Direct Research Journal of Engineering and Information Technology Vol.1 (2), pp. 23-37, August 2013Available online at http://directresearchpublisher.org/drjetISSN 2354-4155© 2013 Direct Research Journals PublisherOriginal Research Paper

A Study on Eco Friendly use ofRecycled Rubber TyresRajesh Kumar Jain

Bharat Group of Colleges, Punjab, India.

ABSTRACT

The 20-40mm particle size of crushed aggregate andrubber aggregate showed 100% passing but riversand showed zero%. The 10 mm particle size of riversand showed 100% passing. Hydraulic conductivityranges from 0.5 cm/sec (0.2 inches/sec) forcompressed 10–38 mm (0.4–1.5 inches) shreds tomore than 20 cm/sec (8 inches/sec) for 25–64 mm (1–2.5 inches) loose shred. Thermal conductivity variesfrom 0.0838 Cal/meter-hour-degree C (5.6X10E-5Btu/ft-hr-degree F). Increasing the tire chip content insand increase the initial friction angle, with values ashigh as 65o being obtained for dense sands and tirechip contents of 30% (by volume). Because of theirlower specific gravity (1.15 to 1.21) relative to that ofsoil solids (2.55 to 2.75), tire chips, alone or in

mixtures with soils, offer an excellent light-weightand strong fill material for use as backfill in earthenstructures. The shear strength decreases as the tyrechip percentage increases beyond 30%. sandy silt–tyre chip mixtures showed an improvement instrength as the percentage of rubber increased (from10% to 20%) . In comparing the minimum dampingratio at a shear strain of 5x 10-4 % increase dampingwith tire inclusion. The addition of fine aggregaterubber from 10% up to 40% rubber contentmaintained a linear decrease in slump values.Maximum compressive strength was obtained for40% CRA and FRA at 7 and 28 days A significantinitial plastic compression under load takes place.This was high as 40% of the initial placementthickness for pure tire chips. The constrained modulifor initial loading vary between 500 and 30,000 kPa.

Key Words: Shear strength, Compressive strength, Splittensile strength, Flexural strength.

*Corresponding Author E-mail: rajmeerutcity@gmail.comAccepted 17 July 2013

INTRODUCTION

With the change in living style and attitude of peoplethere is increase in number of automobiles, the kilometercoverage by the vehicles is also increasing. All thisresulting in increased demand of tyres as originalequipment and as replacement has also increased. About1.1 millions all types of new vehicles are added each yearto the Indian roads. In India top 7 large tyre companiesaccount for over 85% of tyre production, the sale ofautomobile tyres was 70 million units in 2002 which hadincreased to 97.7 million in the year 2012, representingthe growth rate of more than 100% in ten years. Thedisposal of these used tyres has become a globalproblem because tyre is made of natural rubber (alsocalled virgin rubber), styrene-Butadien Rubber (SBR),Polybutadienc Rubber (PBR), Carbon black, Nylon tyrecord, rubber chemicals, steel tyre card and Butyl rubber.Achemical composition has been studied by UNEP(1999)Estimated for a 7Kg Car tyre and given below in

Table1.Most of the times, tyres are being dumped onground or added to garbage create environmental andhealth problems. Developed and industrialized countriesare facing a monu -mental problem in the disposal ofused tyres. Hence, developing large tyre consumptionprocedure is needed in countries like India. Indiagenerated approximately 45 million waste tires in 2012,About three-quarters of tires, or 30 million tires, werediverted to constructive uses in 2012, but 15 million tireswere not. These tires were shredded and disposed inpermitted solid waste land -fills, stored at permitted sites,or otherwise illegally disposed of around the state. Whilethe majo -rity of tires are reused, a significant amount,one-quarter, are not. Thus, recycling needs to beconsidered imaginatively and the solutions must besustainable. In India out of 36 tyre manu -facturers thetyre recyclers are about 20, the major players numberonly about four or five. In these M/S Gujrat Reclaim has

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Table1.Chemical composition of tyre.

Chemical Name Remark Content(%weight)

Content (kg)

Copper Compound Alloying constituent of the metallic reinforcing material (steel cord) Approx. 0.02% Approx. 0.14gZinc Compound Zinc oxide retained in the rubber matrix Approx. 1% Approx. 70 gCadmium On trace levels,as Cadmium compounds attendant substance of

the Zinc OxideMax. 0.001% Max.0.07g

Lead and Lead Compound On trace levels,as attendant substance of the Zinc Oxide Max. 0.005% Max. 0.35gAcidic Solution or acids in solidform

Stearic acids in solid forms Approx. 0.3% Approx. 21g

Organo-halogen compoundsother than substances inAnnex to the basal convention

Halogen n butyl rubberContent ofHalogenMax. 0.10%

Content ofHalogenMax. 7g

Estimated for a 7Kg Car tyre (UNEP,1999)

an annual turnover of over Rs.15 Crore from its Haridwar(Uttra -khand) tyre recycling plants, with a production of20 tonnes of reclaim rubber per day.There arepossibilities of using these tyres in civil engineeringapplications such as highway embank -ments andbackfills behind retaining structures over weak orcompressible soils. New uses for the valuable rawmaterials embodied in whole tires and tire shreds.Whentyres are disposed of they have lost only a few grams oftheir original mass (Adhikari and Maiti, 2000). However,with the growing awareness on environmental protection,this issue have gain wider attention. The definingcharacteristic of a civil engineering application is that thetyre-derived material produces a cost-effectiveengineering benefit. Many researchers have assessedsome fundamental engineering properties of tyre crumbs-soil mixtures, such as compaction characteristics,compressibility, and permeability, shear strength,modulus of elasticity, and poisson’s ratio. Thereclamation and recycling of waste rubber has beenreviewed extensively in Adhikari and Maiti (2000). Raoand Dutta (2006) reported compressibility and triaxialcompression tests on admixtures of sand and tyre chipsvarying in size and content. They stated that sand tyrechip mixtures of up to 20% could be a potential materialfor highway and embankment construction due to theincrease in the stress behavior when compared to thesand material alone.

