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Rutting Characteristics of Styrene-Ethylene/Propylene-Styrene Polymer Modified Asphalt
Mohammad Rahi1; Ellie H. Fini, M.ASCE2; Pouria Hajikarimi3; and Fereidoon Moghadas Nejad4
Abstract: Asphalt binder resistance to permanent deformation at intermediate temperature significantly affects overall pavement resistanceto rutting. To enhance asphalt resistance to permanent deformation, researchers have used various modifiers and additives such as styrenebutadiene styrene (SBS) block copolymer, ethylene vinyl acetate (EVA), polyvinyl acetate (PVA), styrene butadiene rubber (SBR), andnatural rubber latex. In this paper, the effectiveness of modification of asphalt binder by styrene-ethylene/propylene-styrene (SEPS) in orderto enhance asphalt binder’s resistance to permanent deformation has been investigated using two different evaluation methods: the Superpavespecification parameter, G�=sin δ, and the cross model for calculating zero shear viscosity (ZSV). The experiments were conducted usingdynamic shear rheometer (DSR) frequency sweep test performed on both the neat sample and modified ones at 40, 50, and 60°C. Utilizingthese two approaches, rutting resistance of modified asphalt binders are determined and then normalized using the rutting resistance of theneat asphalt binder as the control data in order to calculate the rutting resistance improvement ratio. The research results showed thatregardless of the evaluation method, SEPS can significantly improve anti-rutting performance of asphalt binder. However, it was foundthat ranking of modified asphalt binders according to the two approaches is different. DOI: 10.1061/(ASCE)MT.1943-5533.0001102.© 2014 American Society of Civil Engineers.
Author keywords: Styrene-ethylene/propylene-styrene (SEPS); Rutting resistance; Zero shear viscosity (ZSV); The cross model;Pseudoplastic; Permanent deformation; Pavement distress; Rheology.
Introduction
Permanent strain or rutting is one of the most important forms ofdistress in asphalt pavements. Rutting has been distinguished as aprimary distress mechanism and a major design criterion for flex-ible pavements (Parker and Brown 1990). Permanent deformationappears in the wheel path as longitudinal surface depressions andcauses poor serviceability of the pavements causing vehicles’ ridesto become rough and dangerous (Golalipour 2011). The main rea-son for the rutting of asphalt pavements is known to be the accu-mulated strain which is a consequence of traffic loading (Sybilski1996; Philips and Robertus 1996).
The accumulation of permanent strain in the asphalt surfacelayer is considered to be the major origin of rutting, althoughthe rutting observed on flexible pavements can be measured bythe total sum of accumulated permanent strains in one or morelayers of the pavement structure (Garba 2002). The rutting suscep-tibility of a pavement is also influenced by aggregate skeleton,
aggregate-asphalt interactions, and void ratio in mineral aggregates.However, the characteristics of the binder are known to be adominant factor, especially for modified asphalt binders that areclaimed to improve the rutting resistance of asphalt pavements.One of the most common modifiers that are used to improve hightemperature characteristics of asphalt binder is styrene-butadiene-styrene (SBS) (Sengoz and Isikyakar 2008). Styrene-ethylene/butylene-styrene (SEBS) and styrene-ethylene/propylene-styrene(SEPS) are two products of thermoplastic elastomer (TPE) families.While the effect of SEBS modification on the rheological propertiesof asphalt binder has been thoroughly investigated (Lu et al. 1999;Lu and Isacsson 2000), SEPS has been well received in asphaltliterature and has not been considered as an asphalt modifier. Assuch this paper will investigate the effectives of SEPS on improvinganti-rutting performance of asphalt binder.
The properties related to rutting should be observed in the upperrange of pavement service temperatures because rutting is moredominant at high temperatures than at low temperatures (Petersenet al. 1994). Several research works have been devoted to perma-nent deformation in order to formulate a specification parameterthat can explain and measure the anti-rutting characteristics ofan asphalt binder (Coenen 2011; Desmazes et al. 2000; Centenoet al. 2008; van Rooijen and de Bondt 2004; Rowe et al. 2002).
