Influence of Crumb Rubber Concentration on theRheological Behavior of a Crumb Rubber Modified

Bitumen

F. J. Navarro, P. Partal,* F. Martınez-Boza, and C. Gallegos

Departamento de Ingenierıa Quımica, Universidad de Huelva,Facultad de Ciencias Experimentales, Campus del Carmen, 21071, Huelva, Spain

Received November 22, 2004. Revised Manuscript Received June 23, 2005

Bitumen is widely used in construction. Its applications go from binder for road asphalts tobuilt-up roofing membranes and other waterproofing uses. The use of crumb tire rubber as abinder modifier may contribute to solving a waste disposal problem and to improving the qualityof road pavements. This paper deals with the effect that rubber concentration exerts on thethermorheological properties of rubber-modified bitumens. The results obtained confirm that therheology and storage stability of these modified binders are largely dependent on rubberconcentration. The linear viscoelastic measurements carried out revealed an improvement of bothlow and high in-service properties of the binder. The viscous flow measurements showed animportant increase in viscosity and a noticeable reduction in temperature susceptibility. A swellingprocess of the crumb rubber by light components of the maltenic fraction was confirmed bymodulated differential scanning calorimetry (MDSC). The stability tests, performed on samplesstored at 180°C, demonstrated that the storage stability of rubber-modified bitumen was improvedas rubber concentration increased.

Introduction

Bitumen is usually defined as a dark-brown to blackmaterial, natural or manufactured, mainly composed ofhigh molecular weight hydrocarbons, which can beclassified in two main groups in terms of polarity andsolubility in n-heptane: asphaltenes and maltenes.1Bituminous binders have been widely used in construc-tion applications, mainly for flexible road pavements,waterproofing, roofing, joint sealant, etc.2 In general, abinder must remain flexible enough to withstand sud-den stresses without cracking at low temperaturesduring winter, but it must also resist permanentdeformation or viscous flow at high in-service temper-atures. In this way, bitumens should resist stresses dueto traffic loads and low temperatures in pavementapplication, as well as extensions and contractions inroof construction.3

To achieve and improve the desired in-service proper-ties, bitumens are usually modified, mainly with poly-mers, increasing the cost of the final product. In thisway, waste rubber from discarded tires, from environ-mental and economical perspectives, can provide analternative source of elastomeric polymer material forbitumen modification, being a substitute to virginpolymers.4-7 This could favor the development of a high

potential market for optimum environmental recyclingof scrap tires and further the use of crumb rubber inpavement structures. This application is also in linewith the European directive that will prohibit all landfillof scrap tires by 2006. In this sense, the European TireRecycling Association (ETRA) has stressed the increas-ing importance of material recycling, such as the use ofrecycled postconsumer tires in asphalt pavement mix-tures, to the European Commission.8

Different studies have remarked on the relationshipbetween the rheological characteristics of asphalt bind-ers and field performance,9 illustrating the importanceof understanding the rheological behavior of bitumen.Generally, for pavements and applications concerningbuilding construction, the binder is assumed to sustainsmall deformations, and consequently, its linear vis-coelastic properties can adequately describe its in-service behavior. Thus, these properties may define theresistance of the bitumen to traffic loading (rutting) andcracking due to fatigue.10 However, when road pave-

* To whom correspondence should be addressed. Phone: +34959219989. Fax: +34 959219983. E-mail: partal@uhu.es.

(1) Corbett, L. W. Anal. Chem. 1968, 41, 576-579.(2) Akmal, N.; Usmani, A. M. Polym. News 1999, 24, 136-140.(3) Ait-Kadi, A.; Brahimi, H.; Bousmina, M. Polym. Eng. Sci. 1996,

36, 1724-1733.(4) Takallou, H. B.; Hicks, R. G.; Esch, D. C. Transp. Res. Rec. 1986,

1096, 68-80.

(5) Coplantz, J. S.; Yapp, M. T.; Finn, F. N. Review of Relationshipsbetween Modified Asphalt Properties and Pavement Performance;SHRP-A-631; Strategic Highways Research Program, National Re-search Council: Washington, DC, 1993.

(6) Roberts, F. L.; Kandhal, P. S.; Brown, E. R.; Lee, D. Y.; Kennedy,T. W. Hot Mix Asphalt Materials, Mixture Design and Construction,1st ed.; NAPA Education Foundation: Lanham, MD, 1991; pp 68-382.

