factors affecting the viscosity of crumb rubber-modified asphalt
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Factors Affecting the Viscosity of CrumbRubber–Modified AsphaltD. Q. Sun a & L. H. Li aa Key Laboratory of Road and Traffic Engineering of the Ministry ofEducation , Tongji University , Shanghai, P. R. ChinaPublished online: 19 Aug 2010.
To cite this article: D. Q. Sun & L. H. Li (2010) Factors Affecting the Viscosity of CrumbRubber–Modified Asphalt, Petroleum Science and Technology, 28:15, 1555-1566
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Petroleum Science and Technology, 28:1555–1566, 2010
Copyright © Taylor & Francis Group, LLC
ISSN: 1091-6466 print/1532-2459 online
DOI: 10.1080/10916466.2010.497007
Factors Affecting the Viscosity of CrumbRubber–Modified Asphalt
D. Q. SUN1 AND L. H. LI1
1Key Laboratory of Road and Traffic Engineering of the Ministry of
Education, Tongji University, Shanghai, P. R. China
Abstract The effect of crumb rubber modifier (CRM) content, CRM particle size,
mixing time, and curing temperature on the viscosity of crumb rubber–modified asphalt(CRMA) was studied. The results show that the viscosity of CRMA increases as CRM
content is increased, and there is a critical point in CRM content (15%) significantlyaffecting the increased speed of viscosity. With a decrease in crumb rubber particle
size, the maximal viscosity occurring in the entire process increases at first and thendecreases. At different curing temperatures, three typical changes of the viscosity
of CRMA over mixing time were found. Grey correlation analysis was applied toinvestigate the sensitivity of each factor influencing viscosity. The significance orders
for the factors that influence the viscosity of CRMA are, in turn, CRM content, curingtemperature, CRM particle size, and mixing time.
Keywords asphalt, crumb rubber, crumb rubber–modified asphalt, Grey correlationanalysis, viscosity
Introduction
There are approximately 120 million scrap tires generated annually in China (Z. F. Yang
et al., 2005), 299 million in the United States (Rubber Manufacturers Association, 2006)
and 1.5 billion in the world (Su, 2004). China and many other countries are facing
serious challenge to disposal and utilization of the huge number of waste tires. In the
past 20 years, the study of and interest in use of crumb rubber modifier (CRM) in
asphalt pavements has been increasing to improve the engineering performance of asphalt
pavements, protect the environment, and save resources.
CRM is recycled rubber that is obtained by mechanical shearing or grinding tires into
small particle sizes, which can be used in asphalt mixtures by dry or wet processes (Sun
et al., 2008). The dry process is a method that mixes the crumb rubber with the aggregate
before the mixture is charged with asphalt binder. The CRM acts as a rubber aggregate
in the paving mixture (Cao, 2007). The wet process refers to blending crumb rubber with
asphalt at an elevated temperature before incorporating the binder into the asphalt paving
materials. By the wet process crumb rubber–modified asphalt (CRMA) is obtained and
the performance of asphalt cement can be improved significantly. CRMA can be used as
binder in hot mix asphalt (State of California Department of Transportation, 2003).
Address correspondence to Daquan Sun, Department of Road and Airport Engineering, TongjiUniversity, 4800 Caoan Road, Shanghai 201804, P. R. China. E-mail: [email protected]
1555
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1556 D. Q. Sun and L. H. Li
As illustrated in a number of studies and engineering applications (Billiter et al.,
1997; B. Huang et al., 2002; Jorgenson, 2003; Wong and Wong, 2007; S. C. Huang,
2008), crumb rubber–modified asphalt binders seem to enhance some properties of
asphalt pavement, especially rutting resistance, which has mainly resulted from the
increased viscosity of asphalt–rubber binder. At high temperature and mixing conditions,
the interaction between asphalt cement and CRM has been reported in the literature as
two main mechanisms: particle swelling and degradation (Zanzotto and Kennepohl, 1996;
M. A. Abdelrahman and Carpenter, 1999). Because CRM and asphalt are immiscible
because of the obvious difference of properties, CRM cannot melt in asphalt but can
absorb aromatic oils from the asphalt cement. A CRM particle can swell to three to
five times its original size when blended with hot asphalt (Mathias Leite et al., 2003).
The large number of swelled rubber particles in asphalt would result in a significant
viscosity increase. In addition, by the degradation, which includes devulcanization and
depolymerization, a small amount of CRM components such as rubber, short fibers, and
filler material break away and disperse in asphalt (Xiao et al., 2006). The effect of CRM
degradation on the viscosity of CRMA is complicated. The degraded CRM components
modify the nature of asphalt cement, which leads to improvement of asphalt viscosity.
