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LABORATORY EVALUATION OF FATIGUE DAMAGE AND HEALING OF
ASPHALT MIXTURES
By Jo Sias Daniel,1 Associate Member, ASCE, and Y. Richard Kim,2 Member, ASCE
ABSTRACT: The changes in the stiffness of two asphalt concrete mixtures due to temperature, fatigue damagegrowth, and healing during rest periods are evaluated using the impact resonance method. The impact resonancemethod is a means of determining the dynamic modulus of elasticity of a specimen nondestructively. Thedynamic modulus of elasticity decreases as temperature increases and as microcrack damage growth occurs inthe specimen due to fatigue. The impact resonance method also detects increases in dynamic modulus of elasticityafter the application of rest periods. A gain in flexural stiffness was also observed from measurements and isattributed to closure of microcracks or healing during the rest period. The amount of healing or stiffness gainappeared to increase when specimens were subject to a higher temperature during the rest period. A qualitativestudy of the two asphalt mixtures showed that there is a difference between the two with respect to healingperformance.
INTRODUCTION
The fatigue performance of asphalt pavements is importantnot only for new design but in consideration of rehabilitationand maintenance alternatives for existing structures. Fatigueoccurs due to the repetitive nature of traffic loading; longitu-dinal cracks in the wheel path appear on the pavement surfaceand propagate into alligator cracking patterns as loading con-tinues. The surface cracks can be created through shear forcesat the pavement surface or tensile forces at the bottom of theasphalt layer. This research project uses a beam fatigue ma-chine to induce flexural fatigue damage in testing specimens,similar to the mechanism that creates bottom-up cracking inasphalt pavements. However, the concept of healing duringrest periods to increase fatigue life applied to both mechanismsof crack initiation. The significance of rest periods in extend-ing the fatigue life of asphalt concrete has been well estab-lished by researchers in the last three decades (Raithby andSterling 1970; McElvany and Pell 1973; Bonnaure et al.1982). More recently, research has shown clear evidence of ahealing mechanism occurring in asphalt concrete during restperiods (Whitmoyer and Kim 1994; Kim et al. 1995; Kim andKim 1997). The healing mechanism in polymers has been welldescribed by Prager and Tirrell (1981):
When two pieces of the same amorphous polymeric mate-rial are brought into contact at a temperature above theglass transition, the junction surface gradually develops in-creasing mechanical strength until, at long enough contacttimes, the full fracture strength of the virgin material isreached. At this point the junction surface has in all respectsbecome indistinguishable from any other surface that mightbe located within the bulk material: we say the junction hashealed.
Healing results in a partial regain of stiffness, and fatigue lifeis extended. It is important to understand that the beneficialeffects of rest periods due to healing are mainly due to theclosing of microcracks within the asphalt concrete. Cracks that
1Asst. Prof., Dept. of Civ. Engrg., Univ. of New Hampshire, Durham,NH 03824. E-mail: [email protected]
2Prof., Dept. of Civ. Engrg., North Carolina State Univ., Raleigh, NC27695. E-mail: [email protected]
Note. Associate Editor: Dallas Little. Discussion open until May 1,2002. To extend the closing date one month, a written request must befiled with the ASCE Manager of Journals. The manuscript for this paperwas submitted for review and possible publication on February 18, 1997;revised October 17, 2000. This paper is part of the Journal of Materialsin Civil Engineering, Vol. 13, No. 6, November/December, 2001.qASCE, ISSN 0899-1561/01/0006-0434–0440/$8.00 1 $.50 per page.Paper No. 15113.
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can be seen with the naked eye, or macrocracks, do not closeunless external forces are applied to press the cracked facestogether. In this research, all healing occurred without externalpressure. Increased levels of healing are expected at highertemperatures and have previously been observed by Bonnaureet al. (1982).
This study was part of a project that also included researchon uniaxial constitutive modeling of asphalt concrete duringfatigue damage growth and healing, and in situ evaluation offatigue damage growth and healing of asphalt pavements usingthe stress wave method. Thus, one of the purposes of this studywas to support the findings of these two other research proj-ects, which are summarized in the following section.
