Construction and Building Materials 25 (2011) 3204–3212

Contents lists available at ScienceDirect

Construction and Building Materials

journal homepage: www.elsevier .com/locate /conbui ldmat

Laboratory comparison of the crumb-rubber and SBS modified bitumenand hot mix asphalt

Baha Vural Kök a,⇑, Hakan Çolak b,1

a Firat University, Engineering Faculty, Civil Engineering Department Elazig, Turkeyb 17, Regional Directorate of Province Bank Trabzon, Turkey

a r t i c l e i n f o a b s t r a c t

Article history:Received 9 November 2010Received in revised form 12 February 2011Accepted 1 March 2011Available online 29 March 2011

Keywords:Crumb rubberSBSModificationRheologyMechanical properties

0950-0618/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2011.03.005

⇑ Corresponding author. Tel.: +90 424 2370000/541fax: +90 424 234 01 14.

E-mail addresses: bvural@firat.edu.tr (B.V. K(H. Çolak).

1 Mobile: +90 533 662 11 23.

The use of crumb rubber (CR) recycled from waste tires using an ambient grinding process was evaluatedat two stages in asphalt formulation. First, bitumen modified with crumb rubber was evaluated by rota-tional viscometery (RV), dynamic shear rheometry (DSR) and conventional binder tests. Hot asphalt mix-tures including crumb-rubber-modified bitumen were then evaluated by determining the permanent andfatigue characteristics and stiffness moduli of control and modified mixtures. The properties of thecrumb-rubber-modified bitumen and asphalt mixtures were compared to different contents of sty-rene–butadiene–styrene (SBS) modified-bitumen and asphalt mixtures. The tests showed that to achievethe same performance, as with SBS-modification, the CR-content must be used at much higher than SBS.8%-CR modification was determined as the most suitable content according to both binder and mixturetests.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

To a large extent, the stiffness and strength of mixtures deter-mines the performance of asphalt pavement. Recently, numerousstudies have evaluated formulation modifications designed toimprove the performance of asphalt mixtures [1–3]. Antistrippingadditives and polymer modifications are two common modifiersused to improve the fundamental properties of asphalt binders,as these properties are closely related to the performance of as-phalt mixtures. The results of these studies have shown that theoverall performance of polymer-modified mixtures is more desir-able than those of unmodified mixtures or of mixtures modifiedwith antistripping additives. Polymers have been found to improverutting performance, adhesion, and cohesion of an asphalt binder[4]. Currently, the most commonly used polymer for bitumen mod-ification is styrene–butadiene–styrene (SBS). SBS block copolymersare classified as elastomers that increase the elasticity of bitumen,and they are probably the most appropriate polymers for bitumenmodification. Prior studies have shown that SBS-modified mixturesexhibited good performance in both asphalt cements and mixtures[5–7]. However, in some cases the modifiers not only decreased theworkability of HMA but also failed to provide a cost-effective

ll rights reserved.

8; mobile: +90 506 718 9410;

ök), hcolak23@hotmail.com

solution. The escalating cost of bitumen and polymer modifiersand the lack of available resources have motivated highway engi-neers to explore alternatives for the construction of new roads.The usage of waste materials has thus become an important issuein this respect. Reclaimed rubber obtained from waste tires hasbeen used for paving as an elastic binder additive. Using this wastematerial may contribute to the solution of a waste disposal-prob-lem and will also facilitate economic sustainability by reducingthe construction cost of roads.

Asphalt-rubber binder results from the chemical reaction of amixture of liquid asphalt binder with 5–22% crumb rubber ob-tained from used tires and added to liquid asphalt. Asphalt-rubberis mixed and applied to roads mainly using either of two tech-niques: the dry and wet processes. In a dry process, crumb rubberis used as a part of the aggregate in the hot mixture to replacesome of the solid fraction [8]. In a wet process, crumb rubber isadded to the asphalt cement mixture.

Navarro et al. investigated the thermorheological behavior ofbitumens modified with 9 wt.% crumb tire rubber at in-serviceand handling temperatures. They concluded that the addition ofground tire rubber to bitumen increased both the linear viscoelas-tic modulus and the viscosity at high in-service temperatures.Additionally, the use of rubber-particle sizes less than 0.35 mmand high shear rates during manufacturing operations was highlyrecommended [9]. It has also been reported that the addition ofrecycled tire rubber to asphalt mixtures using a dry process canimprove the engineering properties of asphalt mixtures, and therubber content has a significant effect on the performance with

Fig. 1. SEM images of crumb rubber obtained by grinding processes.

Table 1Physical properties of the aggregate.

Properties Standard Specificationlimits

Coarse Fine Filler

Abrasion loss (%)(Los Angeles)

ASTM D 131 Max 30 29 – –

Frost action (%)(with Na2SO4)

ASTM C 88 Max10 4.5 – –

Flat and elongatedparticles (%)

ASTM D 4791 Max 10 4

Water absorption (%) ASTM C127 Max 2 1.37Specific gravity (g/cm3) ASTM C127 2.613 – –Specific gravity (g/cm3) ASTM C128 – 2.622 –Specific gravity (g/cm3) ASTM D854 – – 2.711

B.V. Kök, H. Çolak / Construction and Building Materials 25 (2011) 3204–3212 3205

respect to resistance to permanent deformation and cracking [10].Nevertheless, compared to the wet process, the dry process hasbeen a far less popular method for crumb-rubber-modified (CRM)asphalt production. This is due to problems regarding the compat-ibility of mixtures. In a study investigating the effects of differentsizes of crumb-rubber modifier on high-temperature susceptibil-ity, it was concluded that by using 10% CR (by total weight of bitu-men) in the wet processes, the mixtures modified with 0.15 mm CRexhibited the best effect on a dense-graded mixture, whereas mix-tures modified with 0.60-mm CR exhibited the best effect on anopen-graded mixture of porous asphalt [11]. Xiao et al. concludedthat the addition of crumb rubber was helpful in increasing thevoids in mineral aggregate in Superpave mix design and improvingthe rutting resistance of asphalt mixtures regardless of rubber sizeand type [12]. Over the last four decades, several researchers havedemonstrated the improved performance of bituminous mixeswith crumb rubber [13–15]. Increased fatigue life, reduced reflec-tive cracking and low-temperature cracking, and improved tensilestrength were cited as the advantages of crumb-rubber-modifiedmixtures [16].

