Construction

Construction and Building Materials 21 (2007) 66–72

and Building

MATERIALSwww.elsevier.com/locate/conbuildmat

Polymer modified asphalt binders

Yetkin Yildirim *

Department of Civil Engineering, University of Texas at Austin, 3208 Red River CTR 318, Austin, TX 78705, USA

Received 19 August 2004; received in revised form 5 July 2005; accepted 21 July 2005Available online 19 September 2005

Abstract

This paper is a review of research that has been conducted on polymer modified binders over the last three decades. Polymermodification of asphalt binders has increasingly become the norm in designing optimally performing pavements, particularly inthe United States, Canada, Europe and Australia. Specific polymers that have been used include rubber, SBR, SBS and Elvaloy�.Specifications have been designed and pre-existing ones modified to capture the rheological properties of polymer modified binders.The elastic recovery test is good at determining the presence of polymers in an asphalt binder, but is less successful at predicting fieldperformance of the pavement.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Polymer modified binder; Asphalt; Binder specifications; Elastic recovery; SBR; SBS; Elvaloy; Rubber

1. Introduction

The addition of polymers, chains of repeated smallmolecules, to asphalt has been shown to improve per-formance. Pavement with polymer modification exhib-its greater resistance to rutting and thermal cracking,and decreased fatigue damage, stripping and tempera-ture susceptibility. Polymer modified binders havebeen used with success at locations of high stress, suchas intersections of busy streets, airports, vehicle weighstations, and race tracks [1]. Polymers that have beenused to modify asphalt include styrene–butadiene–sty-rene (SBS), styrene–butadiene rubber (SBR), Elvaloy�,rubber, ethylene vinyl acetate (EVA), polyethylene,and others. Desirable characteristics of polymer mod-ified binders include greater elastic recovery, a highersoftening point, greater viscosity, greater cohesivestrength and greater ductility [1,2].

0950-0618/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.conbuildmat.2005.07.007

* Tel.: +1 512 232 1845; fax: +1 512 475 7914.E-mail address: yetkin@mail.utexas.edu.

2. History, use, and benefits

Processes of asphalt modification involving naturaland synthetic polymers were patented as early as 1843[3]. Test projects were underway in Europe in the1930s, and neoprene latex began to be used in NorthAmerica in the 1950s [1]. In the late 1970s, Europewas ahead of the United States in the use of modified as-phalts because the European use of contractors, whoprovided warranties, motivated a greater interest in de-creased life cycle costs, even at higher initial costs. Thehigh preliminary expenses for polymer modified asphaltlimited its use in the US [4]. In the mid-1980s, newerpolymers were developed and European technologiesbegan to be used in the US [5,6]. At the same time,the prevalence of a long-term economic outlook in thecountry increased [1]. In Australia, the current NationalAsphalt Specification includes guides and specificationsregarding polymer modified binders [7].

The United States Federal Highway Administration(FHWA) has developed a life cycle cost analysis ap-proach, which can be used to evaluate the life cycle costs

Y. Yildirim / Construction and Building Materials 21 (2007) 66–72 67

of pavement containing asphalt rubber binders as wellas other treatments. The findings indicated that asphaltrubber is cost effective as it is used, for example, in Ari-zona and California, although the estimated life of thepavement is based on interviews and engineering judg-ment, and can be refined as the pavement ages andlong-term field performance is included in the model [8].

A 1997 survey of state departments of transportationin the United States found that 47 states of the 50 re-ported that they would be using modified binders inthe future, 35 of them saying that they would use greateramounts [9]. Several research teams around the worldhave worked on evaluating the benefits of polymer mod-ification on pavement performance, and tests and spec-ifications for binders are continually being developed.

In a 2001 study for the Ohio Department of Trans-portation, Sargand and Kim [10] compared the fatigueand rutting resistance of three PG 70–22 binders, oneunmodified, one SBS modified, and one SBR modified.It was found that the modified binders were more resis-tant to both fatigue and rutting than the neat binder,even though all three had the same performance grade.