(7) Airey, G. D.; Rahman, M. M.; Collop, A. C. Int. J. Pavement Eng.2003, 4, 105-119.

(8) European Tire Recycling Association (ETRA). Summary of thePost-Consumer Tire Market; France, 1999.

(9) Anderson, D. A.; Christensen, D. W.; Bahia, H. U.; Dongre, R.;Sharma, M. G.; Antle, C. E.; Buttom, J. Binder Characterization andEvaluation. 3. Physical Characterization; Strategic Highway ResearchProgram, National Research Council: Washington, DC, 1994.

1984 Energy & Fuels 2005, 19, 1984-1990

10.1021/ef049699a CCC: $30.25 © 2005 American Chemical SocietyPublished on Web 07/28/2005

ments and roofing membranes form ruts or crack, theyare submitted to larger stresses and strains resultingin a nonlinear viscoelastic behavior. In addition, at hightemperatures, when bitumen is handled and mixed withmineral aggregates, compacted, or laid, the flow proper-ties of the binder drastically influence these operations.

A large number of papers dealing with crumb tirerubber modified bitumens (CTRMBs) illustrate thatthey can improve the mechanical properties of thebinder and the resulting asphalt mixes.4-7 Some authorspostulate that the addition of crumb rubber to a bitumenmay improve some mechanical properties of the asphaltpavement.4-6 However, several problems concerningrubber-modified bitumen still remain unresolved, andthe rheological behavior of these systems has not beencompletely described.

The main objective of this work was to study theinfluence of rubber concentration on the rheologicalproperties and microstructure of CTRMBs, over a widerange of temperatures, to obtain information for roadpaving at low rubber concentration and for roofingapplications at high rubber content. High-temperaturestorage stability was also evaluated.

Experimental Section

Materials. The rubber used, supplied by Alfredo MesallesS. A. (Spain), was derived from ambient grind. In other words,the tire was torn at room temperature and then ground. Themean particle size of the particles, determined by screening,was 0.29 mm. The composition of the crumb rubber is shownin Table 1. A neat bitumen, 60/70 penetration grade and asoftening point (ring and ball) of 52 °C, donated by Construc-ciones Morales (Spain), was used as the base material forpolymer modification. The bitumen composition, determinedby the procedures outlined in ASTM D3279 and ASTM D4124,was 5.8% saturates, 54.4% aromatics, 19.6% resins, and 20.2%asphaltenes.

Preparation of Mixtures and Samples. Blends of bitu-men and crumb rubber were prepared by mechanical stirringin an open low-shear batch mixer (80 mm diameter) using anIKA RW-20 agitator (Germany) equipped with a four-bladepropeller (50 mm diameter). Crumb rubber particles wereslowly added at the initial stages of the mixing process andspecial care was taken to avoid the production of lathers.Samples were processed for 1.5 h at 180 °C with a rotationalspeed of 1200 rpm. After the material was mixed, the resultingbinder was poured into a small can and then stored in a freezerat -25 °C to retain the obtained morphology. The rubberconcentration was varied from 5 to 40 wt %. Higher temper-atures (210 °C) were necessary to prepare the modifiedbitumens with the largest rubber contents (30 and 40%).

Methods of Analysis. Frequency sweep tests between 0.01and 100 rad/s, in the linear viscoelasticity range, wereperformed in a controlled-stress Haake RS150 rheometer(Germany) using a serrated plate-and-plate geometry (10 and20 mm diameter). Stress sweep tests, at the frequency of 1Hz, were previously carried out on each sample to determine

the linear viscoelasticity region. Time sweep tests, at aconstant stress and frequency, were also performed to confirmthat no structural modifications occurred during the timerequired for each test. Measurements were done in a temper-ature range between -10 and 90 °C. Dynamic mechanicalanalysis tests at a constant amplitude, within the linearviscoelastic region (<1%) and frequency (0.1 Hz), were per-formed at a heating rate of 1 °C/min from -20 °C to 90 °C.

Flow measurements, at 50 °C and 135 °C were performedin a controlled-strain Rheometrics Ares rheometer (Germany)using a plate-and-plate geometry (25 and 50 mm diameters).

