Construction and Building Materials 68 (2014) 685–691
Contents lists available at ScienceDirect
Construction and Building Materials
journal homepage: www.elsevier .com/locate /conbui ldmat
Evaluation of fatigue behavior of hot mix asphalt mixtures
preparedby bentonite modified bitumen
2014 Elsevier Ltd. All rights reserved.
⇑ Corresponding author. Tel./fax: +98 2177240565.E-mail address:
[email protected] (R. Babagoli).
Hasan Ziari, Rezvan Babagoli ⇑, Mahmoud Ameri, Ali AkbariSchool
of Civil Engineering, Iran University of Science and Technology,
Narmak, Tehran 16846, Iran
h i g h l i g h t s
�Marshall stability, flow and MQ values of modified mixtures are
higher than the control mixture.� The resilient modulus of mixtures
prepared with modified bentonite bitumen is higher than the control
mixture.� Mixtures containing 10% and 15% bentonite modified
bitumen have longer fatigue lives.� Mixtures containing 10% and 15%
of modified bentonite bitumen have higher dissipated energy than
the control mixture.� Models for prediction of the fatigue behavior
of control and modified HMAs under different strain levels were
a r t i c l e i n f o
Article history:Received 5 September 2013Received in revised
form 11 May 2014Accepted 29 June 2014Available online 26 July
Keywords:BentoniteFatigue lifeFour point beam fatigue
testIndirect tensile strengthResilient modulus
a b s t r a c t
The objective of this research study was to investigate and
evaluate effects of bentonite on fatigue prop-erties of hot mix
asphalt (HMA) mixtures. The experimental program for this study
included use of fivepercentages of bentonite (10%, 15%, 20%, 25%
and 30%) by weight of bitumen for modifying base bitumen.Several
tests such as: marshal stability, indirect tensile strength,
resilient modulus and fatigue test wereconducted. The fatigue tests
were based on four-point bending test in strain-controlled mode at
3 micro-strain levels (600–800–1000 lm/m) with sinusoidal loading.
The fatigue life of mixtures has been evalu-ated based on the 50%
reduction of the initial stiffness modulus. The results show that
fatigue life ofasphalt mixtures prepared with bentonite modified
bitumen is longer than conventional HMAs. Also,bentonite leads to
relative increase in indirect tensile strength and resilient
modulus of asphalt mixtures.Finally, based on experimental results,
a model is proposed to describe the fatigue behavior of
asphaltmixtures containing bentonite modified bitumen.
� 2014 Elsevier Ltd. All rights reserved.
Fatigue cracking, is a load associated cracking that is caused
dueto repeated traffic loading. This type of cracking is considered
to beone of the most significant distress modes in flexible
pavements.The fatigue life of an asphalt pavement is directly
related to variousengineering properties of hot mix asphalt (HMA)
mixtures. Thecomplicated microstructure of asphalt concrete is
related to thegradation of aggregate, the properties of
aggregate–bitumeninterface, the void size distribution, and the
interconnectivity ofvoids. As a result, the fatigue property of
asphalt mixtures is verycomplicated and sometimes difficult to
predict [1–4]. Many studieshave been conducted to understand the
occurrence of fatigue andhow to extend pavement life under
repetitive traffic loading [3,5].Various admixtures are used to
elongate the service life of
pavements via prevention or retardation of cracks in
pavementswithout negatively affecting the diverse performance
parametersof asphalt mixtures [6,7].
In several studies conducted, it was determined that thestrength
of HMA mixtures against permanent deformation [8,9],fatigue 
and moisture induced damage [11,12] increase afterutilization of
SBS in bitumen modification.
Nowadays, a great amount of mineral, organic, natural
andindustrial additives are used for improvement and modificationof
some properties of asphalt binders such as resistance to thermaland
shrinkage cracking, reduction in permanent deformation andasphalt
bleeding as well as reduction of hardness due to aging ofasphalt
binder ; however, considering geographic conditionsand existent
facilities in various countries, selecting an appropriatemodifier
differs from one country to another.
Most laboratory and field experiments have indicated that useof
rubberized asphalt concretes (RAC), in general, increase
durabil-ity, reduction of crack reflection, fatigue life and skid
Fig. 1. Grading curves of aggregates.