In this study the performance of crumb-rubber and SBS-modified bitumens and mixtures including these bitumens werecompared in a wet process. In the first stage, bitumens modifiedindividually with SBS and crumb rubber were evaluated by rota-tional viscometery (RV), dynamic-shear rhemotry (DSR) and soft-ening-point tests. Next, SBS and crumb rubber modified hotbituminous mixtures were evaluated by repeated creep test, indi-rect tensile fatigue test, indirect tensile stiffness modulus test. Itwas attempted to determine the appropriate content of crumb rub-ber in hot mixtures, which would exhibit performance equivalentto an SBS modified hot mixtures and so reduce the cost ofpavement.

2. Material and methods

2.1. Material and sample preparation

An asphalt cement, B 160–220, obtained from Turkish Petroleum Refineries wasused as bitumen for mixture preparation. The SBS polymer used was KratonD-1101, supplied by the Shell Chemicals Company. Kraton D-1101 is a linear SBSpolymer in powder form that consists of different combinations polystyrene(31%) and polybutadiene blocks of a very precise molecular weight [17]. Theseblocks are either sequentially polymerized from styrene and butadiene and/or cou-pled to produce mixed coblock polymer chains. Five SBS polymer modified bitumen(PMBs) were produced. The polymer contents ranged from a low polymer modifica-tion of 2% to higher degrees of modification, up to 6%, with a 1% increment. To pro-duce asphalt rubber, the crumb rubber must be cut and scraped into small particlesand reduced to powder size; it is then added into the conventional asphalt. Process-ing scrap tires into crumb rubber can be accomplished through either ambientgrinding or cryogenic grinding technologies. In ambient ground rubber processing,scrap tire rubber is ground or processed at or above ordinary room temperature.Cryogenic processing uses liquid nitrogen to freeze tire chips or rubber particlesprior to size reduction (�120 �C) [18]. The crumb rubber (CR) used in this studywas obtained by an ambient process. Shen et al. indicated that the surface area ofthe ambient CR was twice as large as that of the cryogenic product, leading to amuch higher complex modulus and phase angle of the CRM bitumens [19]. The par-ticles of crumb rubber obtained by grinding processes have an irregular particleshape with high specific surface as can be seen in Fig. 1.

Five crumb rubber modified bitumens were produced. The crumb rubber con-tents ranged from 3% to 15%, with a 3% increment. The modified bitumens wereproduced with a laboratory-scale mixing device with a four-blade impeller (IKA)at a temperature of 180 �C for one hour at a rotation speed of 1000 rpm.

Limestone aggregate was used in the asphalt concrete mixture. The propertiesof the aggregate are given in Table 1. A crushed coarse and fine aggregate, with amaximum size of 19 mm, was selected as the dense-graded asphalt mixture. Thegradation of the aggregate mixtures is given in Table 2. The asphalt mixture was de-signed in accordance with the standard Marshall mix design procedure. The speci-mens were compacted by using 75 blows on each side of cylindrical samples in101.6 mm diameter and 63.5 cm thickness. Mixing and compaction temperatures,which were determined according to viscosity values, were taken into account dur-ing the sample preparation. The mixing and compaction temperatures were deter-mined for mixtures including different bitumens by using the 170 ± 20 and280 ± 30 cP viscosity values, respectively [20] The optimum bitumen content was

found to be 5.0% by weight of aggregate for the unmodified asphalt mixes. This ratiowas chosen for all mixtures so that the amount of bitumen would not confound theanalysis of the test data. For the mechanical tests, SBS-modified mixtures were pre-pared with 2%, 3%, 4%, and 5% SBS modified bitumen and CR modified mixtureswere prepared with the 4%, 6%, 8%, and 10% CR modified bitumen. The physicalproperties of the mixtures such as air voids (Va), voids filled with asphalt (Vfa),voids in mineral aggregates (Vma) Marshall stabilites and flow values are givenin Table 3.

2.2. Dynamic-shear rheometer (DSR) test

At present, the most commonly used methods for the fundamental rheologicalcharacterization of bitumen are dynamic mechanical methods using oscillatory-type testing, generally conducted within the region of linear viscoelastic (LVE) re-sponse. These oscillatory tests are undertaken using dynamic-shear rheometers(DSRs).

Table 2Aggregate gradation.

Sieve size (mm) 19 12.5 9.5 4.75 2.36 1.18 0.6 0.3 0.15 0.075Passing (%) 100 95 88 65 39 24 18 14 9.5 5

3206 B.V. Kök, H. Çolak / Construction and Building Materials 25 (2011) 3204–3212

The DSR test was performed on SBS and crumb rubber modified bitumens usinga Bohlin DSRII rheometer. The test was performed under controlled stress loadingconditions using frequency sweeps between 0.01 and 1 Hz at temperatures be-tween 40 �C and 80 �C. The principal viscoelastic parameters obtained from theDSR are the magnitude of the complex shear modulus (G�) and the phase angle(d). G� is defined as the ratio of maximum (shear) stress to maximum strain andprovides a measure of the total resistance to deformation when the bitumen is sub-jected to shear loading [21]. It consists of elastic and viscous components, which aredesignated as the (shear) storage modulus (G0) and the (shear) loss modulus (G00),respectively. These two components are related to the complex (shear) modulusand to each other through the phase (or loss) angle (d), which is the phase or timelag between the applied shear stresses and shear-strain responses during a test.

