Construction and Building Materials 63 (2014) 242–248

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Construction and Building Materials

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Physical and rheological properties of epoxidized natural rubbermodified bitumens

http://dx.doi.org/10.1016/j.conbuildmat.2014.04.0260950-0618/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +60 147365425.E-mail address: ramizizzi@gmail.com (R.A. Al-Mansob).

Ramez A. Al-Mansob a,⇑, Amiruddin Ismail a, Aows N. Alduri a, Che Husna Azhari b,Mohamed Rehan Karim c, Nur Izzi Md. Yusoff a

a Sustainable Urban Transport Research Centre (SUTRA), Dept. of Civil & Structural Engineering, Universiti Kebangsaan Malaysia, Malaysiab Dept. of Mechanical and Materials Engineering, Universiti Kebangsaan Malaysia, Malaysiac Dept. of Civil Engineering, Universiti Malaya, Malaysia

h i g h l i g h t s

� Adding ENR to bitumen decreased bitumen temperature susceptibility.� ENR modified bitumen increased in the complex modulus and decreased in the phase angle.� ENR modified bitumen decreased the rutting for the binder and improved the fatigue behavior at low temperatures.

a r t i c l e i n f o

Article history:Received 9 December 2013Received in revised form 10 March 2014Accepted 2 April 2014

Keywords:Polymer-modified bitumenEpoxidized natural rubberRheologyStorage stabilityCompatibilityTemperature susceptibility

a b s t r a c t

The use of polymers as bitumen modifiers in pavements has been growing rapidly in the last decadesbecause of the distresses on asphalt pavements. However, using polymers as a modifier mostly changesthe properties of the asphalt pavement to be more stable and stiffer at high temperatures and more flex-ible at low temperatures. Therefore, this study was conducted to investigate the empirical and rheologicalproperties of bitumen modified with epoxidized natural rubber (ENR). Four percentages (3%, 6%, 9% and12%) of epoxidized natural rubber were used as modifiers. The effects of the modifier on the conventionalproperties, storage stability and rheological properties were investigated. Requirement tests (e.g., pene-tration, softening point, ductility and viscosity) and rheological analysis (e.g., isochronal plot, mastercurves, Black diagram and SHRP parameters) using a dynamic shear rheometer (DSR) were conductedto characterize the ENR-modified bitumen. The fundamental parameters were used to describe signifi-cant benefits of ENR as a modifier. The results indicated that the storage stability of epoxidized naturalrubber modified bitumens (ENRMBs) depended mainly on the ENR content. Based on the results obtainedfrom the DSR test, ENR reduces the temperature susceptibility and facilitated polymeric modificationusing a highly elastic network within the bitumen. This elastic network increases the viscosity, stiffnessand elastic behavior of the ENRMBs. ENR improves rutting resistance and fatigue behavior at hightemperatures and low temperatures respectively. The best results were recorded for polymer-modifiedbitumen containing 6% of ENR.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Over the past decades, research on polymer-modified bitumens(PMBs) and its benefits has dramatically increased and has beenreported in many studies [1–3]. The use of PMBs can improvethe performance of asphalt mixtures and substantially increasethe service life of highway surfaces. However, the addition of poly-mers significantly improves various bitumen properties, such as

elasticity, cohesion, stiffness and adhesion, thus resulting in anoverall performance improvement of asphalt pavements, whichbecome more stable and stiffer at high temperatures and moreflexible at low temperatures. In addition to rutting resistance, agood polymer can supply a degree of flexibility or elasticity to anasphalt mixture, thereby improving the fatigue and thermal crack-ing properties of the asphalt mixture [4–6].

