Relating Adhesion and Cohesion of Asphalts to the Effect of Moisture on Laboratory Performance of Asphalt Mixtures

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    Transportation Research Record: Journal of the Transportation Research Board,No. 1901, Transportation Research Board of the National Academies, NationalResearch Council, Washington, D.C., 2005, pp. 3343.

    Antistripping additives and polymer modifications are two common mod-ifiers used to improve the fundamental properties of asphalt binders asthose properties relate to the performance of asphalt mixtures. Adhesionand cohesion are two important related properties of asphalt binders thatcan affect asphalt mixture performance before and after water condi-tioning. The purpose of this study was to quantify the effects of antistrip-ping additives and polymers on the adhesion and cohesion of bindersand to relate these effects to the performance of mixtures as measuredin the laboratory before and after water conditioning. The performancetests of asphalt mixtures included indirect tensile strength, uniaxialcompression permanent deformation, and Hamburg wheel tracking.Asphalt mixtures were produced with different modified binders andwith two aggregate types. The binders were modified with antistrippingadditives and polymers and by chemical treatment and oxidizationmethods. Granite and limestone were selected as two types of aggregatesources. The results indicate that the performance of asphalt mixturesis highly dependent on modification techniques and water conditioning.The overall performance of polymer-modified mixtures is more desir-able than those of unmodified mixtures and of mixtures modified withantistripping additives. Polymers are found to improve rutting perfor-mance, adhesion, and cohesion of an asphalt binder. In contrast, theantistripping additive can improve only the adhesion without changingother properties. The results of this study also illustrate that the adhe-sion and cohesion of an asphalt binder are good indicators of the per-formance of asphalt mixtures in the laboratory when they are conditionedwith water.

    It is well known that antistripping additives and polymers are widelyused modification techniques for improving the performance ofasphalt binders and mixtures. Antistripping additives improve theresistance of asphalt mixtures to moisture damage by reducing thesurface tension of asphalt binders, which increases the adhesion ofbinders to aggregate surfaces (1). The polymers are also success-fully used for modifying binder properties by changing their micro-structure and enhancing the rheological properties and the damageresistance characteristics of asphalt binders and mixtures (2). Most

    research on modification of asphalts has concentrated on evaluatingthe role of polymer-modified asphalt in resisting fatigue cracking,thermal cracking, and permanent deformation; however, few studieshave been done to evaluate the effect of polymer-modified asphaltin reducing moisture susceptibility of asphalt mixtures.

    An attempt was made in a recent study by the authors to predictthe resistance of asphalt mixtures to the moisture damage from theasphalt binder properties. These binder properties include cohe-sion and adhesion, which are directly related to bonding failure inasphaltaggregate systems (3). It was found that that these propertiesare closely related to the indirect tension strength of asphalt mixturesbefore and after water conditioning. This paper expands on the initialstudy by including more mixture testing results and several binderswith various modification techniques, antistripping additives, andtesting conditions. Mixture tests include the Hamburg wheel tracking(HWT) test and the newly developed simple performance test proce-dure. Test conditions include high and intermediate pavement designtemperatures.

    The general objective here was to understand how antistrippingadditives and polymer modification differ in their effects on mixtureperformance before and after water conditioning and what causesthese differences.

    OBJECTIVES

    The specific objectives of this study were the following:

    1. To evaluate and compare the effects of antistripping additiveand polymers on the fundamental properties of asphalt mixturesmeasured in the laboratory before and after water conditioning and

    2. To correlate the adhesion and cohesion properties of as-phalt binders to the performance of asphalt mixtures after waterconditioning.

    MATERIALS

    Two main types of modification techniques were used on theasphalt binders for this study. The first type was the antistripping-additive modification. The original asphalt binder [performancegrade (PG) 5828] was selected as the base asphalt and was modi-fied with three of the most widely used antistripping additives in

    Relating Adhesion and Cohesion of Asphalts to the Effect of Moisture on Laboratory Performance of Asphalt Mixtures

    Kunnawee Kanitpong and Hussain Bahia

    Asphalt Pavement Research Group, Department of Civil and Environmental Engi-neering, University of WisconsinMadison, 2210 Engineering Hall, 1415 Engineer-ing Drive, Madison, WI 53706.

  • 34 Transportation Research Record 1901

    TABLE 2 Effect of Antistripping Additives and Polymers on Rutting Resistance of Asphalt Binders

    Testing GV at 1 sec. of Loading (Pa) Strain @ 50 Cycles (mm/mm)

    Temperature COV COVBinder (C) No. 1 No. 2 Avg (%) No. 1 No. 2 Avg (%)

    Antistripping additives

    A 46 1020 1160 1090 9.08 4.93 4.30 4.62 9.65AS1 46 968 941 955 2.00 5.17 5.32 5.25 2.02AS2 46 797 823 810 2.27 6.28 6.08 6.18 2.29AS3 46 867 868 868 0.08 5.78 5.77 5.78 0.12

    Polymers

    A 58 169 164 167 2.12 29.63 30.44 30.04 1.91AP1 64 610 563 587 5.67 8.19 8.88 8.54 5.72AP2 64 3530 4060 3795 9.88 1.32 1.15 1.24 9.73AP3 64 842 849 846 0.59 5.93 5.88 5.91 0.60

    All tests were performed at the stress level of 100 Pa.