The principle objective of the research presented in this pa-per was to qualitatively compare the healing potentials of twodifferent asphalt mixtures when subject to fatigue loading.This was accomplished using a third-point bending beam fa-tigue machine to induce damage and the impact resonancemethod to evaluate the stiffness of the specimens through cy-cles of damage and healing. The practicality of this researchis in demonstrating the potential difference in fatigue lives ofmixtures made with different asphalt cements. If asphalt ce-ments, showing a greater potential for healing during rest pe-riods, are used in construction, the fatigue lives of pavementswill be extended.
SUPPORTING RESEARCH
Constitutive Modeling of Asphalt Concrete
A uniaxial viscoelastic continuum damage model was de-veloped by applying the elastic-viscoelastic correspondenceprinciple to separate out the effect of viscoelasticity and thenemploying internal state variables, based on work potentialtheory, to account for damage evolution under loading andmicrodamage healing during rest periods. Through the verifi-cation study, it was found that the constitutive model has theability to predict the hysteretic behavior of the material underboth monotonic and cyclic loading up to failure, varying load-ing rates, random rest durations, multiple stress/strain levels,and different modes of loading—controlled-stress versus con-trolled-strain (Lee 1996; Kim et al. 1997a,b; Lee and Kim1998). The testing was performed on the same mixtures thatwere used in this study. Model coefficients developed to de-scribe the constitutive behavior during rest periods and refrac-ture of the healed material quantitatively showed that one mix-ture (AAM) performs better with respect to healing than theother (AAD). The two mixtures are described in more detailin a following section.
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TABLE 2. Batch Weights for Aggregate
Sievesize
Passing(%)
Retained(%)
Cumulativeweight per batch
(5,000 g)
1 in. 100 0 03/4 in. 95 5 2501/2 in. 79 16 1,0503/8 in. 67 12 1,650
#4 47 20 2,650#8 33 14 3,350
#16 22 11 3,900#30 14 8 4,300#50 9 5 4,550
#100 5 4 4,750#200 — 5 5,000
TABLE 1. Summary of AAM and AAD Asphalt Properties
Property AAMa AADb
Grade AC-20 AR-4000Penetration at 777F 64 135Penetration at 39.27F 4 9Ductility 4.6 1501
aStrategic Highway Research Program AAM-1 asphalt cement.bStrategic Highway Research Program AAD-1 asphalt cement.
In Situ Evaluation of Asphalt Pavements Using StressWave Method
A nondestructive technique based on the stress wave prop-agation method was used to detect fatigue damage growth andmicrodamage healing in asphalt pavements with different as-phalt concrete (AC) layer thicknesses and viscosities. Thistechnique was applied to four asphalt pavements in the FederalHighway Administration (FHWA) Turner Fairbank HighwayResearch Center that were loaded by the Accelerated LoadingFacility (Kim and Kim 1997), and to the low volume (loadedwith a calibrated truck) and mainline (subject to actual inter-state traffic loading) test roads at the Minnesota (MN) Roadpavement research facility near Minneapolis, Minnesota (Kimet al. 1998). Test results from both sites showed that micro-crack damage growth and healing of asphalt concrete pave-ments in the field could be measured using the stress wavemethod. They were able to demonstrate that the effective mod-ulus of the asphalt layer increases during rest periods as mi-crocracks heal, and that these periods of healing enhance theperformance of asphalt concrete pavements.
MATERIALS AND SAMPLE FABRICATION
The specimens used in this research were fabricated usingWatsonville granite aggregates and Strategic Highway Re-search Program (SHRP) AAD-1 and AAM-1 asphalt cements(hereinafter denoted AAD and AAM). Basic properties of bothasphalt cements are shown in Table 1. The medium gradationused in the SHRP A-003A project was selected for this re-search, and is shown in Table 2. Aggregates were mixed with5.0% asphalt by weight of dry aggregate (4.8% by weight ofmixture) at 1407C for 4 min.
