Construction and Building Materials 23 (2009) 2701–2708

Contents lists available at ScienceDirect

Construction and Building Materials

journal homepage: www.elsevier .com/locate /conbui ldmat

Variance analysis and performance evaluation of differentcrumb rubber modified (CRM) asphalt

Shutang Liu a,*, Weidong Cao a, Jianguo Fang b, Shujie Shang a

a School of Civil Engineering, Shandong University, No. 73 Jingshilu, Lixia District, Jinan 250061, PR Chinab Highway Bureau of Transportation Department of Shandong Province, No. 19 Shungenglu, Lixia District, Jinan 250002, PR China

a r t i c l e i n f o

Article history:Received 17 July 2008Received in revised form 7 December 2008Accepted 9 December 2008Available online 12 January 2009

Keywords:Crumb rubberCRM asphaltVariance analysisSoftening pointLow temperature ductilityPenetration indexRutting factorCreep stiffness modulus

0950-0618/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2008.12.009

* Corresponding author. Tel.: +86 0531 86358168;E-mail addresses: gtreesl@sdu.edu.cn, logtt@126.co

a b s t r a c t

In order to sort some important factors affecting the performances of CRM asphalt and evaluate the per-formances of different CRM asphalt, this study encompassed one kind of base asphalt binder, four kinds ofcrumb rubber, tread rubber (TR), heavy truck (HT), small truck (ST) and agriculture tire (AT) crumb rub-ber, two kinds of particle size, 60 mesh and 80 mesh, and three contents. Softening point, low tempera-ture ductility, and penetration index are selected as basic evaluation indicators in analysis. Varianceanalysis shows that the crumb rubber content is the primary affecting factor in general, followed bythe crumb rubber type, and particle size comes last. The greater the crumb rubber content is, the higherthe softening point and the penetration index are, and the smaller the low temperature ductility is. Mod-ified effects between 60 mesh and 80 mesh don’t manifest significant difference. The modified effects ofTR-crumb rubber and HT-crumb rubber are better while the low temperature ductility of AT-CRM asphaltis the worst. Also according to the results of DSR and BBR test, TR-CRM asphalt has a better and compre-hensive performance at the crumb rubber content of 20% by weight of base asphalt binder, and, if theprice is also considered, selecting HT-CRM asphalt with the crumb rubber content of 15% is ideal. Thestudy also finds an interesting phenomenon that the rutting factors of CRM asphalt may be remarkablydifferent, although the softening points of them are the same.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

The research and application of CRM asphalt can date back toseveral decades ago in the United States, Canada and other coun-tries [1,2]. Past research and application show that CRM asphalthas many good characteristics such as improved resistance to rut-ting due to higher viscosity, higher softening point and better resil-ience, improved resistance to surface initiated, reduced fatigue/reflection cracking, reduced temperature susceptibility, improveddurability and lower pavement maintenance costs, and saving inenergy and natural resource by using waste products etc. [1].Crumb rubber is the second used polymer to modify asphalt, fol-lowing SBS [3]. In China, the research and application of CRM as-phalt began in the 1980s. In recent years, with the rapiddevelopment of automobile industry, the volume of scrap tireshas followed the rapid increasing. According to statistics, in 2004,the new tire production is 239 million and scrap tire amount isover 112 million [2]. In 2006, the tire production of China is as highas 280 million, ranking first in the world. At the same year thescarp tire also is up to 140 million. It is estimated that, by 2010,China’s auto volume will keep 70 million, and scrap tire output will

ll rights reserved.

fax: +86 0531 88395204.m (S. Liu).

reach 200 million [4]. In this background, the research and applica-tion of CRM asphalt in road engineering in China are gained moreattention, and some cities have already developed their own tech-nology standards [5,6], despite the corresponding Chinese normshas not yet been at present.

A great deal of researches on factors affecting the CRM effecthave also been done, and they find that the improvement degreeof the base asphalt performance depends on many factors suchas the crumb rubber particle size, surface characteristics of the rub-ber particles, blending conditions, the manner in which crumb rub-ber devulcanizes, the chemical/physical properties of the baseasphalt, as well as its source [3,7–10].

Crumb rubber types are various, their sources are broad and thecomponents of them are different, so their modified effect on thebase asphalt binder will not be the same. The asphalt rubber mate-rials must be properly selected, designed, produced, and con-structed to provide the desired improvements to pavementperformance [1]. Considering the technical and economic issues,how to select the suitable or the best crumb rubber from greatvarieties on the basis of the existed investigation is worthy of fur-ther study.

This study representative selected four types of crumb rubberon the basis of a nationwide investigation to the types and prices,which were tread rubber (TR), heavy truck (HT), small truck (ST)

2702 S. Liu et al. / Construction and Building Materials 23 (2009) 2701–2708

and agriculture tire (AT) crumb rubber, and then used ‘‘wet pro-cess” to make CRM asphalt with the same base asphalt. Using soft-ening point, low temperature ductility at 5 �C and penetrationindex as basic evaluation indicators, the paper analyzed the signif-icant influences of the type, particle size and content on the perfor-mance of CRM asphalt, and sorted above affecting factors accordingto the variance analysis. On the basis of performance analysis, thispaper selected HT-crumb rubber, ST-crumb rubber and TR-crumbrubber as modifiers to modify the base asphalt binder, and thenfurther compared rheological and creep properties of the threetypes of CRM asphalt through the dynamic shear rheometer(DSR) and the bending beam rheometer (BBR) tests.

