performance based characterisation of crumb rubber asphalt modified using the wet process

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Performance Based Characterisation of CrumbRubber Asphalt Modified using the Wet ProcessHussain A. Khalid a & Ignacio Artamendi aa Department of Civil Engineering , University of Liverpool , Brownlow Street, Liverpool,UK , L69 3GQ E-mail:Published online: 20 Sep 2011.

To cite this article: Hussain A. Khalid & Ignacio Artamendi (2003) Performance Based Characterisation ofCrumb Rubber Asphalt Modified using the Wet Process, Road Materials and Pavement Design, 4:4, 385-399, DOI:10.1080/14680629.2003.9689955

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Road Materials and Pavement Design. Volume 4 – No. 4/2003, pages 385 to 399

Performance Based Characterisationof Crumb Rubber Asphalt Modifiedusing the Wet Process

Hussain A. Khalid — Ignacio Artamendi

Department of Civil EngineeringUniversity of LiverpoolBrownlow StreetUK-Liverpool L69 3GQ

[email protected]

ABSTRACT. Current EU Directives have justified a fresh look at the use of crumb rubber fromused tyres in asphalt for paving applications. A study was undertaken to investigate theinfluence of crumb rubber modification on binder and mixture performance-based properties.Crumb rubber from two origins, truck- and car-tyre was interacted with two penetrationgrade bitumens from different crude origins at various concentrations. Performance-related,rheological properties of the neat and rubber modified binders were evaluated at high andlow temperatures. Based on their rheological performance, selected binders wereincorporated in a Stone Mastic Asphalt mixture whose mechanical properties were evaluated.Emphasis was placed on the resistance of mixtures to permanent deformation, measured inthe Wheel Tracking Test, and to fatigue cracking, measured in four-point bending fatigue. Anattempt has been made to link performance-based binder and mix properties which areconsidered to correspond to one another.

KEYWORDS: Tyre Waste, Crumb Rubber Modifier, Rheology, Permanent Deformation, Fatigue.

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386 Road Materials and Pavement Design. Volume 4 – No. 4/2003

1. Introduction

The environmental problem of waste tyre rubber continues to increaseworldwide. In the EU, the estimated annual arisings of used tyres are in excess of2.5M tonnes, of which approximately 1M tonnes are disposed into landfills. Thenew EU Landfill Directive coming into force in July 2003 prohibits landfill of wholetyres and, with effect from July 2006, of shredded tyres. Moreover, the End-of-lifeVehicle Directive requires the re-use, recycling and recovery of vehicle componentsto be raised to 85% by 2007. A number of studies have emerged over the pastdecade in which tyre rubber was used as modifier in asphalt mixtures in an attemptto mitigate the impacts of this environmental problem. Given the scale of theproblem and the imminent need for further mitigation, however, there is still room toenhance and improve the current level of knowledge.

Crumb Rubber Modifier (CRM) from tyre waste has been used in asphaltmixtures in two approaches, namely the wet process and the dry process. The wetmethod involves dispersing the CRM particles in the bitumen to produce a rubber-modified binder, which is then mixed with aggregate to form a mixture. In the drymethod, on the other hand, the CRM is mixed with the aggregates beforeintroducing the binder to the mixture, i.e. CRM acts as a partial replacement to someof the aggregate sizes. This paper reports the results of a recent laboratory study tomeasure the effect of crumb rubber on performance-based asphalt properties,modified by the incorporation of CRM using the wet technique. A special referenceis made with respect to the influence of origin of both binder and CRM, and CRMcontent on the material properties.

2. Preparation of crumb rubber

Crumb from tyre rubber was prepared by ambient shredding, followed bymechanical grinding to specific sizes. The shredded tyre rubber is usually fed into agranulator where it is cut down to approximately 4mm granules. It is then airconveyed through ducting to various grinders. Cyclones are used to separate the airfrom rubber particles, before they drop into vibratory sieves, the size of which canbe changed to suit the required production. Material that is of the correct size passesthrough the sieve and goes to a bagging unit. The sieving unit is sealed, so the crumbcan only go through the bags, or over, back to the grinders. This is to prevent anyoversize crumb from getting into the bags.

