Construction and Building Materials 25 (2011) 2014–2022

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Construction and Building Materials

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Performance evaluation of gap graded Asphalt Rubber mixtures

E. Pasquini ⇑, F. Canestrari, F. Cardone, F.A. SantagataDipartimento di Idraulica, Strade, Ambiente e Chimica, Università Politecnica delle Marche, via Brecce Bianche, 60131 Ancona, Italy

a r t i c l e i n f o a b s t r a c t

Article history:Received 25 May 2010Received in revised form 27 September2010Accepted 13 November 2010Available online 9 December 2010

Keywords:Asphalt RubberMechanistic Empirical Pavement Design

GuideMechanical properties

0950-0618/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2010.11.048

⇑ Corresponding author. Tel.: +39 071 220 4507; faE-mail address: e.pasquini@univpm.it (E. Pasquini

In this research, an extensive laboratory test program was conducted in order to investigate the mechan-ical performance (stiffness, fatigue, permanent deformation and thermal cracking) of a gap gradedAsphalt Rubber Asphalt Concrete (ARAC) establishing an important database of ARAC engineering perfor-mance. Additional tests were performed on Asphalt Rubber binder and ARAC mix in order to verifywhether the new Mechanistic Empirical Pavement Design Guide (MEPDG) can be used effectively forAR materials. Results were compared with those of reference materials tested in laboratory or found inliterature.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction dicted by using mixture volumetric and asphalt properties. As far

Asphalt Rubber (AR) binder is a blend of plain bitumen andcrumb rubber produced from reclaimed tires (ASTM D8). Manyworldwide researches have demonstrated that AR is able to en-hance the mechanical performance of bituminous mixes [1–9]simultaneously creating an environmental benefit by re-using awaste material that otherwise would be disposed or burned. More-over, AR binder seems to reduce rolling noise due to the lower stiff-ness of the bituminous mix which positively influences themechanism of noise generation from vibration source [9–11].

This paper summarizes results obtained from an extensive lab-oratory test program conducted to carefully assess the mechanicalperformance (stiffness, fatigue, permanent deformation and ther-mal cracking) of a gap graded AR mix produced at the plant. More-over, additional tests were performed on AR binder and AsphaltRubber Asphalt Concrete (ARAC) mix in order to verify whetherthe new Mechanistic Empirical Pavement Design Guide (MEPDG)[12] can be used effectively for AR materials. The MEPDG, devel-oped under the NCHRP Project 1-37A, utilizes material propertiesto predict distresses in pavement structures. As far as asphalt pave-ments are concerned, the Design Guide utilizes three hierarchicallevels of analysis in decreasing order of accuracy from Level 1 toLevel 3. The input required for Hot Mix Asphalts will vary accord-ing to the selected level of analysis. In particular, at Level 1, theHMA dynamic modulus E* is calculated from a master curve thatis constructed from laboratory data concerning complex modulusmeasurements and binder tests; while, at Levels 2 and 3, E* is pre-

ll rights reserved.

x: +39 071 220 4510.).

as the asphalt binder is concerned, two alternatives for providingtest data are available for Levels 1 and 2: Superpave (complexmodulus G* and phase angle at 10 rad/s) and conventional bindertest data (softening point, penetration and viscosity). On the con-trary, the binder input for Level 3 does not require laboratory testdata, but they are estimated on the basis of typical temperature–viscosity relationships. However, the calibration process under-taken for the MEPDG did not include any AR mixes. Thus, collectedresults could be usefully employed as input data for MEPDGimplementation.

2. ARAC characteristics

An ARAC mixture produced at the plant using basaltic coarseaggregates was selected to be investigated within this researchproject. The mixture composition is detailed in Table 1 while mainbinder characteristics are reported in Table 2. 50/70 penetrationbase asphalt was employed to prepare the AR binder at the asphaltplant. Air void content, voids in mineral aggregate (VMA) and VMAfilled with binder (VFB) of ARAC, according to EN 12697-8, werealso calculated as mean values of all tested samples. Specimenswere prepared using shear gyratory or roller compactor in orderto obtain a target air void content of 6%.

3. Test methods

3.1. AR binder characterization

MEPDG requires a viscosity–temperature relationship, given inEq. (1), as binder properties input data needed for all design levels:

Table 1Characteristics of Asphalt Rubber Asphalt Concrete (ARAC).

Sieves (mm) Granulometriccomposition(% passing)

Gradation limits(% passing)

20 100.0 100.012 99.2 85.0–100.08 69.3 57.0–71.04 29.7 24.0–35.02 18.7 12.0–20.00.5 8.2 8.0–14.00.25 5.6 5.0–9.00.063 2.5 2.0–5.0

AR binder content (%) 7.9Rubber content (% on bitumen) 20%Air void content (%) 5.8VMA (%) 22.0VFB (%) 73.6

Table 2AR binder characteristics.

Characteristic Test method Unit Value

Maximum crumb rubber size ASTM D5644 mm 0.85Penetration at 25 �C EN 1426 0.1 mm 48Softening point EN 1427 �C 59Fraass breaking point EN 12593 �C �14Dynamic viscosity at 175 �C EN 13302 mPa s 1800Elastic recovery at 25 �C EN 13398 % 70After RTFOT EN 12607-1Change of mass EN 12607-1 % 0.42Retained penetration at 25 �C EN 1426 % 46Increase in softening point EN 1427 �C 14Elastic recovery at 25 �C EN 13398 % 66

E. Pasquini et al. / Construction and Building Materials 25 (2011) 2014–2022 2015

LogLogg ¼ Aþ VTSLog TR ð1Þ

where g is the binder viscosity (cP), TR is the temperature (�R), A isthe regression intercept and VTS is the regression slope of viscositytemperature susceptibility.

