NON-LINEAR VISCOELASTIC AND FATIGUE PROPERTIES
OF ASPHALT BINDERS
A paper submitted for presentation at the 1999 Annual Meeting of the
Association of Asphalt Paving Technologists
Session: Asphalt and Modified Asphalt Properties
By
Hussain U. Bahia
HuachuD lhai
Karen Bonnetti
Sadi Kose
The Asphalt Research Group
Department or Civil and Environmental Engineering
The University of Wisconsin-Madison
Madison , WI 53706
[email protected]
July 1998
Bahia. lhOl. 8/lnnelli. & K05t
2
Abslnd
A study was conducted to relat~ linear visco-tlastic properties of asphalt binders
of the same PG·Grade to rheological properties of asphalt mixtures of same aggregates
soun::e, gradation. and volumetric proportions. The only variable was the type of modifier
used in production of the binder. Results indicated that linear properties are not good
indicators of mixture response. The study was expanded to evaluate the role of non·linear
viscoelastic propert i~s in mixture response. The results collected indicate that modified
and un-modified asphalt binders vary significantly in their non-linear behavior. The non·
linear study was expanded 10 inchKk fatigue perfonnance of binders at vari~s strain levels
in the linear and non-linear range.
This paper is written to discuss the need for using large-strain non-linear shear
testing to evaluate binder contributions to mixture behavior. It provides numerical
analyses showing thai strain domains within binders are much higher than mixture bulk
strain. It also suggests that because binders exit as thin films, they can be perfonning in
the non·linear region d~ to the significant difference between modulus of mineral
aggregates and modulus of binders, and because of the boundary conditions.
The paper presenlS a parametric study showing factors that influence strain
dependency of asphalt binders of a wide variety of chemical composition and
modification techniques. The results of fatigue testing of the binders are also presented to
show factors that influence fatigue behavior. It is found that both strain dependency and
fati gue are highly sensitive 10 composition of binders. type of additive. temperarure,
heating rate, aging and interaction of these factors. It is also found that binders can
recover from damage caused by large strains or by fatigue. The recovery trends clearly
indicate that the healing of binders is a time dependant phenomenon that is sensitive to
chemical composition of binders and other factors.
This paper is intended to stimulate the research on the non·linear behavior of
binders and to suggest thai non-linear properties are essential for effective estimation of
role of binders in mixture behavior.
Bahia, ZJrai, Bonnelli, If: KoSt
J
Background
As part of a study funded by the FHWA office of technology application, five
asphalt binders of the same grade (PG 76-22) were used to produce asphalt mixtures
using one selected gradation for aggregates from the same source. The objective was to
evaluate the relation between asphalt binder rheology and mixture rheology using testing
protocols developed for the Superpave binder and mixture evaluation systems. The
results showed that the grnde of the binder could affect the mixture performance
significantly (1). The results, however, indicated that the five binders of the same PG
grades did not show the same relationship with mixture performance despite the fact that
the aggregate gradation, mixture preparation, and testing conditions were kept as idmtical
as possible (see Figure I).
,"
;;
•!!!•
••
•
•••
1000
900
800 .
BInder Type
700
i-+- LS
600
___ 01
__ 02
SOO
-,
400
~" 300
•" 200 ;;" 100
-;O- P2
-+-P
•
•
0
0
100000 200000 300000 400000 500000 600000
Mixlure Stlffness (KPa)@O.01"1.5trJin
Figure I : Relationsbip between Mixture Stiffness and Binder SliffDesJ for different
Mndified Binden at ODe Binder Strain Level
Binder (LS) is a PG 64-22 unrnooified asphalt. The other binders are PG76-22
grade with different modifications. Binders 01 and 02 are oxidized asphalts while the
others are polymer modified asphalts.
A thorough analysis of the results indicated that one of the impanan\ factors that
were nat carefully considered in the analysis was the Slrllin amplitude during the testing
of the binders and the mixtures. Rt-testing of the binders under controlled conditions to
separate the effects of frequency from strain amplitude indicated that at the temperaturts
the analysis was conducted (20 - 40 C), these modified binders afe highly sensitive to
,
/k,II/II. lhai. Bmmrlli. & KosI.'
strain amplitude. It v..-as observed that the strain amplitude used for the binder testing
pla)'s a significant role in defining the binder effect on mixture rhwlngical behavior (see
Figure 2).
700
r------------------,
~l %Slra)n
...... 4%Slfain
-"" 10% Strain
'"*"'20% Strain
•• 300
••
..... 30%Slrain
•
;; 200
•"
,"
•
100
oL-~----------~
o 100000 200000 300000 400000
Mixture Stiffness (KPa)@O.01%Straln
Figure 2: Relationship between Mixture Stiffness aDd Hinder Stiffness At different
Strain Level (37 C)
These preliminary analyses showed that the role of the asphalt binder could be
bener defined if the strain dependency of the different binders is considered. It is.
however. necessary to define the actual range of strain amplitude in a binder used in a
typical mixture. Estimating the distribution of binder domains within a mixture is a
complex task. It is expected that the aggregate gradation. aggregate shapes and the
distribution of binder within a mixture have significant effects on the distribution of film
thicknesses. The strain amplitude within the binder is a function oflhe film thickness and
thus will show a distribution that is related to the distribution of film thicknesses. The
concept of estimating an average film thickness based on idealized aggregate shapes is
not factual. Due to the randomness of shapes and sizes of aggregates. it is expected to
find a wide distribution of asphalt film thickness within a typical mixture.
