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COMPOSITE ASPHALT BINDERS: EFFECT OF MODIFIED RPEON ASPHALT
By A. A. Yousefi,1 A. Ait-Kadi,2 and C. Roy3
ABSTRACT: Recycled polyethylene (RPE) modified asphalts were prepared and characterized. The RPE wasmodified with different copolymers before incorporating into the asphalt. A special morphology was found forthis system. The RPE particles are highly swollen by asphalt. This morphology is attributed to physical inter-actions. The presence of copolymers in RPE was found to change the RPE’s affinity toward asphalt constituents.Aging of asphalt, oil absorption by polymer particles, and interactions of polymer particles with asphaltenesaffect the overall rheological properties of polyethylene modified asphalts. Attempts to approach the rheologicalbehavior of polymer modified asphalt with rheological models for filled polymers were not found to be conclu-sive. Other specific features of asphalt must be taken into account. These include, for example, changes in thestructure that were evidenced by their relaxation time spectra and molecular weight distributions. The polymermodification of asphalt affected the asphalt properties. Well controlled thermomechanical history experiencedby the asphalt is needed to differentiate between the chemical changes of the binder due to the mixing processand the beneficial effect of the polymer for high-performance asphalts.
INTRODUCTION
Improving the properties of bitumen has been a subject ofparticular concern due to the importance of this constituent ofroad paving materials. Many approaches have been used toimprove the properties of bitumen as a pavement binder.Chemical and physical methods have been used to modify thechemical and colloidal structure of bitumen. Polymeric mate-rials are often considered as the most important family of bi-tumen modifiers. Depending on their nature, polymers are ableto impart higher viscosity at high temperatures to asphalt bind-ers and/or ductility at low temperatures. A lack of these prop-erties leads to two types of distresses in pavement; rutting athigh temperatures (permanent deformation) and cracking atlow temperatures.
High molecular weight polymers have profound effects onthe properties of bitumen. However, as the molecular weightof polymers increases, their compatibility with bitumensharply decreases (‘‘Key facts’’ 1989). This results in a dra-matic phase separation at high temperatures; that is, the dis-persed phase (polymer) forms another continuous phase withinthe relatively short storage time at high temperatures (say1607C). Thereafter, the modified bitumen loses its ultimateproperties, and the existence of the grossly separated secondphase induces high cracking susceptibility. Some authors havetried to stabilize the thermodynamically unstable polyethylene-(PE-) bitumen emulsion system [Jew et al. (1986), Hesp andWoodhams (1990, 1991), Hesp et al. (1993), Morrison et al.(1994), Ait-Kadi et al. (1996); see also Hesp et al., ‘‘Bitumen-polymer stabilizer, stabilized bitumen-polymer compositionsand method for the preparation thereof,’’ U.S. Patent No.5,280,064 (1994)]. The first to use the concept of in situ stericstabilization to stabilize PE particles against coalescence wasHesp and Woodhams (1991), who found that coalescence fol-lowed by creaming is the main reason for the breakdown ofthe emulsions of PE fine particles in bitumen. The steric bar-
1PhD Student, Dept. of Chemical Engrg., Laval Univ., CERSIM, Que-bec, PQ, Canada G1K 7P4.
2Prof., Dept. of Chem. Engrg., Laval Univ., CERSIM, Quebec, PQ,Canada G1K 7P4; corresponding author.
3Prof., Dept. of Chem. Engrg., Laval Univ., Quebec, PQ, Canada G1K7P4.
Note. Associate Editor: Tinh Nguyen. Discussion open until October1, 2000. To extend the closing date one month, a written request mustbe filed with the ASCE Manager of Journals. The manuscript for thispaper was submitted for review and possible publication on June 1, 1998.This paper is part of the Journal of Materials in Civil Engineering, Vol.12, No. 2, May, 2000. qASCE, ISSN 0899-1561/00/0002-0113–0123/$8.00 1 $.50 per page. Paper No. 18482.
J. Mater. Civ. Eng.
rier that these authors used, was a low molecular weightpolybutadiene, which was chemically anchored on the surfaceof micronized particles of maleic anhydride grafted PE (Hesp1991). According to their experiments, the emulsions werecompletely stable at high temperatures (1107C, a temperaturebelow the melting point of low density polyethylene) for about3 h. Morrison et al. (1994) also attempted to use end-chainfunctionalized synthetic and pyrolytic polybutadiene to pre-pare stable emulsions. Later, Ait-Kadi et al. (1996) used anamorphous ethylene propylene diene monomer (EPDM) elas-tomer to stabilize the high density polyethylene (HDPE) par-ticles suspended in bitumen medium. According to the opticalmicroscopy pictures reported by Ait-Kadi et al. (1996), theresulting suspensions were stable for long enough times. Intheir work, they studied the rheological properties of this typeof modified asphalt and optimized the level of modification aswell. In various tests, the best results were obtained for 1%PE modified bitumen.
