Construction and Building Materials 47 (2013) 1342–1349

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Construction and Building Materials

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Analysis on fatigue crack growth laws for crumb rubber modified (CRM)asphalt mixture

0950-0618/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.conbuildmat.2013.06.014

⇑ Corresponding author. Tel.: +86 29 82334798.E-mail addresses: wanghainian@aliyun.com (H. Wang), zhaofeng

wuying@163.com (Z. Dang), cdlilian@126.com (L. Li), zyou@mtu.edu (Z. You).1 Tel.: +86 29 82334824.2 Tel.: +86 9319 8259502.3 Tel.: +1 906 487 1059.

Hainian Wang a,⇑, Zhengxia Dang a,1, Lian Li b,2, Zhanping You c,3

a Key Laboratory for Special Area Highway Engineering of Ministry of Education Chang’an University South Erhuan Middle Section, Xia’n, Shaanxi 710064, Chinab School of Civil Engineering, Lanzhou University of Technology, Lanzhou, Gansu 730050, Chinac Department of Civil and Environmental Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931-1295, United States

h i g h l i g h t s

� Crumb rubber concentration has significant influence on CRM asphalt mixture’s fatigue property.� It is beneficial for the CRM mixture’s fatigue property to use smaller size rubber powders.� The fatigue life becomes longer and crack grows slower with the increasing of loading frequency.� Gap aggregate gradation behaves better than continuous gradation for CRM mixtures.

a r t i c l e i n f o

Article history:Received 9 March 2013Received in revised form 2 June 2013Accepted 4 June 2013

Keywords:Crumb rubber modified (CRM) asphaltmixtureNotched semi-circular bending (SCB) testFatigue crack

a b s t r a c t

In recent years, crumb rubber has been applied widely in asphalt pavement, and many researchers haveindicated that crumb rubber modified (CRM) asphalt mixture is an environmentally friendly material. Inthis study, the notched semi-circular bending (SCB) test was employed to study the fatigue crackingproperty for CRM asphalt mixture. Then the cracking growth length was obtained by image processingtechnology, and its correlation with the fatigue number was established and studied in this paper. Con-currently, the influence of gradation type, asphalt content, test temperature, stress ratio, loading fre-quency, rubber powder concentration and rubber powder size on CRM asphalt mixtures’ fatigue lifeand crack growth laws were investigated by this method. The results indicated that the gap-gradedCRM asphalt mixture had a longer fatigue life and a lower crack growth rate than the continuous gradedmixtures Moreover, at the optimum asphalt content, the fatigue life was much longer and the crackgrowth rate was much lower at smaller loading times with higher loading frequency at the CRM asphaltmixture concentration of 20% using the smaller 80 mesh fine crumb rubber size.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

With the rapid development of automobile industry and quickincrease of cars annually, over 65 million scrap tires have been pro-duced every year in China [1]. And moreover, the number of scraptires has been increasing at a rate of 20% annually in the past dec-ade, which gives great pressure to the environment. The questionof how to deal with these scrap tires is an important issue all overthe world [2]. In recent years, it is very popular to employ scrap tireinto asphalt pavement. Crumb rubber modified (CRM) asphalt canconsume a considerable number of scrap tires, moreover adding

crumb rubber into asphalt can improve its performance [3]. Manystudies have proved that CRM asphalt pavement increase pave-ment life by, resistance to cracking and rutting, decrease trafficnoise, and overall reduction of the maintenance costs. In addition,some researchers have found that the use of crumb rubber into as-phalt binders can enhance the fatigue resistance [4–7].

Fatigue cracking of asphalt mixture is one of the main damagesin asphalt pavements, which influences the pavement performancesignificantly. The main influencing factor of fatigue cracking is thematerial property [8,9]. Cao and Bai employed crumb rubber pow-ders from different sources and virgin asphalt of 70# (penetrationgrade) and 90# to evaluate the performances of asphalt rubberstress absorbing membrane interlay [10]. The test results indicatedthat CRM asphalt binder with truck tire crumb rubber has betterperformances than that of binders with car tire crumb. CRM as-phalt binder using the 70# asphalt has a better high-temperatureperformance than using 90# asphalt. However, 70# CRM asphaltbinder’s low temperature performance and fatigue resistance were

Table 1Properties of 90# virgin asphalt binder.

