effect of crumb rubber modification on binder-aggregate coating

Effect of crumb rubber modification on binder-aggregate coating

Carl Christian Thodesen1 & Inge Hoff

2

1

Research Scientist,

SINTEF Road and Railway Engineering,

Høgskoleringen 7A, 7465 Trondheim, Norway

[email protected]

2

Professor,

Norwegian University of Technology and Science: Civil Engineering Department,

Lerkendalsbygget 2-051, Høgskoleringen 7a, 70347491, Trondheim, Norway

[email protected]

ABSTRACT:

Binder-aggregate adhesion is a property of critical importance with regards to pavement

durability. As such transportation agencies often have specific requirements with regards to

aggregate-binder adhesion. In many Nordic countries a suite of specifications have been

developed to evaluate the adhesive properties of various modifiers. Due to the limited use of

crumb rubber modified binders in Nordic countries, little data is available with regards to

how asphalt rubber and other crumb rubber modified binders fare when evaluated using

these test methods. While asphalt rubber is known to decrease rutting and increase pavement

flexibility, to date few studies exist which evaluate how the addition of crumb rubber may

affect adhesion between aggregate and binder.

The purpose of this investigation was to evaluate how crumb rubber modification of

binders affects aggregate-binder adhesion when evaluated using the rolling bottle test

method. Specifically, the investigation will establish how binder-aggregate adhesion of

crumb rubber modified binders varies depending on the type of anti strip additive (hydrated

lime and amine) used.

Binder-aggregate adhesion was evaluated after time intervals of 6, 24, 48, and 120

hours to gain an idea of how stripping develops as a function of time in the rolling bottle

evaluation. Testing involved the evaluation of a reference binder, a 10% CRM binder, and an

asphalt rubber binder. A Norwegian aggregate with average quartz content was mixed with

all the above mentioned binders, in addition samples were also prepared using 1.5%

hydrated lime and 0.5% liquid ASA. Through this evaluation a comprehensive picture of how

rubber modification affects aggregate-binder adhesion was obtained.

KEYWORDS: Asphalt rubber, adhesion, rolling bottle test, hydrated lime, amine

Effect of Crumb Rubber Modification on Binder-Aggregate Adhesion 2

1. Introduction

1.1 Background

Moisture damage of asphalt pavements is an issue that is of concern to

transportation agencies, specifically in Norway where the harsh winters

and significant quantities of snow can have a negative effect on asphalt

pavements. For this reason the use of anti-strip additives (ASAs) are now

mandatory in all Norwegian asphalt mixes, where the specified ASA is

generally a liquid amine. In recent years the use of polymer modified

binders has also increased in Norway, specifically in areas where excessive

levels of permanent deformation were problematic. In fact the quantity of

PMB used has risen dramatically since 2005, when less than 0.5% of all

asphalt pavements required PMB to 2010 when 13% of all pavements built

by the Norwegian Public Roads Administration (NPRA) specified PMB

(Jørgensen, 2010). In addition to this, the NPRA has also expressed an

interest in developing road construction technologies that are more

sustainable. Recycling of asphalt pavements has gained some interest

(Ruud & Dørum, 2004), but so too has the potential for using crumb rubber,

specifically in regards to its use in porous pavements (Lerfald, 2008;

Snilsberg et al., 2003).

While crumb rubber is generally not specified as an adhesion promoter, the

summary report from the TRB national seminar on moisture sensitivity of

asphalt pavements identifies binder stiffness (viscosity and the use of

modifiers) as a potential cause for moisture related stress. In addition to

this binder film thickness is mentioned as a key factor in determining

moisture sensitivity of asphalt pavements (D'Angelo, 2003). The

conclusion of this seminar states that a research gaps in this field was

whether or not the use of asphalt rubber affects the moisture susceptibility

of a pavement.

Independent studies have suggested that mixtures produced with SBS and

EVA polymer produced HMA mixtures with reduced stripping potential

Asphalt Rubber Conference 2012 3

and moisture susceptibility compared with HMA mixtures prepared with

conventional asphalt binder (Gorkem & Sengoz, 2009). Additionally it has

been shown there is a strong correlation between Tensile Strength Ratio

(TSR) values and the asphalt binder film thickness. (Sengoz & Agar, 2006).

