Characterization of Fatigue and Healing in Asphalt Binders

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Characterization of Fatigue and Healing in Asphalt BindersShihui Shen, A.M.ASCE1; Ho-Ming Chiu2; and Hai Huang3

Abstract: Asphalt mixtures have two healing mechanisms: adhesive healing at the asphalt-aggregate interface and cohesive healingwithin asphalt binders. This study investigates the effects of cohesive healing exclusively, without considering the interaction of aggre-gates. This study also introduces a methodology of quantifying healing using the dissipated energy approach and the dynamic shearrheometer test with a specifically designed intermittent loading sequence. The ratio of dissipated energy change approach, which is basedon the fundamental concept of dissipated energy and has been successfully applied to study hot mix asphalt �HMA� mixture healing, isapplied to the binder. By doing so, a healing rate, defined as the rate of dissipated energy recovery per unit of rest time, is used to quantifythe healing potential of asphalt binders. The results indicate that binder type, strain level, and temperatures have important impact onhealing. It is recommended that the research methods introduced in this study be further applied to asphalt mastics and sand asphalt mixesto study adhesive and cohesive healing in HMA mixtures and provide fatigue-healing information for pavement design.

DOI: 10.1061/�ASCE�MT.1943-5533.0000080

CE Database subject headings: Asphalts; Binders, material; Fatigue; Energy dissipation.

Author keywords: Asphalt binder; Fatigue; Healing; Dissipated energy; Rest period.

Introduction

Pavement structures are susceptible to damage in the form ofcracks, which form deep within the structure where detection isdifficult and repair is very expensive. In flexible pavements,cracking usually starts from invisible microcracks, leading to me-chanical degradation of asphalt materials. Under the applicationof traffic and environmental loading, damage accelerates and thecracks propagate until visible macrocracks appear. Once macroc-racks appear, maintenance and rehabilitation work become neces-sary. Pavement engineers have often believed that cracking inpaving asphalt materials is not reversible. Once cracks hadformed in the asphalt materials, they were assumed to continu-ously propagate regardless of the future loading conditions.

Recently, an increasing number of researchers �Raithby andSterling 1970; Bonnaure et al. 1982; Bazin and Saunier 1967;Pronk and Hopman 1991; Sias 1996; Williams et al. 2001; Littleet al. 1999; Kim et al. 2002, 2003; Song et al. 2005; Carpenterand Shen 2006; Shen 2006� have demonstrated phenomenologi-cally that asphalt mixtures have the capability to heal cracks.Healing can be defined as the self-recovery capability of asphaltmaterials under certain loading and/or environmental conditions,especially during rest time. Studies �Raithby and Sterling 1970;Bonnaure et al. 1982; Bazin and Saunier 1967; Pronk and Hop-man 1991; Sias 1996; Williams et al. 2001; Little et al. 1999; Kim

1Assistant Professor, Washington State Univ., Pullman, WA 99164�corresponding author�. E-mail: [email protected]

2Graduate Research Assistant, Washington State Univ., Pullman, WA99164. E-mail: [email protected]

3Research Assistant Professor, Washington State Univ., Pullman, WA99164. E-mail: [email protected]

Note. This manuscript was submitted on January 29, 2009; approvedon December 20, 2009; published online on January 5, 2010. Discussionperiod open until February 1, 2011; separate discussions must be submit-ted for individual papers. This paper is part of the Journal of Materialsin Civil Engineering, Vol. 22, No. 9, September 1, 2010. ©ASCE, ISSN
0899-1561/2010/9-846–852/$25.00.
846 / JOURNAL OF MATERIALS IN CIVIL ENGINEERING © ASCE / SEPTE

J. Mater. Civ. Eng. 201

et al. 2002, 2003; Song et al. 2005; Carpenter and Shen 2006;Shen 2006� have shown that when given a certain amount of resttime among continuous loading sequence in a fatigue test, themodulus of hot mix asphalt �HMA� mixtures can have some in-crease after the rest time and the overall fatigue life can be in-creased. These studies have led to a new way of describing theperformance of asphalt paving materials: asphalt materials arehealable. A thorough qualitative and quantitative understanding ofasphalt material healing mechanism and potential will help withthe design and selection of asphalt materials with higher healingpotential, higher damage resistance, and longer service life.

