36

Transportation Research Record: Journal of the Transportation Research Board,No. 1962, Transportation Research Board of the National Academies, Washington,D.C., 2006, pp. 36–43.

This paper documents and discusses the development of a compact ten-sion (CT) test for the grading of asphalt binders. The geometry was cho-sen because it provides an easy way to obtain the plane-strain fracturetoughness (KIc), fracture energy (Gf), and crack tip opening displace-ment (δt) in brittle failure on a small sample. It is believed that themethod will allow for a better ranking of binders in regard to their low-temperature fracture resistance. CT specimens were prepared in dif-ferent sizes with varying notch depths. KIc was found to be constant,regardless of the notch depth or specimen width for both straight andmodified binders. Gf was found to decrease with notch depth; this isthought to be the result of energy-absorbing mechanisms away from thecrack tip. Deeper notches or an energy correction is able to account forthat issue. A slightly different fracture energy, Jf , was obtained in a moredirect fashion from the slope of a plot of the normalized failure energy(Uf/B) versus notch depth (a). Jf provided results close to those obtainedwith an energy correction from a single notch depth. Reproducibility ofthe fracture test was found to be good with pooled standard deviationsof 5% to 10% for KIc and 15% to 20% for Gf , which is typical for suchtests. Given the fact that brittle fracture properties can vary by ordersof magnitude for binders of the same Superpave® grade, it is concludedthat the test method has a high ability to reveal statistically significantdifferences in toughness.

Often, asphalt pavements in northern climates fail through the com-bined actions of exposure to low temperatures and traffic loading.Winter temperatures in much of Canada and the northern UnitedStates can fall to extreme lows, causing severe tensile stresses inthe surface layers of the pavement. When such stresses surpass thestrength of the material, transverse cracks can result. Sometimes trans-verse cracking starts after just months of service (1). The fact that thistype of distress is hardly controlled is evident to anyone who hasrecently driven on roads in Canada or the northern United States. Low-temperature cracks are ubiquitous, and the associated cost of repairand reconstruction is annually in the billions of dollars. Such experi-ences illustrate the need for the development of new and improvedmethods for asphalt binder grading.

Current grading methods make use of the bending beam rheome-ter (BBR), at times in combination with the direct tension tester(DTT), both developed under SHRP (2). It has been observed on morethan one occasion that binders of the same grade sometimes showvastly different cracking distress in service [see, for instance, Iliutaet al. (3) and Button and Hastings (4)]. The observed anomalies mayhave several explanations; one of them is likely the variation in brit-

tle fracture properties that are currently ignored in BBR and DTTgrading. Hence, it would be desirable if a convenient, accurate, andreproducible test for the binder were available that quantifies thesebrittle state properties. Research that started more than 10 years agoon the development of the single-edge-notched bending (SENB) testhas now evolved to a point at which it is possible to show with a highdegree of precision the large differences in fracture performancebetween straight and various modified binders (5–10).

The object of this paper is to document and discuss the develop-ment of a new compact tension (CT) test for asphalt grading. Thegeometry was recently used successfully for the testing of binders(11) and mixtures (12, 13) on which this study builds. It is smallerthan the SENB geometry and therefore more compatible with cur-rently available DTT equipment. The ultimate aim of this effort isto gain industry acceptance of the method as part of a group of frac-ture grading tests that should prevent premature failures and assistin the selection of binders with superior performance.

BACKGROUND

Standard Test Methods for Fracture

The fracture testing approach investigated in this paper originates inthe metals and polymer fields. ASTM first published a test methodfor the determination of the fracture toughness of metallic materialsin 1970. The current designation, E 399-05 Standard Test Methodfor Linear-Elastic Plane-Strain Fracture Toughness KIc of MetallicMaterials, is used to measure resistance to fracture in either three-point bending or CT (14). These geometries are preferred in thatthey yield bending stresses around the crack tip zone that produce ahigh state of confinement. As such, smaller specimens are requiredto approach plane-strain conditions. The plane-strain fracture tough-ness, KIc, is a material property because it is independent of speci-men size and geometry. It also reflects the worst-case resistance tofracture of a material and hence should be particularly useful forspecification grading in which failure is to be avoided.

