Materials and Design 35 (2012) 873–877

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Materials and Design

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Design and evaluation of gap-graded asphalt rubber mixtures

Yamin Liu a,⇑, Sen Han a, Zhongjie Zhang b, Ouming Xu c

a Key Laboratory for Special Area Highway Engineering of Ministry of Education, Chang’an University, Xi’an 710064, Chinab Louisiana Transportation Research Center, Baton Rouge, LA 70808, USAc School of Material Science and Engineering, Chang’an University, Xi’an 710061, China

a r t i c l e i n f o a b s t r a c t

Article history:Available online 17 September 2011

Keywords:Elastomers and rubberMechanicalPerformance indices

0261-3069/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.matdes.2011.08.047

⇑ Corresponding author. Tel.: +86 15129098106; faE-mail addresses: liu_yamin@yahoo.com (Y. Li

(S. Han).

This paper describes the evaluation of the design and performance of gap-graded asphalt rubber mixtures(GGAR) under laboratory conditions with one type of asphalt binder, three crumb rubbers, and four gap-graded gradations of one aggregate type. The resulting mixture samples were tested and evaluated fortheir high-temperature performance and fatigue resistance and their performance was compared withthat of conventional Stone Matrix Asphalt mixtures (SMA). For mixture design, GGAR can have mineralfiller content less than that of SMA and have the volume of air voids larger than typical values for anSMA. As a result, the optimum asphalt content is also larger than that of SMA. The laboratory testingresults indicate that GGAR performs as well as SMA with respect to high-temperature and has better fati-gue resistance than a conventional SMA. More field tests should be carried out on GGAR in various envi-ronmental conditions to determine its optimum manufacture and long term performance.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The use of crumb rubber in asphalt pavements has becomemore popular in recent years due to property improvements inthe base asphalt binder and the fact that it is a recycled productfrom waste tyres [1–3]. The blend of base asphalt binder andcrumb rubber of a certain size, also called asphalt rubber, has bet-ter physical properties by comparison with base asphalt binder andequals the performance of polymer-modified asphalt binders whenused in asphalt mixtures [4,5]. As a result, there are generally manybenefits from using asphalt rubber mixtures: e.g. lower road noise,increased low-temperature performance, improved durability, lessroad maintenance, etc. [6].

The paving industry in China is rapidly absorbing the technol-ogy of asphalt rubber and numerous test sections were constructedin various areas but a national standard for the material does notexist. Therefore, the design of the asphalt rubber mixtures is vari-able and the resulted performance reflects this variation. Further,asphalt rubber is mostly used in dense-graded asphalt mixturesin China [7–9], and little research has been performed on othertypes of mixtures such as gap grading and open grading. For exam-ple, Stone Matrix Asphalt mixture (SMA) is renowned for its excel-lent performance due to a stone-on-stone contact derived from agap grading [10–12]. This gives rise to the question ‘what wouldbe the performance for a mixture combining asphalt rubber with

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gap-graded gradation aggregates?’ Expanding the area of asphaltrubber usage will definitely have an environmental benefit forthe disposal reduction of waste tyres and a potential benefit in pro-ducing an improved asphalt mixture. Therefore, it is important toinvestigate the physical, engineering properties and performanceof asphalt rubber mixtures with gap aggregate grading by compar-ison with the performance of conventional SMAs.

The objective of the research work presented here is to evaluatethe design and performance of gap-graded asphalt rubber mixtureunder laboratory conditions. Three crumb rubbers and four gap-graded gradations of one aggregate type were prepared and usedwith one asphalt binder for a detailed mixture design. The result-ing samples were evaluated for their high-temperature perfor-mance and fatigue resistance.

2. Materials and experimental program

2.1. Asphalt binder

ESSO A-90 asphalt binder was used in this study. The physicalproperties of the base binder were measured in the laboratoryaccording to Chinese National Standards [13], and are shown inTable 1.

2.2. Aggregate

Aggregates were selected based on their ability to meet the spec-ified criteria of SMA in the Chinese National Specification [14]. Thenproperties of gneiss and artificial sand, which are common in China,

Table 1Physical properties of asphalt binder.

