90

Transportation Research Record: Journal of the Transportation Research Board,No. 2051, Transportation Research Board of the National Academies, Washington,D.C., 2008, pp. 90–97.DOI: 10.3141/2051-11

Therefore, it becomes a priority to study and determine the effectsthat various types and percentages of RAP have on combined asphaltbinders (RAP binder and virgin binder) and mixture properties.

Previous research (1–6) has shown that the structural performanceof asphalt mixtures containing RAP is generally similar to that ofthe conventional virgin asphalt mixtures. This research also showedthat the properties of the mixtures containing RAP are influencedmainly by the aged RAP binder properties and the amount of RAPin the mixture. Aging causes an increase in the viscosity and modu-lus of the asphalt, which is beneficial for the resistance to permanentdeformation at high service temperatures; however, it also causes anincrease in stiffness and brittleness at intermediate and low servicetemperatures, resulting in reduced resistance to fatigue and low-temperature cracking. Research described elsewhere has shown thatblending charts based on the performance grade (PG) specificationlimits can be developed to select a recycling agent to produce a spe-cific PG binder (7) or to determine the maximum and minimumamount of virgin or RAP asphalt binder (8).

The dynamic modulus of asphalt mixtures is related to the majordistress modes, such as permanent deformation, fatigue, and low-temperature cracking (9). An updated mechanistic–empirical pave-ment design guide (MEPDG) proposed the dynamic modulus ofasphalt mixtures as the key parameter in the flexible pavementdesign that controls the permanent deformation and fatigue crack-ing resistance of asphalt pavements (10). Low-temperature crackingis a predominant distress in asphalt pavements built in the northernUnited States and Canada because of the thermal stress that buildsup in the extreme climate. These thermal cracks will result in the for-mation of the transverse cracks along the pavement and ultimatelyaccelerate the deterioration of the structure. Therefore, the evaluationof fracture resistance for the asphalt mixtures containing RAP is ofinterest to owners and agencies seeking better performing pavementsin these northern climates.

The new MEPDG simulates the thermal cracking by performingthe indirect tensile creep test and indirect tensile strength test. How-ever, this approach does not directly address the crack propagation andthe postpeak behavior of the tested materials as a result of a nonrepre-sentative fracture test. It has been demonstrated that tensile strengthvalues from the indirect tension test (IDT) are not sensitive to parame-ters such as polymer modification type and level and aggregate type(11). As an accelerated performance test developed during the Strate-gic Highway Research Program, the thermal stress restrained speci-men test (TSRST) was widely applied to simulate low-temperaturecracking of asphalt concrete (12–15). Beam or cylindrical specimenswith a length of 250 mm are required in this test, whereby thermally

Effect of Reclaimed Asphalt Pavement(Proportion and Type) and Binder Gradeon Asphalt Mixtures

Xinjun Li, Mihai O. Marasteanu, R. Christopher Williams, and Timothy R. Clyne

Reclaimed asphalt pavement (RAP) has been used in the United Statesfor more than 25 years because of the benefits in costs and environmentalstewardship. The recent substantial increases in asphalt prices have ledasphalt technologists to examine the increase in RAP use. The evalua-tion of the performance of the asphalt mixture containing RAP is thereforea priority for the asphalt materials community. This paper investigates theeffect of RAP percentage and sources on the properties of asphalt mix-tures. Ten asphalt mixtures, including two different RAP sources, threeRAP content percentages (0%, 20%, 40%), and two different asphaltbinders (PG 58-28 and PG 58-34) were investigated in this study. Thecomplex dynamic modulus was performed on all mixtures at differenttemperatures and frequencies, and semicircular bend (SCB) fracturetesting was performed for all mixtures at three low temperatures. Exper-imental results indicate that asphalt mixtures containing RAP have higherdynamic modulus values than the control mixtures containing no RAP.The stiffer asphalt binder was found to result in higher dynamic modu-lus values for both the control and the RAP-modified mixtures. Exper-imental data also show that the RAP source is not a significant factor forthe dynamic modulus at low temperatures, although it significantly affectsdynamic modulus values at high temperatures. In addition to test tem-perature, the RAP percentage was found to significantly affect the SCBfracture resistance of mixtures. However, for the dynamic modulus,values for the softer binder were higher than for the stiffer one at lowtemperatures. No significant statistical relationship between dynamicmodulus and fracture energy was found.

