effect of crumb rubber characteristics on crumb rubber modified (crm) binder viscosity

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Construction and Building Materials 23 (2009) 295–303

and Building

MATERIALS

Effect of crumb rubber characteristics on crumb rubber modified(CRM) binder viscosity

Carl Thodesen *, Khaldoun Shatanawi, Serji Amirkhanian

Department of Civil Engineering, Clemson University, 110 Lowry Hall, Box 340911, Clemson, SC 29634-0911, USA

Received 31 May 2007; received in revised form 29 November 2007; accepted 10 December 2007Available online 29 January 2008

Abstract

Asphalt binder viscosity is of great importance during the production process of hot mix asphalt mixture as typically asphalt plantswill store binders between 149 �C and 177 �C. SHRP guidelines state that asphalt binder viscosity must not exceed 3 Pa s. Therefore,given the documented increases in asphalt viscosity when modified with crumb rubber modifier (CRM) it is necessary to produce asphaltbinder that fulfills the SHRP criteria while not exceeding plant mixing and storing requirements. This paper reports the results of aninvestigation of the importance of CRM properties on viscosity of CRM binder. Two binder sources were modified at four concentrationlevels using four different crumb rubber sources; the viscosities of the produced binders were evaluated by AASHTO T 316. Crumb rub-ber properties were evaluated by elemental analysis using a scanning electron microscope (SEM) and by determination of glass transitiontemperature (Tg) using a differential scanning calorimeter (DSC). In general, results indicate that processing procedure and tire type playsan important role in the determination of CRM binder viscosity.� 2007 Elsevier Ltd. All rights reserved.

Keywords: CRM binder; Ambient grinding; Cryogenic grinding; Particle effect; Interaction effect

1. Introduction

Research has shown that the addition of crumb rubberto virgin asphalt produces binders with improved resis-tance to rutting, fatigue cracking, and thermal cracking[1,2] as well as reducing the thickness of asphalt overlaysand reflective cracking potential [3]. As well as producinga superior product, CRM binder is also an environmentallyresponsible product.

The Rubber Manufacturers Association (RMA) esti-mates that 188 million scrap tires were in stockpiles as of2005, in addition to over 290 million scrap tires producedin the United States each year [4]. Approximately 12% ofthe scrap tires generated were ground up for rubber modi-fied asphalt and other applications and 16% were used forother civil engineering projects. CRM binder has been

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

doi:10.1016/j.conbuildmat.2007.12.007

* Corresponding author. Tel.: +1 864 650 3929; fax: +1 864 656 6186.E-mail address: [email protected] (C. Thodesen).

identified as a solution to the scrap tire issue, some studieseven suggest that if only 10% of all asphalt pavements laideach year in the US contained 3% rubber, all scrap tiresproduced for that year would be utilized [5].

1.1. Scrap tire composition

Tires are composed of three main components: rubber,steel, and fiber. In order to grind the tires into crumb rub-ber, the ambient or cryogenic grinding procedures are used.Rubber contributes the greatest amount of material to thetire (approximately 60% by weight). However, some vari-ability exists within the rubber as well as with differentcompounds utilized in different areas of the tire.

Tires are a composite material, typically natural/iso-prene rubber is used for both truck and passenger car tiresin the tread, sidewall, belt, carcass ply, and inner liner. Dif-ferences arise in the amount of styrene butadiene rubberused where in truck tires higher amounts of styrene butadi-ene rubber in the carcass ply and base tread are used,

mailto:[email protected]

Cryogenic ground Ambient ground

Source 2 Source 3

Crumb rubber

Differential ScanningCalorimeter

Scanning ElectronicMicroscope

Morphology

Source 1 Source 4

ElementalAnalysis

Same testingprocedures as source 1

Fig. 1. Experimental Design for CRM properties.

Fig. 2. Experimental Design for CRM binder testing.

296 C. Thodesen et al. / Construction and Building Materials 23 (2009) 295–303

similarly higher amounts of butadiene rubber may befound in the base tread as well [6].

