Fuel 97 (2012) 603–611

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Fuel

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The effect of long-term aging on the rheology of warm mix asphalt binders q

Ambarish Banerjee a,⇑, Andre de Fortier Smit b,1, Jorge A. Prozzi a,2

a Department of Civil, Architectural and Environmental Engineering, The University of Texas at Austin, ECJ Bldg., Ste. 6.10 (C1761), Austin, TX 78712, United Statesb Center for Transportation Research, The University of Texas at Austin, ECJ Bldg., Ste. 6.10 (C1761), Austin, TX 78712, United States

a r t i c l e i n f o

Article history:Received 13 October 2011Received in revised form 29 January 2012Accepted 30 January 2012Available online 22 February 2012

Keywords:Long-term agingRheologyWarm mix asphaltMaster curve

0016-2361/$ - see front matter Published by Elsevierdoi:10.1016/j.fuel.2012.01.072

q Importance of Paper: This paper proposes a novelmaster curves for the complex shear modulus of asphshift factor equation has been estimated outside thehas led to propagation of biases from one model toproposed in this paper uses joint estimation to determtemperature and aging duration on the rheological proFurthermore, the study analyzes the effect of aging ona control PG 64-22 binder and four different WMA binRediset WMA binder has the lowest stiffness in shortrather slowly as compared to the other binders includewas noticed that the Sasobit WMA binder though hasthe PG 64-22, due to its slower rate of gain in stiffnessthan the control over time. It is therefore expected thon the rheology of WMA binders will translate to bet⇑ Corresponding author. Tel.: +1 512 507 8605; fax

E-mail addresses: ambarish03@yahoo.com (A. Bane(A. de Fortier Smit), prozzi@mail.utexas.edu (J.A. Proz

1 Tel.: +1 512 906 5495; fax: +1 512 475 7314.2 Tel.: +1 512 471 4771; fax: +1 512 475 7314.

a b s t r a c t

The study described in this paper aims at quantifying the long-term aging effects on the rheologicalproperties of warm mix asphalt (WMA) binders. Under the scope of this study, the authors focusedon four warm mix asphalt additives: Sasobit, Rediset, Cecabase and Evotherm, of which the first oneis an organic and the latter three are synthetic additives. The rheology of the aforementioned additivesadded to a PG 64-22 binder were studied using a frequency sweep test performed over a range of tendifferent loading frequencies and three different temperatures with different degrees of exposure tooxidative aging.

The paper proposes a more efficient methodology for developing master curves for asphaltic materi-als based on joint estimation of the model parameters that eliminates any possible propagation ofbiases from one model to the other. The developed master curves accommodate the simultaneousquantification of the effect of loading time, temperature and aging on the rheological properties ofthe WMA binder.

The paper finally addresses the effect of aging on different WMA binders. Results indicated that theRediset WMA binder had the lowest shear modulus, followed by the Evotherm, Cecabase and SasobitWMA binders. However, a different rate of gain trend was observed in the modulus values where,again, Rediset was slowest of all but followed by Sasobit, Evotherm and Cecabase binders. This impliesthat, of all the binders investigated in this study, the Sasobit WMA binder will have a significantlylower modulus over time as compared to the control PG 64-22 binder and the Rediset WMA binder willhave the lowest modulus in the short-term as well as over time.

Published by Elsevier Ltd.

1. Introduction

The area of pavement engineering has been steadily evolvingover the last two decades due to growing concerns over global

Ltd.

methodology for developingalt binders. Traditionally, themaster curve equation whichthe other. The methodologyine the effect of loading time,perties of the asphalt binder.the rheological properties of

ders. It was observed that the-term and also gains stiffnessd in this study. In addition, ita higher initial stiffness than; it will have a lower stiffnessat the reduced effect of agingter fracture resistance.: +1 512 475 7314.rjee), asmit@mail.utexas.eduzi).

warming and air quality. Until recently, the effort has been to-wards increased use of emulsified asphalt technologies in orderto reduce the carbon footprint of the highway construction indus-try. Over the last decade, researchers around the world havestarted exploring another significant area that promises to serveidentical needs. This new technology attempts to reduce the mix-ing and compaction temperatures through the incorporation ofwarm mix additives to conventional asphalt binder.

Prominent warm mix technologies can be primarily classifiedinto three groups based on the technique used to lower the mix-ing and compaction temperature. These include: (1) organic addi-tives like Sasobit and Asphaltan, (2) synthetic zeolite-basedadditives like Advera and Aspha-Min and (3) emulsion basedtechnologies like the Evotherm 3G and Evotherm DAT. A fourthgroup includes the injection of water to produce a foaming effect,but this group is out of the scope of this paper since it does notuse additives.