    Wisconsin. The second type was the polymer modification. Thesame binder (PG 58 28) was also used as the base asphalt andwas modified with different polymers, including styrenebutadiene(SB), styrenebutadienestyrene (SBS), and Elvaloy. Other modi-fication techniques, such as oxidization (air blown) and chemicaltreatment (acid), were also evaluated. Table 1 shows a list of asphaltbinders used in this study. To produce asphalt specimens for mix-ture testing, granite and limestone aggregates were selected asaggregate sources.

    EXPERIMENTAL TESTING AND RESULTS ANALYSIS

    In this study, the testing was separated into two main parts: asphaltbinder testing and asphalt mixture testing. The binder testing includedbinder creep and recovery testing to determine the rutting resistanceof asphalt binders, cohesion testing to evaluate the resistance of a

    binder to separation within a thin film, and adhesion testing to evalu-ate the bonding strength of an asphalt binder to an aggregate surfacebefore and after water conditioning.

    The mixture testing included the indirect tensile strength test(IDT) (AASHTO T283), the uniaxial compression permanent defor-mation test, and the HWT test. The following sections describe theexperiments conducted and the results collected.

    Asphalt Binder Testing

    Binder Creep and Recovery Test

    The binder creep and recovery test was conducted to measure theresistance of asphalt binders to accumulation of permanent strainunder repeated application of loading. Through selection of theloading time, the stress applied, and the temperature, the trafficspeed and traffic loading conditions can be simulated. The accu-mulated permanent deformation (strain) during each cycle of load-ing, the rate of the accumulation as a function of cycles, and theviscous component (GV) of the creep stiffness can be used as indi-cators of the rutting resistance of asphalt binders. In this study,binders modified with selected antistripping additives and selectedmodifiers were tested. The top of Table 2 shows a comparison ofthe testing results of the original binder and the same binder mixedwith three antistripping additives commonly used in Wisconsin.The creep and recovery tests of these binders were conducted at46C. It was found that the original binder showed lower totalaccumulative strain at 50 cycles and higher GV at 1 s of loadingthan the antistrip-modified binder. It was thus indicated that someantistripping additives could reduce the resistance of binders torutting (higher accumulated strain and lower GV). The bottom ofTable 2 shows a comparison of testing results conducted at 58Cand 64C for the original binder and three polymer-modifiedbinders, respectively. The results show that all three polymer-modified binders perform better (lower accumulated strain andhigher GV) than the original binder, particularly for the SBS-modified binder.

    The conclusions that can be derived from this testing are that theantistripping additives do not appear to have a large effect on the rut-ting resistance of the original binder and in some case can have neg-ative effects. The polymers, in contrast, can significantly improvethe rutting resistance of a binder.

    TABLE 1 Asphalt Binders Used in This Study

    Test Binder ID PG Modifier

    Binder creep and A PG 5828 recovery AS1 PG 5828 Antistripping 1testing AS2 PG 5828 Antistripping 2

    AS3 PG 5828 Antistripping 3AP1 PG 6428 SBAP2 PG 6428 SBSAP3 PG 6428 Elvaloy

    Adhesion/cohesion Group Abinder testing A1 PG 5828 and mixture A2 PG 5828 Antistrippingtesting A3 PG 6428 SB

    A4 PG 6428 SBSA5 PG 6428 ElvaloyA6 PG 7028 SBSA7 PG 7028 OxidizedA8 PG 7628 SBSGroup BB1 PG 5828 (RTFO) B2 PG 6428 (RTFO) AcidB3 PG 6428 (RTFO) ElvaloyB4 PG 6434 (RTFO) ElvaloyB5 PG 7028 (RTFO) Elvaloy