As a complex composite material, asphalt mixture consists oftwo major components: asphalt binder and aggregates. Williamset al. �2001�, Kim et al. �2003�, Song et al. �2005�, and Bahia andZhai �1999� proposed that in HMA mixtures two main types ofhealing exist: adhesive healing at the asphalt-aggregate interfaceand cohesive healing within the viscoelastic asphalt binder. Thisnormally makes the study of healing in HMA quite complex. It isan appropriate starting point to study asphalt binder only, withoutincluding the interaction of aggregate asphalt in the mix. Thisapproach will allow us to investigate more fundamental asphaltmaterial properties and damage behavior, therefore establish waysto evaluate and quantify asphalt healing capability. The objectiveof this study is to qualitatively and quantitatively examine thecohesive healing that happens mainly within the asphalt binder,and describe how binder types and loading conditions may affecthealing. It is expected that the methodology developed in thisstudy can be extended to future studies of taking into account theadhesive healing at the aggregate-asphalt interface. Because heal-ing has been hypothesized as being the key reason of fatigueendurance limit by balancing the damage created by loading �Car-penter and Shen 2006�, the understanding of healing characteris-tics in asphalt materials will eventually lead to the integration ofthe concepts of healing and “fatigue endurance limit” into
mechanistic-empirical based pavement design procedures.
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Research Approach–Dissipated Energy Approach

This study applied the dissipated energy �DE� concept and theratio of dissipated energy change �RDEC� approach to describethe fundamental asphalt binder damage and healing phenomenonby investigating the energy changes during the process of externalloading. The research findings presented an evaluation of asphaltbinder’s healing potential, a result that supports the healing po-tential determined by the surface energy approach used by otherresearchers �Kim et al. 2003; Song et al. 2005�. At the same time,the results quantified the amount of healing, which was related totemperature, stress/strain level, binder properties, and rest time.This information is especially important for HMA mix design forthe inclusion of healing consideration in mixtures’ fatigue perfor-mance prediction �Shen and Carpenter 2007�.

The DE approach has always been considered one of the mostfundamental approaches to solve engineering problems, espe-cially if damage and cracking are associated with material me-chanical deformation �Pronk and Hopman 1991; Baburamani andPorter 1996; Van Dijk and Visser 1977; Shen and Carpenter 2005;Shen et al. 2006; Carpenter et al. 2003�. In HMA engineering,when sustaining cyclic fatigue loading, the viscoelastic HMA ma-terial traces different paths for the loading cycle �LC� and unload-ing cycle and creates hysteresis loops. The area inside of the loopis the DE. In general, energy dissipated in a LC depends on theenergy applied in the previous cycle, or in other words, it is his-tory �path� dependent. The DE at each LC for a viscoelastic as-phalt material can be calculated by using the following equations:

Controlled strain test: DEi = ��2Gi� sin �i �1�

Controlled stress test: DEi = ��2

Gi�sin �i �2�

where DEi=the dissipated energy at cycle i; � ,�=the controlledstress or strain, �i=the phase angle at cycle i; and Gi

�=the com-plex modulus of asphalt material at cycle i.

Previous work by Carpenter and Jansen �1997�, Ghuzlan andCarpenter �2000�, Carpenter et al. �2003�, Shen and Carpenter�2005�, and Carpenter and Shen �2006� demonstrated that not allDE is responsible for fatigue damage. Only the relative amount ofenergy dissipation coming from each additional load cycle, whileexcluding the energy dissipated through passive behaviors such asplastic DE, viscoelastic damping, and thermal energy, will pro-duce further damage. This incremental value has a direct relationto damage accumulation. A similar description by Delgadillo andBahia �2005� also suggests that the relative change in DE due torepeated cycling is a good direct physical indicator of the initia-tion and propagation of fatigue damage. A low amount of relativeenergy dissipation can be found either in high fatigue resistantmaterials, low external loading amplitude, or both �Shen and Car-penter 2005�. This relative energy dissipation can be correlatedwith fatigue damage of HMA mixtures by using the RDEC todescribe fatigue. The RDEC is calculated based on the followingequation:

For controlled stress mode: RDECn =�DEn − DEm�

DEm � �n − m�

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For controlled strain mode: RDECn =�DEm − DEn�

DEm � �n − m��3�

where RDECn=ratio of dissipated energy change value at cycle n;DEm ,DEn=dissipated energies at cycles m and n; and m ,n=loading cycle, m�n.

