Research ArticleA Study on the Rheological Properties of RecycledRubber-Modified Asphalt Mixtures
Murat Karacasu,1 Volkan Okur,1 and Arzu Er2
1Department of Civil Engineering, Eskisehir Osmangazi University, 26480 Meselik, Eskisehir, Turkey2Department of Civil Engineering, Akdeniz University, 07058 Antalya, Turkey
Correspondence should be addressed to Volkan Okur; email@example.com
Received 14 August 2014; Accepted 17 September 2014
Academic Editor: Turhan Bilir
Copyright 2015 Murat Karacasu et al.This is an open access article distributed under the Creative CommonsAttribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Using waste rubber in asphalt mixes has become a common practice in road construction. This paper presents the results of astudy on the rheological characteristics of rubber-modified asphalt (RMA) concrete under static and dynamic loading conditions.A number of static and dynamic creep tests were conducted on RMAmix specimens with different rubber sizes and contents, anda series of resonant column tests were conducted to evaluate the shear modulus and damping values. To simulate the stress-strainresponse of traffic-induced loading, the measurements were taken for different confining pressures and strain levels. The results ofthe study indicated that rubber modification increases stiffness and damping ratio, making it a very attractive material for use inroad construction. However the grain size of the rubber is very important. Although RMAmay cost up to 100% more than regularasphalt, the advantages it brings, such as an increased service life of the road and proper waste utilization contributing to a moresustainable infrastructure, may justify the added cost.
Asphalt concrete is the leading paving material for roadsand runways. Understanding the characteristics of the asphaltbeing used in a project is important to ensure long-termperformance and stability. Various environmental conditionsand traffic-induced loadings must be taken into account inthe design stage. Sometimes the design does not result inacceptable, safe, and sound usability due to early deterio-ration. Several factors contribute to deterioration, such asthe quality of materials and construction, traffic loading onthe road, road geometry, and environmental conditions. Ingeneral, most of the deterioration is due to rutting or bottomup fatigue and thermal cracking. Enhancing the pavement lifeis possible with somemodifications if possible factors causingdeteriorations are taken into consideration during the designstage.
Plastomeric polymeric materials, such as polyethylene(PE) andpolypropylene (PP), have evoked considerable inter-est among engineers and manufacturers for use in roadpaving modification because of their viscoelastic properties
and good adhesion to mineral aggregates . The mainpurpose of using polymers in asphalt concrete is to increasebinder stiffness at high service temperatures and reducestiffness at low service temperatures . Polymers whichare used for the modification of asphalt concrete can bedivided into three main categories: thermoplastic elastomers,plastomers, and reactive polymers.Thermoplastic elastomersare apparently capable of high elastic response characteristicsand therefore resist permanent deformation by stretching andrecovering their initial shape on the modified binder layer,whereas plastomers and reactive polymers modify asphaltby forming a tough, rigid, three-dimensional network toincrease stiffness and decrease deformations [8, 9]. Because ofits suitability in these conditions, one of the leading polymermodifiers for bitumen among the larger group of copolymersis styrene-butadiene-styrene (SBS) block. SBS is a synthetichard rubber copolymer that is used in applications wheredurability is important and is often substituted in part fornatural rubber based on the comparative raw materials costs. It belongs to the group of copolymers called blockcopolymers, in which the backbone chain is made up of three
Hindawi Publishing Corporatione Scientific World JournalVolume 2015, Article ID 258586, 9 pageshttp://dx.doi.org/10.1155/2015/258586
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segments.Thefirst is a long chain of polystyrene, themiddle isa long chain of polybutadiene, and the last segment is anotherlong section of polystyrene.
SBS is a cost-effective material that is used to stretchdwindling natural rubber resources, especially in tire man-ufacturing. However the disposal of used automobile tireshas causedmany environmental and economic problems.Theannual global production of used tires estimated 17 milliontonnes in which China, European Union Countries, USA,Japan, and India lead the way to produce the largest amountsof tyre . A small percentage of these tires are recappedor reused as lower-quality rubber, but around 80% of thesetires are accumulated in dumps, posing health hazards andadversely impacting the environment .