  • Kanitpong and Bahia 35

    Adhesion Testing

    Adhesion can be measured with the pull-off tensile strength test byusing the pneumatic adhesion tensile tester (PATTI). A schematicdiagram of this device is shown in Figure 1a. Details of adhesiontesting using the PATTI can be found in Youtcheff and Aurilio (4) andKanitpong and Bahia (3). The asphalt is applied to a pull stub, whichis then attached to the aggregate surface as shown in Figure 1b.The film thickness of asphalt is controlled by putting two pieces of14- 14- 212-in. metal blocks under the pull stub. The space underboth the pull stub and the aggregate surface is the film thickness ofthe asphalt specimen. PATTI transmits the air pressure to the piston,which is placed over the pull stub and screwed onto the reactionplate (Figure 1a). The air pressure induces formation of an airtightseal between the piston gasket and the aggregate surface. When thepull stub is debonded from the aggregate surface, the pressure atwhich the cohesive or the adhesive failure occurs is measured andconverted to the pull-off strength (kPa), which can be used as anindicator of the adhesive bond strength of the asphalt binder. Stud-ies by Youtcheff and Aurilio (4) and by Kanitpong and Bahia (3)indicated that this test is considered a rapid, low-cost, reproduciblemethod for measuring the adhesion characteristics of asphalt bindersto commonly used aggregate surfaces.

    Two sets of asphalt binders, eight binders in Group A and fivebinders in Group B (including of the original binder), as listed inTable 1, were used in this study. For each binder, eight specimenswere prepared, four under dry conditions and four after water expo-

    sure for 24 h at 25C. The adhesion test was conducted at the ambi-ent temperature (25C). Two sources of aggregate, limestone andgranite, were used. To provide comparable asphaltaggregate adhe-sion in this test to the adhesion characteristic of the asphalt mixtures,the aggregate surface was prepared from the slice of the asphalt mix-ture specimen produced from the same aggregate source. The pull-off strength was measured to represent the adhesion of the asphaltbinder in both dry and water-exposed conditions.

    Figure 2 shows the results for average pull-off strength for allbinders. The numbers above the bar graph represent the ratio of wet-to-dry pull-off strength. Binders in Group A were analyzed sepa-rately from those in Group B because the binders in Group B wereaged to simulate conditions in the mixtures associated with thisgroup. The results in Figures 2a and 2b show that, before water con-ditioning (dry), all binders on the limestone surface had lower pull-off strength than on the granite surface. After water conditioningfor 24 h, all binders lost some pull-off strength, with the exceptionof Asphalts A2, A3, and B4, whose ratios of wet-to-dry pull-offstrength were close to 1. The reduction, however, was dependent onbinder composition and the type of aggregate surface. The decreasein values of pull-off strength after water exposure on the granite sur-face was larger than that on the limestone surface for most bindertypes. The failure modes changed during water exposure on bothlimestone and granite surfaces. When more than 50% of the aggre-gate surface was exposed, the failure was identified as adhesive fail-ure. When the aggregate surface exposed was less than 50%, thefailure was considered cohesive failure (failure within the asphalt

    Metal

    Asphalt Binder

    Pull-Stub

    AggregateSurface

    (b)

    TemperatureChamber

    ProbeAdhesive

    SolidSurface

    0.01 mm/s

    (c)

    (a)

    Pulling Force

    Pull-Stub

    PressureHose

    Reaction Plate

    Coating Substrate

    GasketGasket

    FIGURE 1 Schematic diagram of the devices for adhesion and cohesion testing of asphalt binders: (a) PATTI devicewith cross-section schematic of piston attached to pull stub, (b) specimen preparation for adhesion (pull-off tensilestrength) test, and (c) cohesion (tack) test.

  • 36 Transportation Research Record 1901

    (a)

    (b)

    Asphalt

    1500

    2000

    2500

    3000

    3500

    B1 B2 B3 B4 B5

    Limestone-DryLimestone-WetGranite-DryGranite-Wet

    Pu

    ll-o

    ff S

    tren

    gth

    (kP

    a)

    0.81

    0.680.83

    0.96

    0.85

    0.92

    0.91

    1.04

    0.82

    0.86

    0

    1000

    2000

    3000

    4000

    5000

    A1 A2 A3 A4 A5 A6 A7 A8

    Limestone-DryLimestone-WetGranite-DryGranite-Wet

    Pu

    ll-o

    ff S

    tren

    gth

    (kP

    a)

    Asphalt

    0.74

    0.83

    1.02

    1.06 0.99

    1.00 0.73

    0.82

    0.86

    0.851.00

    0.76

    0.94

    0.750.84

    0.85

    FIGURE 2 Pull-off strength of (a) eight asphalt binders in Group A and (b) five asphaltbinders in Group B.

    film). In the unconditioned (dry) state, all failures were cohesive fail-ures; in contrast, in the conditioned (wet) state, the failures appearedto break at the asphaltaggregate interface, which is considered anadhesive failure.