Test properties Unit Standardrequirements

Testresults

Penetration (15 �C, 100 g, 5 s) 0.1 mm N/A 33.9Penetration (25 �C, 100 g, 5 s) 0.1 mm 80–100 94.7Penetration (35 �C, 100 g, 5 s) 0.1 mm N/A 147.2Penetration index – �1.5 � +1.0 �0.45Softening point �C P46 47.5Ductility (5 �C, 5 cm/min) cm N/A 11.6Kinematic viscosity (135 �C) Pa s 0.632Density g/cm3 N/A 0.979RTFO aged asphalt residueMass loss % �0.8 � +0.8 �0.34Penetration ratio for asphalt

residue (25 �C)% P61 72.9

Ductility for asphalt residue(5 �C) cm N/A 5.1

Table 2Crumb rubber properties.

Index Property Test results (%)

Physical indexes Specific gravity 1.15Moisture content 0.45Metal content 0.002Fiber content 0.51

Chemical indexes Ash content 3.6Acetone content 11.5Carbon black content 28.4Rubber hydrocarbon content 56.5

Table 3The properties of CRM asphalt binder.

Test properties Unit Test results

Penetration (15 �C, 100 g, 5 s) 0.1 mm 29.6Penetration (25 �C, 100 g, 5 s) 0.1 mm 71.2Penetration (35 �C, 100 g, 5 s) 0.1 mm 114.1Penetration index – 0.19Softening point �C 55.4Ductility (5 �C, 5 cm/min) cm 17.6Equivalent brittle point �20.7Equivalent softening point 51.9Viscosity (177 �C) Pa s 2.8Viscosity (190 �C) Pa s 1.9Elastic recovery % 68.5

H. Wang et al. / Construction and Building Materials 47 (2013) 1342–1349 1343

not as good as CRM asphalt binder using 90# asphalt. All CRMbinders in general had good fatigue and shear resistance.

Wang et al. employed |G�| sind from the Dynamic Shear Rheom-eter test to evaluate the intermediate temperature fatigue propertyof CRM binder with a 20 mesh crumb rubber [11]. The concentra-tion of crumb rubber were 10%, 15%, 20% and 25% by weight of as-phalt binders, respectively. It was found that the addition of crumbrubber could improve the intermediate temperature fatigue life ofasphalt binders significantly. Previous studies have paid moreattention on different crumb rubber modifiers and additive agentsfor rubber asphalt to improve the fatigue resistance property in re-cent years [12–14]. However, there is very limited and detailed re-search work done on fatigue cracking for CRM asphalt mixtures.

Xiao et al. used fatigue beams test to study the long-term per-formance of warm CRM asphalt mixtures with one rubber type(40 mesh ambient crumb rubber), two aggregate sources, twoWMA additives (Asphamin and Sasobit) at 20 �C. It was indicatedthat crumb rubber and WMA additives could effectively extendthe long-term performance of asphalt mixtures when comparedwith conventional asphalt mixtures [15]. Mull employed Semi-Cir-cular Bending (SCB) test to evaluate fracture resistance of chemi-cally modified crumb rubber asphalt (CMCRA) pavement basedon the J-integral concept. The CRA pavement was found to have aslightly higher fracture resistance than that of the control pave-ment [16].

Recently, SCB test, which is popular in Europe and the USA, hasattracted many researcher’s interests. This test can analyze andevaluate asphalt mixtures molded by various methods; specificallyit can directly be used to carry out cracking analysis experimentson pavement cores. Arabani, M. used SCB test to study CRM asphaltmixtures, and found that the asphalt mixtures produced by the wetand dry processes showed better fatigue performance, which even-tually turned out to be an effective evaluation means for the pave-ment service performance of asphalt rubber mixtures [17,18].

