influence of processing conditions on the rheological behavior of crumb tire rubber-modified bitumen

Influence of Processing Conditions on the RheologicalBehavior of Crumb Tire Rubber-Modified Bitumen

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

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

Received 2 August 2006; accepted 6 November 2006DOI 10.1002/app.25800Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Traditional or technological studies aboutcrumb tire rubber-modified bitumens (CTRMBs) do notprovide detailed rheological information. In that way, thisarticle describes the influence of processing conditions onthe linear viscoelastic and viscous behaviors of CTRMBs.The results of the study reveal an exponential increase indissolved/dispersed rubber with processing temperatureand, therefore, lower solid content that affect the rheologicalbehavior in different ways. No influence of the processingdevice was observed, probably due to the fact that process-ing temperature (1808C) was not high enough to break upthe crosslinked network of the rubber. The presence of rub-ber particles avoids the negative hardening effects observedfor unmodified bitumens, resulting in a more flexible binder

at low temperature. At high in-service temperature, betterrutting resistance would also be expected for CTRMBs.Flow results at 1358C indicates that modified bitumens cansatisfy the AASHTO MP1 requirements if high-enoughshear rates are reached. All the CTRMBs studied presentsegregation, due to rubber settling during storage at hightemperature, although an increase in processing tempera-ture seems to enhance its stability. The results obtainedseem to indicate that the optimum CTRMB processing tem-perature is 2108C. � 2007 Wiley Periodicals, Inc. J Appl PolymSci 104: 1683–1691, 2007

Key words: modified bitumen; binder; asphalt; rheology;crumb rubber; stability

INTRODUCTION

Bitumen, from crude oil distillation process, is a com-plex mixture of organic compounds that can be clas-sified, according to their polarity, into two maingroups: maltenes and asphaltenes. Asphaltenes arehigh-molecular-weight species that are insoluble inn-heptane, whereas maltenes have lower molecularweight and are soluble in this solvent.1,2The wide-spread use of bitumen relies on its remarkable water-proofing and binding properties that makes itadequate for road pavement and roofing applica-tions.3

Ideal performance demands that the binder mustbe able to withstand low temperatures and the result-ing thermal stresses that develop pavement shrinks,resist repeated loading and unloading without exhib-iting fatigue failure (cracking), withstand loading toprevent permanent deformation (wheel path rutting),and be capable of being placed, compacted, trans-ported, and stored at safe temperatures.4–7

However, increased traffic factors such as heavierloads, higher traffic volume, and high tire pressure

have tightened pavements specifications, and conse-quently, new materials with improved mechanicalproperties have been developed. The addition of vir-gin polymers to bitumen is nowadays a good way toobtain modified bitumens with improved properties.In that sense, a large number of papers focused on theuse of these polymeric modifiers, like thermoplasticelastomers, polyethylene, block copolymers, etc., havebeen published.5,6,8,9 However, the high cost of thesematerials related to bitumen increases the price of theresulting binder and limits the maximum amount ofpolymer, making the recycled polymers an interestingalternative from both economical and environmentalperspectives.10 In this sense, powdered rubber, fromdiscarded tires, is one of the most interesting alterna-tives, since the European legislation prohibited alllandfill of scrap tires by 2006.11 Thus, an extensivestudy on the use of ground tire rubber in asphaltbinders has been performed, pointing out theimproved mechanical properties of the binder andthe resulting asphalt mixes.12–16 These improved ben-efits include increased fatigue life or fatigue resist-ance, reduced reflective cracking and low tempera-ture cracking, improved tensile strength, ductility,toughness, adhesion, resilience, tenacity, durability,skid resistance, and finally resistance to rutting.14–16

However, the existing literature regarding crumb tirerubber modifiers in bituminous binders has beenfocused on using either conventional binder testingprocedures or the Superpave protocol.12–16 It is

Correspondence to: F. J. Navarro ([email protected]).Contract grant sponsor: Ministerio de Medio Ambiente

(MMA) program, Spain; contract grant number: 51-212/2005/3B.

Journal of Applied Polymer Science, Vol. 104, 1683–1691 (2007)VVC 2007 Wiley Periodicals, Inc.

believed that Superpave binder specifications cannotbe extended to all modified binders, since certain sim-plifications used in the current specification cannotestimate their contribution to pavement performanceunder different traffic and pavement structure condi-tions.17 In this sense, a more detailed study of therheological properties of crumb tire rubber-modifiedbitumens (CTRMBs) is necessary to understand thebinder microstructure and performance in a widerange of frequencies and temperatures.