Strain Distribution within Binder Domains of I Typical Mixture
It is known that betausc of the large difference in stiffness between the binder and
the aggregates. most of the bulk strain will be in the binder domains. Because of the nonuniformity of the binder domains. it is expected thai the distribution of strains is very
complu . II is also expected that the common procedure of estimating strain in binders
s
Bahia. Zhoi. Bonnel/i. " Kase
based on volumetric proponions cannot give an accurate representation. To demonstrate
the complexity of strain distribution within the asphalt binder, simple finite dement
analysis was used to calculate the range in strain amplirude within a binder of a typical
asphalt mixture. A sample that was tested in the Superpave Shear Tester was sliced and
the exposed surface image was copied using a magnifying camera. The picture was
convened to a digital (binary) image and used to generate a finite element mesh using the
computer program (ABAQUS). The digitized image comprised of 22 percent binder area
and 78 percent aggregate area. Using linear elastic representatio,! af material propenies, it
was observed that applying a shear strain of one percent can result in a strain distribution
within the binder that ranges from OJ percent to 32 percent (Sec Figure 3).
I II
DKX ". 458E-0)
SMN ,, -. 314.696
SHX " . 08584
o
o
o
o
o
o
-. )14696
-. 210192
-. 22568B
- . IB11B4
- .13668
-.092116
- . 041672
-.003168
. 041))6
.0 8584
Figure 3: Finite Element Representation or I Typical Asphalt Mixture
Produced using a Magnirying ScanDing Technique.
There are few important limitations with the imaging techniques used for this
study. It is rather difficult to have an actual representation of the very thin films of binder
that are expected to exist between finer panicles that are smaller than the I-nun range.
Based on this Wldersranding, it is expected that within an actual mixture a wider range of
film thickness will exist and therefore a wider range or strain amplitude could be realized
within the binder domains. It is realistic 10 assume that strains within the binder films can
be as high as 1010100 times of the bulk strains of the 10lal mixture.
6
The Nted Cor Non-Linear Ch ar:u~teriz.alio n
The current Supcrpave birKIer tesling protocols. developed as part of the SHRP
program, focus on the linear visco-elastic properties (2). There were several reasons (0
select (he linear range. Among them. the most important ones are
(I) Good pavement design should limit the defonnalion of the pavement layers.
(2) II is much simpler and more convenient \0 lest in the linear region because of
the reduction of the number of variables.
While these reasons are valid, the work in this study indicates clearly that this
simplification cannot be accepted, particularly for modified binders. The finding thaI
slrain amplitude within tile binder domains can be much higher than the bulk strain
within a mixture makes il necessary 10 re-evaluate the testing protocols used in the
current binder specification. The hypothesis that non-linear viscoelastic behavior is
necessary to establish bener relationships between birKkr and mixture behavior is a
departure from the existing concepts in Superpave system, which relies on the linear
visco-elastic properties of binders and mixtures to predict performance. There are several
important reasons to consider non-linear behavior:
(I) Asphalt binders that show similar behavior in the linear range can show highly
variable. non-linear behaviors. which will require direct cvaluation of the
properties in the non-linear region.
(2) A consideration of pavement structure in the binder testing is necessary for the
selection orthe binders. Pavements can have weak (nexible) structures, such
as the large number of primary and secondary highways built on Wlstabilized
granular bases and subbases. Pavements can have strong structures, such as
the large number of highways that consist of concrete (rigid) layers with
nexible asphalt overlays. A nexible structure will allow more deformations
and thus high strains by design while 8 strong structure will limit these
deformations 10 small values. If binders are functioning in the nonlinear
region. strain level will be a major factor in defining their behavior.
(3) Non·linearity in many materials is indicative of damage. It is considered as
the stage closer to failure than the linear (safe) range. Due to the repeated
nature of traffic load applications, binders performing in the non·linear region
can accumulate damage, such as fatigue and runing, faster and thus show
performanct very different from whaltheir linear behavior is indicating.
(4) More modified asphalt are being used to accommodate the higher and heavier
traffic. Thc majority of these binders are multi-phase systems that are known
to show non-linearity different from the un-modified binders. which are more
homogenous in theiTcomposition.
(5) Linear behaviors of binders do nOI show good relationships to mixture
behavior orlhe same aggregate structure and volumetrics. Non·linearity might
explain some or the discrepancy.
Bahia, 'fhai. Bonnell/. & Kost
7
Experimental Design and Materials
Preliminary testing of the non-linear behavior of asphalt binders done as pan of
the NCHRP Project 9-10 (Superpave Protocols for Modified Binders) indicated thatlhere
are several factors that need to be considered for an objective characterization of Nonlinear behavior (3). The factors seletted for this study were based on the resuhs of the
preliminary testing:
I. Binder types: Although more than 50 modified binders were tested in the 9-10
project, eight binders were included in the first phase of the study. These
included 2 un-modified asphalt, two binders modified with plastomers, two
modified with elastomers, and two modified by oxidation. The experiment
was expanded to include several more modified binders.
2. Strain levels: Strain sweeps were conducted to cover the range of I percent to
50 percent strain. The strain sweeps were used to define the strains at which
to conduct fatigue testing.
1 Temperature: Instead of using the temperature as a factor in this study,
specific values of G· were targeted. This was done because it is known that
linearity limits are highly dependent on the G" values. The values of 2, 20.
200, and 2000 Kpa were targeted.
4. Frequency: It is also known that frequency has an effe<:t on G· and also on
fatigue damage. Three frequencies, 0. 15, I.S and IS Hz, were included.