The RDEC versus the number of LCs’ curve typically can bedistinctly divided into three stages, as shown in Fig. 1, for HMAmixtures using constant strain loading mode. The three stagescorrespond to the three stages of the DE versus LC plot, as givenin Fig. 2, for both HMA mixtures and asphalt binders. The stageof most interest is the second stage where the change in DE andthe RDEC is almost constant, for both constant stress and con-stant strain loading modes. This stage is referred to as the plateaustage by Carpenter et al. �2003�, Shen and Carpenter �2005�, andGhuzlan �2001�, as shown in Fig. 1. By using the change in DE,this approach unifies the different loading modes because theslope of the DE versus LC curve maintained constant in the sec-ond stage for both constant strain and constant stress modes.

The plateau value �PV�, the representative value of the RDECparameter, is defined as the RDEC value that corresponds to thenumber of LC at the 50% initial modulus reduction. In order toobtain a representative PV from fatigue testing data and minimizethe error due to testing data variation, a linear regression wasperformed for a DE versus LC curve, and the slope of the curvewas obtained. The regression follows a standard rule �e.g., start-ing and ending points� to make sure the consistency of the fittingresults. For example, the fitting segment from the beginning ofthe data to 10% initial modulus reduction cycle is selected for thebinders’ controlled stress tests conducted in this study. This is to

III

III

Plateau Stage

Load

RatioofDissipatedEnergyChange

PlateauValue

Fig. 1. Typical RDEC plot for constant strain HMA fatigue test withthree behavior zones �23�

Fig. 2. DE versus LCs for HMA mixtures and asphalt binders testedusing different loading modes

ERIALS IN CIVIL ENGINEERING © ASCE / SEPTEMBER 2010 / 847

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ensure that the curve segment used for regression is in plateaustage where the change rate of DE is almost constant.

This PV was demonstrated in literatures �Shen and Carpenter2005� to be a unique parameter that can be fundamentally relatedto the fatigue life of HMA mixtures, regardless of the materialtype, loading modes, and testing conditions. In this study, thetraditional 50% initial modulus reduction definition of fatiguefailure �Nf50�, which has been widely used by many researchers�Johnson et al. 2007; Tsai and Monismith 2005�, was adopted.Although arbitrary, Tayebali et al. �1993� found that the 50%initial modulus reduction definition of fatigue life �Nf50� can berelated to field failure in asphalt pavement by using a shift factor.Also, Ghuzlan �2001� showed a strong correlation between the50% stiffness reduction failure and the true fatigue �Stage III inFig. 1� based on the DE analysis. The readers are referred toprevious publications by Shen and Carpenter �2005� and Shen etal. �2006� for the details of the RDEC approach. The unique re-lationship between PV and Nf50 is significant because it links thefatigue life of the asphalt material to an energy parameter thatdescribes the fundamental energy behavior of the asphalt. Litera-ture has shown that the PV can be related to HMA mixture’sfatigue endurance limit �Shen and Carpenter 2005�, healing rate�Carpenter and Shen 2006�, and can be predicted based on fun-damental material properties �Shen and Carpenter 2007�.

Fatigue and Healing Study

Healing Test Method

Although it is not a standard testing method to evaluate the fa-tigue characteristics of asphalt binder, oscillatory shear testingusing a dynamic shear rheometer �DSR� has been considered auseful technique and has been extensively used as it allows thestrain amplitude and the timescale �loading frequency� to be var-ied independently �Bahia and Zhai 1999; Giacomin and Dealy1993; Martono et al. 2007�. It is also able to simulate the nature oftraffic loading in the application of asphalt pavements and allowsthe evaluation of stress-strain loops and energy dissipation veryeffectively �Bahia and Zhai 1999�.

