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Evaluation of fatigue behavior of hot mix asphalt mixtures prepared by bentonite modified bitumen Hasan Ziari, Rezvan Babagoli , Mahmoud Ameri, Ali Akbari School of Civil Engineering, Iran University of Science and Technology, Narmak, Tehran 16846, Iran highlights 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 obtained. article info Article history: Received 5 September 2013 Received in revised form 11 May 2014 Accepted 29 June 2014 Available online 26 July 2014 Keywords: Bentonite Fatigue life Four point beam fatigue test Indirect tensile strength Resilient modulus abstract 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 five percentages 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 were conducted. 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 of asphalt 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 asphalt mixtures containing bentonite modified bitumen. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Fatigue cracking, is a load associated cracking that is caused due to repeated traffic loading. This type of cracking is considered to be one of the most significant distress modes in flexible pavements. The fatigue life of an asphalt pavement is directly related to various engineering properties of hot mix asphalt (HMA) mixtures. The complicated microstructure of asphalt concrete is related to the gradation of aggregate, the properties of aggregate–bitumen interface, the void size distribution, and the interconnectivity of voids. As a result, the fatigue property of asphalt mixtures is very complicated and sometimes difficult to predict [1–4]. Many studies have been conducted to understand the occurrence of fatigue and how 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 pavements without negatively affecting the diverse performance parameters of asphalt mixtures [6,7]. In several studies conducted, it was determined that the strength of HMA mixtures against permanent deformation [8,9], fatigue [10] and moisture induced damage [11,12] increase after utilization of SBS in bitumen modification. Nowadays, a great amount of mineral, organic, natural and industrial additives are used for improvement and modification of some properties of asphalt binders such as resistance to thermal and shrinkage cracking, reduction in permanent deformation and asphalt bleeding as well as reduction of hardness due to aging of asphalt binder [13]; however, considering geographic conditions and existent facilities in various countries, selecting an appropriate modifier differs from one country to another. Most laboratory and field experiments have indicated that use of rubberized asphalt concretes (RAC), in general, increase durabil- ity, reduction of crack reflection, fatigue life and skid resistances, http://dx.doi.org/10.1016/j.conbuildmat.2014.06.066 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved. Corresponding author. Tel./fax: +98 2177240565. E-mail address: [email protected] (R. Babagoli). Construction and Building Materials 68 (2014) 685–691 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
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  • 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

    http://dx.doi.org/10.1016/j.conbuildmat.2014.06.0660950-0618/� 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 obtained.

    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 2014

    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.

    1. Introduction

    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 [10] 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 [13]; 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 resistances,

    http://crossmark.crossref.org/dialog/?doi=10.1016/j.conbuildmat.2014.06.066&domain=pdfhttp://dx.doi.org/10.1016/j.conbuildmat.2014.06.066mailto:[email protected]://dx.doi.org/10.1016/j.conbuildmat.2014.06.066http://www.sciencedirect.com/science/journal/09500618http://www.elsevier.com/locate/conbuildmat

  • Fig. 1. Grading curves of aggregates.

    686 H. Ziari et al. / Construction and Building Materials 68 (2014) 685–691

    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[21]. 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 [21].

    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 [22].

    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 determinedcoefficients.

    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 model,respectively.

    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. [23] introduced fatigue life prediction modelusing initial modulus and tensile strain of HMA mixtures. Eq. (5)shows the fatigue life prediction model proposed by Monismithet al. [23].

    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 [24]. In fact, bentonite isa clay mineral, which has high montmorillonite in its structure[25]. 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 Table 2.

    2.2. Additive

    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 ASTM D5821

    Table 2Properties of utilized bitumen considering related standards.

    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).

    Component Percent

    SiO2 70.06Al2O3 14.22Fe2O3 3.04Na2O 2.17K2O 0.39MgO 2.4CaO 1.68LoI 5.32

    H. Ziari et al. / Construction and Building Materials 68 (2014) 685–691 687

    A linear kneading compactor was used for compacting fatigue beam specimensin the laboratory environment [26]. 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[27]. 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 [27] and numbersof loading cycles reported as specimens fatigue life base on AASHTO T321 criteria.

    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 compare

    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 [28]. 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 [28].

    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 [29]. 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 [29]:

    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 inSection 2.3.

    Fracture energy and tensile strength are two parameters thatare used simultaneously to evaluate cracking performance ofasphalt mixtures [30]. 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 [31].

    FE ¼Z dmax

    0PðdÞdd=V ð7Þ

    where FE is the fracture energy density (MPa), P is a load (N), V isthe volume of asphalt mixture (mm3) and d is the deformation.

    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 was 0.35.

    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 [32]:

    e ¼ 12dh� 106=3ðG20 � 4G21Þ ð9Þ

    r ¼ G0P=Bh2 ð10Þ

  • Fig. 2. The setup and sketch of four point bending fatigue test.

    688 H. Ziari et al. / Construction and Building Materials 68 (2014) 685–691

    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 followingequation [33]:

    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 fatigueconstants.

    The dissipated energy in each cycle of loading can be calculatedusing Eq. (13) and the accumulated dissipated energy by Eq. (14):

    D ¼ pre sinð360fuÞ ð13Þ

    where D is dissipated energy (J/m3), f is loading frequency (Hz), u istime lag (s).

    W ¼Xi¼ni¼1

    Di ð14Þ

    where W is cumulative dissipated energy (J/m3), Di is D for ith loadcycle.

    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 [34]. 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 [28].

    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 mixture.

    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 [35]. 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 content.

    Fig. 6. Stiffness of mixtures with different bentonite content.

    H. Ziari et al. / Construction and Building Materials 68 (2014) 685–691 689

    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 [36]. 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 [37].

    4.5. Proposing a model for the behavior of HMA containing bentonitemodified bitumens

    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. (15).

  • Fig. 8. Cumulative dissipated energy of mixtures with different bentonite content.

    Table 6Coefficients of the fatigue models in conventional and modified specimens.

    Asphalt concrete pavements Coefficients of the fatigue models R2

    a b

    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 2.2709 0.9901

    690 H. Ziari et al. / Construction and Building Materials 68 (2014) 685–691

    5. Conclusion

    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

    behavior of control and modified HMAs under different strainlevels were obtained.

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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 procedure

    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

    5 ConclusionReferences


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