اخبار

تاریخ انتشار: دوشنبه 31 شهریور 1393

  Rutting Characteristics of Styrene-Ethylene/

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 resistance

to rutting. To enhance asphalt resistance to permanent deformation, researchers have used various modifiers and additives such as styrene

butadiene styrene (SBS) block copolymer, ethylene vinyl acetate (EVA), polyvinyl acetate (PVA), styrene butadiene rubber (SBR), and

natural rubber latex. In this paper, the effectiveness of modification of asphalt binder by styrene-ethylene/propylene-styrene (SEPS) in order

to enhance asphalt binders resistance to permanent deformation has been investigated using two different evaluation methods: the Superpave

specification parameter, G_=sin δ, and the cross model for calculating zero shear viscosity (ZSV). The experiments were conducted using

dynamic shear rheometer (DSR) frequency sweep test performed on both the neat sample and modified ones at 40, 50, and 60°C. Utilizing

these two approaches, rutting resistance of modified asphalt binders are determined and then normalized using the rutting resistance of the

neat asphalt binder as the control data in order to calculate the rutting resistance improvement ratio. The research results showed that

regardless of the evaluation method, SEPS can significantly improve anti-rutting performance of asphalt binder. However, it was found

that 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 of

distress in asphalt pavements. Rutting has been distinguished as a

primary distress mechanism and a major design criterion for flexible

pavements (Parker and Brown 1990). Permanent deformation

appears in the wheel path as longitudinal surface depressions and

causes poor serviceability of the pavements causing vehiclesrides

to become rough and dangerous (Golalipour 2011). The main reason

for the rutting of asphalt pavements is known to be the accumulated

strain which is a consequence of traffic loading (Sybilski

1996; Philips and Robertus 1996).

The accumulation of permanent strain in the asphalt surface

layer is considered to be the major origin of rutting, although

the rutting observed on flexible pavements can be measured by

the total sum of accumulated permanent strains in one or more

layers of the pavement structure (Garba 2002). The rutting susceptibility

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 a

dominant factor, especially for modified asphalt binders that are

claimed to improve the rutting resistance of asphalt pavements.

One of the most common modifiers that are used to improve high

temperature characteristics of asphalt binder is styrene-butadienestyrene

(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 properties

of asphalt binder has been thoroughly investigated (Lu et al. 1999;

Lu and Isacsson 2000), SEPS has been well received in asphalt

literature and has not been considered as an asphalt modifier. As

such this paper will investigate the effectives of SEPS on improving

anti-rutting performance of asphalt binder.

The properties related to rutting should be observed in the upper

range of pavement service temperatures because rutting is more

dominant at high temperatures than at low temperatures (Petersen

et al. 1994). Several research works have been devoted to permanent

deformation in order to formulate a specification parameter

that can explain and measure the anti-rutting characteristics of

an asphalt binder (Coenen 2011; Desmazes et al. 2000; Centeno

et al. 2008; van Rooijen and de Bondt 2004; Rowe et al. 2002).

In this study, rutting resistance of asphalt binders that are modified

using SEPS is compared to that of neat binder at various

modification scenarios by two different specification parameters:

The Superpave specification parameter, G_=sin δ, and the zero shear

viscosity (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 relationship

between G_=sin δ and rutting is insignificant (Bahia et al. 2001;

DAngelo et al. 2006), it is still being used in preliminary rating

of binders in terms of their rutting resistance. The ZSV is defined

as 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 (corresponding

author). E-mail: [email protected]

2Assistant Professor, Dept. of Civil Engineering, North Carolina A&T

State Univ., 1601 E. Market St., Greensboro, NC 27411. E-mail: [email protected]

ncat.edu

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 and

Environmental Engineering, Amirkabir Univ. of Technology, 15875-4413

Tehran, Iran. E-mail: [email protected]

Note. This manuscript was submitted on September 29, 2013; approved

on April 10, 2014; published online on July 28, 2014. Discussion period

open until December 28, 2014; separate discussions must be submitted for

individual papers. This paper is part of the Journal of Materials in Civil

Engineering, © ASCE, ISSN 0899-1561/04014154(5)/$25.00.

