resilient modulus behavior of rubberized asphalt concrete mixtures containing reclaimed asphalt...

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Resilient Modulus Behavior of Rubberized Asphalt Concrete (RAC) Mixtures Containing Reclaimed Asphalt Pavement (RAP) Feipeng Xiao * and Serji N. Amirkhanian * *Clemson University Department of Civil Engineering 110 Lowry Hall Clemson, SC 29634 [email protected] [email protected] ABSTRACT: The resilient modulus is the modulus to be used with the elastic theory during any analysis of a flexible pavement. It is well known that most paving materials (e.g., asphalt pavements) are not elastic but experience some permanent deformation after each load application. With respect to the complexity of the rubberized asphalt concrete (RAC) containing reclaimed asphalt pavement (RAP), the indirect tensile strength (ITS) and resilient modulus evaluation of modified mixtures are important to understand. The aging of binder containing crumb rubber obviously alters the visco-elastic and plastic characteristics of the modified mixtures. The deformation of the mixture under repeated loading, nearly completely recoverable, should also be considered. The experimental design included the use of two aggregate sources, one rubber type (ambient), four rubber contents (0%, 5%, 10%, and 15%), one crumb rubber size (-40 mesh [-0.425 mm]), and four RAP contents (0%, 15%, 25%, and 30%). The findings indicated that an increase in the rubber content in the modified mixture leads to a decrease in ITS and resilient modulus values regardless of rubber content, and this increase also improves the aging resistance and increases the viscous characteristics of the modified binder. However, as RAP content increased, not only the viscosity and G*sinδ values of the modified binder increased, the ITS and resilient modulus values of the modified mixtures also increased. KEYWORDS: Crumb rubber, Resilient Modulus, Rubberized asphalt, RAP, Viscosity, G*sinδ.

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Page 1: Resilient Modulus Behavior of Rubberized Asphalt Concrete Mixtures Containing Reclaimed Asphalt Pavement

Resilient Modulus Behavior of Rubberized

Asphalt Concrete (RAC) Mixtures Containing

Reclaimed Asphalt Pavement (RAP)

Feipeng Xiao

* and Serji N. Amirkhanian

*

*Clemson University

Department of Civil Engineering

110 Lowry Hall

Clemson, SC 29634

[email protected]

[email protected]

ABSTRACT: The resilient modulus is the modulus to be used with the elastic theory during any analysis

of a flexible pavement. It is well known that most paving materials (e.g., asphalt pavements) are

not elastic but experience some permanent deformation after each load application. With respect

to the complexity of the rubberized asphalt concrete (RAC) containing reclaimed asphalt

pavement (RAP), the indirect tensile strength (ITS) and resilient modulus evaluation of modified

mixtures are important to understand. The aging of binder containing crumb rubber obviously

alters the visco-elastic and plastic characteristics of the modified mixtures. The deformation of the

mixture under repeated loading, nearly completely recoverable, should also be considered. The

experimental design included the use of two aggregate sources, one rubber type (ambient), four

rubber contents (0%, 5%, 10%, and 15%), one crumb rubber size (-40 mesh [-0.425 mm]), and

four RAP contents (0%, 15%, 25%, and 30%). The findings indicated that an increase in the

rubber content in the modified mixture leads to a decrease in ITS and resilient modulus values

regardless of rubber content, and this increase also improves the aging resistance and increases

the viscous characteristics of the modified binder. However, as RAP content increased, not only

the viscosity and G*sinδ values of the modified binder increased, the ITS and resilient modulus

values of the modified mixtures also increased.

KEYWORDS: Crumb rubber, Resilient Modulus, Rubberized asphalt, RAP, Viscosity, G*sinδ.