There are two reasons to explain why the addition of crumbrubber can enhance the asphalt mixture’s anti-fatigue property.One reason is that there is some anti-aging agent element in crumbrubber powders such as carbon black, which can increase asphaltbinders’ or asphalt mixtures’ aging resistance when added into.The other reason is that the viscosity and elasticity of asphalt bin-der will increase when crumb rubber powder is added into asphaltbinder, which will increase the asphalt film thickness on aggre-gates surface and then increase the asphalt mixtures’ aging resis-tance. There are various methods and criteria to evaluate theasphalt mixtures’ fatigue property today. This paper employedthe notched SCB test for asphalt mixture fatigue test. The influenceof gradation type, asphalt content, test temperature, stress ratio,loading frequency, rubber powder concentration and rubber pow-der size on CRM asphalt mixtures’ fatigue property was studied toanalyze the fatigue crack growth laws in CRM asphalt mixture.

2. Materials and test program

2.1. Materials

2.1.1. Asphalt bindersA penetration grade 90# virgin asphalt from Karamay in Xinjiang, China was

used in this paper, and Table 1 shows the properties of this asphalt binder.

2.1.2. Crumb rubber powderIn general, there are many physical indexes such as crumb rubber powder size

or gradation, density, fiber content and metal content that will influence crumbrubber powders’ pavement performance. However, synthetic rubber, natural rub-ber, plasticizer, carbon black and ash are included in the chemical composition ofcrumb rubber powders, and the natural rubber plays a very important role in theproperties of CRM asphalt binders. It is indicated that the increase of natural rubberin crumb rubber powders can speed up the reaction between asphalt binders and

rubber, sharply increasing the viscosity of CRM asphalt binders. An ambient crumbrubber from Beijing, China was used in this study, and the properties of the crumbrubber are shown in Table 2.

2.1.3. Crumb rubber modified (CRM) asphalt binderThe wet process was employed to produce CRM asphalt binders in this study.

Moreover, the wet process defines any method that adds CRM to asphalt, whichis then well blended and interacted at high temperature and high mix speed beforeincorporating the modified binder into the mixer.

In general, the CRM asphalt binders’ production process could be described asfollows in this study: firstly, dry crumb rubber powder was added into the virginasphalt which was slowly heated to 170 �C, then mixed manually for 10 min in ahigh speed shear apparatus. Secondly, the mixture was blended by using the highspeed shear apparatus heated at 180 �C at a blending speed of 6500 rpm for50 min. Table 3 shows the properties of CRM asphalt binder in the laboratory.

2.2. CRM asphalt mixture

A type of diabase stone, whose nominal maximum size of 19 mm, was used forthe mix design in this study with limestone as a mineral filler. In addition, the gapgradation SMAR-16 from Texas and a continuous gradation AC-16 were used forCRM asphalt mixtures. The specimens in this study had a diameter of 150 mmand compacted by the Superpave gyratory compactor (SGC). Furthermore, the Nini,

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Sieve size (mm)

SMAR-16AC-16

19169.54.752.361.180.3

Fig. 1. Gradations of aggregates for SMAR-16 and AC-16.

Table 4Volumetric properties of SMAR-16 CRM asphalt mixture and AC-16 CRM asphaltmixture.

Volumetric property SMAR-16

AC-16

Standardrequirements

Air void 4% 4% 4.0%Voids in mineral aggregates

(VMA)16.1% 13.9% >13.5%

Voids filled with asphalt (VFA) 75% 71.1% 65–75%Filler-asphalt ratio 1.05 1.15 0.6–1.2

Fig. 2. The notched SCB test process.

1344 H. Wang et al. / Construction and Building Materials 47 (2013) 1342–1349

Ndes, and Nmax values were 8, 100, and 160, respectively. Gradation results areshown in Fig. 1. Optimum asphalt content (OAC) was defined as the amount toachieve 4% air voids according to the Superpave mix design method. It was indi-cated that the OAC for SMAR-16 CRM asphalt mixture and AC-16 CRM asphalt mix-ture were 5.7% and 4.7%, respectively. Ultimately, the volumetric properties of thetwo CRM asphalt mixtures are indicated in Table 4.

