This paper presents a laboratory study in which the blending processof reclaimed asphalt pavement (RAP) with virgin mixture was analyzedthrough controlled experiments. One type of screened RAP was blendedwith virgin (new) coarse aggregate at different percentages. A blended mix-ture containing 20% of screened RAP was subjected to staged extractionand recovery. The result from this experiment indicated that only a smallportion of aged asphalt in RAP actually participated in the remixingprocess; other portions formed a stiff coating around RAP aggregates,and RAP functionally acted as “composite black rock.” The resultingcomposite layered structure was desirable to improve the performanceof the hot-mix asphalt mixture.

Reclaimed asphalt pavement (RAP) has been used in hot-mix asphalt(HMA) mixtures for paving purposes since the 1930s. Unlike withrecycled aggregates or crushed portland cement concrete, the possibil-ity of utilizing the old asphalt binder in the newly blended mixturesand, therefore, reducing the required (new) asphalt content, makesthe use of RAP in HMA mixtures more economically attractive (1).

Numerous research studies have been reported in the literatureconcerning methods of using RAP and the performance of HMAmixtures containing RAP (2–5). However, one critical question stillremains unanswered: How much old asphalt is actually blended withnew (virgin) asphalt during the mixing process? Common belief is thatthe RAP does not act in the new mixture merely as a “black rock.”The aged asphalt does blend with the new (virgin) asphalt during themixing of new mixtures. Kandhal and Foo suggested that up to 15%RAP could be used without changing PG binder grade. Between 15%and 25% RAP, the virgin binder grade should be decreased by oneincrement on both the high- and low-temperature grades. Above 25%RAP, blending charts should be used to determine how much RAPcan be used (6 ).

Perhaps the most recent national-level comprehensive study of RAPin HMA was the NCHRP 9-12, during which McDaniel and Ander-son compared the laboratory performance of three sets of mixtures,the black rock, total blending, and actual practice (5). The black rockmixes in NCHRP 9-12 were fabricated by extracting the binder froma RAP mixture and then blending the recovered RAP aggregate in

the proper proportions with virgin aggregate and virgin binder. Thetotal blending mixes were prepared by extracting and recovering theRAP binder and physically blending it into the virgin binder, and thencombining the blended binder with the virgin and RAP aggregates.The actual practice mixes were fabricated by adding the RAP withits coating unprocessed to virgin aggregate and virgin binder, similarto the hot-mix plant situation. The NCHRP 9-12 report concludedthat at 40% RAP content, the black rock exhibited significant dif-ferences in laboratory performance compared with the actual practiceand total blending mixtures. There were no significant differencesbetween the total blending and actual practice mixtures (5 ).

However, neither the black rock nor the total blend situations de-scribed in NCHRP 9-12 exist in an actual hot-mix asphalt plant inwhich RAP is usually mixed with virgin asphalt and aggregate in lessthan 1 min.

Further statistical analyses in NCHRP 9-12 only partially supportthe above conclusion. Table 1 presents results from statistical analysesfrom NCHRP 9-12 for mixtures from Arizona (AZ), Florida (FL), andConnecticut (CT) (1). The statistical analyses presented in Table 1were conducted by the research team of the NCHRP 9-12 study. Inthe table, each cell represents the statistical comparisons of mixtureindices of total blending and black rock to those of actual practice. InTable 1, the shaded cells (labeled “TB,” “both,” or “same”) representthat there is no statistical difference for that particular index betweenthe total blending and actual practice; whereas the unshaded cellsrepresent either that there is a significant difference or that not enoughdata are available. As presented in Table 1, at 40% RAP, only a smallportion of the test indices of the PG64-22 mixtures (14 of 33 cells areshaded) during the NCHRP 9-12 study did not support the conclusionthat actual practice is similar to total blending.

Huang et al. conducted a study on the laboratory fatigue character-istics of asphalt surface mixtures containing screened RAP (1, 7 ). Theyfound that up to 30% RAP, the inclusion of RAP generally improvesthe fatigue performance of asphalt mixtures. This conclusion appearsto contradict the common belief—the more RAP, the more brittlethe mixture, thus, the lower the fatigue resistance. However, similarresults were reported by Sargious and Mushule (8), and results wereeven supported by some of the fatigue test results from the NCHRP9-12 study (5 ).