Page 2: Resilient Modulus Behavior of Rubberized Asphalt Concrete Mixtures Containing Reclaimed Asphalt Pavement

2 Road Materials and Pavement Design. Volume X – No. X/2007

1. Introduction

Resilient modulus, an important material property and analogous to the

modulus of elasticity, can be used to predict the response of a material to

repeated impulse or moving loads, such as those imposed by vehicle tires

on a road surface. It is one of many stiffness indicators of mixtures which

can be determined using laboratory testing methods or non-destructive

field tests such as falling weight deflectometer tests through back

calculations (Shalaby et al. 2004). Resilient modulus is determined from a

repeated load test, peak values of stress and recoverable deformation

occurring in the test are used to calculate the resilient elastic constants

even though peak stress and recoverable deformation do not occur at the

same time in a dynamic test of this type (NCHRP 1997). Previous

research indicated that the response of a pavement surface, subbase and

the subgrade, and the correlation between the temperature and resilient

modulus are able to be performed in accordance with various resilient

modulus testing and analysis methods. In general, good relationships

have been found to exist between flexible pavement performance and the

stresses, strains, and displacements calculated by layered pavement

theories using appropriate resilient modulus of that particular pavement

layer (Park and Lytton 2002; Wahhab et al. 2001).

Moreover, the results of these projects also indicated that the

properties of the various materials (e.g., aggregate sources, binder sources,

etc.) used in a pavement affect the resilient modulus and stiffness values

of the mixtures. Especially, in recent years, some special materials such

as crumb rubber are being used to save money, protect the environment,

and extend the life of asphalt pavements. In addition, the utilization of

reclaimed asphalt pavement (RAP) is an acceptable practice in many

countries all over the world. However, the use of RAP containing crumb

rubber has not been investigated in great detail, so it is essential to

explore whether these materials have any effect on the resilient modulus.

In general, previous experience has shown that the use of RAP has proven

to be cost-effective, environmentally sound, and successful in improving

some of the engineering properties of asphalt mixtures (Kandhal 1997;

NCHRP 2001). Crumb rubber has also been used successfully in

Page 3: Resilient Modulus Behavior of Rubberized Asphalt Concrete Mixtures Containing Reclaimed Asphalt Pavement

Resilient Modulus Behavior of RAC Containing RAP 3

improving the mechanical characteristics of hot mix asphalt (HMA)

mixtures (Bahia and Davies 1994; Airey et al. 2003; Shen et al. 2006;

Xiao et al. 2006). The objective of this research was to investigate the

effects of viscosity, G*sinδ, indirect tensile strength (ITS), resilient

modulus, and their correlations on the performance of various rubberized

asphalt concrete (RAC) containing RAP.

2. Experimental design and procedure

2.1 Materials

In this study, the experimental design included the utilization of two

aggregate sources (aggregates L and C), one rubber type (ambient), four

rubber contents (0%, 5%, 10%, and 15% by weight of the virgin binder),

one crumb rubber size (-40 mesh [-0.425 mm]), two RAP sources (RAPs

L and C), and four RAP contents (0%, 15%, 25%, and 30% by weight of

the modified mixture). The properties of virgin PG 64-22 asphalt binder

and aged binder extracted from RAP, used for preparing samples in this

project, are shown in Table 1. The results indicated that aged binder

extracted from RAP L has the higher viscosity, G*/sinδ values in virgin

and rolling thin film oven (RTFO) aging states, and has a lower G*sinδ

value and a higher stiffness value than that of RAP C in pressure aging

vessel (PAV) aging states.

Table 1 Engineering Properties of Asphalt Binder

Virgin Binder

PG64-22

Viscosity @135oC 0.43

G*/sin(δ) @64 o

C 1.28

RTFO G*/sin(δ) @64 o

C 2.81 - 109.78 - 95.30

G*sin(δ) @25 o

C 4074 - 8000 - 11000

Stiffness @-12 o

C 217 267* 294 258* 277

m-value @-12 o

C 0.307 0.262* 0.241 0.271* 0.243

5.98

58.54

2.55

45.63

Aging

StatesTest Properties

No

Aging

PAV

Extracted Binder

Source L Source C

Note: *: Stiffness and m-value results of extracted binders (No RTFO and PAV aging)

Page 4: Resilient Modulus Behavior of Rubberized Asphalt Concrete Mixtures Containing Reclaimed Asphalt Pavement