2.3. Rotational-viscometer (RV) test

Asphalt binders must remain sufficiently fluid, or workable, at the high temper-atures necessary during the plant mixing, field placement and compaction ofhot-mix asphalt. The bitumen reaches temperatures exceeding 135 �C during theseprocedures. The rotational viscometer measures the rheological properties ofasphalt binders to evaluate their pumpability during delivery and plant operations.

The Brookfield DV-III rotational viscometer was used in this study. The rota-tional viscosity was determined by measuring the torque required to maintain aconstant rotational speed (20 rpm) of a cylindrical spindle while submerged in bitu-men maintained at a constant temperature. Different crumb-rubber and SBS con-tents were tried to obtain acceptable contents with respect to the viscosity,which must be lower than 3000 cP at 135 �C. The rotational viscometer was alsoused to establish equiviscous temperature ranges for selecting HMA mixing andcompaction temperatures. Hence, this study can lead to further studies on theuse of crumb-rubber- and SBS-modified asphalt in hot-mix asphalt.

2.4. Conventional tests

The base, SBS and crumb-rubber-modified bitumens were subjected to penetra-tion and softening-point tests. Three replicates of each bitumen containing differentpolymer contents were prepared for bitumen testing. The temperature susceptibil-ity of the modified bitumen samples was calculated in terms of its penetration in-dex (PI) using the results obtained from the penetration and softening-point tests.Temperature susceptibility is defined as the change in the consistency parameter asa function of temperature. A classical approach related to penetration index (PI) isprovided in the Shell Bitumen Handbook [22], as given by the following equation:

PI ¼ 1952� 500 � logðPen25Þ � 20 � SP50 � logðPen25Þ � SP� 120

ð1Þ

where Pen25 is the penetration at 25 �C and SP is the softening-point temperature ofthe bitumen.

2.5. Dynamic-creep test

The dynamic creep test was developed to estimate the rutting potential of as-phalt mixes. The dynamic creep test is thought to be one of the best methods forassessing the permanent deformation potential of asphalt mixtures. This test wasdeveloped by Monismith et al. [23] in 1970 and is based on the concept of anaxial-compression test. NCHRP reported that, among the five laboratory tests inves-

Table 3Physical properties of the mixtures.

Mixturetype

Mixingtemp.(�C)

Compactiontemp. (�C)

Va(%)

Vma(%)

Vf(%)

Marshallstability(kN)

Flow(mm)

Base 145 130 4.07 14.90 72.64 17.1 2.894%CR 168 154 4.17 15.33 72.76 17.3 2.736%CR 178 165 4.16 15.30 72.78 18.4 2.688%CR 191 178 4.14 15.27 72.85 19.0 2.7110%CR 202 189 4.12 15.23 72.94 19.2 2.642%SBS 165 151 4.08 15.00 72.75 17.6 2.923%SBS 174 162 4.09 15.01 72.74 18.2 2.684%SBS 185 173 4.13 15.06 72.52 19.3 2.875%SBS 191 179 4.10 15.05 72.72 21.3 3.04

tigated, the dynamic-creep test gave a very good correlation with measured rutdepth and had a high capability for estimating the rutting potential of asphalt layers[24].

The point or cycle number at which pure plastic shear deformation occurs isreferred to as the ‘‘flow number’’. The flow number has also been recommendedas a rutting indicator for asphalt mixtures [25,26]. Flow number is based on theinitiation of tertiary flow or the minimum point of the strain rate curve.

In the repeated-creep test, the accumulated axial strain, resilient axial strain,peak vertical stress, resilient modulus and creep stiffness are calculated by the fol-lowing equations [27]:

ec ¼ ðL3n � L1Þ=G ð2Þ

er ¼ ðL2n � L3nÞ=ðG� ðL3n � L1ÞÞ ð3Þ

Er ¼ r=er ð4Þ

Ec ¼ r=ec ð5Þ

where ec is the accumulated axial strain (le), er is the resilient axial strain (le), r isthe peak vertical stress (kPa), L3n is the final displacement level of the transducer forpulse ‘n’ just prior to the application of the stress for pulse ‘n + 1’ (mm), L1 is the ini-tial zero-reference displacement of the transducers (mm), G is the initial specimenlength (mm), L2n is the maximum displacement of the transducers with stress ap-plied for pulse ‘n’, Er is the resilient modulus (MPa), and Ec is the creep stiffness ormodulus (MPa).

The dynamic creep test is usually conducted at 40 �C, with a 1-h loading timeand 0.1 MPa applied stress. However, it has been reported that a stress level of100 kPa is not suitable for investigating the permanent deformation potential of as-phalt mixtures [28]. Therefore, to more clearly evaluate the behavior of the mixturein this test, a loading level of 500 kPa and a temperature of 50 �C were chosen. Thespecimen strain during the cycling load was measured along the same axis as theapplied stress using two linear variable displacement transducers (LVDTs). Asquare-pulse wave was applied, with a 500-ms pulse width and a 500-ms rest per-iod. Prior to testing, the specimens were put into the chamber for 6 h to allow for auniform temperature distribution. The dynamic creep test conditions consisted of600 s of preload (10 kPa) and 4200 load cycles.

2.6. Indirect tensile–fatigue test

Fatigue is considered to be one of the most significant distress modes in pave-ments associated with repeated traffic loads [29]. In this study, a constant-stressindirect tensile–fatigue test was conducted by applying cyclic constant loads of270 kPa with a 0.1-s loading followed by a 1.4-s rest period. The test was carriedout at 25 �C. The universal testing machine (UTM) was used for this purpose. Themachine has a servohydraulic test system. The loading frame was housed in anenvironmental chamber to control temperature during the test. The desired load le-vel, load rate and load duration were controlled by a computer. The deformation ofthe specimen was monitored through linear variable-differential transducers(LVDTs). The LVDTs were clamped vertically onto the diametrical side of the spec-imen. A repeated dynamic compressive load was applied to specimens across thevertical cross-section along the depth of the specimen using two loading strips12.5 mm in width. The resulting total deformation parallel to the applied forcewas measured.