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toward zero (DAngelo et al. 2007). Anderson et al. (2002) have

been shown that ZSV can be used to characterize asphalt binder

contribution to rutting. They reported different methods that can

be implemented in order to estimate the ZSV. Application of the

cross model to dynamic viscosity measurements is one of these

methods that previously was used by Sybilski (Sybilski 1996).

The following sections of the paper provide a brief overview of

previous studies and theoretical basis of Superpave specification

parameter, G_=sin δ, as well as the cross model for determining

the zero shear viscosity, followed by a description of materials

and test methods. The cross model is fitted on complex viscosity

versus angular frequency curves in which complex viscosity is

measured during the frequency sweep test. Rutting specification

parameters for both neat and modified asphalt binders were then

calculated using each of the aforementioned approaches. Each

method prioritized asphalt binders based on the rutting resistance

parameter. G_=sin δ was used in the first method, and the ZSV was

calculated in the second method. Finally, the merit of application of

each method to rank various modifiers in terms of their anti-rutting

performance 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 as

well as in the subgrade. Rutting development can be described as a

two-stage process, densification with volume change of asphalt and

shear deformation inside the asphalt mixture (Sousa et al. 1991).

The second stage is related to plastic deformation of the surface

course. Asphalt mixture is a viscoelastic material, and asphalt

binder is responsible for the viscoelastic behavior of all bituminous

materials (Wang 2011). Therefore, one of the major factors that

affects the plastic flow of bituminous mixture is the type of asphalt

binder 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 characterization.

Inspired by the convenient problem of torsional flow

between parallel plates in fluid mechanics (Bird et al. 1987), applying

oscillatory stresses or strains over a range of temperatures

and loading frequencies to a thin disc of binder, squeezed between

the two parallel plates of the DSR was used.

As the first step, Anderson et al. (1994) introduced G_=sin δ as

the rutting parameter in which both G_ (shear complex modulus)

and δ (phase angle) are outcomes of the DSR test. In order to relate

this parameter to rutting, it is assumed that rutting is caused by the

total dissipated energy.

Some researchers showed that there is no proper correlation

between mixture rate of accumulated strain and the Superpave

specification parameter, G_=sin δ. Therefore, other parameters were

introduced that can be determined using a DSR setup and that attempt

to reflect a better correlation between asphalt mixture properties

and the corresponding asphalt binder.

For the purpose of verification of the relationship between

binder properties and road pavement performance, Sybilski used

the fundamental property of viscosity and introduced zero-shear

viscosity as a rutting parameter of asphalt binder (Sybilski

1996). During the NCHRP 9-10 project of National Cooperative

Highway Research Program (NCHRP), Bahia et al. (2001) proposed

the repeated creep and recovery (RCR) test to identify

non-viscous flow that contributes to the permanent deformation

from 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 performed

by a DSR setup (Delgadillo et al. 2006). Following the RCR test,

DAngelo et al. (2006) developed the multiple stress creep and

recovery test (MSCR) to reduce the number of samples at each

stress level and selected two stress levels, 0.1 and 3.2 kPa, upon

correlation between binder and mixture rutting results for performing

the MSCR test (DAngelo et al. 2007).

Superpave Specification Parameter

At high temperatures, during the deformation of asphalt binder, the

work done is partially recovered by the elastic component of the

strain and partially dissipated by the viscous flow component of

the strain and any associated generation of heat. Anderson et al.

(1994) assumed that rutting is caused by the total energy dissipated

per cycle of loading. Using such an assumption for a sine wave

loading, it can be determined that

ΔU ¼ πτ2

max

1

jG_j

sin δ

ً 1ق

where ΔU = energy loss per cycle or dissipated energy; τmax =

maximum shear stress; jG_j = shear complex modulus; δ = phase

angle.

As can be seen in Eq. (1), jG_j=sin δ is inversely proportional

to the total dissipated energy. Therefore, the increase of G_=sin δ

[the denominator in Eq. (1)] causes the total dissipated energy

to decrease. This in turn leads to the reduction of the rutting susceptibility.

For this reason, the Superpave specification parameter,

G_=sin δ, was used for high temperature performance grading

of paving asphalts to rank asphalt binders based on their rutting

resistance.