Advocates in favor of WMA technologies suggest that the re-duced mixing and compaction temperatures improve the long-term durability of the mix due to reduced short-term aging. Thiswork also studied the effects of oxidative aging on the mechanicalproperties of four different WMA binders.

604 A. Banerjee et al. / Fuel 97 (2012) 603–611

2. Objective

This research study investigates the long-term aging of WMAbinders in comparison to HMA binders. The specific objectives ofthis study are:

� To investigate the rheological properties of one organic (Sas-obit) and three synthetic (Rediset, Cecabase and Evotherm)additives of WMA binders. Please note that the Rediset WMAadditive is an organic additive that needs to be processed andtherefore, for most purposes, is classified as a synthetic additive[1].� To quantify the effects of aging on Sasobit, Cecabase, Rediset

and Evotherm WMA binders and develop a predictive modelbased on experimental data that captures the long-term effectsof aging on the rheological properties of the binder.

3. Literature review

The effects of aging on the mechanical properties of the WMAbinders were investigated for the following four additives:

� Sasobit is a long chain aliphatic hydrocarbon produced byFischer–Tropsch synthesis from coal or natural gas [2]. It formsa homogeneous solution with the asphalt binder and produces asignificant reduction in its viscosity, which results in a reduc-tion of 10–30 �C in the mixing and compaction temperatures.Between 115 �C and 70 �C, Sasobit solidifies into microscopic,regularly distributed, stick-shaped particles that increase Sas-obit modified WMA binder stiffness [2].� Cecabase is an artificial additive manufactured by the Arkema

Group. It reduces the asphalt binder surface tension to improveits wetting characteristics.� Evotherm is a chemistry package that includes adhesion pro-

moters and emulsification agents to improve workability [3].Its bulk properties, like viscosity and particle size distribution,are similar to those of bituminous emulsions. However, thiswarm mix surfactant formulation allows complete coating ofdense-graded aggregate at temperatures as low as 60 �C [4].� Rediset is a chemical additive from AkzoNobel that allows the

asphalt mixing temperature to be reduced by as much as35 �C. By design, it is a surface active agent that improves wet-ting properties by significantly reducing the surface tension ofan asphalt binder at temperatures below its typical mixing tem-perature. However, Rediset differs from the other chemicaladditives (Evotherm and Cecabase) because it is also an anti-stripping agent, which is likely to improve its resistance tomoisture damage. It should be noted that the effect of long-term aging on the rheological properties of the WMA bindersis the primary focus of this study and therefore secondary fac-tors, such as moisture damage, are not addressed in this paper.

Xiao et al. [5] studied the effects of non-foaming WMA additiveson asphalt binder rheology. The study looked at the effect of fourdifferent additives, Rediset WMX, Evotherm, Cecabase and Sasobit,on five different PG binders. Each of the WMA binders slightly re-duced the viscosity of the binder, with Rediset and Sasobit achiev-ing the highest reduction. However, the Sasobit WMA additivenoticeably increased the true grade for all the binder samples. Atthe other end, the Cecabase WMA additive reduced the true gradeof the binder sample. The study reports that the Sasobit WMAadditive reduced the phase angle of the asphalt binder and in-creased the complex modulus for the same strain level at 60 �Camong other WMA additives. Therefore, all the evidence presentedin this study indicates that the addition of the Sasobit WMA addi-

tive will likely result in increased asphalt binder stiffness while theaddition of Cecabase will cause the opposite.

Akisetty et al. [6] studied the high temperature properties ofrubberized binders containing warm asphalt additives, namely As-pha-Min and Sasobit. Their results indicated that the use of Aspha-Min in rubberized asphalt binders translated into a higher viscositywhen compared to the control binder, whereas the use of Sasobithelped lower the viscosity of the asphalt binder. The study sug-gested that the viscosity measured for the WMA binders will grad-ually increase as the reaction period between the additives and theneat binder increases. The authors also investigated the effect ofAspha-Min and Sasobit on the binder’s high temperature grade. Re-sults showed that the high temperature grade significantly in-creased for both the aged and unaged Sasobit specimens whencompared to the control experiment, which indicates increasedbinder stiffness due to the addition of WMA additives. The use ofSasobit warm mix additive and its impact on the high temperatureproperties of the binder was also reported by Hurley and Prowell[7]. Their study showed that the improvement in the rutting prop-erties of the binder was accompanied by a lower tensile strengthratio (TSR) which, in the authors’ opinion, was mostly due to theanti-aging properties of the Sasobit additive. The TSR values weremeasured from moisture-conditioned specimens, in which casethe reported TSR values might were affected by moisture damagealso.

