Needed Characterization of

Modified Bindersin India

S. Anjan Kumar, Department of Civil Engineering, andA.Veeraragavan, Professor of Civil Engineering, Indian Institute of Technology, Chennai.

The thermo mechanical properties of bituminous binder have a major effect on its subsequent in-serviceperformance. The rheology of conventional binders is relatively simple and behavior can be predicted through theuse of traditional tests such as penetration, softening point and viscosity testing at various temperatures. On theother hand the rheology of modified binders is highly complex and, although the results from traditional tests mayindicate a significant improvement in properties, the in-service performance of these binders is not easilycategorized. This paper presents detailed investigations on the physical properties of modified binders incomparison with conventional binder using current Indian specifications. Firstly, it gives a brief description onmodification of bitumen its advantages and limitations. It also looks into the present specifications that arefollowed in India and the need for improved specifications for performance based binder characterization, whichmay provide fundamental explanations with respect to in-service performance.

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IntroductionThe bituminous binder functions asa waterproof, thermoplasticadhesive. In other words, it acts asthe glue that holds the mineralaggregates together to act as astructural layer. In its most commonform, bituminous binder is simplythe residue from petroleum refining.To achieve the necessary propertiesfor paving purposes, binder mustbe produced from a carefullychosen crude oil blend, andprocessed to an appropriate grade.Increasing axle loads, climaticvariations and traffic growth hasposed challenge to paving industryto look into the demands made onthe bitumen pavement construction.In this regard, as early as in 1980smodification to base bitumen wasdone by addition of certain additiveslike polymers, natural rubber, crumbrubber, plastics etc., to enhancethe mechanical behavior of bitumenby physical modification. Alsochemical modifications wasattempted in the form ofpolyethylene, poly-phosphoric acid,etc. The use of modified bitumencan serve a number of purposes. Itcan target a specific improvementin the bitumen, such as permanentdeformation (rutting) or lowtemperature cracking. Benefits thatmay be derived from bindermodification include: Improved consistency Reduced temperature susceptibility Improved stiffness and cohesion Improved flexibility, resilience and

toughness Improved binder aggregate

adhesion Improved resistance to in-service

agingHowever, there needs to be a

way to evaluate whether theperformance of the modifiedbitumen is cost effective.

Study Objective andScopeThe objective of this study can belisted as follow: characterization of the modified

and conventional binders throughan elaborative laboratoryinvestigations

In this investigation, four typesof modified binders and aconventional binder were studied.Modified binders like styrene-butadiene-styrene polymer modifiedbinder (PMB) of two grades viz.,PMB-40 and PMB-70, Crumb RubberModified Binder (CRMB) of twogrades viz., CRMB-55 and CRMB-60, natural rubber modified binder(NRMB), waste plastic modifiedbinder (WPMB) and conventionalbinder (60/70 grade) were studied.From these investigations, theproperties affecting the in-serviceperformance in comparison with thespecifications developed by othercountries was assessed.

BackgroundThere is a marked differencebetween the maximum andminimum temperatures in the

country, so a flexible pavementshould be capable of resisting tothe extreme temperature variationsand to prevent pavement distress.In this regard, a binder modificationis an effective tool to reduce thetemperature susceptibil ity andimprove the strength. Hence anideal modifier should haveenhanced cohesion and very lowtemperature susceptibil itythroughout the ranges of thetemperature to which it will besubjected in service. Its resistanceto permanent deformation andfatigue characteristics should behigh. It should have at least thesame adhesion qualit ies asconventional binders and materialshould be such that the wholecomposition of the mix should behomogenous.

Modified BindersUsed In RoadApplicationsTable 1 shows a genericclassification system that was usedto define and classify modifiers, aswell as other additives inbituminous mixes (IRC: SP: 53-2002).

PolymerA polymer is a very large moleculecomprising hundreds or thousandsof atoms formed by the successivelinking of one or two, or occasionallymore, types of small molecule intochain or network structures (4). Toachieve the goal of improvingbinder properties, a selectedpolymer should create a secondarynetwork or new balance systemwithin binders by molecularinteractions or by reactingchemically with the binder. Theformation of a functional modifiedbinder system is based on the finedispersion of polymer in binder forwhich the chemical composition ofbinders is important. The degree ofmodification depends on thepolymer property, polymer contentand nature of the binder.

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Rubber CrumbsRubber used for these products isderived from pneumatic tyres thathave been processed bymechanical means and should besubstantially free from ground fabric,steel and other contaminants,including moisture. Whenintroduced to the hot binder therubber swells through absorptionof the aromatic fractions of thebinder. As a result of the highblending temperature some of therubber dissolves in the binder andsome is de-vulcanized (14).

