Construction and Building Materials 36 (2012) 381–390

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Construction and Building Materials

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Mechanistic approach for fiber and polymer modified SMA mixtures

Ali Mokhtari a,⇑, Fereidoon Moghadas Nejad b,1

a School of Civil Engineering, Iran University of Science and Technology, P.O. Box 16765-163, Narmak, Tehran, Iranb Department of Civil & Environmental Engineering, Amirkabir University of Technology, Tehran, Iran

h i g h l i g h t s

" We examined the laboratory properties and service life of different SMA mixtures." SBS could significantly improve the performance and service life of SMA mixtures." The difference between two types of fibers was insignificant." Just in the draindown test, fibers were more effective than SBS." Significant correlations were found between different properties of SMA mixtures.

a r t i c l e i n f o

Article history:Received 2 April 2012Received in revised form 11 May 2012Accepted 15 May 2012Available online 23 June 2012

Keywords:Stone matrix asphaltSBSFiberPavement designMoisture susceptibilityCreep

0950-0618/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.conbuildmat.2012.05.032

⇑ Corresponding author. Tel.: +98 911 1163925.E-mail addresses: alimokhtari@civileng.iust.ac.ir

aut.ac.ir (F. Moghadas Nejad).1 Tel.: +98 21 64543004; fax: +98 21 66414231.

a b s t r a c t

In this study, different additives such as mineral fiber, cellulose fiber and styrene–butadiene–styrene(SBS) were used to modify stone matrix asphalt (SMA) mixtures, and performance tests were performedfor modified and unmodified mixtures. A mechanistic–empirical design procedure was employed toassess the effect of different additives in improving the service life of the pavement or reduction in thick-ness of pavement layers. Based on the results, SBS was more effective in improving the performance ofasphalt mixtures compared to the fibers. According to the results of mechanistic–empirical design, theservice life of the pavement system modified with mineral, cellulose and SBS were 1.07, 1.081 and1.243 times more than unmodified mix, respectively.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Stone matrix asphalt (SMA) is a type of gap-graded hot mix as-phalt that acquires its resistance from stone skeleton of coarseaggregate. Coarse aggregate fraction of this type of mixture is gluedtogether with a durable and moisture-resistant mortar comprisingasphalt cement (typically 5.5–7%), filler (typically 8–12%), and sta-bilizing additive [1]. According to the reports, stone matrix asphalthas indicated more durability as a surface asphalt concrete [2].Some of positive properties of SMA mixtures comprise its highrut resistance, high skid resistance, high durability, improved resis-tance to reflective cracking, better drainage condition and reducednoise pollution [3]. The main reason of using a gap gradation inthese mixtures is to establish stone on stone condition to be resis-tant against permanent deformation. Because of both gap graded

ll rights reserved.

(A. Mokhtari), moghadas@

nature of this mixture and high amount of binder, drain downhas been regarded as the main problem in constructing and placingthis type of asphalt concrete. To prevent this problem, the use ofsome additives may be essential. There are mainly two types ofadditives used in these mixtures: Fibers and Polymers. Fibers suchas polyester fiber, mineral fiber and cellulose fiber could relativelychange the viscoelasticity of mixture; improves dynamic modulus,tensile strength and moisture susceptibility, creep compliance, rut-ting resistance and fatigue life; while significantly lowers theamount of draindown of asphalt mixtures. In contrast, a polymeris often used in asphalt mixtures to substantially improve themechanical properties of asphalt mixtures and relatively decreasethe binder drain down [4]. Different types of polymers (such as sty-rene–butadiene–styrene (SBS), ethylene–vinyl–acetate (EVA),polyethylene or polypropylene) may be used to gain desired mix-ture properties based on designer’s idea [5].

In addition, the mechanistic–empirical method, a combinationof theory with empiricism, uses the elements of mechanical mod-eling and laboratory test or field performance observations todetermine the desired thickness of pavement system. The mecha-

Table 1Properties of used aggregates.

Properties Method Requirement Values

Coarse aggregateLos angeles abrasion (%) AASHTO T96 [33] 30 max. 20Water absorption (%) AASHTO T85 [34] 5 max. 0.8Bulk specific density (g/cm3) AASHTO T85 [34] – 2.654Flat and Elongated (3–1) (%) ASTM D4791 [35] 20 max. 12Soundness (sodium sulfate) (%) AASHTO T104

[36]15 max. 4.8

Crushed content (one face) (%) ASTM D5821 [37] 100 min. 100Crushed content (two faces)

(%)ASTM D5821 [37] 90 min. 100

Fine aggregateWater absorption (%) AASHTO T84 [38] – 1.4Bulk specific density (g/cm3) AASHTO T84 [38] – 2.617Mineral fillerBulk specific density (g/cm3) AASHTO T84 [38] – 2.702

Table 2Physical properties of asphalt cement.

Parameter measured Test method Test value

Specific gravity at 25 �C (g/cm3) AASHTO T228 [39] 1.01Penetration at 25 �C (0.1 mm) AASHTO T49 [40] 63Softening point (R&B) (�C) AASHTO T53 [41] 51Viscosity at 120 �C (centistokes) AASHTO T201 [42] 1051Viscosity at 135 �C (centistokes) AASHTO T201 [42] 361Viscosity at 160 �C (centistokes) AASHTO T201[42] 170Ductility at 25 �C (cm) AASHTO T51 [43] >100

Table 3Properties of fibers.