One of the most well-known, widely used category of polymersis thermoplastic elastomers (TE). Thermoplastic elastomers (TE)are polymers with thermoplastic and elastomeric properties [7].TE polymers derive their strength and elasticity from a physical

R.A. Al-Mansob et al. / Construction and Building Materials 63 (2014) 242–248 243

cross-linking of molecules within a three-dimensional network.Styrene–butadiene–styrene (SBS) is the most commonly knownTE that can improve the rheological properties of bitumen [8]. Fur-thermore, SBS increases binder elasticity at high temperatures andimproves flexibility at low temperatures. This improvement led toincrease the resistance to asphalt rutting at high temperatures, anddecrease cracking at low temperatures [9]. Styrene–butadiene–rubber (SBR) is another example of an elastomer polymer and itincreases the ductility of asphalt pavement and consequentlybecomes more flexible and crack-resistant at low temperatures[10]. In addition, other types of additives have also been used asmodifiers, i.e., as TE, such as styrene isoprene styrene (SIS), styreneethylene butadiene styrene (SEBS), ethylene–propylene–diene-ter-polymer (EPDM), isobutene–isoprene-copolymer (IIP), crumb-rub-ber, polybutadiene (PBD), polyisoprene and natural rubber (NR)[11,12]. However, the major types of styrenic block copolymerssuch as SBS, SIS and hydrogenated styrenic block copolymers(HSBC) have the best modifying potential when blended with bitu-men [13–15].

In some cases, the rubbers have been used in a vulcanized state,e.g., crumb-rubber, but are difficult to disperse throughout thebitumen. Ineffective dispersion requires high temperatures andlong mixing times and can yield a heterogeneous binder, withthe rubber acting mainly as flexible filler [11]. However, crumb-rubber is a combination of mainly natural rubber (improves theelasticity of the compound), carbon black and synthetic rubber(both of which materials improve the thermal stability properties).In addition, crumb-rubber has been found to improve rutting resis-tance and to decrease reflective cracking [16]. However, it is foundthat natural rubber shows better reactivity compared to crumb-rubber [17]. The reacted particles become tacky and improve adhe-sion properties. The cohesion between aggregates, thus holding theaggregate together, is one advantage of using natural rubber tomodify bitumen [18]. Natural rubber increases the stiffness ofthe binder, thereby enhancing the latter’s performance at hightemperatures but rendering it brittleness at low temperatures[19]. Natural rubber displays high mechanical strength, outstand-ing resilience and excellent elasticity. However, natural rubber isknown to exhibit poor wet grip properties and poor weather resis-tance [20].

The potential application of epoxidized natural rubber (ENR) asa modifier was realized in the 1980s [20]. ENR is a chemically mod-ified natural rubber created by reacting natural rubber with proxyformic acid, as shown in Fig. 1 [20]. This material has a good prop-erties, offering high strength because of its ability to bear straincrystallization, along with increased glass transition temperature.These properties facilitate increased oil resistance, enhanced adhe-sion properties, damping and reduced gas permeation [21,22]. As amodifier, ENR is able to increase the complex viscosity, the storageand the loss modulus of the blends [23]. However, the details ofENR preparation, are reported in [24–26] and not discussed in thispaper for the sake of brevity.

This study was conducted to present a laboratory evaluation ofbase bitumen and ENR modified bitumen (ENRMB) in terms of con-ventional properties, temperature susceptibility, storage stabilityand rheological properties. The ENRMBs were produced by labora-tory mixing using four polymer contents. The rheological proper-ties (viscoelasticity) of the ENRMBs were determined by means

Fig. 1. Performic epo

of an oscillatory type testing of dynamic mechanical analysis(DMA), generally conducted within the region of linear viscoelasticresponse. DMA allows the viscoelastic nature of bitumen to bedetermined over a wide range of temperatures and loading times(or frequencies) [27–32].

2. Experimental works

2.1. Materials

The bitumen used in this study is 80/100 penetration grade, supplied by thebitumen factory at Port Klang, Malaysia. The ENR was obtained from the MalaysianRubber Board under the trade name of ENR 50, with 53% epoxidization and passedthrough 2.36 mm mesh sieve (before shearing). The physical properties of the bitu-men and the ENR used are shown in Table 1.