Table 5Loading times for different CRM asphalt mixtures at all kinds of crumb rubberconcentrations.

CRM asphaltmixture

Crumb rubberconcentration (%)

Maximumloading (kN)

Loadingtimes

Gap gradationSMAR-16

15 4.870 332418 5.262 405620 5.628 468222 4.852 438925 3.620 3895

Continuousgradation AC-16

15 3.234 256318 3.883 312520 4.639 358722 4.195 323525 3.745 2986

2.3. Test method

The fatigue tests on CRM mixtures were conducted on MTS-810 testing facility.There are two control modes in fatigue tests of asphalt mixture. One is strain con-trol, and the loading strain of mixture specimen is fixed in the fatigue test. In thiscase, with the increasing of testing temperature, the maximum loading time will in-crease due to the decreasing of mixture modulus. Another is stress control, and thestress ratio (loading amplitude vs. maximum loading) in the mixture is fixed in thefatigue loading. By this method, the fatigue life of mixture will decrease with theincreasing of testing temperature. In this paper, the stress mode was applied inthe fatigue test and the stress ratio was set as 0.5.

Notched semi-circular bending (SCB) test was employed in this paper to studythe fatigue properties of CRM asphalt mixtures. The specimen at the middle ofwhich there was a 15 mm long notch, was semicircular with a diameter of150 mm and a thickness of 50 mm, and the notch was cut by a 0.36 mm thick dia-mond cutting slice. Sine loading was used in the SCB test, and the loading and dis-placement values were obtained at every 0.01 s by the material testing system(MTS). Pictures were taken continuously as the loading was applied to measurethe clack length by a camera with 18 million pixels. The SCB sample needed to bestored at the test temperature for more than 3 h before starting the notched SCBtest. The notched SCB test was operated similar to the bending fracture test withthree testing repetitions for each type of mixture. However, in order to enhancethe accuracy of the captured cracking length, gridlines were always added on thepictures to obtain accurate crack lengths for SCB specimens, and the notched SCBtest process is shown in Fig. 2.

There were two cylinders as two fulcrums under the semi-circular specimenand one fulcrum on the semi-circular specimen. One advantage of the SCB test isthat the distance between two fulcrums can be changed to meet test requirements.Moreover, the testing device is very simple, and the asphalt mixture specimen couldbe prepared easily. The fulcrums could reduce the friction between the mixturespecimen and fulcrum to make it easy to analyze the specimen under test loading.Another advantage of the SCB test is that the test is suitable for all kinds of speci-mens, such as Marshall-mixture specimen, SGC mixture specimen and the core ta-ken from serving field pavement.

3. Results and discussions

3.1. Analysis on crack growth at different crumb rubber concentration

It was indicated that the crumb rubber concentration played animportant role in properties of CRM asphalt mixtures. Therefore,five concentrations of 15%, 18%, 20%, 22%, and 25% by weight ofthe asphalt binders, were employed to study the influence ofcrumb rubber concentration on the fatigue property of CRM as-phalt mixtures. In this part, SCB test was operated with a sine load,the frequency of which was 10 Hz and the stress ratio was 0.5 at15 �C using thev40 mesh rubber powders chosen in this study. Ta-ble 5 show the fatigue lives (load repetition times) for differentCRM asphalt mixtures with different crumb rubber concentrationunder different loadings. However, the N-a curves for differentCRM binders with different rubber contents are indicated in Fig. 3.

In general, it can be seen from Table 5 that both for SMAR-16CRM asphalt mixture and AC-16 CRM asphalt mixture, the maxi-mum load and fatigue lives increase with increasing amounts ofcrumb rubber concentration. And the maximum load and fatiguelives achieved the largest values when the crumb rubber concen-trations were 20% for the two CRM asphalt mixtures, respectively.However, the maximum load and fatigue lives decreased withcrumb rubber concentration increase when the concentrationwas larger than 20% for both SMAR-16 CRM asphalt mixture andAC-16 CRM asphalt mixture. Moreover, Table 5 also indicates thatwhen the crumb rubber concentrations were the same, the CRMasphalt mixture SMAR-16 had a larger maximum load and fatiguelife than that of AC-16 CRM asphalt mixture. For example, whenthe crumb rubber concentration was 20%, the maximum load ofSMAR-16 CRM asphalt mixture was 1.21 times as lager as that of

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(b)Fig. 3. N-a curves for different CRM binders with different rubber contents, (a)SMAR-16 and (b) AC-16.