The discs used in the grinding process discharge the crumb in different sizes,referred to as mesh sizes. The smallest mesh sizes at present in the UK are mesh 30and 50 (nominal 600 and 300µm particles respectively). In the US, it has becomecommon practice to obtain mesh 80 crumb (150µm approx.) as well. In this paper,mesh 50 CRM was used, although the research programme found insignificant

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Performance of Crumb Rubber Asphalt 387

differences in binder properties containing mesh 30 and 50 crumb (Artamendi,2003).

The crumb used in the research originated from either car or truck tyres. Car-tyreis made of SBR (Styrene-Butadiene Rubber) treated with oil-extenders of relativelyhigh styrene content to produce low resilience, high hysteresis material that wouldprovide adequate grip with the road surface. Truck-tyre, on the other hand, is madeof a blend of Natural Rubber (NR) and Polybutadiene (circa 80:20). Both of thesepolymers show high resilience and low hysteresis properties, which enable the tyreto absorb and quickly return the energy back to the road to prevent heat build-up.Both types of tyre rubber are treated chemically and physically to reduce theirsusceptibility to degradation due to heating.

3. Digestion conditions

The main sources of crude petroleum from which paving bitumen is sourced inthe UK are Venezuelan and Middle Eastern. Whilst this may be relevant to the UK,it is probably not too different from bitumen available in other countries and theresults reported here could, thus, be of use in other parts of the world. In this study,two 50 Penetration Grade binders were employed from the afore-mentioned sources,i.e. Venezuelan (Ven) and Middle Eastern from Kuwait (KSR). Truck- (NR) andcar-tyre (SBR) CRM were digested in the two binders in 1-litre flasks undercontrolled conditions of shear rate, temperature and duration. The CRM sample wasadded at specified quantities to hot bitumen at 180oC for 1 hr and mixed using ahigh-shear Silverson mixing machine at 2000rpm, with an iso-mantle heater.

Figure 1. Brookfield viscosity of the Ven binder at various truck CRM contents

1E+01

1E+02

1E+03

1E+04

1E+05

110 130 150 170 190

Temperature (oC)

Vis

cosi

ty (

cP).

50 Pen Ven Neat

50 Pen+5%CRM

50 Pen+10%CRM

50 Pen+15%CRM

Mixing Viscosity

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1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

110 130 150 170 190

Temperature (oC )

Vis

cosi

ty (

cP

)50pen KSR Neat

50pen+5%CRM

50pen+10%CRM

50pen+15%CRM

Mixing Viscosity

Figure 2. Brookfield viscosity of the KSR binder at various truck CRM contents

Of initial interest to the investigation was to achieve a basic understanding of thenet effect that the quantity of CRM has on the binder with regard to mixing andconstruction conditions. Figures 1 and 2 show the Brookfield viscosity test results ofthe Ven and KSR binders modified with truck-tyre rubber at varying CRM contentsby weight of bitumen. A line has been drawn to indicate the average binder viscosityakin to feasible field mixing conditions. It can be seen that at more than 10% CRM,mixing temperatures in excess of 180oC would be required to achieve a suitablebinder viscosity for mixing with aggregates. In routine paving operations suchconditions are not practicable to achieve. Therefore, it was decided to limit the CRMcontent to no more than 10% by weight of binder to remain within acceptable limitsfor asphalt field mixing temperatures (� ��

oC) practised in the UK.

4. Binder rheology

4.1. High temperature properties

At extreme shear rates (or shear stresses) bitumen viscosity reaches a constantvalue and in the lower shear rate range, known as the first Newtonian region, thisconstant is referred to as the Zero-Shear Rate Viscosity, ηo (Barnes et al., 1989).The zero-shear viscosity is a unique characteristic of the binder and, since it isindependent of the absolute value of the loading time, it can be used to succinctlydescribe the rheological nature of the binder at a particular temperature(Khalid, 2002). It can be measured in the creep mode using a Dynamic ShearRheometer (DSR) and has been strongly tipped to portray the binder’s contribution

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of the asphalt mixture’s resistance to permanent deformation (Phillips and Robertus,1996; Sybilski, 1996). In this study, ηo has been measured in the DSR at 60oC undershear stresses ranging between 15 and 95Pa to evaluate the effect of CRMmodification. Further details of test method can be found elsewhere (Khalid, 2002).

