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Construction and Building Materials 22 (2008) 2111–2115

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MATERIALS

Investigation of rheological and fatigue properties of asphaltmixtures containing polyester fibers

Shaopeng Wu, Qunshan Ye *, Ning Li

Key Laboratory of Silicate Materials Science and Engineering of Ministry of Education, Wuhan University of Technology, Wuhan 430070, China

Received 1 June 2007; received in revised form 18 July 2007; accepted 21 July 2007Available online 4 September 2007

Abstract

The effects of polyester fiber on the rheological characteristics and fatigue properties of asphalt and its mixture are investigated in thispaper. The viscosity, rheological and fatigue tests are conducted to characterize such related properties of asphalt binder and mixturewith different fiber contents. Test results indicate that the viscosity of asphalt binder is increased with increasing polyester fiber contents,especially at lower temperature. With different polyester fiber contents, the complex modulus and loss modulus of asphalt binders aredecreased at 15 �C and 0.1–100 rad/s frequency range. The dynamic modulus test results for asphalt mixture with 0.3% polyester fibercontent also reveal that the dynamic modulus and phase angle are decreased at the same temperature, which leads to a decrease of fatigueparameter for asphalt mixture. When compared with the control asphalt mixture, the cycle numbers to fatigue failure of fiber modifiedasphalt mixture are increased with 1.9, 2.9 and 3.6 times at 0.5, 0.4 and 0.3 stress ratios, respectively. The parameters of fatigue functionsfor asphalt mixture with or without polyester fiber are obtained and compared, and it confirms that the fatigue property of asphalt mix-ture can be improved by fiber addition, especially at lower stress levels.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Asphalt mixture; Binder; Rheological characteristics; Fatigue property; Stress level

1. Introduction

Bituminous binders have been widely used in flexiblepavements because of their good adhesion to mineralaggregates and viscoelastic properties [1–3]. Unfortunately,asphalt mixture or coating layer shows severe temperaturesusceptibility such as high-temperature rutting, mediumtemperature fatigue and low temperature cracking damage.Therefore, asphalt mixture should be modified in some wayto promote their further application.

Ideally, a modifying agent of bitumen should be easilyincorporated to yield a highly viscous mixture at in-servicetemperatures which remains homogenous on storage, andshould have a viscosity which permits its use in standardmaterial manufacturing and paving equipments. Moreover,

0950-0618/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.conbuildmat.2007.07.018

* Corresponding author. Tel./fax: +86 27 8716 2595.E-mail address: yeqs@whut.edu.cn (Q. Ye).

it should be highly resistant to ultraviolet light, thermalactions, water damage, environmental friends, and beavailable [4–7].

Among various modifiers for asphalt, fibers have gottenmuch attention for their excellent improvement effects.Various fiber modifiers, such as cellulous fiber, polyesterfiber and mineral fiber, have been widely used in differentasphalt mixtures. These include Stone Mastic Asphalt(SMA), Open Grade Friction Concrete (OGFC), etc. [8–11]. Many earlier research projects focus on the influenceof fiber additives on the engineering properties of asphaltor asphalt mixture. Chen and Lin [12] investigated cellu-lose, rock wool and polyester fiber on the engineering prop-erties of asphalt, and the test results indicated that goodadhesion between fibers and bitumen enhances the load-carrying ability of asphalt-fiber mastics. Wu et al. [13] con-ducted dynamic shear rheometer (DSR) test to study therheological properties of asphalt with various fibers. Theirresults indicated that the rutting-resistance property of

2112 S. Wu et al. / Construction and Building Materials 22 (2008) 2111–2115

asphalt with fibers could be improved to a large extent.Putman and Amirkhanian [14] compared the performanceof SMA mixtures containing waste tire and carpet fiberswith mixes made with commonly used cellulose and polyes-ter fibers. The results revealed that the tire, carpet andpolyester fibers significantly improved the toughness ofthe mixtures, but no significant difference in permanentdeformation or moisture susceptibility was found.

Polyester fiber modified asphalt binder and mixture havebeen successfully applied in construction practice. How-ever, the effect of polyester fiber on the pavement engineer-ing performances is profound, and the mechanism of thefiber effect on the bitumen is complex. The main objectiveof this research is to investigate the related properties ofpolyester fiber modified asphalt binder and mixture; suchas viscosity, rheological characteristics, dynamic property,and fatigue performance.

2. Materials and experimental designs

2.1. Raw materials and mix gradation

A base asphalt (AH-70), provided by KOCH AsphaltCo. Ltd. (Hubei Province, PR China) was used for polyes-ter fiber modification, with a penetration of 69 (0.1 mm at25 �C, 100 g and 5 s), ductility of more than 100 cm (at15 �C) and softening point of 48 �C according to ASTMD 3381.