Properties and FieldPerformance ofModified BindersIn order to relate binder propertiesto pavement performance, it isnecessary to know the fundamentalrelationships between binderproperties and mix properties. Tominimize the deterioration of aflexible pavement due to influenceof traffic and climate, the bituminouslayers should be stiff enough atelevated service temperature toavoid permanent deformation(rutting), show good load-associatedfatigue resistance, possess goodstripping resistance (low watersusceptibility), and have goodflexibil ity at low temperature

(resistance to low temperaturecracking. All of these performance-related properties of the mix areinfluenced to some extent by binderproperties (5, 6, 10, 12 & 15).

AgingAging is induced by chemical and/or physical changes and is usuallyaccompanied by hardening of thebinder. In road applications, binderis exposed to aging at three differentstages: (i) storage, (ii) mixing,transport and laying, as well as (iii)during service life. Aging is a verycomplex process in neat bindersand the complexity increases whenmodifiers are added. The agingproperties of neat binders arenormally characterized bymeasuring rheological propertiessuch as viscosity and softening pointbefore and after artificial aging inthe laboratory. This procedure isnot sufficient in the case of modifiedbinder since thermolytic degradationof the modifier may occur duringaging and the fragments formedmay contribute to a lowering of theconsistency. Therefore, whenassessing the aging properties ofmodified binder, furthercharacteristics, such as elasticrecovery and chemical compositionhave to be evaluated (1 & 16).Indications of improved aging

properties by admixture of polymersto the binder have been reported inrecent publications (20 & 22).

TemperatureSusceptibilityIn cold climates, cracking inpavements may be an extensiveproblem. Low temperature crackingis caused by thermally inducedtensile stresses when these exceedthe tensile strength of the pavementmaterial. The main factor influencingthe degree of cracking at lowtemperature is found in the binderproperties. Several papers haveindicated that the addition ofpolymers to binders may increaseresistance to low temperaturecracking (13 & 17 to 20). However,validation of laboratory methods byfield performance tests is necessarybefore a more definite opinion onthe matter can be given.

Specifications Basedon Performance(3)In most countries, current binderspecifications are viscosity orpenetration graded and typicallybased on measurements ofviscosity, penetration, ductility andsoftening point. Thesemeasurements are not sufficient todescribe properly the linearviscoelastic and failure propertiesof binder that are needed to relatebinder properties to mixtureproperties. These specifications andtest methods are not performancerelated, because they, lackadequate low-temperaturemeasurements, do not includefundamental binder propertieswhich may be related tofundamental mixture properties orto pavement performance, are notappropriate for measuringconsistency at the upper servicetemperatures, and do not considerlong-term in-service aging. In mostcases, specification used orproposed for modified binders arederivatives of the specifications of

Bituminous Binders

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neat binders and are supplementedwith tests such as tensile strengthand elastic recovery. This is alsothe reason for developing newspecifications within the StrategicHighway Research Program(SHRP), seen in Table 2. In SHRP,new powerful tools for theevaluation of bituminous bindershave been developed. The SHRPbinder specifications are said to beperformance related. In thisspecification, new testinginstruments, such as the bendingbeam rheometer and the directtension tester, are employed. Theinstruments are used to measuremore fundamental properties suchas the inverse of loss compliance,storage modulus, stiffness and

strain at failure. The parametersdetermined have been proposedcollectively as being related to therutting, fatigue and thermal crackingbehavior of binders (2). Short termaging is simulated using the rollingthin film oven test (RTFOT) andlong term aging using the pressureaging vessel (PAV). It is importantto emphasize that proposed SHRPspecifications are intended for bothneat and modified binders, andallow selection of a binder basedon the climate in which it isexpected to perform.

ExperimentsThe main laboratory experimentprogramme envisages thequantitative analysis in assessment

of rheological and empiricalproperties of both neat and modifiedbinder. This chapter presentsproperties of neat and modifiedbinders. Experiments are conductedunder unaged and TFOT (thin filmoven test) aged conditions.

MaterialsThe materials selected for thepresent investigations are: 60/70: Conventional neat binder PMB-40: SBS modified binder PMB-70: SBS modified binder CRMB-55: Crumb rubber modified

binder CRMB-60: Crumb rubber modified

binder NRMB-70: Natural rubber

modified binder

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WPMB-40: Wasteplastic modifiedbinder

All samples weresubjected to a numberof characterizationevaluations according toIS: 73-2006, IS: 15462-2004 and IRC: SP: 53-2002 in AsphaltLaboratory, IIT Madrasby the methods outlinedin the abovespecifications.