The mixture was compacted into a 76-mm deep, 450-mmsquare wood and steel mold using the rolling wheel compac-tion method. Target air void content was 4 6 0.5%. The slabwas allowed to cool for 24 h before being removed from themold. Then, 76-mm square 3 381-mm beams were sawn fromthe slab after conditioning at room temperature for two days.More details on mixing and compaction can be found in Sias(1996). The beams were then subject to the testing describedbelow.
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FIG. 1. ASTM C 215 Experimental Test Setup
TEST METHOD
The impact resonance method is described in ASTM C 215as a means of determining elastic properties of a given materialbased on the resonant vibrational response due to an impactload. The impact resonance method has been shown to pro-duce very repetitive, consistent results for asphalt concrete(Whitmoyer and Kim 1994). The difference between repetitiveimpact resonance tests for the two mixtures in this researchwas less than 3%. Due to the impact nature of loading, theimpact resonance method measures the modulus of the vis-coelastic asphalt concrete in the glassy (purely elastic) region.Stated another way, the impact resonance method measures thevery short time or low temperature response, which is in theelastic range of a viscoelastic material. Thus, the increase inmodulus after rest periods is not affected by time-dependentrelaxation and is attributed to microcrack healing in the asphaltconcrete. The dynamic modulus of elasticity from the impactresonance test is a ‘‘smeared’’ modulus, where the effect ofthe damaged portion is spread over the whole specimen. Theinexpensive equipment and the small amount of time requiredto set up, conduct, and analyze the results from the impactresonance method are additional advantages for choosing thisas a standard method.
Every cylinder or prismatic bar has particular resonant lon-gitudinal, transverse, and torsional frequencies that can be in-duced by impact loading. These frequencies are a function ofthe specimen’s size, shape, mass, elastic properties, and themode of vibration produced in the specimen (McGonnagle1961). Whitmoyer and Kim (1994) have demonstrated that theabove fundamental principles behind the impact resonancemethod, which were originally developed for testing elasticsolids, can be applied to testing viscoelastic asphalt concrete.
Only the longitudinal mode of testing was performed on thebeam specimens in this research. The test configuration usedin this research is shown in Fig. 1. The impact was producedby a steel ball with a 25-mm diameter. A piezoelectric accel-erometer with an operating frequency range of 100 to 10,000Hz and a resonant frequency of 100 kHz was used. The max-imum data acquisition rate was 500 kHz.
The effect of support type on the dynamic modulus of elas-ticity values are negligible (Whitmoyer 1993). Therefore, a 76-mm 3 38-mm 3 13-mm foam support was placed at each endand in the middle of the beam for the impact resonance testing.
The vibrational signal was sampled at 500 kHz for 0.2 s,and then was processed using Fast Fourier Transform (FFT)technique to determine the maximum resonant frequency ofthe material under the longitudinal testing mode. From theresonant frequency, the elastic modulus was determined usingthe following equations from ASTM C 215
2E = DWN (1)L
where E = elastic modulus (Pa); W = mass of specimen (kg);and NL = resonant longitudinal frequency in Hz for a prism:D = 4 (L/bt) (N s2/kg m2); L = length of specimen (m); andb, t = dimensions of cross section of prism (m).
The third-point bending beam fatigue machine was used toinduce flexural damage in the beam specimens. The third-point
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loading created a constant bending moment in the center thirdof the beam, where the damage was concentrated. A load cellmeasured the force that was applied to the beam specimen andan linearly varying displacement transducer (LVDT) measuredthe resulting deformation at the midpoint of the beam. Fromthese readings, the flexural stiffness of the central, damagedportion of the beam was calculated.
To accurately control the test temperature, a 1.2-m 3 1.2-m 3 2.4-m environmental chamber was constructed to housethe beam fatigue machine. A specially designed air tempera-ture control system was installed to maintain the test temper-ature at 207C. All the laboratory experiments using the beamfatigue machine were performed in the environmental chamberthat controlled the test temperature within 60.67C of the targettemperature.
