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Rutting performance prediction ofwarm mix asphalt containing reclaimedasphalt pavementsFereidoon Moghadas Nejada, Alireza Azarhoosha, Gholam HosseinHamedia & Hossein Roshaniba Department of Civil & Environmental Engineering, AmirkabirUniversity of Technology, Tehran, Iranb Department of Civil and Environmental Engineering, University ofTexas at San Antonio, San Antonio, TX, USAPublished online: 20 Dec 2013.

To cite this article: Fereidoon Moghadas Nejad, Alireza Azarhoosh, Gholam Hossein Hamedi& Hossein Roshani (2014) Rutting performance prediction of warm mix asphalt containingreclaimed asphalt pavements, Road Materials and Pavement Design, 15:1, 207-219, DOI:10.1080/14680629.2013.868820

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Road Materials and Pavement Design, 2014Vol. 15, No. 1, 207–219, http://dx.doi.org/10.1080/14680629.2013.868820

Rutting performance prediction of warm mix asphalt containing reclaimedasphalt pavements

Fereidoon Moghadas Nejada, Alireza Azarhoosha∗, Gholam Hossein Hamedia andHossein Roshanib

aDepartment of Civil & Environmental Engineering, Amirkabir University of Technology, Tehran, Iran;bDepartment of Civil and Environmental Engineering, University of Texas at San Antonio, San Antonio,TX, USA

(Received 12 July 2013; accepted 19 November 2013 )

Nowadays, saving energy and recycling materials are becoming priorities in the road construc-tion industry. This paper presents an experimental study to characterise permanent deformationof a warm mix asphalt (WMA) mixture containing reclaimed asphalt pavement (RAP). TheWMA mixtures containing 0%, 15%, 30%, 50% and 60% of RAP were plant prepared. Toassess the impact of RAP on rutting properties of hot mix asphalt, mixtures were tested usingMarshall and dynamic creep tests. It was found that replacing up to 60% of the virgin aggre-gate with RAP improved rutting properties of the asphalt mixtures because RAP increasedasphalt binder’s viscosity as a main factor of rutting, especially at high temperatures. However,replacement of too much RAP in asphalt mixtures can increase moisture sensitivity of flexi-ble pavements. Therefore, an indirect tensile strength test was conducted to evaluate moisturedamage of the mixtures. The results showed that the minimum permissible tensile strengthratio (TSR) of 70% was satisfied by replacing up to 50% of the virgin aggregate by RAP; butthe mixtures with 60% RAP had a TSR of less than 70%. Accordingly, 50% replacement ofRAP was found to be the optimal replacement level.

Keywords: warm mix asphalt; reclaimed asphalt pavement; rutting; tensile strength ratio

1. IntroductionOver millions of tons of asphalt are annually produced in the world (Muradov & Veziroglu,2005). With concerns about global warming and energy consumption, the asphalt industry isalways looking for ways to lower its carbon footprint. Reclaimed asphalt pavement (RAP) andwarm mix asphalt (WMA) are both steps in that direction (Emily, 2010). Asphalt reclamationtechniques have been developed to reduce the amount of waste caused by removing aged asphalt(Dane, 2005). When hot mix asphalt (HMA) pavements reach the end of their usable service lives,their materials retain considerable value (McDaniel & Anderson, 2001).

Virgin aggregates are a limited resource, and the use of fractionated RAP can replace high per-centages of virgin aggregates. RAP consists of aged asphalt and crushed stones and is obtained bymilling top layers of an aged asphalt pavement. The milled RAP is separated and screened to grada-tions. The fractionated RAP is added as an aggregate to the plant, where it is combined with virginaggregate and asphalt binder to produce a new pavement (Emily, 2010). The National AsphaltPavement Association (NAPA) concluded in their research that high RAP mixtures containing

∗Corresponding author. Email: a.r.azarhoosh@aut.ac.ir

© 2013 Taylor & Francis

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30–40% RAP could be produced although major restrictions towards producing quality RAPmixtures result from extreme stiffness of the RAP asphalt binders. A disadvantage of using RAPmaterials is that RAP asphalt binders may force the producer to use a very soft asphalt binderwithin the mixture, which possibly results in workability issues of the mixture (Newcomb, RayBrown, & Epps, 2007).

