enhancing the performance of crumb rubber-modified binders through varying the interaction...

This article was downloaded by: [University of Hong Kong Libraries]On: 12 March 2013, At: 22:22Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK

International Journal of Pavement EngineeringPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/gpav20

Enhancing the performance of crumb rubber-modifiedbinders through varying the interaction conditionsMohamed Attia a & Magdy Abdelrahman aa Department of Civil Engineering, North Dakota State University, Fargo, ND, USAVersion of record first published: 28 Oct 2009.

To cite this article: Mohamed Attia & Magdy Abdelrahman (2009): Enhancing the performance of crumb rubber-modifiedbinders through varying the interaction conditions, International Journal of Pavement Engineering, 10:6, 423-434

To link to this article: http://dx.doi.org/10.1080/10298430802343177

PLEASE SCROLL DOWN FOR ARTICLE

Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions

This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form toanyone is expressly forbidden.

The publisher does not give any warranty express or implied or make any representation that the contentswill be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses shouldbe independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims,proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly inconnection with or arising out of the use of this material.

http://www.tandfonline.com/loi/gpav20

http://dx.doi.org/10.1080/10298430802343177

http://www.tandfonline.com/page/terms-and-conditions

Enhancing the performance of crumb rubber-modified binders through varying the interactionconditions

Mohamed Attia and Magdy Abdelrahman*

Department of Civil Engineering, North Dakota State University, Fargo, ND, USA

(Received 16 October 2007; final version received 12 July 2008 )

Traditional crumb rubber-modified (CRM) binders produced with coarse particles according to the McDonald method, thewet process, raise engineering as well as environmental concerns for their suitability in Superpave applications. This paperinvestigates the possibility of producing high-performance terminal blending CRM binders suitable for Superpaveapplications through the wet process. This paper re-examines the asphalt–rubber interaction mechanism and focuseson controlling the swelling of crumb rubber particles through varying the interaction conditions. This study considersparameters such as time, temperature, shearing speed and the addition of polymer modifiers to enhance the binderperformance properties to meet the requirements of Superpave mixes. Rubber-modified terminal blends are very differentfrom the McDonald-type binders and so are the conditions required for each binder type. The binder properties of interest arethose related to performance and compatibility.

Keywords: asphalt rubber; modified binder; CRM; superpave application; terminal blend

1. Study objectives

This paper investigates a unique technique in the production

of crumb rubber-modified (CRM) asphalt binders for

Superpave applications and suggests a concept of fine

crumb rubber particles dispersed and/or suspended in the

asphalt liquid phase. The main objective of this study is to

investigate the effectiveness of controlling rubber swelling

during the interaction process to enhance and improve the

binder performance and storage properties.

2. Concerns regarding the use of CRM binders

in Superpave mixes

Traditional CRM binders produced with coarse particles,

4.75 mm (no. 4) sieve, according to the McDonald method

raise engineering as well as environmental concerns for

their suitability in Superpave applications. Engineering

concerns are due to the dynamics presented by the gel

nature of the CRM binders and the need to adjust the mix

design parameters to accommodate the swollen rubber

particles to fit into available voids in the mineral aggregate

and to minimise the potential to interfere with aggregate

contact (Heitzman 1992). Also, the wet process results

in higher binder content because the binder thickness

increases during mixing with aggregates due to higher

binder viscosity. Most of the previous study on crumb

rubber applications in hot-mix asphalt was directed

to modify the mixture design procedures to account

for differing physical properties of CRM binders, with

high-content rubber particles (BAS Engineering Consult-

ants 1992, Takallou and Sainton 1992, Chehovits and

Hicks 1993).

A concern regarding CRM binders in Superpave

applications is the method and degree of process control

required as CRM binder properties change with time and

temperature (Pavlovich et al. 1979, Chehovits et al. 1982).

Another concern is binder stability and/or compatibility

during production and storage that is necessary for a long-

lasting pavement (Takallou and Sainton 1992, Stroup-

Gardiner et al. 1993, Abdelrahman 2006). Incompatibility

leads to premature product failure due to rapid ageing and

loss of properties, including adhesion (Stroup-Gardiner

et al. 1993). The suitability of Superpave ageing processes

on CRM binders is another concern (Asphalt Institute

Research Center 1993, SHRPA-368 1993). Both the rotary

thin-film oven test (RTFOT) and the PAV treatments

include elevated temperature for specified time periods.

Temperature and time are main variables in the asphalt–

rubber interaction process. In an asphalt polymer modifier

study, it was reported that the modifications produced

by the polymer modifier become less significant after the

RTFOT and the PAV ageing processes (Shashidhar et al.