As has been previously reported, the rheologicalproperties of CTRMBs are largely influenced by dif-ferent factors such as rubber concentration, particlesize, rubber-grinding method bitumen composition,processing conditions, etc.18–24 As a consequence, themain objective of this article was to study the influ-ence that several processing devices and curing tem-peratures exert on the rheological and morphologicalcharacteristics of the resulting CTRMB. Additionally,the high temperature storage stability of CTRMBswas also evaluated.

EXPERIMENTAL

A 60/70 penetration grade bitumen, donated by Con-strucciones Morales SA (Spain), was used as a basematerial for all the experimental mixes. As a modify-ing agent, a powdered waste rubber, donated byAlfredo Mesalles SA (Spain), derived from discardedtires without metallic and textile contaminants wasutilized. This crumb rubber was obtained by grindingtires at ambient temperature and subsequent sizeclassification by screening, having a mean particlesize of 0.35 mm. Some compositional characteristicsof the crumb rubber used are shown in Table I.

CTRMBs were prepared by mixing crumb rubber(9 wt %) with base bitumen, at different selected tem-peratures (between 90 and 2508C), for 1.5 h. Two dif-ferent homogenizers were used: a lab-scale mixingdevice and a pilot plant rotor–stator device. The pilotplant device incorporates a rotor–stator mixer SD41SUPER-DISPAX from IKA (Germany) and operates at8200 rpm and a processing temperature of 1808C. Thelab-scale blends were mixed in a low-shear batchmixer using a stirring motor (RW20) from IKA (Ger-many) and different stirrers: a four-blade impeller, ata rotational speed of 1200 rpm; and an anchor stirrer

and a helical stirrer at a rotational speed of 100 rpm.After processing, all the samples were immediatelystored at �58C, to avoid phase separation.

Blank samples, i.e., bitumen processed withoutcrumb rubber, were also obtained from both lab andpilot plant devices at 180 and 2508C (Bitumen-180,Bitumen-250).

Asphaltene content of unmodified bitumens wasanalyzed by the procedure outlined in ASTM D 3279-97. The remaining solid rubber after processing wasdetermined by a filter test described in a previouspaper.25

Linear viscoelastic and viscous flow measurementswere performed using a CS-rheometer, RheoStressRS150, from HAAKE GBR (Germany). Frequencysweep (0.01–100 rad/s) tests were carried out at con-stant temperature (between �10 and 758C), within thelinear viscoelastic range, using a serrated plate-and-plate geometry (10 and 20 mm diameter, 1–3 mmgap). Viscous flow measurements were performed at50 and 1358C with a serrated plate-and-plate geome-try (30 and 35 mm diameter, 1–3 mm gap).

Previously, different studies concerning wall effects(influence of the ratio between plate-and-plate gapand particle size) were performed. Thus, differentgaps with parallel plates (1–3 mm) and mixing rhe-ometry geometries (vane and helical ribbon) wereused. At temperatures below the settling point of theparticles, viscous flow tests conducted with wideplate-and-plate gaps showed viscosity values similarto those obtained with vane geometries, where walleffects are not significant.

The morphology of the resulting CTRMBs wasstudied by optical microscopy. A LTS-350 Heating-Freezing Stage, manufactured by Linkam ScientificInstruments (UK), coupled to a standard OlympusBX51 microscope, from Olympus Optical (Japan), wasused. Samples were placed on the hot stage of the de-vice at 758C.

Storage stability was evaluated by a test proceduredeveloped in our laboratory, storing vertically sam-ples in cooper tubes, at 1808C, for 12 h. More detailedinformation on this method can be found elsewhere.19

The storage stability was followed by carrying out fre-quency sweep tests in the linear viscoelasticity range,at 508C, on samples taken at different heights (0, 10,20, and 30 cm) of the testing tube.

