Construction

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Construction and Building Materials 21 (2007) 1011–1015

and Building

MATERIALS

Study on properties of recycled tire rubber modified asphaltmixtures using dry process

Weidong Cao *

School of Civil Engineering, Shandong University, No. 73, Jingshi Road, Jinan, Shandong 250061, PR China

Received 17 February 2006; accepted 27 February 2006Available online 18 April 2006

Abstract

To minimize waste tires pollution and improve properties of asphalt mixtures, properties of recycled tire rubber modified asphalt mix-tures using dry process are studied in laboratory. Tests of three types asphalt mixtures containing different rubber content (1%, 2% and3% by weight of total mix) and a control mixture without rubber were conducted. Based on results of rutting tests (60 �C), indirect tensiletests (�10 �C) and variance analysis, the addition of recycled tire rubber in asphalt mixtures using dry process could improve engineeringproperties of asphalt mixtures, and the rubber content has a significant effect on the performance of resistance to permanent deformationat high temperature and cracking at low temperature.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Recycled tire rubber; Asphalt mixtures; Dry process; Properties

1. Introduction

With the rapid development of the automobile industryand higher standard of living of people in China, the quan-tity of autos increased sharply, China is facing the environ-mental problem related to the disposal of large-scale wastetires. In accordance with the statistic data, 80 million scraptires were produced in 2002, and with 12% of growth rateevery year, the total number of abandoned tires will beexpected to reach 120 million in 2005 and 200 million in2010 [1]. How to deal with the huge number of waste tireshas become an urgent problem of environment in China.

The disposal of waste tires in the world primarily hasthree ways to deal with such as landfill, burning and recy-cling. Recycled tire rubber applied to pavement may be thebest way to reduce waste tires in large quantities and, at thesame time, improve some engineering properties of asphaltmixtures.

The history of adding recycled tire rubber to asphalt pav-ing material can be traced back to the 1940s when the U.S.

0950-0618/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.conbuildmat.2006.02.004

* Tel.: +86 531 8839 2842.E-mail address: cwd2001@sdu.edu.cn.

Rubber Reclaiming Company began marketing a devulcan-ized recycled rubber product, called Ramflex�, as a dryparticle additive to asphalt paving mixture. In the mid-1960s, Charles McDonald developed a modified asphaltbinder with the addition of crumb rubber called Overflex�[2]. Crumb rubber can be incorporate by a wet process ordry process. Wet process refers to modification of asphaltcement binder with 5–25wt% of fine tire rubber crumb mod-ifier (CRM) at an elevated temperature. The dry processincludes mixing the rubber particles with aggregates priorto addition to asphalt. The main differences between thetwo processes consist in rubber particle size, rubberamount, rubber function, and incorporation facility [3].

Although the dry process presents some advantages inrelation to the wet process, mainly concerning the costsinvolved and to the higher amount of rubber to be used,the research all over the world have concentrated mainlyon the wet process. This choice may be explained by theirregular performance of some experiment sections builtwith the dry process, unlike the wet process, which has pre-sented more satisfactory results [4].

This paper presents an experiment research on recycledtire rubber modified asphalt mixtures using dry process.

Table 1Chemical composition of tire rubber

Chemical compositiona

Acetone extract (%) 15.5Ash content (%) 6.0Carbon black (%) 29.5Rubber hydrocarbon (%) 49.0

a Chemical composition provided by supplier.

1012 W. Cao / Construction and Building Materials 21 (2007) 1011–1015

Special designed dense gap-aggregate gradations wereemployed to give enough space to accommodate rubber.Two laboratory tests were performed to evaluate the per-formance of resistance to permanent deformation at hightemperature and cracking at low temperature of rubbermodified asphalt mixtures. Tested results were comparedand analyzed statistically. The single factor variance analy-sis (ANOVA) has been performed to determine the signif-icance at a certain confidence limit.