Reclaimed asphalt pavement (RAP) has been used in the United Statesfor more than 25 years because of the benefits in costs and environ-mental protection. Current specifications allow various RAP percent-ages depending on the traffic level. In Minnesota, the Department ofTransportation Specification 2360 allows up to 40% based on thetraffic level and binder grade. These values are based on field expe-rience of the performance of asphalt pavements built with RAP.However, there is very little information available about the effect ofRAP on the mechanical properties of the resulting asphalt mixtures.

X. Li and R. C. Williams, Department of Civil, Construction, and EnvironmentalEngineering, Iowa State University, Ames, IA 50011. M. O. Marasteanu, Depart-ment of Civil Engineering, University of Minnesota, Minneapolis, MN 55455-0116.T. R. Clyne, Office of Materials, Minnesota Department of Transportation, Maplewood,MN 55109-2044. Corresponding author: X. Li, lixinjun@iastate.edu.

Li, Marasteanu, Williams, and Clyne 91

induced stress within the specimen develops until fracture. Thesespecimens can be cored or sawed from slabs, whereas there is diffi-culty in preparing specimens from gyratory compacted or field coredcylinders because of the 250 length. By maintaining the test speci-men at constant length during cooling, the TSRST loads the specimenwith thermally induced stress through fracture failure, but it does notaccount for the effects of traffic loading and the evolution of cracksover time.

All these effects are best taken into consideration utilizing frac-ture mechanics. A recently completed pooled fund study on asphaltmixtures’ low-temperature properties showed that the semicircularbend (SCB) fracture test method, which has received considerableattention in asphalt mixture fracture testing because of its simplic-ity in specimen preparation and loading setup, is relatively sensitiveto material properties and testing conditions (16). The SCB frac-ture test was therefore employed in this research to measure thelow-temperature fracture resistance of asphalt mixtures.

In this paper the tools and test methods described above wereused to evaluate the effect of RAP proportion and type on the asphaltmixture properties, in particular the properties at low temperatures.

MATERIAL AND MIX DESIGN

Ten laboratory-prepared asphalt mixtures were studied, consistingof two RAP sources (identified as MS and SS, respectively), andtwo asphalt binders (PG 58-28 and PG 58-34) were used to pre-pare the asphalt mixtures using the following RAP percentages:0%, 20%, and 40%. The asphalt binders were obtained from Koch(Pine Bend, Minnesota refinery), and the RAP materials were pro-vided by Commercial Asphalt. The single source (SS) was milledfrom Interstate 494 in Maple Grove, Minnesota. This was an old pave-ment that had a Minnesota Department of Transportation (Mn/DOT)2361 (gyratory) mix in the surface course and a 2351 (Marshall) mixin the base course. The multiple sources (MS) consisted of RAP col-lected from different pavements around the Twin Cities metro areaand blended together into a single pile at the mixing plant. Fourvirgin aggregates, all of which passed the current Mn/DOT 2360aggregate specification including the quality requirements, wereselected for the mixtures, and the gradations are shown in Table 1.Kraemer 9⁄16-in. chip is coarse limestone, BA 1⁄2 in. is intermediateglacial gravel, Kraemer sand is fine washed limestone sand, andNelson sand is fine granite sand (100% crushed). A summary of the

mixtures prepared is given in Table 2. Note that the mix designa-tions consist of the following sequence: type of RAP (MS or SS),percentage of RAP added (0%, 20%, 40%), and virgin binder PG(PG-28 or PG-34, identified as 28 and 34). The 0% RAP mixture isthe same for MS and SS and it is included in the analysis as 0RAP28and 0RAP34.