In particular, this relationship can be extrapolated from labora-tory test results for both Levels 1 and 2, while binder viscosityinformation (A and VTS) for Level 3 is estimated from typical tem-perature–viscosity relationships [12].

As far as laboratory data are concerned, two alternatives arepossible: Superpave (complex modulus G* and phase angle at10 rad/s) or consistency test data (softening point, penetrationand viscosity).

In this study, direct measurements of AR binder complex mod-ulus G* by means of a dynamic shear rheometer, according to SHRPspecifications [13], were performed at different temperatures (40,46, 52, 58, 64, 70, 76, 82 �C). Next, asphalt stiffness (G* in Pa) andphase angle (d) data for a loading rate of 1.59 Hz were convertedto viscosity units (Pa s) using Eq. (2) [12]. This allowed the viscos-ity–temperature susceptibility parameters A and VTS to be ob-tained according to the above mentioned Eq. (1):

g ¼ G�

101

sin d

� �4:8628

ð2Þ

As an alternative, penetration at 25 �C (EN 1426) and softeningpoint (EN 1427) together with dynamic viscosity determinationsusing the Brookfield Viscometer (EN 13302) at different tempera-tures (60, 100, 135, 150, 160, 175 �C) were also performed on theselected AR binder. The results allowed to determine the viscos-ity–temperature susceptibility of crumb rubber modified asphaltbinder also taking into account the conversion of penetration andsoftening point measurements into viscosity units [12]. In particu-

lar, it is assumed that all asphalts at their softening point will yielda penetration of approximately 800 dmm and a viscosity (g) of1.3 � 106 centipoises (cP), while penetration data (Pen) are con-verted to viscosity units (in Poise) according to the followingequation:

Logg ¼ 10:5012� 2:2601LogðPenÞ þ 0:00389LogðPenÞ2 ð3Þ

Next, dynamic viscosity data together with converted penetra-tion and softening point results allowed to obtain the viscosity–temperature susceptibility parameters A and VTS according to theabove mentioned Eq. (1).

3.2. ARAC stiffness modulus and fatigue resistance

In order to assess the bearing capacity and fatigue crackingresistance of the studied bituminous mixture, Indirect Tensile Stiff-ness Modulus (ITSM) and Indirect Tensile Fatigue (ITF) tests werecarried out on six cylindrical samples at 20 �C by means of re-peated load dynamic equipment. Moreover, stiffness data werealso collected at 0, 10 and 30 �C. ITSM values could be used asthe modulus of the bituminous layer when designing pavementusing multi-layered elastic theory.

ITSM tests were carried out according to EN 12697-26 Annex Cwhile ITF tests were performed in controlled stress conditionsaccording to British Standards BS DD ABF applying three differentstress levels with two repetitions for each stress value. Fatigue lifewas assumed as the number of cycles corresponding to physicalfailure of the test sample.

The specimens were prepared with a shear gyratory compactorat 165 �C compaction temperature. The final dimensions of speci-mens corresponded to a nominal diameter of 100 mm and to athickness of 60 mm in order to obtain a target air void content of6%.

The fatigue test results were also employed to verify the suit-ability of the MEPDG predictive fatigue model given in Eq. (4)[12] to also describe the behaviour of the selected AR gap gradedmaterial:

Nf ¼ k11et

� �k2 1E

� �k3

ð4Þ

where Nf is the number of repetitions to fatigue cracking, et is thetensile strain at the critical location, E is the stiffness of the materialand k1, k2 and k3 are laboratory calibration coefficients. In this dam-age model k1 can be expressed as a function of the total thickness ofthe asphalt layers hac (inches), the effective binder content Vb (%)and the air void content Va (%) as illustrated in Eq. (5):

k1 ¼ 0:00432� 10:000398þ 0:003602

1þeð11:02�3:49hac Þ

� 104:84

VbVaþVb

�0:69

� �ð5Þ

3.3. ARAC dynamic complex modulus

The MEPDG uses the dynamic modulus E* as the primary mate-rial property of HMA mixtures in predicting pavement distresses[12]. E* data of an HMA provide very important information aboutthe linear viscoelastic behaviour of the material. Thus, cyclic uniax-ial unconfined compression tests were also conducted on ARAC inorder to determine the dynamic complex modulus E*. The cylindri-cal specimens (diameter £ = 100 mm; height h = 150 mm) werecompacted using a shear gyratory compactor to obtain a targetair void content of 6%. A set of 6 replicates were tested at four tem-peratures (0, 10, 20 and 30 �C) and six frequencies (20, 10, 5, 2, 1and 0.5 Hz). Each sample was tested in an increasing order of tem-peratures and decreasing order of frequency to cause the minimumdamage to the specimen. To account for the effect of temperature

2016 E. Pasquini et al. / Construction and Building Materials 25 (2011) 2014–2022

and loading rate, the complex modulus master curve, at referencetemperature of 20 �C, was constructed using the time–temperaturesuperposition principle. A well-known sigmoidal function [14],illustrated in Eq. (6), was adopted to mathematically model themaster curve as recommended in the MEPDG design guide [12]:

LogjE�j ¼ dþ a1þ eb�c logðfrÞ

ð6Þ

where fr is the reduced frequency, d and a represent the span ofmodulus value and b and c are the shape parameters of the curve.According to previous studies [15], the well-known Arrhenius law[16], shown in Eq. (7), was selected to describe the shift factors:

LogðaTÞ ¼ C1T� 1

Ts

� �ð7Þ

where aT is the shift factor, C is a material constant (K), T is the test-ing temperature (K) and Ts is the reference temperature (K).