2. Test materials and testing program

2.1. Aggregate, binder, crumb rubber and gradation

Crushed stones of diabase and limestone were used forcoarse aggregate and fine aggregate, respectively. Hydratedlime as mineral filler was obtained from a commercialsource. SBS modified asphalt binder was obtained from acommercial petroleum company. A Cycled Rubber Indus-try Ltd., Shanghai, China, supplied recycled tires rubber.The rubber was granulated under room temperature, andfreed of wire and fabric. The range of rubber particle sizewas 1–3 mm, its appearance was shown in Fig. 1. Table 1presents the chemical composition of the crumb rubbersupplied.

Special designed gap-aggregate gradation wasemployed, which was similar to the gradation of stone mas-tic asphalt (13.2 mm nominal maximum size) specified byChina [5], but sieve of 2.36 mm was omitted to provideenough room for the rubber particles. The proportion ofcoarse aggregate (more than 4.75 mm), which is normallyexpressed in percentage of the total weight of mineral,was adjusted in accordance with the content of rubber.

2.2. Marshall mix design

The Marshall mix design procedure as specified inASTM D1559 was used in this study. Laboratory mixingand compaction temperature for all mixtures were selectedaccording to viscosity criteria. Three rubber contents wereconsidered (1, 2 and 3% by weight of total mix) in dry pro-

Fig. 1. Appearance of crumb rubber.

cess mixes. After the addition of crumb rubber, the blend-ing time of aggregate was prolonged 10–20 s to disperserubber evenly. Three mixtures were compared to a conven-tional mixture, without rubber, herein denominated con-trol mixture. The samples were compressed by 75 blowsper face with the standard Marshall hammer. The optimumasphalt content (OAC) of the mixtures was determinedconsidering 4% of air voids.

2.3. Testing program

The permanent deformation test, i.e., rutting test, wasconducted, employing the wheel-tracking device shown inFig. 2 for evaluation of pavement performance at high tem-perature. Samples, which were mixed with optimum asphaltcontents from Marshall mix design and fabricated by therolling machine, were of dimensions 300 mm · 300 mm incross-sectional area and 50 mm in height. According tostandard test methods of bituminous mixtures for highwayengineering (China), the rutting test was performed using0.7 MPa wheel load at 60 �C temperature under dry condi-tion [6]. During the test, the speed of wheel (N) passing overthe center of the sample was 42 cycles a minute. The curve ofdeformation vs. time was illustrated in Fig. 3, from whichthe testing indicator of dynamic stability (DS) could be cal-culated, which can be expressed by following equation:

DS ¼ ðt2 � t1Þ � Nd2 � d1

Fig. 2. Laboratory wheel tracking.

Fig. 4. Indirect tensile test using MTS 810.

Fig. 3. Curve of deformation vs. time.

Table 2The properties of aggregate and rubber

Test materials Properties Testvalues

Specificationa

Coarse aggregate Apparent specific gravity 2.767 >2.60Bulk specific gravity 2.712 >2.50Crushed stone value (%) 9.5 626L.A. abrasion (%) 10.8 628Percent of flat andelongated particles (%)

10 615

Water absorption (%) 0.8 62.0Fine aggregate Apparent specific gravity 2.700 –Mineral filler Apparent specific gravity 2.715 –Crumb rubber Average specific gravity 1.150 –

a Specification for aggregate is given in Technical Specification forConstruction of Highway Asphalt Pavements (China). Testing methodsfor aggregate are followed by Test Methods of Aggregate for HighwayEngineering (China).

W. Cao / Construction and Building Materials 21 (2007) 1011–1015 1013

The higher DS of asphalt mixtures is the better of theperformance of resistance to permanent deformation athigh temperature.