RESULTS AND DISCUSSION

Although bitumen rheological thermosimplicity isquite controversial,26–28 empirical master curves ofthe linear viscoelasticity functions, as a function offrequency, may be obtained when the values of tan dversus complex modulus for bitumen, at differenttemperatures, match on a unique curve,20 giving aclear insight on its behavior in a wider range of time

TABLE ICompositional Characteristics of the Fresh Crumb

Rubber Used Non-processed

Material wt %

Total rubber hydrocarbon(natural and synthetic rubber) 50

Carbon black 32THF extractable 11Ash 4

1684 NAVARRO ET AL.

Journal of Applied Polymer Science DOI 10.1002/app

and temperature. Thus, as can be seen in Figures 1and 3, the frequency dependence of the linear visco-elastic functions of the different neat and modifiedbitumen studied, obtained at different temperatures(between �10 and 758C), can be empirically super-posed onto master curves, at a reference temperatureof 258C, using an empirical shift-factor aT.

The empirical shift factor always follows an Arrhe-nius-like dependence with temperature:

aT ¼ expEa

R

1

T� 1

T0

� �� (1)

where T0 is the reference temperature, 298 K, and Ea

is the activation energy. Table II shows the activationenergy values estimated from eq. (1). These valuesare quite similar to those previously reported inthe literature for other neat and modified bitumensamples.29

Influence of the processing device onthe rheology of CTRMBs

Previous results obtained by the authors demon-strated that the addition of 9 wt % crumb tire rubber(mean particle size: 0.35 mm) to a 60/70 penetrationgrade bitumen (processed at 1808C for 1.5 h) yieldedsimilar rheological characteristics to those obtainedby the addition of 3 wt % SBS, a polymer widelyused, to the same bitumen.25 Consequently, theseprocessing conditions were selected to study theinfluence of the processing device on the rheologicalcharacteristics of CTRMBs. Figure 1 shows the evolu-tion of the complex modulus and the loss tangentwith frequency, at a reference temperature of 258C,for modified bitumens processed with different devi-ces. Results from blank samples of unmodified bitu-mens have been also included in this figure. Negligi-ble differences among the values of both viscoelasticfunctions for CTRMBs processed in different devicesare observed. These results can be explained takinginto account that all the samples have the sameamount of dissolved/dispersed rubber (Table III).Consequently, similar in-service behavior is expectedfor the samples studied, independent of the selectedprocessing device.

On the other hand, when modified and unmodifiedbitumen samples are compared, it is clearly noticedthat the addition of rubber leads to an increase in thecomplex modulus values in the low frequency regionand also to a slight decrease in the high frequencyzone. Upon the Strategic Highway Research Program(SHRP) reports, this linear viscoelasticity function isrelated to major distress modes in road pavements,and represents a measure of the total resistance of amaterial to deformation when exposed to repeatedpulses of shear stress and can be directly relatedto the stiffness of the material.30,31 Thus, CTRMBsbecome harder at high temperatures and more flexi-ble at low temperatures, and consequently, improvedin-service behavior should be expected.

Figure 1 Master curves of the complex shear modulus(A) and loss tangent (B) for unmodified bitumen andCTRMBs processed in different devices at 1808C (referencetemperature: 258C).

TABLE IIActivation Energies and Zero-Shear-Rate-Limiting Viscosities,

at 258C, for Non-modified Bitumens and CTRMBs Manufacturedat Different Processing Conditions

Sample Process deviceProcess

temperature (8C) Ea (kcal/mol) Z0 (Pa s)

Neat bitumen – – 139 1.227 � 105

Bitumen-180 Laboratory 4-blade 180 142 1.765 � 105

Bitumen-250 Laboratory 4-blade 250 150 4.048 � 105

CTRMB Pilot plant 180 146 1.92 � 106

CTRMB Laboratory anchor 180 150 1.489 � 106

CTRMB Laboratory helical 180 148 1.380 � 106

CTRMB Laboratory 4-blade 180 148 1.750 � 106





RHEOLOGY OF CRUMB TIRE RUBBER-MODIFIED BITUMEN 1685


On the other hand, as can be observed in Figure1(B), a significant reduction in the loss tangent valuesis observed for CTRMBs, mainly in the low frequencyrange. This fact indicates an important increase inbinder elasticity, which significantly contributes tothe improvement of the high temperature propertiesof the resulting asphalt. Additionally, at intermediatefrequencies, a pseudoplateau in tan d is clearlynoticed for modified bitumens, suggesting somemicrostructural changes as a consequence of rubberaddition. Thus, for unmodified bitumens there is asimple transition from the glassy to the terminalregion, whereas for CTRMBs an intermediate regionappears as a result of the modification. Given thatonly a small fraction of the added rubber is dissolvedor dispersed during processing, this behavior shouldbe mainly attributed to the presence of undissolvedrubber after processing. As previously reported, therubber particle size used in this study (> 5 mm) canbe considered large enough to exclude the possibilityof any colloid-chemical activity and neglect Brownianmotion, so that the observed rheological behaviormust be explained in terms of hydrodynamical/mechanical interactions between particles in the bitu-minous phase.20,25,32

