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Laboratory investigation of moisture damage inrubberised asphalt mixtures containing reclaimedasphalt pavementFeipeng Xiao a & Serji N. Amirkhanian aa Department of Civil Engineering, Clemson University, Clemson, SC, USAVersion of record first published: 28 Sep 2009.

To cite this article: Feipeng Xiao & Serji N. Amirkhanian (2009): Laboratory investigation of moisture damage in rubberisedasphalt mixtures containing reclaimed asphalt pavement, International Journal of Pavement Engineering, 10:5, 319-328

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Laboratory investigation of moisture damage in rubberised asphalt mixtures containingreclaimed asphalt pavement

Feipeng Xiao* and Serji N. Amirkhanian

Department of Civil Engineering, Clemson University, Clemson, SC, USA

(Received 11 September 2006; final version received 21 April 2008)

In many parts of the world, highway officials are utilising crumb rubber and reclaimed asphalt pavement (RAP) in order tosave money, protect the environment, and improve the life of asphalt pavement. However, due to the use of these materials,the effects of moisture damage should be investigated for rubberised asphalt concrete (RAC) mixtures containing RAP.The objective of this research involved investigating the moisture susceptibility of RAC containing RAP. The testingconducted included the determination of binder viscosity, toughness and indirect tensile strength (ITS) analysis. Severalmixtures containing different crumb rubber types, two different RAP sources and various percentages of rubber and RAPwere evaluated. The results indicated that, in general, the additional of RAP was beneficial in improving the ITS values andreducing the moisture susceptibility of the mixture although the addition of crumb rubber had a slightly negative effect.

Keywords: moisture susceptibility; rubberised asphalt concrete; reclaimed asphalt pavement; viscosity; indirect tensilestrength; toughness

1. Introduction

Moisture damage, caused by a loss of bond between the

asphalt binder or the mastic and the aggregate under traffic

loading, can cause a decrease of strength and durability in

asphalt mixtures. Moisture damage is relatively prone to

produce the separation and removal of asphalt binder from

the aggregate surface, thus, leading to stripping in the

asphalt pavement and ultimately causing premature

failure. Some researchers identified six contributing

mechanisms that might produce moisture damage:

detachment, displacement, spontaneous emulsification,

pore pressure-induced damage, hydraulic scour and the

effects of the environment on the aggregate-asphalt system

(Taylor and Khosla 1983, Kiggundu and Roberts 1988,

Terrel and Al-Swailmi 1994). However, it is apparent that

moisture damage is usually not limited to one mechanism

but is the result of a combination of many processes. From

a chemical standpoint, the literature is clear that although

neither asphalt nor aggregate has a net charge, but

components of both have nonuniform charge distributions,

and both behave as if they have charges that attract the

opposite charge of the other material (Curtis et al. 1992,

Little et al. 1999, Robertson 2000).

The viscosity of the asphalt binder does play a role in the

propensity of the asphalt mixture to strip. Previous research

presented that high viscosity asphalt resists displacement by

moisture better than those that have a low viscosity. High

viscosity asphalt provides a better retention of asphalt on

the aggregate surface (Khosla 1993, Xiao et al. 2007).

However, a low viscosity is advantageous during mixing

because of increased coat ability, providing a more uniform

film of asphalt over the aggregate particles. Based on the

theory of adhesion, the properties of the binder and aggregate

materials directly influence the adhesion developed between

the mix components (Khosla 1993).

In the US, the Federal Highway Administration

(FHWA) reported that 73 of the 91 million metric tons of

asphalt pavement removed each year during resurfacing

and widening projects are reused as part of new roads,

roadbeds, shoulders and embankments (FHWA 2002).

The recycling of existing asphalt pavement materials

produces new pavements with considerable savings in

material, cost, and energy. Furthermore, mixtures contain-

ing reclaimed asphalt pavement (RAP) have been found to

perform as well as virgin mixtures. The National

Cooperation Highway Research Program (NCHRP) report

provided basic concepts and recommendations concerning

the components of mixtures, including new aggregate and

RAP materials (NCHRP 2001).