According to a 2003 Nevada study, the viscosity ofpolymer modified binders tends to be significantlygreater than that of non-modified binders at 60 �C,although penetration changes only slightly at all temper-atures [11].

In 2003, Newcomb [12] discussed the concept of per-petual pavements in Hot Mix Asphalt, claiming that it isa misconception that fatigue cracking is inevitable.Many full-depth hot mix asphalt (HMA) pavementsbuilt 30–40 years ago have yet to exhibit any fatiguecracking, and Newcomb claims that research shows thatincreasing polymer modified binders at the bottom ofthe asphalt layer may raise the fatigue limit of the pave-ment.

A 2003 US Army Corps of Engineers study [13]points out that for optimal economy, it is desirable tochoose an asphalt modifier that resists multiple dis-tresses, such as rutting, fatigue, thermal cracking andwater damage. It was found that the choice of polymermay have a significant impact on fatigue properties,and that the mixtures boasting the highest fatigue lifecontained reactive styrene–butadiene crosslinked poly-mer. Other polymers tested were a chemically modifiedcrumb rubber, SBR, linear block SBS and a proprietarymodified SBS.

3. Test methods

As discussed by King et al. in the 1999 Journal of the

AAPT [1], there are several test methods that have beendeveloped or altered for modified binders. Previously,both modified and unmodified binders alike were testedaccording to the same methods, supported by the Stra-

tegic Highway Research Program (SHRP). Bahia et al.in their 1998 article for the Journal of AAPT note, how-ever, that this blanket testing method failed to test theextreme grades required by the new, modified binders,resulting in the initiation of new testing protocols formodified binders [14].

New test protocols include measuring the softeningpoint using a ring and ball apparatus (ASTM E 28) todetermine the resistance to flow at high temperaturesand a force ductility test that measures tensile properties[15]. Several tests have been developed to look at elasticrecovery, one of the major areas of improvement in elas-tomer modified asphalt. Thompson and Hagman devel-oped a torsional recovery test, included in Californiaspecifications for identifying the presence of elastomers[1]. The elastic recovery test using a ductilometer, de-scribed later in this paper, is included in the Task Force31 Specifications and is used in the US and Europe [1].

King et al. [1] point out that many tests exist to iden-tify whether modification is present, such as the IR, lowtemperature ductility and torsional recovery. The WestCoast User Producer Group tried to use performancebased asphalt (PBA) specifications, which involve a hightemperature viscosity test and low temperature penetra-tion and ductility tests, as specifications for modified as-phalt, but they were not as good at predictingperformance with modified asphalt as they were withneat asphalt [1].

In 1998, Blankenship et al. [16] conducted field andlaboratory tests in Kentucky and found that PG 70–22made using different methods of modification gave dif-ferent results for laboratory tests. They used the Dy-namic Shear Rheometer (DSR) and Bending BeamRheometer (BBR) tests to identity five different PG70–22 binders, two SBS-modified, one SBR modified,one chemically modified, and one neat and comparedtheir behavior in various tests. These binders were foundto differ as far as rutting, moisture damage and modulustesting, although the rutting difference was no more than10 mm between the binders.

In 2004, Yildirim et al. [17] utilized a design methodfor determining the modification level of asphalt bindersusing waste toner, which contains styrene acrylic copoly-mers. Binder designs were performed including blendingtime, performance grading, storage stability and, mixingand compaction temperature calculations. Test resultsindicated that the stiffness of the blend increases as thepercentage of the toner content increases.

4. Specific modifiers

4.1. Rubber

‘‘Crumb rubber modifier’’ (CRM) and ‘‘asphalt-rub-ber’’ are terms that refer to applications in which ground

68 Y. Yildirim / Construction and Building Materials 21 (2007) 66–72

recycled rubber and paving asphalt are combined [1].Characteristics of asphalt–rubber are dependent on rub-ber type, asphalt composition, size of rubber crumbs,and time and temperature of reaction [1]. Usually, therubber is recycled from used automotive tyres, whichhas the additional benefits of saving landfill space thatwould otherwise be occupied by tyres and reducing cost[1,18].