The main objectives of the study are as following:

(1) To sort the factors including crumb rubber type, content andparticle sizes.

(2) To select the best or the proper one among the used kinds ofcrumb rubber considering the technical or/and economicfactor.

2. Experimental program

2.1. Materials

2.1.1. BindersA base asphalt (AH-70), provided by Binzhou Asphalt Co. Ltd. (Shandong Prov-

ince, PR China) was used in this study, and its relevant performance indexes areshown in Table 1.

2.1.2. Crumb rubber modifierA detailed investigation to the production condition of crumb rubber was con-

ducted in entire country. The investigated crumb rubber included HT-crumb rubber(all-steel radial tire), ST crumb rubber (semi-steel radial tires), TR-crumb rubber(all-steel radial tire), AT crumb rubber (diagonal tires), sole tire crumb rubber, hosecrumb rubber and ball crumb rubber. The study chose the first four types of crumbrubber taking into account the market supply.

The production process of crumb rubber affects obviously the performance ofCRM asphalt. There are mainly ambient and cryogenic crumb rubbers, and the for-mer has roughness surface and large surface area, so its modified effect is better[11,12]. The crumb rubbers used in this study were derived from the grinding ambi-ent process.

There is a separation phenomenon of CRM asphalt, which is an important prob-lem to the projects. The settlement speed of crumb rubber particle in base asphaltnot only depends on the viscosity, density of base asphalt and the particle density,but also is proportional to the square of the radius of crumb rubber particle [13].

Table 1Performance indexes of base asphalt binder.

Performance indexes Test results

Penetration (25 �C, 100 g, 5 s) (0.1 mm) 64.7Penetration index (PI) �1.20Softening point (R&B) (�C) 49.7Ductility at 25 �C, 5 cm/min (cm) >100Density at 15 �C (g/cm3) 1.037

Table 2Density and price of crumb rubber.

Type of crumb rubber Density (g/cm3) Market price (RMB, yuan/ton)

HT (60 mesh) 1.165 3100HT (80 mesh) 1.174 3300ST (60 mesh) 1.149 2900ST (80 mesh) 1.157 3100AT (60 mesh) 1.155 2700AT (80 mesh) 1.167 2900TR (60 mesh) 1.189 3200TR (80 mesh) 1.195 3400

Note: The density standard of crumb rubber is 1.10–1.30 g/cm3 (Beijing city, China)and 1.15 ± 0.05 g/cm3 (Jiangsu province, China), respectively.

Therefore, this study selected finer crumb rubber in order to reduce the segregationdegree and be propitious to the comparison of CRM asphalt’s properties, and the se-lected particle sizes are 60 mesh and 80 mesh, respectively. The density of crumbrubber is an important parameter, and some local standards in China provide astandard value. The price of crumb rubber is interesting. The data manufacturersprovide are shown in Table 2.

2.2. Test program

According to four types of crumb rubber, two particle sizes and three crumbrubber contents, a total of 24 kinds of CRM asphalt can be obtained. In the analysis,softening point, low temperature ductility and penetration index are selected as ba-sic performance indexes, which reflect the high-temperature property, low temper-ature property and temperature susceptibility of the CRM asphalt, respectively.

2.3. Preparation

The CRM asphalt was made using ‘‘wet process”. The processing parameters ofthe wet process include mixing temperature, reaction time and mixing-shear rateand they are all key factors influencing the properties of the CRM asphalt. Severalresearches about technics of the wet process have been done [12,14–16], and thetechnics parameters in the paper are as follows in this study: reaction temperatureis 180 ± 5 �C, mixing duration is 30 min, and mixing-shear rate is 700 RPM.

2.4. Test methods

Softening point, ductility and penetration were measured according to the liter-ature [17]. Softening point is an index that reflects the higher performance of as-phalt binder, and low temperature ductility can reflect the low temperatureproperty of binder. Penetration is the depth which is expressed by the needle pene-trating the asphalt vertically in certain gravity, time and temperature conditions.The temperatures in this test are 15 �C, 25 �C and 30 �C, respectively. Penetration in-dex (PI) is calculated as follows:

PI ¼ ð20� 500AÞ=ð1þ 50AÞ ð1Þwhere A ¼ ðlogðPen at T1Þ � log ðPen at T2ÞÞ=ðT1 � T2Þ ð2Þ

Here Pen is the abbreviation of penetration, T1 and T2 are different temperatures, andT1 > T2. A is penetration temperature index [18].

2.4.1. Rheological tests with a dynamic shear rheometer (DSR) [19,20]The DSR is used to characterize the viscous and elastic behavior of asphalt bind-

ers at high and intermediate service temperatures. The DSR measures the complexshear modulus G* and phase angle d of asphalt binders at the desired temperatureand frequency of loading. G* is defined as the ratio of maximum (shear) stress tomaximum strain and provides a measure of the total resistance to deformationwhen the asphalt is subjected to shear loading. It consists of two components:the storage modulus (G0) or the elastic part, and loss modulus (G00) or the viscouspart. These two components are related to the complex modulus and to each otherthrough the phase (or loss) angle (d) which is the phase, or time, lag between theapplied shear stress and shear strain responses during a test. Complex modulus(G*) alone is not sufficient to characterize asphalt binders; phase angle (d) is alsoneeded.

