Evaluation of Warm Mix Asphalt Mixtures Containing Reclaimed Asphalt Pavement through Mechanical Performance Tests and an Acoustic Emission Approach

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Evaluation of Warm Mix Asphalt Mixtures Containing

Reclaimed Asphalt Pavement through Mechanical

Performance Tests and an Acoustic Emission Approach

Brian Hill,1 Behzad Behnia,1 William G. Buttlar,2 Henrique Reis3

1Rersearch Assistant, Department of Civil and Environmental Engineering, University of Illinois,

205 N. Mathews Avenue, Urbana, IL 61801, USA, [email protected]

1Rersearch Assistant, Department of Civil and Environmental Engineering, University of Illinois,

205 N. Mathews Avenue, Urbana, IL 61801, USA ,[email protected]

2Professor, Department of Civil and Environmental Engineering, University of Illinois, 205 N.

Mathews Avenue, Urbana, IL 61801, USA, [email protected]

3Professor, Department of Industrial and Enterprise Systems Engineering, University of Illinois

at Urbana-Champaign, 104 S. Mathews Ave., Urbana, IL, 61801, USA, [email protected]

Journal of Materials in Civil Engineering. Submitted July 5, 2012; accepted December 7, 2012; posted ahead of print December 10, 2012. doi:10.1061/(ASCE)MT.1943-5533.0000757

Copyright 2012 by the American Society of Civil Engineers

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ABSTRACT

Reclaimed asphalt pavement (RAP) and Warm Mix Asphalt (WMA) have become the primary

methods for enhancing sustainability in the asphalt industry in recent years. To further enhance

sustainability benefits, asphalt producers have begun using RAP and WMA in combination.

Research to date focused on evaluating WMA RAP mixtures in terms of moisture sensitivity and

permanent deformation characteristics as measured in laboratory performance tests. In the

present study, a set of WMA mixtures encompassing a variety of variables, including: four

WMA additives (Evotherm 3G, Rediset LQ, Sasobit, and Advera) and three RAP contents (0, 15,

and 45%), is investigated. A common belief amongst practitioners is that the reduced aging in

the asphalt binder associated with lower production temperatures in WMA mixtures leaves

additional ‘headroom’ for the incorporation of higher amounts of RAP, which is generally a

stiffer, more brittle material. To fully characterize the performance of WMA RAP mixtures, the

present work evaluates the low temperature cracking behavior of these mixtures in conjunction

with moisture and rutting resistance characterization. Low temperature testing of WMA RAP

mixtures was achieved through the Disk-Shaped Compact Tension (DC(T)), Indirect Tension

(IDT) creep compliance, and Acoustic Emission (AE) tests.

Test results showed that chemical additives improved moisture susceptibility according to the

AASHTO T-283 test as well as fracture and bulk stress relaxation characteristics using the

DC(T) and IDT tests, respectively. The organic Fischer-Tropsch wax modified WMA mixtures

performed best among the WMA mixtures in terms of rutting resistance. The introduction of

RAP led to increased resistance to permanent deformation and moisture damage. On the other



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hand, RAP reduced thermal cracking resistance according to the low temperature performance

tests in both HMA and WMA. The same trend was observed in AE test results as WMA RAP

mixtures exhibited warmer embrittlement temperatures as compared to control mixtures and are

therefore expected to be more prone to thermal cracking. Based upon these findings, thermal

cracking resistance remains an issue to be considered in WMA mixtures containing RAP.

Additionally, performance testing has shown to be a valuable tool for the evaluation of RAP and

WMA mix designs to avoid performance issues in the field.

Key Words: Warm Mix Asphalt (WMA), Reclaimed asphalt pavement (RAP), Low temperature

cracking, WMA chemical additives, fracture energy, acoustic emissions, embrittlement

temperature.

INTRODUCTION

Environmental experts define sustainability as meeting the needs of the present without depleting

the resources required by future generations (World Commission on Environment and

Development (1987)). Civil engineering infrastructure materials can significantly contribute to

the sustainability movement through the use of recycled materials and more environmentally

friendly production processes. In the asphalt paving community, the most commonly employed

sustainability practices involve the addition of increasingly greater amounts of reclaimed asphalt

pavement (RAP) and the use of warm mix asphalt (WMA).

RAP is the primary recycled product of asphalt concrete pavements. According to Collins and

Cieslinski (1994), asphalt concrete removal leads to a production of more than 100 tons of RAP



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per year in the United States. The use of RAP leads to several advantages including reduced

material costs, energy savings, and increased rutting resistance. Chiu et al. (2008) determined

through cost analyses that 23% energy savings occurred with the use of RAP. In terms of

performance, RAP can improve rutting resistance through increased asphalt binder stiffness.

Oxidative hardening significantly increases asphalt binder stiffness during pavement service life,

which can provide rutting resistance when RAP is incorporated into a new asphalt paving

mixture (Aurangzeb et al. (2011)).