The consistency (penetration and viscosity) of the asphalt binder plays a relatively greater rolein rut resistance of HMA. Some increased resistance to rutting can be obtained using a stiffer (highviscosity or low penetration) asphalt binder. However, stiffer asphalt binders are more prone tocracking during winter in cold regions, especially if they are used in surface courses (Roberts,Kandhal, Brown, Lee, & Kennedy, 1996).

Along with the increase in heavy traffic volume in roads, rutting effect has turned to one ofthe most important failures, especially in tropical regions. Rutting is caused by the progressivemovement of materials under repeated loading either in the asphalt pavement layers or the under-lying base coarse. Consolidation is the further compaction of HMA pavement by traffic afterconstruction. Rutting is also resulted from lateral plastic flow (permanent deformation) of theHMA from the wheel tracks. The use of excessive asphalt binder is the most common cause ofthis phenomenon. Too much asphalt binder in the mix causes loss of internal friction between theparticles and leads the loads to be carried by the asphalt binder rather than the aggregate structure(Roberts et al., 1996).

In recent years, there has been an increased interest in producing asphalt aggregate mixturesusing greener technologies that would alleviate drawbacks of preparing HMA mixtures. Thesetechnologies are called WMA since they facilitate using lower temperatures when preparingasphalt–aggregate mixtures while satisfying coating and workability requirements. WMA tech-nologies reduce viscosity of the asphalt binder through adding organic or mineral additives,chemical emulsification or foaming using water. These processes allow for producing asphalt–aggregate mixtures at temperatures of 17–55◦C lower than those in the production of traditionalHMA (Ayman, 2010). Reduced mix production and paving temperatures would decrease theenergy required for producing asphalt mixes, reduce emissions and odours from plants and makebetter working conditions at the plant and paving site (Gandhi & Amirkhanian, 2007; Hurley &Prowell, 2005a, 2005b, 2006; Kristjansdottir, Muench, Michael, & Burke, 2007).

Usage of RAP in HMA has been limited by many highway agencies due to the concern thatthese materials might get aged at high temperatures and would potentially lead to early crack-ing of the roads (Sampath, 2010). Using WMA technology, higher percentages of RAP can beincorporated into asphalt mixes because of decreased viscosity of the stiffer asphalt binder inRAP. The resulting mix can be properly compacted in the field due to the reduced viscosity. Withlower temperatures associated with the WMA, there tends to be a decreased age hardening of theasphalt binder which helps in rejuvenation of the RAP asphalt binder. Less ageing of the asphaltbinder during the production and pavement processes increases the service life of the pavementwith fewer occurrences of cracking (Keith, 2008; Zaumanis, 2010). Lowering production andpaving temperature for WMA can cause considerable changes in the properties of bitumen hard-ening in the production process. For long-term in-field performance, the general opinion is thatless ageing during the production and paving processes tends to improve pavements’ flexibility,which reduces susceptibility to fatigue and temperature cracking resulting in improvement ofpavements’ longevity (Perkins, 2009).

1.1. Literature reviewTo date, studies have been conducted on the use of RAP in asphalt mixtures. It has been shown inNational Cooperative Highway Research Program report No. 752 that asphalt pavements contain-

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ing up to 50% RAP in projects with diverse climates and traffic have very positive in-service perfor-mance. Several researchers examined data from experimental sections in the Long-Term PavementPerformance Program to compare overlays with RAP mixes and virgin mixes. Those studies haveshown that the overlays containing 30% RAP have been performing equal to, or better than, virginmixes for most measures of pavement performance (West, Willis & Marasteanu, 2013).

In a study by Luo, Xiao, Hu, and Yang (2013), the fatigue life of rubberised asphalt concretemixtures containing RAP was investigated through a probabilistic analysis. The results indicatedthat probabilistic analysis using the point estimate approach and Monte Carlo simulation method-ologies could be effectively used for exploring probability of fatigue life, require very limitedcomputational effort and thus have the potential and ease as a practical tool for being employedin pavement engineering.

In another laboratory study for evaluating moisture susceptibility of plant-produced foamedWMA containing high percentages of RAP in Tennessee, loose WMA mixtures were collectedand compacted at an asphalt plant and were compared with HMA samples through laboratoryperformance tests. The results indicated that the Superpave indirect tensile test and the dynamicmodulus test had the potential of accurately characterising moisture susceptibility. With the incor-poration of RAP, foamed WMA was expected to perform as well as HMA in terms of moisturesusceptibility (Shu, Huang, Shrum, & Jia, 2012).