EXPERIMENTAL PLAN
The experimental procedure aimed to evaluate the effect ofrest periods and temperature on fatigue damage and healingpotentials of different mixtures. First, impact resonance testswere performed on three undamaged specimens of each mix-ture at several temperatures ranging from 2157C to 507C todevelop modulus-temperature relationships for each mixturetype. Two or three replicate beam specimens of each mixture(AAM and AAD) were then subjected to the following testprocedure:
1. All the specimens were initially tested at 207C to obtainthe longitudinal resonant frequencies. The base temper-ature of 207C was chosen for this procedure becauseclearer signals were achieved and ease of handling thespecimens.
2. Fatigue loading for 3000 cycles at 1.7 Hz was appliedto two specimens that were then tested using the impactresonance method. The number of fatigue cycles for thisfirst loading was small to induce a low level of damagein the specimens.
3. The two fatigued beams were exposed to different heal-ing temperatures, one at 207C and the other at 607C, for4 h. The 607C healing temperature was chosen as a typ-ical high in-service temperature that a pavement mightexperience.
4. The beams were then conditioned to 207C for at least 6h and tested again using the impact resonance method.
5. Loading was applied to the specimens until the totalnumber of cycles reached 10,000 to induce a mediumdamage level.
6. Steps 3 and 4 were then repeated.7. Steps 5 and 6 were then repeated to a total of 20,000
cycles, representing a high damage level.8. The two beam specimens were then fatigued to failure,
which was defined as 50% reduction in the initial flexuralstiffness. Control tests in which the beams were fatiguedto failure with no rest periods were also performed foreach mixture. Number of loading cycles to failure (Nf)was recorded for each specimen.
Load and deflection data were collected periodicallythroughout the fatigue tests to determine the flexural stiffnessof the beam specimens. At the beginning of each loadinggroup, 200 loading cycles were applied to the beams beforethe first flexural stiffness was determined in order to allowstabilization of the loading and measurement devices. Impactresonance tests were also performed at intermediate fatiguecycles to define the shape of the dynamic modulus of elasticityversus number of fatigue cycles curve. The specimens sub-jected to the 607C healing temperature were allowed to coolfor at least 6 h to reach 207C before testing resumed.
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FIG. 2. Comparison of Modulus-Temperature Relationship for AAMand AAD Beam Specimens
RESULTS
Modulus-Temperature Relationship
Undamaged beam specimens were tested in the longitudinalmode at several temperatures ranging from 2157C to 507C todevelop the dynamic modulus of elasticity-temperature rela-tionships for both the AAM and AAD mixtures. As seen inFig. 2, the impact resonance method was able to measure thechange in stiffness of the mixtures as the temperature in-creases.
At temperatures above about 257C, the AAM mixture spec-imens have a greater dynamic modulus of elasticity than theAAD mixture specimens. Below 257C, the two mixtures ap-pear to have similar modulus values, partially due to the log-arithmic scale. In previous uniaxial creep, relaxation, and com-plex modulus testing, higher moduli values were observed forthe AAM mixture, even at the lower temperatures (Kim et al.1998). The fact that this trend is not seen with the impactresonance testing indicates that the temperature range wherethe effect of asphalt binder type becomes relatively insignifi-cant is higher in the impact resonance testing than in the creep,relaxation, and complex modulus testing. This difference isdue to a combination of the faster rate of loading and lowerstrain employed in the impact resonance testing over the othertesting methods.
Evaluation of Microcrack Growth and Healing
The effect of rest periods, healing temperature, and mixturetype on fatigue damage growth was evaluated using the testprocedure described previously. Changes in the dynamic mod-ulus of elasticity and flexural stiffness measured from the con-tinuous fatigue test are presented in Figs. 3(a and b), respec-tively. Both the dynamic modulus of elasticity measured fromthe impact resonance method and the flexural stiffness ob-tained from load and deflection data decreased as more loadingcycles were applied (i.e., the damage in the beam increased)for both mixtures. The damage evolution curves for both AAMand AAD mixtures follow the classic S-shape for load con-trolled fatigue.