Lee et al. [8] investigated the effect of warm mix additives in as-phalt binders containing long-term aged binders. Their studyshowed that the addition of Sasobit reduced the viscosity of the as-phalt binder, while the addition of certain other additives, like As-pha-Min, was likely to increase its viscosity. Secondly, theirresearch also found out that the asphalt binder’s high temperatureproperties were improved with the addition of certain warm mixadditives, such as Sasobit and Aspha-Min. The incorporation ofthese additives, however, was likely to have an adverse effect onthe intermediate and low temperature properties of the binderwhen compared to the control binder.

Kanitpong et al. [9] reported that Sasobit helps lower the mixingand compaction temperatures by significantly reducing the poly-mer modified binder viscosity. Their study noted that the additionof 3% Sasobit greatly improved binder resistance to permanentdeformation and fatigue. On the down side, Sasobit modified as-phalt binders mixed at low temperatures could have a lower TSRthan hot asphalt mixtures due to possible entrapped moisture inthe aggregates.

Diefenderfer et al. [10] reported that the air void content in Sas-obit warm mixes was slightly lower than the hot asphalt mixtures,but the difference was statistically insignificant. The rutting perfor-mance of the Sasobit WMA mixtures and the HMA mixtures werealmost the same. TSR results for the Sasobit WMA and HMA mix-tures were inconclusive. The authors found that the TSR and per-manent deformation resistance for Evotherm WMA mixtureswere relatively poorer than the control HMA mixtures. In anotherstudy, however, the authors found that plant produced SasobitWMA mixtures showed a lower order of rutting than the controlHMA mixes [11]. The relatively improved performance of the Sas-obit mixes was attributed to the stiffening properties of Sasobit attemperatures below their melting point. Field trial studies showedthat the stiffness gain rate after construction was reduced in theSasobit produced WMA as compared to the HMA [12] due to as-phalt binder in-service aging. For Evotherm WMA mixes, theauthors did not observe any difference in the recovered binder per-formance grade when compared to the control HMA mixes, exceptfor an increase in one low temperature grade after being in servicefor two years.

Vaitkus et al. (2009) noticed that the addition of Cecabase RTWMA additive has a negative impact on the Marshall Stability

A. Banerjee et al. / Fuel 97 (2012) 603–611 605

value when compared to the control HMA mix [13]. In addition, theauthor also witnessed lower flow numbers compared to controlmixes, which is unexpected for mixes with lower stability values.However, contrary to the observation made by Diefenderfer, Vait-kus and his associates reported higher air void content in CecabaseRT WMA mixes at the same compaction temperature. While the re-sults presented in their paper was based on a limited dataset, it isworth noting that the compaction effort required might differ sig-nificantly between different warm mix technologies.

Hurley and Prowell (2006) noticed that the addition of WMAadditives significantly reduced rutting potential when comparedagainst the trial HMA mixes [3]. The authors observed that addingEvotherm WMA additive increased the mix’s resilient moduluswhen compared to trial mixes prepared with the same base binder.However, as pointed out by Kanitpong et al. (2007), the researchersreported that Evotherm WMA mixes’ rutting potential was sensi-tive to the mixing and compaction temperature [9]. Kanitponget al. argued that the WMA mixes’ rutting performance is affectedby the amount of in-place aging that they are subjected to overtheir service life. Additionally, the study observed lower TSR valuesin Evotherm WMA mixes than in control mixtures, which might bedue to the inherently low tensile strengths of the Evotherm mixes.

Wasiuddin et al. (2007) studied the effect of WMA binder addi-tives, especially Sasobit and Alpha-Min, on PG 64-22 and PG 70-28binder viscosity and mechanical properties [14]. Results showedthat Sasobit WMA binder viscosity was significantly lower thanthe HMA binders viscosity, which suggested that the mixing andcompaction temperatures could be lowered by 16 �C and 7 �Crespectively without compromising the required workability ofthe binder for proper coating of aggregates or the desired air voidpercentages. However, the Superpave G�= sin d parameter, used formeasuring the high temperature performance of the asphalt bin-der, was found to be higher than the HMA binder for both the agedand unaged samples. This indicates that the Sasobit WMA bindersare stiffer than the regular HMA binders at high temperatures. Asexpected, the Asphalt Pavement Analyzer (APA) rut depth mea-sured at 8000 cycles were seen to show the same trend as theSuperpave G�= sin d parameter.

Prowell et al. (2007) studied the rutting potential of WMA (Evo-therm) and HMA surface mixes [4]. Their study pointed out thatWMA mixes rut more than HMA surface mixes. Their resultsshowed that the increased rutting in WMA mixes could be attrib-uted to the tenderness of the mix. However, the improved work-ability imparted by the Evotherm additive helped attain a lowerair void percentage, which effectively compensated for the lowerstiffness of the mix and thus improved its resistance to permanentdeformation.