The mix design for the 10 asphalt mixtures followed the Mn/DOTSpecification 2360 for traffic level 2 (< 1 million equivalent single-axle loads), which is essentially based on the Superpave mix designprocedure described in SP-2 (17). A control mixture was designedfirst, which served as a baseline to compare with the other mixtures.The subsequent mixtures were prepared using an aggregate grada-tion as close as practically possible to the control mixture in an effortto minimize additional factors that can affect the results. The percentpassing for the five mix designs and the virgin aggregate percentagesused to prepare the test specimens are shown in Table 3.

Sample Preparation

Cylindrical specimens with dimensions of 150 mm in diameter by170 mm in height were compacted in the laboratory using a BrovoldSuperpave gyratory compactor with 5% target air voids for the com-pacted cylinders. Based on previous experience, the 5% target airvoid specimens when cut or cored produced 4% air void test speci-mens. These cylinders were then cored or cut to prepare specimensfor the dynamic modulus testing and SCB fracture testing, respec-tively. For the dynamic modulus testing, the specimens were coredto 100-mm diameter and saw cut to a final height of 150 mm accord-ing to the procedure recommended in Bonaquist et al. (18). For theSCB specimens, the gyratory compacted cylinder was cut symmet-rically from the middle of the specimen into two SCB slices thatwere 25 mm each in height. The SCB slice cut from cylinders wasthen symmetrically cut again into two semicircular bend sampleswith an original notch that was 15 mm in length and 2 mm in width.

Testing Setup and Procedures

Dynamic Modulus Testing

All tests were performed on a Mechanical Testing Systems, EdenPrairie, Minnesota (MTS) servohydraulic testing system. A TestStar IIcontrol system was used to set up and perform the tests and to collectthe data. The software package MultiPurpose TestWare was used to

TABLE 1 Gradations for Four Virgin Aggregates

Sieve Size Kraemer Kraemer Nelson BA (mm) 9/16 Sand Sand 1⁄2 in.

19 100 100 100 100

12.5 96 100 100 99

9.5 46 100 100 97

6.3 25 98 99 89

4.75 2.8 96 97 81

2.36 2.0 63 63 69

1.18 1.8 43 40 58

0.6 1.7 33 26 42

0.3 1.5 23 15 17

0.15 1.2 9.9 7.8 7.5

0.075 0.8 2.7 5.1 5.0

TABLE 2 Summary of Asphalt Mixtures

Asphalt RAPMixture Binder Content % Source % RAP Designation

1 PG 58-28 5.85 MS 0 0RAP28

2 PG 58-28 5.38 MS 20 MS2028

3 PG 58-28 5.29 MS 40 MS4028

4 PG 58-28 5.32 SS 20 SS2028

5 PG 58-28 5.05 SS 40 SS4028

6 PG 58-34 5.85 MS 0 0RAP34

7 PG 58-34 5.38 MS 20 MS2034

8 PG 58-34 5.29 MS 40 MS4034

9 PG 58-34 5.32 SS 20 SS2034

10 PG 58-34 5.05 SS 40 SS4034

92 Transportation Research Record 2051

custom-design the tests and collect the raw test data. Flat, circularload platens were used to apply the cyclic compressive load to thespecimens. Teflon paper was used to reduce friction at the end plates.

Dynamic compression tests were performed at −20°C, −10°C,4.4°C, 21.1°C, and 37.8°C and frequencies of 25, 10, 5, 1, 0.5, and0.1 Hz. The number of loading cycles for each frequency was 200,200, 200, 100, 20, 15, and 15, respectively. The target strain levelwas 150 microstrain. Testing started with the lowest temperature andproceeded to the highest. At a given temperature, testing began withthe highest frequency of loading and proceeded to the lowest. Thedata acquisition system recorded the last five cycles for analysis at

each frequency with 200 points per cycle. Three replicate specimenswere tested for each asphalt mixture for all five test temperatures. Afourth replicate sample was tested at −20°C, −10°C, and 4.4°C only.