Performance issues may arise with the use of higher amounts of RAP in the area of pavement

durability as well. Xiao et al. (2007) determined that the introduction of as little as 15% RAP

significantly increased mixture stiffness, which opened the door for premature development of

various forms of pavement cracking. Behnia et al. (2011) reported reduced cracking resistance

in PG 58-28 mixtures containing up to 50% RAP through Disk-shaped Compact Tension

(DC(T)), Indirect Tensile (IDT) Creep Compliance, and Acoustic Emissions (AE) tests at low

temperatures. The increased stiffness associated with RAP may lead to the selection of a more

costly softer asphalt binder grade and/or limit the amount of RAP that can be used in a given

mixture. A current school of thought in the asphalt industry is that the reduced aging in the

asphalt binder associated with lower production temperatures in WMA mixtures allows for the

incorporation of higher amounts of RAP (Prowell and Hurley (2007)). In essence, it is thought

that the stiffer RAP binder can be ‘counterbalanced’ by virtue of the less aged binder resulting

from the WMA production process which reduces mixture production and laydown temperatures

and hence oxidative hardening, volatilization, etc.



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WMA represents a growing alternative to conventional hot mix asphalt (HMA). This technology

is produced at temperatures approximately 25-30oC less than HMA due to chemical composition

changes during the mixing process (D’Angelo et al. (2008)). At least 20 WMA additives and

processes exist on the market today, and include: foaming additives and processes, organic

additives, and chemical additives. The foaming group utilizes water to foam the asphalt binder

prior to or during the mixing process. The foaming processes subcategory uses water injection

systems to foam the asphalt binder while the additives subcategory includes synthetic zeolites

such as Advera and Aspha-min. Synthetic zeolites are metallic alumino-silicates which contain

approximately 20% water by weight in their microstructure (Prowell and Hurley (2007)). At

approximately 100oC, the zeolite degrades and releases the entrapped water. According to

Prowell and Hurley (2005), foaming additives may have moisture sensitivity issues based on

laboratory evaluation. Organic additives generally include paraffin waxes, montan waxes, and

fatty acid amides. This group of WMA additives stiffens the asphalt binder as shown by Prowell

and Hurley (2005) who determined that the addition of 2.5% Sasobit led to a PG 58-28 asphalt

binder behaving as a PG 64-22. Consequently, organic additives may reduce thermal cracking

resistance for a given binder in a given climate. The chemical additive category includes liquid

and solid chemical packages added to the asphalt binder prior to entering the mixing drum.

Liquid chemical additives generally act as emulsifying agents and contain amine groups which

lead to improved thermal cracking and moisture resistance, respectively.

Several environmental advantages occur with the use of WMA including: energy savings and

emissions reductions. According to the National Asphalt Pavement Association (2007), WMA

can reduce fuel consumption by as much as 10-35%, as fuel usage may decrease by as much as



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3% for each 6oC drop in mixing temperature. European and Canadian researchers have

determined that a 15-70% reduction in SOx, NOx, CO2, and volatile organic compounds (VOC’s)

emissions are generally realized with the use of WMA (D’Angelo et al.(2008)).

Potential disadvantages of WMA include increased rutting, moisture sensitivity, and a lack of

long-term field performance results. In the case of the chemical and foaming groups, mixture

stiffness may be reduced such that rutting resistance could be problematic according to Hurley

and Prowell (2005, 2006). Organic additives, on the other hand, may increase stiffness such that

pavement cracking potential also increases. The lack of long-term WMA performance data in

the field of practice also affects WMA use in the United States (the technology has only been in

place for approximately 8 years). As a result, laboratory performance tests continue to fulfill a

critical role in the design and deployment of existing and emerging WMA technologies.

As stated previously, WMA and RAP have the potential to perform well in combination, not to

mention the two-pronged sustainability benefits that can be realized. Research to date, such as

Doyle et al. (2011), primarily focused on the moisture and rutting resistance aspects of WMA

RAP mixture performance. Their study suggested that moisture and rutting resistance can be

improved through the combination of WMA and RAP. However, low temperature performance

of WMA RAP mixtures remains in question. Therefore, this study will introduce new findings

with respect to low temperature characteristics of WMA RAP mixtures, while considering

rutting and moisture resistance in order to evaluate the overall durability of WMA RAP

mixtures.



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In this experimental investigation, four WMA additives, including one additive from each WMA

group, and three different RAP levels were used to evaluate WMA-RAP mixtures in comparison

to control HMA mixtures through advanced asphalt mixture tests. These tests included: DC(T),

IDT Creep Compliance, AE tests (to evaluate cracking resistance), and Hamburg Wheel

Tracking, and AASHTO T-283 tests (to evaluate rutting and moisture sensitivity, respectively).

MATERIALS

The primary objective of this study was to evaluate the combined effects of WMA additives and

RAP on asphalt mixture low temperature properties. An additional objective of the study was to

compare the rutting resistance and moisture sensitivity of WMA RAP mixtures and control

HMA mixtures to fully characterize WMA-RAP mixture durability properties. At least one

additive from each of the WMA categories was used, namely: Sasobit, Advera, Evotherm 3G,

and Rediset LQ. Sasobit is a paraffin wax product of the Fischer-Tropsch process (Sasol

International (2010)). Sasobit was added at a rate of 3.0% by weight of the asphalt binder.

Advera is a foaming additive synthetic zeolite which was added at a rate of 0.25% by weight of

the mixture. Evotherm 3G and Rediset LQ are liquid chemical additives added at a rate of 0.50

and 0.75% by weight of the asphalt binder, respectively. Sasobit, Advera, Evotherm 3G, and

Rediset LQ will be referred to as F-T wax, Zeolite, Chemical-1, and Chemical-2, respectively.