Approximately, 299 million scrap tyres were generated

in the US in 2005, 82% of which were recycled or reused

(RMA 2006). Rubberised asphalt, the largest single civil

engineering market using crumb rubber, is being used in

increasingly large amounts by many departments of

transportation (DOTs) around the country. Most roads

comprised of experimental asphalt containing crumb

rubber show improvements in durability, crack reflection,

fatigue resistance, skidding resistance and resistance

ISSN 1029-8436 print/ISSN 1477-268X online

q 2009 Taylor & Francis

DOI: 10.1080/10298430802169432

http://www.informaworld.com

*Corresponding author. Email: feipenx@clemson.edu

International Journal of Pavement Engineering

Vol. 10, No. 5, October 2009, 319–328

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to rutting (Hicks et al. 1995, Choubane et al. 1999, Way

2003, Amirkhanian 2003).

Previous research (Xiao et al. 2007) results have

indicated that the use of RAP reduced the asphalt binder

content and increased cohesive strength while the use of

crumb rubber was beneficial in improving low tempera-

ture, reflective and fatigue crack resistance of the mixtures.

The results showed that it is possible to use crumb rubber

and RAP together. Furthermore, these specific mixtures

containing crumb rubber and RAP, have not yet been fully

investigated for moisture susceptibility. The mix proper-

ties such as viscosity of the asphalt influence cohesive

forces of the mixture that are inversely proportional to the

temperature of the mix. Therefore, it has become

necessary to seek a more fundamental understanding of

the relationships between moisture damage process and

viscosity by carefully considering the rubberised asphalt

concrete (RAC) and RAP that influence the adhesive, the

cohesive strength and durability of the mastics.

2. Experimental programme and procedures

2.1 Materials

The experimental design detailed in this study included the

use of two rubber types (ambient and cryogenic), four rubber

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

crumb rubber size [240mesh (20.425mm)], and four RAP

contents (0, 15, 25 and 30% by weight of the modified

mixture). Two granite aggregate sources (designated as L

and C) were used for preparing samples, and two binder

grades from the same source, PG 64-22 and PG 52-28, were

used for this project. The engineering properties of all

binders (virgin and extracted) are shown in Table 1. There

were a total of 34 Superpave mix designs.

TheRAPswere taken from the samegeographical area as

the virgin aggregates to ensure that the aggregates in theRAP

have similar properties to the virgin ones. Both RAP sources

(L and C) were mixed with an original binder equivalent to a

PG 64-22 grade. Aged binders extracted from two types of

RAP according to ASTM D 5402 (standard practice for

recovery of asphalt from solution using the rotary

evaporator) and AASHTO TP 2-01 procedures (standard

test method for the quantitative extraction and recovery of

asphalt binder from asphalt mixtures) were only used for

characterising for the modified binders.

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 30min, a reaction temperature of 1778C (3508F),

and a mixing speed of 700 rpm (Xiao 2006). The blended

components were used for the rheological property tests.

These conditions are the same as field criteria used by

South Carolina Department of Transportation (SCDOT)

for producing rubberised mixtures.

2.2 Mix design

Though the original Superpave mix design system did not

address the use of RAP, several studies were later

conducted on this subject. For example, research has

resulted in the Black Rock Study, the use of the three-tier

approach, the use of linear blending and the development

of technician manuals for proper use of RAP (FHWA

1997, NCHRP 2001, McDaniel et al. 2002). For this paper,

Table 1. Engineering properties of asphalt binders.

Virgin binder Extracted binder

Aging states Test properties PG64-22 PG52-28 Source L Source C

No aging Viscosity@1358C (Pa-s) 0.430 0.213 5.982 2.55G*/sin(d)@648C (kPa) 1.279 0.398 58.542 45.625

RTFO G*/sin(d)@648C (kPa) 2.810 0.825 109.780 95.298

PAV G*sin(d)@258C (kPa) 4074 821 8000 11000Stiffness@ 2 l28C (MPa) 217 60.4 294 277m-value@ 2 128C 0.307 0.476 0.241 0.243

No aging LMS (%) 21.42 11.94 38.45 34.74MMS (%) 59.99 51.89 34.42 33.53SMS (%) 18.59 36.17 27.15 31.72

RTFO LMS (%) 22.26 12.74 – –MMS (%) 59.07 52.62 – –SMS (%) 18.67 35.62 – –

PAV LMS (%) 30.25 13.65 – –MMS (%) 51.05 52.34 – –SMS (%) 18.70 34.01 – –

Note: LMS, MMS and SMS: large, medium and small molecular size.