In an experimental study performed by Ghazavi(2004), on the rubber grains, showed that the direct sheartests on rubber grains when mixed with suitableproportions of sand varying in size and content that anaddition of 10- 20% rubber to the sand was optimal toobtain the greatest friction angle and he further quotedthat these rubber materials alone had a friction angle of31º. Moo-Young et al. (2003) conducted tests todetermine the chemical and physical properties of tyreshreds.

The results of the direct shear tests on scrap tyres ofvarious particle sizes indicated that as the particle size

and density increased the shear strength of the scraptyres increased as well. The Triaxial test results on tyrechips alone and rubber-sand mixtures reported by Lee etal. (1999) showed a nearly linear relationship betweendeviator stress and axial strain at the confining pressuresof 28 kPa, 97 kPa and 193 kPa for the tyre chip materialsalone. On the other hand, the mixtures of sand and tyrechips (rubber-sand) presented a response which isintermediate between those of pure sand and pure tyrechips. Many researchers have assessed somefundamental engineering properties of rubber/soilmixtures, such as compaction characteristics,compressibility, permeability, shear strength, modulus ofelasticity, and Poisson’s ratio (Humphrey and Manion1992; Edil and Bosscher 1994; Masad et al. 1996; Lee etal. 1999).

In addition, waste tires can be used as fuel for cementkilns, as feedstock for producing carbon black, and asreefs in marine environments (Paul 1985). Early studieson the use of worn-out tires in asphalt mixes were verypromising. They showed that rubberized asphalt hadbetter skid resistance, reduced fatigue cracking, andachieved longer pavement life than conventional asphalt(Adams et al. 1985; Esch 1984; Estakhri 1990; Khoslaand Trogdon 1990).

The workability, defined as the ease with whichconcrete can be mixed, transported and been put intomoulds, is affected by the interactions of tyre rubberparticles and mineral aggregates. Rubberised concretehas been found to be less workable than conventionalconcrete as the rubber content increases (Khatib andBayomy, 1999; Albano et al., 2005; Oikonomou et al.,2006). It was also observed that mixtures made with finecrumb rubber were more workable than those made withcoarse tyre chips or a combination of tyre chips andcrumb rubber (Khatib and Bayomy,1999).

A shift in paradigm is now necessary from developingwith environmental concern as a small part of the processinto integration of all building projects within the widercontext of environmental agenda (Abidin, 2010). The use

Direct Res. J. Eng. Inform.Tech. 25

Table 2.Properties of rubber (taken from CWA 14243-2002 (CWA, 2002)).

Compacted density 2.3–4.8 kN/m3 compared to soil at 15.6–19.5 kN/m3

Compacted dry unit weight 1/3 that of soilCompressibility 3 times more compressible than soilDensity 1/3 to 1/2 less dense than granular fillDurability Non-biodegradableEarth Pressure Low compared to soil or sand, up to 50% lessFriction characteristics Higher compared to soilHorizontal Stress On weak base: lower than with conventional backfillModulus in elastic range 1/10 of sandPermeability Greater than 10 cm/sPoisson’s ratio 0.2–0.3 corresponding to Ko values of 0.3–0.4Specific Gravity ±1.14–1.27 kg/m3 compared to soil at2.20–2.80 kg/m3Thermal insulation 8 times more effective than gravelUnit weight Half the typical unit weight of gravelVertical stress On weak base: smaller than granular backfill

of recycled tyre rubber as an engineering material hasincreased significantly in the last decade. Tyre crumbs-soil mixtures are currently used in wide range of Civilengineering systems like lightweight fills for slopes,retaining walls, embankments etc.

This project will offers a unique opportunity for theDepartment of Civil and Building Engineering .It will bringtogether individuals, academic and corporate bodies, whoare involved in the design, manufacture, sale, use ortesting of recycled building material ,related productsand/or associated technologies, or who teach or conductresearch about such products.This report will highlightthe benefits both in terms of CO2 reduction and costsavings of using recycled tyres in construction. Thisproject will develop rigorous assessment methods andwill broaden applications. It is expected that the projectwill have an impact on construction practice and will leadmore research in this area.

MATERIALS AND METHODS

Mechanical methods , maturity method, methods basedon acoustics are used for testing of early age properties.The tyre crumbs used in this study are the by-product ofthe shredding process of used tyres and were obtainedfrom Local tyre crumb distributor . Large tire chunksproduced by coarsely shredding waste tires at aprocessing facility or stockpile site. In order to eliminatethe uncontrolled effects of the particle-size distribution,tyre crumbs, which varied in size from 4.75- 0.0075 mm,were graded and then mixed according to a desiredgradation curve. 1 mm and 2 mm sized tyre crumbs werechosen for this study.

Soil

Two typical locally available soil samples are consideredfor the study, one is the red soil and the other is sand.

Index and engineering properties of both the soils aremeasured by carrying out series of laboratory test. Theseinclude: workability, compressive strength, split tensilestrength, flexural strength.

Sand

We used naturally occurring fine grained granular sandmaterial and was chosen due to its availability andrelatively low cost. Furthermore, its unit weight was foundto be relatively easy to control to provide differentdensities for comparison. Properties of rubber have takenfrom CWA 14243-2002, (Table 2).