PG 64-22 was used as the base asphalt binder in this study. This neat asphalt binder is

commonly used across Illinois and much of the United States in applications with low to

moderate traffic levels. Aggregates were sampled from a local central Illinois hot-mix asphalt

producer (Open Roads Paving, LLC in Champaign, IL), including CM16 (9.5 mm nominal



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maximum size) coarse aggregate; FM20 (manufactured) and FM02 (natural) sand, and; a

limestone-based mineral filler. The CM16 and FM20 stockpiles consisted of dolomitic

limestone. Researchers fractionated the virgin aggregate prior to mixture production to reduce

variability caused by material sampling. RAP was sampled from a stockpile of material

reclaimed through milling of the surface of I-72 in central Illinois, and fractionated through a

9.5mm (3/8”) screening deck. The post-extraction RAP gradation can be found in Table 1 with

the estimated RAP asphalt content, effective and bulk specific gravity (Gse and Gsb), and

maximum theoretical specific gravity (Gmm). All specific gravities, asphalt content estimations,

and RAP gradations were verified with the Illinois Department of Transportation (IDOT),

Bureau of Materials and Phyical Research laboratory, located in Springfield Illinois. AASHTO

TP2 was used to extract the asphalt binder to measure the RAP asphalt content while the Gse was

used to indirectly measure Gsb.

Optimum asphalt contents were chosen based on Superpave mixture design. To determine the

HMA mixing and compaction temperatures, which were found to be 160 and 150oC,

respectively, the Asphalt Institute Superpave mix design method for selecting mixing and

compacting temperatures based upon asphalt binder viscosity was followed (Roberts et

al.(1996)). Separate WMA mixture designs were not used in this study. This approach was

taken in order to compare WMA and HMA mixtures with equivalent aggregate skeletal

structures. The WMA mixing and compaction temperatures were selected to comply with

manufacturer recommendations. Consequently, the mixing and compacting temperatures of 135

and 125oC, respectively, were selected. The number of gyrations was chosen to be 70 which

meets the IDOT standard for medium-to-low volume roads receiving a 20 year traffic intensity of



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3 to 10 million Equivalent Single Axle Loads (ESALs). Additionally, all mixtures met the

9.5mm nominal maximum aggregate size (NMAS) surface mixture gradation requirements

designated by Superpave. RAP contents of 0, 15, and 45% were included in this study to

evaluate the interaction of WMA additives and RAP. A 15% RAP content was chosen to

correspond to the maximum allowable RAP content for Illinois surface mixtures (Illinois

Department of Transportation (2011)). The 45% RAP mixture was selected to evaluate the

characteristics of a high RAP content mixture containing WMA additives.

Figure 1 displays gradation plots for the virgin (0% RAP), 15% RAP, 45% RAP mixtures. The

outer red lines represent the Superpave control points for 9.5mm NMAS mixtures. As shown in

Figure 1, the gradations are approximately similar at all points, with a maximum deviation

between curves of 1.5%. The 15 and 45% RAP mixtures contained slightly more material

passing the #200 sieve than the virgin mixture, due to the relatively high content of RAP material

passing the #200 sieve.

Mixture volumetric properties are summarized in Table 2. Similar VMA and VFA levels were

achieved for the three mix types (i.e., three RAP levels). The 15 and 45% RAP mixtures

contained virgin asphalt contents of 5.9 and 3.9% and asphalt binder replacement percentages of

11.9 and 37.9%, respectively.



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EXPERIMENTAL METHODS

Thermal Cracking Evaluation

To characterize the cracking behavior of WMA RAP mixtures, a suite of fracture, creep, and

acoustic emission (AE) tests were performed. Generally, temperature-induced transverse (or

thermal cracking) in asphalt pavements is thought to predominantly occur in a Mode I opening

manner. This is supported by field observations, where evidence of fracture mode-mixity

(curvilinear crack trajectory) is fairly minimal. In other words, thermal cracks are generally

found to propagate perpendicular to the direction of traffic and vertically through the pavement

depth. Since thermal cracks are easier to handle from an experimental and theoretical standpoint

as compared to traffic-induced fatigue cracks or reflective cracks, they are directly addressed

with the mode-I-type low-temperature tests selected for this study. However, it is likely that the

mixture characteristics that promote higher resistance to thermal cracking (higher binder content,

reduced binder aging, etc.) will also tend to reduce other forms of pavement cracking. Wagoner

et al. (2005) determined that the most viable test configuration available for asphalt mixture

Mode I fracture was the DC(T) geometry. This configuration, adjusted from ASTM E-399 for

metals, contains a sufficiently large fractured surface area to reduce test variation and is easily

fabricated from field cores or laboratory-produced gyratory specimens. Furthermore, studies

such as Dave et al. (2008) demonstrated that the DC(T) test can accurately capture the thermal

cracking potential of asphalt concrete mixtures. In 2007, ASTM specified the DC(T) test as

ASTM D7313.

The DC(T) test evaluates the fracture energy associated with propagating a crack perpendicular

to the applied load through the material. Fracture energy can be calculated by measuring the



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area under the load-crack mouth opening displacement (CMOD) gauge curve, shown in Figure 2,

and normalizing it by the fractured surface area. Researchers in this study tested all specimens at

-12oC which corresponded to the ASTM recommendation for PG 64-22 asphalt binders.

Furthermore, all tests were run at a CMOD opening rate of 1.0mm/min.

The IDT creep test was employed to evaluate the creep compliance characteristics of the WMA

RAP and HMA mixtures. According to Buttlar et al. (1994), this test can be used to accurately

predict the low temperature behavior of asphalt concrete. This study complied with the

AASHTO T-322 procedure using three replicates per mixture tested for 1000 seconds. In this

case, specimens were loaded using a step-type creep load at 0, -12OC, and -24oC. The horizontal

and vertical displacements at the center of each side of a given specimen were measured using

highly sensitive extensometers (Epsilon model 3910). Then, creep compliance was calculated

using Equation 1.