2.2. Preparation of binders

Five ENRMBs were produced by mixing ENR-50 (0%, 3%, 6%, 9% and 12% contentby weight of bitumen) with base bitumen. All ENRMBs were prepared using a Silv-erson high shear mixer at 160 �C (±1 �C) under 4000 rpm of speed for one hour. ENRwas added to the bitumen after the temperature stabilized at 160 �C. The differentENRMBs were coded as ENR (polymer), MB (modified bitumen) and (ENR content),e.g., ENRMB3 corresponds to the bitumen modified with 3% epoxidized natural rub-ber content (by weight).

2.3. Conventional binder tests

According to ASTM standards, many conventional tests were conducted on thebase bitumen and the modified bitumen, such as penetration, softening point, duc-tility, viscosity (using the Brookfield Model DV-III) and storage stability test [33–36]. The penetration test is used as a measure of consistency. Higher values of pen-etration indicate softer consistency. The sample is melted and then cooled to25 ± 0.5 �C. The penetration is measured with a penetrometer by means of whicha standard needle is applied to the sample at a temperature of 25 ± 0.5 �C [33]. Inthe softening point test, two horizontal disks of bitumen, cast in shouldered brassrings, are heated at a controlled rate in a liquid bath while each supports a steel ball.The softening point is reported as the mean temperatures at which the two diskssoften sufficiently to allow each ball, enveloped in bitumen, to fall 25 mm [34]. Duc-tility testing provides one measure of tensile properties of bituminous materials. Ina ductility test, bitumen is stretched in a water bath lower than 10 �C and at astretching velocity of 5 cm/min. Ductility length is measured after the sample iscut [36].

2.4. Temperature susceptibility

Changing the temperature susceptibility with ENR was investigated with a cal-culating penetration index (PI) and a penetration viscosity number (PVN). Thehigher the PI and PVN values of bitumen, the lower is its temperature susceptibility.For PI, the temperature susceptibility for the binders is measured by calculating thePI using the penetration at 25 �C and softening point results.

Penetration index is calculated using an equation, shows as follows [11]:

PI ¼ 1952� 500 log Pen� 20S:P50 log Pen� S:P� 120

ð1Þ

where Pen is the penetration test at 25 �C and S.P is the softening point.The penetration viscosity number (PVN) is calculated based on penetration at

25 �C and viscosity at 135 �C, such that

PVN ¼ log L� log Xlog L� log M

� 1:5 ð2Þ

where PVN is the penetration viscosity number, L is the logarithm of viscosity at135 �C for PVN of 0.0, X is the logarithm of viscosity at 135 �C, and M is the logarithmof viscosity at 135 �C for a PVN of �1.5.

The viscosity values of L and M can be determined using the following equa-tions. The equation for the line representing a PVN of 0.0 is:

L ¼ 4:25800 � 0:79670 log Pen ð3Þ

xidation of NR.

Table 1Materials properties for the base bitumen and ENR.

Material Properties Value Specification

Bitumen 80/100 Specific gravity 1.028 ASTM D70Penetration @ 25 �C 82 ASTM D5Softening point (�C) 45.7 ASTM D36Viscosity @ 80 �C (Pa s) 12.6 ASTM D4402Ductility (cm) @ 10 �Cand 5 cm/min

20 ASTM D113

ENR Size (before shearing) 2.36 mm –Specific gravity 0.94 –

244 R.A. Al-Mansob et al. / Construction and Building Materials 63 (2014) 242–248

The equation for the line representing a PVN of �1.5 is:

M ¼ 3:46289� 0:61094 log Pen ð4Þ

where Pen is the penetration at 25 �C.

2.5. Storage stability

In storage stability testing, the binder is poured into an aluminum foil tube,30 cm in height and 3 cm in diameter. The binder is kept vertically in an oven at163 ± 5 �C for 48 h without disturbance. Then, the binder is allowed to cool, afterwhich it is cut horizontally into three pieces. The top and the bottom parts areplaced separately into beakers and tested for their softening point. If the softeningpoint difference is less than 2.5 �C, then the binder can be considered a storage sta-ble blend [37].