4. Experimental design

This research focuses on quantifying the effects of long-termaging on the rheological properties of the asphalt binder. The PG64-22 obtained from Jebro Inc. served as the control experimentfor the first dataset while the PG 76-22 from Valero Ardmorewas the control for the second dataset. The warm mix additivesconsidered for this study include the following:

� Sasobit� Cecabase RT 945� Rediset WMX� Evotherm 3G

Each of the two control binders were subjected to short-termaging in the Rolling Thin Film Oven (RTFO) at 163 �C. The warmmix binders were short-term aged at 143 �C based on the premise

that the addition of warm mix additives allows a reduction of20 �C on average in the mixing temperature. Both the controland warm mix binders were long-term aged within pans placedin an environmental room at 60 �C with film thicknesses notexceeding one millimeter. To evaluate the long-term aging effectson the binder’s rheological properties, it was necessary to obtainexperimental data on asphalt binders exposed to varying degreesof aging. Therefore, the rheological testing of the binder samplesaged in the environmental room was conducted at the followingtime intervals:

� ‘‘0’’ days (unaged sample)� 2 days� 5 days� 11 days� 22 days� 35 days� 67 days� 132 days

This aging duration was based on pilot studies that determinedthe effect of aging on the rheological properties of the binder. Itwas noticed that on a log–log scale, the duration of aging bears alinear relationship with the complex shear modulus. This observa-tion led the study team to select the aging duration evenly spacedon a logarithmic scale such that errors due to extrapolation areminimized.

The mechanical properties of the asphalt binder samples wereevaluated using a frequency sweep test on the dynamic shear rhe-ometer (DSR) at three different temperatures (i.e. 40 �C, 52 �C and64 �C) and at ten different frequencies that ranged from 0.1 Hz to25 Hz. However, the PG 76-22 binder produced stiffness valuesthat were higher than the torque capacity of the instrument. Thisled the study team to abandon that specific dataset because itcould not be used for further analysis due to issues caused byextrapolation errors.

5. Methodology

A master curve is used to express the rheological properties ofasphalt binder as a function of temperature and loading rate. Astandard reference temperature is selected; in this case 60 �C(140�F), then the measured data at various temperatures isshifted with respect to loading frequency until the curves mergeinto a single smooth function [15]. There are two primary math-ematical forms that have been proposed for modeling the mastercurve of asphalt binders. Jongepier and Kuilman (1970) proposedthat the relaxation spectrum for asphalt binders follow a log-nor-mal distribution [16]. This model is quite accurate when thephase angle varies from 10� to 70�. The model works well fortemperatures and frequencies extending into the glassy region,but at low frequencies, where the response of the material ap-proaches that of a viscous fluid, the model’s predictions areinconsistent with measured values. Dickinson and Witt (1974)proposed a practical hyperbolic model, but the fundamentalparameters, (e.g. the glassy modulus and the steady state viscos-ity) are statistically estimated, which can lead to miss-estimationof modulus in certain cases [17].

The inaccuracy of the model proposed by Jongepier and Kuil-man at low frequencies led the authors to adopt the statisticalmodel proposed by Dickinson and Witt. The expression of Wil-liams, Landel and Ferry (WLF) is widely used to describe the tem-perature dependence of the rheological properties of the material,as given in Eq. (1) [18].

logðaTÞ ¼ eþ hð60� TÞ ð1Þ

606 A. Banerjee et al. / Fuel 97 (2012) 603–611

where aT is the shift factor corresponding to the temperature T in �Cand e, h the model parameters that are estimated statistically.

In developing master curves for asphalt binders, researchershave traditionally used a reference temperature and the modelhas been developed to capture the dependence of the complexmodulus on the loading time, while the shift factors were calcu-lated using the WLF equation. In this context, the use of a referencetemperature does not make a difference as the same model can berepresented without a reference temperature as shown below.

logðaTÞ ¼ eþ hð60Þ � hðTÞlogðaTÞ ¼ e0 � hðTÞ

ð2Þ

Since the model parameters for the shift factor and the mastercurve are estimated separately, the standard error of the shift fac-tors is reflected in the master curve, which results in a higher errorin the predictions obtained. Joint estimation of the model parame-ters will produce unbiased parameters and can significantly reducethe standard error, thus improving the overall accuracy of the pre-dictions obtained from the model. Therefore, for the purpose of thisstudy, the Dickinson and Witt model was modified as given below.

logðG�Þ ¼ dþ a1þ ebþc log½f�10d�hðTÞ �

) logðG�Þ ¼ dþ a1þ ebþc log fþc log½10d�hðTÞ�

) logðG�Þ ¼ dþ a1þ ebþc log fþcðe�hðTÞÞ

) logðG�Þ ¼ dþ a1þ eb0þc log fþh0T

) logðG�Þ ¼ dþ a1þ ebþc log fþhT

ð3Þ

Fig. 1. Effect of frequency (left) and Age (right) on the complex shear modulu

where G� is the complex shear modulus (kPa), f the frequency (Hz),T the temperature (�C), and a, b

0, c, d, h is the model parameters.