2.7. Indirect tensile–stiffness–modulus test

The stiffness modulus of asphalt mixtures measured in indirect-tensile mode isthe most popular form of stress–strain measurement used to evaluate their elasticproperties and is considered a very important performance characteristic of a pave-ment formulation. The indirect tensile stiffness modulus (ITSM) test defined by BSDD 213 is a nondestructive test. The ITSM Sm in MPa is defined as:

Sm ¼ FðRþ 0:27Þ=LH ð6Þ

where F is the peak value of the applied vertical load (repeated load) (N), H is themean amplitude of the horizontal deformation obtained from five applications ofthe load pulse (mm), L is the mean thickness of the test specimen (mm), and R isthe Poisson ratio (here assumed to be 0.35). The test was performed with controlleddeformation, again using the universal testing machine (UTM). The magnitude of theapplied force was adjusted by the system during the first five conditioning pulsessuch that the specified target-peak transient diametral deformation was achieved.A value of 6 lm was chosen so as to ensure that the signal amplitudes obtained fromthe transducers were sufficient to yield consistent and accurate results. During test-ing, the rise time, which is the time taken for the applied load to increase from zeroto a maximum value, was set at 124 ms. The load-pulse application was set to 3.0 s.The test was performed at 25 �C.

05

10152025303540455055

0.001 0.01 0.1 1 10Frequency (Hz)

Mod

ific

atio

n in

dice

s 3%CRM

6%CRM

9%CRM

12%CRM

15%CRM

Fig. 4. Modification indices of CR modified bitumens.

0

5

10

15

20

25

30

0.001 0.01 0.1 1 10Frequency (Hz)

Mod

ific

atio

n in

dice

s 2%SBS

3%SBS

4%SBS

5%SBS

6%SBS

Fig. 5. Modification indices of SBS modified bitumens.

y = 2251.6e0.1713x

R2 = 0.9963

25000

30000

35000

s (P

a)

SBS@0.1Hz SBS@1Hz

CRM@0.1Hz CRM@1Hz

B.V. Kök, H. Çolak / Construction and Building Materials 25 (2011) 3204–3212 3207

3. Results and discussion

3.1. Dynamic-shear-rheometer test results

The frequency dependence of the complex modulus (G�) of theSBS and CR modified bitumens, obtained by producing rheologicalmaster curves at a reference temperature of 50 �C, are given inFigs. 2 and 3, respectively.

The complex modulus master curves showed similar behavioraltrends between the SBS and CRM bitumens. The complex moduli ofboth bitumens increased with increasing frequency. The modifica-tion indices of the modified bitumens, which were obtained bydividing the complex modulus of the modified bitumen by thatof the base bitumen, both at 50 �C, are given in Figs. 4 and 5. Thesignificance of frequency on the complex modulus is seen in thesefigures. The complex modulus of the 5% SBS modified bitumen was15 times and 6 times higher than that of the base bitumen at0.01 Hz and 1 Hz respectively. The corresponding values were 24times and 6 times for the 12% CRM bitumen. The modulusimprovement effects of different additive contents of grew closerat higher frequency. While the complex modulus increased withincreasing frequency, the rate of this increase leveled off for bothtypes of modified bitumens. The decrease in modification indiceswas more pronounced at higher additive contents. The results indi-cate that there were significant differences in the modificationindices above 4% SBS content and 9% CR content. These valuescan be considered as the thresholds for transition to superiorperformance.

The variations in complex moduli with additive contents at thefrequencies of 0.1 and 1 Hz are given in Fig. 6. The values were ta-ken from the master curves. The complex moduli increased withincreasing additive content. The effect of additive content on com-plex modulus was exponential for both SBS modified and crumbrubber modified bitumens. This indicates that much higher values

0.1

1

10

100

1000

10000

100000

0.0001 0.001 0.01 0.1 1 10

Frequency (Hz)

Com

plex

mod

ulus

(Pa

)

0%CRM

3%CRM

6%CRM

9%CRM

12%CRM

15%CRM

Fig. 2. Rheological master curves of the CR modified bitumens.

0.1

1

10

100

1000

10000

100000

0.0001 0.001 0.01 0.1 1 10Frequency (Hz)

Com

plex

mod

ulus

(Pa

)

0%SBS

2%SBS

3%SBS

4%SBS

5%SBS

6%SBS

Fig. 3. Rheological master curves of the SBS modified bitumens.

y = 1342.2x1.5727

R2 = 0.9988

y = 133.34x2.1277

R2 = 0.9957

y = 253.27e0.2383x

R2 = 0.9963

0

5000

10000

15000

20000

0 2 4 6 8 10 12 14 16

Additive content (%)

Com

plex

mod

ulu

Fig. 6. The variations in complex moduli with additive contents.

of crumb rubber are more effective than lover values. The slopes ofthe curves decreased with decreasing frequency. The decreasingslope of the curves with decreasing frequency showed similar ratiofor the SBS and CRM bitumens. However, to satisfy the same per-formance with SBS modified bitumens, the CRM bitumens mustbe used at much higher content than SBS, particularly at lowfrequency. Fig. 7 was plotted by determining the crumb rubbercontents that provide the same complex modulus as the corre-sponding SBS contents at 0.1 and 1 Hz. The good curve fits, withhigh correlations, ensure the determination of crumb rubber con-tents equivalent to given SBS contents regarding the same complexmoduli. The same G� values as the 3% SBS modified bitumen wereobtained by 6.9% and 7.1% crumb rubber modified bitumens at0.1 Hz and 1 Hz, respectively. As for the 5% SBS modified bitumen,the same performance was obtained by 11.8% and 11.9% crumbrubber modified bitumens at 0.1 Hz and 1 Hz, respectively. As thecurves were similar at 0.1 Hz and 1 Hz, this indicates that the re-sponse of SBS and CRM bitumens to frequency changes was similarwith respect to the complex modulus, as described above.

y = 0.0185x2 + 0.0821x + 1.5053

R2 = 0.9995

y = 0.0112x2 + 0.2278x + 0.8645

R2 = 0.9979

0

1

2

3

4

5

6

7

0 2 4 6 8 10 12 14 16Crumb rubber content (%)

SBS

cont

ent (

%)

0.1 Hz

1 Hz

Fig. 7. CR contents versus SBS contents providing the same complex modulus.