The dynamic linear viscoelastic behavior of thesesystems may be described by a generalized Maxwellmodel:

=G�= ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiGe þ

XNi¼1

GiðoliÞ2

1þ ðoliÞ2 !2

þXNi¼1

Gioli

1þ ðoliÞ2 !2

vuut(2)

tan d ¼�N

i¼1Gioli

1þðoliÞ2

Ge þ�Ni¼1Gi

ðoliÞ21þðoliÞ2

(3)

where Ge is the elastic modulus. For the sake ofclarity, the recalculated viscoelastic functions havenot been included in Figure 1.

This model considers a superposition of a series ofN independent relaxation processes, each processhaving a relaxation time li and a relaxation strengthGi. The resulting distribution or spectrum of relaxa-tion times may be used to obtain the zero-shear-limit-ing viscosity of the material, Z0, as follows:

Z0 ¼XNi¼1

Gili (4)

The values of Z0 estimated from eq. (4), at 258C, arepresented in Table II. As expected, the zero-shear-rate-limiting viscosity values of the CTRMBs pro-cessed at 1808C are quite similar. On the contrary,they are much higher than those corresponding tounmodified bitumen samples.

On the other hand, the shear rate dependence of vis-cosity, at 508C, for the aforementioned unmodifiedbitumen and CTRMBs samples is presented in Figure 2.As can be observed, all the flow curves show an appa-rent limiting viscosity in the low shear rate range,Z0, followed by a power-law decrease in viscosity. TheCarreau model fits these flow curves fairly well.

Z ¼ Z0

1þ lc _gð Þ2� �ð1�nÞ=2 (5)

where Z0 is the zero-shear-rate limiting viscosity, n isa dimensionless parameter, n � 1 being the slope ofthe shear-thinning region, and lc is a characteristictime of the material, defined as lc ¼ 1=_gc, where _gc isthe critical shear rate for the onset of this intermediateregion. The values of the Carreau parameters are pre-sented in Table IV.

TABLE IIISolubilized Rubber Percentage in the Modified

Bitumen Samples Studied

SampleProcessingdevice

Processingtemp. (8C) % solubilized

Raw rubber 11CTRMB Pilot plant 180 15CTRMB Laboratory anchor 180 15CTRMB Laboratory helical 180 15CTRMB Laboratory 4-blade 180 15CTRMB Laboratory 4-blade 90 11CTRMB Laboratory 4-blade 120 11CTRMB Laboratory 4-blade 210 25CTRMB Laboratory 4-blade 250 45

Figure 2 Viscous flow curves, at 508C, for unmodifiedbitumen and CTRMBs processed in different devices at1808C.

1686 NAVARRO ET AL.


At this temperature, 508C, CTRMBs show, onceagain, similar viscosity values, as a consequence ofcomparable rubber digestion levels.

As can also be observed, rubber addition to bitumenhas different effects on Carreau’s model parameters.Thus, an increase in viscosity is noticed, a fact that mayfavor an increase in the rutting resistance of theblends.33,34 On the other hand, a change in the viscousbehavior is shown, since rubber addition produces aremarkable decrease in n and, therefore, an increase inthe slope of the shear-thinning region. In addition, lcclearly increases for CTRMBs and, consequently, theNewtonian region is shifted to lower shear rates whencomparedwith unmodified bitumens.

Finally, it is worth pointing out that, although thereis no influence of the processing device characteristicson the rheological behavior of CTRMBs, some differ-ences are observed for unmodified bitumens. Thus,samples processed in the pilot plant device displaylarger viscosity values at 508C than those processed atlab scale. This is related to an enhanced bitumen oxi-dation in the pilot plant device that yields an increasein asphaltene content (Table V).35,36

Influence of processing temperatureon the rheology of CTRMBs

Empirically superposed master curves of unmodifiedbitumen and CTRMBs samples processed, at differenttemperatures, in a lab-scale device with a four-bladeimpeller are portrayed in Figure 3. As can be observed,an increase in processing temperature does not quali-tatively change the linear viscoelastic behavior ofunmodified bitumen, although the complex shearmodulus values increase and the loss tangent isshifted to lower values in the whole experimental fre-quency window. The increase in /G*/ can be consid-ered as a hardening process as a consequence of pro-cessing (primary aging). Bitumen ageing is a verycomplex process that produces a variation of bothchemical composition and colloidal structure.35,36 The

increase of asphaltene content, reported in Table IV,as a consequence of processing would explain theseeffects.