686 H. Ziari et al. / Construction and Building Materials 68
and resistance to permanent deformation of asphalt overlay
andthe stress absorbing membrane (SAM) layers [14,15].
The use of crumb rubber (CRM), expanded to HMA
mixtures,continues to evolve since the CRM bitumens enhance the
perfor-mance of asphalt mixtures by increasing the resistance of
thepavements to permanent deformation and thermal and
fatiguecracking. Many researchers have found that utilizing crumb
rubberin pavement construction is both effective and
economical[16–20]. High temperature rutting and low temperature
crackingare two disturbing drawbacks of unmodified and pure
bitumen. Clay based chemicals are pioneered as one of the
mostwell-known and profitable new generation of bitumen
additives.In the recent decades, bentonite clay and organically
modified ben-tonite (OMBT) were used as reinforcement in order to
modify bitu-minous pavements. In the literature, a vast amount of
experimentsperformed on bitumen and the variation of softening
point, viscos-ity and ductility as a function of clay content and
clay type werereported. Bending beam rheometer test results for
aged specimensthrough RTFO and PAV indicated that, modifying
bitumen withbentonite and OMBT, will improve low temperature
properties ofbitumen and significant improve resistance of asphalt
mixturesto cracking .
Although various additives such as polymers and rubberpowder may
improve the performance of bitumen, suitable perfor-mance of a
special additive should not be the criterion for choosingit, but
there are also some other factors such as economical
issues,production of modifier and environmental compatibility
thatshould be considered when selecting an additive.
The simplest fatigue models consider the fatigue prediction
onthe basis of either the strain-controlled mode or
stress-controlledmode. Eqs. (1) and (2) show the simplest fatigue
models for con-trolled-strain and controlled-stress modes,
respectively. This typeof fatigue model does not consider effects
of temperature, modu-lus, and loading frequency on the behavior of
HMA mixtures. Therelationships between fatigue life and
stress–strain level were con-sistently confirmed in the SHRP
project for the ranges of stressesand strains under laboratory
measurements of the asphalt speci-men .
Nf ¼ að1=eÞb ð1Þ
Nf ¼ að1=rÞb ð2Þ
where e = tensile strain at the bottom of specimen (in./in.), r
=applied tensile stress (psi), and a, b = experimentally
In addition to the simple models 1 and 2, there are different
fati-gue models that were used by different agencies or were based
ondifferent considerations, such as the Asphalt Institute model
andthe Shell model. The major role of these models is to provide a
rela-tionship between mixture properties, pavement response
(strain),and load repetitions to failure. The parameters of these
modelsare mainly based on a continuous-loading sequence, and the
coef-ficients are determined from empirical data regression. Eqs.
(3) and(4) show the Asphalt Institute model and the Shell
Nf ¼ 0:0796ðetÞ�3:291ðEtÞ�0:854 ð3Þ
Nf ¼ 0:0685ðetÞ�5:671ðE1Þ�2:363 ð4Þwhere E1 is the initial
flexural modulus of asphalt concrete (psi).
Monismith et al.  introduced fatigue life prediction
modelusing initial modulus and tensile strain of HMA mixtures. Eq.
(5)shows the fatigue life prediction model proposed by Monismithet
Nf ¼ k1ð1=etÞk2 ð1=EÞk3 ð5Þ
where k1, k2, k3 = experimentally determined coefficients,E =
asphalt concrete initial modulus (psi).
In this research study, bentonite is used to modify
bitumen.Bentonite is a sedimentary rock consisting, to a large
proportionof clay minerals with a typical 2:1 layered structure
(smectites)and a high concentration in sodium ions . In fact,
bentonite isa clay mineral, which has high montmorillonite in its
structure. Iran is located in a point of the world in which
there arenumerous sources of bentonite. Studies on the present data
pro-vided by the Geological Organization of Iran show that the
richsources of bentonite in Iran, which are mainly located in the
cen-tral region of Iran. Considering low cost of bentonite
comparedwith other additives and existence of numerous sources of
benton-ite in Iran, evaluation of modified asphalt binders by
bentonite hasbeen the main reason for this research.