Test items Unit Value Standard

25 �C penetration 0.1 mm 81 T060410 �C ductility cm 72 T0605Softening point �C 47.5 T0606Mass loss % 0.26

RTFOT25 �C penetration ratio % 63 T060910 �C ductility cm 12

Table 2Physical properties of coarse aggregate.

Test items Unit Value Standard

10–15 mm 5–10 mm 3–5 mm

Crushing value % 13.4 – – T0316Los angeles abrasion % 11.8 – – T0317Apparent relative density – 2.856 2.857 2.812Bulk relative density – 2.781 2.762 2.709 T0304Water absorption % 0.51 0.63 0.59Flat or elongated particles % 4.0 6.7 – T0312

Table 3Physical properties of fine aggregate.

Test items Unit Value Standard

Apparent relative density – 2.797 T0328Mud content (percent of <0.075 mm) % 1.1 T0333Sand equivalent % 95.5 T0334Angularity s 57.1 T0344

Table 4Physical properties of mineral filler.

Test items Unit Value Standard

Apparent relative density – 2.799 T0352Water absorption % 0.2Grain sizes <0.6 mm % 100.0<0.15 mm % 95.0 T0351<0.075 mm % 90.1Hydrophilic coefficient – 0.60 T0353

0

20

40

60

80

100

1.180.60.30.150.075

Sieve Sizes (mm)

Perc

ent P

assi

ng (

%)

Shaanxi

Shandong

Sichuan

Upper Limit

Lower Limit

Fig. 1. Gradations of five crumb rubbers.

874 Y. Liu et al. / Materials and Design 35 (2012) 873–877

were determined. Tables 2–4 show the physical properties of coarseaggregate, fine aggregate and mineral filler, respectively.

2.3. Crumb rubber

Three crumb rubbers that are widely used for asphalt mixturesin China were selected in this research. They were produced inSichuan, Shandong, and Shaanxi, separately. Their gradations werewithin the bounds of the ADOT’s specification in USA, as presentedin Fig. 1.

2.4. Asphalt rubber preparation

Asphalt rubber was produced in the laboratory by an openblade mixer at a blending speed of 1000 rpm and a blending tem-perature of 180–190 �C for 1 h. The percentage of crumb rubberwas 18% by weight.

Asphalt rubbers were prepared by mixing one asphalt binderwith three different crumb rubbers separately. Their physical prop-erties were measured according to the standard [13]. One asphaltrubber with the greatest viscosity at 180 �C was chosen for the

subsequent specimen preparation and mechanical property testing[15,16].

2.5. Specimen preparation

In keeping with standard procedures for asphalt mixtures inChina, specimens were prepared using a Marshall CompactionDevice [14]. First, asphalt rubber was mixed with aggregates thor-oughly at 170–180 �C for about 90 s. Then, mineral filler was addedinto the mixtures and the blending lasted for a further 90 s. Finally,the hot mixtures were placed in a steel mold and at 160–170 �Ccompacted under 75 blows with a Marshall hammer on each sideof the specimen measuring 101.6 mm in diameter and 63.5 mmin height.

Employing asphalt rubber and four gap-graded gradations ofaggregates, the specimens were used for determining volumetricproperties that allowed the optimum asphalt content of mixturesto be found in the subsequent steps.

2.6. Mechanical property testing

Common problems experienced by asphalt rubber pavementsconstructed in the past in China are premature rutting and fatiguecracking under repeated loading due to mixture design and con-struction problems [7,15–17]. In this study, wheel tracking andfatigue tests were conducted in addition to the Marshall stabilityand flow test to evaluate high-temperature performance and fati-gue resistance of the gap-graded asphalt rubber mixtures for theoptimum asphalt rubber content. The testing procedures werecomplied with Chinese National Standard [13].