As an alternative, it has already been mentioned that, for Levels2 and 3, MEPDG suggests to predict dynamic modulus values byusing the Eq. (8) [12]:

LogjE�j ¼ 3:750063þ 0:02932q200 � 0:001767ðq200Þ2 � 0:002841q4 � 0:058097Va � 0:802208

Vbeff

Vbeff þ Va

� �

þ 3:871977� 0:0021q4 þ 0:003958q38 � 0:000017ðq38Þ2 þ 0:00547q34

1þ e½�0:603313�0:313351 logðf Þ�0:393532 logðgÞ� ð8Þ

where, for a given temperature, E* is the dynamic modulus (psi),q200 is the passing the No. 200 sieve (%), q4 is the retained on theNo. 4 sieve (%), Va is the air void content (%), Vbeff is the effectivebitumen content (% by volume), q38 is the retained on the 3/8 insieve (%), q34 is the retained on the 3/4 in sieve (%), f is the loadingfrequency (Hz) and g is the bitumen viscosity (106 Poise) at the se-lected temperature.

Given the fact that the calibration of the predictive Eq. (8) didnot include any AR mixes, the aggregate gradation, the volumetricproperties and the binder viscosity data of ARAC were utilized toverify if this predictive equation can also be adopted effectivelyfor AR materials comparing measured E* with predicted E* for bothLevels 2 and 3.

3.4. ARAC permanent deformation resistance

Triaxial Cyclic Compression Test (TCCT) and Wheel TrackingTest (WTT) were carried out to assess the permanent deformationresistance of the selected bituminous mixture.

TCCT was carried out according to EN 12697-25 at 50 �C. Testsamples were 100 mm diameter and 80 mm thick cylindrical spec-imens manufactured with a shear gyratory compactor to 6% targetair voids. A haversine load at 3 Hz was selected. The constant con-fining pressure was fixed at 150 kPa while the peak axial load was600 kPa. Three replicates were tested for 10,800 loading cycles. Thecumulative axial strain of the test specimen is determined as afunction of the number of load applications. The cumulative per-manent strain curve is generally defined by three zones. In the pri-mary zone, permanent deformations accumulate rapidly, thenincremental permanent deformations decrease reaching a constantvalue in the secondary zone. Finally, they increase again and per-manent deformations accumulate rapidly in the tertiary zone.The creep characteristics of the specimen are generally determinedin terms of maximum permanent deformation strain, creep rate,

i.e. the slope of the (quasi) linear part of the creep curve, and flownumber, i.e. the number of load repetitions at which tertiary phasestarts [17]. Practically, the flow number is the number of repeti-tions corresponding to the minimum rate of change in permanentstrain evolution. The TCCT test results were also used to verify thesuitability of the predictive rutting model used in the MEPDG andgiven in Eq. (9) [12] to describe the behaviour of the selected ARgap graded material also:

ep

er¼ a1Ta2 Na3 ð9Þ

where ep is the accumulated plastic strain at N repetitions of load, er

is the resilient strain of the asphalt material, T is the temperature(�F) and a1, a2 and a3 are non-linear regression coefficients. In thisdamage model er is a function of the complex modulus E*, the Pois-son’s ratio l and the three-dimensional stress state as illustrated inEq. (10).

er ¼1E�ðrz � lrx � lryÞ ð10Þ

The WTT was carried out according to BS 598-110 for a durationof 45 min. The test was carried out at 45 �C and 60 �C to investigatethe temperature dependency of ARAC rutting resistance. This testused 30 mm height slabs (305 � 305 mm2) prepared with a rollercompactor according to EN 12697-33. Two replicate samples weretested for each temperature. Slabs were subjected to a wheel loadof 520 N which moves backwards and forwards at 21 cycles/minfor a 230 mm travel distance. Final rut depth, BS rut rate, i.e. theasymptotic rate of increase in rut depth over the final 15 min ofthe test, and Dynamic Stability (passes/mm) defined in Eq. (11),i.e. the inverse of the slope of creep curve between 30th and45th min, were selected to evaluate the mixture’s ruttingresistance.

DS ¼ N150

d45 � d30ð11Þ

where N150 is the number of loading passes in 15 min and d45 � d30

is the change in rut depth (mm) in the last 15 min of the test.Observing Eq. (11), it is clear that higher DS values mean betterrut resistance.

3.5. ARAC thermal cracking resistance

The selected ARAC mix was also subjected to Indirect TensileStrength tests (AASTHO T322) at 0 �C and �10 �C in order to createa database of AR mixture thermal cracking properties as suggestedby the new MEPDG. Mixture cracking performance was indicatedby Indirect Tensile Strength (ITS), Fracture Energy to Failure(FEF), i.e. the area under the load-vertical deformation curve untilthe maximum axial load is reached, and Total Fracture Energy(TFE), i.e. the area under the whole load-vertical deformationcurve. Test samples were cylindrical specimens, 150 mm in diam-eter and 38 mm height, sawed from gyratory fabricated samples

Table 4Summary of A and VTS parameters.

Superpave data Consistency data Default data

A 7.8023 8.3676 10.5254VTS �2.5366 �2.7264 �3.5047R2 0.9901 0.9928 –

1.0

1.2

E. Pasquini et al. / Construction and Building Materials 25 (2011) 2014–2022 2017

characterized by 6% target air voids. Three replicate specimenswere used to investigate thermal cracking characteristics of ARAC.