Two different types of discarded tyres were used;granulated' rubber (often called 'chipped') and 'shredded'rubber, representing the sizes available for recyclepurposes . The granulated rubber consisted of smallangular particles that passed the 5mm sieve in anyorientation, whereas the shredded rubber had a length ofapproximately 30mm and passed the 3.35mm sieve in itslongitudinal direction only. The two rubbers had generallyclean cuts and only a small percentage of steel wireswere exposed.

This made triaxial compression tests difficult due tomembrane penetration and hence a magnet was used toremove the steel component. 'Free' steel was not presentin the rubber. The material was sieved and split into twogroups of tyre aggregate: coarse rubber aggregate (19-10mm) and fine rubber aggregate material (10-4.75 mm).These groups will be referred to as CRA (Coarse RubberAggregates) and FRA (Fine Rubber Aggregates).

Sample preparation

All the shear strength tests were performed on a 300mmsquare direct- shear machine ('shear-box'). The machine

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Table 3.Grading of Aggregates.

Sieve Size Percentage passingRiver sand Crushed Aggregate Rubber Aggregate

40mm - 100 10020mm - 100 10010mm 100 33 384.75mm 97 4 62.36mm 86 0 01.18 mm 74 - -600 micron 42 - -300 micron 23 - -150 micron 5 - -Specific Gravity 2.65 2.65 1.14Water Absorption 0.8 0.5 -

represents a typical commercial machine, originallydesigned for testing free-draining materials containingparticles up to 37.5mm in diameter. All the shear testsand sample preparation were performed in accordancewith British Standard BS1377:1990. The normal stress isapplied through a hydraulic loading system and isdesigned to provide normal stresses from 111-1100kPa.The horizontal shear stress is applied through amotorised hydraulic loading system providing a constantrate of displacement of the box (between 0.001mm/min to5mm/min). The horizontal force is measured by acalibrated load ring and vertical displacement ismeasured by a calibrated dial gauge. The requiredamount of sand or pre-mixed sand-rubber composite waspoured steadily into the shear box using acircular motion.The material was then spread as evenly as possible andthe required density achieved through vibration .Thenormal force was applied to the specimen to give thedesired vertical (normal) stress n measured in kPa.

The normal stress range was set between 333kPa and777 kPa, roughly representing lower fill surcharges in anembankment over 20m in height (for an example unitweight of 15 kN/m3). The large shear box is designed totest at these large normal stresses and the results of thetesting programme should yield conservative results forany lower stresses. Tests on natural sand samples wereperformed on a 100mm by 100mm direct-shear machineat a range of normal stresses and the friction anglesattained were found to be in good agreement with thelarge shear box at higher normal loads. Initially threedifferent densities were chosen, namely loose, medium-dense and dense. Preliminary tests on the materialindicated that approximately 15 minutes was required toensure that full settlement of the sand-rubber compositehad occurred. The settlement of the material wasrecorded and the vertical strain calculated. A constantrate of displacement was then applied to the material andthe test stopped when the horizontal displacement

reached 45mm (close to the limit of the machine)corresponding to a horizontal strain of 15.0%.A =Dimension less elastic stiffness parametera =Soil constant used for normalized shear modulusversus shear strain relationb =Soil constant used for normalized shear modulusversus shear strain relationDmin =Minimum damping ratioe =Void ratioGmax =Maximum tangent shear modulusk =Dimensionless parameter related to soil plasticityindexn =Dimensionless elastic parameterOCR= Over consolidation ratiopa= Atmospheric pressure =Shear strainr =Reference strain0= Effective mean principal stresstmax =Shear stress at failure.

Devulcanization

Surface modified rubber refers to a process that enablescrumb rubber particles to chemically bond with othermaterials in compounds, thereby becoming an integralpart of the material structure rather than just a filler. Inthis process, the particles are chemically treated to alterthe surface of the rubber particles, creating reactivefunctional groups on the surface. These reactivefunctional groups react with a broad spectrum of rubberand plastic materials. They give tenacious adhesion topolymers with which they are combined, enhancingstrength and other performance properties of thecompound. Devulcanization is another method ofrestoring reactivity of crumb rubber with other virginrubber by reactivating vulcanization reaction sites.

Direct Res. J. Eng. Inform.Tech. 27

Figure 5.Particles size distribution of material used.

Table 4. Density of whole and shredded tires depends upon size, depth, and compaction.

Whole Tires (Loose) 7.5 lbs/cubic foot

Laced Passenger Tires 10 lbs/cubic foot

Stacked or Laced Truck Tires 14 lbs/cubic foot

Baled Tires 30 lbs/cubic foot

Shreds (Loose-Surface Compacted) 22–50 lbs/cubic foot

Shreds (Compacted CE Uses) 37–60 lbs/cubic foot

Analysis of total carbon

All samples were analysed after oven drying at 85°C andthen powdered. The samples were powdered until themaximum particle size was <75 μm. This operationtypically required approximately 30 grams of material tobe milled for a period of 5 minutes, although a certainamount of judgment was used in the mass of the chargesince larger particle sizes occupied more volume in themill. The 30 grams of sample for powdering was splitfrom the main sample mass using a riffle to ensure that itwas representative. After powdering the maximumparticle size of the sample was checked by passing itthrough a 75 μm wire mesh screen, oversized materialwas re-milled. The sample was then placed in a cleanglass jar and labelled. A second stage of drying (105°C,

overnight) was carried out immediately prior to analysis toremove any moisture from the sample that might havebeen adsorbed during storage.This ensured that the drymass of the sample was measured prior to any analysis.

RESULTS AND DISCUSSION

Utilization of waste materials in construction earthworkwill continue to increase and demand engineering insightfor modeling behavior of earth structures formed of suchmaterials. One aspect of this study is the development ofmixture rules to assist in predicting the variation offundamental properties with the ratio of one particulatematerial to another. Table 3 and Figure 5 showed theparticle size and their passing percentage. 20-40mm

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Figure 6.Tyre crumb percentage vs. Density in g/cc.