Properties Value

Cellulose fiberCellulose content (%) 80 ± 5%Ave. fiber length (mm) 1.1Ave. fiber thickness (mm) 0.045pH-value (5 g/100 ml) 7.5 ± 1Bulk density (g/cm3) 0.5 ± 0.05

Mineral fiberAve. fiber length (mm) 6.0Ave. fiber thickness (mm) 0.005Bulk density (g/cm3) 2.7 ± 0.25

Table 4Properties of styrene–butadiene–styrene.

Properties Test method Unit Typicalvalue

Specificationlimits

Specific weight at23 �C

ASTM D792[44]

g/l 0.94 0.92–0.95

Styrene content ASTM D1416[45]

%wt 32 30.5–33.5

Ash content ASTM D1416[45]

%wt 0.9 61

Oil content ASTM D1416[45]

phr 0 <2

382 A. Mokhtari, F. Moghadas Nejad / Construction and Building Materials 36 (2012) 381–390

nistic component is further related to the calculation of differentdesign parameters of pavement such as stress, strain and deflectionbased on input data such as loading and environmental conditions.Obtained structural responses should be then related to the pave-ment performance by means of empirical distress model [6].

To review the previous researches regarding the effect of poly-mers and fibers on performance of SMA mixtures, Al-Hadidi andTan [7] investigated the influence of Low Density Polyethylene(LDPE) on SMA mixtures, laboratory tests such as Marshall, mois-ture sensitivity and low-temperature cracking test were then per-formed to assess the performance of constructed SMA samples. Thebenefit of modification was evaluated by using a mechanistic–empirical method. As they reported, it was observed that LDPEcould efficiently reduce the binder drain down and improve theproperties of asphalt binder as well as SMA mixture; 6% of LDPEwas reported to be the best dosage to construct SMA mixtures.Moreover, the injection of LDPE to the SMA mixture resulted in alife of pavement 1.359 times more than unmodified SMA mixture.Kumar et al. [8] added different types of fibers (natural and pat-ented) and crumb rubber to SMA mixtures. Based on the results,natural fiber was comparable to the patented fiber in terms of Mar-shall, rutting and flexural fatigue tests; however, natural fiber re-sulted in a better condition in terms of aging of asphalt concrete.In addition, crumb rubber indicated better efficiency in improvingthe properties of SMA mixtures compared to the fibers. As pre-sented by Al-Hadidi and Tan [2] , polypropylene could also en-hance the performance of SMA mixtures in terms of Marshall,moisture sensitivity and compressive strength. There was no needto add any fiber to decrease the amount of binder drain downwhen using polypropylene. This type of additive could also im-prove the service life of the SMA mixture 1.48 times more thanunmodified one. A comparative study performed by Tayfur et al.[9] explored the rutting performance of SMA mixtures modifiedwith different additives such as amorphous polyalphaolefin, cellu-lose fiber, polyolefin, bituminous cellulose fiber and styrene buta-diene styrene. As the main results of this research, SBS showed thehighest resistance to permanent deformation and fibers indicatedthe least improving effect among different used additives. Behbah-ani et al. [10] used different fibers to construct SMA mixtures.Based on this research, German cellulose fiber had a better effecton properties of SMA mixtures rather than Iranian cellulose andmineral fibers. Al-Hadidy and Tan [5] also investigated the effectof SBS and Starch on performance of SMA mixtures. Different per-formance tests such as Marshall, indirect tensile strength and resil-ient modulus were conducted to evaluate the SMA mixturesmodified with 5% of aforementioned additives. Based on their re-port, Starch could be a good replacement of SBS; however, in mostcases, SBS showed a better effect on properties of SMA mixtures;however, Starch was more efficient in improving the moisture sus-ceptibility of SMA mixtures compared to SBS.

The main objective of this study is to perform a laboratoryinvestigation on SMA mixtures containing polymers and fibers,and then conduct a mechanistic–empirical approach to determinethe effect of different additives in increasing the service life of thepavement or reduction of the pavement layers thickness.

Volatiles content ASTM D1416[45]

% 0.6 <1

Hardness ASTM D2240[46]

ShoreA

75 72–78

2. Materials and methods

2.1. Materials used

Aggregate was supplied from an asphalt plant (Asb-cheran mine) in Roudehen,north-earth part of Tehran, Iran. In addition, the used mineral filler was rock dust,Passed through sieve No. 200 (0.075 mm). The properties of aggregate were pre-sented in Table 1.

Asphalt cement for this study was AC-60/70, provided from Tehran Refinery andPasargad Oil Company, Tehran, Iran. Physiochemical properties of this cement wereprovided in Table 2.