(1)

where:

D(t) = Creep compliance at time t.

ΔX = Trimmed mean of the normalized horizontal deflections at time t.

Davg = Average diameter of all replicates.

tavg = Average thickness of all replicates.

Pavg = Average applied creep load.

L = Gauge length.



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Cc = Correction Factor to account for 3D stress and strain fields as a function of specimen

aspect ratio (t/D) and Poisson’s ratio (Buttlar and Roque (1994))

Creep compliance master curves were produced using the principle of time-temperature

superposition and the Power-law model shown in Equation 2 was fit to the data.

(2)

A least-squares fitting method was used to determine the parameters D0, D1, and m. The m-

value relates to the stress relaxation and creep deformation rate of viscoelastic materials.

Typically, larger relative m-values correspond to more compliant and relaxant asphalt mixtures,

which are more resistant to thermal cracking.

An Acoustic Emission (AE) is defined as a spontaneous release of localized strain energy in a

stressed material in the form of transient stress waves. As a recognized nondestructive testing

(NDT) method, AE has been proven to be a powerful tool for examining the behavior of

materials deforming under stress. The AE method has application wherever the stresses in a

material stimulate the release of energy as a detectable indication of the possible failure.

Transient elastic AE waves result from sudden internal micro-displacements in the stressed

material. AE waves from a growing flaw travel within the material and are detected by sensitive

surface-mounted piezoelectric sensors. Figure 3 schematically illustrates crack nucleation and

propagation and corresponding AE wave transmission and detection for material under stress.

AE testing can provide comprehensive information on the nucleation of a flaw in a stressed

component and can also provide information pertaining to the initiation and propagation of a



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crack as a component is subjected to stress. This technique has been extensively applied for

condition assessment and damage detection in many materials such as steel, concrete, wood, etc.

This study employed the AE technique to obtain a relative comparison of the expected low

temperature cracking threshold of WMA mixtures containing RAP materials. Mixture specimens

of 150 mm diameter semicircular shape with 50 mm thickness were prepared for AE testing.

This geometry was selected in order to be able to reuse specimens previously tested in the IDT

and/or DC(T) test. AE samples were positioned on a steel block in the cooling chamber as

shown in Figures 4. AE tests were conducted in a polystyrene box containing dry ice as the

coolant. Wideband AE sensors (Digital Wave, Model B1025) with a nominal frequency range of

20 kHz to 1.5 MHz were utilized to monitor and record acoustic activities of the sample during

the test. High-vacuum grease was used to couple the AE sensors to the test sample. AE Signals

were pre-amplified 20dB using broad-band pre-amplifiers to reduce extraneous noise. The

signals were then further amplified 21 dB (for a total of 41 dB) and filtered using a 20 kHz high-

pass double-pole filter using the Fracture Wave Detector (FWD) signal condition unit. The

signals were then digitized using a 16-bit analog to digital converter (ICS 645B-8) using a

sampling frequency of 2 MHz and a length of 2048 points per channel per acquisition trigger.

The outputs were stored for later processing using Digital Wave software (Wave-Explorer TM

V7.2.6). Sample temperature was continuously recorded using a K-type thermocouple placed on

the specimen surface. A typical temperature versus time cooling plot is shown in Figure 5. The

average cooling rate was around 0.8oC/min.



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Moisture Sensitivity Evaluation

Moisture sensitivity analyses were performed using the AASHTO T-283 procedure. This test,

which is the final step of the Level I Superpave Mix Design method, consists of conditioning,

freezing, thawing, and testing stages. In the conditioning phase, researchers vacuum saturated

half of all specimens with water to a degree of saturation between 70 and 80% using a vacuum

pressure no less than 260mm Hg. In the freezing and thawing stages, the previously conditioned

specimens are placed in a freezer for 16 hours at -18oC and subsequently thawed for 24 hours at

60oC. Afterwards, the conditioned and unconditioned specimens were brought to 25oC prior to

measuring their indirect tensile strengths. The quotient of the average indirect tensile strengths

of the conditioned to unconditioned specimens is calculated to determine the tensile strength

ratio, or TSR parameter. It is generally agreed that TSR results greater than 80% are acceptable,

although other thresholds are sometimes used by certain agencies. Finally, a visual rating

between 0 (not stripped) and 5 (completely stripped) is given following procedures outlined in

AASHTO T-283. The required indirect tensile strength testing was conducted at a rate of

50mm/min on six total gyratory specimens compacted to 95.0mm and 7.0% air voids for each

WMA and HMA mixture. Curved loading heads of 19-mm width were used as per AASHTO T-

283.

Rutting Resistance Evaluation

The Hamburg Wheel Tracking test was used to evaluate the permanent deformation

characteristics of the HMA and WMA mixtures investigated. The Hamburg test, specified in

AASHTO T-324, is conducted in a water immersed state at 50oC to induce both permanent

deformation and moisture damage. A steel wheel applies a load of approximately 158lbs. to

each specimen and external linear variable differential transducers (LVDT) measured the rut



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depths at regular intervals during each pass of the wheel. PG 64-22 mixtures are considered

satisfactory in terms of permanent deformation resistance if they can withstand 10,000 wheel

passes prior to reaching a 12.5mm rut depth in order to conform with Texas Department of

Transportation standard TEX-242-F. The presence of stripping can be validated by visually

examining the tested material. Finally, the maximum rut depth is defined as the rut depth present

at the end of the test.