However, CRM degradation would cause swollen rubber particle volume reduction, which
results in decrease of viscosity of CRMA.
As mentioned above, the resulting viscosity increases that occur when CRM is added
into asphalt are due to CRM particles swelling and degradation. Hence, the viscosity of
CRMA mainly depends on the type and content of CRM, curing temperature, and mixing
times.
The type of CRM mainly includes crumb rubber composition, grinding procedures,
and particle size. Tire is composed mainly of rubber, carbon black, steel, and fiber.
Generally, the greater quantities of natural rubber will increase the viscosity of CRMA
for improved modification. Ambient or cryogenic grinding procedures are used to grind
tires into CRM (Thodesen et al., 2009). Research results have indicated that binder
modified by ambient ground CRM will exhibit greater viscosity than binder modified
by cryogenically ground CRM, which the results from the greater interaction between
ambient CRM with the binder because the ambient CRM particle has a larger sur-
face area and more fine particles than cryogenically ground CRM (Shen et al., 2009).
The particle size of CRM is a more influential factor on the viscosity of CRMA.
West et al. (1998) reported that CRM with greater specific surface area and more
irregularly shaped particles produced asphalt–rubber binders with higher viscosities.
However, Shen et al. (2009) concluded that a CRM blend with larger sizes produced
a larger complex modulus; that is, better rutting resistance. On the effect of CRM
content, research has shown that the viscosity of CRMA increases as CRM content
is increased, regardless of rubber type (Bahia and Davies, 1996; Lougheed and Papa-
giannakis, 1996).
The curing temperature and mixing times affect the swelling and degradation of
CRM to a great extent. The swelling of CRM increases as the temperature and time
increase, which will result in a viscosity increase. But if the temperature is too high or
the time is too long, the rubber will experience depolymerization, which may cause a
gradual reduction in viscosity (M. Abdelrahman, 2006).
Viscosity is a key index not only for the performance of CRMA but also for the
interaction between asphalt cement and CRM. This article investigates the gradual change
in the viscosity of CRMA over time at different elevated temperatures and analyzed the
influences of blend method, crumb rubber particle size, and content on viscosity.
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Viscosity of Crumb Rubber–Modified Asphalt 1557
Table 1
Components of CRM
Component Value
Carbon black content, % 29
Rubber hydrocarbon, % 49
Ash content, % <10
Metal content, % <0.08
Table 2
Gradations (retained, %) of the four CRM blends used
Sieve size, mm 15 Mesh 30 Mesh 60 Mesh 100 Mesh
2.26 0 0 — —
1.18 32.3 0.2 0 —
0.60 62.1 15.2 5.4 —
0.30 5.3 66.2 71.5 0
0.15 0.4 16.6 22.5 45.1
0.075 — 1.9 0.6 50.7
<0.075 — — — 4.2
Experimental
Materials
Ambient crumb rubbers of four different sizes of CRM of 15, 30, 60, and 100 mesh were
used to make the CRMA. The component and gradations of the said CRM are listed in
Tables 1 and 2, respectively.
The base asphalt was obtained from Sinopec Zhenhai Refining & Chemical Company
(Ningbo, China). The physical properties of this asphalt are listed in Table 3.
Preparation of CRMA and Test Methods
About 600 g base asphalt was heated to the set temperature in a cylindrical metal container
(approximately 1,200 mL) and then preweighed CRM was added to the base asphalt. The
Table 3
Properties of the base asphalt
Properties Value
Penetration (25ıC, 100 g, 5 s), 0.1 mm 67
Softening point (R&B), ıC 49
Ductility (15ıC, 5 cm/min), cm 113
Viscosity (60ıC), Pa.s 213
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1558 D. Q. Sun and L. H. Li
base asphalt and CRM were mixed by an X-shaped mixer (at about 500 rpm) at the set
temperature (˙5ıC). At different curing time, the CRMA samples were taken out to
test the viscosity at 60ıC using a Brookfield Rotational Viscometer (Model HV-2000,
Cannon Instrument Company, Philadelphia, PA) by Standard Test Methods of Bitumen
and Bituminous Mixtures for Highway Engineering (T0625, JTJ 052-2000; Ministry of
Transport of P. R. China, 2000).
Results and Discussion
Change in the Viscosity of CRMA Overmixing Time at Different
Curing Temperatures
The four CRMA binders modified by 15% (by weight of the base asphalt) 30 mesh CRM
were made at 150ıC, 175ıC, 200ıC, and 225ıC, respectively. Figure 1 shows the change
in the viscosity of the four CRMA binders over mixing time.