In this study, rutting resistance of asphalt binders that are modi-fied using SEPS is compared to that of neat binder at variousmodification scenarios by two different specification parameters:The Superpave specification parameter,G�=sin δ, and the zero shearviscosity (ZSV). The Superpave specification parameter, G�=sin δ,has been used for many years as a rutting parameter (Anderson et al.1994). Although it has been demonstrated that the relationshipbetween G�=sin δ and rutting is insignificant (Bahia et al. 2001;D’Angelo et al. 2006), it is still being used in preliminary ratingof binders in terms of their rutting resistance. The ZSV is definedas the viscosity related to a constant strain rate as the stress tends
1Head of Research and Development Dept., Pasargad Oil Company,Tondgooyan Highway, Rajaei Shahr, 19395-4598 Tehran, Iran (corre-sponding author). E-mail: [email protected]
2Assistant Professor, Dept. of Civil Engineering, North Carolina A&TState Univ., 1601 E. Market St., Greensboro, NC 27411. E-mail: [email protected]
3Ph.D. Student, Dept. of Civil and Environmental Engineering,Amirkabir Univ. of Technology, 15875-4413 Tehran, Iran. E-mail:[email protected]
4Associate Professor, Head of Transportation Group, Dept. of Civil andEnvironmental Engineering, Amirkabir Univ. of Technology, 15875-4413Tehran, Iran. E-mail: [email protected]
Note. This manuscript was submitted on September 29, 2013; approvedon April 10, 2014; published online on July 28, 2014. Discussion periodopen until December 28, 2014; separate discussions must be submitted forindividual papers. This paper is part of the Journal of Materials in CivilEngineering, © ASCE, ISSN 0899-1561/04014154(5)/$25.00.
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toward zero (D’Angelo et al. 2007). Anderson et al. (2002) havebeen shown that ZSV can be used to characterize asphalt bindercontribution to rutting. They reported different methods that canbe implemented in order to estimate the ZSV. Application of thecross model to dynamic viscosity measurements is one of thesemethods that previously was used by Sybilski (Sybilski 1996).
The following sections of the paper provide a brief overview ofprevious studies and theoretical basis of Superpave specificationparameter, G�=sin δ, as well as the cross model for determiningthe zero shear viscosity, followed by a description of materialsand test methods. The cross model is fitted on complex viscosityversus angular frequency curves in which complex viscosity ismeasured during the frequency sweep test. Rutting specificationparameters for both neat and modified asphalt binders were thencalculated using each of the aforementioned approaches. Eachmethod prioritized asphalt binders based on the rutting resistanceparameter. G�=sin δ was used in the first method, and the ZSV wascalculated in the second method. Finally, the merit of application ofeach method to rank various modifiers in terms of their anti-ruttingperformance of modified asphalt binders is discussed.
Background
Brief Overview of Previous Studies
Rutting can take place in any one or more of the pavement layers aswell as in the subgrade. Rutting development can be described as atwo-stage process, densification with volume change of asphalt andshear deformation inside the asphalt mixture (Sousa et al. 1991).The second stage is related to plastic deformation of the surfacecourse. Asphalt mixture is a viscoelastic material, and asphaltbinder is responsible for the viscoelastic behavior of all bituminousmaterials (Wang 2011). Therefore, one of the major factors thataffects the plastic flow of bituminous mixture is the type of asphaltbinder used in the mixture (Centeno et al. 2008).
In 1993, Superpave introduced the dynamic shear rheometer(DSR) as an apparatus to measure asphalt binder mechanical char-acterization. Inspired by the convenient problem of torsional flowbetween parallel plates in fluid mechanics (Bird et al. 1987), ap-plying oscillatory stresses or strains over a range of temperaturesand loading frequencies to a thin disc of binder, squeezed betweenthe two parallel plates of the DSR was used.
As the first step, Anderson et al. (1994) introduced G�=sin δ asthe rutting parameter in which both G� (shear complex modulus)and δ (phase angle) are outcomes of the DSR test. In order to relatethis parameter to rutting, it is assumed that rutting is caused by thetotal dissipated energy.