In this paper, the preparation, rheological properties, andresults of the conventional and new performance evaluationtests of different recycled PE (RPE) modified bitumens arereported. Following Ait-Kadi et al. (1996), the concentrationof different modifiers in RPE were first optimized (Yousefi etal. 1997, 1998) and then 1% of the resulting blends were in-troduced into asphalt.
MATERIALS AND EXPERIMENTAL PROCEDURES
Materials
As a base binder, a 150/200 penetration grade bitumen fromUltramar Co. (Quebec) was used, and all modifications werecarried out on the same bitumen. The high density RPE pro-vided by Transplastek Inc. (St. Bruno, Quebec) was used asreceived and in modified forms. Two different EPDMs (HulsCo., Somerset, N.J.) and a plastomer EXACT4041 (ExxonCo., Houston) were used to modify the RPE. Some of thephysical properties of the modifiers are listed in Table 1.
Mixing
The RPE and the modifiers were mixed at 1807C and 60rpm for 10 min using a batch mixer (Haake Buckler, Rheomixsystem 40). The details of this procedure are reported else-where (Yousefi et al. 1998). The nonmodified and the modifiedRPEs were added to bitumen at 1607C at a level of 1% byweight (;1.1% by volume) modification, and mixing wasmaintained for 3 h to obtain a finely dispersed minor phase
JOURNAL OF MATERIALS IN CIVIL ENGINEERING / MAY 2000 / 113
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TABLE 1. Some Physical Properties of Modifiers
Modifier(1)
Density(g/cm3)
(2)Comonomer
(3)Mooney viscosity ML(1 1 4)
(4)
Tensile strength(MPa)
(5)C = C/1,000&C
(6)
EXACT4041 (Amorph.) 0.878 Butene 10 (1257C) — —EPDM447 (Cryst.) 0.86–0.87 Propene (25.5 6 4%) 85 6 5 (1007C) 18 8 6 1.2EPDM541 (Amorph.) 0.86–0.87 Propene (44 6 5%) 110 6 5 (1007C) 14 5 6 1.2
(<10-mm particle diameter). To form droplets from the filmsof RPE, the speed of the mixer was kept at a moderate or lowlevel, whereas after 1 h, a high mixing speed was used todecrease the size of the particles. After mixing, the resultingdispersion was poured into a small can, ice-cooled to roomtemperature, and then stored in a freezer to retain the obtainedmorphology. This procedure is based on our earlier experiencewith PMAs (Ait-Kadi et al. 1996) and was scrupulously re-spected for the preparation of all polymer modified asphaltmixtures of this study.
Characterization
Rheological Analysis
The dynamic measurements were carried out on a BohlinCVO mechanical spectrometer (stress controlled rheometer)using parallel plates geometry. Three different plate diameters(10, 15, and 40 mm) were used depending on the test tem-perature. The temperature varied from 215 to 907C. The testswere performed at frequencies ranging from 0.002 to 125 rad/s. Stress sweep tests were first carried out to ensure that therate sweep tests are performed in the linear viscoelastic zone.For the Strategic Highway Research Program (SHRP) tests,the same rheometer was used in 0–907C temperature range at10 rad/s. The tests were repeated for all asphalts at a 57Cinterval in this temperature range. Creep tests were also carriedout at 607C on the same rheometer. In this test, a stress (100Pa) was applied on the sample for 1 min. The stress was thenremoved and the recovery of the deformation was monitored.
Morphological Analysis
A sample of 5–10 mg of bitumen was heated and slowlypressed between glass slides, then micrographs (using ZeissFX optical microscope) were taken using a photo camera (35-mm Yashica FX 2000). Scanning electron micrographs (SEMs)of washed particles coated with a gold-platinum alloy werealso obtained using a JEOL JSM-III microscope.
Emulsion Stability Analysis
After the mixing period, part of the prepared mixture wastransferred into two aluminum toothpaste tubes (15 3 3 cm)(SHRP specification). The tubes were immediately immersedin an oil bath at 163 6 57C for 48 h. The tubes were then ice-cooled to room temperature and stored in a freezer. After 24h, the tubes were cut into three equal parts. The upper andlower parts were then melted into separate small cans and la-beled as T and B, respectively. The softening points (Ring andBall test, ASTM D 36-76) of T and B materials were deter-mined and their morphology and rheological properties werestudied as before.
Penetration Test
The penetration tests were carried out at 257C according toASTM D 5-73. The bitumen was thermostated in a water bath,and the penetration of a standard needle under a standard load(50 g) was measured and reported in tenths of millimeters.
114 / JOURNAL OF MATERIALS IN CIVIL ENGINEERING / MAY 2000
J. Mater. Civ. Eng
Fraass Test
Fraass breaking point tests were performed according to theIP-80 standard. A film of bitumen (0.40 6 0.01 g) was formedon a flat standard steel plaque. The Fraass breaking point isdefined as the temperature at which a break or a crack appearson the film of asphalt coating the steel plaque. The plaque issubjected to successive flexions under determined cooling con-ditions.