From Figures 3 and 4, it is clear that truck-tyre CRM reacts relatively more witheither binder than car-tyre CRM. It can also be seen that the neat Ven binder givesslightly higher viscosity at 60oC than the KSR binder. However, after reaction with10% truck CRM, the KSR binder gives the highest viscosity. These differences areattributed to the different compositions of binders and rubbers and, hence, the levelof interaction between them. Labile components from the bitumen migrate into therubber causing it to swell and leads to the hardening of the binder. It is clearlyevident that the KSR binder reacts best with the truck tyre CRM. These observationsare in line with findings by Oliver (1982) who reported superior levels of interactionachieved between a binder from Kuwaiti crude with NR crumb compared to SBRcrumb.

If the viscosity data at construction conditions (� ��oC) in Figures 1 and 2 were

viewed in conjunction with the ηo values at 60oC in Figures 3 and 4, there appears tobe a cross-over between the neat Ven and KSR binders, which would have occurredat some intermediate temperature. This cross-over feature was investigated(Artamendi, 2003) using other penetration grades from the same crude origins andwas confirmed to be consistent.

4.2. Low temperature properties

At low temperatures, the binder stiffens and may exhibit brittle behaviour,especially if it has undergone any loss of its light components due to interaction withCRM. The complex modulus, G*, and the phase angle, δ, represent the binder’smost vital properties which best express its response to applied loading. The G*sinδparameter is the out-of-phase component of G* and is considered to reflect theamount of energy lost by the binder in response to an applied stress, hence the nameloss modulus. The loss modulus of binders measured in the DSR was included in theSHRP specifications in which limits have been set in terms of a maximum allowablevalue, after a specified laboratory ageing procedure, to safeguard against fatiguecracking of the mixture containing the binder.

Goodrich (1991) considered G*sinδ of a binder as a measure of its ability torelieve strain-induced stresses by viscous flow. He hypothesised that G*sinδ doesnot achieve a zero value as a result of material stiffening at low temperatures (orhigh frequencies); it reaches a peak close to the glass transition temperature, Tg, andthen diminishes. Based on this hypothesis, at low (<10oC) temperatures, a binderwith lower G*sinδ is expected to peak in the Tg region at a lower temperature than abinder with higher G*sinδ value and, consequently, is expected to exhibit lessembrittlement. As regards resistance to cracking at low (<10oC) temperatures,

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1E+2

1E+3

1E+4

1E+5

1E-04 1E-03 1E-02 1E-01 1E+00 1E+01

Shear Rate (1/s)

Vis

cisi

ty (

Pa.s

)

Neat 50 pen Ven

5% CRM Truck10% CRM Truck

5% CRM Car10% CRM Car

60oC

1E+02

1E+03

1E+04

1E+05

1E-04 1E-03 1E-02 1E-01 1E+00 1E+01

Shear Rate (1/s)

Vis

cosi

ty (

Pa.s

)

Neat 50 pen KSR

5% CRM Truck

10% CRM Truck

5% CRM Car10% CRM Car

60oC

Goodrich highlighted the role of the loss tangent (tanδ) as the indicative parameterof performance in these conditions. He concluded that at low temperatures,resistance to cracking is improved with binders that maintain viscous flowcharacteristics; that is with binders which have high low-temperature tanδ values.

Figure 3. Zero shear viscosity of the Ven binder at various truck and car CRMcontents

Figure 4. Zero shear viscosity of the KSR binder at various truck and car CRMcontents

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The isotherms in Figures 5 and 6 show that for both binders similar G*sinδvalues were obtained, with the lowest ones achieved predominantly with the CRMfrom truck-tyres and the KSR binder, although only marginally. Similarly, themarginally higher tanδ values were achieved with the KSR binder. The 5% CRMKSR binder achieved the highest tanδ values. The influence of CRM content on thelow temperature properties, however, appears to be insignificant. The -10oC waschosen as a sufficiently low temperature at which noticeable differences in binderproperties could be seen. Some of the results obtained at frequencies above about13Hz may have been close to machine compliance.

Figure 5. Isotherms of loss modulus for the Ven binder at various truck and carCRM contents

1E+07

1E+08

1E-01 1E+00 1E+01 1E+02Frequency (Hz)

G*s

in δ

(Pa )

0% CRM10% CRM Car10% CRM Truck5%CRM Car5% CRM Truck

-10 oC

0.0

0.5

1.0

1E-01 1E+00 1E+01 1E+02

Frequency (Hz)

Tan

δ

0% CRM10% CRM Car10% CRM Truck5% CRM Car5% CRM truck

-10 oC

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Figure 6. Isotherms of loss modulus for the KSR binder at various truck and carCRM contents

5. Mixture properties

5.1. Specimen preparation and material design and evaluation

At this stage the role that the binder plays in establishing the performance of themixture was sought. The binder results presented in the previous section suggestedthat the KSR binder and CRM from truck-tyres produced a modified binder withimproved properties, which merited further investigation with respect to mixture

1E+07

1E+08

1E-01 1E+00 1E+01 1E+02

Frequency (Hz)

G*s

in δ

(Pa)

...