Studies on the wall effects of the larger particles werepreviously carried out. Thus, different gaps with parallel plategeometries (2-4 mm) and no gap-dependent geometries (vanegeometries) were used to examine wall effects on the platesurface and sample spilling on parallel plate configurations.Furthermore, the experimental conditions were selected toavoid settling of the rubber particles during the measurement.As a result, steady flow tests conducted with wide enough gapsshowed viscosity values similar to those obtained with vanegeometries, where the interactions of particles with themeasurement surfaces are not significant.

Static storage tests were used to determine the storagestability of CTRMBs. The experimental system consisted of atube (3.2 cm diameter and 40 cm height) placed vertically inan oven at 180 °C for 72 h. After storage, four samples weretaken at different heights of 0 (bottom), 10, 20, and 30 cm,and frequency sweeps measurements, at 50 °C, were per-formed on each sample.

MDSC experiments were developed with a Q100 TA Instru-ments (USA) using 10-20 mg samples, sealed in hermeticaluminum pans. An oscillation period of 60 s, amplitude of(0.50 °C, and a heating rate of 5 °C/min were used for alltests. The temperature ranged from -90 to 165 °C. The samplewas purged with nitrogen at a flow rate of 50 mL/min. To makesure that all the samples studied had the same thermalhistory, only the results obtained during the second heatingscan have been handled. The DSC heat flow was initiallycalibrated using a traceable indium standard (∆Hf ) 28.3 J/g),and the heat capacity response was calibrated using a sapphirestandard.

Results and Discussion

Linear Viscoelastic Behavior. One of the mostsignificant aspects of the rheological behavior of bitu-men is its dependence on temperature. Figure 1 showsthe frequency dependence of the storage, G′, and loss,G′′, moduli in a wide temperature window (-10 to 90°C), for samples containing 5 and 40 wt % rubber. Asmay be seen in Figure 1, parts A and B, the viscoelasticfunctions of a 5 wt % rubber CTRMB display the typicalbehavior of a neat bitumen. A crossover between theelastic and viscous moduli is observed at low tempera-tures (-10, 5, and 25 °C), while the values of G′′ becomelarger than those of G′ over the whole frequency windowstudied, as temperature progressively increases. Con-sequently, the transition to the glassy region in themechanical spectrum of these systems is mainly ob-served at low temperature, whereas the terminal orviscous region appears at high temperature. There isno “plateau” region occurring at any of the temperaturesstudied so that entanglement effects are not significant,and therefore, a continuous transition from the glassyto the Newtonian region takes place by decreasing thefrequency.10,11

(10) Dongre, R.; Youtcheff, J.; Anderson, D. Appl. Rheol. 1996, 6,75-82.

(11) Partal, P.; Martınez-Boza, F. J.; Conde, B.; Gallegos, C. Fuel1999, 78, 1-10.

Table 1. Composition of the Crumb Rubber

material wt %

total rubber hydrocarbon(natural and synthetic rubber)

50

carbon black 32THF extractable 11ash 4

Crumb Rubber Modified Bitumen Energy & Fuels, Vol. 19, No. 5, 2005 1985

On the contrary, a CTRMB containing 40 wt % crumbrubber (Figure 1, parts C and D) shows values of thestorage modulus that are higher than those of the lossmodulus in the whole temperature range studied, givingclear indication of the predominantly elastic characterof this system. It is noticeable that, at low frequencies,the curves of G′ exhibit a slight flattening whichbecomes more pronounced as temperature increases,favoring the occurrence of a “plateau” region in the low-frequency zone studied at high temperature. Thisbehavior has often been observed for polymer/rubbercomposites at elevated concentrations and has beenassociated with particle-particle interactions forminga three-dimensional network.12 On the other hand, itis important to underline that the differences betweenthe values of G′, or G′′, at extreme temperatures areclearly dampened as the rubber content in the bitumenincreases, denoting a significant reduction in the mate-rial thermal susceptibility.

