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Effects of water activation of crumb rubber on theproperties of crumb rubber-modified bindersKhaldoun Shatanawi a , Szabolcs Biro a , Carl Thodesen a & Serji Amirkhanian aa Department of Civil Engineering , Clemson University , Clemson, SC, USAPublished online: 13 Jul 2009.

To cite this article: Khaldoun Shatanawi , Szabolcs Biro , Carl Thodesen & Serji Amirkhanian (2009) Effects of wateractivation of crumb rubber on the properties of crumb rubber-modified binders, International Journal of PavementEngineering, 10:4, 289-297, DOI: 10.1080/10298430802169424

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Effects of water activation of crumb rubber on the properties of crumb rubber-modified binders

Khaldoun Shatanawi*, Szabolcs Biro, Carl Thodesen and Serji Amirkhanian

Department of Civil Engineering, Clemson University, Clemson, SC, USA

(Received 2 August 2007; final version received 21 April 2008)

Hot water activation of crumb rubber has been suggested as a method of improving compatibility between crumb rubber andasphalt binder. This procedure removes light oil fractions present in the crumb rubber particles, thus decreasing segregationoccurring between rubber particles and binder.

For this research, a PG 64-22 binder was modified with four different crumb rubber sources at concentration of 15% byweight of binder. Binder properties were evaluated for virgin binders, water-activated crumb rubber-modified (CRM)binders, and for non-activated CRM binders. CRM surface morphologies were studied using a scanning electronmicroscope, molecular size distributions of modified binders were determined using gel permeation chromatography, whilestorage stability was evaluated by cigar tube testing.

Rheological properties of the CRM binder were investigated; improved phase separation properties were observedfollowing water activation, however, these improvements did not result in improved rheological properties of the activatedCRM binder.

Keywords: crumb rubber-modified binder; activated crumb rubber; rheology; stability; asphalt

1. Introduction

Modifying asphalts with crumb rubber from scrap tyres

typically results in the improvement of elasticity, cohesion,

temperature susceptibility and aging characteristics of

virgin asphalts (Dantas Neto et al. 2003, Xiao et al. 2007).

Crumb rubber-modified (CRM) asphalt binders are usually

produced using the wet process (Way 2000). This process

involves the addition of crumb rubber to the binder followed

by certain amount of mixing at elevated temperatures.

Despite the many advantages associated with the wet

process, there are also a number of disadvantages as well

(Defoor and Maurice 2000, Barros and Vasconcellos 2003).

Specifically, these affect the ability of the CRM to become

part of the binder matrix, as opposed to being a foreign

object embedded in the binder (Liang 1999, Memon 2002).

One possible factor responsible for the high settling of

CRM particles in a CRM binder is the lack of solubility of

the CRM particles into the binder matrix. This incompat-

ibility further reduces the reproducibility of the process

(Kasa 1997), the quality of the asphalt mixtures, and may

also result in extreme cases, in large deposits of CRM in the

storage tanks of the CRM binder mixing units. The lack of

consistency in the stability of modified binders can be the

result of varying CRM compositions (Planche 1996), as

different compositions have different solubility (Van

Gooswilligen et al. 2000, Van Heystaeten 2000). Further-

more, the high settlement of CRM binders prepared with the

wet process places restrictions on the practicality of using

CRM binder. Literature states that CRM binders should be

used within four hours of preparation in order to avoid

problems associated with settling CRM particles (Shatnawi

2003). Alternative methods have also been developed over

the past couple of years to remedy this problem, in particular

research has been focused on establishing weak or strong

chemical bonds between the functional groups of the

modifying agents (such as crumb rubber) and asphalt

compounds (Memon 1998, 1999a,b, Liang 2000).

The objective of this research was to study the effects of

the hot water activation process on CRM binder properties.

This activation method has been suggested as an industrial

scale solution due to its ease of implementation. Water

activationof crumb rubber hasbeen hypothesised to improve

the solubility of CRM particles in an asphalt binder (Memon

1999a), however, the exact performance properties of doing

so are still unknown. Rheological testing was employed to

investigate the effect of water activation on CRM binders, as

it is assumed that improved compatibility will have a

substantial effect on the flow properties of the binders.