0100020003000400050006000700080009000

10000

0 2 4 6 8 10 12 14 16Additive content (%)

Vis

cosi

ty (

cP)

SBS@135 °C SBS@165 °C

CRM@135°C CRM@165°C

Fig. 9. The variations in the viscosities of bitumens.

3208 B.V. Kök, H. Çolak / Construction and Building Materials 25 (2011) 3204–3212

The variations in phase angles versus the complex moduli of themodified bitumens are given in Fig. 8. Each curve was plotted withfive points. Each point on the curves represents a different additivecontent. The phase angles decreased with the increase of additivecontent. While the variations in complex moduli were similar atlow and high frequencies, the variation between the complex mod-uli and phase angles of the SBS and CR modified bitumens at 1 Hzwere only similar, despite the fact that a significant difference wasobserved at 0.1 Hz. The CRM bitumens gave lower phase anglesthan SBS modified bitumens at the same complex modulus atlow frequency. The phase angles of the SBS and CR modifiedbitumens were 69.8� and 63.5�, respectively, at a 3.0 kPa complexmodulus at 0.1 Hz. The reduced phase angle at low frequency,where typical vehicle loads are encountered, more effectively indi-cates an elastic response. A distinction was observed between thephase angle increases of the CR and SBS modified bitumens, partic-ularly at higher complex moduli where the additive contents werehigh. Having a lower phase angle than the SBS modified bitumen,the CRM bitumen exhibited more flexible behavior at high additivecontents at low frequency, which is potentially beneficial in resist-ing deformation.

3.2. Rotational viscometer test results

The variations in the viscosities of bitumens at 135 �C and165 �C are given in Fig. 9. The results show an exponential increasein the viscosities with modification. The viscosities give a clearindication of the stiffening effects of crumb rubber modificationat high additive contents. In particular, crumb rubber modificationinduced a significant increase in viscosity above 10%. It was deter-mined that SBS and CRM bitumens reached a viscosity of 3000 cPat 6.7% and 10.5% additive, respectively. Additive contents abovethese values may not be suitable for good workability and

100

1000

10000

100000

50 55 60 65 70 75 80 85

Phase angle (Degree)

Com

plex

mdu

lus

(Pa)

SBS@0.1Hz

CRM@0.1Hz

SBS@1Hz

CRM@1Hz

Fig. 8. Variation in phase angles of modified bitumens.

pumpability according to Superpave specifications. When compar-ing the complex moduli of the 6.7% SBS modified bitumen and the10.5% CR modified bitumen at 1 Hz, it was observed that, althoughthese bitumens had same viscosity, the complex modulus of theCRM bitumen was half that of the SBS modified bitumen. This indi-cates that, at contents above 10%, CR modification is a physical (fil-ler type) modification rather than a polymeric modification. It wasconsidered that insoluble crumb rubber particles remained above10%. The observed increase in viscosity without expected increasein complex modulus can be explained due to these undissolvedparticles.

3.3. Conventional bitumen test results

The effects of SBS and CR modification on the properties of thebase bitumen can be seen in Table 4 as a decrease in penetrationvalues and an increase in softening points with increasing additivecontents. This trend was more pronounced for the SBS modifiedbitumen. The modification reduced the temperature susceptibility(as determined by the penetration index; PI) of the bitumen. LowerPI values indicate higher temperature susceptibility. Asphalt mix-tures containing bitumen with higher PI are more resistant tolow temperature cracking and permanent deformation [30]. Asseen in Fig. 10, both SBS and CR modified bitumens exhibited lesstemperature susceptibility with increasing additive content. How-ever, the slopes of the SBS curves were higher than the CRM curves,especially at higher additive content, indicating more temperaturesusceptibility compared to the CRM bitumen. It was determinedthat 12% CR modification provides the same temperature suscepti-bility as the 4% SBS modification.

At the end of the bitumen test a general assessment was made,as summarized in Table 5. This table shows the crumb rubber con-tent required to exhibit the same properties as the various SBS con-tents according to the panel of bitumen tests. It was observed thateach test presents very different equivalent crumb rubber con-tents. The CR contents required to match the performance of theSBS modifications according to the penetration index are muchhigher than the values obtained by the other tests. Among the vis-cosity, values a slight decrease in CR content was seen at 165 �C,which indicates an increased resistance to high temperature. Thephase angles did not change in accordance with the increase ofSBS content between 0.1 Hz and 1 Hz, whereas the CR require-ments for the equivalent complex moduli remained approximatelyconstant at low and high frequencies for the different SBS contents.A low CR demand for equivalent phase angles at low frequencyindicates an increased elastic response. It was also determined thatthe required crumb rubber content obtained for the softening pointand penetration tests were closer to the values obtained for thecomplex modulus at high frequency than the other rheologicalvalues.

Table 4Physical properties of the bitumens.