At Levels 1 and 2 of the MEPDG, actual laboratory test data arerequired, while for Level 3, a regression equation detailed in Eq.(12) is given for ITS (psi) at �10 �C:

ITS ¼ 7416:712� 114:016Va� 0:304Va2 � 122:592VFB

þ 0:704VFB2 þ 405:71LogðPenÞ � 2039:296LogðAÞ ð12Þ

where Va is the air void content (%), VFB is the voids filled with bitu-men (%), Pen is binder penetration at 25 �C (dmm) and A is the vis-cosity–temperature susceptibility intercept. Collected data allowedthe applicability of this regression equation for AR mixes to beverified.

4. Results and analysis

In the following paragraphs, the main results obtained from theexperimental program detailed in Section 3 are illustrated.

4.1. Viscosity–temperature relationship for the Asphalt Rubber binder

Results obtained from the characterization of AR binder areshowed in Table 3. These data were similar to what can be foundin literature [1,3,6,18]. The elevated softening point and high vis-cosity values of the AR binder suggest a relevant resistance to per-manent deformation for this kind of bituminous material. Complexshear modulus and corresponding phase angle data confirmed thisbehaviour. In fact, according to SHRP specifications [19], the se-lected AR binder is characterized by a Performance Grade withan upper limit of 76 �C.

The data presented in Table 3 were converted to viscosity as ex-plained in Section 3.1 and used to determine the viscosity–temper-ature susceptibility parameters A and VTS. In particular, as far asSuperpave data are concerned, asphalt stiffness (G*) and phase an-gle (d) data were converted to viscosity units using Eq. (2). Thenviscosity converted data were used to obtain A and VTS through lin-ear regression according to Eq. (1). Similarly, as far as consistencytests are concerned, penetration and softening point were con-verted into viscosity units as explained in Section 3.1 and usedwith viscosity data to obtain A and VTS through linear regressionaccording to Eq. (1).

Table 4 shows the results of the regression analysis concerningthe two approaches provided by the MEPDG (Superpave or

Table 3Summary of binder characterization tests.

Temperature (�C) Superpavetests

Consistency tests

G* (Pa) d (�) Penetration(0.1 mm)

Viscosity (cP) Softeningpoint (�C)

25 – – 48 – –40 37,355 66.1 – – –46 17,138 67.4 – – –52 7342 70.3 – – –58 3869 72.3 – – –60 – – – 2,166,667 –64 2345 71.0 – – –70 1513 71.9 – – –76 945 73.1 – – –82 655 73.3 – – –

100 – – – 23,091 –135 – – – 3093 –150 – – – 1446 –160 – – – 1258 –175 – – – 825 –– – – – – 59

consistency tests) together with the recommended default A andVTS values given by the MEPDG for the Level 3. Fig. 1 shows a com-parison of the three alternative viscosity–temperature relation-ships for the studied AR binder. Results denote comparable A andVTS coefficients obtained from complex shear modulus tests andfrom consistency tests. In addition, the high value of the coeffi-cients of determination R2 seem to demonstrate that these charac-terization tests are adequate to describe the viscosity–temperaturesusceptibility of the investigated crumb rubber modified binder.On the contrary, the default values provided by the MEPDG andbased on the binder penetration grade, turned out to be rather dif-ferent to the others. This fact indicates that the use of default A andVTS values is questionable in case of an AR binder. However, theserecommended values can be selected as input parameters only forthe lower accurate Level 3 of the M-E design process.

Comparing A and VTS parameters obtained by laboratory testson the selected AR binder with the default values or with resultsavailable in literature [3,18], it is possible to observe lower A val-ues, desirable for better permanent deformation resistance, andlower absolute values of VTS, typical of binders that are less sus-ceptible to low temperature cracking. These results confirm thewell-known field performance in terms of improved tempera-ture-susceptibility properties of the rubber modified binders.

4.2. Stiffness modulus

Table 5 shows the results of the stiffness modulus analysis car-ried out on the studied AR mixture at 20 �C in indirect tensile con-figuration. In the table, the results obtained for some referencematerials investigated with the same test conditions are also re-

2.70 2.75 2.80 2.85 2.90 2.950.0

0.2

0.4

0.6

0.8

Log

Log

η[c

P]

LogTR [ Ra]

Superpave

Consistency

Default

Fig. 1. AR viscosity–temperature relationships.

Table 5Indirect Tensile Stiffness Modulus at 20 �C.

Mean air voidcontent (%)

Bindercontent (%)

Mean ITSM at20 �C (MPa)

ARAC 5.9 7.9 3297SMA 8 PMB 3.3 7.3 5515AC 12 50/70 8.0 4.6 7028

Table 6ARAC Indirect Tensile Stiffness Modulus.

Test temperature (�C) Mean ITSM (MPa)

0 11,69110 632720 329730 1381

10

100

1000

1000 10000 100000 1000000

Init

ial m

icro

stra

in

Cycles to failure N

Fatigue @ 20°C

ARAC

MEPDG

Finn

CROW (F78)

εε

Fig. 3. Experimental vs. literature fatigue lines (E = 3500 MPa).

Table 7Summary of ARAC dynamic complex modulus.

Freq.(Hz)

0 �C 10 �C 20 �C 30 �C

E* (MPa) s (MPa) E* (MPa) s (MPa) E* (MPa) s (MPa) E* (MPa) s (MPa)

2018 E. Pasquini et al. / Construction and Building Materials 25 (2011) 2014–2022

ported. In particular, a previously studied [7] Stone Mastic Asphalthaving 8 mm maximum aggregate size (SMA 8 PMB) and an As-phalt Concrete having 12 mm maximum aggregate size and manu-factured with a plain 50/70 bitumen (AC 12 50/70) were studied. Itcan be observed that the high amount of AR binder, used togetherwith the selected reduced maximum aggregate size, resulted in alower stiffness for the ARAC mixture. However, the load spreadingability of ARAC mix still remained adequate and comparable withwhat literature data [15,20] thanks to the pronounced rheologicalproperties of the AR binder. On the other hand, the lower ITSM val-ues of ARAC mix could imply a better fatigue behaviour (Section4.3) and enhanced anti-noise properties even if problems in termsof the resistance to permanent deformation could occur. This latteraspect has been investigated in detail in Section 4.5.