The objective of this research study was to gain an
improvedunderstanding of the long-term performance characteristics
(fati-gue behavior) of the modified asphalt concrete mixtures
contain-ing bentonite additive through a series of experimental
tests.Experiments were carried out to evaluate engineering
propertiesof the mixture, such as the marshal stability, mixture
stiffness,indirect tensile strength and fatigue life performance
through flex-ural bending beam fatigue test. At last, based on
experimentalstudies, a model is proposed to describe the fatigue
life of asphaltmixtures containing bentonite modified bitumen.
2. Experimental methods
2.1. Aggregate and bitumen
Aggregates used in this study were obtained from the Boomehen
mine inTehran, Iran. The gradation of the blended aggregates is
shown in Fig. 1. Table 1 listsengineering properties of the raw
material used in the current research.
In this study, a 60/70-penetration grade bitumen was obtained
from Tehranrefinery which was supplied by Pasargad Oil Co, Tehran,
Iran. The physical proper-ties of the bitumen are presented in
The bitumen was modified with bentonite manufactured by
Dorinkashan Co.Five levels of bentonite content were used, namely
10%, 15%, 20%, 25% and 30%by weight of bitumen. The modified
bitumens were prepared by using a high shearmixer. The bitumen was
heated to 140 �C for thirty minutes and then subjected tofifteen
minutes of mixing time with bentonite at 140 �C and 4000 rpm shear
rate.The physical properties and chemical composition of bentonite
are presented inTables 3 and 4, respectively.
2.3. Mix design procedure
The mix design of the asphalt mixtures was performed by using
the standardMarshall mix design procedure with 75 blows on each
side of cylindrical samples(10.16 cm in diameter and 6.35 cm thick)
for compaction. Marshall Samples werecompacted and tested by the
following standard procedures: bulk specific gravity(ASTM D2726),
stability and flow test (ASTM D1559). For each test (Marshall
stabil-ity and flow, indirect tensile strength, resilient modulus
test, fatigue test) three testspecimens were used.
Table 1Engineering properties of aggregate source.
Aggregate tests Aggregate Test method
Bulk specific gravity 2.493 ASTM C127Absorption coarse aggregate
(%) 2.2 ASTM C127Absorption fine aggregate (%) 4.2 ASTM C128Los
Angeles abrasion loss (%) 22.3 AASHTO T96Two fractured faces (%) 94
Table 2Properties of utilized bitumen considering related
Test Method Criteria Result
Penetration at 25 �C, 100gr, (0.1 mm) ASTM D5 60–70 67Softening
point (�C) ASTM D36 45–54 47Ductility at 25 �C (cm) ASTM D113 +100
100Flash point (�C) ASTM D92 +250 304Fire point (�C) ASTM D70 +230
317Specific gravity at 25 �C (gr/cm3) ASTM D70 1.01–1.06
1.045Kinematic viscosity @ 120 �C (mm2/s) ASTM D2170 – 810Kinematic
viscosity @ 135 �C (mm2/s) ASTM D2170 – 420Kinematic viscosity @
150 �C (mm2/s) ASTM D2170 – 232Penetration index (PI)a – (�2) to
(+2) �1.12Penetration viscosity number (PVN)b – – �0.56
a PI = [1952 � 500log(Pen25) � 20SP]/[50log(Pen25) � SP � 120].b
PVN = [�6.387 + 1.195log(Pen25) + 1.5 log(Visco135)]/
[0.79511 � 0.1858log(Pen25)].
Table 3Basic properties of bentonite.
Test items Content
Specific gravity (gr/cm3) 2.5Moisture content (%) 6–10
Table 4Chemical composition of bentonite (mass percent).
SiO2 70.06Al2O3 14.22Fe2O3 3.04Na2O 2.17K2O 0.39MgO 2.4CaO
H. Ziari et al. / Construction and Building Materials 68 (2014)
A linear kneading compactor was used for compacting fatigue beam
specimensin the laboratory environment . Fatigue test can be
carried out in either con-trolled stress or controlled strain mode.
For controlled stress test failure is welldefined since specimens
fail shortly after crack initiation. In controlled strain
modefailure is arbitrary defined as the point when the stiffness of
asphalt reaches half ofits initial value. These criteria are used
by AASHTOT321 as standard test method. In this research study,
strain controlled test was used for evaluation of
fatigueperformance of asphalt mixtures. All the specimens were made
with 4% air void atoptimum asphalt content. To avoid a high air
void at the sample surfaces, 10 mmfrom each side of the sample was
cut. The final dimensions of prepared beams were380 * 63.5 * 50 mm
according to the AASHTO T321 standard. All samples tested
atconstant strain of 600 micro-strain, 800 micro-strain and 1000
microstrain withsinusoidal mode of loading. All tests conducted in
an environmentally controlledchamber at temperature of 20 ± 0.5 �C.