SCB Fracture Testing

The SCB test setup is shown in Figure 1. An MTS servohydraulictesting system was used to perform the tests in an environmentalchamber. The samples were symmetrically supported by two fixedrollers with a span of 120 mm. Teflon tape was used to reduce the

TABLE 3 Asphalt Mixture Gradations and Virgin Aggregate Percentages

Sieve SizePercent Passing

(mm) No RAP + 20%MS + 40%MS + 20%SS + 40%SS

19 100 100 100 100 100

12.5 98.1 97.7 97.2 97.1 96.0

9.5 76.9 79.8 81.1 79.6 80.8

4.75 54.5 58.9 60.6 59.1 61.1

2.36 38.3 43.4 46.6 43.2 46.3

1.18 27.8 32.7 36.3 32.1 35.1

0.6 20.2 22.1 23.1 21.5 21.9

0.3 11.7 13.3 14.7 13.1 14.2

0.15 5.5 6.2 6.8 6.6 7.5

0.075 2.7 3.1 3.3 3.5 4.2

Virgin Aggregate Percentages

Kraemer 9/16 42.0 33.0 27.0 33.0 27.0

Kraemer sand 25.0 18.0 13.0 18.0 13.0

Nelson sand 16.0 14.7 9.7 14.9 10.2

BA 1⁄2 in. 17.0 14.0 9.6 14.2 10.0

Notch

CMOD

Frame

DataAcquisition

LLD

SCB Specimen

Botton

FIGURE 1 SCB experiment setup.

friction from the two rollers. An IDT loading plate was used to loadthe SCB specimens. The load line displacement (LLD) was mea-sured using a vertically mounted Epsilon extensometer with a 38-mmgauge length and ±1-mm range; one end was mounted on a buttonthat was permanently fixed on a specially made frame, and the otherend was attached to a metal button glued to test samples. Crackmouth opening displacement (CMOD) was recorded by an Epsilonclip gauge with 10-mm gauges length and a +2.5- and −1-mm range.The clip gauge was attached at the bottom of the specimens. TheCMOD signal was used as the control signal to maintain the test sta-bility in the postpeak region of the test. A constant CMOD rate of0.0005 mm/s was used and the load and LLD were recorded. A maxi-mum contact load of 0.3 kN was applied before the actual loading toensure uniform contact between the loading plate and the specimen.Testing ceased when the load dropped to 0.5 kN in the postpeak region.

Three testing temperatures (−12°C, −24°C, and −36°C) wereselected and three replicates were tested at each temperature for allmixtures. Liquid nitrogen tanks were used to obtain the required lowtemperature. Before testing, the SCB samples were kept in the envi-ronmental chamber at the test temperature for 2 h to avoid any tem-perature gradient within the samples. The temperature was controlledby an MTS temperature controller and verified using an independentplatinum resistive thermal devices (RTD) thermometer.

Experimental Results

Dynamic Modulus Results

The raw data were converted to dynamic modulus data using a mod-ified version of the SINAAT 2.0 program (19). The resulting indi-vidual temperature data were then shifted to obtain master curves ofthe dynamic modulus versus frequency at a reference temperature.This was done based on the assumption that the asphalt mixturesare thermorheologically simple materials and the time–temperaturesuperposition principle is applicable. The master curves were con-structed fitting a sigmoidal function to the measured compressivedynamic modulus test data using nonlinear least squares regressiontechniques (20):

where

log �E*� = log of dynamic modulus,δ = minimum modulus value,fr = reduced frequency,α = span of modulus values,ST = shift factor according to temperature, and

β, γ = shape parameters.

log ( )log

Ee f sr T

∗ = ++ − ( )+[ ]δ α

β γ11

The shift was done by solving the shift factors simultaneouslywith the coefficients of the sigmoidal function. For comparison pur-poses, all master curves were obtained at a reference temperature of4.4°C. The commercial computer program SigmaStat was used tofit the master curve for each set of data. The fitted data were thenused to calculate the average dynamic modulus for each mixture.