Figure 7.Tyre crumb percentage vs. angle of internal frictionfor 1mm and 2 mm sized tyre crumbs.

Figure 8.Compaction curve of 1 mm sized tyre crumb ofmixed with red soil.

particle size of crushed aggregate and rubber aggregateshowed 100% passing but river sand showed zero%. The10 mm particle size of river sand showed 100% passing.

Mechanical characteristics of soils modified with tyrerubber

Density: Tires are slightly heavier than water and will sink

Figure 9.Compaction curve of 2 mm sized tyrecrumb of mixed with red soil.

in water unless entrapped air provides enough buoyancyto allow them to float. This generally occurs only withwhole tires or fine crumb rubber particles. However, tiresand tire products are much lighter than soil or stone. Thedensity of whole and shredded tires depends upon size,depth and compaction as shown in Table 4.

Relative density tests were conducted on sand mixedwith tyre crumbs percentages of 0%, 5%, 10%, 15%,20%, 25%, 30%, 35% and 40% by volume. 1 mm and 2mm sized tyre crumbs were used for the test. The tyrecrumbs soil mixture was tested in dry condition. The sandused in the test was poorly graded coarse sand. Dialgauge was used to measure the settlement of the tyrecrumbs soil mixture. Percentages of 0%, 5%, 10%, 15%,20%, 25%,30%, 35% and 40% by volume are presentedin the Figure 6 to 9. Maximum and minimum density ofthe tyre crumbs soil mixture and the density of the tyrecrumbs soil mixture used in direct shear test. Densitymay increase even further under external pressure suchas material over burden. Result showed the shred densityincreases with decreasing shred size and with increasingoverburden weight, as expected with any solidmaterial,but the flexibility and deformability of tire chipsaccentuates these variations.

Hydraulic Conductivity: Conductivity ranges from 0.5cm/sec (0.2 inches/sec) for compressed 10–38 mm (0.4–1.5 inches) ,shreds to more than 20 cm/sec (8inches/sec) for 25–64 mm (1–2.5 inches) loose shreds.Conductivity increases with larger particle size anddecreases with increasing compaction. Water flowsthrough whole and shredded tires readily, even whenthey are compressed in bales or under heavyoverburden.

Thermal Insulation: Thermal conductivity varies from0.0838 Cal/meter-hour-degree C (5.6X10E-5 Btu/ft-hr-degree F) for 1mm particles in a thawed state with lessthan 1 percent moisture content to 0.147 Cal/meter-hour-degree C (9.8X10E-5 Btu/ft-hr-degree F) for 25 mmfrozen compacted shreds with a moisture content of 5percent. It has observed that thermal conductivity

Direct Res. J. Eng. Inform.Tech. 29

Table 5.Mix Design Detail.

Materials Control mix for M-25 grade ofconcrete

Modified mix with 30% rubber aggregate bymass replacement of coarse aggregate. Therubber aggregate having specific gravity of 1.14with the given concrete control mix, thereduction in density will be 20 %

Free water (kg/m3) 155 155OPC43-Grade (kg/m3) 310 310River sand (kg/m3) 755 570Crushed aggregate (kg/m3) 1180 620Rubber aggregate (kg/m3) - 265Normal Super plasticizer (kg/m3) 3100 3100Density (kg/m3) 2403 1923W/C Ratio 0.5 0.5Slump (mm) 63 25

150 mm 3 cubes average compressive strength (N/mm2)7 days 22.7 9.728 days 32.8 12.3

Figure10.Shear Strength versus Normal Stress for Sand and Sandy Silt alone and in mixtures with Tire Chips(Slope of the curves indicates friction angle).

depends on particle size, reinforcing wire content,compaction, moisture content, ambient temperature, andother variables. So rubber is a poor thermal conductor,conversely providing a better thermal insulator than soilor aggregate.

Leaching Characteristics: Under neutral pH (pH = 7)normally encountered in surface flow-throughapplications, iron and manganese levels increase asthese metals were extracted from exposed tirereinforcing wire. However, both metals are generally

present in soils, and the increases are generally notconsidered to be harmful to people or the environment.The rate of dissolution of wire increases under acidicconditions (pH < 7), and zinc present within surfacerubber can also be leached, but levels generally remainwithin acceptable parameters. Under basic conditions(pH>7), organic compounds can be leached in tracequantities.(Table 3).

Flammability: It was observed that tire shreds have aflash point of 582º F, higher than some other materials.

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Figure11.Shear stress vs. Horizontal displacementcurves for sand,tire buffing and sand tire buffing mixtureat vertical stress of 20 kPa.

Figure 12.Gmax/[OCRkPal-nn]versus void ratio with

treating the volume of rubber as voids (69 kPaconfinement).

The flash point is the temperature at which a materialwill initially ignite, and the temperature to supportcontinuing combustion (fire point) is even higher. Whencrumb rubber is combined with a binder, the binder maycontrol the flammability of the resulting product if thebinder has a lower flash point. Shredded tires exhibitexcellent frictional properties (Figure 10). Therefore, theycan be used to enhance the strength properties of soils

by internal fiber reinforcement. We observed thatincreasing the tire chip content in sand increases theinitial friction angle, with values as high as 65o beingobtained for dense sands and tire chip contents of 30%(by volume). Tire chips can be used in mixtures withsandy silt but increases in strength were not observed inthe clay-tire chip mixtures. In addition, because of theirlower specific gravity (1.15 to 1.21) relative to that of soilsolids (2.55 to 2.75), tire chips, alone or in mixtures withsoils, offer an excellent light-weight and strong fillmaterial for use as backfill in earthen structures.(Edil,2004) (Figure 11).