5. Number of cycles: To evaluate the fatigue behavior at different conditions, it
was decidt:d to run 5000 cycles and 11000 cycles. depending on the level of
damage induced.
6. Rest Periods: In many studies focused on fatigue. it is clearly observed thai
asphalts exhibit different healing behavior. Rest periods of 0.5, 3, and J2
hours were included in this study. The rest periods were used to evaluate the
recovery from strain sweeps and from fatigue damage.
The testing conducted was aU done using the parallel plate geometry in oscillatory
shear mode. Although no predominant test method has emerged among researchers for
measuring non-linear visco-elasticity, oscillatory shear testing has emerged as a useful
tethnique and has bren used extensively because it alloW5 the strain amplitude and the
time scale (loading frequency) to be varied independently (4). It also meets the need to
simulate the nature of traffic loading in the application of asphalt pavements and allows
the evaluation of stress strain loops and the energy dissipation very effettively.
Non-linear (large strain) measurements is known to present a significant
experimental challenge regarding the geometry of testing because of the edge effe<:1S
which can cause significant errors. Parallel plate, cone and plale, Concentric Cylinder.
and sliding plates have been used in a number of studies (5-8). Although the parallel plate
geometry is known to result in a heterogeneous flow, it is used in the Superpave testing
protocols based on the assumption that an average representative strain can be used to
calculate rcpresentive response. In large strain oscillMory shear testing. although the
heterogeneous flow field in the parallel plate greatly complicates the analysis (4). the
geometry has been used by many researchers because of the advantage of extending the
&Ihill. ZlIai. &lI1nl'lI;. & A:OJl
8
strain range that it orrers. The equations required to correct for the heterogeneous flow
have been derived and for moderate levels of strains «400 percent) and it has been
shown to give results comparable to tile cone and plate (6).
In this slIJdy since it is well recognized that there arc advantages aoo
disadvantages 10 almost all geometries, it was decided not 10 change the geometl)' of
parallel plate used in Superpave system and accept the enors resulting from the
heterogeneous flow field as part of the experimental error. A comparison of results from
parallel plate and cone/plate for few asphalts indicated thaI results are very similar and
differences are negligible. A Dynamic Sllear Rheometer made by Bohlin Instruments
(BDR) was used for all the testing. An c)(ample of stress-strain loops that can be
measured for an asphalt binder is shown in Figure 4. The figure shows the loops for a
number of strain levels and shows the irregularity of the loops with increasing strain.
Such data can be used to calculate the Hannonic Distribution Function (HDF) and
determine the effe<:ts of non-linearity on the modulus and phase angle. Although in this
study the analysis of the stress-strain loops were conducted, to simplify presentation of
the imponant concepts in this paper the analysis is presented in terms of the complex
modulus and phase angle calculated from the fundamental harmonic only. The 0 1' and
01 are used as a substitute to the commonly used a" and 0 measured within the linear
region.
' 0000 , -- -- - - --
--,
30000 I
I
~
!
20000
10000
I
-+- Straw! 2%
a
-.I-I---i- --l
__ Strain 25%
""'Il-Strain 50%
a. . \0000 t
-20000
-30000
~oooo
L
·20000
_
_
_ _ _ _ __
-10000
a
10000
--'
20000
Torque
Figure 4: Torque versus Position loops Bosean PG70·22 ror differtDI5tnlin levels It
30 C with 1 Hz rrequency.
Bahia, lha;' Bonnel/i, & Kose
9
Results and Diu uuions
The results are presented in several sections each addressing one of the imponant
factors included in the experiment
Strain Dependency of Binders
Previous research has shown that strain dependency is related to the rigidity of the
material as measured by G" (9). In fact the current version of AASHTO TP5 .• Standard
Procedure for Testing with the Dynamic Shear Rheometer," includes an equation giving
the linear limit of the suain as a function ofG " (AASHTO 93). Since temperature and
frequency both affect the G" ya,lue of asphalt binders it was expected that there is an
interaction between the strain, frequency and temperature.
Temperature and Frequency Effects
Figure 5 depicts strain sweeps for a modified asphalt binder measured al
temperatures ranging between 10 C and S8 C. Figult 6 depicts the effect of frequency on
strain dependency for a different binder tested at one temperature. These sample results
show that the strain dependency is a strong function of temperature and frequency. Both
these factors (temperature and frequency) can be considered by relating non-linearity to
the level of rigidity (Gt ). What is imponant to notice is the significant effect of strain
dependency. [n Figure 5 it is shown that the value of G" at 10 C drops from 1000 Kpa to
a value of 40 Kpa (more than an order of magnitude) by changing the strain from 11050
percent. Similar significant effects are seen at 22 C and 34 C.
Figure 7 depicts the strong inleraction between the strain and frequency . It is
shown that although at low frequencies the strain effect is minimal, al high frequency (15
Hz) the change of sirain from 2 percent to 50 percent resulted in almost an order of
magnitude change in G- . It is observed that strain effect is highly depe nd~nt on frequency
and can not be accounted for by simple shifting, as is suggested for other materials.
Recent works on asphalt mixtures (10) and models of non-linear behavior (I I), have
suggested simple strain or stress ~ift factors to account for non-linear effects. Th~ results
in Figure 7 show thaI non-linearity of some asphalt binders are mon: complex and their
behavior can not be represented by simple shifts on the freq uency scale. The effect of
strain on the phaSt angle is not as signi ficant as the effec t on the Gt . Figure 8 depicts a
sample of Ihe results for a modified asphalt which indicates that Ihe effect is limited to
few degrees change. This marginal effect on the phase angle is observed for many of the
systems that have been tesled.