Natural rubber modification results in better ruttingresistance and higher ductility but the modifier is sensi-tive to decomposition and oxygen absorption. Due to itshigh molecular weight, it has problems of low compati-bility [18]. Recycled tyre rubber reduces reflective crack-ing, which increases durability. There are some practicalproblems in using natural rubber: it needs high temper-atures and long digestion times in order to be dispersedin the bitumen [18].

In 1991, the Intermodal Surface Transportation Effi-ciency Act (ISTEA) section 1038 was passed into law inthe USA. In its Declaration of Policy, ISTEA states ‘‘Itis the policy of the United States to develop a NationalIntermodal Transportation System that is economicallyefficient, environmentally sound, provides the founda-tion for the Nation to compete in the global economy,and will move people and goods in an energy efficientmanner’’ [19]. The act required that, starting in 1994,5% of roads built with federal funds must use pavementmade with crumb rubber, processed recycled tyres, ormodified asphalt. By 1997, 20% of roads built with fed-eral funds were required to use recycled tyres in thepavement [20].

On the other hand, the Used Tyre Working Group[21] describes the United Kingdom as still being in theprocess of evaluating a pilot project involving road sur-facing that contains recycled tyres.

4.2. Styrene–butadiene–styrene

Styrene–butadiene–styrene (SBS) is a block copoly-mer that increases the elasticity of asphalt [18]. Accord-ing to a 2001 review in Vision Tecnologica by Beckeret al. [18], it is probably the most appropriate polymerfor asphalt modification, although the addition of SBStype block copolymers has economic limits and canshow serious technical limitations. Although low tem-perature flexibility is increased, some authors claim thata decrease in strength and resistance to penetration isobserved at higher temperatures. Nonetheless, ‘‘SBS isthe most used polymer to modify asphalts, followed byreclaimed tire rubber’’ [18].

The Danish Road Directorate [22] found that anSBS-modified binder course showed no superior rutresistance compared to other Danish asphalt courses.Asphalt cores taken from the job site indicated thatseparation had occurred, and that the polymer phasewas not homogeneously distributed, which might have

been the cause of the poor performance of the pave-ment.

As reported in the Journal of Material in Civil Engi-neering, transmission electron microscopy was used in2002 to better understand the behavior of SBS in asphaltbinders [23]. Depending on the sources of asphalt andpolymer, morphology varies: there can be a continuousasphalt phase with dispersed SBS particles, a continuouspolymer phase with dispersed globules of asphalt, ortwo interlocked continuous phases. It is the formationof the critical network between the binder and polymerthat increases the complex modulus, an indication ofresistance to rutting.

In 2003, in the Journal of the AAPT, Mohammedet al. [24] looked at the possibility of recycling SBS mod-ified asphalt for resurfacing pavement. They found thatthe impact of the extraction and recovery process on thebinder was minimal. Eight-year-old SBS modified bin-der was recovered from Route US61 in Louisiana, andwas found to have experienced intensive oxidative agehardening. At low temperatures, the binder was quitebrittle. Blends of virgin and recovered polymer modifiedbinder were found to be stiffer than anticipated at bothlow and high temperatures. It was also found that as thepercentage of recovered binder increased, rutting resis-tance increased, while fatigue resistance decreased.

In 2004, the Florida Department of Transportationand FHWA published a report [25] looking at the effectof SBS modification on cracking resistance and healingcharacteristics of Superpavee mixes. They found thatSBS benefited cracking resistance, primarily due to a re-duced rate of micro-damage accumulation. SBS did not,however, have an effect on healing or aging of the as-phalt mixture.