Edge effects in parallel plate setup of the DSR testing havebeen a big concern because such setup can cause heterogeneousflow �plastic flow� of asphalt binder especially at high tempera-tures. Anderson et al. �2001� suggested to perform the binder’sfatigue test at lower temperatures �higher initial complex modulusG�� in DSR testing to ensure the binder fails in the form of “fa-tigue” rather than “instability flow.” For the two testing tempera-tures �15 and 25°C� conducted in this research, hairline crackswere observed propagating toward the center of the binder whenthe plates were removed from the rheometer, as shown in Fig. 3.Also, in the 25°C sample, in addition to the hairline cracks, someplastic flow was observed at the periphery of the sample.

Due to the advantages of extending the strain range and thedisadvantages of other geometries, the parallel plate setup is stillused by many researchers including the authors in this study.Bahia and Zhai �1999�, Delgadillo and Bahia �2005�, and Johnsonet al. �2007� compared the results from parallel plate and cone/plate �cylinder torsion testing� for a few asphalt binders and con-cluded that results are very similar and differences are negligible.They suggested that the geometry of parallel plate can be usedand the errors resulting from the heterogeneous flow field can beaccepted as part of the experimental error.

This study uses a Gemini 150 DSR from Malvern Inc. to con-



duct fatigue and healing testing of asphalt binder. Compared tothe previous generation of DSRs, the newer version of DSR canapply an extended torque range up to 150 mN m without havingcompliance effects. The special design of the Malvern Gemini150’s software and hardware gives it the capability of using anintermittent loading sequence with short rest periods �RPs� addedafter every certain number of cycles. The low inertia �total systeminertia with plates and motor� and the excellent bearing torquemapping technology offered by Malvern records quick and accu-rate results after every RP. The DSR testing can record the bind-er’s complex modulus �G��, phase angle, stress, and strain forevery designated time period at controlled temperature and fre-quency using either controlled stress or controlled strain loadingmodes.

A similar intermittent loading sequence as used by Carpenterand Shen �2006� for characterizing mixture’s fatigue and healingis adopted in this study for binder testing. A short constant RPvarying from 0 to 6 seconds is inserted after every 10 load pulsesto simulate a RP between loads. This loading sequence designprovides better simulation of field loading conditions comparedwith interrupted loading sequence �continuous loading–full stopfor longer RP–resume continuous loading�. At the same time itmaintains the continuous loading nature so that the DE can still becalculated for each load cycle, which allows DE analysis to beperformed.

Materials and Testing Conditions

Two types of asphalt binder, PG64-28 and PG70-28, commonlyused in the State of Washington, were used in this study. Theyrepresent neat binder and polymer modified binder, respectively,and were hypothesized to have different healing capacities. It isworth noting that for the purpose of identifying fundamental heal-ing characteristics of asphalt binder and to investigate variousfactors that may affect healing, an original binder with no agingeffect was considered in this study.

Both binders were tested at two different temperatures �15 and25°C�, 10 Hz frequency, with constant stress loading mode andwith various RPs. The tests used an 8-mm spindle with a 2-mmgap between the plates. Three repetition tests were conducted tocheck the repeatability of the results. Constant stress loadingmode was selected because under constant stress mode, the DSR

Fig. 3. Binder DSR sample �PG64-28, 15°C, 10 Hz, 180 kPa� withhairline fatigue cracking

equipment can have quicker response and thus maintain a stable

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sinusoidal loading wave when a RP is introduced. For the failurecriteria, the traditional 50% stiffness reduction approach is used tobe consistent with asphalt mixture fatigue tests.