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the Superpave system was used to determine the optimum

binder contents (OBC) for all mixtures.

A nominal maximum size of 9.5mm Superpave

mixture was used for all mix designs. This particular mix

design is used as a primary route surface course mix in

many states in the USA. The SCDOT 9.5mm Superpave

volumetric and compaction specifications, shown in

Table 2, were used. The procedures described in AASHTO

PP 19 and AASHTO T 312 regarding the preparation of

hot mixture asphalt (HMA) specimens were followed.

The engineering properties of two aggregate sources L

andCare shown inTable 3. Somedetails of themix designof

two RAP sources are shown in Tables 4 and 5. The RAP

materials were first oven-dried and sieved to obtain particles

with target sizes shown inTable 5. Thesematerialswere then

blended with the virgin aggregate at the specified (target)

mixing temperatures (Xiao 2006). The mixture was heated

for approximately one hour in order to maintain the target

mixing temperature. Finally, themodifiedbinder (rubber and

virginbinder)was added to themixtures and thefinalmixture

was heated for about two hours prior to compaction.

Hydrated lime, used as an anti-strip additive, was

added at a rate of 1% by dry mass of virgin aggregate.

Gradations of the 9.5mm mixtures are illustrated in

Figure 1. All mixes satisfied the requirements as specified

in Table 2 and Figure 1. When the rubber contents were

varied for other mix designs, the same gradation of the

aggregate, as shown in Figure 1, was used.

2.3 Property testing of modified binder and mixture

Three aging states of the virgin and extracted asphalt

binders were tested for several engineering properties (i.e.

viscosity, dynamic shear rheometer, bending beam

rheometer and gel permeation chromatographic). High

pressure-gel permeation chromatography separates an

asphalt binder into fractions of various molecular sizes,

thus establishing a profile of molecular size distribution

plotted with detector responses on the vertical axis and

elution time on the horizontal. Some researchers (Jennings

et al. 1985, Noureldin and Wood 1989, Kim and Burati

1993, Wahhab et al. 1999, Shen et al. 2006) reported that

the variations in the molecular size distribution of virgin

and recycled asphalt binders are associated with

rheological properties of the binder and engineering

properties of the mixture. As shown in Table 1, the aging

process increases the percentage of large molecular size

(LMS), as reported before by many researchers, and

reduces the percentage of small molecular size (SMS).

As expected, the aged binders extracted from RAP have

the larger amount of LMS than other virginal binders.

The viscosity of all binders was obtained using

the procedures described in AASHTO T 316 (viscosity

determination of asphalt binder using rotational viscometer).

Bulk specific gravity (BSG) was determined using the

ASTM D 2726. Moisture susceptibility was conducted by

comparing the indirect tensile strength (ITS) values of

various mixture types (ASTMD 4867). Three wet and three

dry samples were tested at room temperature (25 ^ 18C),

and the specimenswere compacted to 6–8% air voids with a

Marshall hammer. Furthermore, toughness value was

measured and computed to test the moisture sensitivity of

these mixtures.

3. Analysis of test results

3.1 Statistical considerations

Results of the viscosity, ITS and toughness values were

statistically analysed with 5% level of significance. For

these comparisons, it should be noted that all specimens

were produced at OBC. Regression analysis was used to

develop the correlations of the binder viscosity and the

mixture ITS values in this study.

3.2 Viscosity analysis of modified binders

Viscosity values of various modified binders are shown in

Figure 2. The results show that the viscosity of the modified

binder composed of two types of crumb rubber (ambient and

Table 2. SCDOT 9.5mm Superpave volumetric specifications.

