Comparative study of performance of naturalfibres and crumb rubber modified stone matrixasphalt mixtures

Vikas Sharma and Shweta Goyal

Abstract: Stone matrix asphalt (SMA) is a gap-graded mix that contains a high concentration of coarse aggregate,thereby maximizing stone-to-stone contact in the mixture and providing an efficient network for load distribution.Coarse aggregate particles are held together by a rich matrix of mineral filler and stabilizer in the thick asphalt film.This paper presents details on the laboratory studies carried out on stone matrix asphalt (SMA) mixtures with naturalfibres and crumb rubber modified bitumen (CRMB). Indirect tensile strength, retained stability, resistance to moisturesusceptibility, resistance to rutting, resistance to creep, and resistance to permeability and aging were found to improvewith SMA mixtures with CRMB when compared with SMA mixtures with fibres as stabilizers.

Key words: natural fibres, CRMB, SMA mixtures, draindown, moisture damage, creep, rutting, permeability, aging.

Résumé : L’asphalte à matrice de roche est un mélange à granulométrie discontinue qui contient une forte concentra-tion d’agrégats grossiers, maximisant ainsi le contact de pierre à pierre dans le mélange et fournissant un réseau effi-cace de distribution de charge. Les agrégats grossiers sont liés par une matrice riche en charge minérale et enstabilisateur dans ce film asphaltique épais. Le présent article traite en détail des études de laboratoire effectuées surdes mélanges d’asphalte à matrice de roche contenant des fibres naturelles et un bitume modifié par l’ajout de caout-chouc granulaire. La résistance à la traction indirecte, la stabilité conservée, la résistance de la susceptibilité àl’humidité, la résistance à la création d’ornières, la résistance au fluage, la résistance à la perméabilité et au vieillisse-ment ont été améliorées avec les mélanges asphaltiques à matrice de roche comportant du bitume modifié par l’ajoutde caoutchouc granulaire par rapport aux mélanges asphaltiques à matrice de roche comportant des fibres en tant questabilisateurs.

Mots clés : fibres naturelles, bitume de caoutchouc granulaire modifié, mélanges asphaltiques à matrice de roche, écou-lement, dommage par l’humidité, gonflement, création d’ornières, perméabilité, vieillissement.

[Traduit par la Rédaction] Sharma and Goyal 139

Introduction

The use of stone matrix asphalt (SMA) continues to in-crease in the United States and European countries in thepast 20 years. The use of SMA mixtures has been linked tothe ability to withstand heavy traffic without rutting. De-veloped in Germany during the mid 1960s, the original pur-pose of stone matrix asphalt (SMA) was to provide asurfacing that offered maximum resistance to damage bystudded tires. When the use of studded tires was banned in

1975, SMA took on a different role. Over the years, it hasproved to have high resistance to plastic deformation fromheavy wheel loads at high temperature while exhibitinggood performance at low temperatures. The anti-rutting ca-pability of SMA mixtures is normally accredited to the pres-ence of stone-on-stone aggregate skeleton in the mixture(Bellin 1992; Maccarrone et al. 1997; Brown and Mallick1994; Carpenter 1994). The durability of the mixture is pro-vided by a higher binder content composed of asphalt ce-ment, fine aggregate, filler, and a stabilizing additive(Collins 1996). The various advantages of using SMA mix-tures over dense graded asphalt mixtures include increasedresistance to rutting, high skid resistance, smooth riding sur-face, lesser noise level, and improved visibility in wetweather conditions (Stuart and Malmquist 1994; Harris andStuart 1995).

A stabilizing additive such as cellulose fibres, mineralfibres, or polymers is added to SMA mixtures to preventdraindown of the mastics. Stabilizers have the potential ofreinforcing and improving the tensile strength and cohesionof SMA mixtures. The fibres reinforce the binder system,thus causing the increase in the viscosity of the system. Theresulting mixture tends to greater stability and possiblehigher resistance to fatigue cracking especially in the case of

Can. J. Civ. Eng. 33: 134–139 (2006) doi:10.1139/L05-096 © 2006 NRC Canada

134

Received 22 October 2003. Revision accepted 23 September2005. Published on the NRC Research Press Web site athttp://cjce.nrc.ca on 15 February 2006.