Some researchers showed that there is no proper correlationbetween mixture rate of accumulated strain and the Superpavespecification parameter, G�=sin δ. Therefore, other parameters wereintroduced that can be determined using a DSR setup and that at-tempt to reflect a better correlation between asphalt mixture proper-ties and the corresponding asphalt binder.
For the purpose of verification of the relationship betweenbinder properties and road pavement performance, Sybilski usedthe fundamental property of viscosity and introduced zero-shearviscosity as a rutting parameter of asphalt binder (Sybilski1996). During the NCHRP 9-10 project of National CooperativeHighway Research Program (NCHRP), Bahia et al. (2001) pro-posed the repeated creep and recovery (RCR) test to identifynon-viscous flow that contributes to the permanent deformationfrom the total dissipated energy. Implementing the Burgers model,they introduced the viscous component of the creep stiffness which
can be determined using RCR test which is a cyclic test performedby a DSR setup (Delgadillo et al. 2006). Following the RCR test,D’Angelo et al. (2006) developed the multiple stress creep andrecovery test (MSCR) to reduce the number of samples at eachstress level and selected two stress levels, 0.1 and 3.2 kPa, uponcorrelation between binder and mixture rutting results for perform-ing the MSCR test (D’Angelo et al. 2007).
Superpave Specification Parameter
At high temperatures, during the deformation of asphalt binder, thework done is partially recovered by the elastic component of thestrain and partially dissipated by the viscous flow component ofthe strain and any associated generation of heat. Anderson et al.(1994) assumed that rutting is caused by the total energy dissipatedper cycle of loading. Using such an assumption for a sine waveloading, it can be determined that
ΔU ¼ πτ 2max1
jG�jsin δ
ð1Þ
where ΔU = energy loss per cycle or dissipated energy; τmax =maximum shear stress; jG�j = shear complex modulus; δ = phaseangle.
As can be seen in Eq. (1), jG�j=sin δ is inversely proportionalto the total dissipated energy. Therefore, the increase of G�=sin δ[the denominator in Eq. (1)] causes the total dissipated energyto decrease. This in turn leads to the reduction of the rutting sus-ceptibility. For this reason, the Superpave specification parameter,G�=sin δ, was used for high temperature performance gradingof paving asphalts to rank asphalt binders based on their ruttingresistance.
Zero Shear Viscosity
Unmodified asphalt binders are Newtonian fluids at the high tem-peratures in which rutting behavior is dominant. However, mostof the modified asphalt binders show a phenomenon known aspseudoplasticity, or shear-thinning flow behavior, in which viscos-ity decreases by increasing shear rate. Pseudoplastic liquidsbehave similarly to Newtonian liquids at very low shear rates,and their viscosity is defined independent of shear rate or fre-quency. This viscosity, ηo, is called the ZSVor Newtonian viscos-ity (Yildirim et al. 2000). Sybilski demonstrated that the ruttingresistance of asphalt mixtures is proportional to zero shear viscos-ity of asphalt binders. There are several methods to determine theZSV (Sybilski 1996). In this research, the frequency sweep testwas implemented to determine the ZSV through the cross model.The flow curves of pseudoplastic fluids can be described using thecross model, which is a four parameter model (Sybilski 1996;Sybilski 1994)
η� − η�∞η�o − η�∞
¼ 1
1þ ðKωÞm ð2Þ
where η� = complex viscosity; η�o = ZSV; η�∞ = limiting viscosityin the second Newtonian region; ω = angular frequency (rad=s);and K and m = constant parameters.
The frequency sweep test is usually performed between 0.1 and100 rad=s. In this domain, it is an appropriate assumption thatη� ≫ η�∞ (Anderson et al. 2002). With this assumption, the crossmodel can be re-written to
η� ¼ η�o1þ ðKωÞm ð3Þ
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Thus, implementing a curve fitting tool, it is possible to fitEq. (3) on a complex viscosity versus angular frequency curveat each test temperature to obtain the ZSV. The ZSV is directly pro-portional to rutting resistance of asphalt binders. As such, greaterzero shear viscosity indicates better anti-rutting performance of themodified asphalt binder.
