occurred during the

tensile loading, Fig. 8b. A clear and well-defined

boundary of asphalt binder is also visible which was

formed due to fine aggregate. The size of the fine

aggregate is *6 lm.

An interesting observation was the formation of

few asphalt fibrils along the cracked surface as shown

Fig. 6 a, b Surface morphology of CNF modified asphalt

treated with n-heptane, c surface morphology of CNF modified

asphalt showing crack bridging by CNF



in Fig. 9a. The asphalt fibril seems coarser with

approximate diameter of 8 lm reducing to 4 lm at the

break. On another location, as shown in Fig. 9b, a

micro crack was spotted which was magnified to a

factor of 6,000.

Figure 10a shows the SEM image of fracture

surface of 4 % CNF modified HMA at a magnifica-

tion factor of 2000. The CNF dosage of 4 % includes

1.5 % of CNF dispersed in the asphalt binder and

2.5 % in acetone for HMA mixing. A portion of the

Elt. Line Intensity, (c/s) Error, 2-sig Atomic, % Conc UnitsC Ka 272.42 3.301 90.284 85.661 wt.%

O Ka 6.68 0.517 8.094 10.229 wt.%S Ka 72.18 1.699 1.622 4.109 wt.%

Total 100.000 100.000 wt.%kV 25.0, Takeoff Angle 35.0°, Elapsed Livetime 100.0

Fig. 7 Elemental analysis

of ‘‘neat’’ asphaltene

(a) (b)

Fig. 8 SEM image of fracture surface of processed HMA at magnification factor of a 2000, b 6000



same image is magnified to a factor of 6,000 as

shown in Fig. 10b. The SEM images indicate that

CNF density is low and not evenly distributed with

some lumps formed in one side which is also pulled

out due to tensile loading. CNF can be seen sticking

out in the direction of loading. Such effect was more

enhanced when an SEM image was taken at another

location on the fracture surface of CNF modified

HMA, as shown in Fig. 11a, b. It can be seen from

the figures that the density of CNF is high and all the

CNF fibers are pointing out to the direction of tensile

loading. The fibers are also interconnected with a

good network formation.

Figure 12 shows the magnified portion of SEM

images shown in Fig. 11b. The figure illustrates pull-

out behavior of the CNF due to tensile loading.

A cone-shaped asphalt structure can be observed at the

root of the CNF. This indicates that the adhesion

between the asphalt binder and CNF is at least as high

as the cohesion of asphalt. Hence, when the CNF is

pulled out due to tensile loading it pulls the asphalt

binder with it, thus forming the cone-shaped structure

at the root of CNF. Interestingly, formation of nano-

cracks was also observed in the vicinity of the cone-

shaped structure as a result of localized stresses caused

by the tensile loadings. A closer examination of the

(a) (b)

Fig. 9 a Fracture surface morphology of processed HMA at magnification factor of 2,000 with course fibril formation, b micro-crack

formation shown at magnification factor of 6,000

(a) (b)

Fig. 10 Fracture surface morphology of 4 % CNF modified HMA with low CNF density at magnification factors of a 2000 and b 6000



figures reveals that the CNF are completely coated

with asphalt which is further evidence of better

adhesion of CNF with the asphalt binder.

Figure 13a shows the SEM micrograph of crack

morphology of 4 % CNF modified HMA mixtures.

The figure shows that the crack bridging is formed by

the CNF at the micro/nano scale. As discussed earlier,

a cone-shaped structure is formed due to pulling out of

the CNF. This crack bridging mechanism is believed

to suppress localization of micro-cracks and hinder

micro-crack propagation. Figure 13b through 13c

illustrate the SEM images of fracture surfaces of

6.5 % CNF modified HMA mixtures. It should be

noted that the CNF dosage of 6.5 % includes 1.5 % of

CNF dispersed in the asphalt binder and 5 % in

acetone for HMA mixing. The figures show that the

fracture surface morphology is similar to the 4 % CNF

modified HMA mixtures with enhanced properties.

Clear evidence of the high density of CNF in the HMA

mixture, greater connectivity and better network

formation, and higher number of CNF bridging across

the micro-cracks is presented in the SEM images. It is

believed that good conductivity and mechanical

properties, and enhanced performance in resistance

to fatigue cracking and permanent deformation of

CNF modified HMA are expected due to the afore-

mentioned morphological behavior of the modified

mixtures. Khattak and Khattab et al. [16] has shown

the complex shear modulus and fatigue life of CNF

modified binders were substantially higher than the

neat asphalt binders.