DSC Tests
The thermal tests were performed between 90 and 1257Cusing a Perkin-Elmer differential scanning calorimeter (DSC-7) with 15–25 mg samples. The heating rate was fixed at 57C/min, and the test was repeated three times for each modifiedasphalt.
RESULTS AND DISCUSSION
Rheology
Oscillatory Shear Flow
The experimental master curves of h9 and G* over a widerange of frequency (1026–106 rad/s) obtained from the oscil-latory rheological measurements using time-temperature su-perposition principle are reported in Figs. 1 and 2 for the baseand the modified asphalts. As seen in Figs. 1 and 2, the rhe-ological properties of the base asphalt dramatically change onaddition of only 1% by weight of the RPE. More extensivechanges in the rheological properties of the base asphalt areobserved on addition of EPDM modified RPEs. SEMs of poly-mer particles recovered after filtration indicate that the parti-cles are porous spheres with diameters ranging from 6 to 27mm for the unmodified RPE [Fig. 3(a)]. Diameter distributionfor modified RPEs is in general narrower and depends on thenature of the modifier [Figs. 3(b–d)]. The narrower distribu-tion is found for the 7/93% by weight EPDM447/RPE forwhich the diameters vary from 4 to 16 mm [Fig. 3(c)]. As therheological data of Figs. 1 and 2 are obtained at temperatures(T = 215 to 907C) where the polyethylene particles are in thesolid state, it is legitimate to consider the PMAs as particulatecomposites or suspensions. The suspending matrix in this caseis a viscoelastic fluid (asphalt) with a relatively high viscosity[h0 slightly below 2 3 104 Pa ?s at 307C (Fig. 1)]. These con-ditions represent major differences with respect to suspensionsin nonviscous fluids (Metzner 1985). When the viscosity ofthe continuous phase is very high [>102 Pa ?s (Metzner 1985)],the viscous forces imposed on the particles by the fluid in aflow process are so large that particle-particle interactions arenegligible. This is the case for the nonaged base asphalt [Ul-tramar 150–200 at 307C with h0 > 2 3 104 Pa?s (Fig. 1)].The viscosity for the aged asphalt will be even higher. Notethat at this polymer concentration (1% by weight), hydrody-namic interactions are also supposed to be absent. To evaluatethe contribution of these particles to the rheological behaviorof the asphalt matrix, predictions of several equations devel-oped for suspensions will be compared to the experimentaldata of Fig. 1. The spherical particles are supposed to besmooth and nonporous.
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FIG. 1. Dynamic Viscosity of Asphalts at 308C
FIG. 2. Dynamic Complex Modulus of Asphalts at 308C
After the classical work of Einstein, various equations havebeen proposed to describe the viscosity concentration behaviorof suspensions (Goldsmith and Mason 1967; Metzner 1985;Utraki 1988). Of these, the equations by Einstein, Frankel andAcrivos, Thomas, Mooney and Maron-Pierce (Metzner 1985)were used with the original asphalt and none of them is foundto be able to predict the viscosity concentration behavior ofthe studied polymer modified asphalts. The comparison shouldhave been done with the aged asphalt (original asphalt mixedunder the same conditions as the PMAs but without addingthe polymer). These data are not available for the systemsevaluated in this study. Experimental data for which the rhe-ological properties on aged asphalt are available (Yousefi1998) indicate that the increase cannot be predicted by the
J. Mater. Civ. Eng.
aforementioned equation for the aged asphalt as well. Notethat the data of Fig. 1 indicate that the increase depends alsoon the nature of the polymer used to modify the asphalt. Thiscannot be attributed to the polydispersity of the polymerspheres [Figs. 3(a–d)], because for this effect to be important,the solid volume fraction must be relatively high (higher than30%) (Barnes et al. 1989). This, of course, is not the case ofthe data reported in Fig. 1 for which the volume fraction iswell below this limit. According to the results of Yousefi(1998), aged asphalt has a zero-shear viscosity lower than thatof the 1% by weight RPE modified asphalt. This indicates thatthe 7/93% by weight EPDM541/RPE modified asphalt wouldhave a zero-shear viscosity 10 times higher than that of theaged asphalt (Fig. 1). To achieve a 10-fold increase in the
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FIG. 3. SEMs of Distribution of Washed RPE Particles: (a) Unmodified; (b) Modified with 7%EXACT4041; (c) Modified with7%EPDM447; (d) Modified with 7%EPDM541
viscosity of a suspension, a viscoelastic fluid must be chargedby about 40% by volume of solid particles (Metzner 1985).As seen in Fig. 1, the same increase in the zero-shear viscositycan be achieved by adding only about 1% by volume of mod-ified RPE to the base asphalt. When compared to the baseasphalt (as received), the increase observed upon addition ofpolymers is even more pronounced [a 100 times increase ofthe viscosity is observed when only about 1% by volumeof the RPE modified with 7% of EPDM541 is added to thebase asphalt (Fig. 1)]. Even the similar results, which werereported by Ait-Kadi et al. (1996), cannot be interpreted usingthese formulas.