They also can be used in embankments over softground or as backfill behind retaining structures as alight-weight fill material. The unit weight was varied from5 kN/m3 for 100% tire chips to 13 kN/m3 for 50% byvolume mixture of tire chips and soil. Tire chip fills werequite compressible especially when tire chips are notmixed with soils. A significant initial plastic compressionunder load was observed. This was high as 40% of theinitial placement thickness for pure tire chips. Once thematerial is subjected to this level of compression with theassociated reduction in porosity, it behaves like an elasticmaterial. Thus, most of the deformation should occurduring construction.

The constrained deformation modulus in the elasticrange of pure tire chips was about 100 times smaller thanpure sand. However, inclusion of as low as 30% sand inthe tire chip matrix restores the modulus to a levelcomparable of pure sand. The constrained moduli forinitial loading was varying between 500 and 30,000 kPaand depending on the stress level, soil type, and tire chipcontent. We observed that constrained moduli of sand-tire chip, sandy silt-tire chip, and clay-tire chip mixturesgenerally decrease with increasing tire chips content withthe greatest decrease occurring as the tire chip contentwas increased to 30%. For higher tire chip contents, theconstrained modulus does not change significantly.Although there are many advantages to using recycledtires in civil and environmental applications; however,concerns have been raised about their self-combustionpotential and environmental suitability. It is possible touse tire chips alone as a lightweight geomaterial ordrainage layer by following certain precautions anddirectly in mixtures with soils without the fear of self-combustion.

Shear strength

The tyre chips used in construction are mainly 50–100mm long, although longer and bigger shreds can also beused. These chips have friction angles between 20– 30°and cohesion of 3–11.5 kPa based on relevant laboratorytests (Humphrey and Sandford, 1993). Tyre shred sandmixtures (with shreds up to 102 mm in size) with differentpercentages of tyre shreds, shred aspect ratios and sand

Figure13.Average slump test results for each rubberpercentage.

Figure 14. Compressive strength variation with rubbercontent.

matrix relative densities have been tested in triaxialcompression.

According to results, strength was increased withrelatively high friction angles, and this has also beennoted in field observations of sand–tyre chip mixtures.However, the strength decreases as the tyre chippercentage increases beyond 30%. This is attributed tothe performance of the mixture, which behaves more as amass with sand inclusions rather than a reinforced soil(Foose et al., 1996).

Tatlisoz et al. (1997) tested mixtures made of sand,sandy silt and clay and the results of the tests showedthat tyre chips and soil–tyre chip mixtures behaved likesoils, but required more deformation to mobilise theirultimate shear strength. Incorporation of tyre chips insand and in sandy-silt mixtures resulted in increasedshear strength while mixtures made of clay and tyre chipshad the same, or lower, shear strength as soil (that is

Direct Res. J. Eng. Inform.Tech. 31

clay) alone.Mixtures of sandy silt tyre chips and sand tyre chips

give a linear shear strength envelope and have ancohesion intercept which for the sand–tyre mixtures canbe attributed to penetration of sands into the rubberparticles due to elastic deformation under conditions ofno or low normal stress; while the shear strengthenvelopes of the sand–tyre chip mixtures can be non-linear, with no cohesion intercept. However, the additionof rubber, in the form of shreds or fibres, to the mixturescan give non-linear Mohr–Coulomb envelopes.

According to Ghazavi (2004), at a given normal stressapplied on sandy–tyre chip mixtures, the shear strengthof the soil modified with waste rubber was found toimprove compared with that of the sand alone, and thehigher the waste tyre content the higher the shearstrength, provided that the level of compaction wassimilar. A relatively clear peak in the shear resistance ofalmost all samples has been observed, regardless ofcompaction level and rubber contents, except in the caseof rubber grains alone where shear stress increasedslightly with increasing horizontal deformation.

Results of the direct shear tests on the sandy silt–tyrechip mixtures showed an improvement in strength as thepercentage of rubber increased (from 10% to 20%) forthe sandy silt–tyre chip mixtures compared with valuesfor the sandy silt alone; this was attributed to the higherfriction angle and greater cohesion. The increase instrength for sand–tyre chip mixtures is related to theincreased initial friction angle while for the sandy silt–tyrechip mixtures this increase is due to increases incohesion and not in friction angle (Foose et al., 1996).For mixtures made of clay and tyre chips there was noincrease in strength. In fact, a decrease in shear strengthwas noted at low normal stresses. This reduction wasattributed to the weak bonding between tyre rubber andclay. However, according to Cetin et al. (2006), the shearstrength increases by 30% (when fine (<0.425 mm) tyrechips are used) and by 20% (when coarse (2–4.75 mm)tyre chip mixtures are used), cohesion increases as theamount of rubber increases up to 40%, while the angle ofinternal friction decreases. In addition, the mixtures ofsand and tyre rubber particles are materials that exertless lateral earth pressures on retaining structurescompared with those exerted by sand alone. Lee et al.(1999) observed that in contrast to the settlement, thehorizontal pressure of a rubber–sand mixture on aretaining wall was lower than that of a gravel backfill. Thevariation of maximum shear modulus (Gmax) with percentrubber, as examined herein, provides insight to howmixture rules may be developed for particulatemechanics of earth structures that include non-earthmaterials.