There are three test conditions of DSR: the asphalt binder is tested in the DSR inits original (unaged), oven aged (RTFO residue), and PAV aged conditions. Originaland RTFO aged asphalt binder samples are tested at the maximum design temper-ature to determine the binder’s ability to resist rutting. For the rutting resistance, ahigh complex modulus G* value and low d are both desirable. A criterion of SHRP(Strategic Highway Research Program) is G*/sind P 1 kPa for the asphalt binder athigh temperature [21]. The greater the G*/sind is, the better performance the ruttingresistance at high temperature is.

The DSR tests were performed in the light of the AASHTO T315-2 under the fol-lowing test conditions:

� Mode of loading: controlled-strain (12%).� Temperatures: 46, 52, 58, 64 and 70 �C.� Frequencies: 1.59 Hz.

2.4.2. Creep tests with bending beam rheometer (BBR)The BBR tests asphalt binders at low pavement service temperatures to deter-

mine the binder’s propensity to thermal cracking [19]. Creep tests were carriedout at �18 �C using a BBR (Cannon Instrument Compa AT) according to ASTMD6648-01. In tests, the asphalt beam (125 mm long, 12.5 mm wide and 6.25 mmthick) was submerged in a constant-temperature bath and kept at the test temper-ature for 60 min. After pre-loading procedure, a constant load of 100 g was then ap-plied to the rectangular beam which was supported at both ends by stainless steelhalf-rounds (102 mm apart), and the deflection of center point was measured con-tinuously. Creep stiffness (S) and creep rate (m) of the binders were determined atseveral loading times ranging from 8 to 240 s [19,22].

Table 3Performance indexes of different CRM asphalt at three contents.

Crumb rubber content (%) Crumb rubber type Softening point (�C) Ductility at 5 �C (cm) PI

60 mesh 80 mesh 60 mesh 80 mesh 60 mesh 80 mesh

16 HT 61.2 62.0 22.0 22.7 �0.2266 �0.5686ST 58.7 59.3 20.1 21.3 �0.2833 �0.1093AT 63.0 62.7 14.2 14.1 �0.3044 �0.2188TR 64.5 66.2 25.7 26.9 �0.0065 �0.0067

20 HT 68.5 69.0 19.7 19.0 0.5560 0.6644ST 66.3 67.4 18.5 18.9 0.6685 0.3664AT 69.2 71.3 10.1 11.2 0.3274 0.4034TR 68.2 68.7 22.3 22.9 0.6415 0.6876

24 HT 71.0 71.3 18.1 18.0 0.9328 1.0488ST 69.5 70.7 19.2 20.3 0.7290 0.7260AT 68.5 69.7 11.4 12.5 0.5879 0.5974TR 73.9 74.7 20.7 20.0 0.8511 0.8872

S. Liu et al. / Construction and Building Materials 23 (2009) 2701–2708 2703

3. Results and analysis

3.1. Results of basic properties

For basic properties such as softening point, low temperatureductility at 5 �C and penetration, the crumb rubber contents usedare 16%, 20% and 24% by weight of base asphalt binder,respectively.

Softening point, low temperature ductility, and the penetrationat 15 �C, 20 �C and 25 �C of the 24 kinds of CRM asphalt can be di-rectly measured by tests, while the penetration indexes are calcu-lated through the formula (1) and (2). The softening point paralleltests are two times while ductility and penetration parallel testsare three times, and their precision all meet requirements. Theaveraged values of test result are shown in Table 3.

3.2. Variance analysis of conventional physical properties

Variance analysis can determine the impacts of random and hu-man factors on test results. The two types of influence factors usu-ally affect the test results commonly during the test. Varianceanalysis can distinct the difference and fluctuation caused by thetwo kinds of factors, further give them quantitative descriptionand determine the influence degree of factors to performance in-dexes. The paper used variance analysis of non- repeated dual fac-tors test.

3.2.1. Variance analysis principle of non- repeated dual factors test [23]The variance analysis principle of non-repeated dual factors test

is as follows:Suppose there are two variables A and B in a test, A has r levels

which are A1,A2, . . . ,Ar, while B has s levels which are B1,B2, . . . ,Bs,respectively. If one test is done according to each kind of combina-tion level (Ai,Bj), and the result is xij(i = 1,2, . . . ,r; j = 1,2, . . . ,s), thenall the xijs are independent each other and obey the normal distri-bution. The test results are denoted in Table 4.

For each of the test value xij, i represents the corresponding levelof factor A, while j represents the corresponding level of factor B.The total number of the test results is n = rs.

Table 4The denoted results of non-repeated dual factors test.