Binder Composition
The olher important factor 10 consider is the type of binder. Figure 9 includes a
comparison of the strain SWttpS for six Stlected modified binders of the same grade,
produced from the same base asphalt (3), It is clear that strain dependency is also a strong
function of the type of modifier UStd. The data shows that binders modified with some
elastomers, plastomers. and by oxidation show significant strain dependency that can be
BviTiv. ZIT/I!. !lrJnm:1I1. & Kose
10
as high as two orders of magnitude drop in rigidity as a result of changing strain from 2
percent to 50 percent. The drop in G' values can be as low as only 50 percent for other
binders. The trend of change is also important Some binders show a continuous
decrease in G' while others show a plateau region at low strains followed by a sudden
drop and another plateau region at high strains. These trends might be related to the
morphology of the binder and the types of bonds that exist in the binder.
1000
r<C~-------------,
-+-58 C
___ 46C
~ 34 C
.....:. ....,~
,"*"" 22 C
10 ·_··"........
'....0011....101
-+-lO C
, l·~·~·~'~·~'~'~·~'~'~·~·~·~'~'~.~.~.~.~.~.~,~.~.~.~__J
0%
10%
20%
30%
40%
50"10
60%
S tra in (-Jo)
Figure 5: Strain Sweep of PE Unstabilized Modified PG58·28 at Different
Temperatures (1.6Hz)
,
1000000 ,
•
ii 100000
U
-----~'I'"
~
3;;-
.o
~
..... 0 1 !iz ,
!.... lHz
""*" 10 Hz
10000
•• I I I •• I I . I . . I I I ••• "
'000 C,_
0%
_
_
10'4
_
••••
_ _ _ _ _ _ __
20'4
30'J1,
~O%
50'4
50%
Strain
Figure 6: Strain Sweep of Boscan PG58-28 at Different Frequencies (J3C)
Bahia. ZJroi. Bonntlli. & Kosr
II
1000
_._-
--
-
-+- l %Slri ln
___ 4%Sb'aln
.•
~ '00
·-.- l O%Stlalfl
-*-20% Sb'aln
...... 30%Slrain
b
'0 ----------------------~
0.Q1
010
1.00
to.oo
100_00
Frequency (Hz)
Figure 7: Efrect of Strain on Oxidized PG76-22 at Different Frequencies (37C)
65
64
6J
62
~
"
~ O . 15Hz
60
-+- 1.6 Hz
59
' ~15Hz
"
57
56
""
StrlII in
Figure 8: Phase Angle versus Strain of SOS linear Modified PGS8-28 at
Dirrertnl Frequencies (25C)
Bahia. lJlf/i. Ilomrettr. & Kost
900
12
----
BOO
700
600
~ 500
It
;- 400
(!) 300
-e-- Elastomer 1
-
....... Elastomer2
~ Elastomer
3
...... Plaslomer 1
-+- Plastomer 2
200
100
-+- Oxidized 1
---- ---
a
a
20
40
60
Slrain (%)
Figure 9: G· venus Stnia of Dirruent Modifitd Aspbllt (IT)
The data shown in Figure 9 were measured at intennediate temperatures ranging
belween 20 and ]0 C. At higher temperatures ranging between 70 and 82 C less
sensitivity of the binders to strains were observed. Figure 10 includes the strain sweeps
for a set of binders tested al a range of 0" between 1.2 and 1.7 Kpa. There still appears
to be high strain dependency for certain binders.
The resullS shown in Figures 5 through 10 lead to the finding that rheological
measurements of asphalt binders are a strong function of strain level al all temperatures
within the range of pavement performance. The results clearly demonstrate that the small
strain (linear) measurements cannot be used to predict behavior within the large strain nolinear region. If bindel'1 within Iypical asphalt mixtures are subjected to a wide range of
strains that extcnds 10 the levels included in this study (I - 50 percent) then there is a
need to incorporate non-linear characterization in any perfonnance related or perfonnance
based binder testing and spe1:ificalions.
Bahia, Zhai, Bannelfi, & KoSI!
13
1800 ,- - - -- -1700
1600
1500
• 1400
!!:.
b
~tttttt*
-
-
---,
, ..... Elastomer 1
........ t .......
~
....... Elastomer 2
-+-Elastomer 3
-+- Plastomer 1
-.... Plastomer 2
1200
~ (hjdized
1
1100
' " 1-_
o
_
10
_
_ _ _ __
20
30
_
50
-'
60
Strain (% )
Figurt: 10: G* versus Sln iD of DiffereDI Modifi«i Asphalt (H1)
Rest Periods and Healing of the Binders
For many years healing of asph!llt mixtures have been researched and measured.
The significant works by researchers at the University of Texas A&M (12) and more
recently althe North Carolina State University (13) have shown that rest periods can have
a profound effects on recovery of damage in asphalt mixtures. Such behavior, although
generally recognized in asphalt binders, has not been measured and quantified before. In
this study since large strains resulted in significant reduction in 0 ' and marginal inc~
in 0, it was detided to investigate the effect of rest period on the damage (change) caused
by strain sweeps. Figure 11 is a sample of the effect of rest periods on strain damage for
an unmodified asphalL Figure 12 is another sample of data for a modified binder. The
results show that binders show significant healing with time. The results also show that
healing is a time dependent phenomenon, which depends on the composition of binder.