The performance of resistance to cracking at low tem-perature test, i.e., indirect tensile test at �10 �C accordingto China Standard was performed using MTS 810 shownin Fig. 4. The samples, which were mixed with optimumasphalt contents, were fabricated with the standard Mar-shall hammer. The loading velocity was 1 mm per minuteand the displacements were measured using a linear vari-able displacement transducer. The failure stiffness modulus(FSM), which reflects the flexibility of asphalt mixtures atlow temperature, can be quantified by the maximum failureload and displacement. Obviously, the lower failure stiff-ness modulus is the better of the performance of resistanceto cracking at low temperature.

3. Test results and ANOVA analysis

Properties of aggregate and crumb rubber are shownin Table 2 and SBS modified asphalt binder are shown

Table 3The properties of SBS modified asphalt

Properties Specific gravity (25 �C) Penetration (0.1 mm)

Test values 1.019 62.0Specificationa – 60–80

a Specification for aggregate is given in Technical Specification for Construc

in Table 3. Fig. 5 illustrates four aggregate gradationcurves employed in tests. Test results of the Marshallmix design with rubber modified asphalt mixtures andcontrol mixture are summarized in Table 4, which con-tain bulk specific gravity, air voids, voids in mineralaggregates (VMA), voids filled with asphalt (VFA), sta-bility, flow and OAC.

DS of the samples in rutting test and failure stiffnessmodulus in indirect tensile test are shown in Table 5 andFig. 6. ANOVA analysis was conducted to determine theeffect of rubber on properties of asphalt mixtures. In thesingle-factor tests of ANOVA, rubber content was chosenas factor, dynamic stability and failure stiffness moduluswere response, respectively. The results of ANOVA analy-sis are summarized in Table 6.

4. Analytical results and discussion

4.1. Marshall mix design

In Table 4, it is found that bulk specific gravity, stability,flow and OAC of asphalt mixtures are affected by the addi-tion of tire rubber. Because the specific gravity of rubber isfar less than that of aggregate, the bulk specific gravity ofrubber modified asphalt mixtures decrease with theincrease in rubber contents. Due to lower compressivestrength and higher elasticity of rubber, the stability andflow decrease with the increase in rubber contents. The val-ues of stability and flow are both satisfied with the Mar-shall criteria [7]. The OAC has slight increase forabsorption of rubber.

Softening point (�C) Cohesion (135 �C, Pa.s) Ductility (5 �C)

83 2.96 38P50 – P30

tion of Highway Asphalt Pavements (China).

Table 4Test results by Marshall mix design

Rubber contents (%) Engineering properties

Bulk specific gravity Air voids (%) VMA (%) VFA (%) Stability (KN) Flow (0.01 cm) OAC (%)

0 2.392 4.0 17.4 77.0 11.6 28 5.71 2.383 3.8 16.9 77.5 10.8 43 6.02 2.348 3.9 17.2 77.3 10.0 40 6.23 2.314 4.1 17.5 76.6 9.1 38 6.4

0

10

20

30

40

50

60

70

80

90

100

0.01 0.1 1 10 100

Sieve Size (mm)

Perc

ent P

assi

ng (%

)

control

1% rubber

2% rubber

3% rubber

Fig. 5. Aggregate gradation curves.

Table 5Results of rutting test and indirect tensile test

Rubber contents (%) Dynamic stability (cycles/mm) Failure stiffness modulus (MPa)

Average Average

0 2356 2236 2488 2360 1011.8 1200 1009 1073.61 2387 2519 2662 2523 975.3 968.9 983.2 975.82 3236 3007 3418 3220 809.4 816.6 853.9 826.63 4456 4272 4559 4429 725.7 729.9 772.8 742.8

1014 W. Cao / Construction and Building Materials 21 (2007) 1011–1015

4.2. Rutting test and indirect tensile test

Based on Table 5 and Fig. 6, the values of DS and FSMare affected by the addition of tire rubber. In contrast to aconventional mixture without rubber, DS of rubber modi-fied asphalt mixtures increase with the increase in rubbercontent, while FSM of rubber modified asphalt mixturesdecrease. It could be concluded that the addition of tirerubber in asphalt mixtures using dry process could improvethe properties of resistance to permanent deformation athigh temperature (60 �C) and cracking at low temperature(�10 �C).