The possibility of using SBS-modified binders in In-dia has been investigated recently [26]. Calculations indi-cated that the surface life of the Delhi–Ambalaexpressway would be almost doubled while the thicknessof the bituminous layers would be reduced, although thecost per km would be greater for polymer modifiedbinders.

4.3. Styrene–butadiene–rubber

Styrene–butadiene–rubber (SBR) has been widelyused as a binder modifier, usually as a dispersion inwater (latex). An Engineering Brief from 1987 availableat the US Federal Aviation Administration website [2]describes the benefits of SBR modified asphalt inimproving the properties of bituminous concrete pave-ment and seal coats. Low-temperature ductility is im-proved, viscosity is increased, elastic recovery isimproved and adhesive and cohesive properties of thepavement are improved. The benefit of latex is that therubber particles are extremely small and regular. Whenthey are exposed to asphalt during mixing they disperse

Y. Yildirim / Construction and Building Materials 21 (2007) 66–72 69

rapidly and uniformly throughout the material and forma reinforcing network structure.

According to Becker et al., SBR latex polymers in-crease the ductility of asphalt pavement [18], which al-lows the pavement to be more flexible and crackresistant at low temperatures, as found by the FloridaDepartment of Transportation [25]. SBR modificationalso increases elasticity, improves adhesion and cohe-sion, and reduces the rate of oxidation, which helps tocompensate for hardening and aging problems [25].

In a 1999 laboratory test at the Texas TransportationInstitute, it was found that coating smooth, rounded,siliceous gravel aggregates with cement plus SBR latexfor use in HMA increased stability according to Hveemand Marshall standards, as well as tensile strength, resil-ient modulus and resistance to moisture damage. Coatedaggregates have greater resistance to rutting and crack-ing [27].

Water-based SBR latex has been widely used to im-prove chip retention in emulsions, but SBS has gradu-ally replaced latex because of its effect of greatertensile strength at strain, and because it is compatiblewith a broader range of asphalts [1]. Elastomers suchas SBR and SBS have a significant effect on the resultsof the ductility test at both 4 and 25 �C; while SBR mod-ified asphalts have high ductility at all temperatures,SBS modified asphalts tend to have lower ductility [1].

4.4. Elvaloy�

The Duponte website [28] describes Elvaloy� as anethylene glycidyl acrylate (EGA) terpolymer that chem-ically reacts with asphalt. As a result of the reaction,problems with separation during storage and transpor-tation are avoided. Roads using Elvaloy� have been inuse since 1991.

In 1995 Witczak, Hafez and Qi [29] studied the labo-ratory performance of asphalt modified with Elvaloy�

at the University of Maryland. Two different grades ofasphalt were each modified by 0%, 1.5% and 2.0% Elva-loy� by weight of binder. The susceptibility of the mix-tures to moisture damage was found to be greatlydecreased by the addition of Elvaloy�. In addition, an

% Recovery ¼ Initial elongation�Observed elongation after rejoining sample

Initial elongation� 100

analysis of repeated load permanent deformation behav-ior showed that increasing concentrations of Elvaloy�

resulted in a marked decrease in deformation.In a study on low-temperature rheological properties

of polymer modified binders, the FHWA [30] found that

Elvaloy� in combination with granite had a significantlyhigher (poorer) fracture temperature than with diabase,limestone or granite aggregate treated with hydratedlime.

At the DuPont Institute, Babcock et al. [31] devised alap shear test for high temperature binder properties,which appears to agree with high temperature DSRmeasurements. The results indicated that binder failureat temperatures above 6 �C tends to be cohesive failure,due the loss of integrity within asphalt. On the otherhand, around 6 �C and colder, failure is adhesive, froma loss of adhesion between the binder and the substrate.Since this indicates that cold temperature failure of aroad may be the result of loss of adhesion to the aggre-gate, a chemically reactive polymer is expected to per-form better, and reactive elastomeric terpolymer doesin fact perform better in this test than SBS or the controlneat bitumen.