The effect of aging on the rheological properties was a key as-pect of this study and the properties of the material have beenstudied at different points in time. Therefore, a master curve wasdeveloped to account for age-dependency of the material’s rheo-logical properties.

Long-term aging due to polar functional group oxidation in-creases the binder’s complex shear modulus which causes thematerial to harden over time. Thus, aging affects the rheologicalproperties in a similar fashion as that of loading frequency. Theage dependence of the material can be represented using anexpression that accounts for the necessary shift in the frequencydue to age difference between samples similar to that of the WLFequation. Instead of proposing a separate model to estimate theshift factors for age dependence, an alternative approach couldbe revising the master curve equation to account for long-termaging effects. It was observed that aging has a linear relationshipwith the complex shear modulus in a logarithmic scale. The rela-tionship between the logarithms of the two parameters becomesnon-linear at high levels of exposure to the aging conditions, butthe effect of one on the other can be approximated with a straightline without losing significant accuracy. Fig. 1 illustrates the rela-tionship between log (G�) and frequency as well as age. It is evidentfrom the figure that both frequency and age have a similar effect onG�, but their respective magnitudes differ. Therefore, the effect ofage in the master curve can be captured by correcting the modelin light of the aforementioned discussion. The revised master curveequation proposed is given below.

logðG�Þ ¼ dþ a1þ ebþc log fþhTþk log Age

ð4Þ

s of the Cecabase WMA binder on PG 64-22 (up) and PG 76-22 (down).

Table 1Lower and upper tangents (d, a) to the master curve for different binders.

Coefficient Control Cecabase Rediset Sasobit Evotherm

d �0.32 �0.52 �0.82 �0.52 �0.89

Parameter assumed to be zeroa 7.54 7.56 7.58 7.57 7.52Averaged to 7.55

A. Banerjee et al. / Fuel 97 (2012) 603–611 607

where Age is the number of days of aging for the specified sample,and k the model parameter to account for the effect of aging on theloading frequency.

The model parameters in Eq. (4) can be estimated individuallyfor each of the different binders. However, it is unlikely that thedifferences between binders in the model parameters will be sta-tistically significant. Based on the information presented in theprevious section, one can expect similarity in the model parame-ters between the control PG 64-22 and the Sasobit WMA binder.While the control PG 64-22 was exposed to higher levels of shortterm aging in the RTFO to simulate the real world scenario, the Sas-obit WMA binder has been reported by many to have higher stiff-ness values due to the composition of the additive [11,14]. On thecontrary, the chemical additives Cecabase RT, Rediset and Evo-therm 3G have all been reported on different occasions to have rel-atively lower modulus values. For example, Vaitkus et al. (2009)reported noticeably lower Marshall Stability values for Cecabasewarm asphalt mixes. Prowell et al. (2007) and Diefenderfer et al.(2007) both reported poorer rutting performance for Evothermwarm asphalt mixes. Although it is not necessary that the poor rut-ting performance of these mixes is due to the lower modulus of theasphalt binder, it has been pointed out on many occasions that theproperties of dense-graded asphalt mixtures are dictated more bythe asphalt binder than by the aggregates. It can be hypothesizedthat the poorer laboratory rutting performance of the Evothermand Cecabase warm mixes is due to the relatively lower complexmoduli of the warm asphalt binder, which can partly be attributedto reduced exposure to oxidative aging. There seems to be a cleardistinction between the five binders that were studied as part ofthis research and similarities between the model parameters ofcertain binders are expected.

Factor analysis is a statistical method used to describe variabil-ity among observed variables in terms of a potentially lower num-ber of unobserved variables called factors. It was used in this studyas a data reduction technique to segregate variables that have astatistically significant effect on the response variable from thosethat do not. As mentioned, the study aims to develop a mastercurve for the complex shear modulus as a function of time, temper-ature and aging conditions for the control PG 64-22 binder and thefour different WMA additives included in this study. The use of fac-tor analysis helps the authors meet this objective by identifyingvariables that do not have a statistically significant effect on thecomplex shear modulus so that the proposed equation includesonly those variables that influence the G�.

The second objective of this study is to quantify the effect ofaging on different warm mix additives. To meet this objective,the authors calculated the marginal value of the complex shearmodulus with respect to the duration of aging in order to quantifyits effect on the rheological properties of warm mix additives. Mar-ginal value is the ratio of the dependent variable’s change to that ofthe independent variable and, therefore, it is a measure for theslope of the function with respect to the independent variables. Ahigher marginal value indicates a greater change in the responsevariable due to unit change in the independent variable and viceversa.