0% CRM10% CRM Car10% CRM Truck5% CRM Car5% CRM Truck

-10 oC

0.0

0.5

1.0

1E-01 1E+00 1E+01 1E+02

Frequency (Hz)

Tan

δ

0% CRM10% CRM Car10% CRM Truck5% CRM Car5% CRM Truck

-10 oC

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characteristics. To achieve this, a 14mm Stone Mastic Asphalt (SMA) was designedbased on volumetric considerations and used as a reference mix, which had 5.5 and5.1% binder and voids content respectively. The binder included cellulose fibres at0.3%. At 5% CRM, the design binder content was the same as that of the referencemix, i.e. 5.5% by weight of total mix. However, at 10% CRM modification, thebinder content had to be increased to compensate for the reduced amount of bitumenin the binder and to restore the mobility of the binder to coat aggregates, which isslowed down due to absorption of its mobile components by the rubber. Thus 6.0%binder by weight was added to the modified mixtures, which resulted in 4.0% voidcontent.

Slab specimens measuring 300mm2 and 65mm thick were prepared in thelaboratory using a roller-compacter. For the measurement of volumetric data,stiffness modulus, retained modulus and unconfined dynamic creep, 100mmdiameter samples were cored from the slabs. For fatigue testing, beam specimens50mm2 in cross-section and 300mm long were cut from the slabs. Figure 7 illustratesthe two varieties of specimens. Table 1 gives a summary of mixture design andevaluation data.

Figure 7. Cylindrical and beam specimens obtained from 300mm2 slabs

Table 1. Mix design and mechanical properties of the neat and modified SMA mix

BinderContent

(%)

CRMContent

(%)

VoidsContent

(%)

Initial ITSMat 20oC(MPa)

RetainedITSM(%)

Permanent AxialStrain at 40oC

(%)

5.55.56

05

10

5.15.24.0

470042004150

918689

1.9031.2621.106

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The Indirect Tensile Stiffness Modulus (ITSM) and the Permanent Axial Strain(obtained from a form of unconfined dynamic creep test) values were measured inaccordance with standard procedures (BSI 93, BSI 96). Specimens subjected to theITSM test were subsequently passed through a water immersion regime and were re-tested to determine their retained ITSM values, as given in Table 1. The regimeinvolved vacuum saturation of specimens at ambient temperature beforeconditioning them in water at 60oC for 6 hrs followed by 16 hrs conditioning inwater at 5oC. The retained ITSM, measured after three conditioning cycles, can beconsidered as a measure of the sensitivity of the material to moisture damage. Withregard to the ITSM data in Table 1, it is not uncommon to obtain (slightly) lowerstiffness values with dense mixes containing binders modified with elastomers thanwith those containing the unmodified binder (Preston, 1996). Yet, the lowerpermanent axial strain values of the CRM modified materials are in line withobserved, improved deformation resistance in dynamic creep of asphalt modified bythe addition of an elastomer (Brown and Gibb, 1996). The retained stiffness valuesin Table 1 suggest that the durability, in the shape of sensitivity to moisture damage,of the CRM materials is not impaired under the given conditions of materials andtest procedure used.

5.2. Effect on rutting resistance

A widely used method of assessing the resistance to rutting performance ofasphalt is the Wheel Tracking Test (WTT). The WTT was conducted on themanufactured slabs in accordance with the BS598 method (BSI, 1997) at 45 and60oC. These are typical WTT temperatures commonly used in the UK, hence the ηo

measurements. Each slab was tracked twice for a duration of 45 minutes, one at eachtemperature with the tracks offset from each other by 150mm across the width of theslab. The performance was measured in terms of rutting rate (mm/hr) and rut depth(mm). A number of eminent researchers have used either or both of these parametersas rutting indicators and have correlated the WTT performance with the zero-shearviscosity, ηo, of the binder measured in the DSR (Sybilski, 1996; Phillips andRobertus, 1996; Carswell, 2000). Figure 8 shows such correlations at the two testtemperatures for the neat KSR and the two truck-CRM modified binders. The shearrate at which ηo was considered for the correlation is 0.002 1/s. Sybilski (1996)showed that at this level of shear rates, the viscosity of a wide range of bindersassumes a constant, plateau-like behaviour, which is characteristic of the binder. At45oC, ηo values were obtained from the DSR measurements at the stated shear rate.However, at 60oC it was predicted using a linear regression equation of therelationship in Figure 4.