1.2. Asphalt binder viscosity

Achieving asphalt binder viscosity requirements is ofutmost importance; generally asphalt is stored in asphaltplants between 149 �C and 177 �C depending on the gradeor viscosity [7]. However, fulfilling these requirementsbecomes more difficult with the increasing viscosity dueto modifications made, for example, by crumb rubber [8]as well as the specifications established by SHRP indicatingthat asphalt viscosity should not exceed 3 Pa s at 135 �C [9].

Research has shown that CRM binder viscosityincreases as CRM concentration is increased, regardlessof rubber type [8]. All combinations of rubber and binderproduce a uniquely modified binder, the resulting viscosityincreases are due to the amount of aromatic oil absorptionand rubber particle swelling. Viscosity of CRM binder isknown to be dependent on CRM rubber content [10,11],particle size [11], processing method [5,12], mixing temper-ature and duration [13–17], CRM surface area [18,19], andrubber type (passenger tire or truck tire) [20].

In order to understand the nature of the effects of theCRM, the Particle and Interaction effects of CRM binderswere studied. These are identified as the contribution to thebinder properties given purely by the interaction betweenCRM and the binder (IE) and as the effect of the CRM par-ticles as inert filler in the binder (PE) [21]. They may be cal-culated using Eqs. (1) and (2):

IE ¼ Drained� Base

Baseð1Þ

PE ¼ CRM �Drained

Baseð2Þ

where IE is interaction effect, PE is particle effect, Drainedis drained binder property, Base is virgin binder propertyand CRM is CRM binder property.

Eqs. (1) and (2) produce unitless parameters which maybe applied to either the viscosity or the G*data. The IE isdefined as the change from the base to the drained binderfor a given CRM binder relative to the base binder. ThePE is defined as the change from the drained to theCRM binder relative to the base binder [21].

2. Experimental procedure

The specific focus of this study was to identify crumbrubber properties having a significant effect on binder per-formance. Figs. 1 and 2 provide the experimental proce-dure utilized in this study for examining CRM propertiesand for preparing samples in the laboratory.

SEM images of the different CRM sources wereobtained. Furthermore an elemental analysis was con-ducted on individual CRM particles to identify elementspresent within the particles and also to determine variabil-ity between individual particles. In addition, binders were

modified with crumb rubber in the laboratory, subse-quently tests were performed on the crumb rubber to estab-lish differences between sources, and finally binders were

C. Thodesen et al. / Construction and Building Materials 23 (2009) 295–303 297

evaluated to determine the effects of crumb rubber varia-tions on binder viscosity.

2.1. Sample preparation

The wet process was used when the CRM reacting withthe binders. CRM concentrations of 5%, 10%, 15%, and20% by weight of binder were used to react the materialsusing a reaction time of 30 min while maintaining a con-stant binder temperature of 177 �C. This temperature wasselected as it is a common temperature used to produceCRM binders in the field in South Carolina. The mixingspeed, duration, and impellor were all studied in a preli-minary study performed by Putman et al., this study con-cluded that a mixing speed of 700 RPM was the optimumspeed when using a high-shear radial flow impeller at amixing temperature of 177 �C.

A reaction time of 30 min was selected as previousresearch by Putman et al. evaluated the performance ofCRM binders manufactured using the method previouslydescribed, it was determined that there was no significantdifference between the properties of binders mixed fordurations of 15, 30, or 45 min (Putman et al.). Based onthe results of this study, a mixing duration of 30 min wasused for all of the binders produced in this study [22].

Drained binders were also prepared in order to studyparticle and interaction effects of the CRM on the binder.Separation of the binder from the CRM was done by heat-ing the CRM binder to a temperature of 163 �C in an oven,the binder was then mixed thoroughly to ensure uniformityand 100 g of each binder was poured into a 76.2 mm diam-eter 80 mesh (0.18 mm) sieve and allowed to drain for30 min in an oven maintained at 149 �C. Binder recoveredfrom this process was then subjected to the same tests asthe CRM binder.