For completeness, Table 6 shows the indirect tensile stiffnessof ARAC mixtures at different test temperatures. These valuescould be useful when designing pavement for different seasonalconditions.

4.3. Fatigue resistance

Fig. 2 shows the Indirect Tensile Fatigue behaviour of ARACmixture compared with reference materials. It is worth noting thatARAC and SMA mixes, having similar gradation and binder content,demonstrated similar fatigue life despite the different air void con-tent. Moreover, the experimental data demonstrate a remarkablefatigue resistance of ARAC material compared to reference asphaltconcretes. These results seem to confirm the noteworthy proper-ties of the AR binder, comparable to those of a ‘‘hard’’ polymermodified asphalt. Moreover, these results confirmed the perfor-mance exhibited by similar AR materials and mentioned in previ-ous studies [20].

In Fig. 3, the experimental fatigue data are compared with othertransfer functions currently used in pavement design [12,21,22]and plotted assuming a stiffness modulus equal to 3500 MPa. Con-sidering that controlled stress tests underestimate fatigue life, re-sults showed that a good fatigue behaviour of the ARAC materialcan be anticipated. In particular, it is important to highlight thatthe experimental data collected in the investigated tensile strain

10

100

1000

1000 10000 100000 1000000

Init

ial m

icro

stra

in εε

Cycles to failure N

Fatigue @ 20°C

ARAC

SMA 8 PMB

AC 12 50/70

Fig. 2. ARAC fatigue line.

range for the ARAC mixture fitted well the fatigue life predictedby the MEPDG. However, the ARAC fatigue line is characterizedby a lower slope denoting better fatigue resistance for low strainvalues and vice versa [5,23].

4.4. Dynamic complex modulus

Tables 7 and 8 show the test results of dynamic modulus E* andphase angle / for the ARAC Gap Graded mixture at each test tem-perature and frequency obtained through uniaxial compressiontests. These data are expressed as the average of the six replicates.Standard deviation s of the six replicates is also indicated. It isworth noting that axial complex modulus absolute values werenot pronounced as experienced by the indirect tensile tests (Sec-tion 4.2). Moreover, being equal the test temperature and fre-quency (2 Hz), ITSM values (Table 6) demonstrated to be quitelower than those obtained in the uniaxial compression configura-tion. Uniaxial E* data are plotted in Fig. 4 in terms of isochronaland isotherm curves, while Fig. 5 shows the experimental resultsin a Black space (dynamic modulus vs. phase angle) and in aCole–Cole plane (elastic component E1 vs. viscous component E2),respectively. The experimental results showed a higher data scat-tering at 0 �C test temperature. Finally, Fig. 6 shows the smooth

0.5 8957 586 5802 554 2992 129 1422 1031 10,154 945 6704 643 3580 164 1816 1482 11,966 1225 7494 558 4066 179 2167 1335 13,116 1306 8776 748 5292 187 2606 24510 13,041 984 9750 814 6104 190 3420 43620 15,117 1373 10,857 901 6977 513 4053 420

Table 8Summary of ARAC phase angle.

Freq. (Hz) 0 �C 10 �C 20 �C 30 �C

/ (�) s (�) / (�) s (�) / (�) s (�) / (�) s (�)

0.5 16.4 1.4 22.2 1.3 29.2 0.3 31.2 1.91 15.2 1.0 21.0 1.5 26.8 0.5 28.4 1.02 15.4 1.3 20.1 1.4 24.9 2.2 28.3 0.65 15.0 1.1 18.6 1.4 23.0 0.8 27.3 0.710 14.4 1.3 17.5 1.1 21.1 1.6 26.3 0.720 11.8 1.2 16.3 1.0 20.8 0.7 25.0 1.5

1000

10000

100000

0 5 10 15 20 25 30

E*

(MPa

)

Temperature °C

f = 20 Hz

f = 10 Hz

f = 5 Hz

f = 2 Hz

f = 1 Hz

f = 0.5 Hz

1000

10000

100000

0.1 1.0 10.0 100.0

E*

(MPa

)

Frequency (Hz)

T = 0 °C

T = 10 °C

T = 20 °C

T = 30 °C

Fig. 4. ARAC isochronal and isotherm curves.

0

5000

10000

15000

20000

0 10 20 30 40

E*

(MPa

)

Phase Angle

0

2000

4000

6000

0 5000 10000 15000 20000

E2

(MPa

)

E1 (Mpa)

T = 0 ºC

T = 10 ºC

T = 20 ºC

T = 30 ºC

T = 0 ºC

T = 10 ºC

T = 20 ºC

T = 30 ºC

Fig. 5. ARAC black diagram and Cole and Cole curves.

1.0E+03

1.0E+04

1.0E+05

1.0E-02 1.0E-01 1.0E+00 1.0E+01 1.0E+02 1.0E+03 1.0E+04

E*

(MPa

)

Reduced Frequency (Hz)

T = 0 ºC

T = 10 ºC

T = 20 ºC

T = 30 ºC

Master Curve @ 20 ºC

Fig. 6. ARAC master curve at 20 �C.

E. Pasquini et al. / Construction and Building Materials 25 (2011) 2014–2022 2019

sigmoidal master curve at 20 �C reference temperature developedusing the average values of the six replicates.