V. Sharma1,2 and S. Goyal. Civil Engineering Department,Thapar Institute of Engineering and Technology, Patiala,Punjab-147004, India.

Written discussion of this article is welcomed and will bereceived by the Editor until 30 June 2006.

1Corresponding author: (e-mail:vickysharma1981@rediffmail.com).

2Present address: L.R. Kadiyali and Associates, X-15, HauzKhas, New Delhi-110016, India.

mixtures with discontinuous grading (Kandhal et al. 1996;Brown et al. 1997). The durability of SMA mixtures mayalso be improved by the use of higher binder content in themixture. The two types of fibres commonly used in SMAmixtures are natural (like mineral and jute fibres) Encyclo-pedia Britannica; Pandey and Majumdar 1990) and man-made fibres (like cellulose, polyester, polypropylene) (Bellin1992; Carrick et al. 1991).

Generally, nowadays harder grades of bitumen are beingused as the binder in SMA mixtures. Polymer modifiedbinders (PMBs) are chosen as a means of reducing binderdrainage without the need for fibres, largely because of thedifficulties perceived in adding fibres to drum mixing plantspredominantly used in North America, although subsequentdevelopments in fibres addition systems have largely over-come that difficulty. In hot climates, PMBs are considered toprovide additional resistance to bleeding, taking out some ofthe risk associated with high binder contents and narrowmargin for error in overfilling of voids with binder (Stuartand Malmquist 1994).

Importance and objectives of present study

The primary purpose of this study is to compare engineer-ing properties of stone matrix asphalt mixtures containingorganic and polymeric additives. Jute, a natural fibre, andcrumb rubber modified bitumen (CRMB) were used in SMAmixtures. The effect of change in binder type and stabilizingagent on the performance parameters, such as draindown,moisture-induced damage, resistance to creep, rutting, andaging, has been evaluated.

Laboratory investigation

AggregateQuartzite aggregates from Delhi Quarry, were used in the

present study and their test results are tabulated as shown inTable 1. Gradation adopted for the SMA mixture design wasin accordance to the Ministry of Road Transport and High-ways (MoRTH) specifications (2001) (equivalent toAASHTO specifications) and is presented in Table 2.

Conventional bitumenConventional bitumen obtained from the Mathura Refin-

ery, Uttar Pradesh, was of 60/70 penetration grade. The pen-etration, softening point, ductility at 27 °C, relative density,flash point, and fire point were 64, 50 °C, +72 cm, 1.02,240 °C and 270 °C, respectively.

Reclaimed crumb rubberReclaimed crumb rubber is an elastomeric polymer ob-

tained from waste tyres, which is added in crumb form tobase asphalt under agitation. The properties of the modifiedbinder as per IRC: SP: 53 (2002) are shown in Table 3. Athigh service temperature (40 and 50 °C) the behavior of theCRMB is characterized by greatly increased viscosities andshear rates by comparison with those of unmodified binders.

Natural fibresJute, a natural fibre, which is available in plenty in India

and primarily used for packaging in the form of woven ma-

terial, has been used as a fibre in the present study. Jutefibres basically contain lignin and cellulose including hemicellulose besides waxes, sugars, minerals, etc. Since syn-thetic fibres are not economical and have to be imported, anattempt has been made to use jute fibres having excellentphysico-chemical properties as a stabilizing additive in SMAmixtures. Some of the advantages of using jute fibres in-clude high strength, biodegradability, excellent absorbency,and environment friendliness. The physical properties of jutefibres have been presented in Table 4.