Results and Discussions

Traditional Fatigue AnalysisIn order to be comparable with most existing fatigue study results,the traditional strain-fatigue life �Nf50� relationship was devel-oped for the binders’ controlled stress fatigue testing with no RPadded. In the controlled stress loading modes, the initial strain, anaverage of the strain from cycle 170 to 300, was selected asrepresentative strain. As shown in Fig. 4, under the log-log plot,the strain-Nf50 follows a linear relationship for each binder andtesting condition. Specifically, the lower temperature producedshorter fatigue life at the same initial strain level. This informa-tion confirms many existing research findings and supports cur-rent design considerations, which related fatigue damage to strainof asphalt mixtures �Bahia and Zhai 1999; Monismith et al. 1985;Tayebali et al. 1994�.

Fig. 5 gives a complex shear modulus G� versus LC curve forthe PG64-28 binder at 60-kPa controlled stress level with variousRPs. As observed in this study and other studies �Bahia and Zhai1999�, phase angle is not sensitive to fatigue damage. During thefatigue test, the phase angle only has a marginal �limited to a fewdegrees� increase. Therefore, the complex shear modulus �G�� canbe used to represent the deterioration of material performance due

Fig. 4. Traditional strain-Nf relationship for binder’s fatigue testingresults

6 sec rest

4 sec rest

2 sec restno rest

Fig. 5. G� deterioration curve for the PG64-28 binder with differentRPs

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to cyclic fatigue loading. As shown, although starting from almostthe same initial complex modulus G� value, the G� in the testswith longer RPs traveled much longer before they reached the50% initial G� reduction. Fig. 5 provided a qualitative indicationof the effect of healing due to RPs. In the next sections, an energyanalysis will be provided to quantitatively compare the healingpotential of different binders under different testing conditions.

DE Change and the RDEC ApproachUnder a controlled stress loading mode, the development of DE ina binder’s healing test shows two distinctive stages �Figs. 6 and 2lower right figure�: �1� the DE increases gradually with an almostconstant slope �constant energy changes� and �2� the DE increasesdramatically �damage propagates and true fatigue failure hap-pens�. This behavior is similar to the DE evolution found in aHMA fatigue test �Shen and Carpenter 2005� but without the firststage of energy stabilization, which may be mainly associatedwith aggregate reorientation and densification inside of the mix.Similar DE-LC plots �Fig. 6� have been found in both testedbinders �PG64-28 and PG70-28�, at different temperatures �15and 25°C�, and at various controlled stress levels, which indi-cated the possibility of applying the RDEC approach to evaluatebinder’s fatigue and healing behavior.

In this study the RDEC and the PV for all binders tested atdifferent conditions were calculated following the same proce-dure, as detailed in the “Research Approach–Dissipated EnergyApproach” section. The results were plotted versus fatigue lifeNf50 in Fig. 7. As shown, no matter the binder type �PG64-28 andPG70-28�, temperature �15 and 25°C�, stress level �varies from60 to 230 kPa�, and with or without RP �0–6 s�, all data pointsfollow a unique PV-Nf line with an R-square value of 0.975. This

Fig. 6. Typical DE-LC curve for a binder fatigue testing

Fig. 7. PV versus fatigue life �Nf50� plot for all tested binders


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provides more substantiation that the DE based RDEC approachand the unique PV-Nf relationship, which has been validated forHMA mixtures �Shen and Carpenter 2005, 2007�, is also appli-cable for asphalt binders.

Also indicated in Fig. 7 is the unique PV-Nf curve developedfor 19 typical Illinois HMA mixtures using linear regression. Asshown, the binder’s PV-Nf curve has a similar slope as the mix-ture’s, but with a different interception point. The effect ofaggregate-binder interaction is hypothesized to be responsible forthe difference between the two curves, which requires furtherevaluation.

Quantifying Healing Based on PV-RP RelationshipThe PV values for the binder’s healing tests were calculated usingthe RDEC analysis described in the “Research Approach–Dissipated Energy Approach” section. These PV values were thenplotted against RPs in Figs. 8 and 9, using log-log format. Inorder to include zero RP into the log-log plot, the RPs were trans-formed into �RP+1�. As it can be seen, with the increase of RPfrom 0 to 6 s, the energy parameter, PV, decreases, which

Fig. 9. PV− �RP+1� for the PG70-28 binder

Fig. 8. PV− �RP+1� for the PG64-28 binder

Table 1. Healing Rates for Different Testing Conditions

Binder typeTemperature

�°C�Stress�kPa�

Initi

PG64-28 25 60

PG64-28 25 70

PG64-28 15 180

PG70-28 25 60

PG70-28 25 70

PG70-28 15 230



uniquely corresponds to an extended fatigue life. This trend issimilar for both PG64-28 neat binder and PG70-28 polymermodified binder.