2.6. Dynamic viscoelastic properties

The dynamic shear rheometer (DSR) used in this study is the Rheometer HAAKERheoStress 600 from Thermo Electron Corporation and was used to determine thebinder properties. Frequency sweep was applied between 0.1 and 10 Hz. Tempera-ture sweep ranged from 10 to 80 �C. Two plates were used in DSR, namely an 8 mmdiameter spindle with a gap of 2 mm and a 25 mm diameter spindle with a gap of1 mm. The 8 mm spindle was used for temperatures in the range 10–30 �C, and the25 mm spindle was used for temperatures in the range 20–80 �C. The lower platemust be warmed to establish a good bond with the binder, and the gap must be50 lm longer than the targeted gap [29]. The final step after trimming is adjustingthe target gap. The chosen strain should as small as possible to ensure that the mea-surement is in the linear area, and it must be large enough to obtain sufficient tor-que readings at low frequencies and high temperatures and sufficient strain at highfrequencies and low temperatures.

Dynamic viscoelastic properties analysis, using a dynamic (oscillatory) typetest, has been used as a fundamental rheological testing method for bitumen. In thisstudy, sinusoidal shear strains were imposed on binder samples. Binder sampleswere placed between two parallel plates under a dynamic shear rheometer instru-ment and applied various frequencies under several temperatures [28]. Sinusoidallyvarying shear strain is expressed as

cðtÞ ¼ c0 sin xt; ð5Þ

with resulting stress

sðtÞ ¼ s0 sinðxt þ dÞ ð6Þ

where is peak stress, is peak strain, x is radian frequency, t is time, and d is phaseangle.

After the phase change, the sinusoidally varying strain and the stress can beexpressed as complex quantities with the following equation:

p2

: c� ¼ c0eixt ð7Þ

and

s� ¼ s0eiðxtþdÞ ð8Þ

The complex shear modulus is then given by the following equation:

G� ¼ G0 þ iG00 ¼ s�

c�¼ s0

c0

� �eid ¼ s0

c0

� �ðcos dþ i sin dÞ ð9Þ

The final equation can be written as:

jG�j ¼ s0

c0¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiG0q þ G002

qð10Þ

G0 ¼ jG�j cos d ð11Þ

G00 ¼ jG�j sin d ð12Þ

tan d ¼ G0=G00 ð13Þ

where G� is the complex shear modulus, G0 is the storage modulus, and G00 is the lossmodulus. Phase angle is the phase difference between the stress and the strain in theoscillatory test; it is a term to describe a material’s viscoelastic behavior. If d = 90�,then the bitumen behaves as purely viscous, whereas if, then the bitumen behavesas purely elastic. If, then the bitumen will exhibit a combination of viscous and elas-tic behaviors [27].

2.6.1. Isochronal plotIsochronal plot is a curve on a graph representing a system’s behavior at a con-

stant frequency or loading time. In a DSR test, curves of the complex modulus G�

versus temperature at constant frequencies constitute an isochronal plot [27].

2.6.2. Rheological master curveIn a master curve of dynamic mechanical data, the complex shear modulus (G�)

and the phase angle are plotted against frequency for various temperatures on alog–log scale. Master curves are valid for the reference temperature, which is30 �C in this study. Time temperature superposition principle (TTSP) must beapplied to calculate the rheological properties at other temperatures. The tempera-ture dependency of the viscoelastic behavior of bitumen is indicated by means ofshift factors and expressed using the Williams, Landel and Ferry (WLF) equation[38]:

log aðTÞ ¼ C1ðT � TrÞC2 þ ðT � TrÞ

ð14Þ

where a(T) is the shift factor at temperature T, Tr is the reference temperature and C1

and C2 are empirically determined coefficients that were chosen between 7 and 17for C1 and between 46 and 133 for C2, depending on ENR content [38].

2.6.3. Black diagramBlack diagram is a graph of the magnitude of the complex modulus G� versus

the phase angle d obtained from a dynamic test. The frequency and the temperatureare therefore eliminated from the plot, thus allowing all the dynamic data to be pre-sented in one plot without performing TTSP manipulations of raw data. A smoothcurve in a black diagram is a useful indicator of time–temperature equivalency,while a disjointed curve indicates the breakdown of the TTSP and the presence ofa high wax content bitumen, a high asphaltene structured bitumen or a highly poly-mer-modified bitumen [39].