Following Palierne (1990), Grabelling et al. (1993) pro-posed an equation for the complex modulus G* of suspensionsof rigid particles → `) in a viscoelastic media. The pro-(G*pposed equation can be written as follows:
31 1 f
2S DG*(v) = G*(v) (1)m 1 2 f
where G*(v) and = suspension and matrix complexG*(v)m
moduli, respectively; and f = volume fraction of particles.This equation predicts a very small increase in G* of the sus-pension. As shown in Fig. 2, G* of the modified asphalts aremuch larger than G* of the base asphalt (with the same careas for the viscosity concerning aging effect of the asphalt ma-trix).
All of these comparisons indicate that the behavior of PE
116 / JOURNAL OF MATERIALS IN CIVIL ENGINEERING / MAY 2000
J. Mater. Civ. Eng.
particles in asphalt is very different from that of rigid and inertparticles in a suspension. The asphalt itself has a very complexcolloidal structure (Abraham 1961; Barth 1962; Zakar 1971;Bunger and Li 1981). It is composed of low (oils), medium(resins), and high (asphaltenes) molecular weight constituents.Introduction of other additives (e.g., polymers) into the baseasphalt may disturb its internal structure thereby resulting indramatic changes of some structure-sensitive properties, suchas rheological properties. The structure and the properties ofasphalt are profoundly affected by the following operations:
• Heating and mixing of an asphalt in the presence of airfor an extended time leads to a change in the chemi-cal composition and the internal structure of the asphalt(aging).
• An increase in the asphaltene content extremely affectsthe thermal and rheological properties of the asphalt.
• Addition of solid particles to asphalt results in interactionsbetween particles and asphalt constituents (Rostler et al.1977; Chaala et al. 1996). The particles are able to absorband/or adsorb lower molecular weight ingredients of as-phalt, which results in a change of the composition, andhence the properties of the continuous phase. This, in turn,results in a relatively higher asphaltene content and pos-sible structural change from sol to gel.
From these characteristic properties of asphalt, some extent ofaging may be expected during the mixing process at high tem-
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perature. Moreover at 1607C, PE is in the melt state and canabsorb some oils and also readily release low molecular weightfractions into the asphalt medium. Later, at the end of themixing process when the mixture is cooling, some oils returnto the asphalt phase. This is the property-controlling step. Ahigher compatibility of PE with different asphalt constituents[attributed to polypropylene (PP) impurity in RPE and also toadded EPDMs] facilitates the diffusion of oils into the PEphase. This will result in more oil trapping in the PE phaseduring cooling of the samples, and the resulting modified as-phalt will have superior rheological properties.
The polymers to be introduced into the asphalt have beenthe subject of a systematic study in terms of mechanical andrheological properties as a function of additives (Yousefi et al.1998). It is worth comparing the rheological properties of thepolymer-modified asphalt and those of their correspondingoriginal blend. The EXACT4041 modified asphalt shows, asfor its original blend, lower rheological properties than theRPE modified asphalt, whereas the order of EPDM447 andEPDM541 modified asphalts is inverted with respect to theproperties observed for their blends. Note that this effect isobserved only at low frequencies. At high frequencies, the rhe-ological properties are found to be less sensitive to the pres-ence of the polymer. The effect of the different polymer ad-ditives on the rheological properties is attributed to thedifferent structure of the copolymer added to the RPE. Indeed,the changes in the crystalline structure and the different struc-tures of the modifiers result in different interactions betweenthe asphalt phase and the polymer particles. For example, thesolubility parameter of PP is very close to the solubility pa-rameter’s window of asphalts (‘‘Key facts’’ 1989). Therefore,an increase in propene content of an EPDM rubber leads tolonger PP segments and thus a higher compatibility with as-phalts that results in increased oil absorption by PE particles.As a consequence, a crystalline copolymer such as EPDM447,which contains a lower level of propene, provides a lowercompatibility between the phases. Moreover, the absorbedfractions of asphalt also interact with the structure of the solidpolymer suspended in asphalt. It is indeed expected that thecrystallinity of the polyethylene in the asphalt may be dis-turbed by the presence of the asphalt molecules intimatelymixed with PE molecules. All these reasons account for theinversion of the order of the rheological properties ongoingfrom the blends to the modified asphalts for EPDMs. Theseclaims can be easily justified by the results of differential scan-ning calorimetry (DSC) measurements reported in Table 2.There are 8–127 decreases in the melting point of the PE phaseof the RPE that explain the extent of the changes in the sizeof the crystalline domains of the polymer. These conclusionson the changes in the degree of crystallinity of the PE phaseof the added polymers must be taken with care, because thepercentage of polymer in asphalt is very low, and the concen-tration of the polymer may fluctuate from one sample to theother. In spite of the observed sponge form of the washed PEparticles [Figs. 3(a–d)], a film of these washed particles alsohas a black color that confirms entrapment of the asphalt in-gredients in PE even after severe washing of particles. Anotherconsequence of this observation is that asphalt ingredients areonly physically absorbed in the PE phase of the PMA. Theoptical microscopy provides visual proof for the existence ofasphalt ingredients in RPE particles, as compared with parti-cles of virgin HDPE. This will be discussed later in the mor-phology section.