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Fig. 4. Crack length for SMAR-16 CRM asphalt mixture and AC-16 CRM asphaltmixture with 20% rubber content.

Table 6ANOVA results of crumb rubber concentration on crack length growth.

Source SS df MS F P-value Fcrit

SMAR-16 171.052 4 42.763 29.5597 0.000 2.5252

AC-16 291.749 4 72.937 35.1649 0.000 2.5837

Note: SS: sum of squares; df: degree of freedom; MS: mean squares; F: the F value;Fcrit, the F critical value; The significance level is 0.05.

Table 7ANOVA results of the difference between SMAR-16 CRM asphalt mixture and AC-16CRM asphalt mixture at different crumb rubber concentrations on crack lengthgrowth.

Source (%) SS df MS F P-value Fcrit

15 102.093 1 102.094 29.7482 0.0002 4.844318 110.976 1 110.976 25.3790 0.0002 4.600120 35.824 1 35.824 6.1605 0.02454 4.494022 148.350 1 148.350 25.0691 0.0001 4.543125 112.801 1 112.801 26.3230 0.0002 4.6672

Note: SS: sum of squares; df: degree of freedom; MS: mean squares; F: the F value;Fcrit, the F critical value; The significance level is 0.05.

H. Wang et al. / Construction and Building Materials 47 (2013) 1342–1349 1345

AC-16 CRM asphalt mixture, and fatigue life of SMAR-16 CRM as-phalt mixture is 1.31 times that of AC-16.

However, from Fig. 3 it can also be seen that, both for SMAR-16CRM asphalt mixture and AC-16 CRM asphalt mixture, the cracklength grows faster with the increase of fatigue life. When the loadrepetition times were the same for SMAR-16 CRM asphalt mixturesat different crumb rubber concentrations, the mixture with 20%rubber content had the smallest crack length. Further findings forSMAR-16 CRM asphalt mixture were consistent with AC-16 CRMasphalt mixture. Therefore, it could be learnt that 20% could bean optimum crumb rubber concentration when referring to anti-fatigue property for CRM asphalt mixtures, as the crack growsmuch slower and fatigue life is longer at that concentration thanthat of CRM asphalt mixtures with other crumb rubber contents.Also, the crack length of SMAR-16 CRM asphalt mixture growsmuch slower than that of AR-16 CRM asphalt mixture. Take theCRM asphalt mixtures with 20% for example, as it is indicated inFig. 4, when the load repetition time was lower than 2200, cracklengths grows similarly. However, the crack length grows muchslower in the SMAR-16 CRM asphalt mixture than that of the AR-16 CRM asphalt mixture. Therefore, SMAR-16 CRM asphalt mixturehas a much better anti-fatigue property than AC-16 asphaltmixture.

Furthermore, the statistical analysis of variance (ANOVA) tech-nique was applied to quantify the effects of aggregate gradationand crumb rubber concentrations on the CRM asphalt mixtures’crack length growth, and the analysis results are shown in Tables6 and 7. In this study ANOVA was conducted by using the Excel

program, of which the accuracy of the various models was evalu-ated at a significance level of 0.05 (a = 0.05). It could be learnt thatcrumb rubber concentration had a significant effect on crackgrowth length in notched SCB test for both SMAR-16 CRM asphaltmixture and AR-16 CRM asphalt mixture as Fcrit value was biggerthan F value or P-value was smaller than 0.05. Also, as it is indi-cated in Table 7 the differences between the crack length ofSMAR-16 CRM asphalt mixture and AC-16 CRM asphalt mixtureare significant at different crumb rubber concentrations. Therefore,both crumb rubber content and gradation are very important inCRM asphalt mixture’s fatigue property.