It can be envisioned that if the aged asphalt binder could not be fullyblended with virgin asphalt or rejuvenating agent, it forms a layercoating the RAP aggregate. Owing to long-term aging, this layer ismuch stiffer than the virgin binder. Thus, a composite layered sys-tem exists in the RAP–virgin materials mixture (Figure 1). Such acomposite structure would be favorable in reducing the stress con-centration and potentially would enhance the performance of asphaltmixtures.

Laboratory Investigation of Mixing Hot-MixAsphalt with Reclaimed Asphalt Pavement

Baoshan Huang, Guoqiang Li, Dragan Vukosavljevic, Xiang Shu, and Brian K. Egan

B. Huang, D. Vukosavljevic, and X. Shu, Department of Civil and EnvironmentalEngineering, University of Tennessee, Knoxville, TN 37996. G. Li, Department ofMechanical Engineering, Louisiana State University, Baton Rouge, LA 70803.B. K. Egan, Materials and Tests Division, Tennessee Department of Transportation,6601 Centennial Boulevard, Nashville, TN 37243.

37

Transportation Research Record: Journal of the Transportation Research Board, No. 1929, Transportation Research Board of the National Academies, Washington,D.C., 2005, pp. 37–45.

Carpenter and Wolosick found that when rejuvenating agent is usedin the remixing of RAP, only a portion of aged binder is affectedimmediately after the mixing (9). Over time, the rejuvenating agenteventually diffuses into the aged asphalt film, coating the RAP. Thetime required to diffuse depends on the materials and curing conditions(temperature).

Lee et al. applied β-naphthol as a chemical dye to trace the processof the rejuvenating agent’s diffusion in the RAP mixture and foundthat binder diffusion in asphalt mixtures could take a very long timeto achieve balance (10).

Karlsson (11) conducted a comprehensive study on the diffusion ofvirgin asphalt in RAP mixtures that confirmed the findings of earlyresearchers (9, 10, 12). In Karlsson’s study, Fourier transform infraredattenuated total reflectance spectroscopy was used to monitor thediffusion in asphalt binders.

OBJECTIVE

The objective of the present study is to find out how much aged RAPasphalt binder will be blended into virgin asphalt binder under normalmixing conditions. And, if the composite layered system does exist,

38 Transportation Research Record 1929

to find out how it will influence the performance of the hot-mix asphaltmixtures.

LABORATORY EXPERIMENT OF RAP MIXING WITH HMA

A laboratory experiment was designed to investigate the blendingeffect of the aged asphalt in RAP during a normal mixing operation.It should be noted that diffusion will influence the mixture proper-ties for a long period of time. The process of diffusion is not withinthe scope of this study; therefore, the analysis in this paper will beapplicable to the mixtures shortly after mixing and construction. Thelong-term effect due to diffusion should be included in future studies.

No. 4 sieve–screened RAP materials were blended with coarseaggregates containing all +No. 4 sieve particles. Thus the virgin andRAP aggregates could be visually distinguished after the blending.Staged extraction was used to obtain asphalt binders from differentlayers coating the RAP aggregates. The Abson recovery method wasused to recover the extracted asphalt binder. Rotational rheometer anddynamic shear rheometer were used to characterize the rheologicalproperties of asphalt binders at different layers.

Materials

The RAP considered in this study is a plant-screened material con-taining only −No. 4 particles. The aggregates in the RAP are mostlylimestone; their properties are presented in Table 2. The asphaltcontent obtained from both National Center for Asphalt Technology(NCAT) ignition furnace and trichloroethylene extraction was 6.8%.On the basis of the surface area factor table provided by Roberts et al.,the calculated asphalt film thickness of RAP was 5.3 microns (3).

The virgin aggregate selected in this study was prepared fromNo. 57 coarse limestone. Its properties are presented in Table 3.All −No. 4 particles were screened out before mixing with RAP.

The virgin asphalt binder considered in this study was conventionalPG64-22 asphalt. Its properties are presented in Table 4.