Another way to assess the physicochemical interactions be-tween the asphalt and the polymer and to quantify the presenceof dissolved polymer in the asphalt is to compare the molec-ular weight distribution (MWD) of the original asphalt and thePMA. However, with the RPE based polymers used to modify
J. Mater. Civ. En
TABLE 2. Changes in Melting Point of PE Phase of PolymerBlends in Ultramar 150–200 Asphalt
Sample(1)
Tm
(&C)(2)
DTm
(&C)(3)
1%RPE 120.02 6 0.17 210.931%7%EXACT4041 119.40 6 0.41 28.211%7%EPDM447 119.53 6 0.25 210.61%7%EPDM541 119.71 6 0.06 210.23
the asphalt, it will be difficult to dissolve the polymer dis-persed in the matrix. Moreover, even for asphalt itself, thepresence of a solvent may destroy the existing interactionsbetween some components of asphalt (asphaltenes in a gelstructure), thereby giving rise to a different picture of thestructure. Dissolving the asphalt matrix in a solvent such astetrahydrofuran will also invariably lead to extraction of ab-sorbed oils from the polymer phase. Deducing molecularweight distribution from linear viscoelastic rheological mea-surements may represent an interesting alternative in this case,as the structure of the material is not disturbed in the linearviscoelastic region. Rheological data have been widely usedto obtain the MWD of linear polymers (Tuminello 1986, 1990;Marvidis and Shroff 1993; Mead 1994; Shaw and Tuminello1994; Chambon 1995; Liu and Shaw 1995; Braun et al. 1995,1996; Shroff and Marvidis 1995; Wood-Adams and Dealy1996; Naguen et al. 1996; Anderson et al. 1997). Althoughasphalt has a very complex structure, rheological measure-ments were successfully used to calculate the MWD of poly-mer modified and unmodified bitumens (Zanzotto et al. 1996).In their paper, they compared the MWD of the same asphaltobtained from gel permeation chromatography (GPC) and rhe-ological tests and calculations. They noted that although GPCis more sensitive to low molecular weights, the rheologicalapproach provides a better resolution in high molecular weightband. The larger peak in the low molecular weight region canalso be attributed to the dissociation of low molecular weightmolecules from asphaltenes particles. These changes in as-phaltene will not be detected because GPC is less sensitive tohigh molecular weight constituents. Another advantage men-tioned in their paper is the ability of the rheological methodto detect interfacial relaxation that appears as a negative peakon the rheological chromatograms, whereas GPC cannot pro-vide any information about this phenomenon. On the basis ofthe above observations, the rheological data of the neat andthe modified asphalt binders are further processed to extractthe molecular weight information. According to Zanzotto et al.(1996), the phase angle d(v) is more sensitive to molecularweight changes than G*(v). Using the double reptation theoryas used by Tuminello (1990), the phase angle can be relatedto the molecular weight distribution through the following re-lationship:
`
d(v) = W (M9)c(M9, v) dM9 (2)E0
where W (M9) and c(M9, v) = weight distribution function andthe monodisperse phase angle, respectively. Using the Heavi-side function and the following assumptions:
2aM = kv , x = log v (3)
For a normalized W we have`
d(v) = 1 2 W (M9) d 9M (4)Ex
2aM=k10
Differentiation of this equation using the Leibnitz formulaleads to
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TABLE 3. Parameters of Model (Relaxation Times)
Base asphalt lk(s)(1)
1%RPE lk(s)(2)
1%7%EXACT4041 lk(s)(3)
1%7%EPDM447 lk(s)(4)
1%7%EPDM541 lk(s)(5)
25.1204 928.2934 322.1727 2,015.4339 6,972.79470.3476 37.0996 26.7838 98.2453 122.75160.0165 1.6660 0.6422 5.9026 16.78072.6191e-3 0.0240 9.1958e-3 0.1065 0.31681.5906e-4 1.4129e-3 4.1851e-4 1.9876e-3 0.02371.5695e-4 1.4134e-3 7.8595e-5 1.5793e-5 4.1974e-37.3251e-4 4.8628e-4 7.8834e-5 1.7008e-5 4.1340e-34.0030e-6 1.7536e-5 7.6028e-6 9.9510e-5 1.1892e-48.4742e-7 2.4537e-5 8.3849e-6 9.9585e-5 9.8652e-626.5151e-3 22.5267e-3 21.4314e-4 26.2197e-5 29.6915e-323.7009e-4 26.7114e-4 23.6500e-5 26.2232e-5 29.6668e-3
xa10 ddW (M ) = 2 (5)
ak ln 10 dx
where M = . In an earlier work, Stastna et al. (1994)x2ak10
proposed a fractional model for the phase angle of asphaltbinders
n m1
d(v) = 90 2 arctan(vl ) 2 arctan(vm ) (6)k kSO O Db 1 1
where b = (n 2 m) and lk and mk are relaxation times; n andm are constants; corresponds to the position of the positive21lk
local maxima of 2dd/d log v versus log v curves, whilecorresponds to the position of the local minima on these21mk
curves; and mk is nonzero for the high concentration of poly-mer as is the case for the samples described by Zanzotto et al.(1996).