ResultsThe test results areshown in the Tables 3 to 9.

It is seen from the results bothconventional and modified bindersto satisfy the specificationrequirements. Only in case ofWPMB, it failed to satisfy theelastic recovery criteria on bothunaged and TFOT agedconditions. Elastic behaviorindicates that the binderrecovers most or all of its initialshape when the load thatcaused deformation is removed.The elastic recovery of a binderis commonly used to measurethe fatigue resistance of abinder or its ability to absorblarge stresses withoutnecessarily cracking ordeforming. From this, it may beobserved that use of wasteplastics as modifier forms a

rigid phase or network, impartingno elastic recovery properties to thebase binder but induces a highstiffness. Loss in weight is alsohigher than the specified limits incase of WPMB, which may attribute

that use of waste(recycled/ non-virgin)plastic as a modifier tothe base bindergradually changes overtime due to heat,oxidation, ultra violetradiation and loss ofvolatile components.Viscosity measurementsof unaged PMBs werehigher than the specifiedwhich may due tohigher concentration ofpolymer itself. Bothelastic recovery andviscosity of NRMB was

lesser than the CRMBs. This maybe due to use of stiffer base binderin case of CRMBs.

Discussions andConclusionsThe aim of this paper is toprovide a summary ofinformation found in the currentliterature regarding testmethods, specifications andperformance of modifiedbinders. The main purpose ofmaterial testing is tocharacterize the material inquestion, in such a way thatthe characteristics measuredcan be used to predictbehavior in practice. For testson binders to be valid, thetests must be sensitive toproperties of the bituminous

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pavements, such as resistance torutting, load associated fatigue andlow temperature cracking. Ingeneral, when assessing the qualityof modified binders, traditionalmethods developed for testing neat

binder, are used.These specificationsare in general basedon empirical testmethods, such asp e n e t r a t i o n ,softening point,ductility and viscositymeasurements, thep e r f o r m a n c erelations of whichare not alwaysobvious. There areseveral reasons whyempirical methods,nevertheless, areused. Above all,

these test methodshave been used for avery long time, andpresent knowledgeon binders is for themost part based onresults obtainedusing these methods.To describe theproperties of modifiedbinders in an overallsituation, the modifiedbinder specificationsshould be supplemented by other testparameters, such asdynamic mechanicalanalysis andcompatibility.

There are threecritical workingranges forbituminous binders:a range of hightemperature andlong loading timesduring which thebinders may flow,entailing a risk ofrutting of the mixes,a range of lowtemperature and

long loading times during whichthe mixes are liable to crack underthe effects of thermal stress and arange of low temperature and shortloading times during which thebinder is brittle and may give rise

to mechanical cracking. The testsused currently for binderspecifications yield little informationon the behavior of binders in thesecritical ranges. Fortunately,advances in rheometers have madeit possible to perform dynamic testsat a wide range of temperaturesand frequencies, from whichvarious rheological parameters (e.g.,complex modulus and phase angle)in different conditions can beobtained. However, recent research(1, 2, 10 & 15) has indicated that dynamicparameters are useful for predictingperformance-related properties.

Aging occurs during theproduction of the bituminous mixand during its service life aspavement layer. The circumstancesduring different aging stages varyconsiderably. Standardized agingtest methods simulate the agingthat occurs during the production ofthe pavement. To simulate longterm aging in service, the PAV testhas been developed in SHRP. Thetest is performed after RTFOT orTFOT aging.

The chemistry of binder is verycomplex and is even more complexafter the admixture of modifier. Nospecifications including requirements on the chemical compositionof binder or modified binder havebeen found in the literature. It isdoubtful whether this type ofrequirement should be included atall in specifications, at least not forplain binder. When characterizingthe aging properties of modifiedbinders, some chemical tests couldbe suitable for specificationpurposes.

ConclusionIn conclusion, using the traditionallytests used to characterize the binder,it is very difficult to analyze andpredict its in-service performancedue to the complexity of the variousmodified binders as a function ofbase binder and the type as wellas the content of the modifier.Hopefully, the dynamic mechanical

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analysis and rheological studies,which are intended for both neatand modified binders, may be moresuitable in predicting binderperformance on roads comparedwith conventional tests. Thepressure aging vessel (PAV)simulates the age hardening ofbitumen during the first 5-10 yearsof pavement service life. Thepressure aging vessel conditioningallows further testing by the dynamicshear rheometer and bending beamrheometer to evaluate the bindersperformance following aging toevaluate whether the performance

of the modifiedbitumen is costeffective in its serviceperiod.

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