4 Road Materials and Pavement Design. Volume X – No. X/2007

The gradations of two aggregate sources are shown in Table 2. The test

results of base engineering properties of two aggregates are shown in

Table 3, where the aggregate C (crushed granite), composed

predominantly of quartz and potassium feldspar, exhibits lower LA

abrasion loss, absorption, and specific gravity values than that of

aggregate L (crushed limestone) composed mainly of calcite. In addition,

aggregate C shows a lower sand equivalent (clay content) and higher

hardness than aggregate L. On the other hand, aggregate C has greater

unit weight, compressive strength, shear strength, and modulus of rupture.

Both of two aggregate sources meet current South Carolina Department

of Transportation (SCDOT) specifications for HMA. Obviously, when

using aggregate source C, these physical properties should be beneficial

in improving the workability of the asphalt mixtures.

Table 2 Gradations of Various Mixtures

12.5 mm 9.5 mm 4.75 mm 2.36 mm 0.60 mm 0.150 mm 0.075 mm

1/2" 3/8" #4 #8 #30 #100 #200

#789 stone 100 90 35 6.3 1.4 0.7 0.4

Reg. Scr. 100 100 99.8 96 60.5 22.3 12

Man. Sand 100 100 99.4 82.5 47.2 7.6 2.3

#789 stone 100 87.5 22.9 4.3 1.5 0.9 0.6

Reg. Scr. 100 100 100 81.4 45.5 24.2 16.4

Man. Sand 100 100 99.7 75.7 31.7 8.3 3.2

Type of

Aggregate

Aggregate

Source

L

C

Reg. Scr.: Regular Screenings and Man. Sand: Manufactured Sand

Table 3 Engineering Properties of Aggregate Sources L and C

Agg.

Source

LA Abr.

Loss (%)

Absorption

(%)

Sand

Equi.Hardness

Dry

(bulk)

SSD

(bulk)

37.5 to 19.0

(mm)

19.0 to 9.5

(mm)

9.5 to 4.75

(mm)

L 51 0.70 2.650 2.660 0.3 0.2 0.3 76 5

C 23 0.50 2.610 2.620 0.2 2.4 1.0 60 6

Soundness % Loss at 5 CyclesSpecific Gravity

There were a total of 22 Superpave mix designs conducted. The RAPs

(RAPs L and C) were taken from the same geographical areas as the new

Page 5: Resilient Modulus Behavior of Rubberized Asphalt Concrete Mixtures Containing Reclaimed Asphalt Pavement

Resilient Modulus Behavior of RAC Containing RAP 5

aggregates (aggregates L and C) to ensure that the aggregates in the RAP

have similar properties to the virgin aggregates. The RAP materials

passed the 12.5 mm (1/2 inch) sieve and retained at the 4.75 mm (No. 4)

sieve was referred to as +4 RAP, while the RAP passed the 4.75 mm (No.

4) sieve was referred to as -4 RAP. These +4 RAP and -4 RAP materials

were heated and blended with the virgin aggregate at the proper mixing

temperatures. The analysis of the binder content and aggregate gradation

of RAPs was separated according to these two types (+4 RAP and -4

RAP). Table 4 shows two aggregate gradations of RAPs and their aged

binder contents (i.e., the aggregate gradation analysis were performed

after removing the binder from each RAP source).

Table 4 Aggregate Gradations and Binder Contents of RAPs

9.5 mm 4.75 mm 2.36 mm 0.60 mm 0.150 mm 0.075 mm

3/8" #4 #8 #30 #100 #200

+4 RAP 97 59 45 30 14 8 4.66%

-4 RAP 100 100 88 57 24 14 6.96%

+4 RAP 84 43 33 21 9 5.4 4.46%-4 RAP 100 100 90 56 16 8 5.66%

C

Aggregate

Source

Type of

RAP

Asphalt

Binder

L

A nominal maximum size 9.5 mm Superpave mixture was used

for all mix designs. Gradations of the 9.5 mm mixtures are illustrated in

Figure 1. These particular mix designs are used as a primary route surface

course mixes in many states including South Carolina. The 9.5 mm

Superpave volumetric and compaction specifications described in

AASHTO PP 19 and AASHTO T 312 procedures were followed for the

preparation of HMA specimens.