On the contrary, processing temperature has amore complex influence on the linear viscoelasticityfunctions of CTRMBs. Thus, an increase in processingtemperature from 90 to 1208C yields higher /G*/ val-ues in the low frequency region, while a furtherincrease in temperature up to 2508C results in lowervalues of this viscoelastic function. On the contrary,in the high frequency region, the sample processed at2508C shows the highest /G*/ values, while they areminimum for the sample processed at 2108C.

On the other hand, Figure 3(B) demonstrates thattan d shifts to lower values as CTRMB processingtemperature increases, above all at low frequencies,which indicates an increase in binder elasticity. Fur-thermore, a remarkable enhancement of the pseudo-plateau region in tan d at intermediate frequencies,and a decrease in its values in the low-medium fre-quency region are observed for CTRMBs processed attemperatures higher than 1208C. This result is some-how surprising and seems to indicate some micro-structural changes related to tire digestion in theresulting binder as a consequence of processing.

Figure 4 shows the values of the complex modulus(at �10 and 758C, and 10 rad/s) as a function ofbinder processing temperature. As can be noticed, anincrease in processing temperature for unmodified bi-tumen and CTRMBs generally gives rise to highervalues of /G*/, at 758C, although almost constant val-

TABLE IVInfluence of Processing on Carreau’s Model Parameters, at 508C, for Unmodified Bitumen and CTRMBs

Sample Processing device Processing temp. (8C) Z0 (Pa s) lc (s) n

Bitumen Neat – 4.65 � 103 6.6 0.904Bitumen Laboratory 4-blade 180 5.88 � 103 27.4 0.916Bitumen Pilot plant 180 6.35 � 103 22.2 0.899Bitumen Laboratory 4-blade 250 1.96 � 104 805 0.936CTRMB Pilot plant 180 5.88 � 104 91.5 0.788CTRMB Laboratory anchor 180 5.51 � 104 109 0.780CTRMB Laboratory helical 180 5.38 � 104 105 0.796CTRMB Laboratory 4-blade 180 5.65 � 104 112 0.809CTRMB Laboratory 4-blade 90 2.75 � 104 116 0.859CTRMB Laboratory 4-blade 120 3.05 � 104 114 0.854CTRMB Laboratory 4-blade 210 5.33 � 104 99 0.787CTRMB Laboratory 4-blade 250 4.97 � 104 102 0.804

TABLE VAsphaltene Content for Unmodified Bitumen as a

Function of Processing Conditions

Process temp. (8C) Processing device % asphaltenes

Neat bitumen – 20.2180 Laboratory 4-blade 21.2180 Pilot plant 21.9250 Laboratory anchor 23.8



ues are apparent for CTRMBs for processing tempera-tures higher than 1808C. In this sense, an increase incomplex modulus values in the high in-service tem-peratures range causes enhanced rutting resistance ofthe resulting pavement.5,37 In addition, the behaviorat low in-service temperatures must also be takeninto consideration, since an increase in /G*/ makesthe binder less flexible and more fragile, and moresusceptible to develop thermal cracking.6 Thus, at�108C, a continuous increase in the complex modulusvalues with processing temperature is observed forunmodified bitumen, whereas the values of this linearviscoelasticity function for CTRMBs pass through a

minimum for the sample processed at 2108C. It isworth mentioning that, above 908C, CTRMBs showlower values of /G*/ at �108C than unmodified bitu-men, and the differences are more pronounced asprocessing temperature increases. Consequently, highprocessing temperatures cause negative effects on theperformance of unmodified bitumen, while on thecontrary, the addition of crumb rubber induces betterin-service properties at both low and high tempera-tures. The results obtained seem to indicate that theoptimum CTRMB processing temperature is 2108C.