Gmax and Dmin

Hardin (1978) developed an empirical equation of

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Figure15.Effect of replacement of coarse aggregate by tire chips on (a)compressive strength, and (b) split tensilestrength at 7 and 28 days.

maximum shear modulus Gmax for particulate materialsas:

Where

A = dimensionless elastic stiffness parameter,pa = atmospheric pressure,0 = effective mean principal stress,F(e) = 0.3 + 0.7 e2, defining effect of void ratio on elasticstress-strain relationship,e = void ratio,OCR = over consolidation ratio,k = parameter related to soil plasticity index, andn = elastic parameter.Based on this equation, the Gmax values found in thisstudy were normalized by dividing the Gmax values by Pa

with the estimation of n = 0.5 and OCR = 1. Thisis partly due to the small mass of the water in the undercompation samples which is moving with the particleframe. Hardin (1961) noted there was a reduction in thestiffness of the frame when water was added to a similarparticle size of sand. Further, the samples prepared byhand-spooning were observed to be less uniform than byunder compaction. Consequently, more sand clustersexisted in the samples. The vibrational stress waveslikely propagated faster in a sample with some clusters ofsands than in a uniformly distributed mixture of rubberand sand. In other words, the rubber dominates thestrength behavior in a uniform mixture, thus resulting inlower modulus.

In comparing the minimum damping ratio at a shearstrain of 5x 10-4 % increase damping with tire inclusion isconsistent regardless of preparation method and isjudged to be less related to the sample preparationmethod. It was not possible to determine Dmax in this

study due to the large shear strains that would berequired to achieve Dmax.

This trend appears to be close to that for a typicalsaturated cohesive soil, which was generated using theempirical equation as indicated by Hardin and Drnevich(1972):

wherea, b = soil constants,exp = base of natural logarithms,= shear strain, andG = shear modulus.It is interesting to see how the normalized curves for thegranulated rubber/soil mixtures compared to the wheatsamples tested by K. O. Hardin (1987). He suggestedthat a = 1.3 and b 5= 0.1 The modulus of rubber is verylow relative to that of soil, hence the contribution ofrubber to shear modulus in the mixture may not besignificant. If the volume of rubber in a rubber/soil samplecould be treated as voids; that is, voids in the mixtureinclude the volume of rubber and air. The void ratio at 69kPa, assuming rubber solids to be a part of the voids,was computed for each of the specimens. The value ofGmax / [OCRk(Pas0)0.5] versus this void ratio at 69 kPaconfinement is shown in Figure12.

Workability of fresh concrete (Slump test)

Table 5 and Figure13 show average results for eachrubber percentage from the slump test. These showedthat most mixes for the rubber sizes and percentagesused in this study, with the exception of the 40% mixesfor either fine or coarse aggregates and the 30% mix for

Direct Res. J. Eng. Inform.Tech. 33

Table 6. Average compressive strength loss for mixes containing rubber aggregate.

Curingtime(Days)

Compressive StrengthCoarse Rubber Aggregates ( CRA) Fine Rubber Aggregates ( FRA)

10% 20% 30% 40% 10% 20% 30% 40%7 55% 72.2% 85.1% 90.1% 35% 58% 80.3% 87.4%28 60% 74.5% 86.8% 89.0% 40.2% 63.6% 78.8% 85.2%

fine aggregates, had slump values corresponding to highto normal workability levels. In fact, for small percentagesof rubber (10%) the workability based on slump resultscomparable with those of the control mixes (0%). This isconsistent with results reported by Raghavan et al.,(1998) for mortars containing rubber particles. Withfurther increase in rubber content, for both the fine andcoarse rubber specimens, the mix became stiffer andless workable, which was reflected in the significantdecrease in slump values. The 40% coarse rubber tiremix in particular had too low slump values and wasmanually unworkable. The addition of fine aggregaterubber from 10% up to 40% rubber content maintained alinear decrease in slump values. This was not the casefor the coarse aggregate rubber content whichexperienced a decrease of about 50% from a 10% rubbercontent to a 20% rubber content, a very small decreasein slump between 20% and 30% of rubber aggregate andthen again a large decrease of about 33% between 30%and 40% of rubber aggregate. No particular trend wasobvious as to whether the CRA or FRA mixes were moreworkable.

Consolidation

Consolidation is an important property if the soil is to besubjected to high compressive stresses. Using tyre chipsand tyre chip–soil mixtures, the consolidation decreasesas stress level increases. Sand–tyre and sandy silt–tyremixtures have been examined for compression andshowed similar stress– strain curves regardless of thetype of soil or the amount of tyre chips. However clay–tyre chip mixtures exhibited higher strains. Compressionon sand–tyre mixtures was found to increase significantlyfor tyre rubber contents greater than 30% (by weight ofsand) (Edil et al., 1990). Additionally, clay–tyre chipmixtures compress relatively more compared with sandysilt–tyre chip and sand–tyre chip mixtures, whilespecimens of pure tyre chip compress the most.Therefore, it can be concluded that backfills of soil mixedwith tyre may be less compressible than those consistingof pure tyre chips (Edil, 2004).

Bearing capacity ratio

Laboratory tests have been conducted on soils and, inparticular sand, reinforced with tyre rubber in order to

investigate the effect of rubber particles on the bearingcapacity of the soil. Results on sand–tyre shred mixturesshowed an increase in the bearing capacity ratio and adecrease in post-peak resistance reductions, dependingon the rubber content and the aspect ratio. Furthermore,it can be concluded that a minimum length of shred mustbe provided for better results. However, after an optimumlevel of rubber content and size, the bearing capacityseemed to decrease (Hataf and Rahimi, 2006). Accordingto Oikonomou et al., (2007), preliminary studies onsandy-mud mixtures with rubber granules (2 mm size)showed lower values for California bearing ratio (CBR)compared with those for the sandy mud alone. However,further studies on this property need to be made, bychanging the size and the amount of rubber particles aswell as the type of soil.