The recovery of the modified binder, which is produced from the same base asphalt
shown in Figure I I, shows a much rapid recovery than the base asphalt. This indicates
thaI modification tcchniques can alter the healing perfonnance. It also raises interest in
evaluation the effect of asphalt chemistry on healing of damage caused by higher strains.
In recent experiments, it is found that asphalt chemistI)' of UMlodified binders has an
imponanl effect. 111;s has been documenied in research on asphalt mixtures in recent
studies (1 4).
Bulnu. !.hm. Bvnnl'lIi. & Kuse
14
<SO
-4-0 Hr
"0
....... Retest (No Delay)
3"
-+- Retest {12 hrJ
300
•~• 150
•0 200
150
100
"
0
20%
10%
0%
". ".
Strain
so.
'0%
Figure 11 : Effect of Rest Period 00 Stnin Sweep of BO~caD PGS8 af IT (1.6 Hz)
160000
..... OHr
160000
140000
""*" Retest (1 Hr)
120000
-
-+- Retest (3 Hr)
Retest(12 Hr)
'ii' 100Xl0
!'.
"
60000
60000
",m
20000
0
0
1000
2000
3000
4000
5000
6000
Cycles
Figurt 12: Effect of Rest Period on Stnin Sweep of PE Unslabilized Modified PGS8
al IT (1.6 Ht)
Bahia. Zhai, Bonnet/i. & KosI'
IS
Fatigue of Asphalt Binders
For many years fatigue of asphalt concrete mixtures has been the subject of
investigation. Although it is recognized that fatigue damage is mainly caused by cracking
or damage within the asphalt binders, very few studies have used binder testing to
evaluate fatigue of binders (\5). There is a significant lack of information about the role
of binder composition or rheological properties of binders in fatigue damage. In a
previous study the effect of mechanical working (fatigue) on rehological properties of
modified binders was evaluated by testing for 1000 cycles (3). The results indicated that
for small strains, there is minor amount of damage occurring in 1000 cycles. The testing
done using large sltllins stimulated the idea of considering fatigue testing at higher
strains, within the non-linear region. Based on the information collected for the strain
dependency a fatigue experiment was designed to include testing for a larger nwnber of
cycles at different strains levels, frequency levels, and for different modified and un·
modified binders. The testing also included rest periods to evaluate the effect of healing
on fatigue damage. The following sections present the results of fatigue testing under
various conditions within the non-linear region.
Effect o/S/rain Level on Fallgue
The first experiment conducted was \0 test for 5000 (or I 1O(0) cydes at 1 percent,
10 percent, 20 percent and 50 percent strain at multiple levels of G' . Figure 13 shows the
results for a modified asphal! and an un-modified asphah at a frequel"lCy of 1.6 Hz. The
results show that for both asphalts significant reduction in GO is observed as higher
strains are used in testing. The effects are highly signi ficant. At GOof 200 Kpa, both
binders are showing a drop of more than one oroer of magnitude (a drop to less than 20
Kpa) in the G' values within only 5000 cycles. In light of the discussion about the strains
within binder domains presented earlier in this paper, 20 percent strain is very plausible in
typical mixtures under normal traffic conditions. The drop in complex modulus is
expected to have significant consequences on fatigue damage in the pavement.
Figure 14 depiclS the effect of fatigue on the phase angle for the same binders.
Compared to the effects on the G' , the effect of fatigue on change in phase angle is only
marginal and limited to few degrees increase. This was observed for many binders tested
which indicates that phase angle is nol very sensitive to fatigue damage.
The strain levels represent the possible effect of pavement structure. Weak
pavement structures are expected to allow higher strains than strong pavemem SlruClure
under same level of traffic. The high sensitivity of fatigue to strain level observed in this
study is nOI surprising. It confirms the find ings of many research works that have related
fatigue damage to strain of asphalt mixtures (16-18). What it is showing, however, is that
these effects can be detected in binder testing and this can be included in a binder
specification systcm that is beller related to pavement performance.
Bahia. lAm. 8onlll'lli. & KQse
16
2SO
200
'"*"" PE unmodified 10%
.......PE unmodified 20%
..... Boscan 10%
...... Boscan20%
.150
•
!'.
b 100
50
0
0
4000
2000
6000
Cye'"
Figure 13: G· vtrSus Cytles of Bosun PGS8 and PE Unslabiliud Modified PGS8 al
Diffmnl Strain Levels (1.61h:) (200Kpa)
---
70
69
"
67
...... PE I.I"IITJ:XiIied 10%
"
-w.-PE unmodified 20%
..... Boscan 10%
~"
64
~ Boscan2D%
62
61
60
0
4000
2000
6000
C",..
Figure 14: /) Vt!1U5 Cydes orBman PGS8 and PE Unslabilized Modified PGS8 at
Differenl Strain Levels (1.6Hz) (200Kpa)
Bahia. ZhiJi. Bonnelli. & Kase
17
EJJtrt oj LO(Jding ROlt (Troffic Spud) on Fatigue
Trame speed can vary significantly in the field . It is known that fatigue
performance of different materials, particularly visco·elastic materials, can be sensitive to
loading rate or frequency. Figure 15 includes the fatigue results for a modified asphalt at
three different frequencies (0.1, 1.0 and 10 Hz). All testing was done at 20 percent strain.
The results clearly show the significant effect of the frequency. At 10 Hz repeated
loading at 20 percent for approximately 3000 cycles resulted in reduction of the G" by an
order of magnitude. After only 2000 cycles the G- value at 10 Hz has reduced to the
value measuredat 1.0 Hz. At 1.0 Hz, 3000 cycles are sufficient to reduce the G" value to
the value measured at 0.1 Hz.