Therefore, this study identified crumb rubber modifier as a candidate for

producing moisture resistant binder-aggregate combinations due to its

ability to form thicker film thicknesses (Turgeon, 1992) and due its

significantly increased viscosity (Thodesen et al., 2008). According to

Hicks (1987), high viscosity asphalt cements generally resist displacement

by water to a greater degree than low viscosity asphalts; therefore given the

significant increases in viscosity due to rubber modification, effects on

stripping should be visible as rubber concentration increases (Figure 1).

Figure 1: Effect of crumb rubber concentration and type on asphalt binder

Brookfield viscosity at 135oC (Thodesen et al., 2008)

1.2 Asphalt stripping

In asphalt pavements stripping of the asphalt binder from the mineral

aggregate is an indication of moisture damage within the asphalt-aggregate

system. Kiggundu and Roberts (1988) define stripping as:


“The progressive functional deterioration of a pavement mixture by loss of

the adhesive bond between the asphalt cement and the aggregate surface

and/or loss of the cohesive resistance within the asphalt cement principally

from the action of water.”

Practically speaking, this equates to a removal of the glue that holds the

stones together, and is also the starting point for numerous other pavement

deterioration mechanisms.

1.2.1 Mechanisms

There are a number of proposed mechanisms by which moisture removes

the asphalt film from the aggregate, according to reputable sources (Shell

Bitumen, 2003; Kiggundu & Roberts, 1988) these include: Detachment,

displacement, spontaneous emulsification, pore pressure, and hydraulic

scouring.

1.2.2 Causes

Stripping can be influenced by many of the variables present in an asphalt

pavement. As seen in Table 1, research has narrowed the factors

influencing moisture damage down to some principal contributing factors.

Table 1: Factors influencing moisture related distresses (Hicks et al., 2003)


This study will address mix design issues by addressing the use of rubber

and ASAs. The evaluation of these additives on asphalt stripping will be

evaluated through the use of the rolling bottle test.

2. Objectives

The objectives of this research project are to evaluate the following: issues:

• Does crumb rubber modification of the binder affect aggregate-

binder adhesion?

• How are current anti-strip additives (amine and hydrated lime)

affected by the use of crumb rubber modified binder?

• Which combination (rubber concentration and ASA) yields the

least stripping susceptible mix?

• Which combination yields the optimum results with regards to

Norwegian rolling bottle requirements?

3. Experimental materials and methods

3.1 Experimental plan

The experimental plan is illustrated in Figure 2 and includes the evaluation

of three (0,10%, and 20%) rubber concentrations in conjunction with three

ASA alternatives (no ASA, Amine, and Hydrated lime). The aggregate-

binder coverage of these was evaluated after five (0, 6, 24, 48, and 120

hours) time periods.


Figure 2: Experimental setup

3.2 Materials

During testing, materials often used in Norwegian asphalt pavements were

evaluated. The aggregate material was from Ottersbo in Norway, while the

binder used was a Nynas 70/100 which fulfilled the requirements specified

in Table 2. The crumb rubber was obtained from Svevia in Sweden, with a

gradation falling within the specifications of conventional rubber for

asphalt rubber projects. The crumb rubber was added at the specified

concentrations to the binder which was preheated to 177oC, subsequently

the rubber and the binder were blended in a high shear mixer for 30

minutes. Following this the binder was blended with the aggregate to

Binder A Aggregate A

0 % rubber 10 % rubber 20 % rubber

No

ASA

Amine Hydrated

lime

0

hours

6

hours

24

hours

48

hours

120

hours

Same

as 0 %

rubber

Same as

no ASA

Materials

Rubber

content

ASA

Testing

time


produce the necessary samples for rolling bottle testing.

Table 2: Specified properties of 70/100 binder used in testing (Nynas, 2012)

Test Method Unit Min. Max.

Penetration at 25oC NS-EN 1426 Mm/10 70 100

Ring and ball softening point NS-EN 1427 oC 43 51

Flashpoint (COC) NS-EN-ISO 2592 oC 230

Solubility NS-EN 12592 % weight 99.0

Viscosity at 60oC NS-EN 12596 Pa s 90.0

Kinematic viscosity at 135 oC NS-EN 12595 mm2/s 230

Fraas NS-EN 12593 oC -10

Resistance to hardening at 163oC

Change in mass NS-EN 12607-1 % weight 0.8

Retained penetration NS-EN 1426 % 46

Softening point after hardening NS-EN 1427 oC 9.0

The hydrated lime was added using a slurry method at a concentration of

1.5% by weight of total mix. The amine was added directly to the binder at

a concentration of 0.5% by weight of total mix in accordance with NPRA

specifications (Håndbok 018, 2011).