Factors B1 B2 ... Bs

A1 x11 x12 ... x1s

A2 x21 x22 x2s

... ... ... ... ...Ar xr1 xr2 ... xrs

The basic variance analysis steps of non-repeated dual factorstest are as follows:

3.2.1.1. Calculation of the mean value. Let

�x ¼ 1rs

Xr

i¼1

Xs

j¼1

xij

�xi� ¼1s

Xs

j¼1

xij

�x�j ¼1r

Xr

i¼1

xij

Then

�x ¼ 1r

Xr

i¼1

�xi� ¼1s

Xs

j¼1

�x�j

where �x represents the arithmetic mean value of all the measure-ments, known as the total average value; �xi� represents the arithme-tic mean value of measurements obtained when factor A is held atlevel i; �x�j represents the arithmetic mean value of measurementsobtained when factor B is held at level j.

3.2.1.2. Calculation of the sum of squares of deviations. The sum ofsquares of deviations is

SST ¼Xr

i¼1

Xs

j¼1

ðxij � �xÞ2 ¼ SSA þ SSB þ SSe

where SSA represents the sum of squares of deviations caused byfactor A, SSB represents the sum of squares of deviations causedby factor B, and SSe represents the sum of squares of error.

Let

Ti� ¼Xs

j¼1

xij; T �j ¼Xr

i¼1

xij; Q i� ¼Xs

j¼1

x2ij; Q �j ¼

Xr

i¼1

x2ij;

T ¼Xr

i¼1

Xs

j¼1

xij ¼Xr

i¼1

Ti� ¼Xs

j¼1

T �j

and Q ¼Xr

i¼1

Xs

j¼1

x2ij ¼

Xr

i¼1

Q i� ¼Xs

j¼1

Q �j

then the following equations can be deduced:

SST ¼ Q � T2

rs¼ Q � T2

n

SSA ¼1s

Xr

i¼1

T2i� �

T2

n

SSB ¼1r

Xs

j¼1

T2�j �

T2

n

SSe ¼ SST � SSA � SSB

Table 5Variance analysis table of non-repeated dual factors test results.

Source SS df MS F significance

A SSA r�1 MSA ¼ SSAr�1 FA ¼ MSA

MSe

B SSB s�1 MSB ¼ SSBs�1 FB ¼ MSB

MSe

Error SSe (r�1)(s�1) MSe ¼ SSeðr�1Þðs�1Þ

Sum SST rs�1

2704 S. Liu et al. / Construction and Building Materials 23 (2009) 2701–2708

3.2.1.3. Calculation of the degree of freedom. The degree of freedom(df) of SSA, SSB and SSe is r�1, s�1 and (r�1)(s�1), respectively.

3.2.1.4. Mean value of sum of squares of deviations. We employ MSA,MSB and MSe to denote the mean value of sum of squares of devi-ations corresponding factors A, B and error, respectively, then

MSA ¼SSA

r � 1

MSB ¼SSB

s� 1

MSe ¼SSe

ðr � 1Þðs� 1Þ

3.2.1.5. F test. Let

FA ¼MSA

MSe

FB ¼MSB

MSe

where FA ratio has an F distribution with degree of freedom of (r�1,(r�1)(s�1)). For a given significant level a, if FA > Fa (r�1,(r�1)(s�1)), factor A has a significant effect on test results, other-wise, no significant effect. FB ratio has an F distribution with degreeof freedom of (s�1, (r�1)(s�1)), if FB > Fa (s�1,(r�1)(s�1)), factor B

Table 6Calculation process of variance analysis for softening point.

Softening point (�C) Particle size

60 mesh 80 mesh

HT 61.2 62ST 58.7 59.3AT 63 62.7TR 64.5 66.2T�j 247.4 250.2T2�j 61206.76 62600.04

Q�j 15320.38 15674.22

Table 7Variance analysis of crumb rubber type and particle size at fixed crumb rubber content.

Crumb rubber content(%)

Performanceindexes

Influence of factor A (crumb ru

FA

valueFA critical value

16 Softening point (�C) 41.48 FA0.05(3,3) = 9.28;FA0.01(3,3) = 29.46ductility at 5 �C

(cm)272.08

PI 2.0720 Softening point (�C) 13.63

ductility at 5 �C(cm) 178.90PI 1.88

24 Softening point (�C) 72.90ductility at 5 �C(cm)

111.25

PI 41.70

Note: ‘‘**” Means significant under the significant level of 0.01, ‘‘*”means significant und

has a significant effect on test results, otherwise, the effect is notsignificant. Finally, variance analysis is listed in Table 5.

3.2.2. Variance analysis at fixed crumb rubber contentThe crumb rubber type and particle size are the influence fac-

tors of the test results when crumb rubber content is fixed. Wetake the test results of softening point from Table 3 (The corre-sponding crumb rubber content is 16%) as an example to explainthe calculation process of the variance analysis (where factor A iscrumb rubber type and factor B is crumb rubber particle size),which is summarized in Table 6 and as follows:

In Table 6, r = 4, s = 2 and n = 8.then

SST ¼ Q � T2

rs¼ Q � T2

n¼ 30994:6� 497:62

8¼ 43:88

SSA ¼1s

Xr

i¼1

T2i� �

T2

n¼ 1

2� 61985:22� 497:62

8¼ 41:89

SSB ¼1r

Xs

j¼1

T2�j �

T2

n¼ 1

4� 123806:8� 497:62

8¼ 0:98

SSe ¼ SST � SSA � SSB ¼ 43:88� 41:89� 0:98 ¼ 1:01

MSA ¼SSA

r � 1¼ 41:89

4� 1¼ 41:89

3

MSB ¼SSB

s� 1¼ 0:98

2� 1¼ 0:98

MSe ¼SSe

ðr � 1Þðs� 1Þ ¼1:013� 1

¼ 1:013

FA ¼MSA

MSe¼ 41:89

3� 3

1:01¼ 41:48

FB ¼MSB

MSe¼ 0:98� 3

1:01¼ 2:91

Ti� T2i� Qi�

123.2 15178.24 7589.44118 13924 6962.18125.7 15800.49 7900.29130.7 17082.49 8542.69T = 497.6

P4i¼1T2

i� ¼ 61985:22P2j¼1T2

�j ¼ 123806:8Q = 30994.6

bber type) Influence of factor B (crumb rubber particle size)

Significant FB

valueFB critical value Significance

* 2.91 FB0.05(1,3) = 10.13;FB0.01(1,3) = 34.12** 5.97

0.03** 7.74** 0.85

0.04** 0.60** 16.78 *

** 2.20

er the significant level of 0.05, while blank means no significant.

0

10

2030

40

50

60

5 10 15 20 25

Crumb rubber content (%)G

*/sin

(del

ta)(k

Pa)

(TR

-CR

M a

spha

lt)

46°C52°C58°C64°C70°C

05

10152025303540

5 10 15 20 25

Crumb rubber content (%)

G*/s

in(d

elta

)(kPa

)(H

T-C

RM

asp

halt) 46°C

52°C58°C64°C70°C

0

10

20

30

40

5 10 15 20 25

Crumb rubber content (%)

G*/s

in(d

elta

)(kPa

)(S

T-C

RM

asp

halt) 46°C

52°C58°C64°C70°C

a

b

c

Fig. 1. Relationship lines between G*/sind and crumb rubber content of differentCRM asphalt (unaged) at different temperature.

S. Liu et al. / Construction and Building Materials 23 (2009) 2701–2708 2705

According to the above calculation process, other results of varianceanalysis can be obtained, and all calculation results are shown in Ta-ble 7.

When crumb rubber content is 16%, 20%, and the significant le-vel a is 0.05, crumb rubber type has a significant effect on softeningpoint and low temperature ductility, but can not affect the pene-tration indexes. When the content is 24%, crumb rubber type canaffect softening point, low temperature ductility and penetrationindexes all significantly under the significant level a = 0.01. Crumbrubber particle size can affect ductility at 5 �C significantly onlywhen the crumb rubber content is 24% and significant level is0.05. In addition, when significant level a is 0.05, the size of crumbrubber particle has no significant impact on above three indexesregardless of the crumb rubber content.

The above analysis show that the impact of crumb rubber typeon properties of the CRM asphalt is more significant comparedwith particle size. So the selection of crumb rubber type is neededto be considered carefully, especially when the content is high;while the impact of crumb rubber particle sizes (60 mesh and 80mesh) on properties of the CRM asphalt is not significant, andthe suitable particle size is naturally 60 mesh considering the eco-nomic issue.

3.2.3. Variance analysis at fixed crumb rubber particle sizeThe crumb rubber type and content are the influence factors of

the test results when crumb rubber particle size is fixed, and thecalculation results are shown in Table 8.

No matter the particle size is 60 mesh or 80 mesh, crumb rubbertype has a significant effect on softening point, but little effect onlow temperature ductility and penetration indexes when the sig-nificant level a is 0.01; while the crumb rubber content signifi-cantly affects all the three indexes when the significant level aequals 0.05 even it is 0.01. These show that the effect of crumb rub-ber content on properties of CRM asphalt is more significant com-pared with crumb rubber type at fixed particle size.

It can be concluded that among the three affecting factors ofcrumb rubber type, particle size and crumb rubber content, the lastone is the most key factor, followed by crumb rubber type and par-ticle size comes last in sequence.

3.3. Analysis of effect of different crumb rubber type on CRM asphaltperformance

The data in Table 3 and the variance analysis results show that:(1) when the content is the same, the softening point, ductility at5 �C and penetration index don’t have significant difference be-tween the asphalt modified by 60 mesh and 80 mesh crumb rub-ber. That is to say the basic properties of asphalt modified by 60mesh and 80 mesh crumb rubber are approximately the same.Meanwhile, TR-CRM asphalt and HT-CRM asphalt show highersoftening point and the ductility at 5 �C of TR-CRM asphalt is al-most the best while AT-CRM asphalt is the worst. When crumbrubber content is 24%, HT-CRM asphalt shows largest penetration

Table 8Variance analysis of crumb rubber type and content at fixed size of crumb rubber particle

Crumb rubber particlesize (mesh)

Performance indexes Influence of crumb rubber t

F value F critical valu

60 Softening point(�C) 59.48 F0.05(3,6) = 4.F0.01(3,6) = 9.

ductility at 5 �C (cm) 3.15PI 4.76

80 Softening point (�C) 29.71ductility at 5 �C (cm) 2.40PI 0.830

index that means temperature-susceptibility performance is thebest. (2) For the two particle sizes the softening point and penetra-tion indexes increase significantly with the increasing of the crumbrubber content, while the ductility at 5 �C decrease significantly.The increasing of penetration indexes shows temperature-suscep-tibility performance makes better.