The AAM mixture has higher initial values for both dy-namic modulus of elasticity and flexural stiffness in Fig. 3.The higher modulus value reflects the fact that AAM is astiffer material than AAD. A higher load level, and thereforeloading rate as the frequency does not change, is partially re-sponsible for the higher initial flexural stiffness value seenwith the AAM mixture.
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FIG. 3. Typical Fatigue Damage with No Rest Periods: (a) DynamicModulus of Elasticity; (b) Flexural Stiffness
The number of cycles to failure depends on the load applied,the test temperature, and the type of mixture tested. In thisresearch, the test temperature was fixed at 207C. In an attemptto enable a direct comparison of the two mixtures, trial testswere performed at different load levels until both mixturesfailed around 30,000 cycles. The value of 30,000 cycles waschosen as a reasonable amount of time (5 h) to conduct asingle test. Based on these initial tests, loads of 1023 N and712 N were applied to the AAM and AAD specimens, re-spectively. Rest periods were introduced at 3,000, 10,000, and20,000 cycles, which respectively represent low (10% of total),medium (33% of total), and high (67% of total) levels of dam-age.
Changes in the dynamic modulus of elasticity and the flex-ural stiffness during the healing tests for the AAM mixture arepresented in Figs. 4 and 5, respectively. Although only theAAM mixture is presented, these are typical of the curves seenin both mixtures. Due to the destructive nature of the testing,each fatigue or healing test was performed on a different spec-imen, introducing sample to sample variability. This is the rea-son for different initial stiffness and modulus values as wellas the shape or rate of decrease of the curves. In both Figs. 4and 5, the beneficial effect of the healing periods on both thedynamic modulus of elasticity and flexural stiffness is obvious.An increase in the dynamic modulus of elasticity and flexuralstiffness occurs after each rest period. This phenomenon hasalso been observed with uniaxial tensile testing of cylinders(Kim et al. 1998) and in surface wave testing of in-servicepavements (Kim et al. 1995; Kim and Kim 1997), which weredescribed previously.
Rest periods extend the fatigue life of the specimen by es-
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FIG. 4. Typical Dynamic Modulus of Elasticity through Fatigue Dam-age and Rest Periods: (a) 207C Healing; (b) 607C Healing
sentially shifting the fatigue curve to the right. After each restperiod, the modulus or stiffness follows a negative powercurve until it reestablishes the curvature that would have beencreated if no rest periods were applied. This point can be foundby drawing a horizontal line from the point prior to the restperiod until it intersects with the fatigue curve after the restperiod. This is the point where the healed microcracks havereopened to the extent prior to the rest period and additionalcrack growth starts to occur. The benefit of the rest period hasdiminished, and the stiffness continues to decrease as thoughno healing had occurred. Since there was a period where orig-inal crack growth was not happening (instead refracture of thehealed surfaces), the new fatigue curve is shifted to the right,i.e., the same level of damage to virgin (unhealed) material isoccurring at a greater number of cycles, as shown in Fig. 6.This increases the ultimate number of cycles the specimen en-dures before failure. This could be thought of as stretching thefatigue failure curve with the application of rest periods, whichRaithby and Sterling (1970) have showed previously.
Although it was not observed in every case, increases indynamic modulus of elasticity and flexural stiffness for 607Chealing periods were generally greater than the correspondingincreases for the 207C healing periods. This follows the intu-itive thought that the healing capacity of the asphalt cementshould be enhanced as a result of its increased ability to flowand further close more microcracks at higher temperatures.