Then, the volumetric properties and the binder viscosity datawere utilized to compare measured E* with predicted E* accordingto the MEPDG predictive model. In particular, three different E*

were calculated for each test condition using the binder viscositydata coming from Superpave tests, consistency tests and defaultvalues, respectively (Section 4.1). The experimental data togetherwith the predicted E* are plotted in Fig. 7, while Table 9 summa-rizes the sigmoidal function regression parameters (d, a, b, c) andthe Arrhenius constant C for each case. As opposed to what hasbeen found in previous studies [3], it is worth noting in Fig. 7 thatpredicted dynamic modulus values clearly overestimate measuredE* in all cases. However, E* calculated with binder viscosity datacoming from Superpave DSR tests, which represents the moreaccurate design procedure, were similar to experimental datawhen high test temperatures and low test frequency were consid-ered. On the other hand, the E* predicted when utilizing MEPDGdefault binder characteristics were noticeably higher than thosemeasured in laboratory confirming its low level of accuracy. Theconstruction of the master curves, whose parameters are listed inTable 9, confirmed that AR material is characterized by reduced dy-namic modulus at both warm and cold temperatures than thosecalculated according to MEPDG predictive equations. It should beremembered that a lower stiffness at cold temperatures is advis-able for better cracking resistance, while a low modulus at hightemperatures would not assure ARAC mix has a good resistanceto permanent deformation. In this sense, previous studies [2–4,18] have found that results of unconfined tests are not suitable

for a good characterization of AR performance against permanentdeformation. Finally, the Arrhenius constant found in each casewas consistent with that reported in literature [15,16].

4.5. Permanent deformation

Fig. 8 and Table 10 show the results of Wheel Tracking Test at45 �C and 60 �C comparing ARAC performance with previouslystudied reference materials. In particular, the already mentionedSMA 8 PMB (Section 4.2) and a dense graded Asphalt Concrete

E*

(MPa

)

1.0E+03

1.0E+04

1.0E+05

1.0E+06

1.0E+03 1.0E+04 1.0E+05 1.0E+06

Pred

icte

d E

* (M

Pa)

Measured E*(Mpa)

Superpave

Consistency

Default

1.0E+03

1.0E+04

1.0E+05

1.0E+06

1.0E-02 1.0E-011.0E+001.0E+011.0E+021.0E+031.0E+041.0E+05Reduced Frequency (Hz)

Experimental data

MEPDG - Superpave

MEPDG - Consistency

MEPDG - Default

Fig. 7. Measured vs. predicted E*.

Table 9Master curve regression parameters.

C (�K) d a b c

Experimental data 9270 1.2252 3.1780 �1.0022 0.4229MEPDG – Superpave 7791 1.6599 3.5913 �0.2161 0.3514MEPDG – consistency 9418 1.9566 3.3210 �0.4865 0.3466MEPDG – default 13,609 2.5455 2.7458 �0.5615 0.3492

2020 E. Pasquini et al. / Construction and Building Materials 25 (2011) 2014–2022

(AC 12 70/100) having 12 mm maximum aggregate size, manufac-tured with 6% of a plain 70/100 bitumen and characterized by 4.5%of air voids [24] were selected. At 45 �C, the results for the AC 1270/100 mixture were not available. According to literature [1,3–6,20,25], experimental results in terms of Rut Depth (RD), BS RutRate (BS RR) and Dynamic Stability (DS) show that the gap gradedmixture prepared with AR binder demonstrated very good ruttingresistance comparable with that of SMA notwithstanding the high-er AR binder content employed. It is important to remember thatgap graded mixtures, such as ARAC and SMA, are high rut resistantmaterials thanks to their strong interlocking coarse aggregatestructure. On the other hand, it is well known that the higher thebinder content the lower the permanent deformation resistance.Thus, ARAC material clearly outperformed AC 12 70/100 referencemixture thanks not only to the gap graded gradation but also to thepeculiar properties of the AR binder. These assumptions have alsobeen validated by literature results obtained through the same testprocedure [20,25,26]. Finally, it is worth noting in Table 10 thatARAC material had similar performance at both 45 �C and 60 �C,while SMA mixture demonstrated higher temperature susceptibil-

0.00

0.50

1.00

1.50

2.00

2.50

0 15 30 45 60

Rut

dep

th (

mm

)

Time (min)

WTT @ 45 °C

ARAC

SMA 8 PMB

Fig. 8. WTT experimental results at 45 �C (l

ity. This fact highlighted the remarkable properties of the selectedAR binder.

For a complete characterization of the ARAC mix, this materialwas also subjected to the performance based Triaxial Ciclic Com-pression Test according to EN 13108 as required by the fundamen-tal approach for Asphalt Concrete specifications. In particular, theEuropean specifications require to express the resistance to perma-nent deformation in the triaxial compression test in terms of creeprate fc defined as B1 � 104, where B1 is the slope from the leastsquare linear fit of the (quasi) linear part of the creep curve (stage2). fc can be declared in the CE conformity marking of the material.The results obtained are summarized in Table 11 and Fig. 9 wherethe least square linear fit of the (quasi) linear part of the creepcurve is also depicted. It seems clear that ARAC mix showed a sig-nificant permanent deformation resistance. In fact, based on thecreep rate value provided by the EN 13108 for Asphalt Concretespecification, materials showing the best permanent deformationbehaviour are characterized by a fc value lower than 0.2 lstrain/cycle.

Furthermore, it should be noted that it was not possible to cal-culate the flow number (FN) because the tertiary phase did not oc-cur before the test ended. This fact can be demonstrated if the rateof change in axial strain curve is depicted. The FN is defined as thelowest point of this curve [17] and Fig. 10 shows that this pointwas not reached yet.