Taking into account the small low temperature ductility or badlow temperature performance of AT-CRM asphalt, and the lessmarket supply of AT-crumb rubber, despite the lower price, thestudy no longer used it in the following tests. The three crumb rub-ber types of TR-crumb, HT-crumb and ST-crumb rubber are se-lected as modifiers, and the particle size of 60 mesh is used. Thehigher crumb rubber content can result in the smaller ductility atlow temperature of CRM asphalt and it should be reduced, and

.

ype Influence of crumb rubber content

e Significant F value F critical value Significance

76l;78

** 10.97 F0.05(2,6) = 5.14;F0.01(2,6) = 10.92

**

32.10 **

** 109.3 **

** 7.51 **

24.95 **

25.86 **

2706 S. Liu et al. / Construction and Building Materials 23 (2009) 2701–2708

was determined as 10%, 15% and 20% by weight of asphalt,respectively.

3.4. Rheological measurement results and discussion

The complex modulus G* and phase angle d of the CRM asphalts(unaged) was measured at different temperatures, and at the sametime the rutting parameter G*/sind was calculated. The mastercurves of relationship between G*/sind and crumb rubber contentare showed in Figs. 1 and 2. (In all Figs. the sin (delta) is sind).

Rutting parameter G*/sind of the three kinds of CRM asphaltshows a different changing law as crumb rubber content increases(Fig. 1a–c). For TR-CRM asphalt, G*/sind rises with the increasing ofcrumb rubber content and reaches the maximum value at 20% con-tent for different temperatures. For HT-CRM asphalt and ST-CRMasphalt, G*/sind shows parabola change of opening down andreaches the peak at 15% content, but the parabola shape is differ-ent. The G*/sind of 20% content is bigger than that of 10% contentfor ST-CRM asphalt, while the situation is opposite for HT-CRMasphalt.

The comparison of G*/sind at different temperatures for threekinds of CRM asphalt is shown in Fig. 2. Firstly, the G*/sind of dif-ferent kinds of CRM asphalt is different at the same crumb rubbercontent and temperature, but the differences tend to decrease withthe temperature increasing for all crumb rubber contents. Sec-

05

101520253035

46 52 58 64 70

Temperature (°C)

G*/s

in(d

elta

)(kPa

)(C

onte

nt o

f 10%

) TR-CRM asphaltHT-CRM asphaltST-CRM asphalt

0

10

20

30

40

50

Temperature (°C)

G*/s

in(d

elta

)(kPa

)(C

onte

nt o

f 15%

)

TR-CRM asphaltHT-CRM asphaltST-CRM asphalt

0

10

20

30

40

5060

Temperature (°C)

G*/s

in(d

elta

)(kPa

)(C

onte

nt o

f 20%

) TR-CRM asphaltHT-CRM asphaltST-CRM asphalt

a

b

c

46 52 58 64 70

46 52 58 64 70

Fig. 2. G*/sind values comparison at different temperature of three kinds of CRMasphalt (unaged) for different crumb rubber content.

ondly, the G*/sind at 70 �C of all kinds of CRM asphalt is greaterthan 1.00, and meet the requirement of the SHRP’s criterion. Whenthe crumb rubber content is 10%, the G*/sind of TR-CRM asphalt isthe biggest among the three CRM asphalts at 46 �C, while the valueof HT-CRM asphalt increases slightly bigger than that of TR-CRMasphalt with the temperature rising, but the G*/sind of the threekinds of CRM asphalt, in general, is close (Fig. 2a). When crumbrubber content is 15% and temperature is 46 �C, the G*/sind ofTR-CRM asphalt is still the largest, but smaller at the rest temper-atures, while the values of G*/sind of HT and ST-CRM asphalt is lar-ger and very close (Fig. 2b). When crumb rubber content is 20%, theG*/sind of TR-CRM asphalt is observably bigger than that of HT-CRM asphalt or ST-CRM asphalt at all temperatures, and the valueof HT-CRM asphalt and ST-CRM asphalt is close basically (Fig. 2c).

Comparing the results of DSR test with those of softening point,we can give the following findings:

� For TR-CRM asphalt, DSR and softening point test results areconsistent, that is, with the increasing of the crumb rubber content,both G*/sind and softening point increase.� But to HT-CRM asphalt and ST-CRM asphalt, with the increasingof crumb rubber content, G*/sind increases first, and then lowersafter reaching the peak. This is inconsistent with the increasingtrend of softening point.� When crumb rubber content is 20%, softening point of TR-CRMasphalt and HT-CRM asphalt is very close while the G*/sind valueof the former is more than twice the latter.