Much of the work that led to the development of successful frac-ture tests for plastics started in Europe and was recently reviewed byone of the principal investigators from that effort (15). The originalmethod was based on the work of Technical Committee 4 of the Euro-pean Group on Fracture, which, in turn, based its initial discussionson the ASTM metals standard (15). The first testing protocol from thisgroup was drafted in 1990 (16). On the basis of this protocol, ASTMpublished its first standard test method, D 5045-99 Plane-Strain Frac-ture Toughness and Strain Energy Release Rate of Plastic Materials,for the determination of the plane-strain fracture toughness (KIc) andthe plane-strain critical strain energy release rate (GIc) of plastics (17). ISO followed with its own standard, ISO 13586 Plastics—Determination of Fracture Toughness (GIc and KIc) in 2000 (18).

Compact Tension Testing of AsphaltBinders at Low Temperatures

Michelle A. Edwards and Simon A. M. Hesp

Department of Chemistry, Queen’s University, Chernoff Hall, 90 Bader Lane,Kingston, Ontario K7L 3N6, Canada.

Edwards and Hesp 37

Although the specimen size requirements to ensure plane-strainconditions and to avert plastic collapse appear to be similar in themetals and plastics standards, the latter has a number of modifica-tions in other respects (15, 16 ). Corrections for indentation, samplecompression, and system compliance are made because plastics areof much lower stiffness than metals. Furthermore, the sharpening ofthe notch in plastics is done through a razor blade being slid into apreviously machined notch rather than through fatigue cycling. Inaddition, the plastics standard includes instructions on how to mea-sure GIc, which is not covered for metals. The inclusion of GIc appearsto be mainly for historical reasons because the parameters are relatedand hence should provide very similar information. Finally, the dis-cussion on the determination of the crack initiation point that definesKIc for plastics showing nonlinearity before fracture is rather arbitrary.The method of determining crack initiation is based on the ASTMstandard for metals in which the initial compliance of the specimenis measured and a 5% increase is used to determine the point of crackinitiation (14, 16–18). This approach has been shown as approxi-mate because for materials that show appreciable nonlinearity, the5% rule can produce a significant overestimate of the true fractureenergies as determined from a visual observation of the crack initi-ation (19). Some issues above as they relate to the testing of asphaltare discussed in this paper.

Effort of Ontario, Canada, to Developan Improved Asphalt Binder Specification

Ongoing research in Ontario, Canada, has revealed that many pre-mature failures and performance differences between asphalt bindersof the same grade can be explained by three deficiencies in the currentAASHTO M320 specification (1, 3, 11):

1. Insufficient chemical aging in the rolling thin film oven (RTFO)and pressure aging vessel (PAV),

2. Insufficient reversible aging (physical hardening) in the BBR,and

3. Absence of fracture mechanics–based binder tests in both theductile and brittle states.

The first issue is currently being addressed in an NCHRP project andhence is not discussed here. The second deficiency of the currentspecification is addressed in Ontario’s new BBR test procedure, LS-308 (Draft)—Determination of Performance Grade of PhysicallyAged Asphalt Cement Using Extended Bending Beam RheometerMethod (20), which assesses a binder’s ability to meet a grade afterconditioning times of 1, 24, and 72 h. The limiting stiffness and m-value temperatures after 24 h and 72 h of conditioning at both10°C and 20°C above the pavement design temperature will be usedin a future specification to minimize failures.

Along with the extended BBR protocol, the addition of two frac-ture tests is expected to prevent premature failures and to help withthe design of superior-performing pavements. In the ductile state, theessential work of fracture approach, as developed by Cotterell, Mai,and Reddel (21) and adopted for asphalt binder and mixture testingby Andriescu et al. (22–24), is used to assess a binder’s ability to resistductile failures. The test is described in LS-299 (Draft)—AsphaltCement Grading for Fracture Performance Using Double-Edge-Notched Tension Procedure (25), which assesses a binder’s essentialand plastic works of fracture, we and wp, as well as the critical cracktip opening displacement, δt, in double-edge-notched tension. Thedeveloped test was able to predict the relative performance of vari-

ous binders used in FHWA’s accelerated loading facility, a task inwhich the binder loss modulus, G*sinδ, failed (24).