2.6.4. SHRP rutting and fatigue parametersThe specification criterion for rutting is calculated as the complex modulus

divided by the sine of the phase angle, G�/cos, whereas that for fatigue is calculatedas the complex modulus divided by the sine of the phase angle, G�/sin [40].

3. Results and discussion

3.1. Conventional physical properties

The penetrations and softening points for the binders, i.e.,ENRMBs, are shown in Table 2. All ENRMBs exhibit decreased pen-etration with increasing ENR content. The increase in binder hard-ness can be attributed to a hardening effect caused by the additionof ENR to the base bitumens. The softening points, shown inTable 2, increased after polymer modification, thus indicating anidentical increase in hardness or stiffness or in ENRMB stiffness,as observed in the penetrations. Relying on the results from thesetwo conventional tests, the mechanism of modification of ENRMBswould seem to correspond only to a stiffening of the base bitumen.

Table 2 shows that the ductility values decreased for ENRMB3and then increased again, thus indicating that increasing the ENRcontent in turn increased ductility. This increase moreover indi-cates a performance improvement at low temperatures. Conse-quently, the compatibility between ENR and bitumen isreinforced because of increasing ductility at low temperatures.The addition of ENR increased the viscosity of the base bitumen,implying a good rutting resistance as cited in previous studies[11]. Fig. 2 shows that viscosity increased with the addition ofmore ENR, a variation that became more evident at lower temper-atures. The same consistent increasing in viscosity was recordedwith SBS modified bitumen in a previous study [8]. The viscositiesof both the base and the modified bitumen decreased as tempera-tures rose because of the Newtonian properties. The viscosity of

Table 2Conventional properties for the base bitumen and modified binders.

Binder Penetration @ 25 �C(dmm)

Softening point(�C)

Penetration index(PI)

Penetration viscosity number(PVN)

Ductility @ 10 �C(cm)

Temperature difference in isolationtest (�C)

ENRMB0 82 45.70 �1.16 �1.180 19.50 –ENRMB3 73 50.60 �0.11 �0.342 4.00 1.00ENRMB6 64 53.80 0.32 �0.118 9.70 1.25ENRMB9 52 56.80 0.46 0.017 12.50 >2.50ENRMB12 53 57.00 0.55 �0.136 21.50 >2.50

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ENRMB12 decreased more than did that of ENRMB9, likely becauseof the segregation between the base bitumen and the ENR. Viscos-ities and modification indices at 100, 135 and 160 �C for the basebitumen and modified binders are shown in Table 3. However,the modification indices at 100 and 135 �C are approximately sim-ilar for each bitumen group.

3.2. Temperature susceptibility

Penetration index (PI) and penetration viscosity number (PVN)are frequently used to approximate the expected temperature sus-ceptibility for bitumen. The PI value for base penetration bitumensnormally ranges from approximately �3 (for high temperaturesusceptible bitumens) to approximately +7 for highly blown (lowtemperature susceptible) bitumens [11]. In this study, the highestvalues (0.55) were obtained with the addition of ENR, and PVNincreased subsequently. Therefore, ENR decreases the base bitu-men’s temperature susceptibility.

3.3. Storage stability

The isolation test was conducted for the modified bitumen toevaluate its storage stability at high temperatures. Table 2 showsthat the softening point differences between the ENRMB3 andENRMB6 binders are less than 2.5 �C, thus indicating these binderscan remain stable if stored at high temperatures. However, thisequilibrium between bitumen and polymers can be reached with

Fig. 2. Rotational viscosities of ENRMBs.

Table 3Rotational viscosities of ENRMBs.