Superpave 9.5mm mix specifications

Maximum density at Ndes (%) 96VMA (%) .15.5Voids filled (%) 70–80Maximum density at Ni (%) ,89Maximum density at Nm (%) ,98Dust to asphalt ratio 0.6–1.2

Table 3. Aggregate engineering properties.

Specificgravity

Soundness % loss atfive cycles

Aggregatesource

LA abrasionloss (%)

Absorption(%)

Dry(BLK)

SSD(BLK) Apparent 11/2–3/4 3/4–3/8 3/8–#4

Sandequivalent Hardness

L 51 0.70 2.650 2.660 2.690 0.3 0.2 0.3 76 5C 23 0.50 2.610 2.620 2.640 0.2 2.4 1.0 60 6

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cryogenic), increases as the percentage ofRAP increases due

to the increasing amount of LMS in the components. Inmost

cases, binders containing the same percentage crumb rubber

(ambient and cryogenic) exhibited similar viscosity values at

the percent of 0, 5 and 10% rubber, while the modified

binders containing 15% ambient rubber had a higher

viscosity value than those made with 15% cryogenic rubber

regardless of the RAP percentage and types. Furthermore,

the viscosity of the binder blended with a binder graded as

PG 64-22 shows a greater value than the binder blendedwith

the soft binder (PG 52-28) when using the same percentage

of RAP (30% RAP). Figure 2(b) presents the ITS values of

themodifiedmixtures containingRAPC,where the viscosity

property of the modified binders is similar to Figure 2(a).

3.3 OBC analysis

For this study, the optimum asphalt binder content was

defined as the amount required to achieve 4.0% air voids at a

given number of design gyrations (Ndesign ¼ 75). Table 6

shows OBC for mix designs with various percentages of

RAP, rubber and rubber type. Table 6 also shows, as

expected, that the OBCs of the mixtures decrease slightly as

the percentage of RAP increases for both rubber types

(cryogenic and ambient). In most cases, the OBCs of the

cryogenic modified binders are found to be slightly higher

than those of the ambient binder. Table 6 further illustrates

that an increase in the percentage ofRAP leads to an increase

of aged binder and a decrease of virgin binder in the

mixtures. Thus, the higher mixing and compacting

temperatures were needed to lower the viscosity of the age

binder in order to coat the aggregate surface. As the

percentage of crumb rubber increased, the OBCs in the

mixtures also slightly increased. Previous research indicated

that the rubber particles in modified binders swell in the

presence of the asphalt due to the absorption of some of the

lighter fractions (aromatic oils) of the binder (Heitzman

1992, Bahia and Davis 1994, Zanzotto and Kennepohl 1996,

Green and Tonlonen 1997, Kim et al. 2001, Airey et al.

2003). These crumb rubber particles form a viscous gel

causing an increase in the overall viscosity of the modified

binder. Due to the increased viscosity, more modified binder

is needed to achieve the target air void of the mixture at the

specified mixing and compacting temperatures.

3.4 BSG analysis

In this study, the BSG value of the compacted paving

mixtures was measured according to the AASHTO T166.

Previous research indicated that compared to the virgin

binders, the high temperature viscosity, complex modulus

and elastic response of rubberised mixtures show

considerable increases (Khedaywi et al. 1993, Airey et al.

2003). To achieve a target air void of the rubberised

mixtures during the mixing and compaction process at

design gyrations, the higher temperature, greater compac-

tion pressure or extensively modified binders will be

needed than conventional mixtures. At the same time, the

incorporation of the RAP in the mixtures also affects the

BSG values. Table 6 shows that increasing the percentage

of crumb rubber causes a reduction in the BSG values

regardless of the rubber types and aggregate sources.

The increase of crumb rubber also results in a decrease

in weight of specimens possessing identical volumes and

air voids with the conventional HMA specimens. This

decrease is due to the fact that the BSG of the crumb

rubber is significantly smaller than the fine aggregate in

the mixture. However, Table 6 shows that as the

percentage of RAP increases, the BSG value of

the mixtures also increase. Therefore, the aged binder in

the mixture plays a key role in achieving the target air

voids, mixing and compacting temperatures. A similar

trend is attainable when using the PG 52-28 virgin binder

in place of PG 64-22, mixed with 30% RAP.