Pure SBS content (%) CR content (%)

2 3 4 5 6 3 6 9 12 15

Penetration 190 128 97 82 64 53 116 100 81 62 53S.point (�C) 41.5 47.8 53.8 58.2 62.7 68.6 46.4 52.0 56.3 62.2 67.9PI 0.365 0.922 1.576 2.047 2.219 2.755 0.130 1.222 1.585 2.032 2.637

y = 1.6737Ln(x) - 0.2513

R2 = 0.9995

y = 1.465Ln(x) - 1.4909

R2 = 0.9808

0

0.5

1

1.5

2

2.5

3

0 2 4 6 8 10 12 14 16

Additive content (%)

Pene

trat

ion

inde

x

SBS

CRM

Fig. 10. Penetration index of the bitumens.

00.5

11.5

22.5

33.5

44.5

5

0 1000 2000 3000 4000 5000

Pulse count

Acc

umul

ated

str

ain

(%) Base

4%CR

6%CR8%CR

10%CR

Fig. 11. Accumulated strain versus pulse counts for CR modified mixtures.

00.5

11.5

22.5

33.5

44.5

5

0 1000 2000 3000 4000 5000

Pulse count

Acc

umul

ated

str

ain

(%) Base

2%SBS

3%SBS4%SBS5%SBS

Fig. 12. Accumulated strain versus pulse counts for SBS modified mixtures.

B.V. Kök, H. Çolak / Construction and Building Materials 25 (2011) 3204–3212 3209

3.4. Dynamic creep test results

Figs. 11 and 12 display the results of comparisons between theaccumulated strain of the control (base) and modified mixtures at50 �C and a 500 kPa stress level. The accumulated strain versuspulse counts of the repeated creep test results were obtained fromthree specimens each of the control and modified mixtures. How-ever, the figures represent the average specimen strain pulse countvalues. The pulse count was selected as 4200. It was observed thatall specimens showed a minimum slope lover than a 4200 pulsecount, which means that the mixtures passed the tertiary stage.It is seen from these figures that the accumulated strain of the mix-tures decreased with increasing additive content, indicating aresistance to permanent deformation and an increased service life.The figures show that the 2%, 3%, 4%, and 5% SBS modified mixturesexhibited similar performance compared to 4%, 6%, 8%, and 10% CRmodified mixtures, respectively. The accumulated strain values ofthe mixtures at 4200 pulse count are given in Table 6. Here, a muchgreater affinity was observed between the 3% SBS and 6% CR mod-ified mixtures according to the accumulated strain values.

Another comparison was made with the flow numbers (FN) ofthe mixtures, taken at the minimum points of the strain ratecurves; see Fig. 13. The increase in flow number was more pro-nounced with SBS modified mixtures than in CR modified mix-tures. Because the FN is recommended as a rutting indicator, it isapparent that SBS modified mixtures can better resist to high loadswithout any deterioration than CR modified mixtures at the sameadditive content. However, CR modified mixtures can satisfy thesame performance criteria as SBS modified mixtures when the CRcontents are higher than the SBS contents. As shown in Fig. 13,

Table 5Required crumb rubber content to satisfy the same performance with SBS modification.

SBS content (%) Crumb rubber content (%)

PI Viscosity @ 135 �C Viscosity @ 165 �C G� @ 0.1 Hz

2 5.2 3.2 3.2 4.23 8.1 5.2 4.7 6.94 12.1 6.7 6.5 9.35 14.7 8.0 7.6 11.86 18.1 9.4 9.0 13.4

the required CR content increases with the increase of SBS content.At unfavorable temperatures such as 50 �C, the same performanceas a 5% SBS modified mixture was obtained by a 10% CR modifiedmixture; i.e., two times the SBS content. These mixtures gave2.5-fold higher flow numbers than the control mixture. Althoughthe CR demand was twice that of the SBS, when the cost of SBSand CR are taken into account it is obvious that using CR in themixtures provides a cost reduction.

3.5. Indirect tensile fatigue test results

Based on the results of the fatigue test, the number of load rep-etitions to failure was determined for the base, SBS modified andCR modified mixtures. Figs. 14 and 15 show examples of the

G� @ 1 Hz d @ 0.1 Hz d @ 1 Hz Softening point Penetration Mean

3.5 2.6 3.0 3.7 2.0 3.407.1 4.7 6.5 7.3 6.2 6.309.5 7.3 8.9 9.9 8.7 8.66

11.9 9.0 10.9 12.8 11.9 10.9413.4 10.9 13.8 15.4 12.5 13.16

Table 6Accumulated strain values of mixtures.

Base SBS content (%) CR content (%)

2 3 4 5 4 6 8 10

Acc. strain (%) 4.409 2.737 1.751 1.560 1.461 2.173 1.721 1.660 1.252

0

500

1000

1500

2000

2500

3000

3500

0 2 4 6 8 10 12

Additve content (%)

Flow

num

ber

SBS

CR

Fig. 13. Flow number-additive content relation.

0

0.5

1

1.5

2

2.5

3

3.5

4

0 500 1000 1500 2000 2500

Load cycle

Acc

umul

ated

def

orm

atio

n (m

m)

Base

4% CR

8% CR

6% CR

10% CR

Fig. 14. Accumulated deformation versus load repetitions for CR modifiedmixtures.

0

0.5

1

1.5

2

2.5

3

3.5

4

0 500 1000 1500 2000 2500 3000

Load cycle

Acc

umul

ated

def

orm

atio

n (m

m)

Base

2% SBS 4% SBS3% SBS

5% SBS

Fig. 15. Accumulated deformation versus load repetitions for SBS modifiedmixtures.