The TCCT test results were also used to depict the rate of accu-mulation ep/er for the ARAC mixture compared with the one fromthe MEPDG (Fig. 11). er was calculated according to Eq. (10) usingthe stress state applied during TCCT test and the complex modulus

0.00

0.50

1.00

1.50

2.00

2.50

0 15 30 45 60

Rut

dep

th (

mm

)

Time (min)

WTT @ 60 °C

ARAC

SMA 8 PMB

AC 12 70/100

eft) and 60 �C (right) test temperature.

Table 10Summary of WTT results.

45 �C 60 �C

RD(mm)

BS RR(mm/h)

DS(passes/mm)

RD(mm)

BS RR(mm/h)

DS(passes/mm)

ARAC 0.34 0.15 16,706 0.55 0.16 15,556SMA 8 PMB 0.17 0.14 17,723 0.51 0.24 10,613AC 12 70/100 – – – 2.08 1.62 1560

Table 11Summary of TCCT results.

Final permanent strain ep (%) Creep rate fc (lstrain/cycle)

ARAC 0.91 0.04

εp = 4E-06N + 0.87R² = 0.9574

0.00

0.20

0.40

0.60

0.80

1.00

0 3600 7200 10800

Axi

al p

erm

anen

t str

ain

(%)

Loading cycles

TCCT @ 50 °C

Experimental data

(quasi) linear part

Fig. 9. TCCT experimental results.

0.00

0.20

0.40

0.60

0.80

1.00

0 3600 7200 10800

Axi

al p

erm

anen

t str

ain

slop

e (μ

ε/se

c)

Loading cycles

TCCT @ 50 °C

Fig. 10. Rate of change in permanent strain vs. loading cycles.

1

10

100

000010011

ε p/ε

r

Loading cycles

TCCT @ 50 °C

Experimental data

MEPDG

Fig. 11. ep/er relationships.

Table 12Summary of the Indirect Tensile Strength tests.

Test temperature (�C) Pmax (N) ITS (kPa) FEF (kN mm) TFE (kN mm)

0 16,035 1742 37.5 132.6�10 >20,000 >2234 – –

> 2234

24782321

920

0

500

1000

1500

2000

2500

3000

Measured Predicted Superpave

Predicted Consistency

Predicted Default

ITS

@ -

10°C

(kP

a)

Fig. 12. Measured vs. predicted ITS.

E. Pasquini et al. / Construction and Building Materials 25 (2011) 2014–2022 2021

E* deduced from the master curve. The experimental data demon-strated to be very different with respect to those predicted by theMEPDG. In particular, the ARAC material was characterized by low-er ep/er strain ratios. However, it is worth mentioning that theMEPDG rutting model was calibrated on the basis of unconfined re-peated permanent deformation tests while the performed TCCTtests are characterized by 150 kPa confining pressure.

4.6. Thermal cracking

Results from the Indirect Tensile Strength tests for the ARACmix are summarized in Table 12 in terms of maximum axial load

Pmax, tensile strength ITS, energy until failure FEF, and fracture en-ergy TFE. First of all, it should be said that it was not possible to as-sess the ARAC tensile strength at �10 �C because the machine usedfor the indirect tensile tests is equipped to apply 20 kN maximumvertical load while the investigated material would have required ahigher vertical load value to fail.

Comparing these experimental data with literature results [3] itcan be noted that the investigated ARAC mixture showed highertensile strength than reference gap graded AR mixtures and com-parable with dense graded reference mixtures prepared with poly-mer modified binders. As lower thermal cracking would beexpected for mixtures with higher tensile strength values, theexperimental results presented in this research seem to be consis-tent with well-known field observations for AR mixes and withprevious studies [4,20,27]. Fig. 12 shows the comparison betweenthe measured ITS value and the predicted ITS values at �10 �Caccording to the MEPDG model described in Eq. (12). ITS predictedvalues were calculated with the viscosity–temperature susceptibil-ity intercept A coming from Superpave tests, consistency tests ordefault values (Table 4). It is worth noting that assuming a defaultvalue for the A parameter results in an underestimated tensilestrength for the ARAC mixture, while laboratory based A values re-turn rational ITS results.

Previous studies [3,27] indicated FEF or TFE as a better indicatorof thermal fracture than ITS values. Generally, lower thermal crack-ing should be expected as the energy at failure or fracture energy is

2022 E. Pasquini et al. / Construction and Building Materials 25 (2011) 2014–2022

increased. In comparison with literature results [3,27], the extre-mely high TFE values showed in Table 12 demonstrated once againthe excellent properties of the investigated ARAC material. More-over, it is possible to note a big difference between the energy untilfailure and the Total Fracture Energy. This reflects in a high postpeak energy, i.e. in a low propagation of thermal cracking whichmeans high ductility.

5. Conclusions

In this research, a gap graded Asphalt Rubber (AR) mixture wasstudied through an extensive laboratory experimental program inorder to assess its main mechanical properties. Based upon thisstudy, the following conclusions can be drawn:

– the great amount of AR binder used together with the selectedreduced maximum aggregate size reflected in limited stiffnessfor the ARAC mixture;

– fatigue and rutting behaviour of the investigated material wascomparable with that of a previously studied Stone MasticAsphalt, demonstrated to clearly outperform traditional asphaltconcretes;

– permanent deformation properties of the ARAC mix seemed lesssusceptible to test temperature than that of the SMA;

– the ARAC mixture showed excellent thermal cracking resistancebeing characterized by high tensile strength and energy to fail-ure values;

– the comparison between experimental data and the MEPDGpredicted performance suggested that some refinements areneeded to better predict the AR performance that specially con-cern dynamic modulus and permanent deformations.