Table 4. Blends of two aggregate sources.

Types of Superpave mixture

0%RAP

15%RAP

25%RAP

30%RAP

Specification L C L C L C L C

Percentage by weight of aggregate

Stone 789 59 50 52 49 56 – 53 47R.S. 22 18 12 15 8 – 8 7M.S. 18 31 19 20 10 – 8 14Lime 1 1 1 1 1 – 1 124RAP 0 0 9 9 15 – 18 18þ4RAP 0 0 6 6 10 – 12 12

Note: L and C: aggregate sources L and C; R.S. and M.S.: regular screenings andmanufactured screenings.

Table 5. Components of two RAP sources.

Aggregatesource

Type ofRAP

9.5mm3/8 in.

4.75mm#4

2.36mm#8

0.60mm#30

0.150#100

0.075#200

Asphaltbinder (%)

L þ4 RAP 97 59 45 30 14 8 4.6624 RAP 100 100 88 57 24 14 6.96

C þ4 RAP 84 43 33 21 9 5.4 4.4624 RAP 100 100 90 56 16 8 5.66

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3.5 Toughness analysis

Toughness was defined as the area under the tensile stress-

deformation curve up to a deformation of twice that incurred

at maximum tensile stress (Freeman et al. 1989, Putman and

Amirkhanian 2004). As shown in Table 7, statistical analysis

of the average toughness results of the specimens with the

same percentage of rubber or RAP shows no significant

differences with the mixtures made with PG64-22 binder.

However, in most cases, the toughness values of specimens

containing 15% rubber are significantly lower than those

specimens containing rubber in respective amounts of 0, 5

and 10% regardless of the RAP percentage and the rubber

type. This shows that the greater percentages of rubber in

specimens results in a greater loss of bond strength between

the asphalt binder and the aggregate. In Table 7, in general,

statistical analysis further illustrates that dry specimens, as

expected, show higher toughness values than wet specimens

made with the same percentages of rubber and RAP

regardless of the rubber type. However, an analysis of the

toughness results of bothwet and dry specimens indicate that

the mixture made with PG 52-28 asphalt binder has a

significantly lower toughness values compared to that made

with PG 64-22.

3.6 Indirect tensile strength and tensile strengthratio analysis

The ITS test is often used to evaluate the moisture

susceptibility of an asphalt mixture. A higher ITS and TSR

values typically indicate that the mixture will perform well

with a good resistance to moisture damage. At the same

time, mixtures that are able to tolerate higher strain prior to

failure are more likely to resist cracking than those unable

to tolerate high strains. The viscosity is an important factor

in determining the mixing and compacting temperatures of

the mixture. Temperature plays a key role in determining

asphalt film thickness, thus, affecting the cohesion and air

voids of the mixtures. As such, it is necessary to analyse

the relationship between viscosity and ITS values.

Figure 3 shows that the ITS values of the wet specimens

generally decrease as the rubber percentage of modified

mixture increase regardless of the RAP percentage.

The addition of crumb rubber can increase the viscosity

of an asphalt rubber binder, which results from the effects of

increase in volume of rubber particles due to the light oil

absorption of rubber. Therefore, this decrease in oil likely

inhibits the ability of themodified binder to adequately coat

the surface of the aggregate, thereby leading to the potential

loss of bonds between the rubber, binder and the aggregate.

In addition, as the RAP percentages increase in a mixture,

the ITS values of the wet specimens increase. The addition

of RAP in a HMA mixture might require the need for a

higher compaction temperature to achieve target air voids.

The specimensmadewith the aggregate source C, as shown

in Figure 3(c), indicated the same trend ITS values as for

aggregate source L.

As shown in Figure 4, the tensile strength ratio (TSR)

values of the specimens with 15% rubber (ambient and

cryogenic) containing 0 and 15% RAP are less than 85%, a

minimum TSR value set forth by SCDOT. These values

illustrate that the specimens containing 15% rubber have

more significant moisture susceptibility. In this case, it

might be necessary to include a higher percentage of RAP

to achieve the satisfied TSR values or the need for

additional anti-striping additive to improve the mixture’s

moisture damage resistance.