Gyratory specimens, 130mm in height, were cut in half, and sawn along one edge to produce a

flat face to produce a geometry suitable for the Hamburg test (using a the cylindrical geometry

option). The heights of the two sides of each gyratory specimen were adjusted to reach equal

heights to avoid dynamic loading. All Hamburg tests were conducted until either 20,000 passes

was reached or 20.0mm of rut depth was induced. Finally, all specimens were compacted to

approximately 7.0% air voids to comply with AASHTO T-324 standards and four replicates per

mixture were tested.

RESULTS AND DISCUSSION

Moisture Resistance Results

The AASHTO T-283 results from this study are presented in Figure 5 and Table 3. The virgin

mixture TSR results showed that the chemical additive, Chemical-2 and Chemical-1, mixtures

performed approximately 13 and 28% better than the control HMA mixture, respectively in

terms of tensile strength ratio (TSR). These results were anticipated due to the inherent anti-

stripping capabilities of these chemical additives. The Zeolite additive reduced TSR moisture



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resistance by 23% in comparison with the control HMA mixture. This result agreed with the

foaming additive moisture resistance results found by Prowell and Hurley (2005) and could be

due to the additional moisture released into the asphalt concrete during mixing. This moisture

likely has the potential to disturb and subsequently weaken the interface between the asphalt

binder and aggregate. Similar to Zeolite mixtures, F-T wax modified virgin WMA mixtures

reduced TSR moisture resistance. This result agreed with those found by Kanitpong et al. (2008)

in which the employment of an organic additive and reduced production temperatures led to

reduced moisture resistance. Reduced aggregate coating by the modified asphalt binder may

have led to a reduced capability to resist moisture damage. In all, four of five virgin mixtures

failed to meet the 80% minimum set in AASHTO T-283 which would call for anti-stripping

agents or hydrated lime to be added to the mixtures. However, in this study, researchers chose

not to add these moisture resisting additives to avoid interaction effects with WMA additives.

The addition of RAP into the 15 and 45% WMA RAP and HMA mixtures did not affect the

rankings of the TSR results. For all three RAP contents, chemical additive modified mixtures

performed the best, followed by the control HMA, F-T wax additive, and Zeolite foaming

additive modified mixtures. The consistency of the rankings showed that the interaction of RAP

and WMA additives did not improve or detrimentally affect the moisture resistance of mixtures

characterized using the AASHTO T-283 protocol.

The addition of RAP increased both conditioned and unconditioned indirect tensile strengths in

all cases except for the 15% RAP Advera WMA mixture. This finding agrees with those

reported by Li et al. (2004) and Doyle et al. (2011), where tensile strength was found to increase



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with increasing RAP content. Doyle’s study suggested that mixing of virgin and RAP binder

occurs, at least to some degree, even at reduced production temperatures in the presence of

WMA additives.

As shown in Table 6, tensile strength ratio was found to increase with the addition of RAP. The

TSR results for RAP mixtures differed from the Li et al. (2004) study as they found the inclusion

of RAP reduced tensile strength ratios. However, in this study, similar to the Doyle et al. (2011)

study, the hardened asphalt binder films coupled with the fact that the RAP aggregate was

thought to be of higher quality than the virgin aggregate likely led to the observed increase in

TSR with increasing RAP content.

Hamburg Wheel Tracking Test Results

The permanent deformation results via the Hamburg test are shown in Figures 6 and 7. The error

bars represent the high and low rut depths found using the Hamburg test. The Sasobit WMA

mixture performed approximately 12% better than the control HMA mixture in terms of total

number of passes reached prior to reaching 12.5mm of rut depth. These results agree with those

reported by Gandhi (2008). The chemical and zeolite foaming additives investigated reduced the

maximum number of wheel passes to reach 12.5 mm of rutting in the Hamburg device by 40 and

38%, respectively. These results agreed with virgin mixtures evaluated in Doyle et al. (2011)

and Prowell and Hurley (2005) and likely occurred because chemical additives potentially

emulsify the asphalt binder to soften it while foaming additives potentially reduce the adhesive

characteristics between the asphalt binder and aggregate.



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In the 15% RAP mixtures, the number of wheel passes reached prior to 12.5mm rut depth

increased for all WMA and HMA mixtures. Similar to the AASHTO T-283 results, it was

concluded that virgin and RAP asphalt binder mixing occurred to a degree, leading to increased

rutting resistance in the mixtures. The increased rutting resistance of 15% RAP mixtures as

compared to virgin mixtures agreed with the results found by Doyle et al. (2011). In terms of

rankings, the Zeolite and Chemical-1 WMA mixtures remained the same as the virgin set while

the control HMA proved to exhibit greater rutting resistance than the F-T wax WMA mixture.

The 45% RAP mixtures exhibited the most rut resistance among all three data sets. Each of the

WMA and HMA mixtures in the 45% RAP group met the 10,000 pass TEX-242-F minimum

requirement. Similar to the 15% RAP mixtures, the increased rutting resistance of the 45% RAP

mixtures as compared to the 15% RAP mixtures seemed to indicate that at least partial mixing of

the RAP and virgin asphalt binders occurred. As shown in Figure 7, the control HMA mixture

exhibited a lower average rut depth at 10,000 passes as compared to the F-T wax WMA mixture.