    The statistical analysis of the pull-off strength results for bindersin Group A, which was conducted to evaluate the variables that sig-nificantly affect the pull-off strength values, showed the following:

    Effect P-Value for Pull-Off Strength

    Replicate 0.6472Binder 0.0000Aggregate 0.0000Conditioning 0.2811Binder Aggregate 0.5622Binder Conditioning 0.0000Aggregate Conditioning 0.0308

  • Kanitpong and Bahia 37

    A4, and A5) including SB, SBS, and Elvaloy showed a significantlyhigher tack factor than the original binder (A1) at a given |G*| value.Within the same PG grade and with three different polymer modi-fiers (A3, A4, and A5), Elvaloy resulted in the highest tack factor,while SB and SBS resulted in approximately the same tack factor.Within the same PG grade, the polymer-modified asphalt (A6) alsoshowed a higher tack factor than the oxidized asphalt (A7). Whenthe same modifier (SBS) was used with different PG grades, A8 (PG76 28) showed the highest tack factor compared with A4 (PG 64 28) and A6 (PG 70 28).

    The variation in tack factor as a function of additive used at agiven |G| value, as shown in Figure 3a, clearly indicates that theviscosity was not the only factor controlling cohesion; the additivetype needed to be considered in estimating cohesive strength. Theresults in Figure 3a are, however, promising in the sense that theyindicate a potential that commonly used asphalt modifiers can workeffectively to improve the cohesion property of asphalts.

    To predict the performance of asphalt mixtures produced by usingthese particular binders (Groups A and B), the tack factor was mea-sured for each binder at the temperatures at which mixture testingwas conducted. Group A was tested at 25C and 47C for predict-ing the indirect tensile strength and the permanent deformation ofthe asphalt mixtures, respectively. Group B was tested at 50C forpredicting the rutting resistance under water exposure measuredwith the HWT tester. Figure 3b depicts a comparative bar chart ofthe average tack factors measured at 25C and 47C for binders inGroup A and at 50C for binders in Group B. The results in Figure3b show similar trends to those for the pull-off strength test shownin Figure 2 in that tack factor values vary significantly depending onadditive type, grade, and testing temperature.

    Asphalt Mixture Testing

    Mixtures produced with a selected set of binders, Groups A and B,were tested using three test types: indirect tensile strength, uniaxialrepeated creep in compression, and HWT tester. Results were analyzedand correlated with binder properties as discussed next.

    IDT

    The maximum indirect tensile strength for all mixture specimenswas measured for two different conditions, dry and wet. Figure 4shows a comparison of results for all mixtures and depicts the effectof water conditioning on the indirect tensile strength for both thelimestone and the granite mixes. Figure 4 shows that the wet strengthis always lower than the dry strength because conditioning in wateraccording to ASTM D4867 induces moisture damage. Compar-ing the results in Figure 4, the granite mixes show lower tensilestrength than the limestone mixes, which could be due to the use ofa higher-percent optimum asphalt content in the granite mixes andthe differences in aggregate shape and texture. However, the lime-stone and granite mixes show similar susceptibility to water condi-tioning. Mixtures produced with different binders, however, arefound to result in different dry strength and different resistance tomoisture damage. For example, Mixture A1 (original) has a rela-tively greater reduction in strength after conditioning than MixtureA2 (antistripping). Compared with Mixture A1, other mixturesusing polymer-modified binders have a lower reduction in strengthafter conditioning, with the exception of Mixture A4. Binder A4 was

    The variables in this analysis were asphalt binders, aggregatesources, and conditioning of specimens (dry or water-exposed). Thetesting replication was included as another variable for evaluatingtest repeatability. Three main effects and two-way interaction effectsof all variables were determined. It is clear that, at a significancelevel of 0.05 in the t-distribution, the asphalt binders and aggregatesources significantly affected the pull-off strength. The interactioneffect between the binder and the condition also showed a signifi-cant effect, which was expected because different binders can havedifferent effects on the pull-off strength in the dry and the water-exposed conditions. For example, the water-exposed condition ofAsphalts A2 and A3, which were modified with an antistrippingagent and SB, respectively, did not show significant effects on pull-off strength for either aggregate source, while the water-exposedcondition of other asphalts showed significant effects on pull-offstrength (Figure 2a). The interaction effect between the aggregateand the condition was also significant because the pull-off strengthsof limestone and granite specimens showed different sensitivitiesto the water conditioning. It is clear in Figure 2a that, for the granitesurface, there was a significant reduction of the pull-off strength formost binders after water exposure, with the exception of A2 andA3. In contrast, the interaction between the binder and the aggre-gate showed an insignificant effect on the pull-off strength results.Although it was expected that the interaction between the asphaltand the aggregate should be important for a test related to moisturesensitivity, it appeared that the main effects of the binder composi-tion and aggregate type were so overwhelming that the interactioneffect did not appear to be a significant factor.