2.6.1. Marshall stability and flow testThe Marshall stability and flow test determines the resistance to

plastic deformation of asphalt mixture at 60 �C. Based on T0709 inthe standard [13], a compressive loading was applied to the spec-imen at a rate of 50.8 mm/min till it failed. The maximum loadingat material failure is termed the ‘Marshall stability’ (MS) and theassociated plastic flow (deformation) of specimens is termed ‘flowvalue’ (FV). Greater MS values and lower FV values are expected forstrong asphalt mixtures.

2.6.2. Wheel tracking testThe wheel tracking test can simulate the application of an

actual wheel load on a pavement at high-temperature. Hence, itcan directly estimate the internal resistance of asphalt mixturesin terms of rutting depth. According to T0719 in the standard[13], a specimen was prepared in a slab with the dimensions of300 mm (length) � 300 mm (width) � 50 mm (height) and testedat the temperature of 60 �C with a wheel-pressure of 0.7 MPa. A

Table 6Gradations for mixture design.

Gradation Percentage passing through different sieve sizes (%)

16 13.2 9.5 4.75 2.36 1.18 0.6 0.3 0.15 0.075

G-a 100 97.9 63.5 29 25.2 21.4 17.6 13.8 10 6G-b 100 97.9 63.5 29 24.8 20.6 16.4 12.2 9.3 4G-c 100 97.9 63.5 27 23.5 20 16.4 12.9 9.7 6G-d 100 97.9 63.5 27 23.2 19.4 15.6 11.8 8 4

Table 7Volumetric properties of asphalt rubber mixtures with different gradations.

Y. Liu et al. / Materials and Design 35 (2012) 873–877 875

parameter of dynamic stability was employed to characterize thehigh-temperature stability of the tested mixture and was deter-mined using by the following equation:

DS ¼ 630d60 � d45

ð1Þ

where d60 is the rutting depth (mm) at 60 min, d45 is the ruttingdepth at 45 min, 630 is the number of load cycles applied between45 and 60 min.

A higher dynamic stability value usually indicates an excellenthigh-temperature performance (rutting resistance) of the mixture[18].

Gradation VV (%) VMA (%) VFA (%) VCAmix (%) VCADRC (%)

G-a 6.4 17.8 64.0 41.5 35.3G-b 5.8 17.3 66.5 41.2 35.3G-c 5.5 17.2 68.6 39.2 48.5G-d 5.0 16.6 69.3 39.1 48.5

2.6.3. Fatigue testThe fatigue test can assess the resistance to cracking of asphalt

specimens under a repeated load. It is a stress-controlled test andperformed using American Materials Testing & SimulationMachine (MTS) in accordance with T0715 of the specification[13]. A set of test conditions were set as follows: the test temper-ature was 15 �C; the specimen was a beam with a size of 250 mm(length) � 40 mm (width) � 40 mm (height), and the loading ratewas 50 mm/min.

The fatigue resistance was evaluated according to the fatiguecurves generated by testing, which introduces the relationshipbetween fatigue strength and fatigue life. The fatigue equation inthis study was calculated using the formula given in the followingequation:

log Nf ¼ n� logrþ k ð2Þ

where Nf is the fatigue life (in cycles); r is the fatigue stress (MPa)applied during the test. The equation provides a linear relationshipbetween them using a denary logarithm, in which n is the gradientand k is the intercept. High values of n indicate greater sensitivity tocracking, which infers poor fatigue resistance. Conversely, the largerk is, the higher is fatigue life and longer fatigue life will result.

3. Mixture design results and discussion

The design of an asphalt rubber mixture is divided into threeparts: asphalt rubber selection, gradation design and optimumasphalt content determination.

3.1. Asphalt rubber selection

Table 5 shows the properties of asphalt rubbers produced by theasphalt binder and three crumb rubbers used in this study.

From Table 5 it can be seen that asphalt rubber with Shaanxicrumb rubber has the highest viscosity. Comparing other proper-ties of the three asphalt rubbers, it can be seen that those of asphaltrubber with Shaanxi crumb rubber are generally superior. Hence,this material was chosen for the rest of the investigations.

Table 5Physical properties of asphalt rubbers with different crumb rubbers.