The inconsistent phenomena mentioned above are an interest-ing question, which may be explained by the different physicalsenses of softening point and rutting factor. The former is a tem-perature showing state changing of material, while G*/sind is aphysical quantity depicting the mechanical properties of material,and their essences are significantly different. The latter should beable to better reflect the anti-rutting performance of asphalt bind-ers. If these phenomena are universal, then, that CRM asphalt madeof different types of crumb rubber may have the same soften-ing point but different rutting factor, that is, high-temperature

405060708090

100110

5 10 15 20 25

Crumb rubber content (%)

Cre

ep s

tiffn

ess

S(M

Pa)

TR-CRM asphaltHT-CRM asphaltST-CRM asphalt

0.3

0.35

0.4

0.45

0.5

5 10 15 20 25

Crumb rubber content (%)

Cre

ep ra

te m

TR-CRMasphaltHT-CRMasphaltST-CRMasphalt

a

b

Fig. 3. Results of BBR test for the three CRM asphalt (unaged).

S. Liu et al. / Construction and Building Materials 23 (2009) 2701–2708 2707

performance may be different. Of course, the issue is necessary tobe further identified and studied.

3.5. Creep test results and analysis

The creep stiffness modulus (S) and creep rate (m) of three CRMasphalt (unaged) are determined by the BBR software at 60s load-ing time and showed in Fig. 3. Stiffness modulus and m value of allkinds of CRM asphalt meet the requirements. The former is nomore than 300 MPa and m value is larger than 0.300 [19].

It indicates that the values of stiffness modulus (S) of TR-CRMasphalt and HT-CRM asphalt, except for ST-CRM asphalt, decreasewith the increase of crumb rubber content (Fig. 3a); while the mvalues for each kind of CRM asphalt shows its respective changelaw instead of a unified rule of change (Fig. 3b).

As the stiffness modulus (S) increases, the thermal stressesdeveloped in the pavement due to thermal shrinking also increase,and thermal cracking becomes more likely. On the other hand, asthe m value decreases, the rate of stress relaxation decreases andthe ability of the HMA pavement to relieve thermal stresses byflow decreases [19]. Therefore, binders with the smaller stiffnessmodulus and the larger m value have a good low temperatureanti-cracking property. So this paper defines the coefficient k = S/mand further calculates the k value according to stiffness modulus(S) and m value. The smaller the k, the better the low temperatureperformance is. The relationship between k and crumb rubber con-tent of the three kinds of CRM asphalt is shown in Fig. 4.

For TR-CRM asphalt, when crumb rubber content increases thecoefficient k decreases dramatically, and the k-value of HT-CRM as-phalt firstly decreases then changes gently; while the k-value ofST-CRM asphalt firstly rises, then declines slightly. This impliesthat, when crumb rubber content increases, there are differentchange laws of low temperature anti-cracking property for thethree CRM asphalts: the low temperature performance ofTR-CRM asphalt becomes better and better, the HT-CRM asphalt’sdoes not increase after the 15% content, while the ST-CRM asphalt’sis worse rather than better.

In addition, the relationship of k-value among the three CRM as-phalts at different crumb rubber content is not steadfast. For exam-ple, the k of TR-CRM asphalt is the biggest among the three CRMasphalts at the content 15%, but at the content of 20% the largestk is for ST-CRM asphalt and the k-values of the TR-CRM asphaltand HT-CRM asphalt are approximately equal. Compared TR-CRMasphalt with HT-CRM asphalt, the latter is found to have a betterlow temperature anti-cracking property at the crumb rubber con-tents of 10% and 15%.

Comparing the results of BBR test with those of the ductility at5 �C we can find that, for ST-CRM asphalt, the results of BBR andductility at 5 �C are approximatively consistent, that is, with theincreasing of the crumb rubber content, the decrease of low tem-perature property of ST-CRM asphalt is reflected by the BBR and

0.3

0.35

0.4

0.45

0.5

5 10 15 20 25

Crumb rubber content (%)

Cre

ep ra

te m

TR-CRMasphaltHT-CRMasphaltST-CRMasphalt

Fig. 4. Ratio of S to m for the three CRM asphalt (unaged) at different crumb rubbercontent.

low temperature ductility tests at the same time, and for TR-CRMasphalt and HT-CRM asphalt, the changing trends of BBR and duc-tility at 5 �C with the increasing of crumb rubber content are incon-sistent. Like the different changing trends between the DSR andsoftening point results for HT-CRM asphalt and ST-CRM asphalt,this inconsistent phenomenon in low temperature property ofCRM asphalt is also interesting and need to be researched further.

4. Conclusion and recommendation

Variance analysis shows that among the three factors of crumbrubber type, particle size and content, the content is foremost fac-tor affecting the performance of CRM asphalt, followed by crumbrubber type, and particle size comes last. The basic performancesof asphalt modified by 60 mesh and 80 mesh crumb rubber, forthe same type, have no significant difference, so the 60 meshshould be used in projects according to its lower price. TR-crumbrubber and HT-crumb rubber show better modified effect, whilethe low temperature ductility of AT-CRM asphalt is the worst, thatis, its low temperature performance is bad and it should not beused in cold region.

DSR tests show that the anti-rutting factor rises with theincreasing of crumb rubber content for TR-CRM asphalt, which isconsistent with the changing trend of softening point; but thereis an optimum crumb rubber content of 15% for HT-CRM asphaltand ST-CRM asphalt, which is inconsistent with the increasingtrend of softening point. This is an interesting phenomenon andneeds to be further identified and studied.