SCB Fracture Testing

As previously described, the fracture tests in this study were per-formed at relatively low temperatures. The fracture energy in thisstudy was calculated according to the RILEM (Réunion Internationaledes Laboratoires et Experts des Matériaux) Technical CommitteeTC 50-FMC (21) that has been extensively used in the study of port-land cement concrete. Fracture work is the area under the loading-deflection curve and fracture energy (Gf) can be obtained by dividingfracture work with ligament area. Ligament area is the product ofligament length and thickness of a specimen:

where

Wf = fracture work = ∫ pdu

where

p = loading (force),u = deflection or displacement, and

du = differential of u, andAlig = area of a ligament.

For calculating the fracture work, the tail portion of the loading–deflection curve can be reasonably obtained by fitting data in thepostpeak region; this procedure is detailed elsewhere (22). Usingthis procedure, the data from 60% of peak load in the postpeak partof the curve to the end of the test are fitted with a power curve andthe tail curve is obtained by extrapolating this fitted power curvefrom regression.

A total of 10 mixtures consisting of 90 SCB specimens were testedunder three different temperatures. Fracture energy was calculatedfor all mixtures and all temperatures using the methods previouslydescribed. The average values for the fracture energy and coefficientof variation were calculated and are summarized in Table 4.

DATA ANALYSIS AND DISCUSSION

Dynamic Modulus Testing

The master curves for dynamic modulus were built based on themethod previously described and these curves are plotted in Figure 2.

GW

Af

f=lig

( )2

TABLE 4 Fracture Energy and Coefficient of Variation (COV)

Temp (°C) Mixture ID 0RAP28 MS2028 MS4028 SS2028 SS4028 0RAP34 MS2034 MS4034 SS2034 SS4034

−12 Gf (J/m2) 742.3 530.3 511.0 577.4 446.9 1104.3 1009.5 697.6 792.8 527.9COV (%) 13.64 0.96 14.09 9.64 6.52 22.85 14.96 15.51 4.99 11.20

−24 Gf (J/m2) 275.9 266.0 217.9 281.4 250.5 440.3 381.9 263.6 328.9 280.0CO V(%) 16.07 22.59 14.50 10.6 4.80 14.20 19.76 4.42 6.58 13.90

−36 Gf (J/m2) 225.3 246.5 166.1 215.3 165.0 298.0 252.6 197.4 244.6 219.6COV (%) 5.35 8.89 18.62 2.83 10.55 12.03 17.01 19.69 3.80 16.72

Li, Marasteanu, Williams, and Clyne 93

0

5

10

15

20

25

-20 -10 0 10 20 30 40 50Temperature, °C

|E*|

, GP

a

0RAP34

SS2034

SS4034

FIGURE 3 Effect of SS addition on dynamic modulus of PG 58-34control mixture (frequency � 1.0 Hz).

94 Transportation Research Record 2051

A visual inspection of the data contained in these figures indicatesthat for most frequencies (or temperatures) the dynamic modulusof the mixtures containing RAP is higher than the modulus of the mix-tures with no RAP, and this trend is obviously more significant for thelower frequencies or higher temperatures. This is in agreement withprevious results published by other researchers (23).