At low curing temperature (150ıC) the viscosity increases continually, which prob-
ably indicates that swelling of CRM is continual over the entire mixing time. At the
intermediate temperature (175ıC or 200ıC), the viscosity goes through three stages
of development; that is, increases rapidly at first (stage 1), then changes very slightly
(stage 2), and lastly decreases (stage 3). The rapid increase in viscosity in stage 1 may be
attributed to swelling that is quickly occurring at the beginning of mixing. In stage 1 the
viscosity increases more significantly at 200ıC than at 175ıC, which is probably why
the higher temperature could improve the extent of swelling. In stage 2, the swelling
and depolymerization of CRM should be in equilibrium, which probably means that the
increase in viscosity by the swelling of CRM is offset as the depolymerization of CRM,
which leads to the decrease in viscosity. Hence, the viscosity changes very slightly in
stage 2. In stage 3, the depolymerization of CRM is predominant over the swelling, which
diminishes the volume and number of CRM. The higher curing temperature will cause
more severe depolymerization, so at 200ıC the viscosity decreases more remarkably than
that at 175ıC at the end of mixing time.
When the curing temperature is raised to 225ıC, the viscosity reaches a maximum
value in 5 min and subsequently decreases rapidly. At the end of mixing time the viscosity
of CRMA is close to that of the base asphalt. The significant reduction in viscosity shows
Figure 1. Change in the viscosity of the four CRMA binders over mixing time.
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Viscosity of Crumb Rubber–Modified Asphalt 1559
Figure 2. Maximal viscosity and corresponding mixing time at different curing temperatures.
that the binder loses most of its modified properties, which is probably caused by almost
full depolymerization of CRM at very high temperature for a long time.
According to Figure 1, at different curing temperatures one maximal viscosity occurs
during the entire process. Because the swelling and depolymerization of CRM are the
key factors for the viscosity of CRMA as mentioned above, it may be presumed that the
maximal viscosity occurs when swelling and depolymerization are in equilibrium. The
maximal viscosity occurring at different curing temperatures is illustrated by Figure 2.
The data indicate that with the increase of curing temperature the maximal viscosity
increases at first and then decreases. Moreover, the time when the viscosity reaches
maximum value varies under different curing temperatures. As illustrated by Figure 2, it
takes less time to reach maximum viscosity with increase in curing temperature for faster
speed of swelling and depolymerization of CRM. It is of value to select the technical
parameters, e.g., curing temperature and mixing time for CRMA production, by the
change in the viscosity of CRMA over mixing time at different curing temperatures,
especially by the maximum viscosity for the given asphalt and CRM materials.
Effect of CRMA Particle Size on Viscosity
The four CRMA binders were prepared by addition of 15, 30, 60, and 100 mesh CRM,
respectively. The CRM content was 15% by weight of the base asphalt and curing
temperature was 175ıC.
Figure 3 presents the gradual change in the viscosity of the four CRMA binders over
mixing time. For the coarse CRM, 15 mesh, the viscosity increases continually over the
entire mixing time. For the intermediate size CRM, 30 or 60 mesh, a peak in viscosity
occurs at different mixing times. For the fine CRM, 100 mesh, the viscosity changes
slightly during the whole process of mixing.
Figure 4 shows the maximal viscosity of four CRMA binders taking place in the
mixing process. The data indicate that with a decrease in CRM particle size the maximal
viscosity increases at first and then decreases. At 60 mesh, the maximal viscosity reaches
a peak value.
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1560 D. Q. Sun and L. H. Li
Figure 3. Change in viscosity of four CRMA binders over mixing time.
The change in viscosity of the four CRMA binders may be attributed to the effect
of CRM particle size on the extent and the speed of swelling and depolymerization of
CRM, especially swelling, because rubber depolymerization mainly depends on curing
temperature and time. The smaller CRMA particles have larger specific surface areas,
which increases asphalt–rubber interaction areas and consequently improves the extent
and speed of swelling.
Figure 4. Effect of CRM size on the maximal viscosity of CRMA.
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Viscosity of Crumb Rubber–Modified Asphalt 1561
When blending asphalt with the coarse CRM, for example, 15 mesh CRM, it takes
more time to swell CRM to the greatest extent. Hence, in the limited mixing time the
peak in viscosity did not occur because of continual swelling, as illustrated by Figure 3.