The viscosity shows a good correlation with the tracking rate (R2 = 0.82), whichgives emphasis to ηo as a useful performance indicator. However, the correlationwith rut depth is not as good (R2 = 0.57) which reduces the level of certainty in theprediction process. It is important to note that whilst ηo measurements were

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conducted within the linear domain of the binder, the WTT performance indicatorsare representative of non-linear behaviour of the asphalt mixture. Thus, despite theusefulness of the correlation between tracking rate and ηo, it may not be feasible todraw universal conclusions as binder and mix parameters were measured at differentbehavioural domains (Des Croix and Di Benedetto, 1996). The argument presentedhere in response to the hypothesis by these workers is that measurement of ηo in thenon-linear region would not be contemplated due to increased risks of poorrepeatability of these measurements.

Figure 8. Wheel tracking parameters correlated with viscosity

5.3. Fatigue behaviour

Fatigue cracking is one of the most critical distress modes that asphalticmaterials may undergo during their service life. Fatigue is principally controlled bybinder characteristics and volumetric proportions of the mixture (Brown, 1995) and,although there are many techniques to describe fatigue behaviour, it is commonlyassessed using a form of repeated-load bending tests applied either under controlledstress or strain modes. One of the known effects of crumb rubber addition is themigration of mobile components from the bitumen into the rubber, which leads it toswell. This interaction causes the binder to harden as a consequence and the knock-on effects on the entire asphalt mixture could be significant. While these effects maylead to an improvement in the asphalt’s resistance to permanent deformation as

0

1

2

3

4

5

1E+03 1E+04 1E+05

Viscosity (Pa.s)

Whe

el T

rack

ig R

ate

(mm

/hr) ..

.

0

1

2

3

4

5

Rut

Dep

th (

mm

)...

WT Rate 60CWT Rate 45C

RD 60CRD 45C

RD

WTR

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shown above, the influence on fatigue and low temperature cracking propertieswould be of particular interest if the current level of use of crumb rubber from wastetyres in asphalt were to be enhanced.

Fatigue testing was performed in a four-point bending (4PB) test apparatus undercontrolled strain conditions. All tests were conducted at a frequency of 10Hz and ata temperature of 10oC, under sinusoidal loading with no rest periods. Strainamplitudes selected varied between 150 and 200µm/m for the conventional,reference mix and increasing to 215µm/m for the modified variety. Failure wasreached when the initial stiffness modulus was reduced by 50%. Figure 9 is theWohler diagram for the three materials showing the beneficial effect of CRM onfatigue life, which becomes more prominent at 10% CRM content.

Figure 9. Wohler diagram showing effect of CRM content on fatigue life

In section 4.2, it was hypothesised that G*sinδ may be used to describe thecapacity of the binder in mitigating fatigue damage to the asphalt mix caused byrepeated loading. To test this hypothesis, G*sinδ of the neat and truck-CRMmodified binders from Figure 6, at -10oC and 10Hz, have been plotted against Nf50

which corresponds to 175µε interpolated from Figure 9 for the three materials. Thisstrain level represents the middle of the range of strain amplitudes adopted in thefatigue study. Although the number of points in Figure 10 is small which hascontributed to the weakness of the correlation (R2 = 0.47), the trend suggests that thelower G*sinδ is at low temperatures, the longer the fatigue life is. Similarly, tanδ for

1E+04

1E+05

1E+06

1E+07

1E+08

1E+01 1E+02 1E+03

Initial Strain (µε )

Nf 5

0...

0% CRM

5% CRM

10% CRM

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the three binders deduced from Figure 6 at 10Hz has been plotted against Nf50.Again, the level of predictive uncertainty is high (R2 = 0.33). However, the trendsuggests that, at low temperatures, the more viscous the binder is (i.e. high tanδ) themore likely that the mixture will endure longer fatigue life.