5. Elastic recovery test

Elastic recovery (or elasticity) is the degree to which asubstance recovers its original shape following applica-tion and release of stress. A degree of elastic recoveryis desirable in pavement to avoid permanent deforma-tion. ‘‘When a tire passes over a section of pavement,it is desirable for that pavement to have the ability to�give�, but it is equally important for it to recover to itsoriginal shape,’’ according to the Asphalt Institute web-site [32].

5.1. Measurement and calculation

The elastic recovery of asphalt is measured with theaid of a ductilometer, which is used to elongate an as-phalt specimen at a constant rate. After a period of time,the elongated specimen is cut and then allowed to rest.After the period of rest is complete, the distance betweenthe ends of the cut specimen is measured [33].

The elastic recovery is the ratio between the differencein elongation between cutting and the end of the restperiod, and the total elongation applied [33].

5.2. Binder characterization

The elastic recovery test is used to test polymer mod-ified binders by the departments of transportation of sev-eral states in the US and several other countries, as well

70 Y. Yildirim / Construction and Building Materials 21 (2007) 66–72

as by researchers around the world. According to theSpring 2002 edition of the Asphalt Technology News[34], Kansas, Louisiana and Texas require use of the elas-tic recovery test to ensure that binders have been modi-fied. Michigan also uses it, although it does not requireit, and Kentucky uses it to test PG 76–22 binders. It isalso used to characterize polymer modified binders inQuebec, Sweden, Finland and Switzerland [35].

In 1981, Oliver developed the elastic recovery testfor the Australian Road Research Board to measuredeformation response of rubber modified binders[36,37]. He found that binders with natural (truck-tyre)rubber showed greater elastic recovery than synthetic(car-tyre) rubber. In 1997, it was reported to the Aus-tralian Asphalt Pavement Association that even lowconcentrations of SBS caused an increase in elasticrecovery, softening point, viscosity and cohesivestrength [38].

In 1990, Valkering and Vonk [39] compared SBS andEVA modified binders to neat binders and found thatSBS modified binders had significantly higher elasticrecovery than neat binders. Compared to SBS, EVAmodified binders showed a lesser degree of improvementin elastic recovery and also lost ductility and elasticrecovery much more rapidly.

Braga and Corrieri [40] used the elastic recovery testto compare the resistance to thermal degradation ofSBS and heterophasic polyolefin (TPO) modified bind-ers. Aged SBS polymers following showed a lower resis-tance to thermal degradation than aged TPO binders.

Several studies have investigated the relationship be-tween measurements of elastic recovery and other mea-sures of performance, in both laboratory and field tests.

In the Transportation Research Record 1996, Bone-mazzi et al. [41] compared the performance of bindersmodified with an array of polymers (atactic propylene–ethylene copolymer, low- and high-density polyethylene,ethylene/propylene rubber, ADFLEX, ethylene methac-rylate copolymer, EVA, thermoplasticpolyolefinic ter-polymer, and SBS linear and radial block copolymers)in tests of penetration and elastic recovery as well asthe rheometer dynamic test. All the tests were shownto be good measurements of polymer contribution tobinder performance.

In 1997, Oliver [42] examined the relationship betweenthe rheological properties of asphalt mixes and ruttingresistance using the wheel tracking test. While a relation-ship between polymer consistency and rut resistance wasfound, no relationship was apparent between rut resis-tance and elastic recovery or softening point.

In the Journal of the AAPT, 1998, Bahia, Perdomoand Turner [9] compared five modified binders, measur-ing elastic recovery, ductility and resilience. They foundthat these conventional measurements were inconsistentin ranking the suitability of polymer modified binders.Specifically, rankings changed as strain level changed.

Superpavee testing results were equally inconsistent inranking modified binders.