Mineral fiber, cellulose fiber and SBS were added to the SMA mixtures as draindown inhibitors. The properties of the used fibers are provided in Table 3. As sug-gested by National Cooperative Highway Research Pavement (NCHRP) Report No.425 [11], the added amount of mineral and cellulose fibers were 0.4% and 0.3% byweight of the mixture, respectively. Fibers were uniformly mixed with aggregatebefore incorporating the asphalt cement. Table 4 presents the specification of SBSused in this study. Based on the previous researches [12], 5% of SBS (by weight of

Table 5SMA mixtures specifications for marshall hammer compacted designs [11].

Property Requirement

Air Void (%) 3–4VMA (%) 17 min.VCAMIX Less than VCADRC

TSR (%) 70 min.Drain down @ production temperature (%) 0.30 max.

A. Mokhtari, F. Moghadas Nejad / Construction and Building Materials 36 (2012) 381–390 383

binder) was selected to add to the preheated binder (170 �C). At this temperature,the rotation speed was regulated at 1500 rpm. After about 30 min the speed in-creased up to 4000 rpm for 1 h to make a homogeneous binder.

2.2. Mixture design

The mixture design method used for SMA was as proposed by NCHRP ReportNo. 425 [11]. To evaluate volumetric properties of mixtures with different gradationlimits, a number of 12 samples (four identical samples for each) were fabricatedaccording to the Marshall mix design (ASTM D1559 [13]) procedure as the mostcommon design procedure for asphalt concrete in Iran. The mixing and compactiontemperatures for constructing SMA specimens were selected in a way that the vis-cosities are 170 ± 20 and 280 ± 30 cSt, respectively [1]. This criterion was consid-ered to determine the mixing and compaction temperature base binder. However,construction temperatures for SBS modified binder were 175 �C and 165 �C as rec-ommended by manufacturer. Laboratory specimens were prepared using fiftyblows of the Marshall hammer per side. Seventy-five compaction blows were notused since they would not result in a significant increase in density over that pro-vided by 50 blows [2] and would also cause more breakdown in coarse aggregate[14]. The optimum gradation should be selected in such a way that the VCA2 ratiowas less than 1 in order to establish stone on stone condition in all SMA samples.After optimizing the gradation for SMA mixtures, it is important to select optimumbinder content (OBC) to meet the requirements presented in Table 5. With this regard,three series of SMA mixtures should be prepared for each set of the mixture to calcu-late the OBC for control, SBS-, cellulose- and mineral-modified mixtures. Differentpercentages of binder should be added to each subset to both evaluate volumetricproperties of mixtures and determine the amount of OBC. It should be noted thatthe OBC for each subset in this section was considered so that all specimens met4% of air void.

2.3. Drain down

In order to evaluate the potential of drain down, a test procedure suggested byNCHRP Report No. 425 [11] was conducted for SMA mixtures. Thus, for each set ofmixture, a standard wire basket containing uncompacted SMA sample along with aplate were placed in an oven at the mixing temperature. After about 1 h, theamount of binder drain down was calculated by the following equation:

Draindown ð%Þ ¼ B� AW� 100 ð1Þ

where A is the Initial plate mass, B is weight of plate plus drained materials and W isthe Loose sample mass.

2.4. Performance tests

2.4.1. Marshall testAlthough Marshall test is usually carried out for hot mix asphalt (HMA) mix-

tures in order to perform the process of mixture design, Marshall stability and flowvalues for SMA mixtures are generally measured for information but not for accep-tance. The Marshall stability for SMA mixtures is significantly lower than that fordense graded mixtures [1]. This is not an indication that dense graded mixturesare superior to SMA mixtures but it declares that Marshall stability may not beappropriate for SMA mixtures. To design SMA mixtures, the volumetric propertiesmight be more applicable than Marshall stability.

Marshall quotient is calculated as the ratio of the Marshall stability to the flow.This ratio clearly represents the performance properties of asphalt mixture such asstiffness, resistance to the shear stress and permanent deformation [15]. The higherMarshall quotient value a mixture has, the more resistant to cracking it should be.

To prepare desired specimens, three identical samples for each type of SMAmixtures were compacted at 4% of air void. Designing SMA mixtures at 3% of airvoid would definitely result in a higher probability of fat spots and permanentdeformation [16]. All specimens were submerged in 60 �C water for about 40 minbefore the test.

2 Void in coarse aggregate.

2.4.2. Moisture damageDue to detrimental effect of water, failure rate of pavements may considerably

increase when water can easily get into the pavements. There is a close relationshipbetween the permeability of the mixture and possibility of moisture damage to oc-cur. If asphalt pavements are impenetrable, moisture damage would seldom hap-pen; however, it may result in a excessive permanent deformation of pavementunder heavy traffic load and bleeding phenomenon at high temperatures [17]. Insome cases, failure occurs as a complete separation of the asphalt mixes just withina few years after pavement construction [18].

In the case of moisture damage measurement, tensile strength is one of themost critical parameters which should be always considered for performance eval-uation. This parameter is measured to determine the tensile properties of the as-phalt concrete, which can be further related to the cracking properties of thepavement. Thus, the tensile strength is primarily regarded as a function of the bin-der properties. The amount of asphalt binder in a mixture and its stiffness affect thetensile strength. Tensile strength also depends on the absorption capacity of theused aggregates [19].