TABLE 1 Relationship of Actual Practice Case to Other Cases [5 ]

RAP Binder RAPFS at 20°C FS at 40°C SS IDT Creep

Content Grade Stiffness 0.01 Hz 10 Hz 0.01 Hz 10 Hz 20°C 40°C −10°C 0°C −10°C −20°C

40% PG 52-34 High (AZ) Both Both Same TB BR BR TB TB TB TBMedium (CT) Diff TB Same Diff TB Same Same TB TB TB TBLow (FL) Same TB* TB TB*

PG 64-22 High (AZ) TB* BR Diff Diff Diff Diff Same SameMedium (CT) Diff TB Diff Diff TB TB Diff TB Same BothLow (FL) Diff Same Same TB* Diff TB Same

BR: actual practice = black rockTB: actual practice = total blendingTB*: actual practice = total blending and black rock = total blending, but actual practice ≠ black rockSame: actual practice = black rock = total blendingDiff: actual practice ≠ black rock ≠ total blendingBoth: actual practice = black rock and actual practice = total blending, but black rock ≠ total blendingBlack cells are inconclusive.FS: frequency sweep at constant height.SS: simple shear at constant height.RSCH: repeated shear at constant height.IDT: indirect tensile test.

IDTStrength

RSCH

RAPbinder

Virginaggregate

RAP aggregate

Virginbinder

RAP mixture

FIGURE 1 Composite layered system in RAP–virginmaterials mixtures.

Blending Virgin Aggregate–RAP Mixture

An extreme case was first considered in which RAP was blended withvirgin aggregates only, that is, without any new virgin asphalt binderbeing introduced. The purpose of this procedure is to find out to whatextent the aged asphalt will “get away” from the RAP particles underpure mechanical blending. Because the virgin aggregates were all+No. 4 coarse materials whereas the RAP particles were all screenedby No. 4 sieve, they were easily separated after the mixing.

Three RAP proportions, 10%, 20%, and 30%, were considered. Theblending was performed at 190°C, and the mixing time was 3 min.This blending condition was considered to be more favorable for auniform mixture than the actual plant mixing in which both the mix-ing temperature and mixing time will be below 190°C and 3 min.Figure 2 shows the materials before and after the dry blending. Afterthe RAP and virgin aggregate were separated from the blended mix,NCAT oven ignition tests were performed on both fine and coarseaggregates to obtain the corresponding asphalt contents.

Blending Virgin Asphalt Binder–VirginAggregate–RAP Mixture

To simulate the actual plant mixing, 20% RAP was blended with vir-gin aggregates and PG64-22 virgin asphalt binder. The mixing tem-

Huang, Li, Vukosavljevic, Shu, and Egan 39

perature was still 190°C, and mixing time was set to 3 min. It shouldbe pointed out that this lab mixing condition would tend to producemore uniform mixtures than the actual plant mixes because of theslightly higher mixing temperature and longer mixing time. Virginmixtures consisted of coarse aggregates, and RAP consisted of onlyfine particles separated afterward (Figure 3).

Staged Extraction and Recovery

To determine how much virgin asphalt binder was “cut” into the agedasphalt coating RAP aggregates, staged extraction was used in whichthe RAP mixture was first soaked in trichloroethylene solution for3 min, and then the solution was decanted. This batch of extractedbinder was considered to be the first (outermost) layer of RAP par-ticles. The same mixture was soaked in trichloroethylene again for3 min to obtain the asphalt binder of the second layer, and so on. Atotal of four batches of staged extraction, representing four differentlayers of asphalt, were performed. The 3-min soaking time was deter-mined through trial and error to produce a similar amount of binderfrom each of the batches. The last batch was washed with solventto remove all of the remaining asphalt binder. In addition, coarse(virgin) aggregate mixture was washed with trichloroethylene solutionto determine the extent that aged asphalt “contaminated” the virginasphalt binder. Figure 4 presents a schematic flowchart for the stagedextraction.

Abson recovery was used to recover the asphalt binder from theasphalt–trichloroethylene solution. The recovered asphalt binderwas subjected to rheological tests.