Colbert and You (2012) evaluated the influence of fractionated RAP materials on the perfor-mance of asphalt mixtures. On average, among all RAP mixtures, the addition of RAP decreasedrutting by 24% and increased resilient modulus by 52% due to the addition of RAP asphalt binderand aggregates, which stiffened the mixture under higher temperature and in heavier loading condi-tions. Dynamic modulus results indicated a statistically significant difference for high-percentageRAP mixtures.

Reyes-Ortiz, Berardinelli, Alvarez, Carvajal-Muñoz, and Fuentes (2012) focused on evaluatingthe effects of partial and total replacements of aggregates by RAP on the mechanical responseof dense-graded HMA mixtures. The corresponding results suggested that the highest indirecttensile strength (ITS) and resilient modulus values in both dry and wet conditions were obtainedfor the HMA mixtures produced with 100% replacement of granular material by RAP. Also, in theresearch by Valdes, Pérez-Jiménez, Miró, Martínez, and Botella (2011), the mechanical behaviourof bituminous mixtures containing 40% and 60% RAP was assessed. Their mechanical propertieswere then studied by determining stiffness modulus and ITS, cracking and fatigue behaviour.The results showed that high amounts of recycled material could be generally incorporated intobituminous mixes by proper characterisation and handling of RAP stockpiles.

Hajj, Sebaaly, and Shrestha (2009) presented the findings of a laboratory-based research projectthat evaluated the impact of three RAP sources at three levels of RAP content (0%, 15% and30%) on the mechanical properties of the final mix. Depending on the RAP source and content,the addition of RAP to a mixture with an unmodified target asphalt binder resulted in either abetter or worse fatigue resistance. On the other hand, the addition of RAP to a mixture with apolymer-modified target asphalt binder resulted in worse fatigue resistance regardless of the RAPsource and content. Mogawer et al. (2012) focused on obtaining plant-produced RAP mixtures todocument the mixture production parameters and to evaluate the degree of blending between thevirgin and RAP binders. The result of this research showed that the cracking resistance and therutting resistance improved as the percentage of RAP in the mixtures increased. In another study,Visintine, Khosla, and Tayebali (2013) used the Strategic Highway Research Program (SHRP)A-003A surrogate models and the Asphalt Institute (AI) models to predict pavement performanceusing the results from the frequency sweep testing and the repeated simple shear test. The generaltrend was that as the percentage of RAP increases in the mixtures, the fatigue life and rut lifeincreased.

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1.2. ObjectiveThe objective of this research was to investigate rutting of WMA mixtures containing RAP in termsof RAP replacement value and temperature. Marshall and dynamic creep tests were used to assessthe rutting behaviour of control and modified asphalt samples. Rutting behaviour of the mixtureswas compared with each other and the effects of each mixture parameter were investigated. Also,the ITS test was used to determine the permissible limit of RAP in the mixtures.

2. Materials2.1. Aggregate and asphalt binderLocally available granite aggregates from a mine in Tehran, Iran, were used in this study to preparethe mixtures. The aggregates were randomly obtained from quarry stockpiles and transported to thelaboratory. Physical properties of the granite are given in Table 1. Also, gradation of the aggregatesused in the present study (median limits of American Society for Testing and Materials (ASTM)specifications for dense aggregate gradation) is given in Table 2. The virgin asphalt binder whichwas used for this project was a 60/70 penetration grade base bitumen (AC 60/70) produced byPasargad Oil Company, an Iranian oil company. Their rheological characteristics are presentedin Table 3.

2.2. AdditivesThe milled RAP materials were collected from a highway with medium to high traffic volumein Tehran, Iran. The RAP materials used in this project contained an asphalt content of 5.3%.Penetration of the asphalt recovered from the RAP was 9.1 mm at 25◦C.

Table 1. Physical properties of the aggregate.