The jute, with known initial weight, was dipped in lowviscosity asphalt and placed in a thermostatically controlledforced draft oven maintained at 135 °C for half 1 h for al-lowing absorption of asphalt. The jute fibres were removedfrom the beaker and placed in oven at 135 °C for 0.5 h to al-low the release of extra asphalt from the jute. The jute fibrescoated with asphalt were cured for 24 h at ambient tempera-ture and weighed. The average absorption of asphalt wasfound to be 2.0% by weight. The jute fibres were cut intosmall pieces for uniform distribution while adding the mix-ture.

DraindownThe draindown test developed by the National Center for

Asphalt Technology (NCAT) was selected to determine theefficiency of the CRMB and natural fibres as stabilizers to

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Sharma and Goyal 135

Test description Results

Combined flakiness and elongation index (%) 27Relative density 2.67Water absorption (%) 0.40Impact value (%) 16

Table 1. Physical properties of aggregates.

Sieve size (mm) Gradation adopted Permissible limits

19 100 10012.5 90 85–95

9.5 47 28–754.75 24 20–282.36 20 16–240.600 14 12–160.300 13 12–150.075 9 8–10

Table 2. Aggregate gradation of the SMA.

Property Test results

Softening point (°C) 63Penetration (0.1th of mm) 38Elastic recovery (%) 62

Thin film oven test on residueSoftening point (°C) 67Increase in softening point (°C) 4Penetration (0.1th of mm) 35Elastic recovery (%) 48

Table 3. Physical properties of CRMB.

prevent draindown of the binder and mineral filler (NAPA1994). A loose sample of the SMA mixture was prepared inthe laboratory for testing. The sample was placed in a wirebasket, which was positioned on a pre-weighed paper plate.The sample, basket, and the plate were kept in a forced airoven for 1 h at a prescribed temperature of 170 °C. At theend of 1 h, the basket containing the sample was removedfrom the oven along with the paper plate, and the paper platewas weighed to determine the amount of draindown that oc-curred. The final weight of the mixture was measured, andthe percent drainage was calculated as

Loss (%) = 100 × (Original weight

– Final weight)/Original weight

Losses of less than 0.2% suggest that drainage should notoccur, although losses of up to 0.3% are still acceptable(NAPA 1994). Test results indicated that the percentagedraindown for SMA mixtures with fibres and CRMB werefound to be 0.028 and 0.011, respectively.

Mixture design

The Marshall method of mixture design was adopted forcompacting SMA mixtures with fibres and CRMB. Labora-tory samples were prepared according to the ASTM standardD-1559 (ASTM 1989) at varying binder contents by giving50 blows on each face of Marshall moulds. Optimum bindercontents were based on 4% air voids in the mixture. The tar-get mixing and compaction temperature were 150 and160 °C, respectively. It was observed that the optimumbinder content for SMA mixtures with fibres (0.3% byweight of total mixture) and SMA mixtures with CRMBwere found to be 6.0% and 6.2% by weight of aggregates.The final fibre mix, however, does not include the absorbedasphalt content. The test results at optimum binder contentfor both SMA mixtutres with fibres and SMA mixtures withCRMB are given in Table 5.

Moisture damage evaluation

Tensile strength ratioTensile strengths and tensile strength ratio (TSR) of the

SMA mixtures are summarized in Table 6. The TSR valuesfor both SMA mixtures with fibres and SMA mixtures withCRMB were found to be 87 and 93, respectively, which aregreater than 85%, that is, well within acceptable limits.However, test results indicate that the SMA mixture withCRMB is more resistant to moisture damage as comparedwith SMA mixture with fibres.

Diametrical modulusFrom Table 6, it was found that the SMA mixtures tested

had a diametrical modulus (Md) greater than 70%. The re-tained ratio for SMA mixtures with fibres and CRMB werefound to be 81% and 89%, respectively, thereby indicating

that SMA mixtures with CRMB were found to be less sus-ceptible to moisture damage.