TABLE 2 Properties of RAP Aggregate

Sieve Size % Pass

No. 4 100

No. 8 81

No. 30 46

No. 50 30

No. 100 23.2

No. 200 19.3

Gsb 2.545

FAA 41

FAA � fine aggregate angularity; Gsb � bulk specific gravity.

TABLE 3 Properties of Virgin Aggregate

Sieve Size % Pass

37.5 mm 100

25.4 mm 97.6

19 mm 77.7

12.7 mm 35.3

9.5 mm 14.3

4.75 mm 1.9

Gsb 2.648

TABLE 4 Asphalt Binder Properties

Binder Status Binder Test Test Results Specification

Original binder Rotational viscosity at 135°C, Pa*s 0.52 3 Pa*s maxDSR, G*/sin δ, kPa 70°C 0.78 1.00 kPa min

64°C 1.63

RTFO aged binder DSR, G*/sin δ, kPa 70°C 1.66 2.20 kPa min64°C 3.54

PAV aged binder DSR, G*sin δ MPa, 25°C 3725 5000 kPa maxBBR creep stiffness S, MPa 238 300.0 MPa maxBBR creep slope, m value 0.310 0.300 min

PG Grading 64-22

RTFO � rolling thin-film oven; PAV � pressure aging vessel; BBR � bending beam rheometer.

Binder Rheological Tests

The rotational viscometer test was used to characterize the rheolog-ical properties of asphalt binder at high (mixing) temperature. At thetesting temperature of 135°C, the test followed AASHTO TP48.

The dynamic shear rheometer (DSR) test was used to characterizeasphalt binder properties at high and intermediate service tempera-

40 Transportation Research Record 1929

tures. The test protocols followed AASHTO TP5. In this study theDSR test was performed at 25°C, 64°C, 70°C, and 76°C.

Discussion of Results

Figure 5 presents the results of asphalt contents and percent of asphaltbinder loss of RAP particles due to pure mechanical dry blending.It appeared that regardless of RAP proportion (between 10% and30%), the asphalt content of RAP reduced from 6.8% to 6.0%, whichaccounted for about 11% of binder loss due to pure mechanicalblending.

Results from pure mechanical blending indicated that aged asphalttended to “stick” with the RAP aggregate. Only a small portion (about11%) of the aged binder will be available to blend with virgin asphalt.Thus, the RAP particles were ready to form a three-layered compositein the blended mixture.

Figure 6 presents the thicknesses of corresponding layers from thestaged extraction. The thicknesses of the asphalt layers around RAPparticles (from the first to the fourth layer) were 2.0, 1.1, 1.8, and1.6 microns. The asphalt layer thicknesses were calculated based

(a)

(b)

FIGURE 2 Virgin aggregate–RAP mixture (a) before dry blendingand (b) after dry blending.

FIGURE 3 Virgin asphalt binder–virgin aggregate–RAP mixture.

Trichloroethylene TrichloroethyleneTrichloroethylene Trichloroethylene

Wash withsolvent

Soak for3 minutes

Soak for3 minutes

Soak for3 minutesRAP mixture

Fourth layer Third layer First layer Second layer

Trichloroethylene

Coarse aggregatemixture

Wash withsolvent

Extent of aged asphalt “contaminated”the virgin asphalt binder

FIGURE 4 Schematic flowchart of staged extraction.

Huang, Li, Vukosavljevic, Shu, and Egan 41

on the amount of recovered asphalt binder from each batch. It wasassumed that the asphalt film thickness for different aggregate sizeswas the same. The aggregate surface area factors were taken fromRoberts et al. (3). Figures 7 through 9 present the rheological prop-erties of asphalt binders at different layers of RAP particles. It wasclear that asphalt viscosity increased going from the outside layers tothe inside. The asphalt in Layers 3 and 4 was much stiffer than theasphalt in Layers 1 and 2. In regard to the film thickness, about 60%of the total thickness had asphalt properties close to pure RAP agedbinder, whereas the outside 40% of the binders were blended withvirgin binder.