While a general trend is that the addition of RAP increases thedynamic modulus values, the percentage of RAP contained in themixture was found to have various effects on the dynamic modulusat different frequencies or temperatures. With the exception of themixtures made by the MS RAP and the PG 58-28 asphalt binder, allother mixtures investigated in this study have higher dynamic mod-ulus values with 20% RAP than with 40% RAP at high frequenciesor low temperatures. As for the lower frequencies or higher tempera-tures, the 40% RAP mixtures have higher or nearly the same dynamicmodulus compared with the mixtures containing 20% RAP. Anexample is shown in Figure 3, which illustrates the modulus versustemperature at a fixed frequency relationship for the mixtures with thesingle source RAP at the various percentages and the −34 binder. Athigh temperatures, the addition of more RAP resulted in an increasein modulus, whereas this trend is reversed at low temperatures, andthe addition of 20% RAP resulted in higher modulus values than theaddition of 40% RAP.

The effect of the stiffer asphalt binder contained in the RAP isresponsible for the higher dynamic modulus values for the mixturescontaining RAP than for the control mixtures only made by virginasphalt binders. However, this does not explain the aforementioned

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

1.E-05 1.E-03 1.E-01 1.E+01 1.E+03 1.E+05 1.E+07

Frequency, Hz

|E*|

,GP

a

0RAP28

MS2028

MS4028

(a)

(c) (d)

0RAP28

SS2028

SS4028

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

1.E-05 1.E-03 1.E-01 1.E+01 1.E+03 1.E+05 1.E+07

Frequency, Hz

|E*|

,GP

a

(b)

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

1.E-05 1.E-03 1.E-01 1.E+01 1.E+03 1.E+05 1.E+07

Frequency, Hz

|E*|

,GP

a

0RAP34

MS2034

MS4034

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

1.E-05 1.E-03 1.E-01 1.E+01 1.E+03 1.E+05 1.E+07

Frequency, Hz

|E*|

,GP

a

0RAP34

SS2034

SS4034

FIGURE 2 Average dynamic modulus mixtures: (a) �E*� for 0RAP28, MS2028, and MS4028; (b) �E*� for0RAP28, SS2028, and SS4028; (c) �E*� for 0RAP34, MS2034, and MS4034; and (d) �E*� for 0RAP34, SS2034,and SS4034.

various trends at different temperatures. At high temperatures, theproperties of asphalt mixtures are mainly determined by the asphaltbinder and a stiffer asphalt binder results in a higher modulus mixture.One hypothesis that may explain the behavior of asphalt mixture at lowtemperatures is that the addition of the aged and brittle binder con-tained in the RAP results in the formation of microcracks. Currently,there are no simple experiments that can provide physical evidenceof this phenomenon. A much simpler explanation is provided by the

difficulties of measuring �E*� at low temperatures. The experimentaldata show a much higher variability at −20°C and −10°C than at theother test temperatures and the addition of RAP further increased thevariability of the test data. Other researchers (24) arrived at similarconclusions and related the scatter in the data to a number of reasons:the significant effect of the electronic noise in the sensors on thesmall deformations occurring at the low temperatures; the changein stress distribution due to nonuniform contact of the loading pla-tens, which becomes more important at low temperatures; machinecompliance, which has a significant effect at low temperatures.

As expected, the dynamic modulus of the mixtures is directly relatedto the dynamic modulus of the virgin binder used in the mixturepreparation: a higher modulus binder results in a higher modulus mix-ture. This trend was observed for both the control and the RAP-modified mixtures: the dynamic modulus of the mixtures made withPG 58-28 asphalt binder was always higher than the dynamic modu-lus of the corresponding mixtures prepared with the softer PG 58-34asphalt binder, for the same temperature–frequency combination.

The data also indicate that the source of the RAP is an importantfactor that affects the mixture modulus at high temperatures. It was

observed that the addition of the SS leads to a greater increase inmodulus at higher temperatures than the MS, under similar condi-tions. However, no clear trend was observed for the effect of RAPsource on the dynamic modulus at low temperatures. This is possi-bly due to the different asphalt binders contained in these two RAPsources. An extensive investigation on the asphalt binders recoveredfrom the mixtures tested in this research showed that the SS-basedbinders are stiffer than the MS-based binders (25). The stiffness ofthe asphalt binder contributes more to the mixture’s properties, suchas the dynamic modulus, at a higher temperature than at a lower one.