3.2. Analysis of crumb rubber powder size on crack growth

Since it was indicated that 20% crumb rubber concentrationcould be an optimum content in evaluating the fatigue propertyof CRM asphalt mixture, this paper also employed three sizes ofcrumb rubber powders (20 mesh, 40 mesh, 80 mesh) by 20%weight of asphalt to investigate the fatigue property of CRM as-phalt mixture by notched SCB test. Also the effect of crumb rubberpowder size on fatigue property in different gradations of CRM as-phalt mixture was studied. SCB test was operated with a sine load-ing, the frequency of which was 10 Hz and the stress ratio was 0.5

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Fig. 5. Maximum loading and fatigue life for SMAR-16 CRM asphalt mixture andAC-16 CRM asphalt mixture with different rubber powder size, (a) maximumloading and (b) fatigue life.

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Fig. 6. N-a curves for SMAR-16 CRM asphalt mixture and AC-16 CRM asphaltmixture under different crumb rubber sizes, (a) SMAR-16 and (b) AC-16.

Table 8ANOVA of rubber powder size on crack lengths in notched SCB test.

Source SMAR-16 AR-16

20 mesh 40 mesh 80 mesh 20 mesh 40 mesh 80 mesh

40 mesh – S – S80 mesh – –

Note: N: non-significant; S: significant. The significance level is 0.05.

1346 H. Wang et al. / Construction and Building Materials 47 (2013) 1342–1349

at 15 �C. The maximum loading and fatigue life of SMAR-16 CRMasphalt mixture and AC-16 CRM asphalt mixture with differentcrumb rubber powders sizes at 20% rubber concentration areshown in Fig. 5. It can be seen that both maximum loading and fa-tigue life increase as the crumb rubber powder size decreases inboth SMAR-16 and AC-16 CRM asphalt mixtures. For example,the maximum loadings for 40 mesh and 80 mesh SMAR-16 CRMasphalt mixture were 114.14% and 124.38% of that for 20 meshmixture when the crumb rubber content was 20%, respectively.While, for fatigue life the values were 123.86% and 141.85%,respectively. It was shown that for the two gradation mixtureswith the same crumb rubber size and content, SMAR-16 CRM as-phalt mixture had a lager maximum loading and a longer fatiguelife than AC-16 CRM asphalt mixture.

The N-a curves which describe the relationships between load-ing repetition times and crack growth lengths for both SMAR-16and AC-16 CRM asphalt mixtures with different crumb rubber sizesare indicated in Fig. 6. It could be seen that when the fatigue lifewas the same, both SMAR-16 and AC-16 CRM asphalt mixtureswith smaller size rubber powders had a smaller crack growthlength. Therefore, the crack grew the most slowest in CRM asphaltmixture with 80 mesh rubber powders than that with the othertwo sizes of rubber powders.

The ANOVA was employed to analyze the significant effects ofcrumb rubber powder size and gradation on crack growth length,and the results are shown in Table 8. It was identified and verifiedthat crumb rubber powder sizes had significant effect on crack

length growth in notched SCB test in both SMAR-16 and AC-16CRM asphalt mixtures. As indicated in Fig. 6 and Table 8, mixtureswith smaller crumb rubber powder size would have better ant-fa-tigue property.

3.3. Analysis of test temperature on crack length growth

As it is known that an asphalt mixture is a viscoelastic material,and its mechanical properties are influenced significantly by tem-peratures, therefore, in order to study the effect of temperature onthe fatigue property of CRM asphalt mixture, three temperatures:5 �C, 15 �C and 25 �C were chosen in notched SCB tests. The loadingwas a sine loading with a frequency of 10 Hz, and the stress ratiowas 0.5. In addition, crumb rubber powder size in this study was40 mesh, and the content was 20% by the weight of virgin asphaltbinders. The maximum loadings and loading times of SMAR-16CRM asphalt mixture and AC-16 CRM asphalt mixture at differenttemperatures are shown in Table 9.

Table 9Maximum loading and loading times of SMAR-16 CRM asphalt mixture and AC-16CRM asphalt mixture.