NUMERICAL ANALYSES OF RAP–HMA COMPOSITE

From the limited laboratory study, it was clear that at least in the earlystage, the influence of RAP on the virgin asphalt binder in the newmixture was very limited. However, on the basis of numerous labora-tory studies, mixtures containing RAP exhibited a significant increase

0

2

4

6

8

10

10 20 30

RAP Content in the Mixture (%)

Asp

hal

t C

on

ten

t o

f R

AP

(%

)

Before "dry" blending After "dry" blending

0

5

10

15

0 10 20 30

RAP Content in the Mixture (%)

Per

cen

t o

f A

sph

alt

Bin

der

Lo

ss o

f R

AP

40

(a)

(b)

FIGURE 5 Asphalt content and percent of asphalt binder loss ofRAP particles.

RAP aggregate

Fourth layer

Third layerFirst layer

Secondlayer

FIGURE 6 Layers of asphalt binder coating RAP aggregate.

0.0E+00

5.0E+02

1.0E+03

1.5E+03

2.0E+03

2.5E+03

3.0E+03

3.5E+03

4.0E+03

layer 1 layer 2 layer 3 layer 4

Ro

tati

on

Vis

cosi

ty (

cP)

FIGURE 7 Viscosity from rotation viscometer at 135�C.

42 Transportation Research Record 1929

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0C

om

ple

x S

hea

r M

od

ulu

s, G

*, 6

4oC

(M

Pa)

layer 1 layer 2 layer 3 layer 4

FIGURE 8 Complex modulus at 64�C from dynamic shear rheometer.

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

Co

mp

lex

Sh

ear

Mo

du

lus,

G*,

25o

C (

GP

a)

layer 1 layer 2 layer 3 layer 4

FIGURE 9 Complex modulus at 25�C from dynamic shear rheometer.

in stiffness and even improved fatigue resistance (1, 7, 8). Thisphenomenon could be explained through the following compositenumerical analysis of the RAP–HMA mixture system.

FINITE ELEMENT MODELING

The RAP modified asphalt mix is a particulate-filled compositematerial. On the basis of Eshelby’s equivalent medium theorem, thistype of composite material can be assumed to be a virgin asphaltmastic layer coating black rock aggregates dispersed in an equivalentvirgin asphalt mix. The black rock again is a two-phase compositebody with an aggregate particle coated with an aged asphalt masticfilm. Similar considerations have been used in estimating the elasticmodulus of asphalt concrete, in analyzing thermal stress of polymermodified asphalt, and in predicting the tensile strength of asphalt con-crete (13–15). Consider a cylindrical sample under indirect tensile test.Because the load is applied along the axial direction during the splittensile test, macroscopically it can be treated as a plane-strain body.By using two parallel planes (perpendicular to the axial direction)to isolate a unit thickness piece from the sample, a circular cross-sectioned plane-strain body is obtained. Microscopically, this unitthickness piece is a composite body consisting of virgin asphalt mas-tic coated black rock aggregates and virgin asphalt mastic coated plainaggregates. For simplicity, the aggregate particles are assumed to bespheres in the plane-strain body. To better consider the interactionsamong the neighboring coated aggregates, five coated particles aredistributed in a face-centered pattern and are surrounded by an equiv-alent virgin asphalt mix (Figure 10). On the basis of a previous study(14 ), it is found that the stress concentrates more at large particlesthan at small particles. Therefore, all five particles in Figure 10 are

Huang, Li, Vukosavljevic, Shu, and Egan 43

assumed to be No. 4 RAP particles. The distance between the particlesin Figure 10 is determined by finding the number of RAP particles perunit area by assuming that (a) RAP particles are uniformly distributedin the mix, (b) the RAP volume fraction is 30%, and (c) the probabil-ity for any cross sections of the coated aggregates to be cut by a planeis the same.

The COSMOS/M software package was used to obtain the stress–strain distributions for the model in Figure 10. The mechanical prop-erties used in the analysis are shown in Table 5. In the finite elementmodeling, a three-node plane-strain element, TRIANG, was used toautomatically mesh the composite structure. A total of 12,654 elementswere used to mesh the model. A unit linear load was applied tosimulate a split tensile test.