SCB Fracture Testing

The average fracture energy data shown in Table 4 are plotted inFigure 4. The testing temperature was found to have a significant effecton the fracture energy for all mixtures. Fracture energy increases withthe increase in testing temperature. However, the effect of tempera-ture on the fracture energy varies for different temperature ranges.Specifically, the fracture energy decreases very quickly when the tem-

Tab 5

Fig. 5

Li, Marasteanu, Williams, and Clyne 95

(a)

0

100

200

300

400

500

600

700

800

-48 -36 -24 -12

Temperature (°C)

0RAP28

MS2028

MS4028

Fra

ctu

re E

ner

gy

(J/m

2 )

(b)

0RAP28

SS2028

SS4028

0

100

200

300

400

500

600

700

800

-48 -36 -24 -12

Temperature (°C)

Fra

ctu

re E

ner

gy

(J/m

2 )

(c)

-48 -36 -24 -12

Temperature (°C)

0

200

400

600

800

1000

1200

Fra

ctu

re E

ner

gy

(J/m

2 )

0RAP34

MS2034

MS4034

(d)

-48 -36 -24 -12

Temperature (°C)

0

200

400

600

800

1000

1200

Fra

ctu

re E

ner

gy

(J/m

2 )

0RAP34

SS2034

SS4034

FIGURE 4 Fracture energy for mixtures: (a) fracture energy for 0RAP28, MS2028, and MS4028; (b) fracture energy for 0RAP28,SS2028, and SS4028; (c) fracture energy for 0RAP34, MS2034, and MS4034; and (d) fracture energy for 0RAP34, SS2034, andSS4034.

96 Transportation Research Record 2051

perature changes from −12°C to −24°C, whereas the fracture energycurves are relatively flat in the temperature range from −24°C to−36°C. This is most likely due to the material property transition frombeing a relatively ductile to brittle material.

The experimental data show that the RAP percentage has a signif-icant effect on the fracture energy. At −12°C, the mixtures withoutRAP have the highest fracture energy and the fracture energydecreases with the increase of RAP percentage, with other variableskept the same. The mixtures made with 20% RAP and PG 58-28binder have very close fracture energy values with the control mix-tures at −24°C, and a similar trend was observed at −36°C. Actually,the fracture energy for the mixture MS2028 is a little higher than thatof 0RAP 28 at −36°C. However, the mixtures with 40% RAP and PG58-28 binder were found to have significantly smaller fracture energyvalues at the two lower temperatures. For the mixtures made with thePG 58-34 binder, the fracture energy was found to decrease with theaddition of the RAP at the two lower temperatures. Overall, the mix-tures with 20% MS and PG 58-28 binder exhibit fracture resistancethat is similar to that of the control mixtures, while the other mixturesshow lower fracture resistance than that of the control mixtures, andthe fracture resistance decreases with the addition of RAP.

As anticipated, the fracture energy of the mixtures was found to bedirectly related to the low limit of the virgin binder’s performancegrade used in the mixture preparation: a lower low-limit binder resultsin higher fracture energy, with the other variables constant. Thistrend was observed for both the control and the RAP-modified mix-tures: the fracture energy of the mixtures made with PG 58-34 asphaltbinder was always higher than the fracture energy of the correspond-ing mixtures prepared with the stiffer PG 58-28 asphalt binder. Thisis consistent with the dynamic modulus values at low temperaturesor high frequencies, as analyzed in the previous section.

The experimental data show no conclusive trend for the effect ofRAP source on the fracture energy. This is also consistent with thedynamic modulus values at low temperatures as described in the pre-vious section. No clear trend was observed because of the variabilityof experimental data within a low test temperature (e.g., −12°C,−24°C, and −36°C) and addition of RAP.