Specimens were pre-conditioned at 20 �Cfor a minimum period of 2 h.
The frequency of loading was 10 Hz  and numbersof loading
cycles reported as specimens fatigue life base on AASHTO T321
3. Test methods
3.1. Marshall stability, flow and Marshall Quotient tests
Marshall stability and flow (ASTM D1559), bulk specific
gravity(ASTM D2726), and air void content were determined to
and evaluate cracking performance of control and bentonite
mod-ified mixtures. The ratio of stability (kN) to flow (mm) is
known asthe Marshall Quotient (MQ) (kN/mm) was also calculated. MQ
canbe used as a measure of the material’s resistance to
permanentdeformation in service . A higher value of MQ
indicates a stifferand more resistant mixture. It is well
recognized that the highervalue of the MQ represents the more
resistance of material to shearstresses and permanent deformation
3.2. Indirect Tensile Strength (ITS) test
In an indirect tensile strength test, a cylindrical sample is
sub-jected to compressive loads between two loading strips,
whichgenerate a relatively uniform tensile stress along the
vertical dia-metrical plane. It is commonly used to evaluate the
potential ofstripping and fracture properties of asphalt mixtures.
Failure usu-ally occurs by splitting along this loaded plain .
The IDT testfollowing the ASTM D6931-12 was performed at constant
rate of50.8 mm/min and temperature of 20 �C. The tensile strength
ofthe specimen is determined by the following equation :
ITS ¼ ð2PmaxÞ=ðpDtÞ ð6Þ
where ITS: is the tensile strength of specimens in kPa, Pmax is
theapplied load at failure in kN; D is the diameter of the
specimenin mm; t is the thickness of the specimen in mm. Three
speci-mens were prepared for each asphalt mixture mentioned
Fracture energy and tensile strength are two parameters thatare
used simultaneously to evaluate cracking performance ofasphalt
mixtures . The fracture energy is defined as the workdone to
create a unit area of crack in the specimen, which is equalto the
area under the curve of load–deformation of mixture failure.To
determine fracture energy density from indirect tensile
strengthtest, the fracture energy is divided by the volume of
mixture. Thefracture energy can be calculated according to the
following equa-tion .
FE ¼Z dmax
where FE is the fracture energy density (MPa), P is a load (N),
V isthe volume of asphalt mixture (mm3) and d is the
3.3. Resilient modulus test
This test was performed on the cylindrical samples by using
ahaversine load pulse at 1 Hz and 0.9 s of rest period at 25 �C
withUniversal Testing Machine (UTM-5P). The load and
deformationwere continuously recorded and resilient modulus was
calculatedwith the following equation. The assumed Poisson’s ratio
Mr ¼ Pðv þ 0:2734Þ=dt ð8Þ
3.4. Four point bending fatigue tests
The fatigue resistance of the beams was evaluated in four
pointbending beam fatigue test according to AASHTO T321-07. The
pur-pose of these tests was to obtain the fatigue life of the beams
underdifferent stain levels. The setup and sketch of four point
bendingfatigue test is shown in Fig. 2. The test was conducted at
20 �C witha frequency of 10 Hz. The initial flexural stiffness was
calculatedfrom the measured force and displacement after the
fiftieth cycle(n = 50) according to the following equations
e ¼ 12dh� 106=3ðG20 � 4G21Þ ð9Þ
r ¼ G0P=Bh2 ð10Þ
Fig. 2. The setup and sketch of four point bending fatigue
688 H. Ziari et al. / Construction and Building Materials 68
S ¼ 1000r=e ð11Þ
where e is the maximum microstrain applied on the beam, d is
thepeak deflection at the center of the beam, h is the average
beamlength (mm), G0 is the outer gauge length (355.5 mm), G1 is
theinner gauge length (118.5 mm), r is the maximum tensile
stress(kPa), P is the peak force (kN), B is the average beam width
(mm),and S is the flexural stiffness of the beam (MPa).