Zero Shear Viscosity

Unmodified asphalt binders are Newtonian fluids at the high temperatures

in which rutting behavior is dominant. However, most

of the modified asphalt binders show a phenomenon known as

pseudoplasticity, or shear-thinning flow behavior, in which viscosity

decreases by increasing shear rate. Pseudoplastic liquids

behave similarly to Newtonian liquids at very low shear rates,

and their viscosity is defined independent of shear rate or frequency.

This viscosity, ηo, is called the ZSV or Newtonian viscosity

(Yildirim et al. 2000). Sybilski demonstrated that the rutting

resistance of asphalt mixtures is proportional to zero shear viscosity

of asphalt binders. There are several methods to determine the

ZSV (Sybilski 1996). In this research, the frequency sweep test

was implemented to determine the ZSV through the cross model.

The flow curves of pseudoplastic fluids can be described using the

cross model, which is a four parameter model (Sybilski 1996;

Sybilski 1994)

η_ η_

η_

o η_

¼ 1

1 ًKωقm

ً 2ق

where η_ = complex viscosity; η_

o = ZSV; η_

= limiting viscosity

in 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 and

100 rad=s. In this domain, it is an appropriate assumption that

η_ η_

(Anderson et al. 2002). With this assumption, the cross

model can be re-written to

η_ ¼ η_

o

1 ًKωقm

ً 3ق

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Thus, implementing a curve fitting tool, it is possible to fit

Eq. (3) on a complex viscosity versus angular frequency curve

at each test temperature to obtain the ZSV. The ZSV is directly proportional

to rutting resistance of asphalt binders. As such, greater

zero shear viscosity indicates better anti-rutting performance of the

modified asphalt binder.

Experimental Program

Materials

The base bitumen with an 85=100 penetration grade was acquired

from the petroleum refinery of Pasargad Oil located in Tehran,

Iran. The SEPS polymer used was Kraton G1780M supplied by

KRATON Polymers (Houston, Texas). Kraton G1780M polymer

is a clear multi-arm polymer based on styrene and ethylene/

propylene with a polystyrene content of 7% in powder form, and

it has a star structure.

Preparation of SEPS Modified Asphalt

The SEPS modified asphalt binder samples were prepared by

means of an IKA (Germany) high shear mill rotating at

5000 rpm. In preparation, the base binder was heated to fluid condition

(170180°C). The SEPS polymer was then added slowly to

the base binder. The temperature was kept constant, and the blending

process continued for 45 min. The SEPS Kraton G1780M concentrations

in the base binder were chosen to be 2, 4, and 6% by

weight of base binder in accordance with prior research that investigated

the effect of modification of asphalt binder using SBS and

SEBS (Elseifi et al. 2003; Lundström and Isacsson 2004; Awanti

et al. 2008; Kök and Çolak 2011; Lu et al. 1998).

Test Methods

High Temperature Storage Stability Test

In order to ensure high-temperature storage stability of SEPS

polymer modified specimens, consistent with the European test

method for the determination of storage stability of modified bitumen

(EN 13399 2010), modified asphalt binders were poured into

an 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 vertical

position at room temperature followed by storage for several hours

in a freezer at 20°C before being cut into three equal sections.

After that, the specimens obtained from the bottom and top sections

were used to evaluate the storage stability of the SEPS polymer

modified asphalt binders by measuring their softening points.

If the difference of the softening points between the bottom and

top sections was less than 5°C, the modified asphalt was considered

stable under high-temperature storage condition. Results

of the storage stability test are depicted in Table 1. As can be observed

from this table, the differences between the top and bottom

softening points in all samples are less than 5°C. This indicates that

SEPS modified specimens have appropriate high-temperature

storage stability.

Frequency Sweep Test

To determine high-temperature characteristics of the base and

SEPS polymer modified asphalt binder, the frequency sweep test

was performed at 40, 50, and 60°C. In this study, the frequency

sweep tests were conducted by implementing an Anton Paar

rheometer (Austria) using 25 mm diameter parallel plates and

a 1 mm gap opening. These tests were performed under the

controlled-strain conditions at frequencies between 0.1 and 100.

To remain within the linear viscoelastic region, a shear strain of

1% was selected. Also, the test temperatures were chosen to

achieve dynamic oscillatory shear test in the upper range of pavement

service temperatures in which rutting is dominant.