1995). A study by McGennis (1995) tested the

applicability of Superpave testing on CRM binders. The

behaviour of CRM binders during the RTFOT ageing was

unlike that of the base asphalt, indicating that the effect of

the RTFOT treatment on CRM binders is not the same as

that on regular asphalt.

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

q 2009 Taylor & Francis

DOI: 10.1080/10298430802343177

http://www.informaworld.com

*Corresponding author. Email: [email protected]

International Journal of Pavement Engineering

Vol. 10, No. 6, December 2009, 423–434

Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

22:

22 1

2 M

arch

201

3

The effectiveness of crumb rubber on binder modifi-

cation when compared to other polymer modifiers, for

example, styrene–butadiene–styrene (SBS), can be

explained by the nature of each modifier. Crumb rubber

keeps its physical shape and behaves as a flexible particulate

filler in the binder producing a non-homogeneous nature.

Polymer modifiers disperse completely in the asphalt and

cause changes in the molecular structure of the asphalt

binder, which is more favourable. The literature concluded

that both modifiers have the effects on the Superpave test

parameters but the main change is in rigidity, at both high

and low temperatures, while only SBS has the effects on

elasticity, as expressed by the phase angle (d) (Bahia 1995).

Thus, SBSmodifications will have more effect on the rate of

stress relaxation and energy dissipation, as they are mainly

functions of d. Resistance to rutting is achieved by two

binder properties, stiffness and elasticity. The higher the

complex modulus (G*) value, the stiffer the binder. The

lower the d value, the more elastic the binder. High-

temperature properties, G*/sin d as implemented by the

Superpave protocol, are the area where the CRM binder is

most effective (Oliver 1981, Bahia 1995). Shenoy suggested

(G*/(1 2 (1/tand sind)) as a parameter for high-temperature

performance grading of asphalt and modified binders

(Shenoy 2001, 2004). In this study, the complex modulus

(G*) and the phase angle (d) will be tracked separately.

Environmental concerns are mainly due to the higher

interaction temperatures, which may produce harmful

gases (Rouse 1994). The higher the crumb rubber

concentration and the coarser the particle size, the higher

the temperature required to complete the interaction

process. Owing to the viscous nature of CRM binders, a

high temperature is necessary in mixing with aggregates.

3. Asphalt–rubber interactions

Traditional asphalt–rubber interaction is not chemical in

nature (Heitzman 1992). The reaction does not result in the

melting of the crumb rubber into the asphalt cement.

Rather, rubber particles are swollen by the absorption of

the asphalt’s oily phase at high temperatures (160–2208C)

into the polymer chains, which are the key components of

the CRM asphalt to form a gel-like material. The change in

rubber particle sizes and formation of gel structures results

in a reduction in the inter-particle distance and produces a

modified gel that produces a viscosity increase of up to

a factor of 10 (Oliver 1981, Heitzman 1992, Bahia and

Davies 1994). Rubber swells in a time- and temperature-

dependent manner. If the temperature is too high or the

time is too long, the swelling will continue to the point

where the rubber is dispersed into the asphalt as the rubber

experiences depolymerisation due to long exposure to the

high temperatures, an undesirable occurrence (Heitzman

1992, Chehovits 1993).

Crumb rubber particle size controls the swelling

mechanism over time and affects the binder matrix.

Buckly and Berger (1962) showed that the time required

for swelling increases with the particle radius squared.

Fine particle sizes may require almost no time to react.

Two main interaction mechanisms are reported as the

binder matrix particles and the liquid phase (Abdelrahman

and Carpenter 1998, 1999). Fine rubber swells faster and

depolymerises faster, affecting the liquid phase more than

the matrix of the binder. Coarse rubber has more effect on

the binder matrix but has less effect on the liquid phase

than fine rubber. Liquid-phase modifications are more

stable than matrix modifications. Using high shear mixing

rate (or high frequency) reduces the particle size of coarse

crumb rubber, allowing the interaction process to progress

with greater speed (Abdelrahman and Carpenter 1999).

Additional shearing at the same temperature increased G*and reduced d significantly, which indicates an

improvement in the material behaviour over time

(Abdelrahman 2006). A study by Texas Transportation

Institute theorised that the increased surface area per

volume or weight of the fine particles enhances the ability

of the particles to be swollen by, and thus bond with, the

binder (Billiter et al. 1995).