The brittle state SENB and CT fracture tests are now describedin LS-296 (Draft)—Asphalt Cement Grading for Fracture Perfor-mance Using Single-Edge-Notched Bend Procedure (26) and LS-298(Draft)—Asphalt Cement Grading for Fracture Performance UsingCompact Tension Procedure (27), which assess a binder’s fracturetoughness, energy, and crack tip opening displacement on a notchedspecimen in SENB and CT, respectively. The object of this paper isto document and discuss the development of the CT test.

To support the three new methods, the Ministry of Transportationof Ontario has embarked on several new pavement trials. The first ofthese was constructed north of Timmins, Ontario, in summer 2003 (3);two other trials are in the planning stages. These three experimentswill provide a unique opportunity to validate the improved gradingapproach proposed by the authors.

MATERIALS AND EXPERIMENTAL DETAILS

Binder Information

The first asphalt used for the development of the CT procedure wasa Bow River 85/100 penetration grade obtained from the MaterialsReference Library of the Strategic Highway Research Program (MRLCode AAN). It was aged according to standard procedures in boththe RTFO and the PAV. The continuous low temperature BBR gradewas determined to be −27.9°C, making this a PG-22 binder. It wastested in compact tension at both −22°C (Tgrade) and −16°C (Tgrade + 6),according to the procedures of LS-298 (27 ).

The second binder tested was a California Valley AR-2000 gradeobtained from Golden Bear Oil Specialties of Oildale, California (usedas a substitute for MRL Code AAG-2, which was no longer available).It was modified with 5% of linear styrene–butadiene–styrene (SBS)polymer (Kraton D1192) to produce a higher toughness binder. Thismaterial was tested unaged. Its continuous low-temperature BBRgrade was determined to be −26.7°C, making this also a PG-22. It wastested at both −28°C (Tgrade − 6) and −22°C (Tgrade).

A third set of seven binders was obtained from a pavement trial onHighway 655 in northern Ontario for which the detailed propertiesare given elsewhere (3).

Experimental Methods

The CT test was performed using Instron’s AsphaltPro 5525 machine.The instrument was equipped with a 500-N load cell, which washigh for this application because failure loads were mostly below100 N. The temperature bath was able to hold the test temperatureconstant to within approximately 0.1°C.

Figure 1 provides a schematic of the setup and the sample geom-etry. The tensile tester was fitted with two special grips with integralknife-edges that fit into the aluminum end pieces of the asphalt spec-imens. This method of testing is similar to that described in ASTMMethod E 1304-97 Standard Test Method for Plane-Strain FractureToughness (Chevron-Notch) of Metallic Materials (28).

Samples were prepared with the aid of silicone molds in whichthe inserts were placed before the samples were poured to the geom-etry of Figure 1. A set of samples was sharpened before testing bysliding a razor blade through the notch. However, most sampleswere tested as obtained directly from the molds. The loading rate ineach test was kept constant at 0.01 mm/s, and the temperature wasvaried. Samples were conditioned for 20 h before testing.

38 Transportation Research Record 1962

The load versus displacement record was used to obtain a mea-sure of peak load and failure energy from the area under the curve.The peak load was used to calculate the fracture toughness, KIc,according to the equations given in the various standards (16–18):

where

Pf = failure load (N),B = specimen width (m),W = specimen depth (m),x = a/W, anda = notch depth (m).

A generic fracture energy, Gf, was calculated from the uncor-rected energy under the load versus displacement curve. Althoughsome tests on unnotched samples were conducted to separate theenergy absorbed away from the crack tip zone, it was decided thatfuture results would not be corrected for such energy absorbingeffects for the sake of simplicity. Further comments on this issue areprovided in the results section. Gf was calculated according to therelations given in the various standards (16–18), which all followMarshall et al. (29):

φ =+ − − +( ) −( )1 92 19 12 2 51 23 22 20 54 12 3 4. . . . .x x x x x

119 12 5 02 69 68 82 16 1

2 1 91

2 3. . . .

.

− − +( ) −( )⎡⎣+

x x x x

++ − − +( )⎤⎦19 11 2 51 23 23 20 54

4

2 3 4. . . .