The polyester fiber adopted in this research is a commer-cial product (Tianhui Fiber Materials Co., Ltd., Wuhan,PR China), which is made of polypropylene and with anaverage length and diameter of 6 mm and 20 lm, respec-tively. The specific gravity of polyester fibers is 1.35 g/cm3, and the tension strength is 520 MPa. The meltingpoint of polyester fiber is 248 �C, thus remaining intactduring high mixing temperatures for preparation of asphalt

Fig. 1. SEM morphological image of polyester fiber.

binder or mixes. The microscopic morphology of polyesterfiber is shown in Fig. 1. It can be seen that the cross-sectionof these fibers is quite round and their surface is smooth.

Asphalt mixtures were obtained with a 12.5 mm Super-Pave gradation. Basalt coarse aggregate and fine aggregatewere used in the specimens preparation, properties ofbasalt aggregate are shown in Table 1. Limestone was usedas mineral filler. Table 2 presents the selected mixgradation.

2.2. Specimens preparation and test procedure

The polyester fiber modified asphalt binder wasobtained by a constant mixer at 165 �C. Fiber contents of0.1, 0.3 and 0.5 percent by weight of the asphalt. To obtainhomogenous bitumen-fiber mastics, the polyester fiberswere added slowly into the preheated pure asphalt andmixed for 2 h. Next, the viscosity test and dynamic sheartest were carried out to investigate the viscosity and rheo-logical properties of the fiber modified asphalt binder andthe control sample. The binders were aged in the RTFOand frequency sweep dynamic shear tests were conductedat 15 �C.

Standard SuperPave mix procedures were employed toprepare the dynamic modulus test specimen with 170 mmheight and 150 mm diameter after the mixtures have beenshort-term oven aged for 4 h at 135 �C, Then such speci-mens were sawed and polished to the required specimengauge for dynamic modulus test and fatigue test. The diam-eter and height of specimens for dynamic modulus testwere 100 mm and 150 mm, and for indirect tension fatiguetest were 150 mm and 38 mm. The optimum binder contentfor mixtures without fiber was found to be 4.8%, while formixtures with 0.3 percent polyester fiber (by weight of theasphalt mixture) the optimum binder content was 5.0%.The desired air voids for all specimens was approximately3.0%. When compared with conventional asphalt mixes,the differences for preparation of polyester fiber asphaltmixes are the mixing procedure and the mixing time. Inlaboratory study, the polyester fibers must be mixed withdry aggregates for about 30 s prior to the mixing withasphalt and mineral filler. In field application, the polyesterfiber can be blasted into the mixer plant, and the dry mix-ing time for fibers and aggregates is about 8 s. The layingand compaction for polyester fiber asphalt mixes are thesame with the conventional asphalt mixes.

Table 1Properties of basalt aggregate

Test items Measured values Standard

Specific gravity (g/cm3) 2.96 ASTM C-127Water absorption (%) 0.65 ASTM C-127Frost action (%) (with Na2SO4) 7.05 ASTM C-88Abrasion loss (%) (Los Angles) 15.6 ASTM DC-131Polishing value 0.62 BS-813

Table 2Selected mix gradation

Sieves [mm] 19 13.2 9.5 4.75 2.36 1.18 0.6 0.3 0.15 0.075

Passing [%] 100 93.0 79.6 50.6 33.2 23.1 16.8 12.0 8.7 5.3

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

40 60 80 100 120 140 160 180 200

Temperature / °C

Vis

cosi

ty /

Pa·s

0.10%0.30%0.50%Origin

Fig. 2. Viscosity of asphalt mixing with polyester fibers versustemperatures.

0.E+00

5.E+06

1.E+07

2.E+07

2.E+07

3.E+07

3.E+07

4.E+07

1.00E-01 1.00E+00 1.00E+01 1.00E+02

Angular Frequency / rad/s

G*/

Pa

35

40

45

50

55

60δ

/ Deg

ree

Origin PF-0.1%PF-0.3% PF-0.5%Origin PF-0.1%PF-0.3% PF-0.5%

Fig. 3. Complex modulus and phase angle versus angular frequency fororiginal and polyester fiber modified asphalt.

S. Wu et al. / Construction and Building Materials 22 (2008) 2111–2115 2113

The viscosity tests of asphalt mastic with and withoutpolyester fiber were conducted by a Brookfield viscometer(model DV-II+Pro) according to ASTM D 2170. The vis-cosity test temperatures covered a range of 60–180 �C.