The slope of the PV− �RP+1� curve indicates the rate of PVdecreasing with the increase of rest time per second, showing adifferent healing rate for the specific binder tested at a specifiedtesting condition �temperature and stress/strain level�. The higherthe slope of the PV− �RP+1� curve �healing rate�, the higher thePV recovery per second of rest time, hence the higher healingpotential the binder has. At intermediate room temperature�25°C�, the PG64-28 neat binder has a healing rate of ��0.967�at 70-kPa controlled stress level and a healing rate of ��1.09� at60-kPa controlled stress level. The negative sign means that thePV is being reduced; i.e., damage is being recovered. The poly-mer modified binder PG70-28 has higher healing rates at both 70-and 60-kPa controlled stress levels, ��1.49� and ��1.60�, respec-tively. This finding agrees with Carpenter and Shen’s �2006� find-ing for mixtures that polymer modified HMA mixtures had ahigher healing rate than neat binder HMA mixtures. In that study,the PG64-22 binder mix was showed to have a healing rate of��0.9069� while the PG76-22 binder had a healing rate of��1.352� at 20°C. Although not directly comparable, the similar-ity of the healing rate in HMA mixtures and binders for neat andpolymer modified binders suggested that the effect of healingfound in mixtures can be attributed mainly to the cohesive healingeffect within asphalt binders.

Factors (Temperature and Stress/Strain) Affecting HealingThe healing tests were conducted at two temperatures and variousstress levels to evaluate the effect of stress/strain and temperature.The results are given in Table 1. Although previous research�Bonnaure et al. 1982; Little et al. 1999� has pointed out thelimited effect of stress/strain level on binder healing, the testingresults from this study suggest differently.

The common assumption is that at lower temperatures, theasphalt binder is more viscous and less flowable; hence, there isless healing effect and fatigue life extension. However, whenlooking at Table 1 and Fig. 8, the healing rate at 15°C is muchhigher than at 25°C for the PG64-28 binder, which translates intoa greater healing effect at a lower temperature. This finding iscontradictory to the common assumption of the temperature effecton healing when other impact factors are not taken into account.In fact, the initial strain level may play a significant role in thehealing scenario. At 15°C, even though the stress level was in-creased to 180 kPa, the corresponding initial strain was only1.74%. This initial strain was much less than the ones at 60–70kPa and 25°C �3.27–3.45%� because of the significant increase inmodulus. Therefore, the increase in the healing rate can be mainlyattributed to the decrease in the initial strain level. In other words,strain will affect healing rates inversely. One possible explanationfor this phenomenon is that at large strains, materials are forced to

n Initial G�

�MPa� Initial DE Healing rate

1.95 5.33 1.09

2.14 6.56 0.97

10.85 7.88 1.49

1.95 5.2 1.60

2 6.9 1.49

6.75 20 0.50

al strai�%�

3.27

3.45

1.74

3.3

3.75

3.5

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deform more, which can result in greater distance at the microc-rack interface, hence, less healing capacity. From a micromechan-ics approach, at higher strains, molecules of asphalt binder arepulled farther away from each other resulting in a larger distancebetween crack surface and smaller molecular forces to attractmolecules moving back to their equilibrium position. Therefore,there is less healing potential to close the microcracks at the crackinterface. Similar discussion has been raised by van der Zwaag�2007�, who suggested that healing is more prone to happen whenthe crack surfaces are in close contact. This also suggests thathealing is much easier to realize in situations of partial crackingin which the noncracked region ahead of the crack tip keeps thefracture surfaces in some registry.