Moisture sensitivity of a mixture could be evaluated by performing AASHTO T-283 [20] test. As defined in this procedure, indirect tensile strength of mixturescould be measured by applying compressive loads along a diametrical planethrough two opposite loading strips for both dry and preconditioned specimens[21]. To begin this test, each SMA mixture should be compacted at 6 ± 1 percentof air void by adjusting the number of blows as described by NCHRP Report No.425 [11]. Two subsets (three identical samples for each) are produced in the labo-ratory for each set of mixtures. One subset is tested dry at 25 �C and the other one ispreconditioned before testing in warm-water bath for 24 h at 60 �C. Maximum ap-plied force is measured for each set; then, indirect tensile strength (ITS) and tensilestrength ratio (TSR) are calculated by Eqs. (2) and (3).

ITS ¼ 2PptD

ð2Þ

TSR ð%Þ ¼ ITSWET

ITSDRY� 100 ð3Þ

where ITS is the tensile strength (kPa), P is maximum load (N), T specimen thickness(mm) and D is the specimen diameter (mm).

As mentioned by NCHRP Report No. 425 [11], SMA mixtures should meet appli-cable specification (70% minimum) as their TSR values. Therefore, mixtures withTSR less than 70% are moisture susceptible and it might be necessary to use anti-stripping additives or adjust mixtures to meet the requirement.

2.4.3. Dynamic creep testAs a general description, rutting is the longitudinal settlement of the pavement

surface along the wheel load path due to the progressive movement of paving mate-rials under traffic loading. The amount of the depression significantly depends onthe quality of asphalt concrete as well as mechanical properties of other layers[22]. Based on previous researches, rutting was considered as the most importanttype of distress in asphalt pavement. Although aggregate, asphalt cement and airvoid are the three important parameters of asphalt concrete, which influence therutting performance of pavements, aggregate shape and texture have a significantrole in determining the level of aggregate interlocking and also the amount of pave-ment rutting [23]. Therefore, the main reason why SMA mixtures have been widelyused is that the texture of these types of mixture causes them to be superior to con-ventional hot mix asphalt in terms of permanent deformation. However, the type ofstabilizer can considerably affect the SMA rut-resistance capability [24].

Various experimental tests such as static creep, dynamic creep, wheel trackingand indirect tensile tests are normally used to assess the potential of permanentdeformation of asphalt mixtures. Among the mentioned methods, dynamic creeptest is found to be one of the best methods to evaluate the rutting performanceof asphalt mixtures [25]. In addition, the findings indicated that there is a very goodcorrelation between the measured rut depth obtained from the wheel tracking testand the deformation strain obtained from the uniaxial creep test [15,26]. The Uni-versal Testing Machine (UTM) is the most commonly used device used to measurepermanent deformation of asphalt mixtures. As indicated in Fig. 1. The creep curvecould be divided into three zones: (1) primary zone, (2) secondary zone, and (3) ter-tiary zone. During the primary zone, the mixture volume decreases due to densifi-cation and thus accumulated strain dramatically increases. In the secondary zoneidentified as a transition zone between the primary and tertiary zones, the relation-ship between accumulated strain and number of cycles is linear. The tertiary zonecould be named as appearance of the second mechanism of rutting in which theshear deformation starts and rutting increases again. To compare rutting perfor-mance of different asphalt mixtures, Kaloush and Witczak et al. [25] used flownumber (FN) as a point at which the tertiary zone in creep curve begins.

In order to evaluate the creep behavior of SMA mixtures UTM-5 apparatus wasutilized. The load on the specimens was uniaxial and dynamic, representing the re-peated axle loads applied on the pavement surface. All specimens were prepared byusing Marshall compactor in order to reach 4 percent of air void. Other conditionswere kept the same for different type of mixtures; a loading level of 400 kPa(58.4 psi) was applied to each specimen; the shape of loading was rectangular with

384 A. Mokhtari, F. Moghadas Nejad / Construction and Building Materials 36 (2012) 381–390

a load and rest period of 1s; all specimens were placed at 45 �C for 4 h before test-ing; the test was continued for each sample until 10000 cycles or the loaded spec-imen reach its tertiary stage.

2.4.4. Resilient modulusResilient modulus (MR) is one of the most important properties of asphalt mix-

tures, which is further used in mechanistic design procedure for pavement struc-tures. It is the measure of pavement response in terms of dynamic stresses andcorresponding strains. Resilient modulus of asphalt mixtures is usually measuredin the indirect tensile mode (ASTM D4123 [27]) and used to evaluate the elasticproperties of asphalt concrete mixtures [28].

Although paving materials are not completely elastic and the occurrence ofsome permanent deformation after each load application is possible, the amountof deformation under loading conditions could be considered recoverable if the ap-plied load is small compared to the strength of the materials. In this condition, theamount of deformation is proportional to the load magnitude and thus the behaviorof materials could be considered as elastic. Also, the sum of instantaneous andrecoverable deformations, which are indicators of elastic and visco-elastic behaviorof the material, is considered as the total deformation of asphalt mixture [29]. As aresult, the MR is generally calculated by Eq. (4) as provided below.