2. Materials and preparation

2.1 Binder

For this study, a PG 64-22 virgin asphalt of Venezuelan

origin was selected, some properties of this asphalt are

given in Table 1.

ISSN 1029-8436 print/ISSN 1477-268X online

q 2009 Taylor & Francis

DOI: 10.1080/10298430802169424

http://www.informaworld.com

*Corresponding author. Email: kshatanawi@gmail.com

International Journal of Pavement Engineering

Vol. 10, No. 4, August 2009, 289–297

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2.2 Crumb rubber sources

Four crumb rubber sources were used in this study, two of

the crumb rubbers used were produced cryogenically

while the other two were produced using the ambient

grinding method. The properties of the crumb rubbers used

in this study and their sources are given in Table 2.

The analysis of the crumb rubbers was done in accordance

with ASTM D 297. The sulphur content was determined

with Inductive Connected Plasma –Atom Emission

Spectroscopy (ICP–AES) method, which was found to

be more accurate than regular analytical methods (Fazekas

2005). The gradation of each crumb rubber (Figure 1) was

determined using ASTM D 5644. All the crumb rubbers

used fell within the specifications set forth by some

agencies in the United States.

2.3 Crumb rubber activation with hot water

The low surface area present in crumb rubber, particularly

cryogenically produced, tends to decrease the reactivity of

the crumb rubber in the asphalt binder. This results in a

slower gelation period, and ultimately less compatibility

between crumb rubber and binder (Liang 1998). There-

fore, research has been undertaken studying methods of

improving the compatibility of crumb rubbers with asphalt

binder. Research has shown that the treatment of crumb

rubber in aromatic oils could be a solution, however, due

to environmental concerns this technique has yet to be

implemented by the industry (Potgieter and Coetsee 2003).

One possible alternative to this procedure is the treatment

of the crumb rubber using hot water to generate an

activated rubber surface (Memon 1999a).

The activation was achieved by soaking the crumb

rubber particles in hot water thus allowing the excess oils

and chemicals present in the crumb rubber particles to be

removed in the hot water slurry. This was accomplished

by mixing 400 g of crumb rubber with 800 g of distilled

water, the mixture was then heated up to 85 ^ 28C

and blended for a period of 60 min. Following this,

the slurry was filtered and dried at room temperature

(Memon 1999a).

2.4 CRM binder preparation

The CRM binder prepared in the laboratory was produced

by adding 15% crumb rubber by weight of binder to a

preheated sample of asphalt binder. The asphalt binder

was preheated to 1778C and upon addition of the crumb

rubber an impellor rotating at 700 rpm was used for one

hour to mix the blend. Previous research has shown that

this reaction time and temperature are sufficient for

reaction of the crumb rubber and binder to occur

(Putman 2005). Table 3 provides a description of all the

binders prepared in the lab for this investigation.

Table 1. Properties of virgin asphalt used.

Properties Test results

G*/sin d, kPa at 648C and 1.59 Hz 2.250Viscosity at 1358C, Pas 0.595Viscosity at 1658C, Pas 0.160After RTFOT, change of mass, %

at 648C and 1.59 Hz20.428

After RTFOT, G*/sin d, kPa 5.044After PAV, creep stiffness at 2128C, MPa 156After PAV, m-value at 2128C 0.364After PAV, G*·sin d, kPa at 258C and 1.59 Hz 3774Flash point (Cleveland method, open cup; 8C) 277

Figure 1. Gradation of the selected crumb rubbers according torequirements.

Table 2. Properties of crumb rubbers used.

Crumb rubber ACR1 ACR2 CCR1 CCR2

Specific gravity 1.042 1.037 1.053 1.062Moisture content (wt%) 0.76 0.67 0.77 0.67Ash content (wt%) 6.01 5.36 4.66 5.61Carbon black content (wt%) 32.98 29.75 30.41 32.74Extract content (acetone and chloroform; wt%) 9.86 11.80 11.69 8.52Sulphur content (wt%) 2.02 1.32 1.24 1.47Source Car tyres Mix of car and truck tyres Car tyres Mix of car and truck tyres

Note: ACR1, crumb rubber from ambient grinding source 1; ACR2, crumb rubber from ambient grinding source 2; CCR1, crumb rubber from cryogenic grinding source 1;CCR2, crumb rubber from cryogenic grinding source 2.