In this sense, Figure 2 shows the effect of rubberconcentration on the linear viscoelasticity functions attwo extreme temperatures, -10 °C and 75 °C, and afrequency of 10 rad/s. As can be observed, at -10 °Cthe values of the storage modulus are higher than thosecorresponding to the loss modulus and a remarkablemodulus reduction is observed with increasing polymercontent. This fact reveals an increase in binder flex-ibility and, therefore, better thermal-cracking resis-tance. On the contrary, at 75 °C a large increase in thestorage and loss moduli values with rubber addition isclearly noticed. In addition, it is worth noting that at lowrubber content, G′′ values are clearly higher than G′ ones;at 30 wt % rubber content the values are quite similar,and at 40 wt % rubber content the elastic modulusexceeds the viscous modulus. Consequently, a signifi-cantly improvement of the rutting resistance of theresulting pavement is expected at this temperature.13,14

Moreover, an increase in crumb rubber concentrationleads to a much more complex thermorheological be-havior, as may be deduced from the Black diagrams,where the loss tangent (tan δ ) G′′/G′) is plotted vs thecomplex modulus (G*) at different temperatures.15 Ascan be seen in Figure 3, neat bitumen and 5 and 9 wt% rubber CTRMBs present a single and continuouscurve for temperatures between -10 and 90 °C and,therefore, can be empirically superposed onto a mastercurve. On the contrary, for higher rubber contents (20,30, and 40%) the curves obtained at each temperatureare clearly dispersed, do not overlap onto a master one,and consequently, they cannot be empirically super-posed. This case is indicative of relaxation times (or

(12) Bousmina, M.; Muller, R. Rheol. Acta 1996, 35, 369-381.

(13) Adedeji, A.; Grunfelder, T.; Bates, F. S.; Macosko, C. W.; Stroup-Gardiner, M.; Newcomb, D. E. Polym. Eng. Sci. 1996, 36, 1707-1723.

(14) Lu, X.; Isacsson, U. Polym. Test. 2001, 20, 77-86.(15) Mavridis, H.; Shroff, R. N. Polym. Eng. Sci. 1992, 32, 1778-

1791.

Figure 1. Frequency sweep tests for selected CTRMBs (5 and 40 wt % rubber) as a function of temperature.

Figure 2. Evolution of the storage and loss moduli (ω ) 10rad/s) with rubber concentration at two selected temperatures(-10 °C and 75 °C).

1986 Energy & Fuels, Vol. 19, No. 5, 2005 Navarro et al.

moduli) having nonuniform temperature dependence,which is characteristic of certain multiphase sys-tems.15,16 It is well-known that anisotropic or biphasicmaterials do not hold the time temperature superposi-tion principle.17 In this sense, it is clear that thenonhomogeneity of a modified bitumen becomes morepronounced as rubber concentration increases, resultingin increasing deviations of the thermorheological sim-plicity.

On the other hand, as can be observed in the Blackdiagrams (Figure 3), lower values of loss tangent areobtained as rubber concentration increases in the high-temperature or low-frequency region, which points outthat the addition of crumb rubber leads to a binder thathas a more elastic response in the above-mentionedcircumstances. It is important to underline that themost significant drop in tanδ is observed for the binderwith 20 wt % rubber concentration, with values higherthan 1 in the low-frequency range and minimum values(lower than 1) at high frequencies. The sample contain-ing 30 wt % rubber presents values close to 1 practicallyin the whole frequency window studied, while the 40wt % rubber binder shows values well below 1 in thelow-frequency region. Furthermore, CTRMBs with lowrubber content (up to 9 wt %) undergo a great variationin loss tangent (over 2 orders of magnitude), whereas

CTRMBs with higher rubber content undergo muchmore moderate changes. These facts confirm the above-mentioned reduction in thermal susceptibility and seemto hint at some microstructural changes as rubbercontent increases.

Viscous Behavior. Figure 4 shows the effect ofrubber concentration on the viscous behavior of CTRMBsat 50 and 135 °C. The viscous flow response changesfrom a Newtonian behavior, in a wide range of shearrates (i.e., neat bitumen 60/70), to a shear-thinningbehavior (rubber-modified bitumen). This shear-thin-ning behavior is more apparent as rubber concentrationincreases. As can be observed in the above-mentionedfigure, the flow curves always show a trend to reach aconstant limiting viscosity at low shear rates, η0, fol-lowed by a power law decrease in viscosity as the shearrate increases. Furthermore, a clear tendency to reacha Newtonian viscosity at high shear rates, η∞, is alsoobserved at high temperature (135 °C). This viscousbehavior may be fairly accurately described by the Crossmodel:

where λ is a characteristic time related to the onset ofthe shear thinning region and n is related to the slopeof the power law behavior. The Cross model parameterscalculated for the different binders are shown in Table2. The results show an evident increase in viscosity, λ,and n with rubber content; similar results have beenreported by other authors.18,19 These results point to thedevelopment of a more complex microstructure as crumbrubber concentration increases.