Figure 1. 9.5-mm mixture gradations.

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When comparing the specimens made with PG 64-22

binder and the softer binder (PG 52-28), as shown in

Figure 3, the ITS values of the specimens made with PG

52-28 is lower. Figure 3 also shows that the specimens

containing ambient rubber produced results very similar to

cryogenic specimens even though there are some

differences in manufacturing process for these two types

of crumb rubber.

3.7 Correlation analysis between viscosity and ITS

The viscosity of an asphalt binder is often used to

determine the mixing and compaction temperatures of

HMA. The mixture blended with higher viscosity binder is

produced at a higher temperature according to ASTM D

2493. This approach is simple and provides reasonable

temperature for the virgin binders. However, some specific

modified binders containing RAP and rubber have

exhibited relatively high mixing and compaction

temperatures. This is due to the fact that the modified

binders containing rubber particles and RAP have different

properties to shear rate compared to the virgin binder.

The previous research (Xiao 2006) was performed to

determine reasonable mixing and compaction tempera-

tures for these specific mixtures. In this study, the

measured viscosity of the modified binder was not used as

the viscosity value of the binder in determining the mixing

and compaction temperatures in accordance with ASTMD

2493. The temperature is an important factor that directly

affect the BSG and optimum asphalt content of the mixture

which are associated with the ITS value and moisture

susceptibility.

Figure 5(a) illustrates the relationships between the

wet ITS value of the mixtures and viscosity value of the

Figure 2. Viscosity comparisons of all binders containing PG 64-22 or PG 52-28 and ambient or cryogenic crumb rubber for RAPsources, (a) L and (b) C.

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modified binders with respect to RAP percentage.

Regression analysis was performed to develop the

predictive models between the ITS and viscosity values

at the same percentages of rubber. It can be seen that, for

each individual curve, the increase of viscosity value

results in an increase of the ITS value regardless of the

rubber percentage and RAP type. Obviously, this

increased viscosity value is caused by the additional

RAP percentage. In Figure 5, the ITS value of the mixtures

without the RAP and crumb rubber is defined as the

control mixture and it is shown as a straight line. Although

Figure 5(a) only presents the relationship between the wet

ITS and viscosity values, the dry ITS specimens exhibited

a similar trend. In addition, the specimens made with

either ambient or cryogenic crumb rubbers showed similar

ITS and viscosity properties, therefore, the effect of

asphalt composition on the rubber types appears to much

less pronounced.

With respect to the effect of rubber percentage, as

shown in Figure 5(b), the additional rubber significantly,

as expected, increases the viscosity values; however, this

increase does not result in an increase of ITS values.

A decrease of ITS value is found for each curve that was

obtained based on the regression logarithmic analysis.

Table 6. OBC and BSG of the mixtures.

Aggregate L Aggregate C

Ambienta Cryogenica Ambienta

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

OBC0 5.40 5.60 5.85 6.35 5.25 6.08 6.11 5.00 5.7515 5.25 5.45 5.75 5.90 5.25 5.85 5.30 5.10 5.5325 4.70 5.02 5.08 5.65 5.02 5.18 5.10 – –30 4.82 4.59 5.12 5.25 4.80 5.30 5.08 – 5.1030b 4.65 4.95 4.90 5.05 – – – 4.85 5.00

BSG0 2.345 2.336 2.322 2.299 2.340 2.297 2.305 2.323 2.30315 2.361 2.345 2.330 2.327 2.344 2.318 2.304 2.347 2.31725 2.364 2.350 2.325 2.322 2.367 2.332 2.352 – –30 2.373 2.376 2.363 2.354 2.372 2.348 2.367 – 2.33830b 2.388 2.373 2.372 2.370 – – – 2.346 2.339

Note: OBC: optimum binder content (%); BSG: bulk specific gravity.aPercentage of rubber by weight of virgin binder.

bPG52-28 asphalt binder.