Green and Tolonen (1977) concluded that tempera-

ture has two effects on the interaction process. The first

effect is on the rate of swelling of rubber particles.

As the temperature increases, for example from 160

to 2008C, the rate of swelling increases. The second

effect is on the extent of swelling. As the temperature

increases, the extent of swelling decreases. Lalwani

concluded that binder elasticity was drastically reduced

by as much as three times when the temperature was

increased from 200 to 3008C, while no significant

differences occurred due to changing the temperature

from 150 to 2008C (Lalwani et al. 1982). A study by the

Western Research Institute interacted crumb rubber with

different SHRP core asphalts, and concluded that asphalt

source controls the CRM binder properties and has a

significant effect on the way asphalt and rubber

interacted at different temperatures (Western Research

Institute 1994).

Agglomeration is another possible mechanism of

increasing G* in filled polymer systems (Shenoy 1999).

Shenoy stated that high shear mixing provides the energy

to break particle–particle bonds and helps reducing

agglomeration. Once the polymer wets the particle, the

bond fails to re-form, leading to better dispersion of the

filler in the polymer matrix (Shenoy 1999).

CRM binder properties can be optimised under specific

combinations of interaction time and temperature. It is

necessary to precisely control the interaction conditions to

improve the production of terminal blending binders

to meet the requirements of Superpave mixes. Rubber-

modified terminal blends are very different from the

M. Attia and M. Abdelrahman424

Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

22:

22 1

2 M

arch

201

3

McDonald-type binders and so are the interaction

conditions required for each binder type.

4. Experimental consideration

The results presented in Figure 1 are based on the data

from published work, and include AC-10 asphalt cement

and crumb rubber product made of truck tyres at 10% of

the asphalt weight (Abdelrahman and Carpenter 1999,

Abdelrahman 2006). Crumb rubber particle sizes were

controlled as 30–40 and 60–80 sizes, according to the US

standard system. Rouse Polymerics International Inc.,

Vicksburg, Mississippi, supplied the crumb rubber

product. Emulsico (Urbana, IL, USA) supplied the

AC-10 asphalt cement. Interaction temperature is con-

trolled at three levels: 160, 200, and 2408C. No shearing

energy was applied.

Asphalt–crumb rubber compatibility varies from one

asphalt combination to another (Western Research

Institute 1994). To ensure the same level of compatibility,

all interactions, as presented in the following sections,

were based on one source of asphalt cement (PG58-28),

with one source of crumb rubber at 5% of asphalt weight.

Asphalt cement was supplied by Flint Hill Company

(Lincoln, NE, USA). A polymer additive, SBS, was used at

2% of asphalt weight. Only the particle size of crumb

rubber varied in four specific particle sizes: 30–40,

40–60, 60–80 and 80–200. Crumb rubber particle size

30–40 indicates that the particles pass the 30 mesh sieve

and retain on the 40 mesh sieve according to the US

standard system. The crumb rubber was cryogenically

processed and supplied by EnTire Recycling Inc.

(Lincoln). Preliminary interactions with asphalt showed

acceptable performance of the supplied rubber source.

A polymer additive, Stereon 841A, which is a high-

efficiency styrene–butadiene multi-block thermoplastic

elastomer in the pellet form, was supplied by Firestone

Polymers (Akron, OH, USA).

A high-speed shear mixer made by ROSS Company

(model no. HSM-100LM-2) was used to mix the binder

during the interaction process. A temperature controller

(Omega 110B – TM618) was used to control the asphalt–

rubber–SBS interaction temperature of a sample of size

1500 g of asphalt plus modifiers. Figure 2 shows that the

asphalt – crumb rubber – polymer interaction passed

through several processing stages. All samples were pre-

interacted before processing by mixing asphalt with crumb

rubber for 10 min at 10 Hz and a temperature of 1708C.

The experiment was then conducted in three stages.

The first stage (effective interaction) included high shear

and high temperature while the binder was mixed with the

crumb rubber for 20–40 min. The blend was sheared at

either 30 or 50 Hz. Temperature was controlled at three

levels: 170, 200, and 2208C. The second stage (mixing

additives) and the third stage (binder stabilisation) were

constant throughout all interactions. SBS was added to the

blend of asphalt and rubber in the second stage. The mix

was then sheared at 30 Hz for 30 min at a temperature of

1708C. The mixing speed was then reduced to 10 Hz while

the temperature was maintained at 1708C in the third stage.