A strain-controlled rheometer (Anton Paar, Austrilia)with parallel plate geometry (8 mm in diameter) was usedto determine the rheological characteristics of asphalt mas-tics. A frequency sweep was applied over the range 0.1–100 rad/s at a fixed temperature of 15 �C. An approximately1.0 g sample was put onto the lower plate. After the samplewas heated to flow, the upper parallel plate was lowered tocontact tightly with the sample and the sample trimmed.The final gap was adjusted to 1.0 mm. All the samples wereheld at the defined temperature for 10 min and then con-ducted the frequency sweep from higher levels to the lowerones. Various viscoelastic parameters, such as, G 0, G00, and dwere collected automatically by the software.

Dynamic modulus test and indirect tension fatigue testin the simple performance test (SPT) of SuperPave wereperformed using the Universal Testing Machine (UTM-25) with an environmental chamber [15]. The dynamicmodulus test was conducted at 15 �C and nine frequencies(0.1 Hz, 0.2 Hz, 0.5 Hz, 1 Hz, 2 Hz, 5 Hz, 10 Hz, 20 Hz,25 Hz). The loading pattern used in the indirect tensile fati-gue test was a haversine load with a loading time of 0.1 sand rest period of 0.4 s. The fatigue test was also conductedat the same temperature at 0.3, 0.4, 0.5 stress levels accord-ing to the splitting strength of asphalt mixtures. All sam-ples were conditioned at the test temperature so that thetest temperature was uniform throughout the mass of thespecimen.

3. Results and discussion

3.1. Viscosity

As shown in Fig. 2, the viscosity of asphalt bindersincreases with increasing polyester fiber contents. Therewas a limited increase in viscosity at a content of 0.1%because the polyester fibers act only as a dispersing mate-rial. If the fiber content was increased, the viscosityincreased by two to three times because the polyester fibersbegan to form a localized network structure. When thefiber content was up to 0.5%, the local networks graduallybegan to interact to initiate a continuous network through-out asphalt, this leads to a 10-fold increase or more in vis-cosity. The network acts as a support structure, reinforcingthe asphalt and resisting deformation. As seen in Fig. 2,these reinforcement effects are more significant at lowertemperatures (60–135 �C) than at higher temperatures(135–180 �C).

3.2. Rheological characteristics of asphalt binder

At any combination of time and temperature within thelinear range, the visco-elastic behavior is usually character-ized by the shear modulus, G*, which is calculated as theratio of the maximum shear stress to the maximum shearstrain. The complex shear modulus (jG*j) and phase angle(d) at 15�C for different concentrations of polyester fibersmixed with asphalt are shown in Fig. 3 For clarity, curvesof complex modulus are plotted as solid line. It can be seenthat the values of complex modulus of asphalt mastic aredecreased with the increase of fiber contents and frequen-cies, but the change of phase angles with or without fibersare limited, which indicates that the addition of polyesterfiber increases the flexibility of asphalt mastic, results in

5000

7000

9000

11000

13000

15000

17000

19000

21000

0.1 0.2 0.5 2 10 20 25Frequency / Hz

Dyn

amic

Mod

ulus

/ M

Pa0

5

10

15

20

25

Phas

e an

gle

/ Deg

ree

E*-Control E*-PF0.3%δ-Control δ-PF0.3%

51

Fig. 5. Dynamic modulus and phase angles of asphalt mixtures with andwithout polyester fiber.

1000.0

1500.0

2000.0

2500.0

3000.0

3500.0

4000.0

4500.0

0.1 10 100Reduced Frequncy / Hz

Fatig

ue p

aram

eter

/ M

Pa

Control

PF0.3%

3000.0

3500.0

4000.0

Fatig

ue P

aram

eter

/ M

Pa

Control PF-0.3%

1

Fig. 6. Fatigue parameters of asphalt mixtures.

2114 S. Wu et al. / Construction and Building Materials 22 (2008) 2111–2115

the improvement of cracking and fatigue resistance forasphalt mixtures.

Fatigue is one of the most severe damage for asphaltpavement. Typically, the initiation and propagation ofcracks are always related to the magnitude of lost energyproduced by outer loading [16]. The loss modulus(jG*jsind) mentioned by the Strategic Highway ResearchProgram (SHRP) is an effective parameter to characterizethe resistance to fatigue cracking of asphalt mixtures.Fig. 4 depicts the loss modulus of various asphalt binders.It also can be found that the loss modulus decreases withthe increase of polyester fiber contents, especially at higherfrequency range (1–100 rad/s). Such results indicate thatthe resistance to fatigue damage of asphalt mixtures isimproved when the fiber modifiers are used. It may be thatthe polyester fiber dispersing in the asphalt can transfer anddisperse the stress caused by loading, which can hold theinitiation of fatigue cracks and their propagations.