Page 6: Resilient Modulus Behavior of Rubberized Asphalt Concrete Mixtures Containing Reclaimed Asphalt Pavement

6 Road Materials and Pavement Design. Volume X – No. X/2007

0

10

20

30

40

50

60

70

80

90

100

Sieve Size (mm)

Per

cen

t P

assi

ng

(%

)

0% RAP

15% RAP

25% RAP

30% RAP

Lower Range

Upper Range

0.07 0.6

0

2.3

6

0 4.750.15 9.5 12.5

Figure 1 Gradations of 9.5 mm mixture

2.2 Experimental Procedure

The aged binders were extracted from the RAP according to AASHTO

TP 2-01 (Standard Test Method for the Quantitative Extraction and

Recovery of Asphalt Binder from Asphalt Mixtures) and ASTM D 5402

(Standard Practice for Recovery of Asphalt from Solution Using the

Rotary Evaporator). A mechanical mixer was used to blend the rubber,

the aged and the virgin binder. The crumb rubber and aged binder were

added to the virgin binder using a reaction time of 30 minutes, a reaction

temperature of 177 C (350 F), and a mixing speed of 700 rpm (Xiao et

al. 2006; Putman 2005). The viscosity values of the modified binders

were obtained in accordance with AASHTO T 316. A rotational

viscometer apparatus is used for viscosity testing at a temperature of

135oC. The modified binders after RTFO and PAV aging were tested.

G*sinδ values, according to AASHTO T 315 that covers the

determination of the dynamic shear modulus and phase angle of asphalt

binder when tested in dynamic (oscillatory) shear using parallel plate test

Page 7: Resilient Modulus Behavior of Rubberized Asphalt Concrete Mixtures Containing Reclaimed Asphalt Pavement

Resilient Modulus Behavior of RAC Containing RAP 7

geometry at a testing temperature of 25oC and a frequency of

approximately 1.59 Hz, were obtained.

For this study, the optimum binder content (OBC) was defined as the

amount required to achieve 4.0% air voids at a given number of design

gyrations (Ndesign= 75). Six ITS specimens, compacted to 7±1 percentage

air voids, were used to evaluate the moisture susceptibility of various

mixtures according to testing procedures described in AASHTO T283

(Resistance of a Compacted Bituminous Mixture to Moisture Induced

Damage). Four specimens, one for destructive indirect tensile test and

others for repeated loading, were also compacted to 7±1 percentage air

voids and then were employed to perform resilient modulus testing at

three different temperatures (5ºC, 25ºC and 40ºC). All specimens had a

height of 75±1 mm and a diameter of 150±1 mm.

The resilient modulus values for all mixtures were determined based

on AASHTO TP31-96 (Standard Test Method for Determining the

Resilient Modulus of Bituminous Mixtures by Indirect Tension) and

ASTM D 4123 testing procedures (Standard Test Method for Indirect

Tension Test for Resilient Modulus of Bituminous Mixtures). The values

of the resilient modulus determined from these test methods is a measure

of the elastic modulus of the HMA materials recognizing certain

nonlinear characteristics. Resilient modulus value can be used with

structural response analysis models to calculate the pavement structural

response to wheel loads, and with pavement design procedures to design

the pavement structure. During testing process, the indirect tensile testing

mode produces a highly nonlinear stress field with the least variability at

the center of the sample, and linear variable differential transducers

(LVDT) were used to measure the response, as shown in Figure 2. A

frequency of 1Hz was used in this study.