The rankings of the Zeolite and Chemical-1 WMA mixtures reversed in the 45% RAP data set.

In general, increasing RAP content improved rutting resistance in all WMA and HMA mixtures.

The interaction of RAP and WMA additives did not seem to significantly alter the rankings of

the mixtures. The chemical and foaming zeolite additive WMA mixtures displayed the greatest

potential for rutting at each of the RAP levels. In addition, the organic F-T wax additive WMA

mixtures performed similarly to the control HMA mixtures due to the stiffening effect of this

additive.



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DC(T) Fracture Results

Figure 8 displays the DC(T) fracture test results. The error bars represent the high and low

fracture energies produced via the DC(T) test among the three replicates tested per set. Table 4

provides the average CMOD fracture energies, average peak loads, and CMOD fracture energy

coefficients of variation (COV) for each mixture.

In the virgin mixtures, the chemical WMA mixtures exhibited slightly greater fracture energy,

approximately 7%, as compared to the control HMA mixture. Higher fracture energy is

desirable from the standpoint of resisting thermal, block, and reflective cracking. On the other

hand, the F-T wax and Zeolite WMA additives had a slight adverse effect on mixture fracture

energy as these mixtures exhibited fracture energies 11 and 12% lower than the control HMA,

respectively. Among the five WMA and HMA virgin mixtures tested in this study, the F-T wax

WMA mixture exhibited the steepest post-peak softening response in its load versus crack mouth

opening displacement (CMOD) curve. Previously published simulation studies have

demonstrated that steep post-peak softening behavior leads to a more brittle fracture with a

higher propensity for crack propagation (Dave et al. (2010)).

The 15% RAP mixtures showed decreased fracture energies as compared to the virgin mixtures.

This result agrees with Behnia et al. (2011), where DC(T) fracture energy decreased with the

addition of RAP for the PG 58-28 mixtures blended with several RAP sources obtained from

Illinois HMA contractors. The 15% RAP HMA mixture performed the best among the 15%

RAP mixtures tested. Thus, perhaps unexpectedly, fracture energy (and hence thermal cracking



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resistance) was not aided by the fact that the WMA mixtures were produced at significantly

lower production temperatures than the reference HMA mix.

The 45% RAP fracture results further demonstrated that increased RAP content led to decreased

fracture resistance. This result differed from Doyle et al. (2011). The 45% RAP mixtures

exhibited significantly higher coefficient of variation as compared to the 0 and 15% RAP

mixtures. It is hypothesized that this increased variability could manifest itself in a greater

likelihood for poor field performance in some sections, and also in difficulties in mixture

production control, especially in meeting end-result or performance-related specifications.

However, it is acknowledged that behavior of field produced mixtures with high RAP content

could vary significantly from that which is characterized in laboratory prepared specimens.

The DC(T) fracture results for WMA RAP and HMA mixtures provided several key

observations although the results were not statistically different. First, the DC(T) test displayed

that mixture fracture resistance can be sensitive to the WMA additive used. Thus, a case can be

made for the importance of a low temperature performance test, such as the DC(T) fracture

energy test. Secondly, irrespective of WMA additives used or reduced production temperatures,

increasingly greater RAP contents likely lead to increased thermal cracking potential.

Consequently, the addition of a softer virgin asphalt binder grade may remain the only option to

combat thermal cracking in high RAP mixtures as WMA employment did not significantly

improve the fracture resistance of RAP mixtures in this study. Advances in asphalt mix plant

design might also aid in the production of WMA RAP mixtures with improved low temperature



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properties, but again, performance tests such as the DC(T) can be used to assess such

technologies.

IDT Creep Compliance Results

Mixture low temperature creep compliance data can be used to assess the ability of a mixture to

resist thermal stress build up upon cooling during a critical low temperature event. Higher

compliance and higher slope at longer loading times (or ‘m-value’) are both desirable from a

standpoint of minimizing stress development and maximizing stress relaxation upon cooling

under restrained conditions. Creep compliance master curve results produced in the Superpave

Indirect Tension Test (IDT) are shown in Figure 9. Similar to the DC(T) fracture test, the

Chemical-modified virgin WMA mixtures displayed the most desirable low temperature creep

performance in terms of greatest creep compliance as compared to the other virgin mixtures. In

addition, the virgin Chemical-1 WMA mixture produced the greatest m-value, which indicates

the greatest capacity for stress relaxation. The control HMA, Zeolite, and F-T wax mixtures

displayed similar creep compliance master curves. The F-T wax WMA mixture exhibited the

lowest m-value and thus the lowest stress relaxation potential among the virgin mixtures tested.

Among the three RAP levels investigated, the virgin mixtures displayed the greatest sensitivity to

WMA additives in terms of creep compliance.

In the 15% RAP data set, RAP addition at this level was found to reduce the m-value computed

from all WMA and HMA mixture master curves. These results agreed with those found by

Behnia et al. (2011) and showed that the presence of RAP led to reduced capacity for stress

relaxation in the case of a thermal event. The m-value rankings remained the same for HMA and



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WMA mixtures in this data set which suggests that RAP interaction with WMA additives did not

improve or reduce creep behavior. The 15% RAP WMA mixtures were slightly more compliant

and therefore better able to relax stress than the control HMA mixture. This result differed from

the fracture test results and likely leads to the conclusion that although the fracture energy may

not be enhanced by WMA technologies, these technologies may improve the bulk material

relaxation capabilities during a cooling event. Modeling and field trials will be needed to fully

evaluate the relative cracking behavior of these mixes; i.e., to determine if the compliance

benefit would outweigh the fracture energy reduction for a given climate and pavement structure.