These significant effects of frequency and the increase io fatigue damage with
frequency were observed for many of the asphalts tested. The sensitivity to fatigue
however is different from ooe bioder to another and so is the innuence of other factors,
such as frequency, strain level, and number of cycles. The consequences of the fatigue
behavior in terms of pavement performance are very important. In the current Superpave
specification the grade of asphalt changes every six degrees interval. On 'average, six
degrees intervals result in increasing the G" by approximately 100 percent (one fold).
The effect of fatigue al moderately high strains appears to be much more signi ficant, for a
low number of cycles. A reduction of G" by 1000 percent (10 fo lds) after only 4000
cycles indicate that a small amount of traffic can change grade by at least 3 temperature
grade intervals. This might clarify the importance of this behavior and the need to
consider fatigue within the non·linear region.
1000
:
~
1-+- 0.1 Hz20%i
-+- 1 Hz20%
100
""*"' 1a Hz 20%
•
~
10 "--
o
- - - - - - - '--""
4000
2000
6000
Cycles
Figure 15: Errect or Frequency on Fatigue of PE uDstabilized PG58 aliT
Bahia. Zhai. Bunnelli. & Kose
18
Mechanical lI'nrk Dislurbance (MWO)
What is also important 10 consider is the temperature range al which this is
happening. As indicated by the results, more damage is observed at lower temperatures in
the range nfG ' values 0[20 to 200 Kpa. This ~havior can have dual effetts. For rulling,
the observed reduction in G" can have adverse effect on perfonnance and allow more
rutting at high 10 intennediate temperatures. At lower temperatures, the reduction in G"
can have a positive effect on reducing the classical strain-amtrolled fatigue damage
because of the softening. For stress-conlrolled fatigue, the reduction in G- can have
adverse effett and result in increasing stress-conlrolled fatigue.
Before any specific relationships could be derived to pavement perfonnance, tllere
is a need to understand what is causing the reduction in Gt and the nature of changes at
the molecular level. In asphalt mixtures, it has been assumed that micro-cracking is the
cause of fatigue damage and the observed cracking at failure is the progressing of the
micro cracking.
The results from this siudy suggest that the nature of micro-cracking assumed
needs to be re-evaluated. In recent literature, there are indications that the hypothesis of
micro-cracking is being questioned. Work. done by Little. et aI (1 4) is proposing using the
tenn micro-damage because it is very difficult to separate cracking from other rypes of
damage. This work even suggests that damage can include positive damage (bealing) or
negative damage (cracking).
Recent modeling techniques are also considering various types of damage. The
concept ofOislurbed Slate Concepl (DSC) introduced by Desai et aI.( 19) is a contribution
of the concept of rontinium damage used by others (20).
It is certainly evident from the results that disturbance is occurring to these
binders. Because of the different nature of consequence that the reduction in G" might
have in tenns of pavement structure, it is suggested thai the tenn Mechanical Work
Disturbance (MWO) be used to replace the classical fati gue damage terminology. MWD
can have positive or negative elTects on mixture properties and pavement performance.
EJJtct of Binder Type and modifier used
It is recognized that classical fatigue damage is a function of tbe microstructure of
tM bindr:r. which is defined by the chemical and physico-chemical nature of the material.
Several UMlooified asphalts and modified asphalts were tested and compared at a range
of strains and frequencies. Figure 16 depicts tbe results for two un-modified binders that
are different in their cbemistry. The (8) asphalt is produced from heavy crude oil and is
relatively high in asphaltenes and polar aromatics. The
asphalt is produced from a
blend of crudes and is relatively low in aspbaltenes. Two grades are shown for each of
the binders. The data indicates that the aspbalts vary significantly in their fatigue
behavior. The daLa also show highly significant reduction in G" that represents several
grade intervals for some of the binders. These results give evidence that the chemical
composition of the binders has a profound elTect on fatigue behavior. or the Mechanical
Work Disturbance (MWD).
en
BtJhitJ, ZhtJi, Bonnelll, & Kose
19
'000000
--- - ...... TPG5810%
.
"
"'"*"" T PG58 20%
.lXXlOO
-+- TPG&410%
-e- T PG&4 20%
!!.
~B
PG5810%
.....-- B PG58 20%
.0000
....- B PG70 10%
~ BPG7020%
.000 L-_ _ _ _ _ _- - '
o
sooo
.0000
Cycles
Figure 16: Timt Sweep ofTwo Nut Asph.11(2 Gndes) at DiffereDt Stu in LeYcis
(200K,.)
To further show the effect of chtmical composition, Figure 17 is prepared to show
the change in fatigue behavior after aging with the PAY, It is known Ihat the PAY
conditioning results in significant oxidation and thus changes in the chemical
composition. The data in figure 17 show thai PA V aging increases the fatigue damage in
some cases, particularly at lowtr strains.
Modification of binders by proct ss (controUed oxidation) or by re:activt additives
(polymtrs) is known to change tht microstructlm of binders and thus are expected to
changt the fat igue performance. Figure 18 shows the results for a selected set of polymer
modified binders that includes EMA, EVA, PE Wlstabilized. SBS tinear, SBS radial, and
SB Diblock modifiers. All binders are tested at 20 percent strain.
There: is a significant difference in fatigue behavior seen in this data. It appears
that the modification with these additives reinforce the asphalts and make them much
more resistance to disturbance. It is important to note that the modified asphalts are made
with the same base asphalts shown in the pre:vious figure . This finding is very imJXlrtant
since it shows the effettiveness of these modifiers in improving the reslstanct to
accumulated damage. It also shows that the properties used for grading binders that are
derived from the linear visco·elastic range, and which ignores repeated loading and
accumulated disturbance, can be misleading.