Experimental Program
Materials
The base bitumen with an 85=100 penetration grade was acquiredfrom the petroleum refinery of Pasargad Oil located in Tehran,Iran. The SEPS polymer used was Kraton G1780M supplied byKRATON Polymers (Houston, Texas). Kraton G1780M polymeris a clear multi-arm polymer based on styrene and ethylene/propylene with a polystyrene content of 7% in powder form, andit has a star structure.
Preparation of SEPS Modified Asphalt
The SEPS modified asphalt binder samples were prepared bymeans of an IKA (Germany) high shear mill rotating at5000 rpm. In preparation, the base binder was heated to fluid con-dition (170–180°C). The SEPS polymer was then added slowly tothe base binder. The temperature was kept constant, and the blend-ing process continued for 45 min. The SEPS Kraton G1780M con-centrations in the base binder were chosen to be 2, 4, and 6% byweight of base binder in accordance with prior research that inves-tigated the effect of modification of asphalt binder using SBS andSEBS (Elseifi et al. 2003; Lundström and Isacsson 2004; Awantiet al. 2008; Kök and Çolak 2011; Lu et al. 1998).
Test Methods
High Temperature Storage Stability TestIn order to ensure high-temperature storage stability of SEPSpolymer modified specimens, consistent with the European testmethod for the determination of storage stability of modified bitu-men (EN 13399 2010), modified asphalt binders were poured intoan aluminium foil tube and kept in an oven at 180°C for three days.The tube was then removed from the oven and cooled in a verticalposition at room temperature followed by storage for several hoursin a freezer at −20°C before being cut into three equal sections.After that, the specimens obtained from the bottom and top sectionswere used to evaluate the storage stability of the SEPS polymermodified asphalt binders by measuring their softening points.If the difference of the softening points between the bottom andtop sections was less than 5°C, the modified asphalt was consid-ered stable under high-temperature storage condition. Resultsof the storage stability test are depicted in Table 1. As can be ob-served from this table, the differences between the top and bottom
softening points in all samples are less than 5°C. This indicates thatSEPS modified specimens have appropriate high-temperaturestorage stability.
Frequency Sweep TestTo determine high-temperature characteristics of the base andSEPS polymer modified asphalt binder, the frequency sweep testwas performed at 40, 50, and 60°C. In this study, the frequencysweep tests were conducted by implementing an Anton Paarrheometer (Austria) using 25 mm diameter parallel plates anda 1 mm gap opening. These tests were performed under thecontrolled-strain conditions at frequencies between 0.1 and 100.To remain within the linear viscoelastic region, a shear strain of1% was selected. Also, the test temperatures were chosen toachieve dynamic oscillatory shear test in the upper range of pave-ment service temperatures in which rutting is dominant.
Consistent with ASTM D 7175-05 (ASTM 2005), two repli-cates were done, and precision and bias specifications werechecked and determined to be acceptable for all modified andnon-modified samples.
Results and Discussion
Superpave Specification Parameter
In order to compare neat and modified asphalt binders, results offrequency sweep test at 40, 50, and 60°C were used to calculate theSuperpave specification parameter (G�=sin δ) for each specimen.These results are depicted in logarithmic scale in Figs. 1–3, respec-tively. As can be seen in these figures, as the concentration of SEPSincreases so does G�=sin δ. In addition, the trend was found tobe consistent at different temperatures. This in turn indicated thatmodification of asphalt binder using SEPS improves rutting resis-tance of the neat asphalt binder.