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3. Testing procedures

The physical properties of the crumb rubber were studied

by analysis of the surface morphology of the crumb rubber

particles using a scanning electron microscope (SEM). Gel

permeation chromatography (GPC) was also performed to

determine the molecular size distribution of modified

binders.

GPC testing involved dissolving the binder with

tetrahydrofuran (THF) at a concentration of dissolution of

0.5% by weight (Putman 2005). Waters GPC equipment

with computerised software was used for chromatographic

analysis of binders. A differential refractive meter (Waters

410) was used as a detector. A series of two columns

(Waters HR 4E and HR 3) were used for separating

constituents of asphalt binder by molecular size. During

testing, the columns were maintained at a constant

temperature of 358C in a column oven. The mobile phase

was THF flowing at a rate of 1 ml/min. Researchers have

classified asphalt binder constituents into several groups

(Nourendin and Wood 1989, Kim et al. 1995, Wahhab

et al. 1999). In this study, a chromatogram profile was

partitioned into 13 slices and three parts; large molecular

size (LMS; slices 1–5), medium molecular size (MMS;

6–9) and small molecular size (SMS; 10–13). Defining

LMS portion as the front five slices was verified in the

previous study (Wahhab et al. 1999). Among quantitative

data of the chromatogram, the LMS value was only used

for evaluation in this study. Statistical analyses were

performed for evaluating correlation of binder rheological

properties with GPC results.

For CRM storage stability testing, the common cigar

tube test was performed, where the binder was placed in a

standard tube (cigar tube), sealed, and conditioned in the

oven at 1808C for 3 days. Following this, the material was

quench-cooled in a freezer at 2208C. The frozen specimen

was retrieved, cut into three equal parts (Youtcheff et al.

2005). Complex moduli (G*) and phase angle (d) of top

and bottom sections were determined at 608C, at a

frequency of 10 rad/s by using a Bohlin II dynamic shear

rheometer. Separation tendencies were determined from

results of top and bottom sections as shown in Equation (1)

(Abdelrahman 2006).

SI ¼ðG*=sin dÞmax 2 ðG*=sin dÞavg

ðG*=sin dÞavg

; ð1Þ

where SI represents the separation index, (G*/sin d)max

represents the higher value of either the top or bottom

section of the tube and (G*/sin d)avg is the average value of

both sections.

Other rheological tests, such as flow and master curve

determination at different temperatures, within the linear

viscoelasticity region were performed by using 25 mm

plate and plate geometry with 2 mm gap; by testing two

replicates form each binder for each test. The range of

temperature used for calculating the master curves was

from 25 to 828C. The 258C was selected so that we keep the

testing geometry the same (spindle size), the 828C was used

based on the limits of the water bath and dynamic shear

rheometer (DSR). The time temperature superposition was

done according to William Landell Ferry equation, in

which was automatically calculated by the Bohlin

software. A temperature of 608C was used when presenting

the G* and d versus the angular frequency as these

parameters are commonly used for predicting rutting

susceptibility.

4. Results

4.1 Surface study and oil extraction

Images obtained using a SEM permitteda closer examination

of the surface morphology of the crumb rubber particles

before and after activation with hot water. Figure 2 shows the

surface morphologies of ambient and cryogenically ground

crumb rubbers, respectively. From these images, it is difficult

to ascertain the effect of the water activation, however no

significant differences were seen. The water activation was

not expected to have a great affect on the surface

morphology, as the goal of this procedure is the removal

Table 3. Prepared modified asphalt samples.