Bahia et al. (2001) have shown that G*sinδ of the binder does not correlate wellwith Nf50 of the mixture based on work in which nine polymer modified/oxidisedbinders tested in the DSR and their mixtures in bending fatigue. The shape of thecorrelation presented by these workers was, however, similar to that given in Figure10. Bahia et al. echoed the concept provided by Des Croix and Di Benedetto as themain reason for the poor correlation obtained. Consequently, they went on torecommend that a different rheological parameter be opted to portray the binder’scontribution to fatigue cracking resistance of the asphalt mixture instead of G*sinδ.

1

2

3

4

5

6

7

8

9

10

10 20 30 40 50

G*sin (MPa)

Nf5

0 (x

106 )

1

2

3

4

5

6

7

8

9

10

0 0.1 0.2 0.3 0.4 0.5

Tan

Nf5

0 (x

106 )

Figure 10. Correlation between binder loss modulus/tangent and fatigue life of theSMA mixture

6. Conclusions

– For the types of binder and rubber used in this investigation, Brookfieldviscosity test results showed that the addition of more than 10% crumb rubber byweight renders the binder too viscous for routine field mixing operations.

– The KSR binder modified with 10% crumb rubber from truck tyre producedthe highest viscosity values at 60oC among all the rubber-binder blends. The KSRbinder modified with 5% CRM from truck tyre produced the lowest loss modulusand highest loss tangent values at -10oC over a range of frequencies.

– Crumb rubber modified Stone Mastic Asphalt (SMA) showed improvedresistance to permanent deformation and fatigue cracking compared to conventionalSMA. For the crumb rubber modified SMA, improvement in rutting and fatigueperformance was only marginal with increase in crumb rubber content from 5 to10%.

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– The zero-shear viscosity, ηo, of the binder at 60 and 45oC showed a reasonablecorrelation with the wheel-tracking rate, which gives support to ηo as a usefulperformance prediction parameter. However, the correlation between ηo and rutdepth was not of adequate certainty.

– Correlations between the binder loss modulus/tangent at -10oC and the SMAmixture’s fatigue life were of inadequate certainty, although the trend was indicativeof the binder’s role. Further work is required to study the binder’s properties withinthe non-linear domain under conditions commensurate with those governing thetested mixture.

Acknowledgements

This work formed part of a research programme funded by the UK Engineeringand Physical Sciences Research Council and supported by a number of industrialpartners including Tarmac, J Allcock & Sons and Shell, to whom the authors areindebted.

7. Bibliography

Artamendi I., “A fundamental study into wet process modification of paving binders andmixtures by crumb rubber from used tyres”, Unpublished PhD Thesis, University ofLiverpool, 2003.

Bahia H.U., Zhai H., Zeng M., Hu Y., Turner P., “Development of binder specificationparameters based on characterisation of damage behaviour”, Association of AsphaltPaving Technologists, Vol. 70, 2001, pp. 442-464.

Barnes H., Hutton J., Walters K., An introduction to rheology, Amsterdam, Elsevier, 1989.

Brown S.F., “Practical test procedures for mechanical properties of bituminous materials”,Transport Jl. Proc. Inst. of Civil Engineers, Vol. 111, 1995, pp. 289-297.

Brown S.F., Gibb L.M., “Validation experiments for permanent deformation testing ofbituminous mixtures”, Association of Asphalt Paving Technologists, Vol. 65, 1996,pp. 255-290.

British Standards Institution (BSI), “Method for determination of indirect tensile stiffnessmodulus for bituminous materials”, DD213, London, BSI, 1993.

British Standards Institution (BSI), “Method for determining resistance to permanentdeformation of bituminous mixtures subject to unconfined dynamic loading ”, DD226,London, BSI, 1996.

British Standards Institution (BSI), “Sampling and examination of bituminous mixtures forroads and other paved areas: method of test for determination of wheel-tracking rate”,BS598, Part 110, London, BSI, 1997.

Carswell J., Green P.J., “Prediction of rutting resistance in hot rolled asphalt using rheologicalparameters”, The Asphalt Year Book 2000, Institute of Asphalt Technology, Middlesex,2000, pp. 27-36.

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Received: 20 July 2003Accepted: 8 November 2003

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performance based characterisation of crumb rubber asphalt modified using the wet process

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