( )

x x x x

G U BWf f= φ ( )3

f xx x x x x( ) =

+( ) + − + −2 0 866 4 64 13 32 14 72 5 62 3 4. . . . .(( )−( )1

2x

3 2( )

KP

BWf xIc

f=⎛⎝⎜

⎞⎠⎟ ( )1 2

1( )

where Uf is the area under the force versus displacement curve (J) and φ is the energy correction dependent on change in sample com-pliance with notch depth (29).

An alternate approach for determining slightly different fractureenergies, Jf, also was investigated (30–33). In the most general terms,fracture energy relates to the energy release rate as follows:

This equation shows that by plotting the change in failure energynormalized to the specimen width, Uf /B, versus the crack length,a, a straight line should be obtained of which the slope provides Jf. This approach, which finds its roots in elastic–plastic frac-ture mechanics, should be consistent with linear-elastic fracturemechanics; hence it was used here to validate the results obtainedfrom Equation 3.

Direct tension tests on dogbone-shaped samples were conductedfor the SBS-modified binder in an attempt to obtain a yield stress at −22°C. However, for loading times similar to those obtained in the CTtest, all samples failed in a brittle fashion. Hence, the failure stress wasused as a conservative measure of the yield stress, which is needed tocheck for size requirements in the test standards. The failure stress wasused to calculate a conservative estimate of the plastic zone size thatwas subsequently compared with the specimen dimensions.

RESULTS AND DISCUSSION OF RESULTS

Test Reproducibility

Good reproducibility is desirable because the method is intended foruse in a specification scheme. Because CT tests generally are moresensitive to alignment issues compared with SENB tests, care was

J B dU daf f= − ( )1 5( )

(a)

a

W

(b)

FIGURE 1 CT test: (a) schematic of test setup, specimen geometry, andcritical variables (a � notch depth, angle of notch � 30�, W � specimenheight), and (b) photographs of actual CT test specimen and test setup.

taken to avoid problems in that regard. Nevertheless, some testsshowed evidence of minor misalignment. Figure 2 provides exam-ples of different sets of repeat tests for various combinations ofnotch depth, width, temperature, and binder. The force versus dis-placement records for some specimens in Figure 2 are somewhatnonlinear. However, close scrutiny of the graphs shows that the mis-alignment is corrected after a few microns of displacement and thatthe area affected is small. An energy correction using unnotchedsamples could account for this although, as will be discussed later,for the sake of simplicity, this step was omitted.

The peak loads in Figure 2 are all highly reproducible, and theydiffer by a significant amount between the two binders and betweengeometries. Hence, the test is able to distinguish differences infracture performance with a high degree of confidence.

Not all samples were as reproducible. However, the figure showsthe capabilities of the test method with good specimens (i.e., those thathave little or no residual stresses from cooling). Some binders pre-sented much higher variability than others, most likely as a result ofstresses that occur during cooling, conditioning, or both. Further workmay be needed to develop protocols that can lower the variability.

A second indication of the reproducibility of the test method is pro-vided by a determination of the pooled standard deviation of the mean.It was found that the standard deviation so obtained ranged from 5%to 10% for the fracture toughness and from 15% to 20% for the frac-ture energy, depending on the geometry and binder tested. The values

are somewhat higher than those reported for polymers (15), but arestill reasonable in the context of typical differences that exist betweenbinders, which can be as high as several orders of magnitude.

Specimens were tested with various notch depths and widths. Incalculating the pooled standard deviation it was assumed that it doesnot change from geometry to geometry. Hence, there is some approx-imation in this approach because the samples with the lower notchdepth of 20% were somewhat less reproducible than those withhigher notch depths of 40% and 60%.

Validation of Plane-Strain Condition

The effects of the sample size and geometry were investigatedthrough the testing of three different specimen widths (B) eachwith three different notch depths (a) for a total of nine geometries.The effect of specimen width should indicate whether under the giventest conditions the plane-strain condition is predominant. On the out-side of the sample the surface is able to contract on straining, whichprovides a certain degree of plane-stress condition. If this zone issmall compared with the other dimensions of the sample, the testis considered to be under near plane-strain.