Binder Viscosity @ 100 �C(mPa s)

Viscosity @ 135 �C(mPa s)

Viscosity @ 160 �C(mPa s)

ENRMB0 2381.5 244.0 52.0ENRMB3 4669.0 469.0 119.0ENRMB6 5619.0 606.5 175.5ENRMB9 7806.5 787.5 237.5ENRMB12 7238.0 694.0 212.5

a medium–high percentage of polymer, around 5–6% by weight,which create a three-dimensional polymer network that com-pletely changes the properties of bitumen; with lower percentage(<3–4%) a continuous bitumen phase with a dispersed polymer isobtained, with higher amount (6–7%) instead is the bitumen thatis dispersed in a continuous polymer phase [41].

3.4. Rheological properties of the ENRMB’s

3.4.1. Isochronal plotIsochronal plots of complex modulus (G�) versus temperature at

0.1 Hz and 1 Hz of frequency are shown in Figs. 3 and 4. The

gENRMB/gBitumen @100 �C

gENRMB/gBitumen @135 �C

gENRMB/gBitumen @160 �C

1.00 1.00 1.001.96 1.92 2.292.36 2.49 3.383.28 3.23 4.573.04 2.84 4.09

Fig. 3. Isochronal plots of complex modulus at 0.1 Hz for the ENRMB binders.

Fig. 4. Isochronal plots of complex modulus at 10 Hz for the ENRMB binders.

246 R.A. Al-Mansob et al. / Construction and Building Materials 63 (2014) 242–248

increase in G� for all binders is similar except ENRMB12 whichshows an extreme increase in G� at high temperature, greater than50 �C, indicating a different compatibility between base bitumenand ENR comparing with the other binders. This behavior was alsorecorded for another elastomer modifier, natural rubber and SBS[8,10]. In addition, ENRMB6 shows a more pronounced increasein G� and an improved temperature susceptibility.

In a previous study, isochronal plots for SBS modified bitumenshow same minor increasing in G� at low frequency, 0.02 Hz, andlow temperatures, less than 20 �C. Another study was done withusing natural rubber latex (NR) and shows same minor increasingin G� for high frequency which indicating improvement in term oftemperature susceptibility [8,42].

Fig. 7. Black diagram for the ENRMB binders.

3.4.2. Rheological master curve

The frequency dependence of G� and d for the ENRMBs wasassessed in Figs. 5 and 6 by producing rheological master curvesat a reference temperature of 30 �C using the time–temperaturesuperposition principle (TTSP) and shift factors determined forthe G� master curves [7]. ENRMB3 and ENRMB6 show similarincreases in the complex modulus with increasing polymer con-tent, whereas ENRMB9 and ENRMB12 behaved differently. How-ever, SBS as modifier showed a similar increase in G� withincreasing modifier content for 3%, 5% and 7% [8]. The effect in

Fig. 5. Complex modulus master curve for unmodified and ENR modified bitumensat a ref. temperature of 30 �C.

Fig. 6. Master curve of phase angle at 30 �C for ENRMB binders.

the latter cases is not as marked as for ENRMB3 and ENRMB6.The differences in rheological behavior can be attributed to vari-ances in compatibility of the different bitumen–polymer blends.This different behavior for ENRMB9 and ENRMB12 was observedin the storage stability and rotational viscosity results. The sameresults have been noted in other research that used a hard bitumenwith a polymer [43].

The phase angle master curves for the ENRMBs showed a reduc-tion in phase angle with modification (Fig. 6); again, the differencebetween the binders occurred at low frequencies, when the natureof the polymer network depends on the properties of the base bitu-men (maltenes composition) and the compatibility of the bitu-men–polymer system. The differences in the plots of phase angleversus frequency can be related to differences in the molecularinteraction (e.g., dispersion, swelling and compatibility) betweenthe base bitumen and ENRs. The rheological properties of theENRMBs are functions of the mutual effect of polymer and bitumenand consequently were influenced by bitumen composition andpolymer nature and content.