This finding is further supported by the PG70-28 binder testingconducted in this study. A stress sweep test, continuous oscillationwith 20-kPa controlled stress increment every 200 cycles startingfrom 150 kPa, was first conducted at 15°C temperature to find astress level that produces a similar initial strain as measured at25°C temperature. This stress level was selected as 230 kPa.Healing tests at 230-kPa constant stress level and 15°C were thenconducted. As shown in Table 1 and Fig. 9, with similar initialstrain, the healing rate of the PG70-28 binder at 15°C is muchsmaller than at 25°C. This should be regarded as the real effect oftemperature on healing: with the decrease of temperature, thehealing potential decreases, and the effect of fatigue life extensiondue to RPs diminishes.

Conclusions and Recommendations

In this study, an energy based RDEC approach is applied to studythe effect of healing on asphalt binder in a qualitative and quan-titative manner. Oscillatory DSR testing based on constant stressloading mode was conducted for two binders: PG64-28 �neatbinder� and PG70-28 �polymer modified binder�. The findingsprovide substantiation that the effect of healing observed in HMAmixtures has an important relation to binder’s healing. This studyalso proposed a promising methodology to study the cohesivehealing in asphalt binders. Without the interaction of aggregates,the healing mechanism within the binders �healing of cohesivebonds� can be investigated more clearly. The results of the paperprovide information for the design consideration that field fatiguelife can be extended due to the existence of RPs between trafficloads.

According to the discussion and analysis described in thispaper, the following findings and future research recommenda-tions are provided:• The specific healing test that was developed for asphalt binder

using DSR testing with intermittent loading sequence is moresimilar to real traffic loading and can provide effective resultsfor evaluating binder’s healing behavior using energy con-cepts.

• Healing, created in the laboratory by introducing RPs in acontinuous DSR oscillation test, is found to extend fatigue lifeto a noticeable degree. The effect of healing for specific bind-ers at specific testing conditions can be quantitatively relatedto a healing rate, the slope of the PV− �RP+1� curve underlog-log plot. The higher the slope of the PV− �RP+1� curve,the higher the healing rate, and thus, the greater healing capac-ity of the binder.

• Binder type has a significant effect on healing potential. Thetesting indicates that the healing/recovery rate of the polymer
modified binder PG70-28 is greater than the neat binder
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PG64-28 tested. With the inclusion of a 6-s RP, the fatigue lifewas extended about 7 times for the PG64-28 binder but 17times for the PG70-28 binder. Further chemical and mechani-cal analysis is recommended to investigate the healing mecha-nism of different binders. Future research should also becorrelated and compared to ongoing surface energy and heal-ing studies conducted elsewhere �Kim et al. 2002, 2003; Bom-mavaram et al. 2009� to investigate the healing potential ofvarious binders and select appropriate binders for pavementstructures.

• The initial strain level can affect the degree of healing. With adecrease of initial strain, the healing rate is increased, giventhat all other conditions are the same. Stress level does notseem to have a clear correlation with the healing effect. Onepossible explanation for such a phenomenon is that the degreeof damage and the contact condition at the crack surface maybe responsible for the correlation between the initial strain andhealing. However, this finding needs further verification. Inparticular, a micromechanics study would help to explain thebehavior of molecules at the crack interface when differentlevels of strain are applied.

• In general, binder becomes less healable at lower tempera-tures, when the same strain is applied. This can possibly beattributed to the material’s less thermodynamic activities atlower temperatures to induce “wetting” and “diffusion,” andthe asphalt binder becomes more viscous and less flowable.

• It is recommended that the method introduced in this paper beapplied to asphalt mastics and sand mix asphalt to qualitativelyand quantitatively evaluate the adhesive and cohesive healingin HMA mixtures.

Acknowledgments

The writers greatly appreciate the financial support from theWashington State University Foundation and from the Washing-ton State University Office of Research.

Disclaimer

The contents of this paper reflect the views of the authors who areresponsible for the facts and accuracy of the data presentedwithin. This paper does not constitute a standard, specification, orregulation.

References

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Characterization of Fatigue and Healing in Asphalt Binders