MR ¼ Pðlþ 0:27Þtdh

ð4Þ

where P is the maximum dynamic load (N), l us Poisson’s ratio, t is specimen length(mm) and dh is the horizontal recoverable deformation (mm).

3. Statistical analysis

To investigate how different properties of SMA mixtures couldrelate to each other, a statistical analysis should be performed. Inthis research, the relationship between performance tests resultssuch as Marshall stability, indirect tensile strength (ITS), resilientmodulus and dynamic creep test was inspected using linear regres-sion by conducting the Statistical Package for the Social Sciences(SPSS V19) program. In addition, for each existed association, therelevant equation with the corresponding information wasprovided.

4. Mechanistic–empirical design

In this research, a mechanistic–empirical design procedure hasbeen developed to assess the benefit of modification in two differ-ent aspects, which could be stated as reduction in pavement layersthickness and extension in service life of the pavement system. Theformer which has been widely called as layer thickness reduction(LTR) is used to determine the allowable decrease in pavement lay-ers thickness when the service life is kept constant for unmodifiedand modified mixtures. However, the later, extremely specified astraffic benefit ratio (TBR) could be put into practice in order to de-fine the permissible additional number of passing axles when thepavement section is maintained the same for the two aforemen-tioned mixes.

Fig. 1. Typical creep curve.

As the two important indices of rutting and fatigue phenomena,vertical compressive strain (ev) at the top of the subgrade layer andhorizontal tensile strain (et) at the bottom of the asphalt concretelayer should be considered, respectively. In this study, BISAR soft-ware was employed to analyze a typical multilayer elastic pave-ment system. The typical pavement section used for analyzing isas presented in Fig. 2. Dual tires of an 80 KN standard axle loadwas considered as applying load on the surface of the pavementsection. A range of thicknesses and resilient moduli of SMA mix-tures obtained from corresponding test should be taken into con-sideration in the multilayer elastic design to estimate theimprovement in service life or reduction in layer thickness as a re-sult of using different modifiers.

As presented by Transport and Road Research Laboratory(TRRL), Eq. (5) could be used to estimate the allowable numberof load repetition in terms of rutting with the reliability of 85%[30].

Nd ¼ 6:18� 10�8 ðevÞ�3:95 ð5Þ

where Nd is the allowable number of repetition to produce a rutdepth of 10.16 mm and ev is vertical compressive strain at the topof subgrade layer.

In addition, the desired equation to predict the allowable num-ber of accumulated standard axles in terms of fatigue cracking fail-ure is as Eq. (6) [30].

Nf ¼ 1:66� 10�10 ðetÞ�4:32 ð6Þ

where Nf is allowable number of repetition to prevent fatigue crack-ing, and et is horizontal tensile strain at the bottom of asphalt con-crete layer.

Thus, for each set of mixture, two values for N could be calcu-lated using Eqs. (5) and (6). The lesser one should be consideredas the allowable number of load repetition since it would be morecritical. In addition, Eqs. (7) and (8) might be used to calculate thevalue of LTR and TBR for modified mixtures.

TBR ¼ NcM

NcU¼ evM

evU

� ��3:95

ð7Þ

LTR ¼ TU � TM

TU

� �� 100 ð8Þ

Fig. 2. Typical pavement section and loading condition for mechanistic–empiricaldesign.

A. Mokhtari, F. Moghadas Nejad / Construction and Building Materials 36 (2012) 381–390 385

where T and c respectively denote thickness of considered layer andcritical number of load repetition and M and U represent modifiedand unmodified pavement systems, respectively.

Chandra [31] used Eqs. (7) and (8) to evaluate the benefit of fi-ber in reduction of layer thickness (LTR) and extension in servicelife (TBR), respectively. Moreover, Al-Hadidy and Tan [2,5,7] evalu-ated SMA mixtures modified with different additives such as SBS,Starch, polyethylene and polypropylene in terms of LTR and TBR.They claimed that using these modifiers respectively resulted inTBR of 1.295, 1.27, 1.359 and 1.48.

5. Results and discussion

5.1. Rheological tests

The rheological tests including Penetration and Softening Pointtests were performed for unmodified and modified binders and thevalue of Penetration Index (P.I.) was measured for each type of bin-der. Penetration Index is a term which describes the sensitivity ofthe binder to the temperature. Temperature susceptibility is therate at which the consistency of binder changes. To evaluate thepotential of temperature sensitivity of asphalt binder, penetrationindex could be calculated by using Eqs. (9) and (10).

P:I: ¼ ½ð20� 500AÞ� ð9Þ

A ¼ ½ðlogðpen:@TÞ � log 800Þ=ðT � TR&BÞ� ð10Þ

where T is the testing temperature; pen:@T is penetration at testingtemperature; TR&B is the Ring and Ball softening point.

The results of binder tests were provided in Table 6, which indi-cates that the penetration and softening point of the base bindersignificantly improved by incorporating 5% of SBS. This improve-ment was about 30.2% and 31.4% for penetration and softeningpoint, respectively. After adding SBS, the binder ductility was stillabove 100 cm; this fact could obviously assert that SBS modifiedbinder was elastic enough. The values of penetration index forunmodified and SBS modified binders clearly illustrate that thetemperature susceptibility of base binder considerably decreasedafter modifying with 5% of SBS so that the range of temperaturesat which the asphalt cement could be resistant extendedsignificantly.