Test Standard Granite Specification limit

Specific gravity (coarse agg.) ASTM C 127Bulk 2.654 ———–SSD 2.667 ———–Apparent 2.692 ———–Specific gravity (fine agg.) ASTM C 128Bulk 2.659 ———–SSD 2.661 ———–Apparent 2.688 ———–Specific gravity (filler) ASTM D854 2.656 ———–Los Angeles abrasion (%) ASTM C 131 19 Max 45Flat and elongated particles (%) ASTM D 4791 6.5 Max 10Sodium sulfate soundness (%) ASTM C 88 1.5 Max 10–20Fine aggregate angularity ASTM C 1252 56.3 Max 40

Note: SSD, saturated, surface dry.

Table 2. Gradation of aggregates used in the present study.

Sieve(mm) 19 12.5 4.75 2.36 0.3 0.075

Lower–upper limits 100 90–100 44–74 28–58 5–21 2–10Passing (%) 100 95 59 43 13 6

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Road Materials and Pavement Design 211

Table 3. Test results for the 60–70-penetration asphalt binder.

Test Standard Result

Penetration (100 g, 5 s, 25◦C), 0.1 mm ASTM D5-73 64Penetration (200 g, 60 s, 4◦C), 0.1 mm ASTM D5-73 23Penetration ratio ASTM D5-73 0.36Ductility (25◦C, 5 cm/min), cm ASTM D113-79 112Solubility in trichloroethylene (%) ASTM D2042-76 98.9Softening point (◦C) ASTM D36-76 51Flash point (◦C) ASTM D92-78 262Loss of heating (%) ASTM D1754-78 0.75Properties of the thin film oven test residuePenetration (100 g, 5 s, 25◦C), 0.1 mm ASTM D5-73 60Specific gravity at 25◦C (g/cm3) ASTM D70-76 1.020Viscosity at 135◦C (cSt) ASTM D2170-85 158.5

Sasobit as a WMA additive was used in this study. Sasobit is a long-chain aliphatic hydrocarbonobtained from coal gasification by the Fischer–Tropsch process. The Fischer–Tropsch process isa catalysed chemical reaction, in which carbon monoxide and hydrogen are converted into liquidhydrocarbons of various forms in the presence of iron and cobalt as catalysts. Sasobit forms ahomogeneous solution with the base asphalt binder upon stirring (1.5% by weight of the asphaltbinder) and yields marked reduction in the asphalt binder viscosity. After crystallisation, Sasobitforms a lattice structure in the asphalt binder, providing structural stability of the asphalt binder(Sasol Wax, Sasobit Technology, 2001).

3. Mix designThe asphalt mixtures were designed using the standard Marshall mix design procedure with 75blows on each side of cylindrical samples. The samples were compacted and tested by deployingthe following standard procedures: bulk specific gravity (ASTM D2726), stability and flow test(ASTM D1559) and maximum theoretical specific gravity (ASTM D2041).

The optimum asphalt content for the mix design was determined by taking average values ofthe following three asphalt contents:

(1) Asphalt content corresponding to maximum stability.(2) Asphalt content corresponding to maximum bulk specific gravity.(3) Asphalt content corresponding to median of the designed limits of percent air voids in

the total mix.

The stability value, flow value and voids filled with asphalt were checked with Marshall mix designspecifications. The optimum asphalt binder contents were found to be 5.5% for the mixtures. Fivedifferent mixes were used. The first mix was WMA, in which all the used aggregates were granite;this mix was called the control mix. In other mixtures, RAP was used as a portion of aggregate.Composition of the recycled mixture had to be determined, which included the granite-RAPaggregates’ contributions and the asphalt content. The amount of new asphalt binder to be addedto the trial mixes of the recycled mixture, expressed as percent by weight of total mix, could becalculated by the following formula:

Pnb = (1002 − rPsb)Pb

100(100 − Psb)− (100 − r)Psb

100 − Psb, (1)

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Table 4. How to replace RAP and asphalt content of the tested mixes.

Granite-RAP aggregate contributions Asphalt content (%)

Mixture Granite (%) RAP (%) New RAP

WMA-control 100 0 5.5 0WMA-15% RAP 85 15 4.7 0.8WMA- 30% RAP 70 30 3.9 1.6WMA- 50% RAP 50 50 2.8 2.7WMA- 60% RAP 40 60 2.3 3.2

where: Pnb is the percentage of new asphalt binder in the recycled mix, r the new aggregateexpressed as percentage of the total aggregate in the recycled mix, Pb the percentage of optimumasphalt binder content and Psb the percentage of asphalt binder content of RAP.