The fatigue test was continued until the flexural
stiffnessdropped to half its initial value. After fatigue testing
of the beamsat 600, 800 and 1000 microstrain with sinusoidal
loading, thefatigue life of the mixture was determined using the
Nf ¼ ae�b ð12Þ
where Nf is the number of loading cycles to fatigue, e is the
micro-strain amplitude used in fatigue testing, a and b are
The dissipated energy in each cycle of loading can be
calculatedusing Eq. (13) and the accumulated dissipated energy by
D ¼ pre sinð360fuÞ ð13Þ
where D is dissipated energy (J/m3), f is loading frequency
(Hz), u istime lag (s).
where W is cumulative dissipated energy (J/m3), Di is D for ith
4. Results and discussion
4.1. Marshall stability and flow
Table 5 presents physical and mechanical properties
includingmarshal stability, flow and MQ of the mixtures
investigated in thisresearch study. The values are the average of
three samples. TheMarshall stability is the ability of asphalt
concrete to resist ruttingand shoving . It is seen that
Marshall stability increases withthe increase in bentonite content.
It appears that addition of
Table 5Results of Marshall stability.
Mixture Bulk specific gravity Air voids content (%) VLimits
– 3–7 >Results
0% BT 2.41 2.35 110% BT 2.31 6.88 115% BT 2.36 4.62 120% BT 2.37
4.48 125% BT 2.34 5.75 130% BT 2.32 6.72 2
bentonite increases in stiffness of bitumen. Thus the mixtures
con-taining bentonite modified bitumen, have higher stability
valuesthan that of control mixtures. It was determined that the
MarshallStability values increased 26.55% when base bitumen is
modifiedwith 20% bentonite.
As seen in Table 5, the specimen with 20% bentonite has
thehighest MQ value too. MQ is a measure of the material’s
resistanceto shear stresses and permanent deformation .
4.2. Indirect Tensile Strength (ITS) test
The average tensile strengths of control specimen and the
spec-imens containing bentonite modified bitumen are shown in Fig.
3.The values are average of three specimens. As seen in Figs. 3 and
4,addition of bentonite increases indirect tensile strength and
frac-ture energy of the mixtures. Considering that fracture energy
isthe sum of elastic energy and dissipated creep strain energy,
addi-tion of bentonite to a certain amount (15%) results in an
increase inboth elastic energy and the dissipated creep strain
energy. While,modification of bitumen with more than 20% bentonite,
reducesthe share of elastic energy and does not show a significant
increasein the total amount of fracture energy in the mixtures.
Whereas,modification of bitumen with higher percentage of
bentonite(30%) has led to decrease in the fracture energy of the
4.3. Resilient modulus (MR) test
Fig. 5 presents the resilient modulus variation of asphalt
mix-tures with different modified bentonite bitumen content
(10%,15%, 20%, 25% and 30% by weight of bitumen). The resilient
modu-lus at low temperatures is somehow related to thermal
cracking. Ithas been shown that stiffer mixtures at lower
temperatures aremore prone to thermal cracking . The results
show that useof modified bentonite bitumen in the mixture initially
increasesthe resilient modulus but the resilient modulus decreases
byaddition of more percentages of bentonite in bitumen.
Resilientmodulus of the mixture containing modified bitumen with
10%bentonite is 1.15 times greater than that of the control
sampleand is 1.09 times greater than that of the control sample
when ben-tonite content of modified bitumen is 30%.
MA (%) Stability (kN) Flow (mm) MQ (kN/mm)
14 >8 2–3.7 –
6 9.475 3.125 3.0329.86 11.305 3.22 3.5117.83 11.64 3.46
3.3648.52 12.01 3.5 3.4319.22 10.51 3.62 3.90.07 9.49 3.72 2.55
Fig. 3. Results of indirect tensile strength tests.
Fig. 4. Fracture energy of mixtures.
Fig. 5. Resilient modulus of mixtures with different bentonite
content at 25 �C.
Fig. 7. Fatigue life of mixtures with different bentonite
Fig. 6. Stiffness of mixtures with different bentonite
H. Ziari et al. / Construction and Building Materials 68 (2014)
4.4. Fatigue life analysis
Fatigue cracking that is associated with repetitive traffic
loadingis considered to be one of the most significant distress
modes inpavements and is related to various properties of HMA.