Asphaltbinder

Crumbrubber

180 �Cviscosity(Pa s)

25 �Cpenetration(0.1 mm)

Softeningpoint(�C)

5 �Cductility(cm)

Elasticrecovery(%)

ESSO A-90#

Shaanxi 3.650 63 59.4 17.4 53

Shandong 1.220 74 55.8 17.9 38Sichuan 3.505 59 57.6 9.7 48

3.2. Gradation design

In order to form a stone-on-stone contact like SMA, four gap-graded gradations were designed by varying the percentage pass-ing through two sieve sizes: 4.75 mm and 0.075 mm. Accordingto the specification [14], the mineral filler content in SMA, that isthe percentage passing through a sieve size of 0.075 mm, is keptat about 10%. However, considering that crumb rubber in asphaltrubber can occupy extra space in mixtures, the mineral filler con-tent was decreased. Table 6 shows the gradations of aggregatestested.

Specimens with four different gradations shown in Table 6 wereprepared with an asphalt rubber content of 6.0% with five speci-mens for each gradation. A total number of 20 specimens weretested and evaluated with their bulk specific gravities measuredby the surface-dry condition method. Then the volumetric proper-ties of specimens, such as Volume of Air Voids (VV), Voids in Min-eral Aggregate (VMA), Voids Filled with Asphalt (VFA) werecalculated, as shown in Table 7.

The results in Table 7 indicate that gradation G-a and G-b haveVoids in Coarse Aggregate in the asphalt rubber mixture (VCAmix)larger than the Voids in Coarse Aggregate in the Dry-Rodded Con-dition (VCADRC), which fails a principle of asphalt mixture design[14]. Too many fine aggregates have destroyed the stone-on-stonecontacts among coarse aggregates [11].

For gradation G-c and G-d, their VCAmix is less than VCADRC,which means that coarse aggregates interlock very well. Finally,gradation G-c was selected for the determination of optimumasphalt content with the consideration that a greater VMA is usefulfor a skeleton structure and deformation of crumb rubber [12].

3.3. Optimum asphalt rubber content determination

Three asphalt rubber mixtures were produced using gradationG-c with asphalt rubber contents of 6.1%, 6.4%, 6.7%, respectively,with five specimens for each asphalt content.

Table 8 shows the volumetric properties of mixtures with dif-ferent asphalt rubber content. It can be seen that the mixture withan asphalt content of 6.1% had the greatest VMA. But the asphaltcontent was too low to ensure that the performance, especiallyfatigue resistance and moisture susceptibility, met the require-ments. So the asphalt content of 6.4% was selected as the optimumvalue, which is larger than the traditional optimum asphalt contentof 6.0% for conventional SMA mixtures in China. Similarly, theresulting SMA mixture in this study, called Gap-graded asphaltrubber mixture (GGAR), had a greater VV of 4.5%, which was morethan the usual value of 4.0% for a conventional SMA mixture [10].

Table 8Volumetric properties of mixtures with different asphalt rubber content.

Gradation Asphaltrubbercontent (%)

VV (%) VMA (%) VFA (%) MS (KN) FV (mm)

G-c 6.1 5.3 17.1 69.0 6.90 3.676.4 4.5 16.7 73.0 7.57 4.696.7 3.7 16.2 77.2 6.20 3.777.0 3.2 16.7 80.8 5.88 4.48

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Time (min)

Rut

dep

th (

mm

)

SMA

GGAR

0 5 10 15 20 25 30 35 40 45 50 55 60

Fig. 3. Wheel tracking test results for GGAR and SMA.

y = -2.4789x + 4.3545

R2 = 0.9906y = -2.6293x + 4.1581

R2 = 0.9765

3.1

3.3

3.5

3.7

3.9

4.1

fatig

ue li

fe (

time)

GGARSMA

876 Y. Liu et al. / Materials and Design 35 (2012) 873–877

4. Mechanical property testing results and discussion

4.1. Marshall test

Fig. 2 shows the Marshall test results for GGAR and theordinary SMA mixture [10]. The MS value of SMA was about 0.2greater than that of GGAR, while the FV value was about 0.1greater. This seemly indicated that GGAR had a lower deformationresistance at high temperature. This could be attributed to thegreater VV in the mixture since a greater VV in laboratory testingleads to a higher stress concentration in specimen under theapplied loading.