3.4.3. Black diagramThe Black diagrams for ENRMBs are shown in Fig. 7. The rela-

tively smooth Black diagram curves shift, with tiny plateau regionsin the modified binders, toward lower phase angles with increasingENR modification. A linear behavior is an indicator of low elasticity.However, the ENRMB12 curve exhibited a different behavior, spe-cifically, a shift toward lower phase angles with increasing polymercontent. In general, the elasticity of the modified binder increased

Fig. 8. Effect of temperature on rutting parameter (G�/sind).

Fig. 9. Effect of temperature on fatigue parameter (G��sind).

R.A. Al-Mansob et al. / Construction and Building Materials 63 (2014) 242–248 247

with the increasing of ENR content. In a previous study, 3% of SBSwas modified bitumen, the effect of polymer modificationappeared insignificant, and the behavior of the modified bindersremained close to that of the base bitumens. After modificationwith a sufficiently high polymer content (P6%), the binders chan-ged fundamentally in their rheological behavior, as indicated by asubstantial decrease in phase angle (a substantial increase in theelastic response) with decreasing complex modulus [44]. These dif-ferences showed that SBS may improve bitumen rheology over awide temperature range, while ENR show its effect mainly with12% of ENR content at high temperature.

3.4.4. SHRP rutting and fatigue parametersThe effect of temperature on the rutting parameter (G�/sind) is

shown in Fig. 8. The rutting parameter G�/sind is defined as thestiffness indicator for evaluating the rutting resistance of bothunmodified and polymer modified bitumen and as the permanentdeformation of each layer of the pavement under repeated loads. Itwas observed that G�/sind increased considerably, except forENRMB12, probably because of the incompatibility between thebitumen and polymer. The same results were recorded in previousstudies, increasing NR latex content as a modifier with bitumenresulted increasing in stiffness where indicating improvement inrutting resistance [42].

Fig. 9 shows the effect of temperature on fatigue parameter(G��sind). Fatigue damage is a complex phenomenon that can occurin asphalt pavements because of repeated bending loads and out-comes in microdamage in pavements. This damage is a competitivepractice between microcracking and healing and results indecreased stiffness, thus declining the applied load capacity andcapability to resist additional damage. The fatigue parameterdecreased with increasing ENR content at less than 30 �C, thusindicating an improvement in fatigue behavior. Between 30 and80 �C, the G��sind parameter increased with increasing ENR con-tent, thus indicating that at high temperatures, ENR negativelyaffects binders in terms of fatigue behavior.

4. Conclusions

This study was conducted to investigate the effect of ENR on theconventional properties, temperature susceptibility, storage stabil-ity and rheological properties of bitumens. Based on penetrationand softening point results, the mechanism of modification ofENRMBs seemed to entail a stiffening of the base bitumen. Theductility decreased initially after adding ENR and then startedincreasing. This increase indicated a performance improvement

at low temperatures. Furthermore, The PI and PVN results indicatethat adding ENR to bitumen decreased bitumen temperature sus-ceptibility. Moreover, Storage stability test results indicated that6% or less of ENR content for binder was stable for use at high tem-peratures. On the other hand, isochronal plots showed thatENRMB6 more pronounce increase in the complex modulus andhence improved temperature susceptibility. Furthermore, ENRMBsincreased in the complex modulus (except 12% of ENR content atlow frequencies) and decreased in the phase angle. However, basedon master curves, ENR provides polymeric modification by meansof a highly elastic network within the bitumen and this elastic net-work increases the viscosity, stiffness and elastic behavior of theENRMBs. In addition, based on the rutting and fatigue parameters,adding ENR decreased the rutting for the binder and improved thefatigue behavior at temperatures lower than 30 �C. As a result, therheological properties indicated that 6% ENR content optimallymodified bitumen.

Acknowledgments

The authors would like to acknowledge the Universiti Kebang-saan Malaysia (UKM) and the Ministry of Science, Technologyand Innovation (MOSTI), Malaysia, for providing research fundingthrough the projects UKM-GUP-2011-037 and 03-01-02-SF0999,respectively, and the Sustainable Urban Transport Research Centre(SUTRA) for providing research facilities. We also thank to theUniversity of Malaya (UM) for providing opportunities for experi-mental collaboration.

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