Sustainability issues and life cycle assessment

The public, governments and industry are all greatlyinterested in green design and engineering approachestowards better environmental quality and sustainabledevelopment. A life cycle assessment (LCA) is a detailedanalysis dealing with the interaction between a productand the environment. In particular, LCA calculates rawmaterials and energy used in order to produce aparticular product (inputs) and the negative impacts of theresulting release of pollutants into the environment and,as a result, impacts on human health (outputs). LCA isconducted in order to produce more ‘green’ products withthe least environmental impact. This can be achievedwith studies on the effects of each phase of the LCA onthe environment. At the same time, these studies canhelp producers to take conservative action aimed atmaking the environmental impact less harmful. There areseveral studies employing the LCA framework ormethodology that have focused on passenger vehicletyres (Amari et al., 1999; Krömer et al., 1999). In generalthere are five stages in the LCA of a tyre (Krömer et al.,1999).

Sustainability of construction materials partialsubstitution of silica for carbon black as a filler could be asolution to the development of tyres with lower rollingresistance (Krömer et al., 1999). Corti and Lombardi(2004) used LCA in order to compare different processesn for the end-of-life treatment of worn tyres such assubstitution of conventional fuels by tyres in cement kilnproduction, combustion in a conventional waste-to energy

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Figure 16. Effect of replacement of fine aggregate by crumb rubber on (a)compressive strength, and (b) splittensile strength at 7 and 28 days.

process and two processes of reusing tyres as a fillingmaterial based on a cryogenic and a mechanicalpulverization process. Results showed that the use oftyres as fuel in cement kilns and combustion in aconventional waste-to-energy process are verysatisfactory in terms of reducing the negative effectsassociated with the use of conventional fuels, with thefirst one to be better than the second one. The other twofilling material processes showed worse results becauseof the high energy consumption related to thepulverization processes.

Compressive strength

The compressive strength of rubber concretes wasstudied using for different days and different sizes,shapesof specimens.(Table 6). Maximum compressive strengthwas obtained for 40% CRA and FRA at 7 and 28 days(Figure14).Cylindrical specimens of 75, 100, or 150 mmin diameter were used by Rostami et al. (1993), Ali et al.(1993), and Eldin and Senouci (1993), respectively.Results of various studies indicate that the mechanicalstrength of rubber concrete mixtures is greatly affected bythe size, proportion, and surface texture of rubberparticles, and the type of cement used in suchmixtures.Various published results show that coarsegrading of rubber granules lowered the compressivestrength of rubber concrete mixtures more than finegrading. For instance, results obtained by Eldin andSenouci (1993) indicate that there was about 85%reduction in compressive strength and 50% reduction intensile strength when the coarse aggregate was fullyreplaced by coarse rubber chips. However, specimenslost up to 65% of their compressive strength and up to50% of their tensile strength when the fine aggregate wasfully replaced by fine crumb rubber Figure 16 a and b.Topcu (1995), and Khatib and Bayomy (1999) alsoshowed that the addition of coarse rubber chips inconcrete lowered the compressive strength more than theaddition of fine crumb rubber. However, results of tests

carried out by Ali et al. (1993), and Fatuhi and Clark(1996) indicate the opposite trend. All results (Khatib andBayomy 1999; Ali et al. 1993; Eldin and Senouci 1993;Fatuhi and Clark 1996) show that the greater the rubbercontent used in rubber conrete mixtures, the lower thecompressive and tensile strengths achieved (Figure15and 16).

This has usually been attributed to the fact that therubber particles act as voids in the cement matrix due tothe lack of adhesion between the rubber and the cementmatrix. It was also suggested that the lower specificgravity of the rubber particles compared to the cementpaste causes cracks around the rubber particles toappear quickly upon loading which accelerates the failureof the specimens (Khatib and Bayomy, 1999).Due to thelow compressive strengths of the mixes containing rubberaggregates which would not be acceptable in mostcases, the tests for the mixes containing 10% of rubberaggregate only, as these showed the best compressivestrength values of all mixes containing rubber aggregate.The 20% and especially 30% rubber mixes do not exhibita real peak, with the 30% FRA curve showing aprolonged plateau throughout the straining of thematerial. This shows evidence of a ductile fracturebehaviour as well as an ability to support loads aftercracks were generated. Khatib and Bayomy (1999) foundthat the 28-day compressive strength of rubber conretemixtures was reduced by about 93% when 100% of thecoarse aggregate volume was replaced by rubber, and by90% when 100% of the fine aggregate volume wasreplaced by rubber. They hypothesized that there arethree major causes for this strength reduction. First,because rubber is much softer than the surroundingcement paste, upon loading, cracks are initiated quicklyaround the rubber particles due to this elastic mismatch,which propagate to bring about failure of the rubbercement matrix. Second, due to weak bonding betweenthe rubber particles and the cement paste, soft rubberparticles may be viewed as voids in the concrete mix.The assumed increase in the void content would certainlycause a reduction in strength. The third possible reason

Direct Res. J. Eng. Inform.Tech. 35

Table 7.Possible Reuse of Tyre.

Natural Rubber Natural rubber predominantly obtained from the sapof Hevea brasilensi tree

Natural rubber accounts for about 30-40% of a car tyre and about 60-70% ofa truck tyre.

Synthetic Rubber All synthetic rubber are made from petrochemicals Synthetic rubber account for about 60-70% of a car tyre nd 30-40% of a truck tyre.Steal cord and beading including thecoating materials andactivators,copper/tin/zinc/chromium

Steel is premium grade and is only manufactured in afew plants around the world due to its high qualityrequirement

Steel is used to provide rigidity and strength to the tyres.It account for 15% of theweight of a car tyre.