Gradation Temperature (�C) Maximum loading (kN) Fatigue life

SMAR-16 5 10.977 894615 5.628 468225 2.342 2245

AC-16 5 8.653 685215 4.639 358725 2.231 1986

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AC-16 5 oCAC-16 15 oCAC-16 25 oCSMAR-16 5 oCSMAR-16 15 oCSMAR-16 25 oC

Fig. 7. N-a curves for AC-16 CRM asphalt mixture and SMAR-16 CRM asphaltmixture at different temperatures.

Table 10ANOVA results of the difference between the fatigue properties of SMAR-16 CRMasphalt mixture and AC-16 CRM asphalt mixture at different temperatures.

Source SS df MS F P-value Fcrit

5 �C 16.224 1 16.224 9.9015 0.0034 4.130015 �C 33.834 1 33.834 6.0524 0.0249 4.451325 �C 32.5125 1 32.5125 21.5275 0.0012 5.1173

Note: SS: sum of squares; df: degree of freedom; MS: mean squares; F: the F value;Fcrit, the F critical value; The significance level is 0.05.

H. Wang et al. / Construction and Building Materials 47 (2013) 1342–1349 1347

From Table 9 it can be seen that for both the SMAR-16 and AC-16 CRM asphalt mixtures, the maximum loading and fatigue life alldecrease as the test temperature increases. As for SMAR-16 CRMasphalt mixture, the maximum loading decreased by 48.73% and58.39% as the test temperature rose from 5 �C to 15 �C and 15 �C

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Fig. 8. The maximum loadings and loading times for SMAR-1

to 25 �C, respectively. However, when referring to fatigue life inthe same test temperature range, the decrease values were47.67% and 52.05% respectively. Therefore, the anti-fatigue prop-erty of CRM asphalt mixture could become weak when the testtemperature increases.

In addition, N-a curves for AC-16 CRM asphalt mixture andSMAR-16 CRM asphalt mixture at different temperatures areshown in Fig. 7. It is indicated that for the same CRM asphalt mix-ture, the crack length grows much faster in the notched SCB test asthe test temperature rises. As stress mode was applied in the SCBfatigue test, and the fatigue life of mixture will decrease with theincreasing of testing temperature. Fig. 7 also indicates that temper-ature has a significant effect on CRM asphalt mixture’s fatigueproperty, and for either SMAR-16 CRM asphalt mixture or AC-16CRM asphalt mixture the crack length grows faster at low temper-ature than at high temperature, therefore, the increase of temper-ature could weaken fatigue life. It is also indicated that at the sametesting temperature, SMAR-16 CRM asphalt mixture has a betterant-fatigue property than that of AC-16 CRM asphalt mixture.However, the differences between the fatigue properties of thetwo mixtures at different temperatures are different. ANVOA wasemployed to study these differences, and the results were shownin Table 10. It can be seen that the differences between fatigueproperties of SMAR-16 CRM asphalt mixture and AC-16 CRM as-phalt mixture are significant at different test temperatures. Whencompared using the P-values, it could be obtained that the differ-ence at 25 �C is the greatest as the P-value is smallest and the dif-ference at 15 �C is the smallest. Therefore, this phenomenon mightrequire a further investigation.

3.4. Analysis of loading frequency on crack growth

Loading frequency was used to simulate the vehicle speed onasphalt pavement to some extent. In general, the vehicle with ahigh speed will have a higher loading frequency on an asphaltpavement. Three sine loadings with frequencies of 5 Hz, 10 Hzand 15 Hz were employed in the notched SCB tests for this part.The test temperature was 15 �C, and 40 mesh crumb rubber pow-der’s content was 20% by weight of virgin asphalt binder. Thenthe maximum loadings and fatigue lives for SMAR-16 CRM asphaltmixture and AC-16 CRM asphalt mixture were obtained in thenotched SCB tests, the results are shown in Fig. 8. It was indicatedthat the maximum loadings for the two mixtures remained thesame when the loading frequency changed stating that in notchedSCB test loading frequency had no influence on the mixture’s max-imum loadings. However, the longest fatigue life emerged at 15 Hzboth for SMAR-16 and AC-16 CRM asphalt mixture. This indicates

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(b)

5Hz10Hz15Hz

Fig. 9. N-a curves for SMAR-16 CRM asphalt mixture and AR-16 CRM asphalt mixture at different loading frequencies.