    Cohesion Testing

    The tack test was initially developed by the authors (3) to measurethe cohesive strength of asphalt binders. The tack test was conductedwith the dynamic shear rheometer following a protocol developedby Paar Physica USA in collaboration with the University ofWisconsinMadison in 2001. A simplified and schematic diagramof this device is given in Figure 1c. The measuring system can moveup at a well-defined speed (0.01 mm/s) and measure the force act-ing between the asphalt and the measuring-system surface. Themeasuring system is interfaced to a computer, which enables auto-matic measurement of the force applied and the time at which filmseparation occurs. The testing temperature and the film thickness ofthe specimen can also be controlled through the temperature cham-ber and the measuring system unit of the rheometer. The forceapplied and the time of separation are measured and related to thestickiness or tack factor (CT) of the asphalt, which is a testing param-eter of the tack test. The tack factor of the binder can be calculatedby integrating the area under the curve for the force applied to pullapart the specimen versus the measuring time.

    The tack factors for all binders in Group A were measured at acommon |G| value to evaluate the tack factor of different bindersand modifiers at a given |G| value at various testing temperatures.Figure 3a shows the average of tack factors of asphalt binders inGroup A, which are measured at the equivalent |G| value of 2.58MPa. At this level of |G|, the asphalt binders were measured forthe tack factors at temperature close to ambient. The results indicatea significant change in cohesion of asphalts because of incorporat-ing different additives. It was found that the antistripping additive(A2) did not result in a significant change of tack factor comparedwith the original binder (A1). The polymer-modified binders (A3,

  • 38 Transportation Research Record 1901

    0

    100

    200

    300

    400

    500

    A1 A2 A3 A4 A5 A6 A7 A8

    Tac

    k F

    acto

    r (s

    N)

    Asphalt Binder

    (a)

    (b)

    0

    100

    200

    300

    400

    500

    A1 A2 A3 A4 A5 A6 A7 A8

    Tac

    k F

    acto

    r (s

    N)

    Asphalt

    0

    10

    20

    30

    40

    50

    60

    B1 B2 B3 B4 B5

    Tac

    k F

    acto

    r (s

    N)

    Asphalt

    (c) (d)

    0

    10

    20

    30

    40

    50

    60

    A1 A2 A5 A8

    Tac

    k F

    acto

    r (s

    N)

    Asphalt

    FIGURE 3 Results of tack test or cohesion test: (a) effect of modifier on cohesion of asphaltbinders (measured at equivalent G* values 2.58 MPa); tack factors of asphalt binder inGroup A tested at (b) 25C and (c) 47C; and (d) tack factors of asphalt binder in Group Btested at 50C.

    observed to have high moisture sensitivity as measured in the adhesionbinder test.

    The adhesion and cohesion tests of Group A binders were con-ducted at 25C so that these properties of asphalt binders could berelated to the indirect tensile strength of the conditioned asphaltmixtures, which was also measured at 25C. The pull-off strength

    under the water-exposed condition (PW) from the adhesion test andthe tack factor (CT) from the cohesion test were combined to predictthe tensile strength of asphalt mixtures after water conditioning. Thereason for using the tack factor instead of the pull-off strength underthe dry condition for representing binder cohesion was that the tacktest has a better-controlled system with well-defined testing tem-

  • and it highlights the importance of binder adhesion and cohesion inthe phenomenon of moisture damage.

    Permanent Deformation Test

    On the basis of a research study for NCHRP Project 9-19 (5), ArizonaState University has developed a laboratory test method for measur-ing permanent deformation of asphalt mixtures. The test protocol wasfollowed in this study to perform rut tests on the conditioned andunconditioned asphalt mixture specimens similar to the specimens pre-pared for ASTM D4867. Four binders in Group A and two aggregatesources (limestone and granite) were used to produce 100- (diameter) 150-mm (height) specimens. All cylindrical specimens were sub-jected to a haversine axial load. This load was applied for 0.1 s andwas followed by a rest time of 0.9 s. The test was performed at 47C(the high pavement temperature reference in Wisconsin). The cumu-lative axial and radial strains were measured with linear variable dif-ferential transformers and recorded during the test.

    According to Witczak et al. (5), two parameters, flow number andpermanent deformation rate, were suggested for the analysis of thepermanent deformation test (PDT) results. The flow number is deter-mined from the rate of change of permanent strain; it is identified asthe number of cycles at which the rate of permanent strain reachesa minimum, which usually occurs before the tertiary creep stagebegins. In this study, most of the asphalt mixtures produced for thePDT did not reach tertiary flow after 20,000 cycles, and some of themfailed after 1,500 cycles. Therefore, the permanent deformation rateand the cumulative strain at 1,000 cycles were selected as the bestresponse parameters for this test.