6. Results and discussion

Shear testing of the control and warm mix binders were con-ducted in the laboratory using the Dynamic Shear Rheometer(DSR). The experimental data was analyzed using statistical toolsto estimate the model parameters, as presented in Eq. (4).

Past research studies have indicated that the complex shearmodulus of the binder approaches the glassy modulus at low tem-peratures and high loading frequency, which in most cases is as-

sumed to be 3 GPa [19]. Although the test environment chosenfor this study does not extend into the glassy region, the shearmodulus of the different binders approaches the same value athigh frequencies and low temperature over extended period ofaging. This suggests that a can be substituted with a single value,irrespective of the WMA additive. In addition, preliminary estima-tion suggests that the parameter d approaches zero for each of thefive different binders. The parameters d and a can be interpreted asthe lower and upper tangents to the sigmoidal curve used for mas-ter curves. The physical interpretation of d approaching zero is thatat high temperature and loading time, the complex shear modulusof the binder approaches zero. The estimates for ‘d’ and ‘a’ for thedifferent binders are provided in Table 1.

In the light of the results presented in Table 1, Eq. (4) can bemodified as given below.

logðG�Þ ¼ 7:551þ ebþc log fþhTþk log Age

ð5Þ

) 1þ ebþc log fþhTþk log Age ¼ 7:55logðG�Þ

) ebþc log fþhTþk log Age ¼ 7:55logðG�Þ � 1

) bþ c log f þ hT þ k log Age ¼ ln7:55

logðG�Þ � 1� �

) bþ c log f þ hT þ k log Age ¼ y

) y ¼ bþ c log f þ hT þ k log Age ð6Þ

where

y ¼ ln7:55

logðG�Þ � 1� �

:

The model presented in Eq. (6) represents a linear relationshipbetween G� and the loading frequency, temperature and aging per-iod and, therefore, the parameters in the equation can be estimatedthrough linear regression individually for the different binders. Themodel parameters for the control PG 64-22 binder and the RedisetWMA additive are summarized in Table 2.

Having determined the master curve parameters for the indi-vidual binders, the next step involved combining the differentmodels into a single master curve that accommodated all the bind-ers. The authors used factor analysis to distinguish between thesignificant and the non-significant variables. The final master curvethat was developed to accommodate the five different binders,including the control PG 64-22, is given in Table 3.

The proposed model allows someone to predict the complexshear modulus as a function of loading time, temperature andaging conditions for each of the four different WMA additives aswell as the control PG 64-22 binder. In addition, the model cap-tures the marginal effect of the loading frequency, temperatureand aging conditions on the complex shear modulus for each ofthe WMA additives with respect to the control PG 64-22. The vari-ables ‘‘Ceca,’’ ‘‘Redi,’’ ‘‘Saso’’ and ‘‘Evo’’ serve as binary variables in

Table 2Master curve parameters for control PG 64-22 and Rediset WMA binders.

Parameter Coefficient t-Stat p-Value

Control PG 64-22b �2.10 �123 <0.00c �0.47 �116 <0.00h 0.04 122 <0.00j �0.30 �65.2 <0.00Master curve for PG 64-22: logðG�Þ ¼ 7:55

1þe�2:14�0:47 log fþ0:04T�0:8 log Age

Rediset WMAb �2.02 �95.4 <0.00c �0.48 �99.8 <0.00h 0.04 102 <0.00j �0.27 �45.4 <0.00Master curve for Rediset WMA: logðG�Þ ¼ 7:55

1þe�2:02�0:48 log fþ0:04T�0:27 log Age

Table 3Master curve for G� for PG 64-22 and Cecabase, Rediset, Sasobit and Evotherm WMAbinders.

Parameter Coefficient t-Stat p-Value

b �2.14 �195 <0.00c �0.475 �231 <0.00h 0.0388 204 <0.00j �0.298 �92.4 <0.00b (Cecabase) 0.117 5.91 <0.00b (Rediset) 0.122 14.4 <0.00b (Sasobit) �0.0157 �2.04 0.04b (Evotherm) 0.133 6.39 <0.00c (Rediset) �0.00922 1.96 0.05h (Cecabase) �0.000799 �2.15 0.03h (Evotherm) �0.00110 �2.96 <0.00j (Rediset) 0.0272 4.34 <0.00j (Sasobit) 0.0325 5.56 <0.00j (Evotherm) 0.0123 2.21 0.03

608 A. Banerjee et al. / Fuel 97 (2012) 603–611

the model for the Cecabase, Rediset, Sasobit and Evotherm WMAadditives, respectively.