Figures 11a, d, and g present the overall distribution of normal stressalong the x-direction, shear stress, and equivalent strain, respectively.Figures 11b, e, and h show the distribution of the normal stress alongthe x-direction, shear stress, and equivalent strain in the interphaselayer, a layer in between the aggregate and the virgin asphalt masticlayer (in this instance with the aged asphalt mastic layer replaced bythe virgin asphalt mastic), respectively. Figures 11c, f, and i illustrate

Equivalent virginasphalt mix

Virgin asphaltmasite

Aggregate

P

P

Aged asphaltmastic

FIGURE 10 Schematics of composite structure.

TABLE 5 Mechanical Properties Used in Finite Element Modeling

Material Elastic Modulus (MPa) Poisson’s Ratio

Equivalent asphalt mix 3,000 0.35

Aggregate 50,000 0.2

Aged asphalt mastic layer 3,000 0.25

Virgin asphalt mastic layer 600 0.25

the distribution of the normal stress along the x-direction, shear stress,and equivalent strain in the interphase layer (in this instance with theaged asphalt mastic present), respectively. From Figure 11, it is clearthat, with the introduction of the aged asphalt mastic layer, the stressand strain distribute more uniformly around the interphase layer. Thissuggests that the aged asphalt mastic layer helps reduce the stress andstrain concentration. The aged asphalt mastic layer was actually serv-ing as a cushion layer in between the hard aggregate and the softasphalt mastic. In such a way, the stiffness changed more gradually,avoiding sudden change in stiffness and reducing the stiffness mis-match. Consequently, the stress and strain concentration was reduced.Table 6 presents the ratios derived from dividing various stress orstrain components having the aged asphalt mastic layer by thosewithout this layer. Table 6 clearly shows that all the stress and straincomponents have been reduced with the introduction of the agedasphalt mastic layer. The reduced stress or strain concentration sug-gests that the strength or ultimate strain could be increased with theblack rock as aggregates. This conclusion was in agreement with testresults (7 ).

44 Transportation Research Record 1929

SUMMARY AND CONCLUSIONS

A study has been conducted to investigate the blending of RAPinto virgin HMA mixtures. One type of screened RAP was blendedwith virgin coarse aggregates under (a) pure mechanical blendingand (b) realistic blending incorporating virgin asphalt binder. Stagedextraction and Abson recovery were employed to study the layeredstructure of RAP particles in asphalt mixtures. Finite element com-posite analyses were performed to explain certain phenomena of lab-oratory mixtures containing RAP. On the basis of the results fromthis limited study, the following can be summarized:

• Mechanical blending affected only a small portion of aged asphaltbinder RAP.

• Staged extraction can be used to analyze different layers of asphaltbinder around aggregate particles.

• Instead of blending with virgin asphalt, the aged asphalt in RAPformed a stiffer layer coating the RAP aggregate particles.

(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

FIGURE 11 Stress and strain distributions in composite model: (a) overall normal stress distribution,(b) normal stress in interphase layer without RAP, (c) normal stress in interphase layer with RAP, (d) overall shear stress distribution, (e) shear stress in interphase layer without RAP, (f ) shear stress in interphase layer with RAP, (g) overall equivalent strain distribution, (h) equivalent strain in interphaselayer without RAP, and (i ) equivalent strain in interphase layer with RAP.

TABLE 6 Various Stress and Strain Ratios

Case P1 σx σy σz τxy �x �y �eq

Ratio 0.86 0.85 0.87 0.89 0.90 0.90 0.91 0.92

• Composite analyses indicated that the layered system in RAPhelped to reduce the stress concentration of HMA mixtures.

• Inclusion of RAP in HMA mixtures had the positive effect of forming a favorable layered system to enhance the pavementperformance.

• The diffusion of asphalt will reduce the effect of the layeredsystem in RAP mixtures, which should be considered for long-termpavement performance.

• Results presented in this paper represent only the materials underthe test conditions in this study. It should also be mentioned that themixtures in this study may not reflect the majority of mixtures usedby the HMA industry. More complete analyses are recommended tocover a wide variety of materials under various conditions.

ACKNOWLEDGMENTS

This study was supported by the Tennessee Department of Transpor-tation. The authors express special thanks to Mike Clouds and DavidMcMackin, who carefully conducted all the tedious testing.

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