    The permanent deformation rate is the slope of the power modelwhen the permanent strain is plotted versus the number of cycles onthe loglog scale. The parameter is not new, and a previous studyindicated that the slope of the power model shows better correlationwith rut depth on the Minnesota road test than other measurableparameters such as intercept, resilient strain, and permanent strainat 2,000 cycles (6). The power model is fitted to the permanent straindata starting at the point where the rate of permanent strain is at a

    Kanitpong and Bahia 39

    300

    400

    500

    600

    700

    800

    900

    1000

    1100

    300 400 500 600 700 800 900 1000 1100

    Limestone

    Granite

    Ind

    irec

    t T

    ensi

    le S

    tren

    gth

    (kP

    a)(C

    on

    dit

    ion

    ed S

    pec

    imen

    s)

    f(Adhesion, Cohesion)

    R2 = 0.94

    0

    200

    400

    600

    800

    1000

    1200

    A1 A2 A3 A4 A5 A6 A7 A8

    Limestone-DryLimestone-WetGranite-DryGranite-Wet

    Max

    imu

    m T

    ensi

    le S

    tren

    gth

    (kP

    a)

    Asphalt

    FIGURE 4 Effect of water conditioning on the indirect tensile strength of limestone andgranite mixes.

    FIGURE 5 Relationship between adhesion and cohesion ofasphalt binder and indirect tensile strength of conditionedasphalt mixtures.

    perature and film thickness. The results of the correlation from asimple multilinear model of tensile strength values as a function ofbinder cohesion and the pull-off strength under the water-exposedcondition (PW) resulted in choosing the following model:

    It includes adhesion (PW) and cohesion (CT) and shows the best-predicted linear model with R2 = .94. The model also has the loweststandard error, 49.67, for all data of limestone and granite mixes.The plots in Figure 5 show the predicted versus the measured ten-sile strength values and their proximity to the equality line. This isan especially encouraging result; it indicates that adhesion and cohe-sion of binders could be reliably used to predict moisture resistance,

    IDT wet PW( ) = + ( ) + ( )273 55 0 026 1 21 1. . . ( )CT

  • minimum, which can be considered the start of the linear regionof the permanent strain curve. A straight line is fitted to the powermodel where the line is tangent to the linear region of the permanentstrain curve, and hence, the intercept and slope (model constants a and b, respectively) are determined as shown in Figure 6.

    Model constants a and b were determined from the linear regionof the permanent strain curve excluding the tertiary flow region. Asmentioned previously, most of the specimens in this study did notreach tertiary flow at 20,000 cycles; therefore, the constants a and bwere estimated from the linear region of the permanent strain curve,which extended to the end of the test. In addition to the permanentstrain rate (the slope, b), the cumulative strain was determined at aspecific cycle for comparison between specimens.

    Table 3 summarizes the average values of permanent deforma-tion rates and cumulative strain at 1,000 cycles for the mixturesproduced with four different binders and limestone and graniteaggregates, respectively, before and after water conditioning ofmixture specimens. For both aggregate sources, it is clear that thepolymer-modified binders (A5 and A8) result in both a lower perma-nent strain rate and a lower accumulative permanent strain than eitherthe original binder (A1) or the antistripping-additive-modified binder(A2). The original binder does not show significantly different re-sults from the binder with antistripping additive.

    40 Transportation Research Record 1901

    For limestone, the water-conditioned specimens show a lower per-manent deformation rate and a lower cumulative strain at 1,000 cycles(except for A5 and A8) than unconditioned specimens. This phenom-enon was unexpected, but it could be explained by the observation thatlimestone shows less sensitivity to moisture than the granite mixes inthe adhesion test; there is also a possibility of pore water pressureeffects. For granite, water conditioning increases the permanent defor-mation rate of mixture specimens produced with A5 and A8, whichwas expected because A5 and A8 show high sensitivity to moisture,as shown in the adhesion test. However, the opposite trend was foundin the granite mixes produced with A1 and A2, and this trend was alsoshown in the cumulative strain at 1,000 cycles for the mixes producedwith A2, A5, and A8. Two replications of the permanent deformationtest were performed in this study due to limited time.

    Similar to the indirect tensile strength, the PW and the tack factor(CT) from adhesion and cohesion binder testing of four binders inGroup A were combined to predict the permanent deformation rateof asphalt mixtures after water conditioning. A multilinear model ofpermanent deformation rate was developed as a function of bindercohesion and adhesion. The best model is shown in the followingequation:

    The model including both adhesion (PW) and cohesion (CT)shows the best-predicted linear model with R2 = .96, and the loweststandard error of 0.030 for all data of limestone and granite mixes.It can be seen from Figure 7 that the predicted and measured per-manent deformation rates are approximately close to the equalityline. Figure 7 also clearly shows that the granite specimens result inhigher permanent deformation rate than limestone specimens.