Figure 14a shows the SEM image of the fracture

surface of 11.5 % CNF modified HMA mixture. The

figure reveals that there is a high density of CNF

concentrated in a single area. Some are individually

placed and others are agglomerated thereby making

clumps. Additionally, micro/nano cracks were

observed all around the asphalt surface and at the

vicinity of CNF. Recall that similar micro/nano cracks

were visible for 4 % CNF mixtures at magnification

factor of 22,000. However, for 11.5 % CNF mixtures,

such cracks can be seen at a relatively low magnifi-

cation factor of 6,000. Figure 14b shows the distribu-

tion of micro/nano cracks at the high magnification

(a) (b)

Fig. 11 SEM image of fracture surface of 4 % CNF modified HMA with high CNF density at magnification factors of a 2,000 and

b 6,000

Formation of Nano-cracks

Cone-shaped formation

Fig. 12 Zoomed SEM image of Fig. 11b of fracture surface of

4 % CNF modified HMA showing CNF pull-out behavior and

nano–micro cracks at magnification factor of 22,000



factor of 22,000 for the 11.5 % CNF modified HMA

mixture. A high density network of the micro/nano

cracks can be found all around the fracture surface. It

should be noted that 11.5 % CNF created a high

percentage of air voids in the HMA mixtures. The high

percentage of CNF also stiffened the asphalt binder

due to high viscosity. Furthermore, due to no adjust-

ment made in the mix design, the asphalt resulted in

insufficient surface coating of the aggregate including

the CNF. All the above factors may result in poor

mechanical performance of the mixtures. The evi-

dence of the poor mechanical performance of HMA

mixtures can be seen through the SEM fracture surface

morphology of HMA mixtures that exhibited

numerous and dense micro/nano crack formation once

subjected to tensile loading.

5 Summary and conclusions

Several tests were conducted to characterize the basic

morphology and microstructure of CNF, neat asphalt

binders, CNF modified asphalt binder, and fracture

surface morphology of CNF modified HMA mixtures.

In order to better visualize the structural components

of the asphalt and their inter-relationships, the asphalt

samples were extracted with n-heptane that removes

the maltenes but not the asphaltenes or CNF. High-

(a) (b)

(c) (d)

Fig. 13 a Fracture surface of 4 % CNF modified HMA

showing CNF crack bridging mechanism at magnification

factor of 6000, b fracture surface of 6.5 % CNF modified

HMA with high CNF density at magnification factor of 2000,

c fracture surface of 6.5 % CNF modified HMA with high CNF

density at magnification factor of 2,000 at another location,

d fracture surface of 6.5 % CNF modified HMA showing CNF

crack bridging mechanism at magnification factor of 6,000



resolution SEM was used to study the binder mor-

phology and fracture surface morphology and the

failure mechanisms. It was observed that binder

morphology of CNF modified asphalt binders indi-

cated a well distributed and interconnected CNF

network. The fibers were bridging across the cracks

and uniformly coated with asphalt, exhibiting better

adhesion between the CNF and asphalt binder. The

CNF-asphalt and CNF-asphaltene was exceptionally

conductive under the SEM electron beam. The fibers

were tightly bound within the asphaltene matrix yet

were still sufficiently interconnected even after

extraction of the maltenes so that they could continue

to provide a continuous conductive path.

The examination of the fracture morphology of

HMA mixtures revealed that CNF density increased

with the increase in CNF dosage. The CNF fibers

established a good network and showed high connec-

tivity. Fiber pull-out behavior under tensile loading as

observed in the SEM demonstrated a cone-shaped

asphalt structure formation at the root of the CNF. This

indicates that the adhesion between the asphalt binder

and CNF is at least as high as the cohesion strength of

the asphalt. Furthermore, the CNF is completely

coated with asphalt which was an evidence of better

adhesion of CNF with asphalt binder. SEM images of

crack morphology also showed that CNF were pulled

out in the direction of tensile loading and exhibited

crack bridging at micro/nano scale. This crack bridg-

ing mechanism is believed to suppress the localization

of micro-cracks and hinder micro-crack propagation.

In addition, formation of nano/micro cracks was

observed in the vicinity of cone-shaped structures as

a result of localized stresses caused by the tensile

loadings. The density of such cracks was significantly

high for the 11.5 % CNF dosage.

Acknowledgments The authors wish to express their sincere

thanks to the University of Louisiana at Lafayette and the

Louisiana Transportation Research Center-Transportation

Innovation for Research Exploration (TIRE) program for their

financial support.

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(b)

Fig. 14 a SEM image of fracture surface of 11.5 % CNF

modified HMA showing CNF density and nano- and micro

cracks in asphalt binder at magnification factor of 6000, b SEM

image of fracture surface of 11.5 % CNF modified HMA

showing nano- and micro cracks around the CNF/asphalt

interface at magnification factor of 22,000



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Documents

Microstructure and fracture morphology of carbon nano-fiber modified asphalt and hot mix asphalt mixt