When CRM is very fine, for example, 100 mesh, CRM particles may swell quickly and
almost completely. Although it takes less time to reach the equilibrium of swelling and
depolymerization, CRM particles are almost entirely swollen, which may result in the
rigid granules disappearing. Therefore, the maximal viscosity of the binder modified by
100 mesh CRM is lower than others. For the intermediate size CRM, most of the swollen
particles still retain a rigid core when CRM is swollen to the greatest extent. Although
the swelling extent is lower than that of the fine CRM, the swelling of rigid granules
significantly increases the viscosity of the blends, as illustrated by Figure 4 for 60 mesh
CRM.
Effect of CRM Content on Viscosity
The four CRMA binders were prepared by addition of 10, 15, 20, and 25% (by weight
of the base asphalt) 60 mesh CRM using a reaction time of 90 min while maintaining a
constant binder temperature of 175ıC.
Figure 5 shows the effect of CRM content on the viscosity of CRMA. The data
indicate that the viscosity of CRMA increases as CRM content is increased. Moreover,
when CRM content is less than 15%, the effect of CRM content on the viscosity is not
significant. When CRM content is more than 15%, the viscosity increases rapidly with
CRM content. For the given materials and reaction condition, there is a critical point
in CRM content (at 15%) affecting the increase speed of viscosity. The critical point in
CRM content is valuable for selecting a feasible CRM content for CRMA production
and application.
Sensitivity Analysis of Factors Influencing Viscosity Using Grey
Correlation Analysis
As analyzed above, these factors, including CRM content, CRM particle size, curing
temperature, and mixing time, have complex influences on the viscosity of CRMA. It
Figure 5. Effect of CRM content on viscosity.
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1562 D. Q. Sun and L. H. Li
is of value to understand the sensitivity of each factor influencing viscosity for the
production control of CRMA. Considering the limited experimental data, the sensitivity
analysis of factors influencing viscosity was decided by Grey correlation analysis.
Methodology
Grey system theory was formulated by Ju-long Deng in 1982. The principle of the Grey
correlation analysis from Grey system theory is based on the macro- or microgeometric
approach between the behavior factors. The more similar the array curves are, the closer
connection they have (Zhang and Zhang, 2007). Grey correlation analysis is an effective
method for the limited experimental data sample to get reasonable analysis results with
good sensitivity (Lin et al., 2007).
The process of Grey correlation analysis is as follows (F. Yang et al., 2008):
Step 1: Identify the analysis sequence—One dependent variable factor (viscosity) and
four independent variable factors (CRM content, CRM particle size, curing tem-
perature, and mixing time) were identified. The dependent variable data constitute
a reference sequence X0 (X0 (1), X0 (2), X0 (3), : : : , X0 (n)) for viscosity, and
the independent variable data constitute four comparison sequences; that is, X1
(X1 (1), X1 (2), X1 (3), : : : , X1 (n)) for curing temperature; X2 (X2 (1), X2 (2),
X2 (3), : : : , X2 (n)) for mixing time; X3 (X3 (1), X3 (2), X3 (3), : : : , X3 (n))
for CRM particle size; and X4 (X4 (1), X4 (2), X4 (3), : : : , X4 (n)) for CRM
content.
Step 2: Calculate the initial images of every sequence Xi
xi D Xi=Yi D .xi.1/; xi.2/; xi.3/i; : : : ; xi.n//
Yi D .Xi.1/ C Xi.2/ C Xi.3/ C : : : C Xi.n//=n
i D 0; 1; 2; 3; 4
Step 3: Calculate the difference sequence �i
�i D jx0.k/ � xi.k/j D .�i.1/; �i.2/; �i.3/; : : : ; �i.k//
k D 1; 2; 3; : : : ; n
i D 1; 2; 3; 4
Step 4: Calculate the correlation coefficient �
�i D .m C �M/=.�i.k/ C �M/ D .�i.1/; �i.2/; �i.3/; : : : ; �i.k//
m D min�i.k/
M D max�i.k/
k D 1; 2; 3; : : : ; n
i D 1; 2; 3; 4
where � is the distinguishing coefficient used to adjust the difference of the
relational coefficient. � is value in (0, 1). The smaller � is, the greater the
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Viscosity of Crumb Rubber–Modified Asphalt 1563
Figure 6. The correlation degree of each factor with viscosity.
difference between correlation coefficients. We adopt � as 0.5 to perform the
grey correlation analysis (GCA) because this value offers moderate distinguishing
effects and good stability (Lin et al., 2007).