(16) Carriedo, G. A.; Gomez-Elipe, P.; Garcıa Alonso, F. J.; Lizaso,E.; Munoz, M. E.; Santamarıa, A. Macromol. Chem. Phys. 2001, 202,3437-3443.

(17) Kim, S. O.; Shin, W. J.; Cho, H.; Kim, B. C.; Chung, I. J. Polymer1999, 40, 6443-6450.

Figure 3. Black diagrams of the neat bitumen and CTRMBsstudied: (A) neat bitumen, 5 and 9 wt % rubber CTRMBs; (B)20, 30, and 40 wt % rubber CTRMBs.

Figure 4. Viscous flow curves of neat bitumen and CTRMBs,as a function of rubber concentration, at 50 (A) and 135 °C (B).

η - η∞

η0 - η∞) 1

1 + (λγ)n(1)

Crumb Rubber Modified Bitumen Energy & Fuels, Vol. 19, No. 5, 2005 1987

It is important to underline that the differencesbetween the flow curves obtained at 50 and 135 °C arelower as rubber concentration increases. This factconfirms the above-mentioned reduction in the thermalsusceptibility of the material, in the referred tempera-ture range, and may be related to a notable improve-ment of the mechanical in-service behavior of themodified bitumen. However, in road applications, bitu-men must be heated at safe temperatures to obtainenough low viscosities for mixing and compaction opera-tions. In this way, AASHTO MP1 requires that thebinder viscosity measured at 135 °C should be less than3 Pa‚s.18 According to this limit, CTRMBs with rubbercontent higher than 5% should not be used for roadapplications. Nevertheless, it is worth mentioning thatthis criterion may be violated if the asphalt can bepumped and mixed at safe temperatures. Thus, al-though the CTRMB containing 9% of crumb rubberexhibits higher viscosity than the above-mentioned limitat shear rates lower than 10 s-1, it satisfies this criterionat higher shear rates, well inside the range where itwould be handled and mixed with mineral aggregates.20

Thermal Behavior and Microstructure. Figure 5shows the evolution of the complex modulus and the losstangent of neat and rubber-modified bitumens withtemperature obtained from temperature sweep tests.Rubber addition to bitumen yields a decrease in thecomplex modulus of the binder at low temperatures,while the opposite effect is observed at high tempera-tures (see Figure 5A). Furthermore, as can be observedin Figure 5B, the loss tangent values are lower as rubberconcentration increases, above all at high temperatures,due to the elasticity of the binder clearly being en-hanced. In summary, the glassy region of the mechan-ical spectrum is shifted to lower temperatures and thebinder presents enhanced viscoelastic properties in thewhole in-service temperature range.

During CTRMBs processing, rubber is normally dis-integrated, dissolved, and/or reduced in molecular size,as a consequence of devulcanization and depolymeriza-tion processes.18,21 However, previous data obtainedwith a 9% rubber CTRMBs indicated that, at 180 °C,only 4% of the structural network of the added rubberwas dissolved or disintegrated during the sample diges-

tion.20,22 Hence, the mechanical properties of the modi-fied bitumen are influenced by the remaining particlesof rubber as well as by the bituminous phase. Moreover,the swelling process of the rubber particles by some lightcomponents of the bitumen may modify both the elasticcharacteristic of the filler and the bitumen colloidalmicrostructure. The above-mentioned swelling processand its effect on the thermal behavior of the modifiedbitumen have been analyzed by MDSC experiments(Figure 6).

MDSC curves for bitumen are extremely dependenton thermal and shear history.23 For this reason, thethermal characterization of bituminous blends has beencarried out on samples that were previously submittedto heating above their melting temperature and furthercooling to -60 °C. Therefore, all the samples had thesame thermal history. As can be seen in Figure 6, MDSCfor neat bitumen and CTRMBs reveal different thermalevents. Thus, the total heat flow curves of these bindersindicate glass transitions at temperatures below 0 °Cand weak transitions at higher temperatures. On thecontrary, fresh crumb rubber undergoes a continuousdrop of the heat flow with temperature, and no glasstransitions are noticeable in the temperature rangestudied. As has been reported by other authors,24 abitumen may show multiple glass transition tempera-tures, since its fractions (i.e., saturates, aromatics,(18) McGennis, R. B. Transp. Res. Rec. 1995, 1488, 62-71.