    HWT Test

    The HWT test measures the combination of the repeated loadingeffect and the moisture damage effect on the asphalt mixtures by usinga steel wheel rolling on the surface of asphalt mixtures immersed inwater at 50C. The HWT testing was conducted and the results wereobtained from Gerald Reinke of MTE Services, Inc., Onalaska, Wis-consin. Five binders in Group B and two aggregate sources (limestoneand granite) were used to produce asphalt mixture specimens. Twocylindrical specimens were then arranged together to provide a pathlength for wheel tracking. A steel wheel, which is 47 mm wide, applies

    permanent deformation rate wet( ) = 0 705 0 00. . 001

    0 002 2

    PW( )

    + ( ). ( )CT

    0.0001

    0.001

    0.01

    0.1

    10 100 1000 104

    Per

    man

    ent

    Str

    ain

    ()

    No. of Cycles (N)

    b

    a

    Log[(N)] = a + b Log (N)

    FIGURE 6 Plot of permanent strain and number of cycles in theloglog scale.

    TABLE 3 Results of Permanent Deformation Test

    Permanent Deformation Rate(1/cycle) Cumulative Strain @ 1000 Cycles

    Mix Type Asphalt Unconditioned Conditioned Unconditioned Conditioned

    Limestone A1 0.520 0.465 0.0069 0.0052A2 0.516 0.422 0.0064 0.0039A5 0.300 0.208 0.0023 0.0030A8 0.342 0.242 0.0008 0.0014

    Granite A1 0.539 0.491 0.0101 0.0142A2 0.645 0.510 0.0156 0.0074A5 0.271 0.403 0.0037 0.0032A8 0.243 0.316 0.0023 0.0015

  • Kanitpong and Bahia 41

    a 702-N load to the specimens. Each specimen is loaded for 20,000cycles or until 22 mm of rut depth occurs.

    The data parameters that were obtained from the HWT test andused in the analysis included the number of cycles to stripping onset,the number of cycles to 12.5-mm rut depth, and the number of cyclesto failure. The number of cycles to stripping onset is the number ofcycles at which the rate of rutting in the specimen rapidly increasesbecause of wheel load, temperature, and moisture effect (7 ). Theother two parameters, cycles to 12.5-mm rut depth and cycles tofailure, can be determined directly from the test results.

    Figure 8 depicts the HWT test results. For all mixes produced withfive binders, Asphalts B3, B4, and B5, which were polymer-modifiedasphalts, resulted in significantly higher resistance to rutting thanB1 and B2. The mixes produced with Asphalt B2, which was a chem-ically modified asphalt, did not perform much differently from thoseproduced with Asphalt B1, which was the original unmodified asphalt.

    To correlate the adhesion and cohesion of different asphalt bindersto the results obtained from the HWT test, PW and CT of these binderswere measured at the same temperature as that used for the HWT test.Similar to results from other mixture performance tests, the PW andthe CT from the adhesion and cohesion binder tests can make goodpredictions of the cycles to stripping onset, cycles to 12.5-mm rutdepth, and cycles to failure of the mixtures, as shown in Figure 9.

    0

    0.1

    0.2

    0.3

    0.4

    0.5

    0.6

    0 0.1 0.2 0.3 0.4 0.5 0.6

    Limestone

    Granite

    Per

    man

    ent

    Def

    orm

    atio

    n R

    ate

    (Co

    nd

    itio

    ned

    Sp

    ecim

    ens)

    f(Adhesion, Cohesion)

    R2 = 0.96

    FIGURE 7 Relationship between adhesion and cohesion ofasphalt binder and permanent deformation rate ofconditioned asphalt mixtures.

    B1 B2 B3 B4 B5

    Cyc

    les

    to 1

    2.5

    mm

    Ru

    t D

    epth

    Asphalt

    LimestoneGranite

    0

    5000

    1 104

    1.5 104

    2 104

    2.5 104

    0

    5000

    1 104

    1.5 104

    2 104

    2.5 104

    B1 B2 B3 B4 B5

    LimestoneGranite

    Cyc

    les

    to F

    ailu

    re

    Asphalt

    B1 B2 B3 B4 B5

    LimestoneGranite

    Cyc

    les

    to S

    trip

    pin

    g O

    nse

    t

    Asphalt

    0

    5000

    1 104

    1.5 104

    2 104

    (a) (b)

    (c)

    FIGURE 8 Cycles to (a) stripping onset, (b) 12.5-mm rut depth, and (c) failure for limestone and granite mixes.

