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

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<ul><li><p>ride</p><p>Sh</p><p>0 L</p><p>29e 29</p><p>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,</p><p>to virgin asphalt produces binders with improved resis-</p><p>ed asphalt and other applications and 16% were used forother civil engineering projects. CRM binder has been</p><p>produced for that year would be utilized [5].</p><p>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,</p><p>* Corresponding author. Tel.: +1 864 650 3929; fax: +1 864 656 6186.E-mail address: cthodesen@gmail.com (C. Thodesen).</p><p>Available online at www.sciencedirect.com</p><p>Construction and Building Materi</p><p>Constructiontance to rutting, fatigue cracking, and thermal cracking[1,2] as well as reducing the thickness of asphalt overlaysand reective cracking potential [3]. As well as producinga superior product, CRM binder is also an environmentallyresponsible product.</p><p>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-</p><p>1.1. Scrap tire composition</p><p>Tires are composed of three main components: rubber,steel, and ber. 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 dierentcompounds utilized in dierent areas of the tire.given the documented increases in asphalt viscosity when modied with crumb rubber modier (CRM) it is necessary to produce asphaltbinder that fullls 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 modied at four concentrationlevels using four dierent 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 dierential 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.</p><p>Keywords: CRM binder; Ambient grinding; Cryogenic grinding; Particle eect; Interaction eect</p><p>1. Introduction</p><p>Research has shown that the addition of crumb rubber</p><p>identied 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 tiresEect of crumb rubber characte(CRM) bin</p><p>Carl Thodesen *, Khaldoun</p><p>Department of Civil Engineering, Clemson University, 11</p><p>Received 31 May 2007; received in revised formAvailable onlin</p><p>Abstract0950-0618/$ - see front matter 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.conbuildmat.2007.12.007stics on crumb rubber modiedr viscosity</p><p>atanawi, Serji Amirkhanian</p><p>owry Hall, Box 340911, Clemson, SC 29634-0911, USA</p><p>November 2007; accepted 10 December 2007January 2008</p><p>www.elsevier.com/locate/conbuildmat</p><p>als 23 (2009) 295303</p><p>and Building</p><p>MATERIALS</p></li><li><p>modied with crumb rubber in the laboratory, subse-</p><p>Cryogenic ground Ambient ground</p><p>Source 2 Source 3</p><p>Differential ScanningCalorimeter</p><p>Scanning ElectronicMicroscope</p><p>Morphology</p><p>Source 1 Source 4</p><p>ElementalAnalysis</p><p>Same testingprocedures as source 1</p><p>Fig. 1. Experimental Design for CRM properties.</p><p>Fig. 2. Experimental Design for CRM binder testing.</p><p>Busimilarly higher amounts of butadiene rubber may befound in the base tread as well [6].</p><p>1.2. Asphalt binder viscosity</p><p>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, fullling these requirementsbecomes more dicult with the increasing viscosity dueto modications made, for example, by crumb rubber [8]as well as the specications established by SHRP indicatingthat asphalt viscosity should not exceed 3 Pa s at 135 C [9].</p><p>Research has shown that CRM binder viscosityincreases as CRM concentration is increased, regardlessof rubber type [8]. All combinations of rubber and binderproduce a uniquely modied 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 [1317], CRM surface area [18,19], andrubber type (passenger tire or truck tire) [20].</p><p>In order to understand the nature of the eects of theCRM, the Particle and Interaction eects of CRM binderswere studied. These are identied as the contribution to thebinder properties given purely by the interaction betweenCRM and the binder (IE) and as the eect of the CRM par-ticles as inert ller in the binder (PE) [21]. They may be cal-culated using Eqs. (1) and (2):</p><p>IE Drained BaseBase</p><p>1</p><p>PE CRM DrainedBase</p><p>2</p><p>where IE is interaction eect, PE is particle eect, Drainedis drained binder property, Base is virgin binder propertyand CRM is CRM binder property.</p><p>Eqs. (1) and (2) produce unitless parameters which maybe applied to either the viscosity or the G*data. The IE isdened as the change from the base to the drained binderfor a given CRM binder relative to the base binder. ThePE is dened as the change from the drained to theCRM binder relative to the base binder [21].</p><p>2. Experimental procedure</p><p>The specic focus of this study was to identify crumbrubber properties having a signicant eect 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.