Bahia. Zlwi. iJol1!u'lIi. & Kuse
20
1000000 ,- - -_ _ _ _,
100000
-+- lS Origrnal 10%
•
-
~ LS
<l.
•
"
Original 20%
l-+- l5 PAV 10'4
10000
i"",*"" LS PAY 20%
1000 L
o
_
_ _ __
5000
10000
-----'
15000
Cycles
Figure 17: Effect or PA V Conditioning on Fatigue al Different Strain
l000000 r--------~
~ _Bosca n
58
.
~-'- SBS Radial
""'*"" SBS Unear
~ 100000
~ SBDiblock
b
..... EVA
~PE
unstabilized
-+-EMA
10000 L _ _ _ _ _ _----.J
o
4000
2000
6000
Cycles
Figure 18: Effeds of Polymer Modilitn on Fatigue (20percent Strain)
Bahia, lhai, Bonnelli, &: Kore
21
Modification by process is becoming well used because of the low cost associated
with it. The issue of oxidized asphalts of equivalent grades 10 other modified binders is
still being widely debated. The fatigue behavior of a set of oxidized binders produced
from the same base asphalts is shown in Figure 19. As seen in the figure the fatigue
disturbance of the oxidi7.ed asphalts are mostly smaller than thai of the base aspbahs.
What is important to notice, however, is the o:o:idized asphalts that vary significantly in
their behavior despite the fact that same base asphalt is used in their production. In
addition, it is observed that polymeric modification is offering much beller resistance to
fatigue disturban<:e that some of the oxidation te(hniques. This finding is important
because it puts the modification by polymeric additives at a different class than
modification by process. It certainly sheds more light on the effect of polymeric additives
and raises some questions about modi fi cations by oxidation. Figure 20 depicts a
comparison between the base asphalts, the same asphalt modified with an elastomeric
modifier, plastomeric modifier, and by controlled oxidation, The results show the
dramatic differences in fatigue perfonnance,
The resuits concerning the somce of asphalt and the type of modification,
although show very important trends, need to be confinned with a larger set of binders. II
is important to ootice that the testing in this study included only three base asphalts. How
much improvement can modifiers offer and what are the critical chemical properties that
can improve resistance of base asphalts to fatigue distmbance is yet need to be
understood.
Efftct 0/ Rest Periods
Healing effects, as detennined by allowing rest periods, have been discussed with
regard to strain sweeps. It is also expetted that healing will have significant effect on
fatigue disturbance. Selected samples were tested after various rest periods. Figure 21 is
prepared to show the effect of re·testing of an wunodified binder within 30 minutes after
completing faligue testing for 5000 cycles and after a rest period of 12 bours. The results
confinn thai healing has a significant effect on recovery from fatigue damage and that
recovery is a time dependent phenomenon, Figure 22 shows the results for a binder
modified with a plastomeric polymer. [t appears that this type of binder can recover
within 3 hours of the initial test. What is more imponant is how much this disturbance is
recoverable. After waiting for 12 hours and retesting the same sample, the trend is very
similar to the initial fatigue curve. This indicates thai no pennanent damage is occurring
and thai the binder can fully recover to its original status, This finding is consistent with
what has been observed in previous research for asphalt concrete mixtures (14), It has
signi fi cant consequences regarding the effcct of the amount of traffic and the frequency of
the traffic on fatigue disturbance of asphalt pavements.
Bahia, lha;. Bonnelli. & Kose
1000000
22
I-~~~~~~~~--'
-_ .._-
100000
-+- T Oxidized1
'"'""'*- T Oxidized1
20%
~ T Oxidized2
10%
10%
•
-e- T Oxid ized2 20%
b
-+- 8 Oxidized1 10%
~
........ a Oxidizedl 20%
10000
........ 80xidized210%
-+- 8 Oxidized2 20%
1000
o
5000
10000
Cyele.
Figure 19. Effects of Process (Oxidized) 00 Fatigue at Different Strain (200Kpa)
1000000
•
100000
-+-LS
, ___ E1
-
0.
•
I ~P
Cl
1-+-01
10000
.~-.-
1000
o
5000
10000
Cycles
Figure 20. Effects of Modifier and Process on Fatigue (20 percent Strain 37C)
Bahia, Zhai, Bonnetti, & Kase
23
-+- 0 II
250
...... Retest (30 min)
"""*- Relesl (12 II)
ii 200
~
l!O
•
0
150
100
50
0
0
1000
2000
3000
<4000
5000
6000
Cycles
Figure 21 : Errttl of Rtst Period on BOSCln PG58 at IT 1,6 Hz 20 percent Strain
250000 , - - - - - - -- -- - ,
200000
.
150000
"
100000
- -- --____ 10% 01'1'
~
-+- 20% ot-t
-+- 20010 3hr
-*- 20% 121-1"
•
.-
------
50000
o~
o
________
1000
2000
3000
4000
5000
~
6000
Cycle
Figure 22: Erred ofRtst Period on PE Unstllbilized Modified PG58 at20e 1.6 H'l20
percent Strain
Bahiu, 7Jwi, BO/met/i. & Ka.le
24
Su mmary of Findings
This study was conducted to evaluate the behavior of asphalt binders in Ihe non-linear
viscoelastic region. The following points summarize the findings from this study.
1. Strain dependency of asphalt binders can playa major role in defining relationships
between rheology of asphalt binders and rheology of asphalt mixtures.