Samples were taken at 10 min, 20 min, 40 min, 4 h and 8 h

intervals. Three additional interactions were pre-soaked

for 6 h at 10 Hz mixing speed and 1708C interaction

temperature before starting high shear mixing to evaluate

the effect of pre-soaking on the binder properties; the

results are shown in Figure 7. Samples in these cases were

taken after 10 min from the beginning of the soaking

period (2360 min) and then at regular sampling times at

10 min, 20 min, 40 min, 4 h and 8 h intervals. Table 1 lists

Figure 1. Property development of basic asphalt–CRM interactions.

International Journal of Pavement Engineering 425

Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

22:

22 1

2 M

arch

201

3

all the interactions discussed in this paper. Material testing

was conducted at 588C and 10 rad/s using the dynamic

shear rheometer (DSR) device (Bohlin Instruments CVO,

Worcestershire, UK). A plate size of 25 mm was used for

the original and RTFOT-aged material and 8 mm for the

PAV-aged material. The sample thickness was controlled

at 2.0 mm in all cases. The strain level during DSR testing

was controlled at 12, 10 and 1.2% for the tank material,

the RTFOT-aged material and the PAV-aged material,

respectively. Not all collected samples were tested for

Figure 2. Production of the CRM binder.


Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

22:

22 1

2 M

arch

201

3

G* and d. The samples taken at 8 h were evaluated for

stability, also known as the separation test using the Cigar

tube procedures according to ASTM D5892-00. The

separation percentage was calculated using the following

equation:

Separation% ¼ 100 £ {ðG *=sin dÞmax2

� ðG*=sin dÞavg}=ðG*=sin dÞavg:

where G* is the shear modulus, d is the phase angle, both

are measured in the DSR test, (G*/sin d)max is the higher

value of either the top or the bottom portion of the tube,

and (G*/sin d)avg is the average value of the two portions.

Scanning electron microscopy (SEM) was performed

on thin samples of the CRM binder and on extracted

rubber particles. The SEM samples were coated with a thin

layer of gold (20–40 nm thick). The JEOL JSM-6300

scanning electron microscope was used to obtain the

images. Asphalt extraction was conducted according to

ASTM D2172, method B.

5. Basic property development

Figure 1 presents the G* data of a traditional asphalt–

crumb rubber interaction and illustrates the expected

relationships with temperatures. At a low temperature

(1608C), swelling is continual over the entire time period

as illustrated by the continual increase in G*. At an

intermediate temperature (2008C), swelling is still

occurring at the beginning of the process. After the first

20min at 2008C, swelling of the fine material is offset as

the swollen rubber particles are depolymerising, releasing

more components back to the liquid phase of the binder

and decreasing G*. After 60min, the G* value of the fine

material stabilises. The 30–40 size reaches a greater

maximum G* value about 15min after the fine material.

There is a significant difference in the G* values between

the two particle sizes. At a high temperature (2408C),

swelling of the fine material has been mostly completed

before the first sample at 5min. TheG* value is decreasing

continually during the time period. The interaction of the

30–40 material is similar to that of the fine material.

Figure 1 confirms an earlier study on the effect of particle

size on the time required for swelling to increase with the

particle radius squared (Buckly and Berger 1962). Figure 1

also confirms the trade-off between interaction tempera-

ture and time required to develop binder properties.

6. Proposed binder production

In terminal blending binder production, two main proper-

ties are of concern: first, the development of performance-

related properties and, second, binder compatibility or

storage stability. The literature indicates that performance-

related properties develop early in the process while

compatibility requires few hours to stabilise (Abdelrah-

man and Carpenter 1999, Abdelrahman 2006). Figure 1

indicates that rubber particles reach their maximum

swelling, and hence maximum G*, in a relatively short

time for both particle sizes with less time for fine particles

at the same interaction temperature (2008C). Interacting

CRM binders for extended hours at a high temperature

would improve compatibility but will accelerate particle

depolymerisation and will have a negative effect on the

development of performance properties. This paper

examines whether swelling can be balanced to optimise

the development of both performance properties and

compatibility. The proposed approach is to reach a high

particle swelling rate, to minimise particle depolymerisa-

tion through reducing the interaction temperature and then

to control the particle size through shearing. This will

result in very fine crumb rubber particles suspended in the

Table 1. List of interactions.