Page 8: Resilient Modulus Behavior of Rubberized Asphalt Concrete Mixtures Containing Reclaimed Asphalt Pavement

8 Road Materials and Pavement Design. Volume X – No. X/2007

(a)

Strain

Ste

ss

(b)

Figure 2 Resilient testing of the mixture (a) indirect tension testing; (b) relationship of

stress and strain during pulse loading

Pulse Load

Pulse Load

LVDTs

Mixture

Specimen

Page 9: Resilient Modulus Behavior of Rubberized Asphalt Concrete Mixtures Containing Reclaimed Asphalt Pavement

Resilient Modulus Behavior of RAC Containing RAP 9

3. Experimental results and discussions

Results of the viscosity, G*sinδ, ITS, and resilient modulus values

were statistically analyzed with a 5% level of significance. An analysis

of variance (ANOVA) was performed to statistically analyze the data. For

these comparisons, it should be noted that all specimens were produced at

OBC.

Figure 3 indicates that viscosity values of the modified binders,

composed of two aged binders (L and C) and crumb rubber, increased as

the percentage of crumb rubber increased regardless of the RAP type (L

and C). For the modified binders, containing the same percentage of

crumb rubber, as expected increasing the percentage of aged binder also

resulted in an increase in viscosity of the binder. The same trends are

observed for all mixtures regardless of the source of aged binders (L or C).

However, the statistical analysis indicated that, in most cases, the

modified binder used with RAP C has a significantly lower viscosity

value than one used with RAP L at the 95% level of confidence.

Figure 4 shows the G*sinδ (fatigue cracking factor) values of the

binders made with crumb rubber containing aged binder. This figure

shows that, for the same percentage of crumb rubber, as the aged binder

content increases, the G*sinδ value also increases regardless of the RAP

type (L or C). The statistical analysis shows that the addition of crumb

rubber significantly decreases these values. In other words, the increase of

crumb rubber percentage decreases the G*sinδ values of modified binders.

Previous studies also indicated that additional crumb rubber could

significantly slower the aging behavior of asphalt binder due to the

absorption of some of the lighter fractions (aromatic oils) of the binder by

the crumb rubber particles (Bahia and Davis 1994; Airey et al. 2004; Xiao

et al. 2006). In general, the crumb rubber is beneficial in improving the

fatigue resistance and extending the performance life of the asphalt binder

(Xiao 2006). Superpave mix design has a specification requirement for

the G*sinδ (less than 5000 kPa: bold dash horizontal line on the figure).

In general, low values of G* and are considered desirable attributes

from the standpoint of resistance to fatigue cracking. Thus, the Superpave

Page 10: Resilient Modulus Behavior of Rubberized Asphalt Concrete Mixtures Containing Reclaimed Asphalt Pavement

10 Road Materials and Pavement Design. Volume X – No. X/2007

specifications promote the use of compliant, elastic binders (PAV aged)

to address fatigue cracking. If the loss modulus value is greater than 5000

kPa, the fatigue cracking of the asphalt pavement occurs easily at the

room temperature (i.e., 25oC).

0

500

1000

1500

2000

2500

3000

3500

0% 5% 10% 15%

Percentage of Rubber

Vis

cosi

ty (

cP)

0%RAP

15%RAP(L)

25%RAP(L)

30%RAP(L)

15%RAP(C)

25%RAP(C)

30%RAP(C)

L: Aggregate L

C: Aggregate C

Figure 3 Viscosity values of the modified binder

0

1

2

3

4

5

6

7

8

0% 5% 10% 15%

Percentage of Rubber

G*S

in(

) M

Pa

0%RAPl 15%RAP(L) 25%RAP(L) 30%RAP(L)

15%RAP(C) 25%RAP(C) 30%RAP(C)

L: Aggregate L

C: Aggregate C

Figure 4 G*sinδ Values of the modified binder

Page 11: Resilient Modulus Behavior of Rubberized Asphalt Concrete Mixtures Containing Reclaimed Asphalt Pavement

Resilient Modulus Behavior of RAC Containing RAP 11

The optimum binder contents (OBCs) and %VMA for mix designs

with various percentages of RAP, rubber, and RAP types are shown in

Table 5. It can be seen that the OBCs of the mixtures decreased slightly

(Table 5(a)), as expected, as the percentage of RAP increased. The OBCs

of the modified mixtures containing aggregate L are found to be slightly

higher than mixtures used aggregate C at the same percentage of RAP.