PermeabilityPermeability tests were carried out on SMA mixtures with

fibres and CRMB. Specimens of the design gradation andbinder content were prepared at different air voids levels inthe total mixture. This was accomplished by varying thenumber of blows while compacting SMA mixtures by Mar-shall hammer. The number of Marshall hammer blows werevaried to produce a range of air voids in the mixture. Afalling-head method was used to determine the coefficient ofpermeability in centimetres per second. Test specimens weresubjected to a vacuum in accordance with AASHTO T-209before testing to achieve saturation. The permeability test re-sults of the SMA mixtures with fibres and CRMB are givenin Table 7. When the criterion of 10–3 cm/s for the FloridaDepartment of Transportation, established by Choubane etal. (1998), is used as a guideline for limiting permeability,the table indicates that SMA specimens with CRMB couldbe considered to be more permeable as compared with speci-mens of SMA with fibres over the range of densities usuallyspecified in the field (5% to 6% voids for SMA mixtures).

Resistance to creepCreep tests were performed on SMA specimens by apply-

ing a repeated unconfined compressive load of 100 kPa. Aloading time of 0.1 s and a rest period of 0.9 s for 1 h wasapplied on the SMA specimens at 40, 50, and 60 °C. Perma-nent deformation tests for SMA specimens were carried outat 4% air voids in the mixture and the test results are pre-sented in Table 8, and it was observed that permanent strainof the SMA mixtures increased with increase in test temper-ature. It was also observed that strain values of the SMAmixtures with CRMB were found to be less as comparedwith specimens of SMA with fibres at the respective temper-atures.

Resistance to ruttingRutting characteristics were studied using the Hamburg

wheel tracking device (HWTD). The wheel tracking deviceconsists of a loaded wheel and a confined mould in whichthe 300 mm × 150 mm × 50 mm sample of SMA mixturewith stabilizer is rigidly restrained on its four sides. A motorand a reciprocating device gives the wheel a to and fro mo-tion of 24 passes/min with a travel distance of 300 mm. Thesolid rubber tyred steel wheel bears a total load of 31 kg andindents a straight track in the specimen. The depth of the de-

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136 Can. J. Civ. Eng. Vol. 33, 2006

Average fibre length 5 mm maximumMoisture content <10% (by weight)Ash content (non-volatile) (%) 20

Table 4. Physical properties of fibres.

SMA test results

Test properties of SMA mixture With fibres With CRMB

Optimum binder content (%),(by weight of aggregates)

6.0 6.2

Bulk density (gm/cm3) 2.312 2.331VTM (%) 4.00 4.00VMA (%) 18.50 18.25VCA (%) 37.41 37.66

Note: VTM, voids in total mix; VMA, voids in mineral aggregates;VCA, voids in coarse aggregates.

Table 5. Test results at optimum binder contents.

formation was recorded at the mid-point of its length bymeans of a rut-depth measuring device. The contact area be-tween the wheel and specimen is about 5.457 cm2, giving amean normal pressure of 5.66 kg/cm2.

Rolling compaction was adopted for SMA slabs toachieve compaction level of 5% to 6% air voids in the SMAmixture to simulate the behaviour of SMA mixtures to fieldconditions. Later, the slabs were cured at room temperaturefor 2 d and placed in the machine completely immersed inwater at 40 °C. The test started 45 min after placing thespecimen in immersed condition. Each slab was subjected toreciprocative load repetitions for 20 000 passes or until a de-formation of 20 mm appeared on the slab surface. Deforma-tions recorded for SMA mixtures with natural fibres andCRMB at regular intervals are presented in Table 9. Test re-sults indicate that at 40 °C, the depth of deformation of slabsfor SMA mixtures with CRMB was less in comparison withSMA mixtures with fibres, thereby indicating that SMAmixtures with CRMB are better resistant to rutting.