First, the same shift factors as those of G9 and G0 wereused to construct phase angle master curves of different bitu-mens. Then, to use this approach, we fit the experimental mas-ter curves of the phase angle of the modified and the un-modified asphalt by setting n = 11 and m = 0 in (6). Theparameter m was set equal to zero because the polymer volumefraction is very low and also no inflection point is detected onthe d(v) master curves. Following Zanzotto et al. (1996), using(5) with a = 0.2131 and k = 2.976, W(M), which represents
118 / JOURNAL OF MATERIALS IN CIVIL ENGINEERING / MAY 2000
J. Mater. Civ. Eng
the relative amount of different molecular weights, can thenbe calculated. Note that since we use this method for com-parison purposes to assess some conclusions about molecularweight changes of the base asphalt as compared to the polymermodified ones; this choice is not critical. The results of thefitted relaxation times lk are reported in Table 3. As seen inthis table, the relaxation times of the modified asphalts arelonger than those of the base asphalt. Another important ob-servation is that the relaxation times of the modified asphaltsare not the same. This means that for similar concentrationsof PE particles and under the same conditions, different asphaltbinders were obtained.
The calculated MWDs of the asphalt represented in Fig. 4clearly illustrate the structural changes. An inspection of thisfigure reveals that the relative amount of different componentsof different molecular weights are changed by the presence ofthe different modifiers. As clearly seen, the largest peak thatcorresponds to the lowest molecular weights is displaced forall modified bitumens, and the second peak of the base asphalthas changed significantly in each case. Again, an intermediatebehavior was observed for the EXACT4041 modified asphalt.In the case of EPDM modified asphalts, these two peaks arealso seriously diminished. These changes are counterbalancedby the appearance of a peak in the very high molecular weightregion.
FIG. 4. Molecular Weight Distribution of Base and Modified Asphalts
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FIG. 5. Creep-Recovery Test for Different Asphalts at 608C
Creep-Recovery Test
Rutting is one of the major mechanisms of the failure ofasphalt pavements at high temperatures (50–607C). This typeof distress creates permanent deformations in the shape of apavement. The ability of the binder in a pavement to recoverdeformations at high temperatures during its service lifetimeis of great importance and can be estimated by creep testing.A lesser deformation of the modified binder is also of valuefor the performance of the binder at high temperatures. Theresults of the creep-recovery tests at 607C are shown in Fig.5. As seen, the base asphalt has a larger shear creep compli-ance in comparison with those of the modified asphalts. Theshear creep compliance is defined as follows (Ferry 1980;Dealy and Wissbrun 1990):
g(t)J (t) > (7)
s
where g(t) and s are shear strain and shear stress, respectively.Although shear stress is constant (100 Pa) for all bitumens, alarger creep compliance translates to a higher deformation dur-ing the same period of time. The values of the creep compli-ance of the bitumens follow the following order:
Base asphalt > 1%7%EXACT4041 > 1%RPE
> 1%7%EPDM447 > 1%7%EPDM541
This order clearly shows the extent of the deformation of allfive bitumens against deformation. Among them, the1%7%EPDM541 binder (1% of 93/7% RPE/EPDM541 in as-phalt) shows the highest rate of resistance against deformation.As seen in Fig. 5, whereas the base asphalt does not show anysign of recovery of the relatively large induced deformation,the modified asphalts recover (slightly) part of the relativelysmall induced deformation. The same order for deformation isobservable for the recovery of the studied asphalts (Fig. 5)(not clearly seen due to the logarithmic y-scale). Consequently,the deforming step is the controlling step, and the recoverystep does not play any role in the rutting resistance of the baseasphalt at high temperatures, whereas the recovery step couldbe important in the case of the modified asphalts.
J. Mater. Civ. Eng.
FIG. 6. (a) PE Particles in Asphalt Medium; (b) Coalescence ofPE Particles on Top of Stability Test after 24 h
Morphology and Its Stability
Addition of the modifiers does not stabilize the RPE sus-pensions against creaming in the asphalt medium. The chosenstability test does not show any difference between the soft-ening point of the lower and higher parts of the test tubes.