In a 1988 Iowa Department of Transportation report,Lee and Demirel [43] compared viscosity, penetration,softening point, force ductility, elastic recovery and sev-eral other characteristics of binders with SBS, polyole-phins, neoprene, SBR latex and hydrated lime. Therewas no correlation between the different types of mea-surement.

John D�Angelo, on the Asphalt Institute web page[44], points out that the literature suggests that mosttests of modified binders may only measure whether apolymer modifier is present, not its effect on the fieldperformance of the modified binder.

6. Conclusions

In the 1980s, polymer modified asphalts began to beused in the US and by 1997 all but three states were al-ready using modified binders or intended to use them inthe future and federal regulations supported their use.Pavements made with modified binders are more resistantto fatigue, thermal cracking, rutting, stripping, and tem-perature susceptibility than neat binders. Polymer modi-fied binders tend to exhibit increased viscosity and elasticrecovery, although penetration does not appear to beinfluenced by modification. An ideal modifier will in-crease binder resistance to multiple types of distresses.Modification is not without its drawbacks, however, sincecompatibility between an asphalt and a modifier is not as-sured, and separation during storage or application, ifnot addressed, can result in poorly performing pavement.

Since Superpavee specifications were designed forneat binders, they are inappropriate for polymer modi-fied binders. In fact, asphalts modified with differentpolymers can behave very differently even when theyhave the same performance grade. Test methods thathave been developed or altered for modified binders in-clude measuring the softening point and elastic recovery,and a force ductility test. There is disagreement aboutwhether bending beam rheometer (BBR) tests, devel-oped for Superpavee, are acceptable for polymer mod-ified binders. In general, it seems that the results ofrheological tests are not indicative of the performanceof polymer modified binders. Several tests exist to iden-tify whether modification is present, such as the IR, lowtemperature ductility and torsional recovery. In 1991,the ISTEA required that an increasing proportion ofroads use modified asphalt.

Different polymers impact characteristics of asphaltto differing degrees.

� Natural rubber improves rutting resistance and ductil-ity, but is sensitive to decomposition and often hasproblems of compatibility.

Y. Yildirim / Construction and Building Materials 21 (2007) 66–72 71

� The use of tyre rubber as an asphalt modifier is envi-ronmentally responsible and results in decreased rut-ting and reflective cracking, but special conditions,such as high mixing temperatures and long digestiontimes, need to be maintained to prevent separationfrom the asphalt binder.� The addition of SBR to asphalt improves low-tem-

perature ductility, increases viscosity, improves elas-tic recovery and improves the adhesive and cohesiveproperties of the pavement. Water-based SBR latexwas used commonly to improve chip retention inemulsions.� SBS has been replacing SBR due to the former�s

wider compatibility and greater tensile strength understrain. SBS is now the polymer most used to modifyasphalt. SBS increases the elasticity of asphalt andSBS modified asphalt can be recycled. SBS modifiedbinders have been found to perform better at lowtemperatures than neat binders or binders modifiedwith chemically reactive polymers.� Elvaloy� is a modifier that forms a chemical bond

with the asphalt, avoiding problems of separationduring storage, transportation and application. Itincreases pavement moisture resistance and resultsin modified asphalt performing better in high temper-ature DSR tests.

Elastic recovery of asphalt, a measurement widelyused to test polymer modified binders, can be measuredby elongating an asphalt sample, cutting it, allowing itto rest, and determining the degree to which the elon-gated specimen returns to its original length.

The elastic recovery test has been shown to be a goodmeasurement of polymer contribution to binder perfor-mance, although no relationship appears to exist be-tween rut resistance and elastic recovery. Elasticrecovery and other conventional measurements areinconsistent in ranking polymer modified binder perfor-mance and may only measure whether or not a modifieris present in an asphalt specimen, not its contribution tothe asphalt�s performance.

Polymer modified binders have had proven successin the field and the laboratory, and a continuing effortis being made to develop a correlation between resultsfrom laboratory tests and field performance.

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