Detailed description of the combined aggregate in the recycled mixture and the optimum asphaltcontent of mixtures are given in Table 4.

All the mixtures were mixed at 135◦C and compacted at a target temperature of 125◦C. Foreach mixture, at least three samples were produced to determine reproducibility of the results(ASTM, 2000).

4. Experimental set-up and procedureThe tests which are generally used to assess resistance of bituminous mixes to permanent defor-mation are Marshall, static creep, dynamic creep and wheel-tracking tests (Arabani, MoghadasNejad, & Azarhoosh, 2013; Khodaii & Mehrara, 2009; Verstraten, 1994). In this study, resistanceto permanent deformation of mixtures was evaluated using Marshall and dynamic creep tests.

4.1. Marshall stability and flow testThe Marshall test procedure involves applying a compressive load to a cylindrical specimen(101.6 mm in diameter and 63.5 mm in height) through semicircular testing heads. Temperatureof the specimen is 60◦C and the load is applied at a rate of 50.8 mm/minute. Temperature of 60◦Chas been selected since it approximates the maximum pavement temperature in summer, therebyproviding the weakest condition for the WMA mixture (Roberts et al., 1996).

One property that is sometimes used to characterise asphalt mixtures is the Marshall stiffnessindex (or Marshall quotient (MQ)), which is Marshall stability divided by flow. Moreover, MQis a measure of the resistance of the materials to shear stress, permanent deformation and rutting(Zoorob & Suparma, 2000). High MQ values indicate that the mixture presents a high degree ofstiffness and is able to spread the applied load and resist creep deformation (Arabani & Azarhoosh,2012).

4.2. Dynamic creep testThe dynamic creep experiment has been used for a long time to determine rutting potential ofasphalt, which is due to its simplicity and logical relation with permanent deformation of theasphalt mixture. Figure 1 depicts this carve typically (Goh & You, 2009).

As shown in the figure, the curve is composed of three major parts: a primary zone, in whichpermanent deformations are quickly accumulated, a secondary zone, in which accumulative strainsare increased with a smooth and constant slope and a tertiary zone, in which the tone of increase

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Road Materials and Pavement Design 213

Figure 1. Typical flow number test result.

in accumulative strain is again increased. The first zone could be identified as the primary ruttingmechanism (densification; decrease in mass and increase of density). The second zone could beidentified as a connector of the two zones. The third zone could be identified as the secondaryrutting mechanism (shear deformation; plastic flow with no change in the volume) (Gokhale,Choubane, Byron, & Tia, 2005).

The dynamic creep test applies a repeated pulsed uniaxial stress on an asphalt specimen and mea-sures the resulting deformations in the same direction using linear variable differential transducers.For the dynamic creep test, cylindrical specimens of 63.5 mm × 101.6 mm (thickness × diameter)were prepared. The test temperatures were considered equal to 40◦C and 60◦C.

Creep modulus is the most important output of the dynamic creep test. The value of creepmodulus is an additional indication of the resistance to permanent axial deformation and forbituminous specimens which are basically obtained from the ratio of applied stress (200 kPa)to the cumulative compressive strain at a defined temperature and time of loading. Mixtureswith a lower creep modulus are known to undergo higher deformation. Thus, knowing the initialheight of the specimen, axial strain, ε, and therefore creep modulus Smix can be determined at anyloading time:

Smix = σ

ε, (2)

where σ is the applied stress and ε the axial strain.

4.3. ITS testIn this study, moisture sensitivity of asphalt concrete was assessed by the tensile strength ratio(TSR). For this purpose, cylindrical specimens with a diameter and height content of 101.6and 62.5 mm, respectively, were tested in dry and wet conditions and at 25◦C. The samples inwet conditions were placed in a vacuum to reach saturation levels of 55–80%. The vacuum-saturated samples were kept in a −18◦C freezer for 16 h and then placed in a 60◦C waterbath for 24 h.

All of the samples were brought to a constant temperature and ITS was measured on bothunconditioned (dry) and conditioned (wet) specimens. The specimen was removed from the bath,its thickness was determined and then it was placed on its side between the bearing plates ofthe testing machine. Steel loading strips were placed between the specimen and bearing plates.