Sample Asphalt (wt%) ACR1 (wt%) ACR2 (wt%) CCR1 (wt%) CCR2 (wt%) H2Oa

A 100 – – – – NoACR1 85 15 – – – NoACR1_H2O 85 15 – – – YesACR2 85 – 15 – – NoACR2_H2O 85 – 15 – – YesCCR1 85 – – 15 – NoCCR1_H2O 85 – – 15 – YesCCR2 85 – – – 15 NoCCR2_H2O 85 – – – 15 Yes

aActivation agent.

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of light oils from the crumb rubber particles, not morphology

changes.

Determining, accurately, the exact amount of oil

extracted is practically impossible, since the light oil

content (acetone and chloroform extract) is around

10–14%. However, not all oil is extracted, and some of

the extracted oil will remain on the rubber particle surface

after drying out the rubber. Table 4 provides the acetone–

chloroform extraction test results which were performed

according to ASTM D-297 ‘Standard Test Methods for

Rubber Products-Chemical Analysis’. The test results

support the theory that oil extraction occurred as results of

the activation process. The specific gravity of the rubber

particles used, before and after activation were also

measured, these results are presented in Table 5.

4.2 Molecular weight distribution

GPC was used to determine the molecular weight

distribution of the various binders. The results indicate

that the hot water activation of the crumb rubber dissolved

the light oil fraction from the crumb rubber. This

phenomenon is observed from Figure 3 whereby it is

clearly seen that the SMS of the activated crumb rubbers is

significantly less than that of the conventional CRM

binder. This finding was expected as the stated purpose of

hot water activation is the removal of light oil fraction

from the crumb rubber. Furthermore, the non-activated

binder showed higher SMS values when compared with

the virgin binder. The reason for that could be contributed

to the hypothesis that light weight oil is released from the

rubber to the asphalt upon mixing. GPC testing indicated

that non-activated binder contained higher oil fractions

than virgin binder, thus resulting in higher SMS values.

4.3 Storage stability

Test results suggest that the hot water activation of crumb

rubber does in fact improve the storage stability properties

of the activated CRM binder. Figure 4 illustrates a

significant decrease in the separation indices for both

cryogenic and ambient CRM binders. Furthermore, test

results suggest that separation index of the activated

cryogenic CRM binders is similar to that of the non-

activated ambient CRM binders. Therefore, it appears that

from this stand point, the hot water activation of the crumb

rubber achieved its goal of improving cryogenic crumb

rubber to comparable ambient crumb rubber properties.

4.4 Rheological characteristics

The acetone and chloroform extract, GPC analysis, and

storage stability tests indicate that the hot water activation

Figure 2. Surface texture of: (a) ambient rubber at 50 £,(b) ambient rubber with water activation at 50 £, (c) ambientrubber at 1600 £, (d) ambient rubber with water activation at1600 £, (e) cryogenic rubber at 50 £, (f) cryogenic rubber withwater activation at 50 £, (g) cryogenic rubber at 1600 £ and(h) cryogenic rubber with water activation at 1600 £.

Table 4. Acetone–chloroform extraction of crumb rubber.

Samplename

Acetone and chloroformextracta, w/w%b before

water activation

Acetone and chloroformextracta, w/w%b after

water activation

ACR1 8.52 4.39ACR2 11.80 5.31CCR1 11.69 5.47CCR2 9.86 3.96

aAcetone–chloroform extraction was according to ASTM D-297.bPercent by total weight of crumb rubber.

Table 5. Specific gravity of the crumb rubber after and beforewater activation.

Samplename

Specific gravitybefore water activation

Specific gravityafter water activation

ACR1 1.042 1.084ACR2 1.037 1.048CCR1 1.053 1.102CCR2 1.062 1.111

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of the crumb rubber did exert some changes on the binder

properties. However, these parameters are unrelated

to performance parameters of the binders. Therefore, the

next section covers the rheological testing of the activated

and non-activated CRM binders.

4.4.1 Flow behaviour

Figure 5(a,b) is an illustration of how the viscosity of the

various binders varies as a function of shear rate, in this

case the water treated crumb rubber was seen to be less

susceptible to variations in shear rate. However, as the

shear rate was increased the water-activated and standard

CRM binders converged yielding little difference between

them. It can also be seen that only the virgin binders

exhibited Newtonian flow. Consistent with the definition

of Newtonian flow the viscosity does not vary with shear

rate, binders modified with CRM tend to exhibit shear

thinning properties whereby as the shear rate is increased

the viscosity decreases.