5.2. Mixture design

By performing the Saybolt-Furol test, the mixing and compac-tion temperatures for base binder which is applicable to preparecontrol, cellulose and mineral included SMA specimens were158 �C and 146 �C, respectively. The 9.5 mm SMA gradation was se-lected to prepare all SMA specimens. The data accumulated from aprevious study performed by Cooley and Brown showed that finegradation SMA mixtures (9.5 mm and 4.5 mm) could be success-fully designed to have stone-on-stone contact and be rut resistant.Additionally, finer SMA mixes are more durable than coarser ones[32]; however, they need more effort to be compacted.

To optimize the gradation, three limits (upper, middle and low-er) were first considered as shown in Fig. 3. In addition, 6.2% of bin-

Table 6Binder rheological test results.

Property Base binder SBS-modified5%

Penetration at 25 �C (0.1 mm) 63 44Softening point (R&B) (�C) 51 67Ductility at 25 �C (cm) >100 >100Penetration index �0.39 2.01

der was initially added to the mixture according to the report. Thevolumetric properties of SMA mixtures were measured to calculaterequired parameters such as VCA, VMA3 and Va4 are presented inTable 7. As shown in Table 7, the VCA ratio for middle limit is0.927 at 6.95 percent of air void. This amount of air void in thesesamples seems to be relatively high to be filled with binder and itdoes not seem logical to select the middle limit as optimum grada-tion. A more wisely way is to select the optimum gradation betweenupper and middle limits in a way that the VCA ratio is nearer to 1.The aim of this is to produce mixtures more economically in termsof the amount of used binder. Therefore, the optimum gradation isas shown in Fig. 3. The properties of specimens prepared with thisoptimum gradation are presented in Table 7. It is obvious that allVMA values were more than 17% specified in Table 5.

The results of binder optimization showed that the amount of7.1% for control/SBS, 6.7% for mineral and 6.8% for cellulose fibermodified mixtures were considered as OBC (Table 8) to reach 4%of air void. To keep the binder consistency constant for all preparedsamples, the same OBC was selected for unmodified and SBS mod-ified mixtures.

The results of the drain down test (Table 8) indicate that thedrain down value for unmodified mixture exceeded 0.3% as the re-quired specification. However, different additives were capable ofreducing the amount of binder drain down. For instance, SBS re-duced the binder drain down by 26.8%. As could be expected, thecapability of mineral and cellulose fiber was significantly morethan SBS which is in support of ideas developed by Brown et al.[24]; however, cellulose fiber was slightly more efficient than min-eral fiber in this regard. Therefore, based on the results, the perfor-mance of all modified specimens was satisfactory in terms ofbinder drain down.

5.3. Marshall test

As indicated in Table 9, all modified mixtures had higher Mar-shall stability and less flow value than control mixtures, whichclearly describes that different additives had positive effect onMarshall properties of SMA mixtures. Among various SMA mix-tures, those modified with SBS indicated the best performancewith the highest values of stability and quotient (6.56 and 2.02,respectively) and the least value of flow (3.24 mm). Based on theresults, mineral and cellulose fiber could improve the performanceof SMA mixtures, but this improvement was insignificant.

5.4. Moisture sensitivity

As could be seen in Fig. 4, modified specimens indicated moreresistance than control mixture in terms of ITS and TSR. Althoughit was not observed any significant difference between the ITSdryvalues of mineral and cellulose modified mixtures, the differencebetween the ITSwet of those two types of fiber-included mixtureswas relatively high so that incorporating mineral and cellulose fi-bers into the SMA mixtures would result in an increase of 4% and7% in TSR, respectively. On the other hand, SBS could effectivelyimprove the moisture sensitivity of SMA mixture so that the TSRvalue for SBS modified mixture was obtained as 89%, which is con-siderably more than that for unmodified and fiber modifiedmixtures.

All in all, the values of TSR for all constructed specimens wereabove the desired requirement. This fact could explicitly indicatethat there would not be any problem in using these mixtures in

3 Void in mineral aggregate.4 Air void.

Fig. 3. Selected gradation for SMA-9.5.

Table 7Volumetric properties of different limits of gradation.

Gradation Binder content (%) Additive VCADRC (%) VCAMIX (%) VCA ratio VMA (%) Va (%)

Upper limit 6.2 None 40.65 41.45 1.02 18.61 5.54Middle limit 6.2 None 41.56 38.52 0.927 19.07 6.95Lower limit 6.2 None 42.73 35.76 0.837 19.68 8.31

Selected gradation 6.2 None 40.73 39.92 0.98 18.78 5.94

Fig. 4. Results of moisture susceptibility test.

Table 8Results of selecting optimum binder content.

Gradation Additive Va (%) OBC (%mixture)

Draindown (%)

Selected gradation(previous section)

None 4 ± 0.1 7.1 0.332SBS 4 ± 0.1 7.1 0.243Mineral 4 ± 0.1 6.8 0.064Cellulose 4 ± 0.1 6.7 0.048

Table 9Marshall test results.