Other reinforcing fabrics Predominantly derived from petrochemicals Used for structural strength and rigidity. Its account for 5% of a radial tyre.Carbon Black Derived from petroleum stock It provide durability and resistance against tear and wear.It accounts for 28% of

car tyre.Zinc Oxide A mineral Zinc is added to provide resistance against UV degradation and vulcanization

control. Zinc oxide accounts for about 1.2% of a passenger tyre.Sulpher including compounds Sulpher is used to vulcanize the rubber Makes up about of the 1% of passenger tyre.Otherb additives and solvents, ageresistors,Processing aids accelators,vulcanizing agents,softeners and filters

The other additive are used in the various rubbercompound to modify handling, manufact -uring and endproducts propeties

The additive makes up about by weigth of a passenger tyre.

Recycled Rubber Recovered from used tyres or other rubber products Used in some rubber compound in the manufacturing of new rubber products andretreated materials

for the reduction in strength is that the strength ofconcrete depends greatly on the density, size, andhardness of the coarse aggregate (Mehta andMonteiro 1993). Because aggregates are partiallyreplaced with relatively weaker rubber, a reductionin strength is anticipated. It was also found (Khatiband Bayomy 1999) that the flexural strength ofrubcrete mixtures decreased with an increase inthe rubber content in a fashion similar to thatobserved for compressive strength, perhaps dueto similar mechanisms.The results showed thatconcrete with rubber aggregate contents higherthan 10% by mass would be unacceptable forprimary structural elements. However, there is anumber of structural applications of medium to lowstrength requirements for which this materialwould be acceptable in terms of strength. Forinstance, a number of recent studies pointed outthat concrete rubber aggregate could be used inthe production of concrete blocks or other precastconcrete units and have the advantage of a lower

unit weight over usual concrete mixes. Concreteblocks are available in compressive strengthsusually ranging from 2.8MPa to 35MPa (solid)and 2.8MPa to 20MPa (cellular and hollow), thatis strengths which were shown to be achievableby concrete including rubber aggregates.

A study conducted by Biel and Lee (1996)suggests that the type of cement used in rubberconcrete mixtures greatly affects the mechanicalstrength. Recycled tire rubber particles were usedin concrete mixtures made with both magnesiumoxychloride cement and Portland cement. Thepercentage of fine aggregate substitution rangedfrom 0 to 90%, increasing by 15% for each set. Itwas observed that 90% loss of the compressivestrength occurred for both the portland cementrubber concrete (PCRC) and magnesiumoxychloride cement rubber concrete (MOCRC)when 90% of the fine aggregate (25% of the totalaggregate) was replaced by rubber.Whether withor without rubber inclusion, the magnesium

oxychloride cement concrete exhibitedapproximately 2.5 times the compressive strengthof the portland cement concrete.The portlandcement concrete samples containing 25% ofrubber by total aggregate volume retained 20% oftheir splitting tensile strength after initial failure,whereas the magnesium oxychloride cementconcrete samples with similar rubber contentretained 34% of their splitting tensile strength afterinitial failure.

The ratio of the MOCRC tensile strength toPCRC tensile strength rose from 1.6 to 2.8 withincreased amounts of rubber. It was argued (Beiland Lee 1996) that the high-strength and bondingcharacteristics provided by magnesiumoxychloride cement greatly improved theperformance of rubber concrete mixtures and thatstructural applications could be possible. Thefeeding flow rate of rubber powder to the extruderwas found to affect the extent of devulcanization,and have an impact on the product viscosity and

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processability.Although CO2 con -centration was a veryimportant factor for the flow ability and extrusion, it wasfound to have little influence on the degree ofdevulcanization. In addition, the type of screwconfiguration employed affected significantly therheological properties of the de-vulcanized rubber due tothe varying amount of deformation that it imparts on therubber crumb. The possible reuse of tyre are shown inTable7.

Conclusion

From experimental study and literature review it can beconcluded that it possesses a number of desirableproperties, such as lower density, higher toughness,higher impact resistance, enhanced ductility, and moreefficient sound and heat insulation compared toconventional concrete. Such engineering properties areadvantageous for various construction applications. Ifrubber concrete is used to its potential rubber concretemixtures usually absorb significant plastic energy andundergo relatively large deformations without fulldisintegration. This property can be utilized in variousstructural and geotechnical projects in which thedeformation at peak load is a primary design concern.Using rubber concrete as a flexible sub-base forpavements, as pipe bedding. The shear modulus of themixtures is strongly influenced by the percentage of therubber inclusion, as expected. Damping ratio increasedslightly with confinement pressure for the 100% rubber,an opposite response from soil. This may be becauseunder increasing confining stress, the size of interparticlecontacts between particles increases significantly due tothe presence of rubber. The Gmax can be estimated fromHardin’s empirical equation, that is, when given a knownpercentage of tire inclusion and air void for a rubber/soilmixture, the maximum shear modulus of the mixture canbe estimated.The reduced compressive strength ofrubberized concrete in comparison to conventionalconcrete there is a potential large market for concreteproducts in which inclusion of rubber aggregates wouldbe feasible which will utilize the discarded rubber tyresthe disposal of which is a environment pollution problem.Rubberised concrete strength may be improved byimproving the bond properties of rubber aggregates. Thelight unit weight qualities of rubberized concrete may besuitable for architectural application, The use of tyrerubber as a lightweight geomaterial for embankments oras backfill against retaining walls is very promising andshould be promoted. Thus, large volumes of waste tyrescan be consumed.

ACKNOWLEDGEMENTS

Authors wish to acknowledge the authorities of MatScience laboratory and Institutes around Delhi/GZB

where the experimental investigations were carried outto evaluate the mechanical properties and durabilityparameters of recycled rubber tyres.

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