2.2. Characterization of CRM

Cryogenic processed particles are known to exhibit acrystalline structure as a result of the fracturing occurringfollowing the cryogenic freezing process, whereas ambientground CRM display rougher edges as a result of the tear-ing action typical of the ambient grinding procedure [23].CRM particles resulting from the two production processescan be identified by magnified imagery. Additionally SEMwas utilized to conduct the elemental analysis of the CRMparticles to establish the elemental compositions of the var-ious CRM sources studied.

Typically truck tires exhibit a higher concentration ofnatural rubber than passenger car tires; however, the exactamounts of each component are proprietary as well as pos-sibly being varied from year to year or from manufacturerto manufacturer. During the crumb rubber productionprocess, in many cases, tires from numerous sources arecollected, shredded, and distributed without particularattention being paid to the nature of each tire type. There-fore, CRM as a result will contain a certain amount of

variability due to the nature of its extraction; of interestis the effect of these elemental and physical inequalitieson the CRM binder properties.

A differential scanning calorimeter (DSC) was used todetermine the presence of various rubber compounds inthe CRM, by determining the number of glass transitiontemperatures present. These corresponding temperatureswere then used to verify the presence of natural and syn-thetic rubber. The glass transition temperature of naturalrubber (NR) is approximately �70 �C whereas the glasstransition temperature of synthetic rubber is approximately�108 �C. Therefore, confirming the presence of such glasstransition temperatures would permit identification of var-ious rubber types present in the CRM.

Samples were analyzed using a TA instruments Q1000DSC, 7.5 mg samples were enclosed in standard aluminumDSC sample pans and entered into the DSC. The temper-ature was then varied from �150 �C to 100 �C at a rateof 20 �C/min. The data was analyzed using Universal Anal-ysis Software, glass transition temperatures were obtainedfrom the inflection point of the step function.

3. Materials

The objective of this study was to study differences inbinder viscosities when modified with different CRMs.Four crumb rubber sources were selected from aroundthe US (South Carolina, Florida, Arizona, and California)in order to provide a wide array of crumb rubber sources.Two of the crumb rubbers used were of cryogenic originwhile the remaining two were derived from the ambientgrinding process. Gradations of the CRM were determinedby ASTM D 5644-01-A, all four CRM sources fell withinthe specifications as shown in Fig. 3. The analysis of crumbrubbers was done in accordance with ASTM D 297, therubber characteristics are given in Table 1.

The experimental procedures shown in Figs. 1 and 2were followed to conduct this study. Two PG 64-22 bindersources were used in the study, Binder A was of Venezuelanorigin while Binder B was a blended binder. Some of thephysical properties of these binders are provided in Table2, with regards to viscosity it can be seen that Binder Ahas a higher base viscosity than Binder B.

4. Results

Upon completion of testing, all results were analyzedusing the Statistical Analysis System (SAS). Fisher’s LeastSignificant Difference (LSD) analysis was employed todetermine the cause of the differences in the CRM binderperformances.

The Fisher LSD procedure is a commonly used statisti-cal analysis tool, it is defined as the observed differencebetween two sample means necessary to declare the corre-sponding differences between population means. If the dif-ference between two population means is found to begreater than the least significant difference calculated using

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0.010.1110Sieve opening size (mm)

Per

cent

pas

sing

(%

)

Upper LimitSource 1Source 2Source 3Source 4 Lower Limit

Fig. 3. Particle Size Distribution of CRM particles.

Table 2Properties of binders A and B

Aging states Test properties Sources

A B

Unaged binder Rotational viscosity @135 �C (Pa-s)

0.703 0.430

G*/sind @ 64 �C (kPa) 2.413 1.279RTFO aged residue G*/sind @ 64 �C (kPa) 6.075 2.810RTFO + PAV aged

residueG*/sind @ 25 �C (kPa) 3352.1 4074.3Stiffness @ �12 �C (MPa) 141.3 217.0m-value @ �12 �C 0.359 0.307

Table 1Crumb rubber properties

Crumb rubber Source1

Source2

Source3

Source4

Specific gravity, wt% 1.04 1.04 1.05 1.06Moisture content, wt% 0.76 0.67 0.77 0.67Ash content, wt% 6.01 5.36 4.66 5.61Carbon black content, wt% 32.98 29.75 30.41 32.74Extract content (acetone and

chloroform), wt%9.86 11.80 11.69 8.52

Sulfur content, wt% 2.02 1.32 1.24 1.47


Eq. (3), then the population means may be declared statis-tically different [24].