Fig. 7, shows that mixtures with bentonite modified bitumenhave
longer fatigue life. As is observed from Fig. 7, mixturesprepared
with modified bitumen containing 10%, 15% and 20% ben-tonite have
an extended fatigue life relative to the control mixture.But
modification of bitumen with 30% bentonite reduces mixturesfatigue
life. In mixtures containing modified bitumen with morethan 20% of
bentonite, the adhesion of bitumen to aggregate wasreduced. However
in mixtures containing 10%, 15% and 20% ben-tonite modified
bitumen, bentonite does not have a negative effect
on the cohesion of bitumen to aggregate . For strain levels
of600 & 800 a similar trend is observed. However in 1000
micro-strain level fatigue life is increased with addition of
bentonite inbitumen modifications. As shown in Fig. 6, flexural
stiffnessincreased by adding bentonite in bitumen. In Fig. 8,
dissipatedenergy of specimens was shown. For two strain levels (600
&800) a similar trend is seen. However in 1000 microstrain
level fati-gue life is increased with addition of bentonite in
bitumen modifi-cations. The higher the dissipated energy is, the
greater the abilityof materials to absorb energy and thus cracking
of asphalt isdecreased and the fatigue life is increased .
4.5. Proposing a model for the behavior of HMA containing
Based on the test results obtained in this experimental
researchstudy a power low function as presented by Eq. (15) was
used tomodel and predict fatigue lives of control mixtures as well
as mix-tures containing bentonite modified bitumen. Table 6,
presentsrelationships between numbers of load applications to
failurebased on induce tensile strains a power law function
presentedby Eq. (15).
Nf ¼ ae�b ð15Þ
Table 6, presents fatigue life for the control mixture as well
asthe mixtures containing bentonite modified bitumens based onEq.
Fig. 8. Cumulative dissipated energy of mixtures with different
Table 6Coefficients of the fatigue models in conventional and
Asphalt concrete pavements Coefficients of the fatigue models
Control mixture 1E+14 3.2337 0.8253Mixture with 10% bentonite
7E+13 3.1051 0.8131Mixture with 15% bentonite 1E+13 2.815
0.7415Mixture with 20% bentonite 5E+12 2.7144 0.8968Mixture with
25% bentonite 2E+12 2.5651 0.9279Mixture with 30% bentonite 2E+11
690 H. Ziari et al. / Construction and Building Materials 68
The aim of this study was to evaluate the effects of bentonite
asbitumen modifier in hot mix asphalt mixtures. Various
laboratorytests were conducted to evaluate the characteristics of
hot mixasphalt by varying contents of bentonite in the base
bitumen.Based on the limited test results obtained in this research
study,the following conclusions are drawn:
� Marshall stability and flow of mixtures containing
bentonitemodified bitumens are higher than the control mixture.�
Marshall Quotient of the mixtures prepared with modified ben-
tonite bitumen is higher than the control mixture. However
theincreasing trend of MQ stops and plateaus where base
bitumencontains more than 20% bentonite.� The resilient modulus of
mixtures prepared with modified ben-
tonite bitumen is higher than the control mixture. The
increas-ing trend stops and declines at a point where base
bitumencontains more than 20% bentonite.� Mixtures containing 10%
and 15% bentonite modified bitumen
have longer fatigue lives than the control mixture.
Modificationof the base bitumen with higher percentage of bentonite
neitherenhances nor improves fatigue life of the mixtures.� The
dissipated energy of the mixtures containing 10% and 15%
of modified bentonite bitumen is higher than the control
mix-ture. However the increasing trend declines at higher
percent-ages of bentonite content.� According to results, models
for the prediction of the fatigue
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Evaluation of fatigue behavior of hot mix asphalt mixtures
prepared by bentonite modified bitumen1 Introduction2 Experimental
methods2.1 Aggregate and bitumen2.2 Additive2.3 Mix design
3 Test methods3.1 Marshall stability, flow and Marshall Quotient
tests3.2 Indirect Tensile Strength (ITS) test3.3 Resilient modulus
test3.4 Four point bending fatigue tests
4 Results and discussion4.1 Marshall stability and flow4.2
Indirect Tensile Strength (ITS) test4.3 Resilient modulus (MR)
test4.4 Fatigue life analysis4.5 Proposing a model for the behavior
of HMA containing bentonite modified bitumens