The addition of RAP is helpful in decreasing the virgin asphalt binder and

aggregate contents. As shown in Table 5(b), VMA values of mixtures

also increase as the rubber content increases, however, the increase of

RAP content results in a decrease in VMA. Generally, these VMA values

are greater than 15.5%, a minimum requirement of SCDOT, only the

mixtures made with 30% RAP containing 0% or 5% rubber have the

VMA values of less than 15.5%. Obviously, additional crumb rubber can

increase VMA values of the modified mixtures while RAP has the

opposite effects.

Table 5 (a) Optimum binder content and (b) VMA of the modified Mixtures

(a)

Rub

RAP 0% 5% 10% 15% 0% 10%

0% 5.40 5.60 5.85 6.35 5.00 5.75

15% 5.25 5.45 5.75 5.90 5.10 5.53

25% 4.70 5.02 5.08 5.65 - -

30% 4.82 4.59 5.12 5.25 4.85 5.10

Aggregate L Aggregate C

(b)

Rub

RAP 0% 5% 10% 15% 0% 10%

0% 16.6 16.7 17.2 18.3 15.7 16.9

15% 16.0 16.5 17.0 17.2 15.1 16.6

25% 14.7 15.5 15.7 16.9 - -

30% 15.1 15.4 15.7 16.1 15.2 15.5

Aggregate L Aggregate C

Note: - = not available

Page 12: Resilient Modulus Behavior of Rubberized Asphalt Concrete Mixtures Containing Reclaimed Asphalt Pavement

12 Road Materials and Pavement Design. Volume X – No. X/2007

The ITS test is often used to evaluate the moisture susceptibility of an

asphalt mixture in Superpave mix design procedures, and also sometimes

used to help evaluate cracking potential of an asphalt mixture. The

mixtures that are able to tolerate high strain prior to failure are more

likely to resist cracking than those unable to tolerate high strains. One of

the issues involved with moisture susceptibility of asphalt mixtures is

known as stripping which produces a loss of strength through weakening

the bond between the asphalt binder and the aggregate. The loss of

strength can be sudden and catastrophic where the asphalt peels off the

aggregate, the cohesion of the mixture is lost, and distresses develop

rapidly. There is a gradual loss of strength over a period of years which

contribute to the development of many distresses including rutting and

shoving in the wheel paths. The use of the anti-stripping additive is

helpful in reducing the moisture damage during a long term performance

of the asphalt pavement if the mixtures are susceptible to moisture

damage.

With respect to the effect of RAP percentage, Figures 5 and 6 show

that, in general, the increase of RAP content, from 0 to 30%, leads to an

increase of ITS values at the same percentages of rubber. With respect to

the effect of rubber content, it can be seen that the increase of rubber

content results in the decrease of ITS values at the same percentage of

RAP regardless of moisture conditioning type (dry or wet).

At the same time, these figures show that, in most cases, the ITS

values of dry specimen is higher than the wet; moreover, the tensile

strength ratio (TSR), shown in Table 6, of various mixtures were higher

than 85% (which is SCDOT‟s specifications).

Page 13: Resilient Modulus Behavior of Rubberized Asphalt Concrete Mixtures Containing Reclaimed Asphalt Pavement

Resilient Modulus Behavior of RAC Containing RAP 13

0

600

1200

1800

Dry Wet Dry Wet Dry Wet Dry Wet

0% 5% 10% 15%

Percentage of Rubber

ITS

Val

ues

(k

Pa)

Control 15%RAP 25%RAP 30%RAP

Figure 5 ITS values of the modified mixture used RAP L

0

600

1200

1800

Dry Wet Dry Wet

0% 10%

Percentage of Rubber

ITS

Val

ues

(k

Pa)

0%RAP 15%RAP 30%RAP

Figure 6 ITS values of the modified mixture used RAP C

Page 14: Resilient Modulus Behavior of Rubberized Asphalt Concrete Mixtures Containing Reclaimed Asphalt Pavement