However, previous studies showed that many asphalt pave-ments could not avoid deformation (rutting) even though theirMS value and FV value met the requirements [11]. So Marshalltesting results should be considered as an adjunct indicator, nota critical one for rutting evaluation.

2.5

2.7

2.9

0.1 0.2 0.3 0.4 0.5 0.6 0.7

fatigue stress (MPa)

Fig. 4. Fatigue curves for GGAR and SMA.

4.2. Wheel tracking test

Fig. 3 shows the test results from the wheel tracking tests. Withincreasing time, the rut depths of the two mixtures increase con-tinuously. SMA always has greater rutting than GGAR does. Thedynamic stabilities of the two mixtures were calculated using Eq.(1). SMA was measured at about 7400 cycle/mm while the valuefor GGAR was about 7700 cycle/mm. This indicates that the latterhas a little better high-temperature performance (rutting resis-tance). The results may be due to coarse aggregates stone-on-stonecontact and the asphalt rubber’s stiffness.

It is well documented that rutting resistance of asphalt mix-tures has a 60% dependency on aggregates skeleton and 40%dependency on the asphalt binder’s cohesiveness [19]. While thetwo mixtures have similar aggregates skeletons with gap-gradedgradation, crumb rubber’s networking effect gives rise to higherviscosity and stiffness of the asphalt rubber at high temperatures.Hence, the rutting resistance of GGAR increases.

6.5

7

7.5

8

8.5

9

9.5

GGAR SMA

MS

(KN

)

4.4

4.5

4.6

4.7

4.8

4.9

5

5.1

FV (

mm

)

MS

FV

Fig. 2. Marshall test results for GGAR and SMA.

4.3. Fatigue test

Fig. 4 shows the fatigue curves of both SMA and GGAR deter-mined by laboratory testing. It can be seen that the n value inthe GGAR fatigue curves was smaller than that of SMA, whichmeans that GGAR exhibited lower fatigue sensitivity. Meanwhile,the k value of GGAR was higher than the one for SMA, which indi-cates that under the same applied load, GGAR would have a longerfatigue life. Conversely, the fatigue stress required to promoted acrack for GGAR was higher for equal fatigue lives. Therefore, GGARhas a superior fatigue resistance to SMA. This can have two possi-ble explanations. Firstly, crumb rubber improves both the interfaceadhesion between the asphalt film and the aggregate surface andasphalt rubber’s flexibility at low temperature. Secondly, more as-phalt content in the GGAR of the same coarse aggregates skeletonwill result in thicker asphalt films on the aggregates’ surface, whichcontributes to the asphalt rubber’s superior ability to relax andheal [20].

5. Conclusions

This paper describes a study of the design and performance un-der test of asphalt rubber mixtures with gap-graded gradation.From the results and analysis, the following conclusions can bedrawn:

� Compared with conventional SMA, GGAR offered equally goodhigh-temperature performance due to the coarse aggregatesstone-on-stone contact and asphalt rubber’s higher stiffness.

Y. Liu et al. / Materials and Design 35 (2012) 873–877 877

� GGAR has superior fatigue resistance than conventional SMA,which results from the asphalt rubber’s flexibility, improvedcohesion and thicker coated asphalt film.� GGAR could exploit mineral filler contents less than those of

SMA allowing enough space for crumb rubber in mixture.� GGAR had a larger volume of air voids than the traditional val-

ues for SMA. As a result, its optimum asphalt content was alsolarger than that of SMA.� Even though the laboratory evaluation in this study favors the

GGAR mixture, field testing sections should be built with GGARunder various environmental conditions to determine its con-struction requirement and long-term performance.� Further study should be undertook on the issues of moisture

susceptibility, aging resistance, noise reduction ability, etc.

Acknowledgment

The authors are grateful for the financial support of ShaanxiDepartment of Transportation of China (No. KY08-01).

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