BBR tests find that HT-CRM asphalt and TR-CRM asphalt havethe best low temperature anti-cracking performance at the crumbrubber content of 15% and 20%, respectively.

Analyzed comprehensively the results of DSR and BBR test, theoptimum or feasible crumb rubber content may be drawn: 20% forTR-CRM asphalt, 15% for HT-CRM asphalt and 10–15% for ST-CRMasphalt, respectively. The TR-CRM asphalt has a better and compre-hensive performance at 20% content, and if the higher price of thecrumb rubber is not considered the TR-crumb rubber should bechosen firstly. If the fixed content of crumb rubber is 15%, the highand low temperature performance of HT-CRM asphalt is good andHT-crumb rubber should be used in place of the other two kinds ofcrumb rubber.

Acknowledgements

This study was supported by the Highway Bureau of Transpor-tation Department of Shandong Province, China, and the authorswould like to acknowledge their financial support.

References

[1] <http://www.dot.ca.gov/hq/esc/Translab/pubs/Caltrans_Asphalt_Rubber_Usage_Guide.pdf,January>. 2003.

[2] <http://info.qipei.hc360.com/2005/11/21104958218.shtml>. November 21,2005.

[3] Yetkin Yildirim. Polymer modified asphalt binders. Constr Build Mater2007;21:66–72.

[4] <http://info.qipei.hc360.com/2007/06/26154680830.shtml>. June 26, 2007.[5] Technical guide of design and construction for crumb rubber of scrap tire and

hot mix asphalt of Beijing city; 2006, unpubished data.[6] Jiangsu province transportation academy of science. Technical specifications

for rubber asphalt pavements of Jiangsu province; 2005, unpubished data.[7] Bahia HU, Davies R. Effect of crumb rubber modifiers (CRM) on performance-

related properties of asplhat binders. J Assoc Asphalt Paving Technol1994;l(63):414–20.

[8] Navarro FJ, Partal P, Martínez-Boza F, Gallegos C. Thermo-rheologicalbehaviour and storage stability of ground tire rubber-modified bitumens.Fuel 2004;83:2041–9.

[9] Lee Soon-Jae, Akisetty Chandra K, Amirkhanian Serji N. The effect of crumbrubber modifier (CRM) on the performance properties of rubberized binders inHMA pavements. Constr Build Mater 2008;22:1368–76.

2708 S. Liu et al. / Construction and Building Materials 23 (2009) 2701–2708

[10] Cao Rongji, Chen Rongsheng. Laboratory study on process parameters ofasphalt rubber and their effects on performance. J Southeast Univ (Nat Sci Edn)2008;38:269–73.

[11] Shen Junan, Amirkhanian Serji, Xiao Feipeng, Tang Boming. Influence ofsurface area and size of crumb rubber on high temperature propertiesof crumb rubber modified binders. Constr Build Mater 2009;23:1304–10.

[12] Thodesen Carl, Shatanawi Khaldoun, Amirkhanian Serji. Effect of crumb rubbercharacteristics on crumb rubber modified (CRM) binder viscosity. Constr BuildMater 2009;23:295–303.

[13] Shen Zhong, Zhao Zhenguo, Wang Guoting. Colloid and surface chemistry. 3rded. Beijing: Chemical industry press; 2004. p. 50–63.

[14] Putman BJ, Thompson JU, Amirkhanian SN. High temperature properties ofcrumb rubber modified binders. In: Mairepav4 – fourth internationalconference on maintenance and rehabilitation of pavements andtechnological control. Belfast, Ireland; 2005.

[15] Shen J, Amirkhanian S, Lee S-J, Putman BJ. Recycling of laboratory-preparedRAP mixtures containing crumb rubber modified binders in HMA. TransportRes Record: J Transport Res Board 2006;1962:71–8.

[16] Wong Cheuk Ching, Wong Wing-gun. Effect of crumb rubber modifiers on hightemperature susceptibility of wearing course mixtures. Constr Build Mater2007;21:1741–5.

[17] Shen Jinan et al. Standard test methods of bitumen and bituminous mixturesfor highway engineering. 1st ed. Bei Jing: China Communication Press; 2000.

[18] Li Jun, Zhang Yuxia, Zhang Yuzhen. The research of GMA-g-LDPE modifiedQinhuangdao bitumen. Constr Build Mater 2008;22:1067–73.

[19] Roberts Freddy L, Kandhal Prithvi S, Brown E Ray. Hot mix asphalt materials,mixture design and construction. 2nd ed. NAPA Reaserch and EeducationFoundation; 1996 [Printed in United States of America].

[20] Airey Gordon D, Rahimzadeh Behzad. Combined bituminous binder andmixture linear rheological properties. Constr Build Mater 2004;18:535–48.

[21] Kennedy TW, Hubert GA, Harrigan ET. Superior performing asphalt pavements(superpave): the product of the SHRP asphalt research program. StrategicHighway Research Program SHRP-A-410; 1994.

[22] lu Xiaohu, Ulf Isacsson. Low temperature properties of styrene-butadiene-styrene polymer modified asphalt. Constr Build Mater 1998;12:405–14.

[23] Chen Kui. Application probability statistics. 1st ed. Beijing: TsinghuaUniversity press; 2000. p. 218–38.