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FIG. 7. Material Functions of Top and Bottom of Stability Test Tube at 308C
This could mainly result from the extensive rate of aging ofthe asphalt during the test (48 h at 1657C). However, opticalmicroscopy [Figs. 6(a and b)] and dynamic rheological tests(Fig. 7) on the top and the bottom parts of the test tubes re-vealed that there is a large difference between the concentra-tions of polymer particles in these two extremes of the testtubes. High concentration of polymer in the top of the testtube results in a plateau in the curves of elastic and loss moduliand a yield behavior of viscosity curve typical of a structuredmaterial. On the other hand, the material functions of the bot-tom of the test tube show no plateau or yield, a behavior whichis typical of a liquid material. Thus, there are probably twofactors affecting the properties of asphalt (i.e., the aging of theasphalt matrix and the polymer concentration).
Inspection of the micrographs [Figs. 6(a and b)] shows thatthe polymer particles are mainly subjected to creaming, as seenby comparing the micrographs of the samples before and afterthe stability tests. On the micrographs, it is evident that thereare small particles in the upper part of the tube. Of course,some large aggregates of polymer particles are also observed,which could result from the collision of particles during thecreaming process.
As seen on SEMs, the particles’ radius ranges between 4and 27 mm [Figs. 3(a–d)]. This would indicate that the Brown-ian motions of the particles are negligible. Therefore, the prin-cipal reason for the breakdown of these suspensions at hightemperatures is creaming, which results in some extent of co-alescence as well. SEMs of the washed PE particles reveal thatthe surface of the particles is not smooth. Before washing outbitumen with toluene, the PE particles are completely swollenby nonasphaltene ingredients of bitumen. The interface of PE-asphalt is very complex and extends into the particles. Nodifference is observed between the surface of the modified andthe nonmodified RPE particles. The presence of copolymersresults in a decrease in particle size in comparison with non-modified RPE. This decrease in particle size may result froma better compatibility of the added RPE/copolymer with non-asphaltene part of asphalt. This probably stems from a largeroil absorption capacity of the modified RPE.
Reducing the size of PE particles in an asphalt medium hasmany important benefits. First, a reduction in particle size willdecrease the buoyancy forces on the particles, which in turn
L OF MATERIALS IN CIVIL ENGINEERING / MAY 2000
J. Mater. Civ. Eng
positively affects the stability of polymer modified asphaltsduring high temperature storage period. The buoyancy forceson a spherical particle can be calculated using the followingexpression:
4 3Buoyancy force = pR Drg (8)3
where R = particle radius; Dr = difference between the particledensity and the matrix density; and g = gravitational acceler-ation. As shown, the buoyancy force on a PE particle in theasphalt medium is directly proportional to the third power ofits radius. Using a macroscopic force balance for a creepingflow around a sphere, the creaming velocity of PE particles isobtained by (Bird et al. 1960):
22gR Drv = (9)c 9m
where vc = creaming velocity; and m = asphalt viscosity. Al-though, on the addition of copolymers to RPE, we partiallyincrease the difference of density between PE phase and bi-tumen phase, the radius of particles decreases [Figs. 3(a–d)],and the zero-shear viscosity of the suspending asphalt matrixincreases (Fig. 1). In this situation, the effect of the enlarge-ment of the density gap can be compensated for and the systemcan be stabilized against coalescence. This is because thecreaming velocity varies as the square of the particle radiusand the inverse of the viscosity [see (9)]. Second, a reducedpolymer particle size enhances the adhesion of the binder tothe aggregates in a pavement. In turn, this will result in a betterwetting of the aggregate by the binder and a decrease in rav-eling, which extends the lifetime of the pavement. A schematicrepresentation of the polymer, asphalt cement, and aggregatesin an asphalt concrete mix can be represented as in Fig. 8.This is the situation after compaction of an asphalt concretemix. The thickness of the film between the aggregates rangesbetween 5 and 10 mm (Rostler et al. 1977). Therefore, polymerparticles of diameter >10 mm could have a negative effect onthe adhesion of bitumen and aggregates. However, small par-ticles can remain in between the film of bitumen and reinforcethe film, as in composities, to enhance the strength of the bi-tumen against thermal and mechanical stresses during the ser-
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FIG. 8. Schematic Representation of Bitumen Film in AsphaltConcrete Mix
FIG. 9. Schematic Representation of PE Particles in AsphaltMedium
vice time of the pavement. This is the case for EPDM modifiedbitumens.
As witnessed by different methods of microscopy (opticaland electron scanning microscopy), the PE particles are dis-persed in an asphalt matrix. Ait-Kadi et al. (1996) and Le-wandowski (1994) reported similar results. A schematic rep-resentation of the structure of the PE-asphalt suspension isillustrated in Fig. 9. The polymer particles behave as physi-cally active fillers that interact with the asphalt medium byabsorption and adsorption mechanisms. This results in the pen-etration of asphalt into the particles, the formation of a layeraround the particles, and creation of many hollow holes in PEparticles after washing out bitumen (Fig. 9). This is just whathappens for every active particle in the asphalt medium(Rostler et al. 1977). As will be seen below, this profoundlyaffects the properties and performance of the original bitumen.