LSDi;j ¼ ta=2

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffis2

w

1

niþ 1

nj

� �sð3Þ

where ni and nj are sample size for population i and j,respectively, t is critical t value for a = a/2 and S2

w is meansquare within samples from the analysis of variance (ANO-VA) table.

4.1. SEM characterization of CRM

As seen in the SEM images given in Fig. 4, the effects ofprocessing procedure on CRM surface characteristics were

confirmed. Two of the CRM sources (Sources 1 and 2)exhibited smooth fractured edges consistent with cryogenicgrinding. The remaining samples (Sources 3 and 4) exhib-ited a rougher morphology typical of ambient groundCRM. The SEM was also utilized to determine the elemen-tal composition of the CRM particles (Fig. 5).

Elemental composition was seen to vary from source tosource, however of the major constituents of the CRM theonly element to vary significantly was the Oxygen in Source4 CRM. This lower oxygen content may be indicative of asignificant presence of truck tire in this source of CRM[25]. Results indicate that the amounts of Carbon were sim-ilar regardless of the grinding procedure; however, Oxygenlevels in the cryogenically ground particles were found tobe statistically greater than those in the ambient groundparticles.

4.2. Glass transition temperature (Tg)

Analysis of the glass transition temperatures was con-ducted, generally CRM glass transition temperatures werefound to be quite similar with the exception of Source 4.Major differences found in the differential scanning calo-rimeter (DSC) profiles of the various CRM types involvedthe presence of more than one glass transition temperaturefor some CRM types. Multiple glass transition tempera-tures are indicative of the presence of more rubber typeswithin the CRM. The glass transition temperatures for nat-ural rubber (NR) were generally lower for cryogenic rub-bers than for ambient ground rubber sources.

Findings from the DSC tests were consistent with theSEM tests, CRM Sources 3 and 4 exhibited no visible Tg

in the synthetic rubber (SR) range thus indicating the pres-ence of a significant concentration of truck tire rubber. Asshown in Fig. 6, CRM Sources 1 and 2 exhibited Tgs atboth the natural rubber (NR) and the synthetic rubber(SR) range thus illustrating significant presence of both

Fig. 4. SEM micrographs of (a) Cryogenically and (b) Ambient Ground CRM at 30� magnification.

0

10

20

30

40

50

60

70

80

90

100

C O Al Si S Ca Cl Fe Zn

% b

y w

eigh

t

Element

Source 1

Source 2

Source 3

Source 4

Fig. 5. Elemental composition by weight.

Fig. 6. DSC Profiles of (a) Source 1 CRM and (b) Source 4 CRM.


types of rubber. Source 3 CRM was seen to exhibit somecharacteristics of truck and passenger tire rubbers.

Fig. 6 clearly demonstrates the differences in the stepspresent in the DSC profiles for the different CRM Sources.

For example, this figures shows that Source 4 exhibited twoclear steps in the profile consistent with the glass transitiontemperatures of natural and synthetic rubber. However,only one step was apparent in the DSC profile of Source


1 CRM, this indicates a lack of material present at the SRglass transition zone. Source 2 CRM illustrated a step sim-ilar to the one shown for Source 1, while Source 3 demon-strated two steps similar to the ones present in Source 4.