2. Based on computer simulations and finite element analysis. it is shown that asphalt
binders can be subjected 10 strain levels much higher than the bulk strain to which
mixtures are subjected. Depending on size distribution of aggregate, binders can be
subjected to strains 10 times higher than the bulk strains of mixtures. It is therefore
necessary to consider large strain testing (non-linear) to accurately estimate the role of
binders in mixture performance.
1 Asphalt binders, particularly modified binders, vary significantly in their strain
dependency. Some binders are morc sensitive to strain level than othcrs due to their
composition or microstructure.
4. Strain dependency is a strong funct ion of temperature and loading time. Lower
temperatures and/or higher frequenc ies result in higher sensitivity 10 strain leveL
Some binders are so sensitive to strain that complex modulus reduces by as much as
10 folds when strain changes from 2percent 10 SOpercen\. Strain levels 20percent to
SOpercent in (he thin films of asphalt are plausible in typical mixture used for paving.
S. There is a significant interaction between strain levels and time loading (frequency).
The data collected for binders show that the interaction is not simple and cannot be
represented by simple shift factors. This complex interaction can have significant
affects on construction of rheological master curves and time temperatW"e shift
functions.
6. SlIain dependency trends are not uniform for all binders. Some binders show simple
transition from small to large strains while other binders show specific transition
regions. The strain dependency behavior can give some indications of the integrity of
binders and the compatibility of additives.
7. Strain level has a very significant affect on classical fatigue behavior of asphalt
binders. Fatigue damage, as defined by reduction in rigidly (a· ) increases rapidly
with increasing strain.
8. Because reduction in a· values and increase in is can have different consequences in
terms of pavement performance. It is proposed that the term Mechanical Work
Disturbance (MWD) be used to replace classical strain-controlled or stress-controlled
fatigue. Effects of Mechanical Work Disturbance (MWD) on a" is much more
pronounced than its effects on phase angle (6). Reduction of a· by as many as 100
times (2 orders of magnitude) is observed as a result of only 10,000 cycles at
moderate strains of 20 percent.
9. There is an affect of loading time (frequency) and temperature on fatigue disturbance.
The effects arc howtver much smaller than those of strain Icvel. Strain level is by far
the most important factor controlling fatigue disturbance.
10. Based on the limited binders tested, fatigue disturbance appears to be a strong
fu nclion of asphalt composition and types of modification. Polymeric modifiers (both
Bahia. Zhoi. Bonnetti. & Kose
25
elastomers and plastomers) are observed to have a profound effect on limiting fatigue
disturbance and resist reduction in 0' values.
11 . Rest periods show significant effect on recovery from shear thinning (strain
dependency) and from accumulation of fatigue damage. Healing is the most probable
mechanism for their recovery. Based on test results collected so far, it appears that
healing is a function of composition of base asphalt and type of modifiers.
Aeknowledgmenl
This work is part of the FHWA Project "Superpave National Training Center II".
The project main contractor is the Asphalt Institute. The authors gratefully acknowledge
the support of FHWA . The authors would like to acknowledge the support of the Project
Officers, Mr. Juhn Bukowski and Mr. John D'Angelo. The support of Dr. Richard May
and Mr. Mike Anderson of the Asphalt Institute are also acknowledged.
DiS(:Jaimer
The opinions and conclusions expressed or implied in the report are those of the
research agency. They are not necessarily those of the Transportation Research Board, the
National Research Council, the Federal Highway Administration, the American
Association of State Highway and Transportation Officials, or of the individual states
participating in the National Cooperative Highway Research Program.
Iklhia. Zhai. Bunnclli. & KoSI'
26
References
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Superpave to Modified Asphalts - A Mixture Study", Preprint of the 77th Annual
Transportation Research Borad Meeting, Washington D.C.. 1998.
2. H. Bahia, W. Hislop, H. Zhai, A. Rangel. "Classification of Asphalt Binders Into
Simple and Complex Binders", preprint of 1998 Annual Meeting, AAPT
3. H. Bahia D. I. Hanson. et al.. Second Interim Report for NCHRP 9- \0 "Superpave
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4. A. 1. Giacomin, and J. M. Dealy, "Large-amplitude oscillatory shear, in: Techniques
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Bahia, Zhai. BQnnt lli, & Kost
27
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,
ASSOCIATION OF ASPHALT
PAVING TECHNOWGISTS
Preprint
SESSION I-Binder/Mastic Rheology
Tuesday, 9:00 AM
Presiding,
Gerald Huber
"Non-Linear Viscoelastic and Fatigue Properties
of Asphalt Binders"
by Hussain Bahia, Huachun Zhai, Karen Bonnetti and Sarli Kose
"Laboratory Invesligation of Polymer Modified Bitumens"
by VIC Isaccson and Xiahou Lu
"The Structure of Polymer Modified Binders and Corresponding
Asphalt Mixtures"
by Vibeke Wegan and Bernard Brull!
"Rheological and Mechanical Properties of Asphalt Binder
Containing Recycled Asphalt Pavement"
by Nikone Soupbarath, K. Wayne Lee, Arun Sbulda, Colin Franco and Francis Manning
-Time-Temperature Dependency of Blended Rejuvenated
Asphalt Binders~
by Hamid SoJeymani, Hussain Bahia and Arthur Bergan
(by title only)
ANNUAL MEETING AND TECHNICAL SESSIONS
March 8-10, 1999
These are considered draft copies of the papers
only. Final publication and discussions will be
in Volume 68 of the AAPT Journa1.