Comparison Between Two Tests

A statistical analysis was performed to determine the significance ofthe different factors on mixture dynamic modulus and low-temperaturefracture resistance. The factors considered were RAP percentage,binder type, test temperature, RAP source, and test frequency (fordynamic modulus only). The main statistical tool employed was theanalysis of variance (ANOVA). The level of confidence used for allanalyses was 95%. The results of the analysis are shown in Table 5.

The statistical results in Table 5 show that the RAP percentage,binder type, and test temperature have a significant effect on both thelow-temperature fracture resistance and dynamic modulus values.The factor of RAP source was found to have no statistical effect oneither the fracture energy or the dynamic modulus.

As a part of this study, the relationship between the fracture energyand dynamic modulus at low temperature was checked. The fractureenergy data from SCB at −24°C and the dynamic modulus data at −20°C and two different frequencies (0.1 Hz and 0.5 Hz) were com-pared here. These data were plotted in Figure 5. With correlation co-efficients smaller than 0.3, no clear trend can be observed between thelow-temperature fracture resistance and dynamic modulus values.

SUMMARY AND CONCLUSIONS

Because of the benefits in costs and environmental stewardship, theuse of RAP in pavement rehabilitation and maintenance has been acommon practice in the United States for more than 25 years. Theevaluation of the performance of asphalt mixtures containing RAPis therefore a priority for the asphalt materials community. Thispaper investigates the effect of RAP percentage and sources on theproperties of asphalt mixtures.

Ten asphalt mixtures, including two different RAP sources, threeRAP content percentages (0%, 20%, and 40%), and two differentasphalt binders (PG 58-28 and PG 58-34) were investigated in thisstudy. The dynamic modulus, which was proposed as the key param-eter in the newly developed mechanistic-empirical design guide, wasperformed on all mixtures at different temperatures and frequenciesand SCB fracture testing was performed for all mixtures at three lowtemperatures.

Experimental results indicate that the asphalt mixtures containingRAP have higher dynamic modulus values than the control mixturescontaining no RAP. The asphalt mixtures containing 40% RAP werefound to have higher or similar dynamic modulus with mixtures hav-ing 20% RAP at high temperatures. However, most mixtures contain-ing 20% RAP were observed to have the highest dynamic modulus atlower temperatures or high frequencies. The stiffer asphalt binder wasfound to result in a higher dynamic modulus for both the control andthe RAP-modified mixtures. Experimental data also show that theRAP source is not a significant factor for dynamic modulus values atlow temperatures, though it significantly affects the dynamic modulusvalues at high temperatures.

In addition to the testing temperature, the percentage of RAP in themixtures is found to significantly affect the fracture resistance. Frac-ture testing results indicate that the control mixtures have the high-est fracture energy and 20% RAP-modified mixtures exhibit similarfracture resistance abilities to the control mixtures. The addition of40% RAP significantly decreases the low-temperature fracture resis-tance. For the dynamic modulus, in contrast, the softer asphalt binderwas found to provide higher fracture resistance at low temperature.Experimental data indicate that the RAP source does not significantlyaffect the fracture resistance for the asphalt mixtures at low temperatures.Finally, no significant statistical relationship between dynamicmodulus and fracture energy was found (R2 values less than 0.3).

ACKNOWLEDGMENTS

The support provided by the Minnesota Department of Transportationand the Local Road Research Board for this research is gratefullyacknowledged.

TABLE 5 Statistical Analysis Results for Significant Factors

Factor Fracture Energy Dynamic Modulus

RAP percent Yes Yes

Binder type Yes Yes

RAP source No No

Test temperature Yes Yes

Frequency N/A Yes

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The Characteristics of Nonbituminous Components of Bituminous Paving Mixtures Committee sponsored publication of this paper.

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FIGURE 5 Comparison between two different parameters.

Li, Marasteanu, Williams, and Clyne 97