Table 1 provides results for the two binders, AAN and AAG-2 + 5% SBS. The data are interesting in several respects. The fracturetoughness appears to be independent of B and a. This is a validation

0

10

20

30

40

0 0.5 1 1.5 2 2.5

Displacement (mm)

For

ce (

N)

(a)

0

30

60

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120

0 0.5 1 1.5 2 2.5

Displacement (mm)

For

ce (

N)

(c)

0

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60

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120

0 0.5 1 1.5 2 32.5

Displacement (mm)

For

ce (

N)

(d)

0

10

20

30

50

40

0 0.5 1 1.5 2 2.5

Displacement (mm)

For

ce (

N)

(b)

FIGURE 2 Representative force–displacement records for the CT test: (a) AAN(T � �22�C, B � 25.4 mm, a/W � 0.4); (b) AAN (T � �16�C, B � 19.1 mm, a/W � 0.2); (c) AAG-2 � 5% SBS (T � �28�C, B � 19.1 mm, a/W � 0.4); and (d ) AAG-2 � 5% SBS (T � �22�C, B � 19.1 mm, a/W � 0.2). Individual curves areshifted for clarity, and no tests were excluded.

Edwards and Hesp 39

40 Transportation Research Record 1962

of the plane-strain condition and, together with the nearly linear forceversus displacement records as provided in Figure 2, also shows thatthe specimens were tested far from their state of plastic collapse.

A second observation, made on many earlier occasions, is that thefracture toughness of the SBS-modified material is twofold to three-fold higher than that of the straight binder. That is why a fracture testwill probably yield valuable information for performance grading.

In addition to the variation in specimen width and notch depth, anumber of tensile tests were conducted. The binder tested wasAAG-2 + 5% SBS, and the test temperature was −22°C. These testswere performed in an attempt to determine the yield stress (σy) fromwhich a reasonable estimate for the plastic zone ahead of the cracktip can be made (7 ):

where rp is the size of the plastic zone, m.The various standards set lower limits on the specimen size

compared with this plastic zone as follows [ASTM 2005 (14)]:

The analysis for this specimen gave a brittle-type failure stress ofapproximately 4.0 MPa and a fracture toughness of approximately150 kN.m−3/2 (see Table 1). This gives a conservative estimate of the(KIc/σy)2 factor of 1.4 mm, which fulfills all the size requirementsgiven.

Effect of Notch Depth on Fracture Energy

The relatively strong effect of the notch depth on fracture energyis evident from the data in Table 2. The reason for this apparentanomaly is explained through the analysis of the energy correctionsthat are usually made for fracture tests on plastics. The various stan-dards all require a correction for the frame compliance, loading pinpenetration, and specimen compression. These were never consid-

W KIc y> ( )5 82σ ( )

B a W a KIc y, , . ( )−( ) > ( )2 5 72σ

r Kp Ic y= ( )π σ8 62

( )

ered to be of significance in the authors’ work because typically theyare reported to be less than 20% (15–18). However, the data fromTable 2 show that significant differences in fracture energy do exist fordifferent notch depths. Hence, this finding is likely due to the ability ofasphalt binders to provide a significant degree of plastic deformationaway from the crack tip zone at relatively low load levels.

To address the issue, a number of unnotched samples were testedin the same fixture but at slower speeds to match the loads on theunnotched samples to the loads and failure times of the correspondingnotched samples (15–18). Because of time constraints, only the datafor the AAG-2 + 5% SBS was assessed at −22°C.

The energy under the force versus displacement curves for theunnotched samples was subtracted from the energy in the notchedtests to obtain the corrected fracture energy. Results of this analysisare provided in Figure 3, in which the error bars reflect the errors asobtained from duplicate and triplicate measurements. The resultspresented show that although differences still exist, the fractureenergies obtained from corrected energy measurements are closertogether than those from uncorrected energies.

The analysis also shows that the Gf corrected for energy-absorbingmechanisms away from the crack tip zone is still only about 50% ofthe energy without the correction. Hence, this suggests that for asphaltthe general observation is different from that reported for polymers(15–18). It also suggests that it is better to test the toughness with deepnotches. It is proposed in future testing to use either a 40% or 60%notch without energy corrections. However, the correctness of thischoice will have to be investigated with further tests on a larger num-ber of binders with known field performance. An alternate approachfor possibly obtaining more correct fracture energies is discussed next.