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A load was applied to the specimen by forcing the bearing plates together in a constant rate of 2(50.8 mm) per minute.

S = (2P)

(πDt), (3)

where S is the ITS (kPa), P the peak value of the applied vertical load (repeated load) (kN), tthe mean thickness of the test specimen (m) and D the specimen diameter (m). Indirect TSR wasdetermined by the following equation:

TSR = 100(Scond/Suncond), (4)

where Scond is the average ITS of the wet specimens and Suncond the average ITS of the dryspecimens.

5. Results and discussion5.1. Marshall stability and flow testMarshall stability and flow of the mixtures are given in Table 5, as the mean of three samples.As shown in the table, when RAP was used, Marshall stability of the samples increased and theflow decreased. Specifically, Marshall stability of the mixture was increased by 49.43% in the60% RAP mixture. Due to using RAP in asphalt mixes, viscosity of the asphalt binder increased.Stability is generally a measure of the mass viscosity of the aggregate–asphalt binder mixtureand is affected significantly by viscosity of the asphalt binder at 60◦C. Therefore, the increasesin viscosity of the asphalt binder increase the Marshall stability. The flow of the mixture wasdecreased by 30.78% in the 60% RAP mixture. High flow values generally indicate a plastic mixthat will experience permanent deformation under traffic.

The relationship between MQ (Marshall stability divided by flow) and type of mixture isprovided in Figure 2. The 60% RAP mixture displayed the highest MQ value, which was ameasure of the resistance of the material to shear stress, permanent deformation and rutting. Ahigher value of stability divided by flow indicated a stiffer mixture and, hence, the mixture waslikely to be more resistant to permanent deformation.

5.2. Dynamic creep testAs shown in Figures 3 and 4, the number of pulses resulting in failure for the mixtures withand without RAP at 40◦C and 60◦C is compared. It was observed that at all temperatures; themixtures including RAP had less permanent deformation. As the amount of RAP is increased inthe mixture, permanent deformation decreased and the descending pace was kept at 60% of RAP.

The temperature rise led to an increase in rutting potential in control and modified asphaltsamples; high sensitivity of the asphalt binder to temperature variation being the cause. As the

Table 5. The Marshall stability and flow for all mixtures.

Mixture type

WMA-15% WMA-30% WMA-50% WMA-60%Property WMA-control RAP RAP RAP RAP

Marshall Stability (kN) 13.27 14.11 15.62 17.03 19.83Flow, 0.25 mm 12.15 11.62 10.78 9.34 8.41

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Road Materials and Pavement Design 215

Figure 2. Effects of using RAP on the MQ values.

Cycles

WMA-Control WMA-15%RAP WMA-30%RAP WMA-50%RAP WMA-60%RAP

Figure 3. Number of cycles versus permanent deformation at 40◦C.

Cycles

WMA-Control WMA-15%RAP WMA-30%RAP WMA-50%RAP WMA-60%RAP

Figure 4. Number of cycles versus permanent deformation at 60◦C.

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WMA-Control WMA-15%RAP WMA-30%RAP WMA-50%RAP WMA-60%RAP

Cre

ep m

odul

us (

MP

a)

Mixtures

Figure 5. Creep modulus values of the control and modified asphalt mixtures.

temperature is increased, viscosity and stiffness of the asphalt binder declined. These factorswere in agreement with SHRP qualification for the rutting property (Kebritchi, Jalali-Arani, &Roghanizad, 2011).

Viscosity of the asphalt binder at high temperature is considered as one of its most importantproperties since it represents the ability of the asphalt binder to be pumped through an asphaltplant, thoroughly coat the aggregate in an asphalt mixture and be placed and compacted to forma new pavement surface (AI, 2003). The use of RAP decreases the temperature sensitivity of theasphalt binder by decreasing its penetration and increasing its viscosity and softening point. Onthe other hand, the procedure of changes in permanent deformation at any temperature followsthe last model.

As shown in Figure 3, difference of permanent deformation between control and RAP mixtureswas not very high at 40◦C. It is believed that the first mechanism of rutting (densification; decreasein mass and increase in density) happens as the curve is passing through the first and the secondzones. It can be concluded that mixtures with 60% RAP had 38.60% less deformation comparedwith the control mixture. This amount equaled 11.32%, 18.82% and 28.75% for 15%, 30% and50% of RAP usage, respectively.