The 45% RAP WMA and HMA mixtures displayed further stress relaxation losses due to the

presence of RAP. In all 45% RAP mixtures, m-values decreased, which further agreed with the

results found by Behnia et al. (2011). The presence of a high RAP content narrowed the

differences in the creep compliance and m-values among the 45% RAP WMA and HMA

mixtures, which naturally follows due to the high percentage of this common ingredient among

the mixtures. Finally, WMA mixtures continued to display slightly better performance as

compared to the control HMA mixture. In addition, the m-value rankings remained the same as

the other two RAP data sets in this study.

The IDT creep compliance data results in this study provided three key findings: (1) WMA

technologies affected the creep compliance characteristics of virgin asphalt mixtures. Therefore,

additives and processes should be considered carefully in terms of their effect upon stress

relaxation characteristics in colder climates or where rapid temperature changes exist; (2)the use

of WMA slightly improved the stress relaxation characteristics of RAP mixtures, whereas



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fracture energy was not improved with the presence of WMA additives, and; (3) although RAP

WMA mixtures performed slightly better than the control HMA RAP mixture, increased RAP

contents led to significantly reduced stress relaxation capabilities in both WMA and HMA

mixtures.

AE Test Results

Acoustic Emission activity of WMA mixture samples subjected to thermal loading (rapid

temperature decrease) was evaluated by analyzing recorded AE event counts, test temperature

and computed AE energy. A typical plot of event counts & AE energy versus temperature is

shown in Figure 10. Temperatures corresponding to two characteristic AE events were

determined: (1) the “Embrittlement” Temperature (TEMB), and; (2) the maximum energy event

temperature (TMAX). Embrittlement temperature is the temperature corresponding to the event

when the first major energy event occurs, as shown in Figure 10. It is hypothesized that the

embrittlement temperature is the onset of the thermally induced micro-cracking damage in the

mixture and it represents a fundamental material state which is independent of material

constraint, sample size (as long as a statistically representative volume or larger is used), and

sample shape (Behnia et al. (2011), Dave et al. (2011), Behnia et al. (2010). The temperature at

which a maximum acoustic energy release occurs, TMAX, was highly repeatable between test

replicates. Similar to embrittlement temperature, this quantity is thought to be an intrinsic

fracture property of the material.

AE test results of WMA mixtures with 0%, 15% and 45% `RAP are summarized in Figure 11.

The provided results are an average of at least four test replicates for each material, and COV



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values in the range of 5% to 10% were fairly typical. Comparing TEMB of WMA mixtures

containing different RAP amounts reveals the effect of the presence of RAP on low temperature

cracking performance of the mixtures. It is observed that embrittlement temperature of WMA

mixtures occurs at warmer temperatures as the RAP content increases. This can be attributed to

the aged-hardened binder in the RAP, which contributes to more brittle behavior. A comparison

of AE test results of different additives indicates that among all utilized additives, F-T Wax

exhibited the most significant increase in TEMB as compared to others. For WMA mixtures with

45% RAP, there was not much difference between TEMB of the control HMA mixture, Zeolite

and Chemical-1 mixtures. As evidenced in DC(T) and IDT tests, this can be attributed to the fact

that as the RAP content increased, the RAP began to dominate the overall material behavior.

This would explain the lack of significant distinction between TEMB of mixtures with 45% RAP

amounts. AE results of WMA mixtures showed that the TMAX values for control mixtures were

close to their virgin binder low temperature PG grades. This was previously reported by Behnia

et al. (2011) in their study on low temperature performance of RAP mixtures. Another

observation is that unlike TEMB, the TMAX of mixtures was not significantly affected by RAP

content or type of additive. More work is needed, including numerical simulation of the AE test,

to obtain a more fundamental understanding of the damage/cracking behavior leading to AE

events, especially at the thresholds of TMAX and TEMB.

CONCLUSIONS

This study investigated the low-temperature durability of WMA and HMA RAP mixtures

through the use of advanced asphalt mixture performance tests. The research presented herein

focused on the low temperature performance of these mixtures using the DC(T), IDT creep



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compliance, and AE tests. In addition, durability concerns such as moisture and permanent

deformation sensitivity were addressed through the AASHTO T-283 and Hamburg tests,

respectively.

Based upon the results obtained through this experimental investigation, the following

conclusions regarding the behavior of the WMA-RAP mixtures investigated were drawn:

Increased RAP contents led to increased resistance to moisture based upon AASHTO T-

283 test results. Therefore, the use of quality RAP material may be advantageous to

avoid moisture damage with the assumption that the AASHTO T-283 results mirror

trends present in the field. In addition, the chemical additive modified WMA mixtures

performed best among the WMA mixtures in terms of moisture resistance due to the

inherent anti-stripping capabilities of these chemical additives.

Rutting resistance increased with increasing RAP contents in all mixtures. The F-T wax

modified WMA mixtures performed best among the WMA mixtures with regard to

rutting resistance due to the stiffening characteristics of this particular organic additive.

DC(T) fracture energy results for virgin mixtures displayed a sensitivity to WMA

additives. The chemical additives improved the fracture resistance of WMA mixtures as

compared to the control HMA mixture. Therefore, careful consideration of WMA

additive options should be made prior to use in the field to avoid thermal cracking.