0

500

1000

1500

2000

2500

3000

Base 2%SBS 4%CR 3%SBS 6

M

Loa

d cy

cle

Fig. 16. Load cycle numb

3210 B.V. Kök, H. Çolak / Construction and Building Materials 25 (2011) 3204–3212

changes in accumulated deformation versus load repetitions. It isreadily observed that, at first, the accumulated deformationsincreased rapidly as the air voids were compressed by the load rep-etition; later, the accumulated deformations of the mixture in-creased linearly. Because the test was performed in stresscontrolled mode, the deformations finally increased rapidly afterinitial cracking occurred. The crack growth of SBS and CR modifiedmixtures exhibited the same trends, except for the 10% CR modi-fied mixture. It can be concluded that, among the modified mix-tures, the 10% CR mixture has low resistance to crack growth.Again, the undissolved crumb rubber particles in the bitumencould have led to this type of deterioration. Conversely, the 6%CR and 8% CR mixtures showed similar deformation characteristicsuntil initial cracking. However, it was observed that the crackgrowth of the 8% CR mixture was lower than that of the 6% CR mix-ture, which indicates an elastic behavior. Therefore, it can be as-sumed that the effective crumb rubber content is 8% by weightof bitumen according to the fatigue test. The load cycle numbersof the mixtures to fracture induction are given in Fig. 16. The val-ues ranged from low to high; thus, the relative performances of themixtures can be compared along the horizontal axis of the graph.The presented data are the mean values of three specimens. It isseen from the figure that the 4% CR modification gave better resultsthan the 2% SBS modification, the 6% CR modification gave betterresults than the 2% and 3% SBS modification, and the 8% and 10%CR modifications gave better results than the 2%, 3%, or 4% SBSmodifications. Any of the CR modified mixtures can reach the per-formance of a 5% SBS modified mixture. The load cycle number ofthe mixture modified with 8% CR, which was previously deter-mined to be the effective CR content, was five times greater thanthat of the base mixture.

3.6. Indirect tensile stiffness modulus test results

The test results for stiffness modulus are shown in Fig. 17. Threespecimens each of the control and natural asphalt mixtures weretested. Each specimen was tested at three different positions.Therefore, to obtain the stiffness modulus value of any type ofmixture, the mean of nine values was used. The force, providinga 6-lm deformation, was applied five times. When the moduliranged from low to high, it was seen that the sequencing did notgreatly differ from that of the fatigue tests. The 8% CR modified

%CR 4%SBS 8%CR 10%CR 5%SBS

ixtures

ers of the mixtures.

800

900

1000

1100

1200

1300

1400

1500

1600

1700

1800

Base 2%SBS 3%SBS 4%CR 6%CR 4%SBS 8%CR 10%CR 5%SBS

Mixtures

Sm (

MPa

)

Fig. 17. Stiffness modulus values of the mixtures.

B.V. Kök, H. Çolak / Construction and Building Materials 25 (2011) 3204–3212 3211

mixture exhibited performance similar to the 4% SBS modifiedmixture and had a 50% higher stiffness modulus value than thebase mixture. It was also determined that there was little variationin the stiffness modulus with the alteration of additive content inthis test method. While the 8% CR mixture had a value only 1.5times higher than the base mixture in the stiffness modulus test,the values rose to 2.3 times and 5 times higher for the dynamiccreep and fatigue tests, respectively. The most effective responseto modification was obtained for the fatigue test.

4. Conclusions

The experimental results of this study show the effect of the useof crumb rubber for the modification of bitumen and hot mix as-phalt mixtures, including conventional and rheological bitumentests and hot mixture performance tests such as the dynamiccreep, fatigue and stiffness modulus tests. The performance ofcrumb rubber modified bitumen and asphalt mixtures was com-pared to SBS modified bitumen and asphalt mixtures to determineif the use of crumb rubber can replace SBS as an additive withrespect to the tested properties of the bitumen and mixtures.

Based on the laboratory test results, the following conclusionswere drawn:

According to conventional bitumen tests, it was determinedthat both SBS and CR modified bitumens exhibited reduced tem-perature susceptibility with increasing additive contents. However,this trend was more pronounced for the SBS modified bitumen.

In the dynamic shear test, it was determined that the improve-ment effects of the contents of the different additives (SBS and CR)were closer at high frequency. The decrease in modification indiceswas more pronounced at higher additive contents. Significant dif-ferences were observed in the modification indices above 4% SBScontent and 9% CR content. These values can be considered to bethresholds for the transition to superior performance properties.The effects of additive content on complex modulus were deter-mined to be exponential for both the SBS modified and crumb rub-ber modified bitumens, indicating that much higher crumb rubbercontents were more effective than lower contents.

The crumb rubber modified bitumen displayed lower phase an-gle than the SBS modified bitumen at the same complex modulusat low frequency, indicating an elastic response.

The viscosities gave a clear indication of the stiffening effects ofcrumb rubber modification at high additive content. It was deter-mined that SBS and CRM bitumens each reached a viscosity of3000 cP at 6.7% and 10.5% addition, respectively.

According to a series of bitumen tests, very different percent-ages of crumb rubber addition were required to exhibit the sameproperties as the corresponding SBS modifications. Depending on

the type of bitumen test, a 6.3–12.1% crumb rubber additionwas required to achieve the same performance as a 4% SBSmodification.

The results of the dynamic creep test demonstrated that acrumb rubber content twice that of the SBS content in the hot mix-tures provided the same performance, and a high affinity between3% SBS and 6% CR modified mixtures was observed with respect tothe accumulated strain values.

The fatigue test results showed that the crack growth of the SBSand CR modified mixtures exhibited the same trends. As to 10% CRmodified mixtures showed a high deformation rate after initialcracking, the 8% crumb rubber modification was determined to bethe maximum useful crumb rubber content. An 8% CR modificationgave better results than the 2%, 3%, or 4% SBS modifications andyielded a load cycle number five times that of the base mixture.

The stiffness modulus test showed that the 8% CR modified mix-ture exhibited performance similar to the 4% SBS modified mixtureand had a 50% higher stiffness modulus value than the base mix-ture. It was also determined that the stiffness modulus test resultswere less severely affected by the additive increases than those ofthe other mixture tests.