Additive Ave. stability(KN)

Ave. flow(mm)

Marshall quotient(KN/mm)

None 5.49 4.38 1.253Mineral 5.58 4.27 1.307Cellulose 5.68 4.22 1.346SBS 6.56 3.24 2.025

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terms of moisture sensitivity; however, some of these mixtureswere more resistant to the detrimental effect of the moisture.

5.5. Dynamic creep test

The variation of accumulated strain versus the number of cyclesfor all constructed SMA specimens is presented in Fig. 5. As couldbe expected, unmodified SMA mixture has the least resistance torutting. Mineral and cellulose fibers had a positive effect inimproving the rutting performance of SMA mixtures; however, thiseffect does not seem to be significant. In addition, SBS was consid-erably capable of decreasing the accumulated strain of asphaltmixture and thus producing a more rut resistant mixture.

The point at which each creep curve enters to its tertiary zone isconsidered as Flow Point and the corresponding number of cyclesis regarded as flow number (FN). Flow number values for different

types of SMA mixtures were obtained from Fig. 5 and are presentedin Fig. 6. As presented in Fig. 6, the SBS modified mixture has theservice life about 6 times more than unmodified mixture in termsof rutting resistance, but the life of mineral and cellulose modifiedmixtures are about 2 and 3 times more than conventional mixture,respectively.

5.6. Resilient modulus

Resilient modulus of different types of SMA mixtures was mea-sured as proposed in ASTM D4123 [27]. For each set of mixture,about 15% of indirect tensile strength was applied on the verticaldiameter of the Marshall-prepared specimens. The frequency, loadand rest duration were adjusted as 1 Hz, 0.1 s and 0.9 s, respec-tively. All specimens were kept at 25 �C for 4 h prior to testingusing a specific chamber to distribute the temperature throughoutthe specimens evenly. Three samples for each set were tested andthe average was selected as MR for that mixture. According toFig. 7, SBS modified mixture indicated the highest MR value. Theresults of resilient modulus test of fiber modified mixtures showedthat there is not any significant difference between two types of fi-

Fig. 5. Dynamic creep curves for SMA specimens.

Fig. 7. Results of resilient modulus test.

A. Mokhtari, F. Moghadas Nejad / Construction and Building Materials 36 (2012) 381–390 387

bers, which could support the fact that both of these fibers affectthe resilient modulus of SMA mixtures similarly.

5.7. Statistical analysis of performance tests results

In order to correlate the results of performance tests, linearregression was used and obtained equations are provided. Theaccuracy of presented equations was verified by the coefficient ofcorrelation (R2). For each test, three identical samples were used.Each point in Fig. 8 represents the average of those three samples.According to Fig. 8, it was observed that there are very good corre-lations between the results of different mixture tests that couldillustrate that it is possible to predict the results of each test fromthe others. The R2 values for all established correlations were high-er than 0.93, which indicate that the predicted equations are accu-rate enough.

5.8. Benefit of modification

By using a mechanistic–empirical design approach, the bene-fit of asphalt pavement modification has been investigated. Tothis end, the maximum amount of compressive vertical strainon the top of subgrade layer and horizontal tensile strain atthe bottom of asphalt layer were calculated using the BitumenStress Analysis in Roads (BISAR) program. To evaluate the benefitof modification in terms of LTR, the thickness of subbase, baseand asphalt concrete (AC) layer was considered constant andthe thickness of SMA layer was varied. Similarly, the AC layerthickness was varied for a constant thickness of subbase, baseand SMA layer. By calculating the amount of vertical compres-sive strain on the top of subgrade layer and horizontal tensilestrain at the bottom of HMA layer, and also using corresponding

Fig. 6. Flow numbers corresponding to different SMA mixtures.

equations, the values of Nf and Nd were calculated for each set ofmixture to determine the critical number of allowable load rep-etition and thus calculate the effect of different additives inimproving the life of the pavement or benefit in layer reduction.Desired information related to the mechanistic–empirical designprocedure for the given pavement system section (Fig. 2) is pre-sented in Tables 10 and 11. It can be inferred from Tables 10and 11 that the vertical compressive strain decreased by modify-ing SMA mixtures; however, the amount of tensile strain at thebottom of HMA layer increased. In addition, the values of Nd

were always less than Nf, which could posit that for each mix-ture, Nd is more critical than Nf. Therefore, Nd was consideredas Ncritical to investigate the benefit of modification. Thus, modi-fied mixtures had higher Ncritical than unmodified ones that couldexplicitly prove the positive effect of different modifiers in mak-ing the SMA mixtures more efficient. Moreover, using mineral fi-ber, cellulose fiber and SBS could respectively result in a TBR of1.07, 1.081 and 1.243. In other words, the life of pavement sys-tem modified with mineral, cellulose and SBS would respectivelybe 1.07, 1.081 and 1.243 times more than unmodified SMA mix-ture. Additionally, based on Table 10, if the life of different mod-ified pavement systems is considered constant, the thickness ofSMA layer could decrease by modifying the SMA layer. For in-stance, using mineral fiber, cellulose fiber and SBS, the thicknessof SMA layer could be respectively reduced by 8.52%, 9.82% and24.9%, compared to the unmodified pavement system. In addi-tion, as presented in Table 11, it would be possible to decreasethe thickness of HMA layer instead of SMA layer, which resultedin a reduction in HMA layer thickness by 9.64, 10.92 and 29.24for mineral, cellulose and SBS modified mixtures, respectively.Therefore, the selection process of an alternative from thesetwo options could depend on various factors such as designer’sidea, economical considerations and materials availability.