Figure 5 shows the viscous flow behavior, at 508C[Fig. 5(A)] and 1358C [Fig. 5(B)], for unmodified bitu-men and CTRMBs manufactured at different process-ing temperatures. Figure 5(A) demonstrates that anincrease in processing temperature always yieldshigher viscosity values, at 508C, for unmodified bitu-men, above all at 2508C, although they are inferior tothose corresponding to CTRMBs processed at anytemperature between 90 and 2508C. Similarly, anincrease in CTRMB processing temperature up to1808C gives rise to higher viscosity values at 508C.However, viscous flow curves are slightly shifted tolower values for CTRMBs processed at higher pro-cessing temperatures. The Carreau model fits the ex-perimental flow curves fairly well. As can be seen in

Figure 3 Master curves of the complex shear modulus(A) and loss tangent (B) for unmodified bitumen andCTRMBs processed, at different temperatures, in a lab-scale device with a four-blade impeller (reference tempera-ture: 258C).

Figure 4 Evolution of the complex shear modulus (at �10and 758C, and 10 rad/s) with processing temperature forunmodified bitumen and CTRMBs.

Figure 5 Viscous flow curves, at 508C (A) and 1358C (B),for CTRMBs processed at different temperatures.

1688 NAVARRO ET AL.


Table IV, the characteristic time, lc, increases withprocessing temperature, and consequently, the criticalshear rate for the onset of the shear-thinning behaviordecreases. The values of the exponent n are close to 1(Newtonian behavior) and slightly increase withprocessing temperature. Rather different behavior isobserved for CTRMBs. Thus, the values of lc remainalmost constant, and n slightly decreases.

At 1358C, unmodified bitumen is always Newtonian,whereas CTRMBs shows shear-thinning behavior atlow and intermediate shear rates, and a clear tendencyto a high-shear-rate limiting viscosity [Fig. 5(B)]. Onceagain, CTRMBs exhibit higher viscosities than un-modified bitumen, as has been reported by otherauthors.19,25,38 In addition, an increase in binder proc-essing temperature gives rise to an increase in unmodi-fied bitumen viscosity at 1358C, being the values wellbelow 3 Pa s (limiting values required by AASHTOMP1). On the contrary, CTRMBs viscosity values areabove this limiting viscosity at low shear rates. Only athigh shear rates, the viscous flow curves tend toapproach this value. The viscous behavior of CTRMBs,at 1358C, can be described by the Sisko model:39

Z ¼ Z1 þ ks _gn�1 (6)

where Z1, is the high-shear-rate limiting viscosity, ksis the consistency index, and n is the power law orflow index. As can be observed in Table VI, theparameter n increases with processing temperature,while the high-shear-rate limiting viscosity, Z1, showquite similar values. From an engineering standpoint,this last result is important, since industrial opera-tions such as pumping, handling, and mixing of bitu-men occur in the high-shear-rate range previouslymentioned. As has been previously remarked, theexisting AASHTO MP1 procedure requires a maxi-mum binder viscosity of 3 Pa s, at 1358C. However,this method has been developed for Newtonian bitu-men and is not well defined for highly shear-thinningmaterials like CTRMBs. Consequently, this criterionmay be violated if the asphalt can be pumped andmixed at safe temperatures, i.e., at high shear rates.25

Relationship between microstructureand rheology of CTRMBs

As has been mentioned earlier, the rheological behav-ior of CTRMBs is not influenced by the processing de-

vice and impeller geometry used for their manufac-ture. Conversely, other authors have found a cleardependence of binder properties with the mixerspeed.23 In this sense, as can be observed in Table III,the rubber percentage dissolved/dispersed in the bi-tuminous phase is always quite similar (15%) for allthe samples processed at 1808C, whereas the percent-age of soluble components in raw rubber is around11%. Consequently, only about 4% rubber is dis-solved or dispersed in the bitumen due to partial de-polymerization (breaking of the backbone of the mainchain) and devulcanization (cleavage of the sulfurcrosslink bonds) of the rubber particles.23,24 Conse-quently, the influence of the processing device wasnot significant, because the temperature (1808C) wasnot high enough to produce rubber depolimeriza-tion/devulcanization to a great extent.24