Overall, the rheological and mechanical test results made itapparent that crumb rubber modification exhibits superior perfor-mance with respect to bitumen and mixture properties. In addi-tion, an 8% crumb rubber content was determined to be the mostsuitable content, yielding much better test results than unmodifiedbitumen and the other mixtures. The use of crumb rubber is pre-ferred over SBS modification because it can provide a significantcost savings due to the high price of SBS and will also preventthe accumulation of this waste material in the environment.

References

[1] Isacsson U, Lu X. Testing and appraisal of polymer modified road bitumens:state of the art. Mater Struct 1995;28:139–59.

[2] Mahabir P, Mazumdar M. Engineering properties of EVA modified bitumenbinder for paving mixes. J Mater Civil Eng 1999;11:131–5.

[3] Ho S, Zanzotto L. The low temperature properties of conventional and modifiedasphalt binders evaluated by the failure energy and secant modulus fromdirect tension tests. Mater Struct 2005;275:137–43.

[4] Kanitpong K, Bahia H. Relating adhesion and cohesion of asphalts to the effectof moisture on laboratory performance of asphalt mixtures. Transp Res Rec2005;1901:33–43.

[5] Sengoz B, Isikyakar G. Evaluation of the properties and microstructure of SBSand EVA polymer modified bitumen. Construct Build Mater 2008;22:1897–905.

[6] Chen JS, Liao MC, Shiah MS. Asphalt modified by styrene–butadiene–styrenetriblock copolymer: morphology and model. J Mater Civil Eng 2002;14(3):224–9.

[7] Valkering CP, Vonk WC, Whiteoak CD. Improved asphalt properties using SBSmodified bitumens. Shell Bitumen Rev 1992;66:9–11.

[8] Adhikari B, De D, Maiti S. Reclamation and recycling of waste rubber. ProgPolym Sci 2000;25(7):909–48.

3212 B.V. Kök, H. Çolak / Construction and Building Materials 25 (2011) 3204–3212

[9] Navarro FJ, Partal P, Martínez-Boza F, Gallegos C. Thermo-rheologicalbehaviour and storage stability of ground tire rubber-modified bitumens.Fuel 2004;83:2041–9.

[10] Cao W. Study on properties of recycled tire rubber modified asphalt mixturesusing dry process. Construct Build Mater 2007;21:1011–5.

[11] Ching WC, Wing-gun W. Effect of crumb rubber modifiers on high temperaturesusceptibility of wearing course mixtures. Construct Build Mater 2007;21:1741–5.

[12] Xiao F, Amirkhanian SN, Shen J, Putman B. Influences of crumb rubber size andtype on reclaimed asphalt pavement (RAP) mixtures. Construct Build Mater2009;23:1028–34.

[13] Lalwani S, Abushihada A, Halsa A. Reclaimed rubber asphalt blendsmeasurement of rheological properties to assess toughness, resiliency,consistency and temperature sensitivity. Proc J Assoc Asphalt Paving TechnolKansas City Kan 1982;51:562–79.

[14] McGennis RB. Evaluation of physical properties of fine crumb rubber-modifiedasphalt binders. J Transport Res Rec 1488. Washington (DC): TransportationResearch Board; 1995. p. 62-71.

[15] Bahia HU, Davis R. Effect of Crumb Rubber Modifiers (CRM) on PerformanceRelated Properties of Asphalt Binders, J. Assoc Asphalt Paving Technol1994;63:414–49.

[16] Oliver JWH. Rutting and fatigue properties of crumb rubber hot mix asphalts.In: Proceedings of the asphalt rubber 2000 conference, Vilamoura, Portugal;2000. p. 221–40.

[17] Preparing blends of Kraton D polymers and bitumen. Shell Technical Bulletin1995.

[18] Fontes Liseane PTL, Trichês G, Pais JC, Pereira Paulo AA. Evaluating permanentdeformation in asphalt rubber mixtures. Construct Build Mater 2010;24:1193–200.

[19] Shen J, Amirkhanian SN, Xiao F, Tang B. Influence of surface area and size ofcrumb rubber on high temperature properties of crumb rubber modifiedbinders. Construct Build Mater 2009;23:304–10.

[20] Zaniewski JP, Pumphrey ME. Evaluation of performance graded asphaltbinder equipment and testing protocol. Asphalt technology program; 2004.p. 107.

[21] Airey GD, Hunter AE, Rahimzadeh B. The influence of geometry and samplepreparation on dynamic shear rheometer testing. Performance of bituminousand hydraulic materials in pavements; 2002. p. 3–12.

[22] Whiteoak D, Read JM. The Shell bitumen handbook. London: Thomas TelfordServices Ltd.; 2003.

[23] Monismith CL, Ogawa N, Freeme C. Permanent deformation of subgrade soilsdue to repeated loadings. J Transport Res Rec 1975:537.

[24] Kaloush KE, Witczak MW. Tertiary flow characteristics of asphalt mixtures.J Assoc Asphalt Paving Technol 2002;71:278–306.

[25] Zhou F, Scullion T, Sun L. Verification and modeling of three-stage permanentdeformation behavior of asphalt mixes. Am Soc Civ Eng 2004;130:486–94.

[26] Witczak MW. Specification criteria for simple performance tests for rutting. In:National cooperative highway research program; 2007.

[27] Feeley AJ. UTM-5P, universal testing machine, hardware referencemanual. Boronia (Australia): Industrial Process Controls Limited; 1994.

[28] Khodaii A, Mehrara A. Evaluation of permanent deformation of unmodifiedand SBS modified asphalt mixtures using dynamic creep test. Construct BuildMater 2009;23:2586–92.

[29] Ye Q, Wu S, Li N. Investigation of the dynamic and fatigue properties of fiber-modified asphalt mixtures. Int J Fatigue 2009;31:1598–602.

[30] Lu X, Isacsson U. Characterization of SBS polymer modified bitumen-comparison of conventional methods and DMA. J Test Eval 1997:383–90.