Interaction stages

Sample no. CRM size Pre-soaking Stage 1 Stage 2 Stage 3

1 30–40 30 Hz, 2008C, 40 min Add SBS at 30 Hz, 1708C, 30 min 10 Hz, 1708C, 410 min2 30–40 10 Hz, 1708C, 6 h 50 Hz, 2008C, 40 min3 40–60 50 Hz, 2008C, 40 min4 40–60 30 Hz, 2008C, 40 min5 40–60 10 Hz, 1708C, 6 h 50 Hz, 2008C, 40 min6 60–80 30 Hz, 2008C, 40 min7 60–80 50 Hz, 1708C, 40 min8 60–80 30 Hz, 1708C, 40 min9 60–80 30 Hz, 2208C, 20 min

10 60–80 10 Hz, 1708C, 6 h 50 Hz, 2008C, 40 min11 80–200 30 Hz, 2008C, 40 min12 80–200 50 Hz, 1708C, 40 min13 80–200 30 Hz, 1708C, 40 min14 80–200 30 Hz, 2208C, 20 min


Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

22:

22 1

2 M

arch

201

3

asphalt liquid phase. Binder compatibility is achieved

through low-temperature storage. Lower storage tempera-

ture is common in terminal blending and saves energy and

reduces binder cost. This paper introduces a binder

production layout that was implemented in the laboratory

and can be implemented in terminal binder production.

Details on the material components and interaction

conditions are described in Section 4 and presented in

Table 1. Figure 2 presents this approach.

The layout presented in Figure 2 is simple, flexible

and will fit in any asphalt terminal blending facility with

high-speed mixing equipment. The proposed system

conserves energy compared to traditional asphalt–crumb

rubber interactions. Effective interaction time is only

20–40 min at 2008C while the total time of a production

cycle is 8 h, mostly at typical terminal storage conditions

(1708C). Lower storage temperature may be used

depending on the used modifiers and the desired level

of separation (compatibility) of the produced binder.

Current asphalt–rubber technologies achieve binder

compatibility through high interaction temperatures, up

to 2608C (Billiter et al. 1997). In addition to the

environmental risk of producing harmful gases, high

interaction temperature results in significant depolymer-

isation of crumb rubber particles and the loss of

modification effectiveness (Asphalt Institute Research

Center 1993). The proposed interaction applies high

shearing energy to reduce the particle sizes as they swell

and keep the particles suspended in the liquid phase of

the binder. As discussed earlier, fine rubber affects the

liquid phase more than the matrix of the binder

(Abdelrahman and Carpenter 1999, Abdelrahman 2006).

SBS is added to enhance the elastic properties of the

produced binder. The effectiveness of SBS is optimised

through balancing the interaction process to improve

compatibility and separation properties of CRM binders

(Abdelrahman 2006). Interaction temperature is then

reduced to slow down the degradation of crumb rubber

and SBS components.

7. Development of performance property

The development of the binder properties, G* and d,

requires the precise control of the interaction conditions

starting with mixing the crumb rubber with asphalt and

then shearing at high temperature. SBS is then added and

sheared into the CRM binder. All components are stored

for about 7 h or more to stabilise the developed properties.

The behaviour of all tested binders was similar, continuous

increase in G* for the first 4 h of interaction and then

stabilisation for the remaining time. The phase angle d is

gradually decreasing throughout the first 4 h of interaction

and then stabilising or slowly improving for the rest of the

interaction process. Differences in binder properties are

noted for different particle sizes under the same interaction

conditions.

Since all interaction conditions are the same in both

stages 2 and 3, the differences in performance properties

are attributed to the processing conditions during the

effective interaction period (stage 1). In addition to the

interaction time and temperature, shearing (to reduce

the particle size) is investigated in this study. The effects

of the effective interaction conditions are compared in the

following. Figures 3–5 present the 30 Hz interactions and

indicate that the effective interaction temperature controls

binder properties at the 30 Hz low shearing. At low

temperature and low shearing (1708C and 30 Hz for

40 min), a difference is noted between particle sizes

similar to that of the basic CRM binder interaction.

The coarser particles had greater G* values than fine

particles through the interaction period. This can be

explained by the greater extent of swelling of coarse

rubber compared to that of fine rubber. At low temperature,

the rubber particles continue to swell through most of the

interaction time (first 4 h), leading to stiffer binder for the

coarser rubber. Increasing the temperature to 2008C

reduces the differences in the G* values among all particle

sizes with insignificant change in the G* value in the

80–200 interaction. Increasing the temperature to 2208C,

even for a shorter period of time (20min) minimises the

Figure 3. Property development at 30 Hz and 1708C.


Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

22:

22 1

2 M

arch

201

3

differences between particle sizes and reduces the final G*values of all interactions. This agrees with the literature

that temperature controls the interaction process. At high

temperature, depolymerisation takes place early in the

interaction and is the major controlling interaction

mechanism (Abdelrahman 2006). SBS minimises the

effect of temperature on both the G* and d development in

both the 170 and 2008C interactions but has a less effect on

the 2208C interaction. The literature explains that the high

temperature affects the polymer structure and may degrade

the effectiveness of polymer modifiers on asphalt proper-

ties (Green and Tolonen 1977, Billiter et al. 1995, 1997).

Temperature controls the interaction mechanism and the

development of binder properties even with the addition of

the SBS polymer. Particle size has a significant effect

on the development of the performance properties.

The coarser the particle size, the higher the G* value

and the lower the d value under the same interaction

conditions at all times (Abdelrahman and Carpenter 1999).

Figure 1 confirms this finding.

Figures 6 and 7 present the interactions of higher

shearing energy, the 50 Hz cases, and show slightly higher

G* values than those produced through the 30 Hz

interactions. Figures 6 and 7 also show the same d values





Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

22:

22 1

2 M

arch

201

3

as those produced through the 30 Hz interactions.

Increasing the shearing speed from 30 to 50Hz does

not have a significant effect on the interaction process

when compared to the temperature and the rubber particle

size.

The discussion on controlling binder properties

through controlling particle swelling extends to pre-

existing swelling. This section investigates the effects of

high temperature and high-speed shearing on pre-soaked

crumb rubber particles in asphalt for significantly longer

time (6 h) at a low interaction temperature (1708C).

The first 6 h in Figure 8 were considered as pre-soaking

(starting at 2360 min) and the zero time in the graph

refers to the beginning of the effective interaction when

high shearing (50 Hz) was applied. Figure 8 agrees with

earlier studies on the effect of temperature on the rate

and extent of crumb rubber particle swelling and

confirms that the rate of particle swelling can be

controlled through temperature control (Green and

Tolonen 1977, Abdelrahman and Carpenter 1999,

Abdelrahman 2006). As shown in Figure 8, the G*develops slowly for 6 h at 1708C. Comparing Figure 8

with Figure 7, at 2008C for 40min, 6 h of pre-soaking

improves the rate of property development. Pre-soaking

crumb rubber in asphalt slightly improved the G* and d

values of all particle sizes.

The two most important aspects related to terminal

blend binders are performance-related properties and

binder stability (compatibility). Table 2 lists final Super-

pave performance testing of selected binders produced for

this study. Table 2 presents the benefits of adding crumb

rubber at the suggested low percentage in improving the

high-temperature binder grade from PG 58-28 to PG 70-22

through controlling the interaction parameters and using

the SBS polymer. An advantage of adding SBS is to

improve elastic properties. The CRM binders of Table 2

achieved 55–60% elastic recovery (ASTM D6084-04)

while controlling viscosity (ASTM D4402) at an

acceptable range (1.10–1.30 Pa s).

8. Storage stability and binder compatibility

Binder compatibility was evaluated using the separation

tube test according to ASTM D5892-00. The results are

presented in Figure 9. The results of this study suggest

that the main factor affecting stability is the shearing

speed. At 30 Hz, all binders have similar and high

separation values. Increasing the shearing speed to 50 Hz

improves binder stability and produces values that are

comparable to those of patent or proprietary products

(Abdelrahman 2006). Particle size shows an effect on

binder stability at high shearing speed. The effect of


Figure 8. Property development by pre-soaking at 50 Hz and 2008C.


Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

22:

22 1

2 M

arch

201

3

Table 2. Performance testing on original and modified CRM binders.

Modified PG 70-22

AASHTO M 320 Measured parameter/test methodTesting

temperature (8C)Original

PG 58-2840–60,

2008C, 30 Hz60–80,

2008C, 50 Hz

Dynamic shearrheometer (DSR)

Unaged G*/sin d, at test temperature(8C), AASHTO T 315

58 1.607 kPa 3.74 kPa 5.38 kPa70 1.10 kPa 1.16 kPa

Unaged d, at test temperature(8C), AASHTO T 315

58 86.68 76.18 76.4870 82.68 82.38

RTFOT-aged G*/sin d, at test temperature(8C), AASHTO T 315

58 3.177 kPa 8.01 kPa 11.14 kPa70 2.45 kPa 2.9 kPa

PAV-aged G*/sin d, at test temperature(8C), AASHTO T 315

19 3246 kPa 3701.6 kPa 3927.9 kPa28 917 kPa 1272.2 kPa

Bending beamrheometer (BBR)

Creep stiffness at test temperature(8C), AASHTO T 313

218 262.067 MPa 231.2 MPa 241.36 MPa212 114.2866 MPa 119.4 MPa

m-value at test temperature(8C), AASHTO T 313

218 0.3312 0.278 0.273212 0.34535 0.33795

Figure 9. Development of binder stability.


Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

22:

22 1

2 M

arch

201

3

interaction temperature on binder stability is not

evaluated in this study because none of the developed

binders is subjected to high temperatures for extended

interaction periods.

9. Monitoring the change in crumb rubber particle

sizes

SEM was used in this study to investigate the degree of

dispersion of the rubber particles with the asphalt and to

investigate the change in rubber particle sizes during the

interaction process. Figure 10 presents SEM images on

sample 3 in Table 1 at two different stages of interaction

(10 min and 8 h). The original rubber particles in this

sample were 40–60 (250–425mm). Figure 10(a) shows

that high shearing was not enough to reduce the size of the

swollen rubber particles after 10 min of the interaction at

2008C, causing the binder stiffness to increase. Figure

10(b) shows the effect of the time–temperature–shearing

combination on the particle sizes and the rubber

concentration. Swelling was reduced and significant

amount of the rubber has been dispersed in the asphalt

matrix. Binder modifications can be attributed to the

partial depolymerisation of rubber and/or other activities.

Agglomeration was not considered as the main possible

reason for increasing G* in this case because of the use of

high shear processing on a low rubber content. Shenoy

stated ‘It is known that high shear mixing helps in reducing

the agglomerates whereas low shear mixing may at times

increase the number of agglomerates’ (Shenoy 1999).

High shear provides the energy to break particle–particle

bonds. Once the polymer wets the particle, the bond fails

to re-form, leading to better dispersion of the filler in the

polymer matrix (Shenoy 1999).

Physical rubber concentration was examined after 8 h

of interaction by extracting the rubber from the binder

using trichloroethylene, ASTM D2172 (Method B using a

filter paper with a retention of 6.0mm). For sample 1 of

Table 1, the results showed that the remaining rubber was

only 0.6% of the total asphalt weight compared to 5%

rubber added at the beginning of the interaction. Figure 11

Figure 10. SEM image of 40–60 crumb rubber and asphaltafter interaction at 50 Hz and 2008C.

Figure 11. SEM image of extracted 40–60 crumb rubber afterinteraction with asphalt at 50 Hz and 2008C.


Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

22:

22 1

2 M

arch

201

3

presents images of the rubber particles after extraction

from the asphalt at the same interaction times (10 min and

8 h). It shows that rubber particle sizes after 10 min are

greater than the rubber particle sizes after 8 h. This could

be attributed to partial depolymerisation of the rubber

particles caused by the combined effects of temperature

and high shearing.

10. Conclusions

This paper demonstrates the possibility of producing high-

performance terminal blending CRM binders suitable for

Superpave applications through a modified wet process.

This paper suggests a concept of very fine crumb rubber

particles dispersed and/or suspended in the asphalt liquid

phase. The suggested approach balances the development

of performance-related properties and compatibility or

storage stability. A main advantage of the suggested

approach is energy savings. Only 8% of the total

interaction time requires high temperature (2008C).

The rest of the interaction period is at relatively lower

interaction temperature (1708C). Hence, a lower storage

temperature can be used.

The study confirms that temperature controls the

interaction mechanism even with the addition of the SBS

polymer. Particle size has a significant effect on the

development of the performance properties. The coarser

the particle size, the higher the G* value and the lower the d

value under the same interaction conditions. The shearing

speed was found to improve storage stability and to

disperse the rubber particles in the binder matrix.

The rubber content was reduced from 5% at the beginning

to 0.6% by the end of the interaction process.

Acknowledgements

The authors would like to acknowledge the NDSU CivilEngineering Department for its support of this research.Nebraska Department of Roads is greatly appreciated for itscontributions.

References

Abdelrahman, M.A., 2006. Controlling the performance ofcrumb rubber modifier (CRM) binders through the additionof polymer modifiers. Transportation Research Record,1962, 64–70.

Abdelrahman, M.A. and Carpenter, S.H., 1998. Controlling lowtemperature properties of asphalt rubber binders. In: 2ndtransportation specialty conference, Canadian Society ofCivil Engineering, Halifax, Nova Scotia, Canada.

Abdelrahman, M.A. and Carpenter, S.H., 1999. The mechanismof the interaction of asphalt cement with crumb rubbermodifier (CRM). Transportation Research Record, 1661,106–113.

Asphalt Institute Research Center, 1993. Background of SHRPasphalt binder test methods. Demonstration Project 101,National Asphalt Training Center, Lexington, Kentucky.