On the contrary, the final properties of CTRMBs arelargely dependent on processing temperature. Asreported in Table III, the rubber solubilized in the bi-tumen phase remains constant and equal to the ini-tial value for processing temperatures comprisedbetween 90 and 1208C, and consequently, these condi-tions are not severe enough to break up the chemi-cally crosslinked network. Thus, the observed rheo-logical behavior would be a consequence of the pres-ence of rubber particles swollen by light componentsof the maltenic fraction.13,20,40 By increasing process-ing temperature, the dissolution/dispersion of rubberinto the bitumen is clearly enhanced due to fasterrates of breaking crosslink bonds. Then, two differenteffects influence the rheological properties of theresulting product. The first one is related to the bitu-minous phase, which is modified by an increasedamount of soluble components.25 The second effect isassociated with the remaining solid rubber, whoseconcentration decreases.20 Additionally, rubber par-ticles are reduced in size as a consequence of the dis-gregation process, which also affects the rheologicalproperties of CTRMBs.19

The resulting morphology is shown in Figure 6.Dispersed non-dissolved rubber particles are clearlyobserved for CTRMBs processed at 90, 120, and1808C. No significant differences are noticed, becauseof the small differences in the amount of dispersed/solubilized rubber. A rather different morphologyappears for binder manufactured at higher processingtemperatures, since only a continuous dark phase is

TABLE VIInfluence of Processing on Sisko’s Model Parameters, at 1358C, for CTRMBs

Sample Processing device Processing temp. (8C) Z1 (Pa s) ks (Pa s)2�n n

CTRMB Laboratory 4-blade 90 2.25 3.30 0.184CTRMB Laboratory 4-blade 120 2.10 5.69 0.255CTRMB Laboratory 4-blade 180 2.49 3.69 0.301CTRMB Laboratory 4-blade 210 2.28 4.02 0.305CTRMB Laboratory 4-blade 250 2.35 2.12 0.497



seen (data not shown). This fact is probably due tothe disgregation of the rubber, which increases theamount of small dispersed particles. Direct observa-tions of a filter paper after filtration tests confirm thisfact. Furthermore, these small particles are responsi-

ble for a reduction in filtration rate during solid con-tent determination.

The referred morphological changes have oppositeeffects on the linear viscoelastic behavior of thesamples. Thus, rubber solubilization give rise to anincrease in elastic and viscous moduli values in thehigh in-service temperature range,25 while a reduc-tion in solid content and nonspherical particle sizedecreases the values of the linear viscoelastic mod-uli.19,20 The results obtained seem to indicate that theoptimum CTRMB processing temperature is 2108C.

High temperature storage stability of CTRMBs

One of the most important technical problems involv-ing road pavements building is the phase separationor segregation during polymer-modified binder stor-age at high temperature. It has been found that rub-ber particle size and concentration are factors thataffect the potential for separation of rubber particlesin the bitumen during storage.12,19,20

In this work, a storage stability index has beendefined as the ratio between the value of the elasticmodulus, at 0.1 rad/s and 508C, at the bottom of thesettling tube and its corresponding value at 30-cmdepth (Fig. 7). A global index of one would indicatethat the binder is stable, whereas higher values giveclear indications of rubber particle sedimentation.Figure 7 demonstrates that all the samples studied areclearly unstable under the selected storage conditions.However, this figure also shows that an increase inprocessing temperature from 120 to 2108C yieldsmore stable modified bitumens, whereas a subse-

Figure 6 Optical microscopy observations of CTRMBsprocessed at different temperatures: (A) 908C; (B) 1208C;(C) 1808C.

Figure 7 Effect of processing temperature on the storagestability index of the modified binders studied.

1690 NAVARRO ET AL.


quent increase in processing temperature up to 2508Cdestabilizes the binder.

CONCLUSIONS

The rheological properties of CTRMBs are largely de-pendent on their manufacturing conditions, speciallyprocessing temperature. Thus at low processing tem-peratures (90 and 1208C), the chemical crosslinkednetwork of the rubber remains practically unaltered,and the corresponding rheological properties are aconsequence of the interactions between bitumen andswollen rubber particles. Higher processing tempera-tures lead to partial depolymerization/devocalizationof the network, increasing the amount of componentsthat are incorporated to the bitumen phase and, con-sequently, reducing both solid concentration and rub-ber particle size. On the contrary, the processing de-vice and impeller geometry used to manufactureCTRMBs (at 1808C) have an almost negligible influ-ence on the rheology of the modified binders studied.All the CTRMBs studied present segregation, becauseof rubber settling during storage at high temperature,although an increase in processing temperature seemsto enhance its stability. The results obtained seem toindicate that the optimum CTRMB processing tem-perature is 2108C.

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