4.3. Viscosity

Research results have indicated that binder modified byambient ground CRM will exhibit greater viscosities thanbinder modified by cryogenically ground CRM [20]. Inaddition, literature also states that greater quantities ofnatural rubber will also increase the viscosity; therefore,it might be assumed that Source 4 CRM would exhibitthe greatest viscosities as CRM tests indicated a substantialpresence of natural rubber within Source 4. This was gen-erally the case as binder modified with Source 4 CRMyielded the highest viscosities for three of the four concen-trations studied. Similarly the ambient ground CRM was

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

Vis

cosi

ty (P

a S)

% CRM by

C B DAC

AB

C

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

5 10

5 10

Vis

cosi

ty (

Pa

S)

% CRM by

AB B BA ABBC C

Source 1 Source 2

Fig. 7. Brookfield Viscosities at 135 �C of (a) Binder A and (b) Binder B (Valstatistically similar values).

seen to consistently provide the higher CRM binderviscosities.

Fig. 7 shows the experimental values of the viscositiesdetermined by Brookfield Viscometer at 135 �C. Both bind-ers clearly exhibit a viscosity increase with increasing CRMconcentrations; however the extent to which these arequantified depends on the binder being modified and theproperties of the crumb rubber used to makemodifications.

At a concentration of 5% CRM by weight of binder, theresults were generally quite similar. However, as the CRMincreased, the differences in the CRM started to manifestthemselves. The CRM binders at 20% provided the greatestcontrasts between the binders, however even then theresults were not consistent with Source 3 exhibiting thehighest viscosity for Binder B while Source 4 had the high-est viscosity for Binder A. When examining the binder vis-cosities using grinding procedure as a blocking factor, the

weight of binder

CC

B

AC

D

A

B

15 20

15 20

weight of binder

A

B

C

AB

C CC

Source 3 Source 4

ues having at least one letter, within given %CRM, in common produced


importance of grinding procedure was confirmed for deter-mination of viscous properties of the CRM binder. Bindersource, in this limited study, played a key role in dictatingthe viscous properties of the CRM binder, when bindersource was used as a blocking factor, statistical differenceswere noted for every CRM concentration.

When the CRM concentration was used as a blockingfactor, the relative effects of the various CRM sourcescould be analyzed. From Fig. 7, it can be seen that Source4 CRM consistently exhibited higher viscosities regardlessof the binder source. When the grinding procedures werecompared, the ambient ground crumb rubber samplesyielded statistically significant higher viscosities regardlessof CRM concentration. This suggests that the morphologyof the CRM plays an important role in the viscous perfor-mance of the binder, as the smooth crystalline particlesgenerated by the cryogenic procedure yielded less viscousbinder than their rougher ambient counterpart. There canbe little doubt that the base binder plays an influential rolein determining the viscosity of the CRM binder, when thebinder source was used as a blocking factor, Binder A con-sistently had a statistically higher viscosity than Binder B.

0.42

0.320.38

0.50

0.180.07

0.050.03

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

10

IE

% CRM by we

10% CRM by we

1.180.28

3.061.72

0.800.50

1.551.15

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

18.00

20.00

PE

a

b

Binder A Source 1Binder B Source 1


Fig. 8. Viscosity: (a)

4.4. Particle and interaction effects on viscosity (PE & IE)

Particle effect and interaction effect were determined forviscosity, permitting the analysis of interaction and particleeffects on binder properties to be investigated. PE and IEidentified as the contribution to the binder viscosity dueto interaction between CRM and the binder (IE) and asthe effect of the CRM particles as inert filler in the binder(PE). Precise identification of the contribution of eachwas necessary to determine the cause of the change in prop-erties of the CRM binder.

Results indicated that at a CRM concentration of 10%,the interaction effect for viscosity was statistically consis-tent regardless of CRM source; this indicates that at 10%CRM the CRM binder effects due to interaction betweenparticle and binder were independent of CRM source.Results indicate that at 10% no significant interactionsoccur regardless of CRM, binder source, or grinding proce-dure when viscosity results are considered.