Based on the strategic highway research program (SHRP) super-pave protocol, to determine the rutting resistance improvement ra-tio, the G�=sin δ of modified asphalt samples at the frequency of1.59 Hz (10 rad=s) is specified and then is divided by G�=sin δof unmodified samples at the same frequency. In this study, theaforementioned ratio was calculated to reflect the amount of ruttingresistance improvement due to addition of SEPS polymer. Fig. 4shows the improvement ratio for all test temperatures. In Fig. 4,the effect of SEPS modification on rutting properties of the neat
Table 1. Results of High Temperature Storage Stability Test of SEPSPolymer Modified Asphalt
Sample
Softening point (°C)
Top Bottom SPTop − SPBottom
Neat 47.7 47.8 0.12% SEPS 53.5 54.4 0.94% SEPS 57.9 59.5 1.66% SEPS 62.2 64.4 2.2
Note: SP = softening point.Fig. 1. G�=sin δ for all samples at the test temperature of 40°C
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asphalt binder is even more significant at high temperature. Further-more, the anti-rutting performance showed consistent improvementwith increasing SEPS content, with 6% SEPS showing the highestimprovement. Introduction of 6% SEPS can increase rutting resis-tance of the neat asphalt binder by 9.8 times (at the test temperatureof 60°C).
Zero Shear Viscosity
As mentioned earlier, the cross model is used to determine the ZSV.The cross model includes three parameters, zero shear viscosity(η�o) and two constants (K and m) that are calculated using multiplenon-linear regression analysis. For this purpose, the MATLABCFTOOL (Curve Fitting Toolbox) is implemented to fit the desiredexpression of Eq. (3) on the curve of complex viscosity versusangular frequency at 40, 50, and 60°C. Fig. 5 shows experimentaldata of complex viscosity versus angular frequency and the corre-sponding fitted curves for SEPS polymer modified asphalts at thetest temperature of 60°C.
To compare the effect of SEPS modification on high-temperaturecharacteristics of asphalt binder, the ZSVof each modified samplewas determined. Dividing the ZSVof each modified binder by thatof neat binder determined an index that served as a measure ofrutting resistance improvement of the base binder. Fig. 6 illustratesthe rutting resistance improvement ratio for all samples at test tem-peratures of 40, 50, and 60°C in a logarithmic scale. Accordingto Fig. 6, the effect of 4 or 6% modification with SEPS is signifi-cantly higher than that of 2%. However, as shown in Fig. 4, theSuperpave specification parameter (G�=sin δ) shows a differenttrend for the rutting resistance improvement ratio. Also, basedon Fig. 6, at the higher temperatures of 50 and 60°C, 4% SEPSwas found to be more effective than 6% SEPS. Therefore, in order
Fig. 2. G�=sin δ for all samples at the test temperature of 50°C
Fig. 3. G�=sin δ for all samples at the test temperature of 60°C
Fig. 4. Rutting resistance improvement ratio of SEPS polymer mod-ified asphalt at all test temperatures based on G�=sin δ
Fig. 5. Complex viscosity versus angular frequency at 60°C
Fig. 6. Rutting resistance improvement ratio of SEPS polymer mod-ified asphalt at all test temperatures based on ZSV
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to thoroughly compare the effectiveness of the two proposedapproaches in evaluating rutting resistance of asphalt binder, aswell as their correlation with the field data, further investigationis needed.
Conclusion
In this paper the effectiveness of modification of asphalt binderby SEPS in order to enhance the asphalt binder’s resistance topermanent deformation has been investigated using two differentevaluation methods: the Superpave specification parameter,G�=sin δ, and the cross model for calculating ZSV. The aforemen-tioned two methods were implemented using DSR tests conductedin frequency sweep mode. In both approaches, there is a particularparameter for determining the rutting resistance improvement ratioof modified asphalt binders in comparison with the base binder.Results of both methods show that SEPS modification of asphaltbinder can improve rutting resistance of the neat asphalt binder.However, there is a difference between the results of the presentedmethods. The first method, the Superpave specification parameter,indicates that anti-rutting performance consistently improves as theconcentration of SEPS polymer increases. However, the ZSVmethod shows that 4% SEPS shows a better anti-rutting perfor-mance than 6% SEPS. Therefore, investigation of rutting resistancebehavior of SEPS modified binder should be extended to the mix-ture level in order to investigate the significance of anti-ruttingresistance improvement due to the introduction of SEPS polymerto neat binder.
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