Asphalt Performance Tests
Conventional Tests
The results of the ring and ball tests are summarized inTable 4. They indicate that addition of different RPE resultsin an increase in the softening point of the resulting asphaltbinders. These increases correspond to the higher consistencyof the binders. The results also show that the resulting bindersare more resistant to permanent deformation at high temper-atures (higher resistance to rutting).
The results of the penetration tests at 257C are also listedin Table 4 for all asphalts. We observe that on the addition of
J. Mater. Civ. Eng.
polymer, the penetration of asphalts decreases. This is anotherindication of an enhanced consistency and resistance againstpermanent deformation of the modified asphalts during theirservice life in pavement.
As reported by Ait-Kadi et al. (1996), the addition of PEand its blends increases the Fraass temperature of modifiedasphalts. They also reported that adding 1% of 10%EPDM541modified virgin HDPE to the same asphalt results in a lowerFraass temperature before and after thin film oven test. Ourresults (Table 4) always show increases in the Fraass pointof the modified asphalts. Among the modified asphalts,1%7%EXACT4041 binder shows the lowest Fraass tempera-ture, which makes it a better candidate for paving in coldareas.
SHRP Tests
The SHRP system of classification and characterization ofasphalt binders put the accent on the rheological methods (Per-formance 1996) to replace the conventional tests of asphalts,which usually have no theoretical background. As asphalt be-havior depends on loading time and temperature (a viscoelasticfluid), SHRP has chosen the dynamic shear rheometer as atool to measure this dependency (Bahia and Anderson 1995).The criterion adopted by SHRP for an asphalt binder is thehighest temperature at which the ratio of G*/sin d is superiorto 1 kPa (G* = complex modulus, and d = phase angle). Thistest is used instead of the softening point test and the resultsfor the base and the modified asphalts are reported in Fig. 10.It is seen in the figure that the addition of RPE increases thisratio. Modification of RPE with the different copolymers re-sults in more increases in this ratio. The values of temperatureat which G*/sin d $ 1 kPa (TSHRP) are reported in Table 4.The 1%7%EXACT4041 modified has a lower TSHRP than thatof the 1%RPE modified binder. As already seen (Yousefi et al.1998), EXACT4041 has lower viscoelastic properties (G9 andG0) than EPDMs and RPE. Therefore, this modifier decreasesthe viscoelastic properties of all RPE blends. Although thisseems to be a negative factor at high temperatures, it can turnout to be a positive character at low temperatures. Indeed,decreasing the elasticity of the asphalt binder at low temper-ature allows it to dissipate the traffic energy. This reveals theadvantage of using EXACT4041 in composition with conven-tional binders, which tend to be brittle at low temperatures.
Other results can be extracted from the rheological data ofFigs. 1 and 2. These are isochrones and crossover frequencydata (Anderson et al. 1994). These data are not shown herefor the sake of brevity.
CONCLUSIONS
Introduction of RPE into asphalt results in the productionof high performance asphalt binders. The PE modified asphaltshave a special composite morphology. The increases in thematerial functions of the modified asphalts are not compatiblewith the models proposed for changes in the material functionsof suspensions after addition of the same volume percentageof particles. The main reason for this difference is compositionchanges of asphalt matrix. The changes in the composition ofasphalt result from two major sources (i.e., the asphalt matrix
TABLE 4. Results of Conventional Tests for Asphalts
Property(1)
Base asphalt(2)
1%RPE(3)
1%7%EXACT4041(4)
1%7%EPDM447(5)
1%7%EPDM541(6)
Penetration (at 257C) 152 95 71 71 70Ring and ball (7C) 39 43 44 45 48Fraass (7C) 217 211 214 211 210TSHRP (7C) 58 74 70 77 78
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FIG. 10. G */sin d versus Temperature for Asphalts at 10 rad/s
aging during the mixing process at high temperature and theabsorption of some asphalt constituents by RPE particles). Thedifferent modified RPE particles have different affinity towardasphalt whereas the extent of aging of asphalts is the same forall modified asphalts. Consequently, starting with the sameasphalt, the resulting modified asphalts show different be-haviors. According to different asphalt performance tests, theEPDM541 modified asphalt shows the best results at high tem-peratures whereas the EXACT4041 modified asphalt has thebest behavior at low temperatures. Finally, the modified RPEscan provide high performance asphalt binders whereas theproblem of the stability of the PE suspensions in asphalt me-dium remains unsolved. In brief, the asphalt-PE system doesnot need compatibilization, but stabilization.
ACKNOWLEDGMENTSThe writers acknowledge the financial support provided by the Natural
Sciences and Engineering Research Council of Canada and the Fondspour la formation de chercheurs at l’aide a la recherche of Quebec. Thewriters also acknowledge Transplastek, Dow Chemical Canada, HimontCanada, Exxon USA, and Huls USA, who gracefully provided us withraw materials. The first writer wishes to express his gratitude for thescholarship provided by the Ministry of Culture and Higher Education ofThe Islamic Republic of Iran for his PhD program.
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