Table 11ANOVA of loading frequency on crack lengths in notched SCB test.

Source (Hz) SMAR-16 AR-16

5 Hz 10 Hz 15 Hz 5 Hz 10 Hz 15 Hz

5 – S S – S S10 – S – S15 – –

Note: N: non-significant; S: significant. The significance level is 0.05.

1348 H. Wang et al. / Construction and Building Materials 47 (2013) 1342–1349

that increasing vehicle speed appropriately could extend asphaltpavement life.

In addition, the N-a curves for SMAR-16 CRM asphalt mixtureand AR-16 CRM asphalt mixture are indicated in Fig. 9. It couldbe seen that when the loading frequency was 15 Hz, the cracklength grew the slowest for both the SMAR-16 sand AC-16 CRM as-phalt mixture after bearing the same loading repetition times. Forexample, when the loading repetition time was 2000 for SMAR-16CRM asphalt mixture, the crack lengths at 10 Hz and 15 Hz were66.55% and 62.21% of that at 5 Hz. Loading frequency play a signif-icant role in CRM asphalt mixture’s fatigue property as indicated inTable 11. Therefore, the vehicle speed is very important in asphaltpavement fatigue property.

4. Summary and conclusion

In this paper, in order to study crack growth in CRM asphaltmixture in notched SCB test, the dynamic expansion length ofcracking was obtained by image processing technology, and its cor-relation with the fatigue number and crack growth length werestudied. Therefore, the influence of gradation type, asphalt content,test temperature, stress ratio, loading frequency, rubber powderconcentration and rubber powder size on CRM asphalt mixtures’fatigue life and crack growth laws were also studied. Therefore,the following conclusions are drawn from the experimental dataobtained in this study:

1. Five crumb rubber concentrations (15%, 18%, 20%, 22% and 25%by weight of virgin asphalt binders) were chosen to study therubber content’s effect on the fatigue property of CRM asphaltmixture. The crumb rubber powder size was 40 mesh innotched SCB test. It was indicated that crumb rubber concentra-tion plays a significant role in CRM asphalt mixture’s fatigueproperty. When the crumb rubber concentration was 20%,CRM asphalt mixture had the best anti-fatigue property, thefatigue life was the longest and crack grew the most slowest.

2. When studying the crumb rubber powder size effect on CRMasphalt mixture’s fatigue property, rubber powders of 20 mesh,40 mesh and 80 mesh were employed in this paper. The testresults show that fatigue lives for both SMAR-16 and AC-16asphalt mixture would grow longer when the rubber powdersize decreases as crack grew more slowly in the notched SCBtest.

3. The notched SCB test was conducted at 5 �C, 15 �C and 25 �C tostudy the effect of temperature on CRM asphalt mixture’s fati-gue property. It was learnt that for the same CRM asphalt mix-ture, crack would grow much faster as temperature rises, andthe fatigue life would decrease. Therefore, temperature had asignificant effect on the CRM asphalt mixture’s fatigue life.

4. Another significant factor to influence CRM asphalt mixture’sfatigue property was the loading frequency. Three loading fre-quencies of 5 Hz, 10 Hz and 15 Hz were employed in notchedSCB test. The fatigue life would be longer as crack grew muchslower when the loading frequency was increased..

5. It was also found in this study that the SMAR-16 CRM asphaltmixture had a better ant-fatigue property than AC-16 CRMasphalt mixture. This indicated that gap gradation is more suit-able for CRM asphalt mixture than continuous gradation.

Acknowledgements

The research is supported by the funds of National Natural Sci-ence Foundation of China (NSFC) (No. 51178056), the Shaanxi Pro-vincial Natural Science Foundation (2011JQ7007) and the SpecialFund for Basic Scientific Research of Central Colleges, Chang’anUniversity (CHD2012ZD013).

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