Table 7. Toughness values of mixtures.

Aggregate L Aggregate C

Ambienta Cryogenica Ambienta

RAP (%) 0% 5% 10% 15% 5% 10% 15% 0% 10%

Dry0 3.25 3.17 3.07 3.03 3.04 2.80 2.88 2.99 3.0215 3.22 3.04 3.16 2.90 2.80 2.72 2.19 3.51 3.1225 3.09 3.05 3.05 2.88 2.92 2.88 2.41 – –30 3.09 3.00 3.05 2.89 2.96 2.76 2.13 – 3.2430b 2.04 2.00 1.81 1.52 – – – 2.35 2.35

Wet0 2.79 2.94 2.99 3.22 2.57 2.75 2.66 3.20 3.8815 2.83 3.08 3.08 2.86 2.22 2.64 1.94 3.66 3.6925 2.81 2.96 2.99 2.78 2.80 2.79 2.36 – –30 2.55 2.93 2.87 2.70 2.66 2.53 2.45 – 3.3130b 1.83 2.00 2.08 1.63 – – – 2.17 2.19

Note: Toughness unit: N/mm.aPercentage of rubber by weight of virgin binder.

bPG52-28 asphalt binder.

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Especially, when using 0 or 15% RAP, the ITS values

decrease although the viscosity values increase signifi-

cantly as the rubber percentage increased. In most cases,

these values were less than the control ITS values.

However, when using 25 and 30% RAP, the ITS values are

significantly higher than the control mixtures. Both RAP

sources, C and L, showed similar results. In the previous

study, it can be seen that the additional RAP is beneficial in

mitigating the loss of bond in the mixture caused by the

additional rubber. On the other hand, the crumb rubber is

effective in improving the long term performance (fatigue

resistance) and diminishing low temperature and reflective

cracking due to the additional RAP (Xiao 2006).

4. Conclusions

The following conclusions were determined based upon

the experimental results obtained from a laboratory

investigation of various HMA mixtures which contain

both RAP and RAC:

(1) Statistical analysis of viscosity showed no significant

differences in the viscosity values of modified binders

between two types of rubber (ambient and cryogenic)

Figure 3. Wet ITS comparisons of all mixtures made with PG64-22 or PG 52-28 for specimen containing: (a) ambient crumbrubber, RAP L, (b) cryogenic crumb rubber, RAP L and(c) ambient crumb rubber, RAP C.

Figure 4. TSR comparison of all mixtures made with PG 64-22or PG 52-28 for specimen containing: (a) ambient crumb rubber,RAP L, (b) cryogenic crumb rubber, RAP L and (c) ambientcrumb rubber, RAP C.

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under identical conditions (0, 5 and 10% rubber). The

binders blended with ambient rubber had higher

viscosity values than those blended with cryogenic

rubber when using 15% rubber regardless of RAP

percentages and types.

(2) In comparison with 15% rubber, in general, speci-

mens containing 0, 5 and 10% crumb rubber had

significantly higher toughness values regardless of

the percentage of RAP and the rubber type (ambient

and cryogenic). The toughness values of mixtures

made with the softer binder showed a lower value

compared to specimens made with PG 64-22 binder.

(3) When using 15% rubber, the specimens exhibited

moisture damage, however, the increased RAP

content significantly improves the moisture resistance

and increase the bonds between the aggregate, rubber

particle, and modified binder.

(4) The viscosity values of modified binder increased,

caused by an increase of rubber content, and the ITS

values of both the dry and wet specimens decreased.

However, the increase of the percentage of RAP

resulted in an increase in viscosity and ITS values.

The additional RAP plays a key role in mitigating the

loss of bond in the mixture due to the influence of the

additional rubber.

(5) Two aggregate and RAP sources showed similar

effects on the viscosity of the binder and the ITS

values of the mixture although there are some

differences in engineering properties of the aggre-

gates and RAP materials.

Acknowledgements

Financial support was possible through a grant from SouthCarolina’s Department of Health and Environment Control(DHEC) and the Asphalt Rubber Technology Service (ARTS) ofClemson University.

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