Resistance to agingThe effect of aging on the SMA mixtures with fibres and

CRMB was examined according to Strategic Highway Re-search Program (SHRP) method M-007 (SHRP 1992). Thetest procedure consists of determining the effects of short-and long-term aging on asphalt mixture stiffness. For short-

term aging, loose mixtures were aged in a forced draft ovenfor 4 h at 135 °C, and then compacted using Marshall ham-mer to an air void level expected in the field, i.e., 5% to 6%.The Md of each compacted sample was measured at 25 °C.

For long-term aging, the compacted mixtures were agedin a forced draft oven for 5 d at 25 °C. The diametricalmodulus (Md) and the tensile strength of each specimenwere then measured, and the test results are presented in Ta-ble 10. Increase in stiffness for the SMA mixtures withfibres, related to short- and long-term aging were signifi-cantly higher than the increase in stiffness for SMA mixturewith CRMB. Table 10 shows how the stone matrix asphaltmixtures varied with the test and the degree of aging.

Test results and discussions

DraindownThe draindown test results indicated that the use of natural

fibres and CRMB as stabilizers can effectively retard thedraindown of binder and mineral filler.

Mixture designThe SMA mixtures prepared by Marshall hammer had

higher density, stability, and lower flow value, and the valueswere well within the specifications given by MoRTH (2001).The optimum binder contents for SMA mixtures with fibresand CRMB were found to be 6.0% and 6.2% by weight ofaggregates. The VMA and VCA, at optimum binder content,of the SMA mixtures with fibres were found to be 18.5%,37.41%, respectively, and for SMA mixtures with CRMBwere 18.25% and, 37.66%, respectively, as shown in Ta-ble 5.

Moisture damageFrom Table 6, it was observed that the tensile strength ra-

tio and diametrical modulus ratio values for SMA mixtureswith fibres were 87% and 81%, respectively, and for SMAmixtures with CRMB were 93% and 89%, respectively, indi-cating that SMA mixtures had less potential for moisturedamage.

PermeabilityPermeability test results indicated that SMA mixtures

with CRMB prepared at different voids content were morepermeable, when compared with SMA specimens withfibres. In general, it was observed that SMA mixtures mayrequire higher field density to produce impermeable hot mixasphalt (HMA) layer.

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Sharma and Goyal 137

SMA test propertiesSMA withfibres

SMA withCRMB

Tensile strength, 25 °CAverage dry (kPa) 547.8 691.3Average wet (kPa) 476.1 640.8Retained ratio (TSR) (%) 87 93

Diametrical modulus, 25 °CAverage dry (MPa) 1085.3 1312.3Average wet (MPa) 880.1 1170.4Retained ratio (MdR) (%) 81 89

Table 6. Comparative TSR and retained stability.

Permeability, k (cm/s)

Air voids (%) SMA with fibres SMA with CRMB

6 0.00037 0.000427 0.00056 0.000668 0.00079 0.000959 0.00158 0.00174

10 0.00531 0.00683

Table 7. Permeability test results for SMA mixtures with fibresand CRMB.

Test propertiesPermanent strain (mm/mm) at 3600cycles

Temperature (°C) 40 50 60SMA with fibres 0.0416 0.0761 0.1167SMA with CRMB 0.0259 0.0651 0.0981

Table 8. Unconfined compressive repeated load test results.

Average depth of deflection HWTD (mm)

No. of wheelpasses SMA with fibres SMA with CRMB

1 000 0.8 1.04 500 1.7 1.3

10 000 2.4 2.115 500 3.2 2.520 000 4.1 3.825 000 5.0 4.0

Table 9. Rutting characteristics of SMA mixtures.

CreepThe permanent strain values at 40, 50, and 60 °C for SMA

mixtures with fibres were found to be 0.0416, 0.0761, and0.1167 mm/mm, respectively, and for SMA mixtures withCRMB were 0.0259, 0.0651, and 0.0981 mm/mm, respec-tively. The test results indicated that strain values of speci-mens of SMA with CRMB were found to be less ascompared with those of SMA with fibres.