An interesting feature in Fig. 5(b) is that the flexural stiff-nesses after rest periods were greater than the original flexuralstiffness. Typically, this was seen with both AAM and AADspecimens and at both healing temperatures. Experimental er-
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FIG. 5. Typical Flexural Stiffness through Fatigue Damage and RestPeriods: (a) 207C Healing; (b) 607C Healing
FIG. 6. Flexural Stiffness versus Number of Cycles to Failure with andwithout Rest Periods
ror associated with repositioning the beam specimen in thebeam fatigue machine and attaching the LVDT to the specimenin the same spot as prior to the rest period was responsible forthis difference. A slight change in placement of the beam orLVDT has the potential to significantly change the measureddeflection, depending on where damage was concentrated. Itwas not possible to separate the effect of any change in place-ment from the flexural stiffness measurements, but the authorsbelieve that errors due to placement cannot account for thewhole stiffness gain observed. Only the flexural stiffness was
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TABLE 3. Healing Comparison Summary
Method
PERCENT INCREASE
AAMa
207C 607C
AADb
207C 607C
Number of cycles to failure 150 280 41 60Damage indicator 42 45 18 15Horizontal increase 52 47 32 25Increase/drop 70 92 50 70
aStrategic Highway Research Program AAM-1 asphalt cement.bStrategic Highway Research Program AAD-1 asphalt cement.
affected by the deflection readings, and hence this phenome-non was not seen with the dynamic modulus of elasticity mea-surements.
Comparison of AAM and AAD HealingCharacteristics
The healing performances of the AAM and AAD mixtureswere compared to determine which mixture has a better heal-ing potential. The four methods of measuring healing potentialused to compare the two mixtures are summarized in Table 3.
The percent increase in number of cycles to failure is cal-culated based on the differences between control tests withoutrest periods and healing tests where the specimens were al-lowed three rest periods
N 2 Nf,healing f,controlPercent Increase = (2)
Nf,control
Using this method, the AAM mixture shows a much greaterpotential for healing than the AAD mixture at both tempera-tures. Also, for both mixtures, the higher healing temperatureresults in greater increases in the number of cycles to failure.The number of cycles to failure for the AAD control tests(;40,000) was an average of four times those for the AAMmixture (;10,000), despite the effort to produce similar fa-tigue lives. This contributes to the large difference betweenthe two mixtures using this healing indicator. The percentagesreported are averages of at least two replicate tests at 4% airvoids.
In an effort to eliminate the effects of the different numberof cycles to failure for the control tests, a damage indicatorwas developed. The damage indicator is defined as the ratiobetween the number of cycles a specimen has endured at aparticular rest period and the number of cycles to failure forthat particular specimen
Nat a particular rest periodDamage Indicator = (3)Nf,healing
Alternatively, the damage indicator can be described as thepercentage of fatigue life consumed at a specific time. Forinstance, if a specimen fails at 30,000 cycles, the damage in-dicator at the third rest period is 20,000 divided by 30,000, or0.67, as shown graphically in Fig. 7. The average percent in-creases in modulus and flexural stiffness at rest periods withsimilar damage indicators are reported in Table 3. These valuesare based on most of the tests run, regardless of air void con-tent and ranged from 30 to 50 for AAM and 10 to 25 for AADfor both healing temperatures. The damage indicator showsthat the AAM mixture is a better healer than the AAD mixturebut does not indicate much difference between the healingtemperatures, for reasons unknown to the authors.
The horizontal increase is the ratio between the total numberof cycles gained from the three rest periods and the numberof cycles to failure
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FIG. 7. Graphical Representation of Healing Comparison Methods
DN 1 DN 1 DNf,1stRP f,2ndRP f,3rdRPHorizontal Increase = (4)
Nf,healing
The number of cycles gained was determined by evaluatingwhen the modulus or stiffness returned to the value prior tothe rest period. This was done by drawing a horizontal linefrom the point at the start of the rest period until it intersectswith the fatigue curve after the rest period and is graphicallyillustrated in Fig. 7. Each value that is reported in Table 3 isan average of at least three specimens. This method is actuallya measure of how long it takes for the healed surfaces to re-fracture once fatigue loading resumes. It indicates the healedmaterial’s resistance to damage evolution. Assuming that ahigher healing temperature only increases the amount or levelof healing and not the strength of the healed material, thisindicator cannot conclusively evaluate the effect of healingtemperatures. At the 607C healing temperature, the number ofcycles gained from the rest periods (numerator) and Nf (de-nominator) increase by the same number of cycles. This shouldshow a small increase in the indicator for the higher healingtemperature. However, the error associated with graphicallydetermining the number of cycles gained from each rest periodwas larger than any difference due to temperature.