As stated in the objective, the study quantified the relative ef-fect of aging on the rheological properties of the WMA binder. Itis important to realize that asphalt binder has a viscoelastic re-sponse when subjected to dynamic loads, which implies that itsproperties will vary depending on the temperature and loading fre-quency. Therefore, it is evident that the effect of aging on the rhe-

Fig. 2. Predicted G� with aging time fo

ological properties of binders will be dictated by the choice oftemperature and loading frequency.

The selection of temperature and loading frequency was madeafter giving due consideration to the three most prominent formsof distresses seen in bituminous pavements: rutting, fatiguecracking and low temperature cracking. High pavement tempera-tures and loading times present the most critical conditions forplastic deformation or rutting. The Superpave performance grad-ing (PG) uses two numbers representing temperatures in degreeCelsius: the first is the average seven-day maximum pavementtemperature and the second is the minimum pavement designtemperature likely to be experienced. For example, a PG 64-22is intended for use where the average seven-day maximum pave-ment temperature is 64 �C and the expected minimum pavementtemperature is �22 �C. Therefore, the temperature and loadingfrequency corresponding to 60 �C and 1 Hz will be appropriateto study the effect of aging on the rheological properties, withrutting as the governing form of distress. Unlike rutting, low tem-perature thermal cracking is critical at low pavement tempera-tures and loading times. Therefore, the time–temperaturecontour chosen to study the effect of aging on the shear moduluswith low temperature thermal cracking as the dominant distresscriteria analysis was 0.1 s and 5 �C, respectively., While low tem-perature cracking is the result of fracture due to thermal stressesin the material that are induced by the restraint provided againstdimensional variations, fatigue cracking is a progressive andlocalized structural damage that occurs when a material is sub-jected to cyclic loading. Fatigue cracking in asphalt binders ismostly associated with intermediate temperatures and high load-ing frequencies [20], so the time–temperature regime correspond-ing to 0.1 s and 25 �C is ideal for studying the effect of aging onthe shear modulus with fatigue cracking serving as the dominantdistress mechanism.

The complex shear modulus was evaluated under each of thesethree conditions and the results corroborated some of the findingsfrom other research studies. For example, the complex shear mod-ulus of the control PG 64-22 is higher than any of the WMA addi-tives (see Fig. 2). The Sasobit WMA additive was next to the controlbinder while the Cecabase and Evotherm WMA binders had similartrends. The Rediset WMA binder was subjected to the least amountof aging over time. In fact, Fig. 2 shows that the G� for the RedisetWMA binder was less than half the G� predicted for the control PG

r different binders at 60 �C, 1 Hz.

Fig. 3. Predicted G� vs. aging time for different binders at 5 �C, 10 Hz.

Fig. 4. Predicted G� vs. aging time for different binders at 25 �C, 10 Hz.

A. Banerjee et al. / Fuel 97 (2012) 603–611 609

64-22 binder. This results presented in Fig. 2 also explain the re-ported tenderness in WMA mixtures by Prowell et al. (2007).

It is important to note that the materials were subjected toaccelerated aging and do not directly relate to the time scale forin-service pavements. The film thicknesses used for this experi-mental study was restricted to a maximum of one mm withconstant exposure to 60 �C; previous studies showed that an agingduration of 38 days in an environmental room maintained at 60 �Cis equivalent to 1 year of field-aging [21]. Therefore, the results re-ported in this study are almost an order of magnitude higher thanwhat would be expected under field aging conditions.

Although the tenderness in WMA binders compromises therutting performance to some degree, the use of WMA binders isespecially beneficial from a cracking perspective as the reduced

aging results in lower stiffness which implies improved fractureresistance. The complex shear modulus calculated at low temper-ature and high loading frequencies is illustrated through Fig. 3.The control and Sasobit WMA binders have the highest moduliover time while the Evotherm and Rediset WMA binders were sub-jected to least aging. It is therefore expected that WMA binderswith the chemical additives would be most resilient to lowtemperature cracking. Although the Sasobit WMA binder has asignificantly higher modulus than the other additives, it is stillexpected to perform better than the control PG 64-22 at low tem-peratures over time.

Similar trends were observed while assessing the effect of agingon G� from a fatigue cracking perspective. While the control andSasobit WMA binders had the highest moduli over time, the

Table 4Marginal values of G� w.r.t. duration of aging (at 70 �C and 1 Hz).

Type of additive

None (PG 64-22) Cecabase Rediset Sasobit Evotherm

Marginal of G� at # days of Aging 1 4.42 4.22 3.69 3.97 4.052 2.33 2.24 1.95 2.08 2.144 1.22 1.18 1.03 1.09 1.138 0.64 0.62 0.54 0.57 0.59

16 0.33 0.32 0.28 0.29 0.3132 0.17 0.17 0.15 0.15 0.1664 0.09 0.09 0.08 0.08 0.08

128 0.04 0.04 0.04 0.04 0.04

610 A. Banerjee et al. / Fuel 97 (2012) 603–611

Cecabase, Evotherm and Rediset additives showed the least sign ofaging over the same interval of time (see Fig. 4). This further high-lights the fact that WMA binders are expected to deliver superiorfracture resistance at conditions critical for fatigue or low temper-ature thermal cracking.