RuttingThe HWTD test results indicated that, at 40 °C, the defor-

mation for SMA slabs prepared by rolling compaction forSMA mixtures with fibres and CRMB were found to be 5.0and 4.0 mm, respectively, for 25 000 number of wheelpasses. From the test results, it was observed that SMA mix-tures with CRMB had significantly less deformation whencompared with SMA slabs prepared with natural fibres.

AgingFrom Table 10, it can be observed that SMA mixtures var-

ied with the test and the degree of aging. It was found thatthe resistance to aging was related to changes in the moduliand strengths. These changes, as shown in Table 10, werecomputed from the average value of each mixture. It was ob-served that degree of aging was found to be significantly lessfor SMA mixtures with CRMB when compared with SMAmixtures with fibres.

Conclusions

Natural fibres (jute) and crumb rubber modified bitumen(CRMB) can be effectively used as a stabilizer in SMA mix-tures to retard draindown of binder and mineral filler. Mar-shall method can be used to design SMA mixtures withfibres and CRMB. Engineering properties like resistance tomoisture damage, rutting, creep, aging, and permeability ofSMA mixtures were found to improve with the use ofCRMB when compared with SMA mixtures with fibres asstabilizers.

Acknowledgements

The authors wish to thank Dr. P.K. Sikdar (Director,CRRI, New Delhi) for granting permission to work at Cen-tral Road Research Institute (CRRI), Mathura Road, NewDelhi-110020, India.

References

AASHTO T-209. 2000. American Association of State Highwayand Transportation Officials, Washington, D.C.

ASTM D-1559. 1989. Test method for resistance of plastic flow ofbituminous mixtures using Marshall apparatus. Standard D-1559-89, American Society for Testing and Materials, Philadel-phia, Pa.

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Brown, E.R., Mallick, R.B., Haddock, J.E., and Bukowski, J. 1997.Performance of stone matrix asphalt (SMA) mixtures in theUnited States. National Center for Asphalt Technology, AuburnUniversity, Auburn, Ala. Report no. 97–1.

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Carrick, J., Macinnes, K., Davidson, K., Schenk, W., and Emery, J.1991. Development of stone mastic asphalt mixes for Ontariouse. Proceedings of the 36th Annual Conference of the Cana-dian Technical Asphalt Association. Vol. 36. Edited by E.Thompson. Polyscience Publications, Morin Heights, Que. pp.267–282.

Choubane, B., Page, G.C., and Musselmar, J.A. 1998. Investigationof water permeability of coarse graded superpave pavements.Association of Asphalt Paving Technologists, 67: 254–276.

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138 Can. J. Civ. Eng. Vol. 33, 2006

Dynamic modulus, Md at 25 °C (MPa) Change in Md (MPa)

Type of SMA mixture Unaged Short-term aging Long-term agingShort-termto unaged

Long-term toshort-term aging Total aging

SMA with fibres 1085 2104 3467 946 797 1743SMA with CRMB 1312 1726 3123 823 692 1515

Tensile strength at 25 °C(kPa)

Change in tensilestrength

Unaged Long-term aging Total aging

SMA with fibres 547 1153 606SMA with CRMB 691 984 293

Table 10. Aging test results for SMA mixtures with fibres and CRMB.

MoRTH. 2001. Specification for road and bridge works. 4th revi-sion. Ministry of Road Transport and Highways, IRC, NewDelhi, India.

NAPA (National Asphalt Pavement Association). 1994. Guidelinesfor materials, production and placement of stone matrix asphalt.National Asphalt Pavement Association, Lanham, Md.

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SHRP. 1992. Short and long term aging. Method M-007. StrategicHighway Research Program (SHRP), National Research Council(U.S.), Washington, D.C.

Stuart, K.D., and Malmquist, P. 1994. Evaluation of using differentstabilizer in the US route 15 (Maryland) stone matrix asphalt(SMA). In Asphalt concrete mixture, design and performance.Transportation Research Record 1454, Transportation ResearchBoard, National Research Council (U.S.), Washington, D.C.pp. 48–57.

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