Due to its elastic nature, crumb rubber can be used inroad construction to improve deformation resistance. Rub-ber-modified asphalt (RMA) is a bituminous mix consistingof blended aggregates, recycled crumb rubber, and bitumen.It is found out that rubber structure is themost important fac-tor affecting elastic properties. Crumb rubber has a granulartexture and ranges in size from a very fine powder (5mm). Choubane et al.  publishedthe findings from a ten-year study of RMA surfaces andconcluded that crumb rubber increases overall strength andreduces surface rutting. RMA has been known to improvethe rheological properties at low and high temperatures andprovides a lifespan that is up to three times longer thanconventional asphalt . There are also other reasons whyrubber is useful in both highways and railways, such asdecreasing thermal instability, increasing resistance to low-temperature cracking, reduction of noise levels, and reduc-tion of vibrations generated by heavy axle loads [20, 21].
The focus of the test program was to examine the per-formance of asphalt concrete whenmixedwith crumb rubberas aggregate. Same type rubber in different sizes and shapeswas added to the mix without any modification to the bitu-men content. The study concentrated on the behavior ofadmixture under cyclic loadings. The digestion mechanismof the rubber sizes is not evaluated in this paper but shouldbe considered in the future research.
2. Experimental Procedure
The appropriate aggregate gradation for hot-mix bitumenwas designed according to the technical specifications of theGeneral Directorate of TurkishHighways (GDTH) 2006 .The aggregates have a mean grain size (
50) between 0.30
and 3.0mm and a coefficient of uniformity () between 2.0
and 3.0. The margins of the GDTH and the prepared gradingcurves are given in Figure 1. The physical characteristics ofthe aggregates and properties of the mix design are given inTables 1 and 2, respectively.
To determine the optimal bitumen content, Marshallstability tests and flow tests are performed according to theASTM D 1559-76 specifications. The optimum bitumen ratiowas found to be 4.89% for the 50/70 penetration grade. Thecrumb rubber selected for this investigation was producedfrom recapping automobile tires. The bitumen was modified
0.0100.1001.00010.000100.000Sieve size (mm)
Lower limitsUpper limits
Figure 1: Aggregate grading curves for asphalt mixtures comparedwith the current GDTH.
Table 1: Physical characteristics of the aggregates used in the tests.
Properties Testvalues Standards
Specific gravity of coarse aggregate,25C, g/cm3 2.62 ASTM C127-07
Water absorption of coarse aggregate, % 0.23 ASTM C127-07Specific gravity of fine aggregate, 25C,g/cm3 2.622 ASTM C128-07a
Water absorption of fine aggregate, % 1.04 ASTM C128-07aSpecific gravity of filler, 25C, g/cm3 2.708 ASTM C128-07aLos Angeles wearing test, % 28.91 ASTM C535-09Freezing and thawing test, % 5.467 ASTM C1646-08aBitumen absorption, % 0.14 ASTM D4469-01
Table 2: Test results of the mix design.
Properties of the mix designUnit weight, kg/cm3 2413.36Marshall stability, N 16500Void, % 3.2Voids filled with bitumen, % 76.4Flow, mm 1.82Coarse aggregate (%) 36.5Fine aggregate (%) 56.5Filler (%) 7
with three different sizes of tire rubber by-products: Type-I(granulated tire rubber that passes a #3/8 sieve), Type-II (515mmpieces of chipped tire rubber), andType-III (powderedtire rubber that passes a #40 sieve). All rubbers were obtainedby shredding and grinding the tire after removing the fabricand steel belts. Photographs and SEM images compares thecrumb rubber gradation and surface texture for three typesin Figures 2 and 3, respectively. Table 3 shows the physicalproperties of the virgin binder. The characteristics of theRMA depend on the concentration amount and the polymer
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Figure 2: Tire rubber after the shredding and grinding: (a) Type-I, (b) Type-II, and (c) Type-III process.
Figure 3: Scanning electron micrographs of waste tire: (a) Type-I, (b) Type-II, and (c) Type-III.
type in which the polymer is mixed in concentrations ofapproximately 0.21.0% by weight of aggregates. Higherconcentration mixes of polymers can be less economical andmay also cause problems related to thematerial properties .
Rubber contents of 0.2%, 0.4%, 0.6%, 0.8%, and 1.0% byweight of aggregates were blended with bitumen for eachrubber size at a mixing temperature of approximately 160C.
To verify the repeatability of