3.3 Test method

Evaluation of the aggregate-binder adhesion was evaluated using the

rolling bottle test specified in NS-EN 12697-1. For ASAs to be accepted

for use by the NPRA they need to exceed a 25% binder coverage rate after

48 hours rolling time (Håndbok 018, 2011).

The rolling bottle test, along with the indirect tensile strength (ITS) test

(NS-EN 12697-12, 2003) and the active adhesion test form the basis of

NPRA requirements for anti-strip additives. However, experience has

shown that the requirement that is the most difficult to achieve with out the

use of ASAs is the rolling bottle test.


The rolling bottle test involves blending 510 g of aggregate with 16g of

bitumen at 165oC. Following this the blended material is divided into three

equal parts of 150±2 g. These individual samples are then transferred into

individual bottles with distilled water. The bottle in this test includes a

glass rod inside to ensure continual exposure of the aggregate-bitumen

mixture to water as well as to prevent clumping of the material. The binder-

aggregate mixture was then removed from the bottle at intervals of 6, 24,

and 48 hours to evaluate the stripping development of the mixture. For the

purposes of this investigation an extreme time interval of 120 hours was

also included to evaluate the long term effects of the various blends.

The actual visual evaluation of the mixture involved the individual visual

evaluation by three operators, the final coverage score was the average of

the three values given by the three individual operators. Scoring was done

whereby a mix was considered to have 0% coverage when the whole mix

resembled the aggregate particle in Figure 3 (a) and considered to have 100%

coverage when the particles resembled Figure 3 (b), however, most

particles fell somewhere between 0 and 100% and were thus judged on

how much coverage the blend was perceived to have.

(a) (b)

Figure 3: Aggregate particles with (a) 0% and (b) 100% coverage


4. Experimental results and discussion

Following completion of the testing the results were analyzed, initially the

coverage curves were studied to evaluate which ASA-rubber combinations

yielded values fulfilling the NPRA requirement of 25% after 48 hours.

Models were also fitted to the various alternatives to evaluate uniformity of

the various options.

A visual analysis was conducted to see how the details of the coverage rate

varied from combination to combination. In other words, did rubber

concentration and ASA types dictate how the aggregate was covered? If so

could it be concluded that specific combinations proved incompatible.

The effects of rubber concentration were evaluated to see if a correlation

could be drawn between rubber concentration and coverage. Doing so

would provide quantification of the relative effects of using rubber with

ASAs. Moreover, doing so would provide an indication of the effects of

binder viscosity on moisture sensitivity. Finally recommendations were

made with respect to suitability to NPRA specifications.

4.1 Numerical evaluation of coverage

From Figure 4 it can clearly be seen that the hydrated lime and amine

samples both comfortable achieved the 25% coverage after 48 hours

specified by the NPRA. The ability of the mixtures with ASAs to achieve

this requirement did not appear to be dependent on crumb rubber

concentration. However, generally speaking, the samples prepared with

hydrated lime tended to achieve the highest coverage rates, deviations

became more apparent as the rolling time increased. From this evaluation it

can be concluded that both 10 and 20% rubber can be used in conjunction

with ASAs to fulfill the NPRA rolling bottle requirement.

Models were fitted to the various sample results, from this it was seen that

both ASA samples underwent a linear stripping rate with respect to rolling


time, while for the samples not using ASAs the exponential model was

found to provide the best fit. For all the models an initial coverage of 100%

was used. The highest R2 values for each individual category were found at

10%, followed by 20% and finally with no rubber. This indicates that the

stripping follows the modeled rate best when rubber modification was used.

A potential benefit of increased adherence to models lays in improved

accuracy of pavement performance prediction models, whereby if materials

more accurately follow a model, maintenance costs can be more accurately

estimated. The findings from this study provide an indication that when

rubber and ASAs are used together a higher level of consistency in

coverage results is achieved.