Examination of the viscosity curves suggest that the

effects of water activation on cryogenically ground CRM

binder are similar to those experienced by ambient ground

CRM binders. As expected, the viscosity of the

cryogenically ground CRM binder is significantly lower

than the ambient ground CRM, but as the shear rate

increases both crumb rubber types tend to follow similar

trends. In this respect, it is also evident that water

activation of cryogenic crumb rubber also produces CRM

binders susceptible to shear thinning.

4.4.2 Elasticity

The creep recovery test evaluates the ability of the binder

to recover from deformation, in this test a constant shear

stress is applied to the sample and the resulting strain is

observed with increasing time. As seen in Figure 6, three

different stresses of 3, 10 and 50 Pa were used. Doing so

evaluated low, medium and high intensity traffic loading

scenarios, respectively. Since the different stresses are

used for comparison purposes only, the only criteria taken

into consideration when selecting the stresses was not to

reach to the level of Newtonian flow because after that the

binder would not recover from the loading. As the

compliance values increased, regardless of stress levels,

higher deformation and lower penetration resistance of the

binder could be anticipated.

Testing showed that the effects of water activation on

creep recovery were more pronounced for the lower

applied stress levels, as the applied stress increased the

effects of water activation appeared to decrease continu-

ously. Also, the findings suggest that creep recovery of the

binder is aided significantly by the addition of crumb

rubber; however, water activation of the crumb rubber did

not appear to affect the performance of the CRM binder.

Water activation of cryogenic crumb rubber was

shown to produce binders with greater rutting suscepti-

bility; Figure 6 suggests that regardless of loading rate

CRM binders experience less permanent deformation

than the virgin binder, hot water-activated CRM binders

produced significantly greater compliance values.

Figure 3. GPC analysis of (a) ambient and (b) cryogenic rubber-modified binders.

Figure 4. Effect of water activation on phase separation.

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Figure 5. Shear rate versus viscosity for (a) ambient CRM binder and (b) cryogenic CRM binder.

Figure 6. Creep-recovery curves at 608C for: (a) 3 Pa stress level using ambient CRM binder, (b) 3 Pa stress level using cryogenic CRMbinder, (c) 10 Pa stress level using ambient CRM binder, (d) 10 Pa stress level using cryogenic CRM binder, (e) 50 Pa stress level usingambient CRM binder and (f) 50 Pa stress level using cryogenic CRM binder.

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These findings indicate that the water activation of

cryogenic crumb rubber produced less elastic CRM binder

than non-activated CRM binder; furthermore, they suggest

that some of the benefits of incorporating crumb rubber in

the binder are offset by using hot water activation.

4.4.3 Temperature and load dependence

Figure 7 is a plot of the Shenoy parameter (Shenoy 2001,

2004a, 2004b), G*/(1 2 (1/tan d £ sin d)), versus fre-

quency. The values of G*/(1 2 (1/tan d £ sin d)) for

ambient rubber were always higher than cryogenic rubber,

this supports the practical experience that ambient rubber

results in more enhanced asphalt rubber, than cryogenic

rubber. The water activation had less of an effect on this

parameter compared to non-activated samples.

Figure 8(a,b) illustrates the change in G* values with

increasing angular frequencies. The G* values were seen

to increase regardless of whether the binder was modified

or not. The CRM binders have higher G* values than the

virgin binder, however no significant difference was noted

between the water-activated CRM binder and the standard

CRM binder. A temperature of 608C was used when

presenting the G* and d versus the angular frequency

Figure 7. Frequency versus G*/(1 2 (1/tan d £ sin d)) for (a) ambient CRM binder and (b) cryogenic CRM binder.

Figure 8. Reduced frequency at 608C versus: (a) G* for ambient CRM binder, (b) G* for cryogenic CRM binder, (c) phase angle forambient CRM binder and (d) phase angle for cryogenic CRM binder.