When temperature increased to 60◦C, it was seen that in the control with 15% and 30% RAPmixtures, the second rutting mechanism (shear deformation; plastic flow with no change in thevolume) occurred and the curve entered the third zone which ended in failure. However, the 50%and 60% RAP mixtures remained in the second zone, where accumulative strains occurred with asmooth slope. At this temperature, permanent deformation of the mixture with 60% was 45.91%less than that of the control mixture. As a result, it can be concluded that RAP could be identifiedas a good replacement aggregate to be used for the main roads in tropical regions, where therutting effect is known to be one of the main failures.

Creep modulus values of control and RAP mixtures are given in Figure 5. In the unmodifiedspecimens, the rate of reduction was more than the modified samples. Compared with the controlmixture, there was substantial improvement in resistance to permanent deformation of the mixturescontaining 60% RAP as indicated by a higher creep modulus at different temperatures. However,with increasing temperature, the creep modulus of all the specimens decreased. This behaviourresulted from high sensitivity of the asphalt binder to temperature in asphalt mixtures.

5.3. ITS testThis test was used to determine the maximal allowable amount of RAP in the asphalt mixtures. Themost frequently referenced relationship between characteristics of the asphalt binder (viscosity)

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Road Materials and Pavement Design 217

Ten

sile

str

engt

h ra

tio

(%)

Mixtures

Figure 6. Results of the moisture susceptibility tests.

in a paving mixture and tendency of the mix to strip related stripping resistance in service. Itwas mentioned earlier that the use of RAP in asphalt mixtures results in progressively lowerpenetration and/or higher viscosity. Low viscosity is desirable during mixing operations becausea low viscosity fluid has more wetting power than the one with high viscosity.

The minimum permissible TSR should be 70% in order to have an asphalt mixture that suf-ficiently resists moisture and water-related damage, otherwise known as stripping. Moisturesusceptibility using an indirect TSR test of the mix at 25◦C is shown in Figure 6. It is revealed thatas the percentage of RAP is increased, the degree of moisture susceptibility of the virgin aggregate(VA)-RAP HMA is increased. However, all the mixes were within the acceptable range, exceptthe 60% RAP mix (TSR: 67%). Therefore, according to the results of other tests, the optimalamount of RAP in the asphalt mixtures was revealed to be 50%. It should be noted that usingRAP containing less asphalt binder or asphalt binder with lower viscosity can increase the optimalamount of them in the asphalt mixtures.

6. ConclusionsThe objective of this study was to investigate rutting characteristics of WMA with the RAPadditive. Resistance to permanent deformation of the mixtures was evaluated by Marshall anddynamic creep tests. Also, the ITS test was used to determine the permissible limit of RAP inmixtures. Based on the experimental results for the asphalt binders and mixes, the followingobservations can be made:

(1) The results obtained by Marshall, dynamic creep and ITS tests for the samples showedthat 50% RAP was an optimal content in asphalt mixtures.

(2) When RAP was used, Marshall stability of the samples increased and flow decreased.Due to the use of RAP in asphalt mixes, viscosity of the asphalt binder increased.

(3) RAP mixtures had a higher value of stability divided by flow, which indicated a stiffermixture and the mixture was likely to be more resistant to permanent deformation.

(4) When RAP increased (at two temperatures), permanent deformation decreased and adescending pace was kept at 60% of RAP. However, with temperature increased to 60◦Cin the control, 15% and 30% RAP mixtures, the second rutting mechanism occurred andthe curve entered the third zone which ended in failure.

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(5) Using RAP increased the moisture sensitivity of asphalt mixtures because it resultedin progressively lower penetration and/or higher viscosity. However, all the mixes werewithin the acceptable range, except for the 60% RAP mix (TSR: 67%). It is recommendedthat the use of RAP as a replacement aggregate should be limited to 50% for the selectedmix.

(6) Temperature increase caused an increase in rutting potential. However, the mixtures withRAP had less sensitivity to temperature changes and increase in permanent deformationin these mixtures was less than the control mixture.

(7) The use of RAP could reduce primary production costs and prevent high accumulationof this material in the environment.

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