Consistent with ASTM D 7175-05 (ASTM 2005), two replicates

were done, and precision and bias specifications were

checked and determined to be acceptable for all modified and

non-modified samples.

Results and Discussion

Superpave Specification Parameter

In order to compare neat and modified asphalt binders, results of

frequency sweep test at 40, 50, and 60°C were used to calculate the

Superpave specification parameter (G_=sin δ) for each specimen.

These results are depicted in logarithmic scale in Figs. 13, respectively.

As can be seen in these figures, as the concentration of SEPS

increases so does G_=sin δ. In addition, the trend was found to

be consistent at different temperatures. This in turn indicated that

modification of asphalt binder using SEPS improves rutting resistance

of the neat asphalt binder.

Based on the strategic highway research program (SHRP) superpave

protocol, to determine the rutting resistance improvement ratio,

the G_=sin δ of modified asphalt samples at the frequency of

1.59 Hz (10 rad=s) is specified and then is divided by G_=sin δ

of unmodified samples at the same frequency. In this study, the

aforementioned ratio was calculated to reflect the amount of rutting

resistance improvement due to addition of SEPS polymer. Fig. 4

shows 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 SEPS

Polymer Modified Asphalt

Sample

Softening point (°C)

Top Bottom SPTop SPBottom

Neat 47.7 47.8 0.1

2% SEPS 53.5 54.4 0.9

4% SEPS 57.9 59.5 1.6

6% 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. Furthermore,

the anti-rutting performance showed consistent improvement

with increasing SEPS content, with 6% SEPS showing the highest

improvement. Introduction of 6% SEPS can increase rutting resistance

of the neat asphalt binder by 9.8 times (at the test temperature

of 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 multiple

non-linear regression analysis. For this purpose, the MATLAB

CFTOOL (Curve Fitting Toolbox) is implemented to fit the desired

expression of Eq. (3) on the curve of complex viscosity versus

angular frequency at 40, 50, and 60°C. Fig. 5 shows experimental

data of complex viscosity versus angular frequency and the corresponding

fitted curves for SEPS polymer modified asphalts at the

test temperature of 60°C.

To compare the effect of SEPS modification on high-temperature

characteristics of asphalt binder, the ZSVof each modified sample

was determined. Dividing the ZSVof each modified binder by that

of neat binder determined an index that served as a measure of

rutting resistance improvement of the base binder. Fig. 6 illustrates

the rutting resistance improvement ratio for all samples at test temperatures

of 40, 50, and 60°C in a logarithmic scale. According

to Fig. 6, the effect of 4 or 6% modification with SEPS is significantly

higher than that of 2%. However, as shown in Fig. 4, the

Superpave specification parameter (G_=sin δ) shows a different

trend for the rutting resistance improvement ratio. Also, based

on Fig. 6, at the higher temperatures of 50 and 60°C, 4% SEPS

was 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 modified

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 modified

asphalt at all test temperatures based on ZSV

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to thoroughly compare the effectiveness of the two proposed

approaches in evaluating rutting resistance of asphalt binder, as

well as their correlation with the field data, further investigation

is needed.

Conclusion

In this paper the effectiveness of modification of asphalt binder

by SEPS in order to enhance the asphalt binders resistance to

permanent deformation has been investigated using two different

evaluation methods: the Superpave specification parameter,

G_=sin δ, and the cross model for calculating ZSV. The aforementioned

two methods were implemented using DSR tests conducted

in frequency sweep mode. In both approaches, there is a particular

parameter for determining the rutting resistance improvement ratio

of modified asphalt binders in comparison with the base binder.

Results of both methods show that SEPS modification of asphalt

binder can improve rutting resistance of the neat asphalt binder.

However, there is a difference between the results of the presented

methods. The first method, the Superpave specification parameter,

indicates that anti-rutting performance consistently improves as the

concentration of SEPS polymer increases. However, the ZSV

method shows that 4% SEPS shows a better anti-rutting performance

than 6% SEPS. Therefore, investigation of rutting resistance

behavior of SEPS modified binder should be extended to the mixture

level in order to investigate the significance of anti-rutting

resistance improvement due to the introduction of SEPS polymer

to neat binder.

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