Generalized Locus Method

The data from Table 2 were checked by the fracture energy normal-ized for specimen width (Uf/B) being plotted versus the notch depth(a) according to Equation 5. Results of this analysis are provided inFigure 4, in which the slopes of the lines equal the fracture energy(Jf). Results are slightly lower than those given in Table 2, but forAAG-2 + 5% SBS at −22°C come quite close to the corrected ener-gies given in Figure 3. This may be because the generalized locusapproach considers only the energy needed for crack propagation.Hence, it can be concluded that the generalized locus method is able

TABLE 1 Effect of Notch Depth and Specimen Width on FractureToughness (KIc in kN.m−3/2) of AAN and AAG-2 + 5% SBS

Specimen Width (B) (mm)

Binder T (°C) a/W 12.7 19.1 25.4

AAN −16 0.2 48 40 470.4 31 57 510.6 86 54 —a

AAN −22 0.2 42 39 410.4 53 47 500.6 52 46 42

AAG-2 + 5% SBS −22 0.2 120 135 1200.4 145 155 1360.6 144 154 148

AAG-2 + 5% SBS −28 0.2 172 144 1210.4 168 178 1580.6 159 160 157

NOTE: Values reported are averages from between 4 and 8 repeat tests. AAN isclose to the ductile state at −16°C so this set of data was not the most reproducibleand some of these results may reflect a mixed-mode fracture toughness. aNot all samples were tested in all nine configurations.

TABLE 2 Effect of Notch Depth and Specimen Width on FractureEnergy (GIc in J.m−2) of AAN and AAG-2 + 5% SBS

Specimen Width (B) (mm)

Binder T (°C) a/W 12.7 19.1 25.4

AAN −16 0.2 13.5 9.9 15.10.4 5.4 9.5 8.20.6 7.7 5.4 −

AAN −22 0.2 8.2 10.9 8.90.4 5.8 4.5 5.70.6 3.7 4.4 3.3

AAG-2 + 5% SBS −22 0.2 54.0 51.3 58.80.4 37.7 43.5 35.30.6 25.7 30.5 30.1

AAG-2 + 5% SBS −28 0.2 89.4 73.8 78.30.4 39.3 52.1 44.70.6 27.5 32.0 30.9

NOTE: Not all data were obtained for all 36 combinations.

to yield a fracture energy that is approached by the ASTM and ISOmethods on deeply notched samples.

Effect of Notch Sharpness

An issue that has attracted significant attention in the polymers fieldis how notch sharpening affects fracture properties. Certain poly-mers are notch sensitive, whereas others are less affected. In asphaltbinders the work by Champion and coworkers (9) and Hesp (11)addresses the effect of sharpening the notch with a razor blade.However, only Hesp (11) compares the specimens directly obtainedfrom the silicone mold with the razor-sharpened ones and finds thatthe two sets of data are statistically the same.

This project compared the fracture results obtained from samplespoured in the silicone molds and tested directly with those for sam-

ples of which the notch was sharpened by sliding a fresh razor bladethrough before testing. The data are presented in Figure 5.

It is apparent that there is little difference between the two sets ofdata. Although the higher-toughness binder (AAG-2 + 5% SBS)appears to be somewhat more sensitive to sharpening by a razor blade,the differences are small, and for now it was decided to test all sam-ples made with the silicone molds. The fracture toughness of asphaltbinders is probably low enough that the notch sharpness is of noconsequence in this particular range.

Effect of Binder Type on Low-TemperatureFracture Performance

A number of PG-34 binders tested were used in a pavement trial onHighway 655 north of Timmins, Ontario (3). These binders were aged

60%40%20%0

20

40

60

80

0

20

40

60

80

Gf, corrected, J.m-2Gf, uncorrected, J.m-2

Notch Depth

FIGURE 3 Effect of energy correction on fracture energy of AAG-2 � 5% SBS at �22�C.Relative errors were calculated from differences in replicate measurements. Higherfracture energies (left) are uncorrected; lower ones (right) are corrected.

0.0

0.1

0.2

0.3

0.4

0.5

0.0 4.0 8.0 12.0 16.0 20.0

Uf/B, J/m

a (mm)

Jf = 3.6 J/m2

Jf = 2.2 J/m2

Jf = 12.1 J/m2

Jf = 22.4 J/m2

FIGURE 4 Generalized locus approach to determine fracture energy, Jf (■ � AANat �16�C; ● � AAN at �22�C; ▲ � AAG-2 � 5% SBS at �22�C; ♦ � AAG-2 �5% SBS at �28�C; error bars give �/�20% of the mean).