  • 42 Transportation Research Record 1901

    3000

    6000

    9000

    1.2 104

    1.5 104

    3000 6000 9000 1.2 104 1.5 104

    Limestone

    GraniteC

    ycle

    s to

    Str

    ipp

    ing

    On

    set

    f(Adhesion, Cohesion)

    R2 = 0.83

    4000

    8000

    1.2 104

    1.6 104

    2 104

    4000 8000 1.2 104 1.6 104 2 104

    Limestone

    Granite

    Cyc

    les

    to 1

    2.5

    mm

    Ru

    t D

    epth

    f(Adhesion, Cohesion)

    R2 = 0.81

    4000

    8000

    1.2 104

    1.6 104

    2 104

    4000 8000 1.2 104 1.6 104 2 104

    Limestone

    Granite

    Cyc

    les

    to S

    trip

    pin

    g F

    ailu

    re

    f(Adhesion, Cohesion)

    R2 = 0.87

    Cycles to StrippingOnset Cycles to 12.5 mm Cycles to Failure

    Model Variables

    Adhesion Cohesion Adhesion and cohesion

    R2

    0.520.820.83

    StandardError

    338120801994

    R2

    0.420.790.81

    StandardError

    435526202518

    R2

    0.350.820.87

    StandardError

    457424192023

    (a) (b)

    (c)

    (d)

    FIGURE 9 Relationship between adhesion and cohesion of asphalt binder and cycles to (a) stripping onset, (b) cycles to12.5-mm rut depth, and (c) cycles to failure of asphalt mixtures from the HWT test, with (d) tabular summary.

    The plots and fitted models show that, as the PW and CT of bindersincrease, all measurement parameters from the HWT test increase.As Figure 9d shows, the cohesion of asphalt by itself could also pre-dict well the mixture performance measured by the HWT test, as theR2 values and the standard error are quite similar. This result demon-strates the significant value of cohesion in controlling the HWT testresults.

    SUMMARY OF FINDINGS

    On the basis of the analysis of data collected in this study, findingsfor this study can be summarized as follows:

    1. The performance of asphalt mixtures under moisture condi-tioning is highly dependent on modification techniques and water

  • conditioning. Different modifiers perform differently under waterexposure conditions.

    2. The antistripping additive does not improve rutting perfor-mance of the asphalt binder tested. On the other hand, the polymersshow significantly better performance in rutting resistance of asphaltbinder.

    3. The antistripping additive can change the adhesion propertybut does not significantly change the cohesion property of an asphaltbinder. The polymers can effectively change both cohesion andadhesion properties of an asphalt binder.

    4. The overall performances of asphalt mixtures modified withpolymers are more desirable than those of unmodified mixtures andmixtures modified with other additives.

    5. The cohesion and adhesion measurements of the asphaltbinders are reasonable predictors of mixture performance in thelab. Although sample size is small in this study, it is believed thatthe cohesion and adhesion testing of asphalt binder can be used asa tool to select desired asphalt binders to improve the resistance ofasphalt mixtures to moisture damage and rutting.

    ACKNOWLEDGMENTS

    The authors acknowledge Gerald Reinke of MTE Services, Inc., forhis support in providing some of the asphalt binder materials and theHamburg wheel tracking test results. The authors thank Judie Ryanof the Wisconsin Department of Transportation for her support in

    Kanitpong and Bahia 43

    providing useful information about the moisture damage problem inWisconsin.

    REFERENCES

    1. Anderson, D. A., E. L. Dukatz, and J. C. Petersen. The Effect of AntistripAdditives on the Properties of Asphalt Cement. Journal of the Associationof Asphalt Paving Technologists, Vol. 51, 1982, pp. 298316.

    2. Bahia, H. U., D. I. Hanson, M. Zeng, H. Zhai, M. A. Khatri, and R. M.Anderson. NCHRP Report 459: Characterization of Modified AsphaltBinders in Superpave Mix Design. TRB, National Research Council,Washington, D.C., 2001.

    3. Kanitpong, K., and H. U. Bahia. Roles of Adhesion and Thin FilmTackiness of Asphalt Binders in Moisture Damage of HMA. Journal ofthe Association of Asphalt Paving Technologists, Vol. 72, 2003.

    4. Youtcheff, J. and V. Aurilio. Moisture Sensitivity of Asphalt Binders:Evaluation and Modeling of the Pneumatic Adhesion Test Results. Proc.,Canadian Technical Asphalt Association, 1997.

    5. Witczak, M. W., K. Kaloush, T. Pellinen, M. El-Basyouny, and H. VonQuintus. NCHRP Report 465: Simple Performance Test for Superpave MixDesign. TRB, National Research Council, Washington, D.C. 2002.

    6. Guler, M. Characterization of Hot Mixture Asphalt Shear Resistance andCorrelation with Rutting Performance, Ph.D. dissertation. University ofWisconsinMadison, 2002.

    7. Reinke, G., S. Glidden, D. Herlitzka, and J. Jorgenson. Interaction of Aggre-gate Type, Mix Design Level, and PG Binder Grade on the Performance ofHMA Mixtures as Measured by the Hamburg Wheel Tracking Test and theDSR Creep Test. Journal of ASTM International (preprint copy), 2003.

    The Characteristics of Bituminous Materials Committee sponsored publicationof this paper.

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