A fourth method of evaluating the healing potential of thetwo mixtures is to compare the drop in modulus and flexuralstiffness before each rest period to the corresponding increaseafter the rest period. To do this, the increase (C-B in Fig. 7)was divided by the drop (A-B), expressing the benefit of therest period as a percentage of the modulus lost during theprevious fatigue cycles. This indicates that AAM has a greaterhealing potential than AAD and that the higher temperatureincreases healing. The values reported are averages for bothflexural stiffness and dynamic modulus of elasticity at all restperiods for specimens with approximately 4% air voids.
All four methods of evaluating the healing potential indi-cated that AAM performs better with respect to healing thanAAD, when subject to flexural fatigue loading. This trend wasalso observed in uniaxial cyclic fatigue testing of the twomixtures (Lee 1996; Kim et al. 1998). Two of the comparisonmethods also showed that the higher temperature encouragedhealing in both mixtures.
CONCLUSIONS
The purpose of this research was to evaluate the change inmodulus of two asphalt concrete mixtures due to temperature,fatigue damage growth, and healing during rest periods usingthe impact resonance method. A third point bending beam ma-chine was used to apply cyclic loading to the specimens, and
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flexural stiffnesses were calculated over cycles of loading andrest periods.
The impact resonance method was able to detect the de-crease in dynamic modulus of elasticity as the temperatureincreased and as fatigue damage accumulated in the specimen.An increase in dynamic modulus of elasticity was observedafter damaged specimens were allowed to rest. The gain inmodulus is attributed to microcrack healing, because the im-pact resonance method measures the elastic response of thematerial (i.e., no viscoelastic relaxation is involved). The cal-culated flexural stiffness also increased after the specimenswere subject to rest periods. Observing and measuring thehealing of microcracks in the laboratory is important as it sup-ports the findings of Kim and Kim (1997) who observed heal-ing of asphalt pavements in the field using the stress wavemethod. Although a more careful study is needed, the higherhealing temperature appeared to increase the amount of heal-ing that occurred during the rest periods.
The qualitative study of the two asphalt mixtures showedthat the AAM mixture has a higher healing potential than theAAD mixture. The same conclusion has been made from an-other study in which continuum damage mechanics and theoryof viscoelasticity are used in modeling uniaxial cyclic behaviorof asphalt concrete cylinders (Lee 1996; Kim et al. 1998). Theauthors are continuing research in this area to develop a sim-plified method for characterizing asphalt mixtures with respectto fatigue and healing. The result of that research could be atool with which asphalt mixtures can be evaluated to determinewhich will have the desired performance prior to being placedin the field.
The major contribution from the research presented in thispaper is not only that it supports the results from other studies,but that it serves as a basis for further research in the fatigueand healing behavior of asphalt mixtures which will eventuallylead to development of better materials and construction ofbetter performing pavements under variable loading condi-tions.
REFERENCES
ASTM. (2000). ‘‘Standard Test Method for Fundamental Transverse, Lon-gitudinal, and Torsional Resonant Frequency of Concrete Specimens.’’C 215, West Conshohocken, Pa.
Bonnaure, F. P., Huibers, A. H. J. J., and Boonders, A. (1982). ‘‘A lab-oratory investigation of the influence of rest periods on the fatiguecharacteristics of bituminous mixes.’’ J. Assoc. of Asphalt Paving Tech-nologists, 51.
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