While Figs. 2–4 illustrate the effect of aging under conditionsconsidered critical for plastic deformation, low temperature ther-mal cracking, and fatigue cracking, it is essential to assess the asso-ciated benefit in quantitative terms. The marginal value of theshear modulus would help quantify the increase in G� correspond-ing to unit change in the duration of aging. The marginal value ofthe shear modulus for the PG 64-22 is as given below.

EG� ;Age ¼dG�

dAge

EG� ;Age ¼ �7:55� ð1þ ebþc log fþhTþk log AgeÞ�2

log810

� G�kek log Age

Age

EG� ;Age ¼ �7:55

ð1þ ebþc log fþhTþk log AgeÞ�2 �kek log Age

log810

� G�

Age

ð7Þ

The marginal values were calculated at 1, 2, 4, 8, 16, 32, 64 and128 days of aging and are summarized through Table 4.

The results presented in Table 4 suggest that, irrespective of theduration of aging, the control PG 64-22 ages faster than any of thewarm mix binders. On the other extreme of the spectrum, the Red-iset WMA binder has the slowest rate of aging followed by the Sas-obit and Evotherm WMA binders. Interestingly, the mixing andcompaction temperatures for hot mix asphalt binders are higherthan those for warm mix binders. These temperature effects weresimulated in the laboratory by selecting a higher aging tempera-ture for the Rolling Thin Film Oven. This translates into reducedshort-term aging, which is reflected by lower moduli for WMAbinders. In addition to short-term aging effects, the warm mixadditives have a significant influence on the long-term aging char-acteristics of the binder. The warm mix binders not only have low-er initial complex shear moduli but they also age at a significantlylower rate than the PG 64-22.

7. Conclusion

This paper reports the findings from an investigation of the longterm effects of aging on WMA binders. The authors evaluated theeffect of aging on the rheological properties, namely the complexshear modulus, of the binders in a laboratory environment. Thescope of this study included four different warm mix additives:one was an organic additive (Sasobit), while the other three weresynthetic additives (Cecabase, Rediset and Evotherm).

The research objective included investigating the long term ef-fects of aging when subjected to varying degrees of oxidative aging.Binder specimens were subjected to a frequency sweep on a dy-namic shear rheometer and the complex shear modulus was mea-sured at ten different loading frequencies and three differenttemperatures at periodic intervals of time in accordance with the

experimental design. The major findings from this study were asfollows.

� The authors proposed an approach for developing master curvesfor asphalt binders based on joint estimation that does notpropagate the WLF equation bias to the sigmoidal master curve,thus improving the efficiency of the model. Given that the effectof aging on the rheological properties is similar to that of tem-perature, an additional shift factor was introduced in the modelto account for differences in age between specimens. The effectof time, temperature and aging differs between binders depend-ing on their respective thermo-rheological characteristics; themodel used switch variables to account for these differences.� The study’s findings suggest that, irrespective of the time–tem-

perature envelope or the duration of aging, the control PG 64-22had the highest expected shear modulus due to the selection ofa higher temperature for short-term aging on the Rolling ThinFilm Oven. The Sasobit wax-based additive had the secondhighest shear modulus for the time–temperature-age profilesused in this study. The Rediset WMA binder was on the otherextreme of the spectra with the lowest shear modulus of allthe individual binders, followed by the Evotherm and CecabaseWMA binders. The observations were consistent across thetime–temperature profiles selected for this study, which sug-gests that the synthetic additives studied in this research areless affected by oxidative aging.� The marginal value of the shear modulus (G�) with respect to

the duration of aging was calculated in order to quantify theeffect of aging on the long term rheological properties of theindividual binders. Results suggest that the rate of aging forthe PG 64-22 is higher than any of the other binders. The Red-iset WMA binder exhibited the slowest rate of gain in the mod-ulus values, followed by Sasobit, Evotherm and Cecabase.Although the Sasobit WMA binder is closest to PG 64-22 interms of the early-age shear modulus, its slow rate of gain instiffness suggests that over time it will have a significantlylower modulus than the control experiment. In general, theWMA additives not only reduce short-term aging effects onthe rheological properties of the binder but they also retardthe rate at which the stiffness grows over time. While reducedmixing and compaction temperature partly explains this find-ing, the other half is attributed to the molecular compositionof the additive.

Acknowledgment

The authors extend their sincerest thanks to Patricia Trujillo forconducting all the necessary laboratory measurements.

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