effect of crumb rubber characteristics on crumb rubber modified (crm) binder viscosity
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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,
to virgin asphalt produces binders with improved resis-
ed asphalt and other applications and 16% were used forother civil engineering projects. CRM binder has been
produced for that year would be utilized .
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,
* Corresponding author. Tel.: +1 864 650 3929; fax: +1 864 656 6186.E-mail address: firstname.lastname@example.org (C. Thodesen).
Available online at www.sciencedirect.com
Construction and Building Materi
Constructiontance to rutting, fatigue cracking, and thermal cracking[1,2] as well as reducing the thickness of asphalt overlaysand reective cracking potential . 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 . Approximately 12% ofthe scrap tires generated were ground up for rubber modi-
1.1. Scrap tire composition
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.
Keywords: CRM binder; Ambient grinding; Cryogenic grinding; Particle eect; Interaction eect
Research has shown that the addition of crumb rubber
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
Carl Thodesen *, Khaldoun
Department of Civil Engineering, Clemson University, 11
Received 31 May 2007; received in revised formAvailable onlin
Abstract0950-0618/$ - see front matter 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.conbuildmat.2007.12.007stics on crumb rubber modiedr viscosity
atanawi, Serji Amirkhanian
owry Hall, Box 340911, Clemson, SC 29634-0911, USA
November 2007; accepted 10 December 2007January 2008
als 23 (2009) 295303
modied with crumb rubber in the laboratory, subse-
Cryogenic ground Ambient ground
Source 2 Source 3
Source 1 Source 4
Same testingprocedures as source 1
Fig. 1. Experimental Design for CRM properties.
Fig. 2. Experimental Design for CRM binder testing.
Busimilarly higher amounts of butadiene rubber may befound in the base tread as well .
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 . However, fullling these requirementsbecomes more dicult with the increasing viscosity dueto modications made, for example, by crumb rubber as well as the specications established by SHRP indicatingthat asphalt viscosity should not exceed 3 Pa s at 135 C .
Research has shown that CRM binder viscosityincreases as CRM concentration is increased, regardlessof rubber type . 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 , processing method [5,12], mixing temper-ature and duration , CRM surface area [18,19], andrubber type (passenger tire or truck tire) .
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) . They may be cal-culated using Eqs. (1) and (2):
IE Drained BaseBase
PE CRM DrainedBase
where IE is interaction eect, PE is particle eect, 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 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 .
2. Experimental procedure
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.
SEM images of the dierent CRM sources wereobtained. Furthermore an elemental analysis was con-ducted on individual CRM particles to identify elements
296 C. Thodesen et al. / Construction andpresent within the particles and also to determine variabil-ity between individual particles. In addition, binders wereCrumb rubber
ilding Materials 23 (2009) 295303quently tests were performed on the crumb rubber to estab-lish dierences between sources, and nally binders were
Buevaluated to determine the eects 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 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.
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 .
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.
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 .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.
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
C. Thodesen et al. / Construction andattention being paid to the natur