The inclusion of RAP led to reduced DC(T) fracture energy as well as IDT creep

compliance. These results showed that the presence of RAP at low temperatures may

lead to increased thermal cracking potential irrespective of the WMA additive employed.



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Acoustic Emission results were sensitive to RAP content as well as to the additive type

used in the WMA mixtures tested. The higher the RAP content, the higher (warmer) the

TEMB of the mixture. In addition, TMAX of control mixtures were close to their virgin

binder low temperature PG grades.

The overall trends of TEMB of WMA mixtures were consistent with the results observed

for fracture energy and creep compliance. This provides more confidence in the use of

the TEMB quantity as a screening tool to quickly assess the cracking resistance of asphalt

mixtures, including those containing RAP and/or WMA.

Advanced mechanical and AE tests such as those presented in this study may be useful in

validating WMA/RAP mixture designs for important paving projects, particularly until

more long-term field performance test results are available. In fact, even when long-term

field performance is available, these tests may continue to serve the asphalt industry for

high-profile projects, since a near infinite combination of WMA additives, RAP sources,

climates, pavement structures, and production/construction variables potentially make

each paving project unique.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the support provided by the sponsor, O’Hare Modernization

Program (OMP), throughout the course of this study, and for the guidance provided by Mr. Ross

Anderson of Bowman, Barrett and Associates. In addition, this study was supported by the

National Cooperative Highway Research Program – Ideas Deserving Exploratory Analysis

(NCHRP-IDEA) program under project #144, “An Acoustic Emission Based Test to Determine



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Asphalt Binder and Mixture Embrittlement Temperature”. Furthermore, the authors would also

like to acknowledge the assistance of Mr. Nathan Kebede and Mr. Salman Hakimzadeh of the

University of Illinois at Urbana-Champaign. The views and opinions expressed in this paper are

those of the authors and do not necessarily reflect the views and opinions of the sponsor.

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List of Tables

Table 1: RAP Gradation and Selected Volumetric Properties

Table 2: Mixture Volumetric Properties

Table 3: AASHTO T-283 Results

List of Figures

Figure 1: WMA-RAP Mixture Gradations

Figure 2: Typical Load-CMOD Plot

Figure 3: Nucleation, Propagation and Detection of AE Waves

Figure 4(a): AE Testing Set-up

Figure 4(b): AE Testing Specimen

Figure 4(c): Typical Temperature vs. Time Cooling Plot

Figure 5: Tensile Strength Ratio Results

Figure 6: Hamburg Test Results (*Additional 45% RAP results shown in Figure 7)

Figure 7: 45% RAP Mixture Rut Depth at 10,000 Wheel Passes

Figure 8: Average DC(T) CMOD Fracture Energy

Figure 9(a): Virgin Mixture Fitted Creep Compliance Master Curves

Figure 9(b): 15% RAP Mixture Fitted Creep Compliance Master Curves

Figure 9(c): 45% RAP Mixture Fitted Creep Compliance Master Curves

Figure 10: Typical Plot of Event Count and AE Energy versus Temperature

Figure 11(a): TEMB and TMAX of Virgin WMA Mixtures, Determined using Acoustic Emission

Technique



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Figure 11(b): TEMB and TMAX of WMA Mixtures with 15% RAP, Determined using Acoustic

Emission Technique

Figure 11(c): TEMB and TMAX of WMA Mixtures with 45% RAP, Determined using Acoustic

Emission Technique



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Table 1: RAP Gradation and Selected Volumetric Properties

Sieve (mm) RAP Gradation RAP Properties 25.0 100 19.0 100

12.5 100 RAP Gsb 9.5 99.3 2.641 4.75 73.8 AC Content (%) 2.36 50.5 5.50

1.18 35.5 RAP Gmm 0.60 25.8 2.492 0.30 18.1 0.15 13.8

0.075 11.2

Accepted Manuscript Not Copyedited



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TABLE 2: Mixture Volumetric Properties

Mix Type Total AC (%) Air Voids (%) VMA (%) VFA (%) Effective AC (%) DP Virgin 6.70 4.0 15.3 73.7 4.9 1.2

15% RAP 6.70 4.0 15.5 74.4 5.0 1.3 45% RAP 6.30 4.0 15.3 73.3 4.9 1.4




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TABLE 3: AASHTO T-283 Results

Virgin Mixtures Mix Type Conditioned Str. (kPa) Unconditioned Str. (kPa) TSR Visual Rating Control 483.3 726.0 67% 5 Zeolite 443.3 859.8 52% 5

F-T Wax 519.9 857.0 61% 5 Chemical-1 635.7 743.9 85% 3 Chemical-2 680.5 902.5 75% 3

15% RAP Mixtures Mix Type Conditioned Str. (kPa) Unconditioned Str. (kPa) TSR Visual Rating Control 869.4 999.1 87% 3 Zeolite 570.9 845.3 68% 5

F-T Wax 648.8 873.6 74% 4 Chemical-1 716.4 758.4 94% 2

45% RAP Mixtures Mix Type Conditioned Str. (kPa) Unconditioned Str. (kPa) TSR Visual Rating Control 1030.1 1354.8 76% 4 Zeolite 812.2 1152.1 70% 4

F-T Wax 917.0 1232.1 74% 4 Chemical-1 1052.1 1218.3 86% 3




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Documents

Evaluation of Warm Mix Asphalt Mixtures Containing Reclaimed Asphalt Pavement through Mechanical Performance Tests and an Acoustic Emission Approach