14 Road Materials and Pavement Design. Volume X – No. X/2007

Table 6 Tensile strength ratios of the modified mixtures

Rub

RAP 0% 5% 10% 15% 0% 10%

0% 86 97 85 78 102 96

15% 86 102 96 76 104 103

25% 88 92 93 90 - -

30% 94 100 100 90 93 93

Aggregate L Aggregate C

Note: - = not available

In the indirect repeated load testing, the resilient modulus is

determined using the recoverable horizontal and vertical deformations

that occur during the unloading portion of the load-unload cycle. The test

is normally performed over a range of temperatures and stresses to

simulate moving vehicles over a pavement structure (e.g., surface,

subbase, and subgrade) during the service life of the asphalt pavement.

Previous research indicated that several factors influence the response of

asphalt mixtures under repeated loading. The most important parameters

are materials‟ volumetric properties, test temperature, frequency of

loading, load magnitude, instantaneous and total deformation, load

duration, cycle rest period, and specimen dimensions (Almudaiheem and

Al-Sugair 1991; Kim et al. 1992; and Shalaby et al. 2004).

The resilient modulus of the mixture is computed by Equations 1 and 2

in according with ASTM D 4123.

tH

PM R

)2734.0(

(1)

27.0359.0 V

H (2)

Where,

P = the repeated load in Newton;

υ = Poisson ratio;

t = the thickness of specimen in mm;

H = recoverable horizontal deformation in mm; and

V= recoverable vertical deformation in mm.

Page 15: Resilient Modulus Behavior of Rubberized Asphalt Concrete Mixtures Containing Reclaimed Asphalt Pavement

Resilient Modulus Behavior of RAC Containing RAP 15

In this study, if the calculated Poisson ratio values were found to be less

than “0.10”, then the Poisson ratio was assumed to equal “0.10”. If they

were greater than “0.50”, then Poisson ratio values were assumed to be

equal “0.50”.

The resilient modulus values of the modified mixtures are shown in

Figures 7 and 8. It can be seen that as the temperature increases, the

resilient modulus values significantly decrease regardless of the rubber

and RAP contents, and RAP source types. These figures also show that

the increase of rubber content results in a significant decrease of resilient

modulus values at the similar environmental conditions.

However, as shown in Figures 7 and 8, increasing the RAP content

results in an increase of resilient modulus value generally, regardless of

rubber contents and testing conditions. This increase may be related to the

repeated traffic loads. Comparing the two type of aggregates (L and C), in

most cases, the statistical analysis shows that the modified mixtures made

with aggregate C had significantly higher resilient modulus value than

those made with aggregate L at the same testing conditions.

Previous researchers indicated that the developed equation might be

used to estimate the resilient modulus of asphalt mixes for design

purposes or to estimate a seed value or boundary limit for the back-

calculated resilient modulus in common pavement evaluation techniques

(Hicks and Monismith 1971; Hielmstad and Taciroglu 1998). To develop

the simple regression models for these specific mixtures, the Pearson

correlation of dependent and independent variables of the modified

mixtures are presented in Table 7. It can be seen that, in most cases, all

independent variables are strongly related to dependent variable (resilient

modulus).

Page 16: Resilient Modulus Behavior of Rubberized Asphalt Concrete Mixtures Containing Reclaimed Asphalt Pavement

16 Road Materials and Pavement Design. Volume X – No. X/2007

0

10000

20000

30000

40000

50000

60000

5ºC 25ºC 40ºC 5ºC 25ºC 40ºC 5ºC 25ºC 40ºC 5ºC 25ºC 40ºC

0% 5% 10% 15%

Percentage of Rubber

Res

ilie

nt

Mo

du

lus

(MP

a) 0%RAP 15%RAP 25%RAP 30%RAP

Figure 7 Resilient modulus values of the modified mixture used RAP L

0

10000

20000

30000

40000

50000

60000

5ºC 25ºC 40ºC 5ºC 25ºC 40ºC

0% 10%

Percentage of Rubber

Res

ilie

nt

Mo

du

lus

(MP

a) 0%RAP 15%RAP 30%RAP

Figure 8 Resilient modulus values of the modified mixture used RAP C

Page 17: Resilient Modulus Behavior of Rubberized Asphalt Concrete Mixtures Containing Reclaimed Asphalt Pavement