(19) Zaman, A. A.; Fricke, A. L.; Beatty, C. L. Transp. Eng. 1995,121, 461-467.

(20) Navarro, F. J.; Partal, P.; Martınez-Boza, F.; Gallegos, C. Fuel2004, 83, 2041-2049.

(21) Zanzotto, L.; Kennepohl, G. J. Transp. Res. Rec. 1996, 1530,51-58.

(22) Navarro, F. J.; Partal, P.; Martınez-Boza, F.; Valencia, C.;Gallegos, C. Chem. Eng. J. 2002, 89, 53-61.

(23) Masson, J. F.; Polomark, G. M. Thermochim. Acta 2001, 374,105-114.

Table 2. Cross Model Parameters of theRubber-Modified Binders Studied as a Function of

Rubber Concentration, at 50 and 135 °C

concn(wt %) temperature η0 (Pa·s) η∞ (Pa·s) n λ (s)

5 50 °C 1.318 × 104 0.32 1.129 8.194 × 104 0.41 1.319 × 101

20 5.356 × 105 0.46 3.5198 × 102

30 1.379 × 106 0.59 5.384 × 102

40 5.676 × 106 0.65 4.530 × 102

5 135 °C 4.458 0.436 0.17 4.349 1.918 × 101 2.639 0.62 7.6420 1.435 × 104 112 1.01 3.462 × 103

30 3.182 × 104 253 0.90 1.512 × 103

40 1.514 × 106 303 0.91 6.14 × 102

Figure 5. Evolution of the complex modulus and the losstangent with temperature for neat bitumen and CTRMBs withdifferent rubber concentration.

1988 Energy & Fuels, Vol. 19, No. 5, 2005 Navarro et al.

resins, and asphaltenes) display at least two Tg’s. In thiscase, no systematic evolution of any glass transitiontemperature has been found.

The nonreversing heat flow curves (Figure 6B) pointout the presence of overlapping thermal effects thatgovern the bitumen metastability, which may be as-sociated with a specific bitumen substructure or phase.23

A broad endothermic background from -60 to 80 °C,approximately, is clearly noticeable for all the bitumi-nous samples. This is the most rapid event and arisesmost likely from the melting of partly crystallinesaturates and from the ordering of simple aromaticstructures. On the other hand, a second observablethermal event is an exotherm that centers around -7°C and can be reduced with time due to self-ordering.This ordering may arise from the diffusion or assemblyof structures larger than those responsible for thebackground, e.g., naphthalene-like aromatics, to producea mesophase.23,24 Further thermal events can be foundin the high-temperature region, between 20 and 80 °Cprobably due to the slow diffusion of relatively large andhigh molecular weight structures (asphaltenes, multi-ring, and multiarm structures) to form independentdomains.23,24

As can be observed in Figure 6, the addition of crumbrubber reduces significantly the extension of the back-ground (first event) and the exotherm (second event),probably due to the absorption of some components ofthe maltene fraction by the rubber.7,25,26 Moreover, 85%of the total endothermic background arises from theisotropisation of the aromatics, whereas only 15% arisesfrom the melting of partly crystalline saturates.23 Theresults obtained would suggest that the swelling processof rubber could be mainly attributed to the aromaticfraction of maltenes.