Edwards and Hesp 41

42 Transportation Research Record 1962

only in the RTFO before testing. Specimens were stored for 20 h at −28°C, followed by testing at 0.01 mm/s and various temperatures.Results for this set of binders are given in Figure 6.

The graph shows a reasonable amount of scatter in the data, mostlikely because only two specimens were tested at each temperature.Furthermore, the fracture data for this figure were obtained closeto the ductile-to-brittle transition temperature, which providesinherently variable results. Despite the high variability, the datashow that there are minor differences in the ductile-to-brittle tran-sition temperature for these binders, but significant differences intemperature susceptibility as reflected in the slopes of the powerlaw fit to the data, even though they all graded between −34°C and−36°C under AASHTO M320. However, close scrutiny alsoreveals that the power law fit provides only limited accuracy andthat there are likely several different processes that are occurringsimultaneously.

The authors envision that when the development of this bindergrading method is completed, a comprehensive specification willhave to set limits on the brittle state fracture energy, the ductile-to-brittle transition temperature, and the temperature susceptibil-ity in the ductile-to-brittle regime as reflected by the slopes inFigure 6.

SUMMARY AND CONCLUSIONS

Given the review of the literature and the results presented in thispaper, the following conclusions are provided:

• The CT test developed in this project appears to be able to pro-duce highly reproducible results for fracture toughness (KIc) andgood reproducibility for fracture energy (Gf) with pooled standarddeviations from 5% to 10% and from 15% to 20%, respectively.

• The KIc measured appears to be independent of specimen size,geometry, and notch depth and hence should be useful for performancegrading of asphalt binders in their brittle state.

• The Gf measured in CT varied with notch depth. Lower ener-gies are obtained with deeper notches. This suggests that a signifi-cant amount of plastic deformation occurs in areas away from thenotch tip, which adds to the apparent fracture energy. An energy cor-rection obtained through the testing of an unnotched sample cansolve this problem although it is recommended that deeper notchesbe used to avoid this extra test.

• The generalized locus method for determining the fractureenergy appears to give results similar to those of the ASTM-basedprotocol and hence may provide an alternate route to the brittle statefracture energy.

• The crack tip opening displacement (δt) is easily measured inthe CT test and may provide a good measure of performance inregard to low-temperature cracking.

ACKNOWLEDGMENTS

The assistance of Serban Iliuta during the early stage of this project isgratefully acknowledged. Financial support for this work was gener-ously provided by the Ministry of Transportation of Ontario throughits 2004 Highways Innovations Funding Program, NCHRP through anInnovations Deserving Exploratory Analysis contract, Imperial Oil ofCanada through a university research award, E.I. du Pont Canada, andthe Natural Sciences and Engineering Research Council of Canada.

REFERENCES

1. Yee, P., B. Aida, S. A. M. Hesp, P. Marks, and K. K. Tam. Analysis ofPremature Low-Temperature Cracking in Three Ontario, Canada, Pave-ments. In Transportation Research Record: Journal of the Transporta-

0

40

80

120

160

200

0 40 80 120 160 200

Directly Tested KIc (kN.m-3/2)

Raz

or S

harp

ened

(K

Ic, k

N.m

-3/2

)

0

20

40

60

80

100

0 20 40 60 80 100

Directly Tested (Gf, J.m-2)

Raz

or S

harp

ened

(G

f, J.

m-2

)

(a) (b)

FIGURE 5 Effect of razor sharpening the notch.

1

10

100

1000

10000

0.00385 0.0039 0.00395 0.004 0.00405

1/T (K-1)

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ctur

e E

nerg

y (J

.m-2

)

FIGURE 6 Ductile-to-brittle transitions in fracture energy forHighway 655 binders (� � 655-1; • � 655-2; ▲ � 655-3;♦ � 655-4; � � 655-5; ▫ � 655-6; and � � 655-7).

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None of the sponsoring agencies necessarily concurs with, endorses, or hasadopted the findings, conclusions, or recommendations either inferred or expresslystated in subject data developed in this study.

The Characteristics of Bituminous Materials Committee sponsored publicationof this paper.

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