Step 5: Calculate the correlation degrees
i D .�i.1/ C �i.2/ C �i.3/ C : : : C �i.n//=n
i D 1; 2; 3; 4
The Grey correlation degree is a quantitative value of the correlation between the factors.
The higher the correlation degree is, the more relevant the reference sequence and
comparison sequence are.
Data Analysis
Table 4 presents the viscosity of CRMA at different experimental conditions for the sen-
sitivity analysis of factors influencing viscosity. The correlation coefficients of viscosity
with four factors were obtained by Grey correlation analysis, as shown in Table 5. The
correlation degrees of each factor with viscosity are given in Table 6. As illustrated by
Figure 6, the correlation degree of CRM content with viscosity is the highest, whereas that
of mixing time is the lowest. Grey correlation analysis results show that the significance
orders for the factors to influence the viscosity of CRMA are, in turn, CRM content,
curing temperature, CRM particle size, and mixing time.
Conclusions
Based on the limited experimental results, the following conclusions can be made for the
given materials:
1. Curing temperature has a great effect on the change in the viscosity of CRMA over
mixing time. At 150ıC the viscosity increases continually, whereas at 225ıC the
viscosity reaches a maximum value in 5 min and subsequently decreases rapidly. At
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Ta
ble
4
Vis
cosi
tyo
fC
RM
Aat
dif
fere
nt
exp
erim
enta
lco
nd
itio
ns
Fac
tors
n1
23
45
67
89
10
11
12
13
14
15
16
Vis
cosi
ty(6
0ıC
),P
a.s
1,2
86
1,2
25
1,3
31
86
99
65
93
61
,24
81
,29
78
06
1,3
86
59
21
,19
71
,41
21
,28
33
,12
66
,04
8
Cu
rin
gte
mp
erat
ure
,ıC
17
51
75
17
51
75
17
51
75
17
52
00
20
02
25
22
51
50
15
01
75
17
51
75
Mix
ing
tim
e,m
in5
15
04
55
45
55
14
05
18
05
50
51
30
90
90
90
CR
Mp
arti
cle
size
,m
esh
30
30
60
10
01
00
15
15
30
30
30
30
30
30
60
60
60
CR
Mco
nte
nt,
%1
51
51
51
51
51
51
51
51
51
51
51
51
51
02
02
5
Ta
ble
5
Co
rrel
atio
nco
effi
cien
to
fv
isco
sity
wit
hfo
ur
fact
ors
�n
12
34
56
78
91
01
11
21
31
41
51
6
� 10
.93
76
0.9
15
20
.95
48
0.8
03
20
.83
06
0.8
22
10
.92
35
0.8
66
40
.73
30
0.8
27
60
.64
60
0.9
87
70
.97
86
0.9
36
40
.60
13
0.3
43
2
� 20
.67
92
0.5
20
90
.91
00
0.7
72
81
.00
00
0.9
00
40
.55
20
0.6
77
00
.41
78
0.6
60
00
.82
73
0.6
97
20
.60
86
0.7
65
90
.70
16
0.3
73
7
� 30
.93
52
0.9
58
60
.76
58
0.4
74
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0.8
72
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0.9
31
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0.8
99
20
.85
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0.9
69
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0.7
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76
7
� 40
.94
08
0.9
18
30
.95
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0.8
05
60
.83
31
0.8
24
60
.92
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0.9
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00
.78
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0.9
80
30
.73
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0.9
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.68
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0.4
01
7
1564
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Viscosity of Crumb Rubber–Modified Asphalt 1565
Table 6
Correlation degrees of each factor with viscosity
Factors Correlation degree
Curing temperature 0.8192
Mixing time 0.6915
CRM particle size 0.7867
CRM content 0.8476
175ıC or 200ıC, the viscosity increases rapidly at first, then changes very slightly,
and lastly decreases.
2. With decrease in CRM particle size, the maximal viscosity occurring during the entire
process increases at first and then decreases. At 60 mesh, the maximal viscosity reaches
a peak value.
3. The viscosity of CRMA increases as CRM content is increased. When CRM content
is more than 15%, the viscosity increases rapidly with CRM content. For the given
materials and reaction condition, there is a critical point in CRM content (15%)
affecting the increased speed of viscosity.
4. According to Grey correlation analysis, the significance orders for the factors to
influence the viscosity of CRMA are, in turn, CRM content, curing temperature,
CRM particle size, and mixing time.
5. More studies are required to investigate the swelling and depolymerization of CRM in
hot asphalt, which is important to understand the interaction between asphalt cement
and CRM and the mechanism of these factors affecting viscosity.
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