References

[1] Bertollo SM, Bernucci LB, Fernandes JL, Leite LM. Mechanical properties ofasphalt mixtures using recycled tire rubber produced in Brazil – a laboratoryevaluation. In: Proceedings of the 83rd TRB annual meeting, Washington DC,USA; 2004.

[2] Kaloush KE, Witczak MW, Sotil AC, Way GB. Laboratory evaluation of AsphaltRubber mixtures using the modulus (E*) test. In: Proceedings of the 82nd TRBannual meeting, Washington DC, USA; 2003.

[3] Kaloush KE, Witczak MW, Way GB, Zborowski A, Abojaradeh M, Sotil A.Performance evaluation of Arizona Asphalt Rubber mixtures using advanceddynamic material characterization tests. Final report, College of Engineeringand Applied Sciences, Department of Civil and Environmental Engineering,Arizona State University, Tempe; 2002.

[4] Cao R. Evaluation of new generation of gap graded Asphalt Rubber mixtures.In: Proceedings of the Asphalt Rubber 2009 conference, Nanjing, China; 2009.

[5] Kaloush KE, Nordgren T, Biligiri KP, Zeiada WA, Rodezno MC, Souliman MI,et al. Laboratory evaluation of Asphalt Rubber gap graded mixture in Sweden.In: Proceedings of the Asphalt Rubber 2009 Conference, Nanjing, China; 2009.

[6] Chiu C-T, Lu L-C. A laboratory study on stone matrix asphalt using ground tirerubber. Constr Build Mater 2007;21:1027–33.

[7] Santagata FA, Canestrari F, Pasquini E. Mechanical characterization of asphaltrubber – wet process. In: Proceedings of the 4th international SIIV congress,Palermo, Italy; 2007.

[8] Partl MN, Pasquini E, Canestrari F, Virgili A. Analysis of water and thermalsensitivity of open graded AR mixtures. Constr Build Mater 2010;24:283–91.

[9] Canestrari F, Pasquini E, Santagata FA, Antunes I. Asphalt Rubber: policydisclosure in Italy. In: Proceedings of the Asphalt Rubber 2009 conference,Nanjing, China; 2009.

[10] Sandberg U, Easement JA. Tire/road noise reference book. Kisa: Informex;2002.

[11] Hanson DI, Donavon P, James R. Tire/pavement noise characteristics of HMApavements. J Assoc Asphalt Paving Technol 2005;74:1005–39.

[12] NCHRP 1-37A. Guide for mechanistic-empirical design of new andrehabilitated pavement structures. Final report, Washington DC, USA; 2004.

[13] Strategic Highway Research Program. Binder characterization and evaluation,vol. 1. SHRP report A-367, Washington DC, USA; 1994.

[14] Fonseca OA, Witczak MW. A prediction methodology for the dynamic modulusof in-place aged asphalt mixtures. J Assoc Asphalt Paving Technol1996;65:532–67.

[15] Francken L, Partl M. Complex modulus testing of asphaltic concrete:RILEM Interlaboratory Test Program. Transport Res Rec 1996;1545:133–42.

[16] Francken L, Clauwaert C. Characterization and structural assessment of boundmaterials for flexible road structures. In: Proceedings of the 6th internationalconference on the structural design of asphalt pavements, Ann Arbor, USA;1988.

[17] Witczak MW, Kaloush K, Pellinen T, El-Basyouny M, Von Quintus H. Simpleperformance test for superpave mix design. NCHRP report 465, WashingtonDC, USA; 2002.

[18] Rodezno MC, Kaloush KE. Implementation of Asphalt Rubber mixes into themechanistic empirical pavement design guide. In: Proceedings of the AsphaltRubber 2009 conference, Nanjing, China; 2009.

[19] Strategic Highway Research Program. Binder characterization and evaluation,vol. 4. SHRP report A-370, Washington DC, USA; 1994.

[20] Widyatmoko I, Elliott R, Grenfell J, Airey G, Collop A, Waite S. Laboratoryassessment of workability, performance and durability of rubberised asphaltmixtures. In: Proceedings of the Asphalt Rubber 2009 conference, Nanjing,China; 2009.

[21] CROW. Guideline on PCN assignment in the Netherlands. Report 05–06, Ede,The Netherlands; 2005.

[22] Finn FN, Saraf C, Kulkarni R, Nair K, Smith W, Abdullah A. The use of distressprediction subsystems for the design of pavement structures. In: Proceedingsof the 4th international conference on the structural design of asphaltpavements, Ann Arbor, USA; 1977.

[23] Gallego J, Castro M, Prieto JN, Vassallo JM. Thermal sensitivity and fatigue lifeof gap-graded asphalt mixes incorporating crumb rubber from tire waste.Transport Res Rec 2007;1998:132–9.

[24] Canestrari F, Bocci M, Ferrotti G, Pasquini E. Mechanical characterization ofenvironmentally friendly mixtures. In: Proceedings of the internationalconference on advanced characterisation of pavement and soil engineeringmaterials, Athens, Greece; 2007.

[25] Wong CC, Wong W. Effect of crumb rubber modifiers on high temperaturesusceptibility of wearing course mixtures. Constr Build Mater2007;21:1741–5.

[26] Gibney A, O’Callaghan R, Lohan G, Moore V. Permanent deformation in hotrolled asphalt and stone mastic asphalt. In: Proceedings of the 3rd Europeansymposium on performance and durability of bituminous materials andhydraulic stabilised composites, Leeds, UK; 1999.

[27] Zborowski A, Kaloush KE. Predictive equations to evaluate thermal fracture ofasphalt rubber mixtures. Road Mater Pavement 2007;8:819–33.