Fig. 8 illustrates the differences in IE and PE for the twobinder Sources (A & B) when modified with CRMs fromfour different sources. Clearly, the IE increases significantly

1.171.20

0.98

1.52

0.530.43

0.470.50

20ight of binder

20ight of binder

5.60

8.57

15.40

17.50

3.054.33

13.46

9.12



IE and (b) PE.


as the amount of CRM is increased, thereby suggestingthat any reactions occurring between the CRM and binderare dependent on the amount of CRM used. The base bin-der used also appears to play a major role on the develop-ment of the IE as the interactions were noted to beconsistently higher for Binder A than Binder B.

As shown in Fig. 8, the PE clearly increases as the CRMconcentration increases, thus suggesting that the effect ofthe particle in the binder matrix is magnified as the numberof particles increases. At 10% CRM, there appears to besome variation between binder and CRM sources However,these differences are more apparent when the 20% CRMconcentration is studied. At 20% rate, the ambient groundCRM sources yielded the highest PEs, furthermore the baseBinder A also tended to exhibit higher PEs than Binder B.These findings suggest that the ability of the particle as aninert filler in the binder matrix to modify the asphalt prop-erties is highly dependent on the base binder viscosity aswell as the morphological properties of the CRM.

Emphasis must be placed on the fact that PE was gener-ally 12 times greater than the IE, thereby suggesting thatthe bulk of the viscous effects due to the presence ofCRM in the binders were attributed to the effects of theCRM as inert filler. The grinding procedure was seen toconsistently affect the properties of the binders where theambient ground binders were generally seen to demon-strate the highest viscosity.

Source 4 CRM consistently produced the most viscousbinder; CRM testing also indicated that this rubber sourcecontained a significant portion of truck tire CRM. There-fore, it appears that the source of tire used as a modifierplays some role in the viscosity of the modified binder. Itis unclear whether this is due to chemical differences in tiresource, and thus interaction between tire and binder. Tirecomposition and properties depend on tire grade, age,and manufacturer [26], differences in natural and syntheticrubber content in tire sources may contribute to differentparticle morphology, such as surface area of particles. Tirecompounds exhibit differing properties with respect toaging (oil resistance, sunlight aging, and oxidation) andphysical distresses (abrasion resistance, tear resistance,and maximum tensile strength); therefore, variability inthese may contribute to varied particle morphologies thusresulting in varied CRM binder properties.

5. Conclusions

The following conclusions were made upon completionof this limited study:

� As a recycled resource, crumb rubber obtained fromscrap tires is inherently a unique material; therefore,its properties may be affected due to many factorsincluding: source, grinding procedure, tire type, variabil-ity of tires in stockpile, and even on the particular batch.As a result inequalities in the crumb rubber tend to res-onate through to the CRM binder viscosity.

� Statistically significant variations in viscosity of CRMbinders were found when CRM type and source werevaried; furthermore, these changes were also foundwhen the same CRMs were used for different bindersources of similar PG grade and base viscosity.� Elemental analysis conducted by SEM and DSC pro-

duced statistically similar results for three out of thefour CRMs tested; this suggested a significant presenceof passenger car tire. The CRM exhibiting unique prop-erties with characteristics indicative of truck tire consis-tently yielded CRM binders with the highest viscosities.When particle and interaction effects were studied largedifferences were not found between the truck tire sourceand the other three.� Ultimately, the properties exhibiting the greatest effects

on CRM binder viscosity proved to be grinding proce-dure of the scrap tires and base binder viscosity. Thesefindings suggest changes in the viscous properties ofthe binder are dependent on the CRM type and source.However, it appears that these changes are due to phys-ical interaction between the CRM particles and binderrather than any chemical interactions.� As interaction effects remained low for all CRM sources,

the reason for the change in viscosity may be due to dif-fering particle morphology and surface area due tounequal grinding and tearing properties of various tirecomponents.

Acknowledgments

This study was supported by the Asphalt Rubber Tech-nology Service (ARTS) at the Civil Engineering Depart-ment, Clemson University, Clemson, South Carolina,USA. The authors wish to acknowledge and thank SouthCarolina’s Department of Health and Environmental Con-trol (DHEC) for their financial support of this project.

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effect of crumb rubber characteristics on crumb rubber modified (crm) binder viscosity

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