The rubber particle size used in this study (.5 µm)can be considered large enough to exclude the possibilityof any colloid-chemical activity and neglect Brownianmotion so that the observed rheological behavior should

be explained in terms of hydrodynamical/mechanicalinteractions between rubber particles swollen by lightcomponents of the bituminous phase, leading to aphysical gelation process with increasing rubber addi-tion.27,28 Thus, binders containing 30% rubber can beconsidered as a critical rubber concentration for a sol-gel transition in a wide range of temperatures, that is,a state of the matter between liquid and solid describedby a three-dimensional macromolecular network madeof particle aggregates.12

Storage Stability. Phase separation in CTRMBsduring storage at high temperature has been reportedin different studies and has been related to the presenceof nondissolved rubber particles.20,29 A storage stabilityindex has been defined as the ratio between the valueof the elastic modulus at 0.1 rad/s and 50 °C, at a givendistance from the bottom of the settling tube, and thesame parameter calculated on a fresh sample (notstored).30 Similar stability indexes, in the whole rangeof settling heights, indicate stable modified bitumens.As can be seen in Figure 7, the storage stability indexof a binder containing 5% rubber presents a remarkablereduction from 0 cm (bottom) to 10 cm of height,remaining practically constant at larger distances fromthe bottom of the settling tube. This fact demonstratesthat the rubber has settled, and consequently, thismodified bitumen is unstable under the selected storageconditions. At higher rubber content the values of thestability index do not change along the tube height, andtherefore, these CTRMBs remain stable after 72 h.However, as a consequence of a further digestion ofrubber particles, the indexes of the stable systemspresent values below one.20 The enhanced storagestability with higher rubber concentration can be ex-plained by taking into account the increased interpar-ticle interactions, leading to the above-mentioned ex-

(24) Masson, J. F.; Polomark, G. M.; Collin,s P. Energy Fuels 2002,16, 470-476.

(25) Frantzis P. J. Mater. Civ. Eng. 2004, 16, 387-390.(26) Miknis, F. P.; Michon, L. C. Fuel 1998, 77, 393-397.

(27) Das, S.; Murthy, V. S. R.; Murty, G. S. J. Mater. Sci. 1999, 34,1347-1352.

(28) Pogodina, N. V.; Winter, H. H. Macromolecules 1998, 31, 8164-8172.

(29) Bahia H. U.; Zhai H. Prepr. - Am. Chem. Soc., Div. Fuel. Chem.1998, 43, 1041-1045.

(30) Gonzalez, O.; Munoz, M. E.; Santamarıa, A.; Garcıa-Morales,M.; Navarro, F. J.; Partal, P. Eur. Polym. J. 2004, 40 (10), 2365-2372.

Figure 6. Modulated differential scanning calorimetry (MDSC) tests carried out on neat bitumen, crumb rubber, and CTRMBs:(A) total heat flow curves; (B) nonreversing heat flow curves.

Crumb Rubber Modified Bitumen Energy & Fuels, Vol. 19, No. 5, 2005 1989

tended network, which increases the bulk viscosity ofthe system, avoiding particle settling. However, rubberconcentration should not be considered as the uniqueand decisive criterion for CTRMBs stability, since otherfactors may govern the settling characteristics of rubberparticles in a bituminous phase, such as particle sizeand shape.20

Conclusions

The results obtained indicate that the rheology,microstructure, and storage stability of the modifiedbinders studied are largely dependent on the rubber

content. The addition of crumb rubber to bitumendecreases the elastic and viscous moduli at low tem-peratures and, therefore, causes an increase in binderflexibility. On the contrary, at high temperatures asignificant increase in both moduli and a notable dropin the loss tangent values are observed, resulting in amore elastic binder. Furthermore, from the linearviscoelastic and viscous flow results obtained, it can bededuced that the thermal susceptibility of the binder isclearly reduced as a consequence of rubber addition.Consequently, enhanced resistance to permanent de-formation, or rutting, low-temperature cracking, andfatigue cracking should be expected in the resultingasphalt mixtures. A maximum amount of 9% should notbe exceeded for a binder to be considered adequate forpaving applications. However, higher rubber contentsare of interest for roofing and other bitumen uses.

After processing, a large amount of rubber particlesremain dispersed, although they are swollen by somecomponents of the light fraction of the maltenes. Therheological changes observed with rubber addition havebeen associated to particle-particle interactions, form-ing a three-dimensional network of particle aggregates.These interactions are responsible for a better storagestability at 180 °C with increasing rubber content.

Acknowledgment. This work is part of a researchproject sponsored by a MCYT-FEDER program, (Re-search Project MAT2001-0066-C02-02). The authorsgratefully acknowledge its financial support.

EF049699A

Figure 7. Effect of rubber content on the stability index ofrubber-modified bitumen.

1990 Energy & Fuels, Vol. 19, No. 5, 2005 Navarro et al.