The models also suggest that the rate of loss of coverage for amine samples

with respect to rolling time is approximately twice that of hydrated lime.

For the amine samples the rate increases as rubber content increases, while

for the samples with no ASA there appears to be no effect on the modeled

rate of loss. These findings indicate that crumb rubber modification does

not improve the moisture sensitivity of aggregate-binder blends. However,

a clear trend can be seen whereby increasing rubber contents negatively

affects blends where amine has been used. The effects of rubber when

hydrated lime is used are less significant.


(a)

(b)

(c)

Figure 4: Binder coverage vs. time for (a) 0% crumb rubber, (b) 10% crumb rubber,

and (c) 20% crumb rubber.

Controly = 100e-0,03x

R² = 0,7037

Hydrated limey = -0,1393x + 100

R² = 0,8792

Aminey = -0,3448x + 100

R² = 0,8974

0

20

40

60

80

100

0 20 40 60 80 100 120 140

Coverage (

%)

Rolling time (hours)

Controly = 100e-0,027x

R² = 0,9306


R² = 0,9847

Aminey = -0,387x + 100

R² = 0,9801

0

20

40

60

80

100

0 20 40 60 80 100 120 140

Covera

ge (

%)


Aminey = -0,466x + 100

R² = 0,9469

Control y = 100e-0,025x

R² = 0,8687


R² = 0,9836

0

20

40

60

80

100

0 20 40 60 80 100 120 140

Covera

ge (

%)



4.2 Evaluation of crumb rubber at different times

From Figure 5 the effect of crumb rubber on aggregate-binder adhesion

becomes more evident. The samples prepared with hydrated lime appeared to

be the least variable with respect to crumb rubber content. For the 6, 24, and 48

hour tests there was no discernible difference between the rubber

concentrations for the hydrated lime samples. This indicates that when hydrated

lime is used, rubber modification does not alter the adhesive properties of the

binder and aggregate.

When evaluating the samples prepared with amine it was apparent that the

coverage level while high, was still lower than that that of the hydrated lime. In

addition to this it can be seen that the amine sample coverage rate also

fluctuated more with respect to the crumb rubber content than did the hydrated

lime samples.

Perhaps the most surprising findings came from the samples with only crumb

rubber. As seen in Figure 5 (b) it is visible that as crumb rubber content

increases coverage decreases. However, 24 hours later that trend is reversed

with increasing crumb rubber contents resulting in increased coverage.

A potential reason for the decrease in coverage for crumb rubber only samples

lies in the fact that as the same binder (binder + rubber) content was used for

both the crumb rubber modified binder samples, there was in reality less binder

for the 20% to attach to the aggregate than there was in the 10% binder sample.

However, after the loose binder was washed away between the 24 and 48 hour

test intervals the remaining modified binder was in fact more resistant to

stripping.


(a)

(b)

(c)

(d)

Figure 5: Binder coverage vs. crumb rubber concentration for (a) 6 hours, (b) 24 hours,

(c) 48 hours, and (d) 120 hours

0

50

100

0 5 10 15 20 25Coverage (

%)

Crumb rubber concentration (%)

0

50

100

0 5 10 15 20 25Coverage (

%)


0

50

100

0 5 10 15 20 25Covera

ge (

%)


0

50

100

0 5 10 15 20 25Covera

ge (

%)



4.3 Visual evaluation of coverage

When visually evaluating the various alternatives it appears that the binder-

aggregate stripping/coverage method is dependent on the ASA used. Consistent

with previous findings it appears to be less dependent on crumb rubber

modification.

Generally speaking, Figure 6 suggests that upon completion of 120 hours in the

rolling bottle test the aggregate areas most stripped are the edges of the

aggregate. This is likely due to the abrasive action of aggregates rubbing

against each other during the rolling period. When examining some particles

closer it is clear that for some mixtures edge stripping due to abrasion appears

to be the predominant factor promoting stripping even after 120 hours in the

rolling bottle. As shown in Figure 7, when hydrated lime was used with no

rubber, most stripping occurred along the edges. The effect of 20% crumb

rubber on ASA samples is seen Figure 8, the wear on the edges of the aggregate

particles is consistent with the wear illustrated in Figure 7.