Fig. 8. Results of statistical analysis.

Table 11Results of mechanistic–empirical design for different thicknesses of HMA layer.

Mixture type HMA thickness was variable

HMA Thickness (cm) LTR (%) et (ls) ev (ls) Nf (�106) Nd (�106) Ncritical (�106) Calculated LTR (%) TBR

Unmodified 5 0 103.0 241.1 27.81 12.06 12.06 0 1.004 20 103.5 250.5 27.23 10.37 10.373 40 101.4 260.4 29.77 8.90 8.90

Mineral modified 5 0 104.7 237.0 25.89 12.90 12.90 9.64 1.074 20 106.1 246.2 24.54 11.10 11.103 40 104.9 255.9 25.68 9.53 9.53

Cellulose modified 5 0 104.9 236.4 25.68 13.03 13.03 10.92 1.0814 20 106.4 245.7 24.24 11.19 11.193 40 105.4 255.4 25.15 9.60 9.60

SBS modified 5 0 107.7 228.2 22.99 14.98 14.98 29.24 1.2434 20 110.2 237.2 20.80 12.86 12.863 40 110.8 246.5 20.32 11.05 11.05

Table 10Results of mechanistic–empirical design for different thicknesses of SMA layer.

Mixture type SMA thickness was variable

SMA thickness (cm) LTR (%) et (ls) ev (ls) Nf (�106) Nd (�106) Ncritical (�106) Calculated LTR (%) TBR

Unmodified 5 0 103 241.1 27.84 12.06 12.06 0 1.004 20 103.2 251.2 27.61 10.25 10.253 40 100.3 261.8 31.22 8.71 8.71

Mineral modified 5 0 104.7 237 25.94 12.90 12.90 8.52 1.074 20 105.6 247.3 25.00 10.91 10.913 40 103.3 258.5 27.49 9.16 9.16

Cellulose modified 5 0 104.9 236.4 25.73 13.03 13.03 9.82 1.0814 20 106.1 246.8 24.49 11.00 11.003 40 103.7 257.9 27.04 9.24 9.24

SBS modified 5 0 107.7 228.2 22.96 14.98 14.98 24.9 1.2434 20 110.3 238.8 20.71 12.52 12.523 40 109.8 250.6 21.12 10.35 10.35

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A. Mokhtari, F. Moghadas Nejad / Construction and Building Materials 36 (2012) 381–390 389

6. Conclusions

According to this limited study, the following conclusions weremade:

� The amount of binder drain down for conventional SMA mix-ture exceeded the allowable level; however, all used additiveswere efficient in reducing the drain down to meet the specifiedrequirement. As expected, mineral and cellulose fiber weremore effective than SBS as drain down inhibitors.� The results of Marshall test may strengthen the idea that

although this test is not perfect to indicate the real performanceof SMA mixtures, it seems accurate to some extent when thepurpose is to make a comparison between different types ofSMA mixtures. This consequence could be made consideringthe results of rutting, ITS and Marshall stability at the sametime.� SBS could be considered as a better modifier in making the SMA

mixture more resistant to the moisture damage; however, therewas observed insignificant improvement when using mineralfiber. Moreover, cellulose fiber affected the moisture suscepti-bility of SMA mixtures in a better way than mineral fiber did.Finally, all prepared SMA mixtures meet the moisture sensitiv-ity requirement; there would not be any problem in this regard.� According to the results of experimental program, it was

observed that there were very good correlations between differ-ent properties of SMA mixtures and these mechanical proper-ties could be estimated from each other.� The results of different performance test for fiber modified mix-

tures could present the fact that cellulose and mineral fibersmight play the same role in SMA mixtures and cause similarexperimental results. Therefore, selecting the type of fibermay more depend on other factors such as cost considerations,availability and ease of application.� The results of mechanistic–empirical design indicated that by

using different additives it would be possible to extent the ser-vice life of the pavement when the pavement section is kept thesame or reduce the pavement layers thickness when the life ofthe pavement system is assumed constant. As resulted, by keep-ing the pavement section constant, the TBR for mineral, cellu-lose and SBS modified mixtures was obtained as 1.07, 1.081and 1.243, respectively. On the other hand, for the same servicelife of the pavement and reduction in SMA thickness layer, min-eral, cellulose and SBS modified mixtures gave LTR values of8.52%, 9.82% and 24.9%, respectively. These values were respec-tively 9.64, 10.92 and 29.24 when the propose is to reduce thethickness of HMA layer.

Acknowledgments

Special thanks to Dr. F.R. Haghighi and Dr. A. Izadi for theirguidance during the study and Mr. Bahadori for his support in com-pleting the experimental phase of the research at Iran University ofScience and Technology Asphalt Mixtures and Bitumen ResearchCenter.

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