Bahia, H., 1995. Critical evaluation of asphalt modification usingthe SHRP concepts. In: The 74th Annual Meeting of theTransportation Research Board, Washington DC.

Bahia, H.U. and Davies, R., 1994. Effect of crumb rubbermodifiers (CRM) on performance-related properties ofasphalt binders. Journal of the Association of Asphalt PavingTechnologists, 63, 414–449.

BAS Engineering Consultants, 1992. Evaluation of the use ofrubber modified asphalt concrete pavement in the state ofIllinois. Submitted to Illinois Department of Energy andNatural Resources.

Billiter, T., et al., 1995. Physical properties of asphalt–rubberbinders. In: The 74th Annual Meeting of the TransportationResearch Board, Washington DC.

Billiter, T.C., et al., 1997. Production of asphalt–rubber bindersby high-cure conditions. Transport Research Record, 1586,50–56.

Buckly, D. and Berger, M., 1962. The swelling of polymersystem in solvents. Journal of Polymer Science, 56, 175.

Chehovits, J., 1993. Binder design procedures. Session 9.0,crumb rubber modifier workshop notes, Federal HighwayAdministration, Office of Technology Applications.

Chehovits, J. and Hicks, R., 1993. Mix design procedures.Session 10.0, crumb rubber modifier workshop notes, FederalHighway Administration, Office of Technology Appli-cations.

Chehovits, J., Dunning, R., and Morris, G., 1982. Characteristicof asphalt–rubber by the sliding plate microviscometer.Journal of the Association of Asphalt Paving Technologists,51, 241.

Green, E. and Tolonen, W., 1977. The chemical and physicalproperties of asphalt-rubber mixtures, part I – basic materialbehavior. Arizona: Arizona Department of Transportation,FHWA-AZ-HPR14-162.

Heitzman, M., 1992. State of the practice-design and construc-tion of asphalt paving materials with crumb rubber modifier.USA: Federal Highway Administration, Report FHWA-SA-92-022.

Lalwani, S., Abushahada, A., and Halasa, A., 1982. Reclaimedrubber-asphalt blends measurements of rheological proper-ties to assess toughness, resiliency, consistency andtemperature sensitivity. Proceedings of the association ofasphalt paving technologists, 51.

McGennis, R., 1995. Evaluation of physical properties of finecrumb rubber modified asphalt binders. Transport ResearchRecord, 1488, 62–71.

Oliver, J., 1981. Modification of paving asphalts by digestionwith scrape rubber. Transport Research Record, 821, 37.

Pavlovich, R., Shuler, T., and Rosner, J., 1979. Chemical &physical properties of asphalt-rubber, phase II productspecification and test procedures. Arizona: Arizona Depart-ment of Transportation, FHWA/AZ-79/121.

Rouse, M., 1994. Application of crumb rubber modifiers (CRM)in asphaltic materials. In: The Meeting of the AmericanChemical Society, Rubber Division, Chicago, IL.

Shashidhar, N., Needham, S., and Chollar, B., 1995. Rheologicalproperties of chemically modified asphalts. In: The 74thAnnual Meeting of the Transportation Research Board,Washington DC.

Shenoy, A.V., 1999. Rheology of filled polymer systems.Netherlands: Kluwer Academic Publishers.


Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

22:

22 1

2 M

arch

201

3

Shenoy, A., 2001. Reifinment of the superpave specificationparameter for performance grading of asphalt. Journal ofTransportation Engineering, 127 (5), 357–362.

Shenoy, A., 2004. A comprehensive treatise of the high temperaturespecification parameter jG*j/(1 2 (1/tan d sin d)) for perform-ance grading of asphalts. Applied Rheology, 14 (6), 303–314.

Strategic Highway Research Program, 1993. Binder character-ization and evaluation. Vol 2: Chemistry. Washington DC:National Research Council, SHRP-A-368.

Stroup-Gardiner, M., Newomb, D., and Tanquist, B., 1993.Asphalt–rubber interactions, Transportation Research BoardAnnual Meeting, January.

Takallou, H. and Sainton, A., 1992. Advances in technology ofasphalt paving materials containing used rubber. TransportResearch Record, 1339, 23.

Western Research Institute, 1994. Fundamental properties ofasphalts and modified asphalts. Semi-Annual TechnicalReport. USA: Federal Highway Administration.


Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

22:

22 1

2 M

arch

201

3

enhancing the performance of crumb rubber-modified binders through varying the interaction...

Documents