3.3. Dynamic properties of asphalt mixture

Fig. 5 shows the changes of dynamic modulus (jE*j) andphase angle (d) at 15 �C for asphalt mixture with fibers andthe control mixture, these cover the frequency range from0.1 Hz to 25 Hz. This illustrates that the dynamic modulusand phase angle decrease with the increase of frequency atthis temperature. The test results indicate that the visco-elastic properties of asphalt mixtures could be changedby the fiber modifiers. Such fiber modifiers can enhancethe viscous property of asphalt mixtures at medium tem-peratures, which result in the reduction of fatigue damagefor asphalt mixtures during their service life. Just like theloss modulus of asphalt mastic, we can define the doubleof dynamic modulus and sine phase angle jE*jsind as thefatigue parameter for asphalt mixture. The fatigue param-eters for control mixture and polyester fiber modifiedasphalt mixture are shown in Fig. 6. The reduction of fati-gue parameters is also observed when polyester fibers areused. The value of E*sind for control mixtures at 10 Hz is3956.8 MPa, and for fiber modified asphalt mixture is3455.7 MPa, which has a decrease about 13%. This reduc-

0.E+ 00

2.E+06

4.E+06

6.E+06

8.E+06

1.E+07

1.E+07

1.E+07

2.E+07

2.E+07

2.E+07

0.1 10 100Angular Frequency / rad/s

Los

s M

odul

ous

/ Pa

Origin

PF-0.1%

PF-0.3%

PF-0.5%

1

Fig. 4. Loss modulus of polyester fiber modified asphalt.

tion of fatigue parameter implies an improvement of resis-tance to fatigue damage for asphalt mixtures.

3.4. Fatigue properties

Fatigue test results of asphalt mixtures at different stresslevels are shown in Fig. 7 and Table 3. Compared with the

1.E+02

1.E+03

1.E+04

1.E+05

0.40 0.60 0.80 1.00 1.20Stress / MPa

Cyc

le n

umbe

rs to

fai

lure

/ T

imes Control PF-0.3%

Fig. 7. Cycle numbers to failure versus stress lever for asphalt mixtures.

Table 3Parameters for fatigue equation of asphalt mixtures

Items Nf Parameters

0.5P 0.4P 0.3P K n

Control 1011 1491 6711 1100.6 3.745PF-0.3% 1911 4271 24141 1171.2 5.044

S. Wu et al. / Construction and Building Materials 22 (2008) 2111–2115 2115

control mixture, the cycle numbers to failure of polyesterfiber modified asphalt mixture are 1.9 times, 2.9 timesand 3.6 times at 0.5 P, 0.4 P and 0.3 P(P is the indirect ten-sion strength of each kind of asphalt mixture), respectively,which reveal that the fatigue property of asphalt mixturescan be improved by polyester fiber modifiers. It is obviousthat the fatigue property of asphalt mixture is enhanceddramatically at lower stress levels, but this improvementis poor at higher stress levels. The fatigue property ofasphalt mixture can be depicted by the fatigue equation,as given in the following equation

N f ¼ KðrÞ�n

where Nf is the cycle numbers to failure; r is the test stress;K, n are constants. After simulating, the constants for as-phalt mixtures with and without fibers can be obtained,which can also be seen in Table 3. It can be observed thatthe values of K and n are all increased when polyester fibersare added, which results in an increase in the cycle numbersto failure for asphalt mixtures, especially at lower stresslevels.

4. Conclusions

1. The viscosity of asphalt mastic increase with the increaseof polyester fiber contents. The contribution effect ofpolyester fiber to increase of viscosity are more signifi-cant at lower temperatures (60–135 �C) than at highertemperatures (135–180 �C).

2. Complex shear modulus of asphalt binder is decreasedwith the increase of fiber contents and frequencies, butthe change of phase angles with or without fibers is lim-ited. The loss modulus of asphalt binder is decreasedwith the increase of polyester fiber content, which indi-cates that the resistance to fatigue crack of asphalt bin-der can be improved.

3. Dynamic modulus and phase angle of asphalt mixturewith 0.3% polyester fiber are all decreased. The fatigueparameter of asphalt mixture with polyester fiber isdecreased about 13% at 10 Hz.

4. Cycle numbers to failure of asphalt mixture modified bypolyester fiber can be increased at all stress levels tested,and this improvement to fatigue resistance is more sig-nificant at lower stress levels.

Acknowledgements

The author is grateful to the Department of Transporta-tion in Hubei Province, China and Headquarters of Han-Yi Expressway in Hubei Province for its financial supportof this work.

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