In Table 6, the case of variance analysis of DS, the valueof F (108.066) is bigger than that of Fcitical (4.06618), it can

be concluded that rubber content has significant effect ondynamic stability (DS). In the case of variance analysis ofFSM, the value of F (19.83701) is also bigger than thatof Fcitical (4.06618), which shows that rubber content hassignificant effect on failure stiffness modulus (FSM). There-fore, the asphalt mixture containing 3% tire rubber has thebest performance both at high temperature (60 �C) and lowtemperature (�10 �C).

5. Conclusions and recommendations

Based on the results of evaluation and analysis, conclu-sions and recommendations of this study are described asthe following:

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

1 2 3 4

Rubber content (%)

DS

(cyc

les/

mm

)

0

200

400

600

800

1000

1200

FSM

(M

Pa)

DS

FSM

Fig. 6. Rubber content vs. DS & FSM.

Table 6Results of ANOVA analysis of rutting test and indirect tensile test(a = 0.05)

SS df MS F Fcitical p-value

Source of variation (DS)

Between 7971837 3 2657279 108.066 4.06618 8.2E–07Within 196715.3 8 24589.42Total 8168552 11

Source of variation (FMS)

Between 197665.3 3 65888.43 19.83701 4.06618 4.62E–04Within 26571.93 8 3321.491Total 224237.2 11

W. Cao / Construction and Building Materials 21 (2007) 1011–1015 1015

1. From the results of Marshall mix design, special gap-aggregate gradation and SBS modified asphalt binderare recommended for recycled tire rubber modifiedasphalt mixtures using dry process.

2. All of stability and flow values are satisfied with theMarshall criteria.

3. Based on the analytical results of rutting test and indi-rect tensile test, the addition of tire rubber in asphaltmixtures using dry process could improve the propertiesof resistance to permanent deformation at high temper-ature and cracking at low temperature.

4. From the results of ANOVA analysis of rutting test andindirect tensile test, rubber content has significant effecton dynamic stability and failure stiffness modulus, andthe asphalt mixture containing 3% tire rubber has thebest performance both at high temperature and lowtemperature.

5. The long-term performance of recycled tire rubber mod-ified asphalt mixtures using dry process will need to befurther studied.

Acknowledgement

The author gratefully acknowledges the guidance for thepaper provided by Professor Lv Weimin from Tongji Uni-versity of Shanghai.

References

[1] Jiang Zhi-Yun. The status and development of waste tireresourcesrecycling in china. Chinaire Resource Recycling;2005. p. 6–8 [inChinese].

[2] Sacramento county DERA and Bollard & Brennan, Inc., report on thestatus of rubberized asphalt traffic noise reduction in sacramentocounty; 1999. p. 3–5.

[3] Roberts FL, Kandhal PS, Brown ER, Dunning RL. Investigation andevaluation of ground tire rubber in hot mix asphalt. National Centerfor Asphalt Technology; 1989. NCAT Report No. 83-3.

[4] Bertollo Sandra A Margarido, Bernucci Liedi Bariani, Fernandes JoseLeomar, Leite Leni Mathias. Mechanical properties of asphaltmixtures using recycled tire rubber produced in Brazil – a laboratoryevaluation. TRB 2004 Annual meeting CD-ROM. Washington (DC):TRB, National Research Council; 2004. p. 5.

[5] Committee of Highway Engineering of Association of China ProjectConstruction Standardization. Technology guide for construction ofhighway pavement using stone matrix asphalt. SHC F40-01-2002,Beijing; 2002 [in Chinese].

[6] Ministry of Communications of PR China. Standard test methods ofbitumen and bituminous mixtures for highway engineering. Beijing;2000 [in Chinese].

[7] Ministry of Communications of PR China. Technical specification forconstruction of highway asphalt pavements. JTG F40-2004, Beijing;2004 [in Chinese].