</p><p>SEM images of the dierent CRM sources wereobtained. Furthermore an elemental analysis was con-ducted on individual CRM particles to identify elements</p><p>296 C. Thodesen et al. / Construction andpresent within the particles and also to determine variabil-ity between individual particles. In addition, binders wereCrumb rubber</p><p>ilding Materials 23 (2009) 295303quently tests were performed on the crumb rubber to estab-lish dierences between sources, and nally binders were</p></li><li><p>Buevaluated to determine the eects of crumb rubber varia-tions on binder viscosity.</p><p>2.1. Sample preparation</p><p>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 eld 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 ow impeller at amixing temperature of 177 C.</p><p>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 signicantdierence 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].</p><p>Drained binders were also prepared in order to studyparticle and interaction eects 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.</p><p>2.2. Characterization of CRM</p><p>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 identied by magnied imagery. Additionally SEMwas utilized to conduct the elemental analysis of the CRMparticles to establish the elemental compositions of the var-ious CRM sources studied.</p><p>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 particular</p><p>C. Thodesen et al. / Construction andattention being paid to the nature of each tire type. There-fore, CRM as a result will contain a certain amount ofvariability due to the nature of its extraction; of interestis the eect of these elemental and physical inequalitieson the CRM binder properties.</p><p>A dierential 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 approximately108 C. Therefore, conrming the presence of such glasstransition temperatures would permit identication of var-ious rubber types present in the CRM.</p><p>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 inection point of the step function.</p><p>3. Materials</p><p>The objective of this study was to study dierences inbinder viscosities when modied with dierent 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 specications as shown in Fig. 3. The analysis of crumbrubbers was done in accordance with ASTM D 297, therubber characteristics are given in Table 1.</p><p>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.</p><p>4. Results</p><p>Upon completion of testing, all results were analyzedusing the Statistical Analysis System (SAS). Fishers LeastSignicant Dierence (LSD) analysis was employed todetermine the cause of the dierences in the CRM binderperformances.</p><p>The Fisher LSD procedure is a commonly used statisti-cal analysis tool, it is dened as the observed dierencebetween two sample means necessary to declare the corre-sponding dierences between population means. If the dif-</p><p>ilding Materials 23 (2009) 295303 297ference between two population means is found to begreater than the least signicant dierence calculated using</p></li><li><p>Bu20%</p><p>30%</p><p>40%</p><p>50%</p><p>60%</p><p>70%</p><p>80%</p><p>90%</p><p>100%</p><p>Perc</p><p>ent p</p><p>assin</p><p>g (%</p><p>)</p><p>298 C. Thodesen et al. / Construction andEq. (3), then the population means may be declared statis-tically dierent [24].</p><p>LSDi;j ta=2s2w</p><p>1</p><p>ni 1</p><p>nj</p><p> s3</p><p>where ni and nj are sample size for population i and j,respectively, t is critical t value for a = a/2 and S2w is meansquare within samples from the analysis of variance (ANO-VA) table.</p><p>4.1. SEM characterization of CRM</p><p>As seen in the SEM images given in Fig. 4, the eects ofprocessing procedure on CRM surface characteristics were</p><p>0%</p><p>10%</p><p>110Sieve open</p><p>Fig. 3. Particle Size Distribu</p><p>Table 2Properties of binders A and B</p><p>Aging states Test properties Sources</p><p>A B</p><p>Unaged binder Rotational viscosity @135 C (Pa-s)</p><p>0.703 0.430</p><p>G*/sind @ 64 C (kPa) 2.413 1.279RTFO aged residue G*/sind @ 64 C (kPa) 6.075 2.810RTFO + PAV aged</p><p>residueG*/sind @ 25 C (kPa) 3352.1 4074.3Stiness @ 12 C (MPa) 141.3 217.0m-value @ 12 C 0.359 0.307</p><p>Table 1Crumb rubber properties</p><p>Crumb rubber Source1</p><p>Source2</p><p>Source3</p><p>Source4</p><p>Specic 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</p><p>chloroform), wt%9.86 11.80 11.69 8.52</p><p>Sulfur content, wt% 2.02 1.32 1.24 1.47conrmed. 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).</p><p>Elemental composition was seen to vary from source tosource, however of the major constituents of the CRM theonly element to vary signicantly was the Oxygen in Source4 CRM. This lower oxygen content may be indicative of asignicant 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 ground</p><p>0.010.1ing size (mm)</p><p>Upper LimitSource 1Source 2Source 3Source 4 Lower Limit</p><p>tion of CRM particles.</p><p>ilding Materials 23 (2009) 295303particles.</p><p>4.2. Glass transition temperature (Tg)</p><p>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 dierences found in the dierential scanning calo-rimeter (DSC) proles of the various CRM types involvedthe presence of more than one glass transition te...</p></li></ul>

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