Resilient Modulus Behavior of RAC Containing RAP 17

Table 7 Pearson correlation matrix for the variables of modified mixture

MR (5ºC) MR (25ºC) MR (40ºC) RP Rb Viscosity G*sinδI T S

MR 1.00 1.00 1.00

RP 0.77 0.93 0.89 1.00

Rb -0.54 -0.23 -0.16 0.00 1.00

Viscosity -0.30 -0.01 0.02 0.23 0.93 1.00

G*sinδ- 0 . 8 7 - 0 . 7 9 - 0 . 7 6 - 0 . 6 1 0 . 6 7 0 . 4 7 1 . 0 0

I T S 0 . 9 0 0 . 9 0 0 . 7 8 0 . 8 0 - 0 . 4 7 - 0 . 2 1 - 0 . 8 4 1 . 0 0

Note: Rp and Rb = percentages of RAP and crumb rubber, respectively

With respect to the relationships among the resilient modulus,

viscosity, G*sinδ and ITS values, the prediction models are developed

according to the following regression method:

54321 FxExDxCxBxAM R (3)

Where,

RM = resilient modulus;

A, B, C, D, E, F = regression coefficients; and

54321 ,,,, xxxxx = percentage of RAP and rubber, viscosity, G*sinδ

and ITS, respectively.

The analysis results, shown in Table 8, were derived from using

regression techniques where statistical analysis system was used to

analyze the data.

Table 8 Coefficients and R2 values of resilient modulus models

Temp. A B C D E F R2

5ºC 25192.6 27056.2 -39241.4 0.3 -1260.5 7.9 0.94

25ºC 8845.9 22703.7 23028.6 -1.0 -1229.2 3.7 0.97

40ºC 7855.4 9747.1 22131.1 -0.9 -926.6 -0.5 0.95

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18 Road Materials and Pavement Design. Volume X – No. X/2007

The predicted and measured values of resilient modulus are shown in

Figure 8. It can be seen that predicted results are close to a perfect match

line regardless of the testing temperature conditions. The resilient

modulus values of the modified mixtures are located in different zones

due to the influence of the temperature. The lower temperature results in a

higher resilient modulus value because of the temperature susceptibility

of the asphalt mixture.

0

10000

20000

30000

40000

50000

0 10000 20000 30000 40000 50000

Measured Resilient Modulus (MPa)

Pre

dic

ted R

esil

ient

Mo

du

lus

(MP

a)

5C 25C 40C

5ºC

25ºC

40ºC

Figure 9 Predicted and measured values of resilient modulus

4. Conclusions

Based on the experimental data shown in this limited study, the

following conclusions are reached:

The viscosity value increases, as expected, as the rubber or/and

RAP contents increase regardless of RAP sources. The increasing

range of the viscosity value is based on the aged binder properties

and crumb rubber contents.

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Resilient Modulus Behavior of RAC Containing RAP 19

The G*sinδ value increases as the RAP percentages increases,

and the addition of crumb rubber is helpful in reducing G*sinδ

value during a long term aging regardless of the RAP source.

The use of RAP in modified mixtures benefit in decreasing the

virgin asphalt binder content and increasing the ITS and resilient

modulus values at various environmental conditions. Also, the

additional crumb rubber in a mixture results in an increase in the

virgin asphalt binder content and a decrease in ITS and resilient

modulus values.

A strong correlation between resilient modulus and percentages

of crumb rubber and RAP, the viscosity, G*sinδ, and ITS value

are found in this study. The predicted and measured resilient

modulus values are very close; therefore, the developed predicted

models could be effectively utilized.

5. Acknowledgement

The authors wish to thank the financial support of South Carolina

Department of Health and Environmental Control (SC DHEC) to conduct

this research work.

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