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as these parameters are commonly used for predicting

rutting susceptibility. Therefore, it was natural to select a

‘higher’ temperature in order to adequately simulate

rutting conditions. The shift factors used in calculating the

master curve are shown in Table 6.

The phase angle, as shown in Figure 8(c,d), tends to

decrease with increasing angular frequencies. The phase

angle obtained from the virgin binder was significantly

higher than the CRM binder, however no significant

difference were noted between the water-activated CRM

binder and the standard CRM binder.

Increases in angular frequency resulted in increased G*values and decreased d values (Figure 8). When treated with

water activation, theG* values of the CRM binder tended to

increase consistently regardless of angular frequency, the

phase angle for the water-activated CRM binder decreased

in a similar fashion to the standard CRM binder. These

findings suggest that the G* and d values of water-activated

CRM binders with varying angular frequency is similar to

that experienced by conventional CRM binder.

5. Conclusions

From the investigation of the effects of hot water activation

of CRM binder the following conclusions were made:

. Hot water activation of crumb rubber was not seen to

alter the surface texture of the crumb rubber regardless

of production process (ambient and cryogenic).. The SMS of the hot water-activated CRM binder

appeared to be significantly decreased with hot water

activation; this decrease is due to the dissolution of the

light oil fraction by the water activation process.. Significant improvements in the storage stability of

the CRM binders were noted with the hot water

activation of the crumb rubber. Findings suggest that

the extent of the improvement of storage stability of

activated cryogenic CRM binder is comparable to that

of non-activated ambient CRM binder.

. The rheological investigation produced results

indicating that hot water activation did not improve

the performance properties of the CRM binder.

Generally, hot water-activated CRM binders exhib-

ited properties consistent with the non-activated CRM

binders.. From this limited study, it can be concluded that hot

water activation of crumb rubber removes significant

quantities of the light oil fraction present in the crumb

rubber, also this activation tends to improve the

storage stability of the CRM binder. However, these

improvements do not transfer to the rheological

properties of the CRM binder, where activated and

standard CRM binders produced very similar results.

Acknowledgements

This study was supported by the Asphalt Rubber TechnologyService (ARTS) at Civil Engineering Department, ClemsonUniversity, Clemson, South Carolina, USA. The authors wish toacknowledge and thank South Carolina’s Department of Healthand Environmental Control (DHEC) for their financial support ofthis project.

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Table 6. Shift factors for the time temperature superposition.

Temperature(8C) A ACR1 ACR1_H2O ACR2 ACR2_H2O CCR1 CCR1_H2O CCR2 CCR2_H2O

20 7.9127 7.6404 7.7940 7.8642 8.0243 7.8341 8.0302 8.1292 7.866525 6.6390 6.6606 6.4156 6.8241 6.7758 6.6509 6.6446 6.5049 6.561830 5.5540 5.5159 5.4334 5.6009 5.5838 5.5430 5.6209 5.4786 5.434235 4.5128 4.4471 4.3923 4.4708 4.4515 4.3797 4.5081 4.3851 4.338440 3.4908 3.4319 3.3799 3.4181 3.4070 3.3114 3.4768 3.3753 3.311245 2.5198 2.4373 2.4449 2.4670 2.4390 2.3799 2.4997 2.4105 2.397350 1.5832 1.5611 1.5593 1.5699 1.5604 1.5394 1.6110 1.5402 1.545455 0.7362 0.7596 0.7383 0.7481 0.7474 0.7515 0.7711 0.7323 0.738860 0 0 0 0 0 0 0 0 065 20.6856 20.7511 20.7519 20.7513 20.7303 20.0821 20.7683 20.7290 20.815270 21.3616 21.4645 21.4442 21.4132 21.4213 21.5089 21.4675 21.4382 21.550675 22.0128 22.1984 22.1881 22.0798 22.1334 22.2032 22.1760 22.1425 2.247680 22.6329 23.1115 23.1249 22.9357 22.9067 22.9459 22.8960 22.8672 2.8962

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