With regards to rubber modification, the principal difference occurring from

other mixtures lies in the "clumping" of the binder along sections of the

aggregate surface. As seen in Figure 9, these clumps are large in size and upon

closer inspection contain rubber particles. The clumping is not seen to occur

when rubber is used in conjunction with the hydrated lime or the amine. These

findings indicate that when the binder is modified with rubber that anti-

stripping behavior is still dictated by the type of ASA used.

The clumping of the rubber and binder does however combine to form a thicker

film thickness which in turn does appear to resist stripping. However, the

increased film thickness does not appear to promote aggregate-binder bonding,

rather it just slows down the rate at which bare aggregate is exposed. These

findings do not however indicate that the addition of rubber at 10 or 20% affect

the ability of the ASAs to resist stripping.


0 % rubber – No ASA 0 % rubber – Hydrated

lime 0 % rubber – Amine





Figure 6: Asphalt binder stripping after 120 hours in rolling bottle test


Figure 7: Edge stripping of hydrated lime with 0% rubber after 120 hours

(a) (b)

Figure 8: 20% crumb rubber after 120 hours with (a) hydrated lime and (b) amine

Figure 9: Clumping of binder to rubber particles 10% rubber no ASA after 120 hours


4.4 Effect of crumb rubber on ASAs

To quantify the combined effects of the various components, Equation 1 was

used to calculate the change in coverage. Figure 10 charts the variation in the

coverage as a function of time, rubber content, and ASA used.

∆ �� =�� ,% �� − �� ,�% ��

�� ,�% ��

Eqn. 1

(a)

(b)

(c)

Figure 10: Difference in coverage due to anti strip additives and rubber at different

concentrations for (a) no anti strip additive, (b) hydrated lime, and (c) amine.

-2000%

0%

2000%

0 10 20

∆∆ ∆∆coverage

du

e t

o

ad

dit

ives

Crumb rubber content (%)

-2000%

0%

2000%

0 10 20

∆∆ ∆∆covera

ge

du

e t

o

ad

dit

ives


-2000%

0%

2000%

0 10 20

∆∆ ∆∆covera

ge d

ue

to a

dd

itiv

es



From Figure 10 it is evident that the benefits of using ASAs become apparent

after 48 hours rolling time, prior to this most of the alternatives do not

demonstrate significant differences. These findings confirm the suitability of

the NPRA specification to be set for the coverage rate after 48 hours as this is a

significant turning point with respect to aggregate-binder coverage. It should

also be noted that in the context of this analysis, the significant changes are of

the order of magnitude of approximately 1400%, thus demonstrating the

sizeable effects of ASAs on resisting stripping.

This analysis also yields quantification of the binder "clumps" that were noted

in the 10 and 20% crumb rubber blends. Figure 10 illustrates that significant

increases in coverage are noted after 48 hours and 120 hours for the crumb

rubber samples. These increases are the result of the concentrated crumb rubber

binder spots with increased film thickness that have formed on the surface of

the aggregate. The results do not indicate an increase in frequency of these

spots with rubber concentration.

The results therefore indicate that crumb rubber at concentrations of both 10

and 20% can be successfully used in conjunction with both hydrated lime and

amine to improve the stripping properties of aggregate-binder blends. Generally,

mixes containing crumb rubber performed slightly better with hydrated lime

than with amine.

5. Conclusions

The objective of this study was to evaluate the use of crumb rubber modifier as

an ASA as well as to evaluate its effect on use with other ASAs such as

hydrated lime and amine. Moreover it was attempted to evaluate whether or not

it was possible to increase stripping resistance by increasing the binder

viscosity through crumb rubber modification. Following the evaluation of

combining 0%, 10%, and 20% crumb rubber with no ASA, 1.5 % hydrated lime,

and 0.5% amine the following conclusions were reached:

• Hydrated lime and amine can be used with rubber concentrations of


0%, 10%, and 20% to successfully achieve the minimum rolling bottle

requirements of 25% coverage after 48 hours set by the NPRA.

• The use of rubber alone did not satisfy the binder coverage

requirements of 25% after 48 hours when using the rolling bottle test,

as such crumb rubber modification alone cannot be considered a

sufficient method for improving aggregate-binder adhesion.

• Increasing the viscosity of the asphalt binder through the addition of

crumb rubber does not lead to any apparent increase in the aggregate-

binder adhesion when the rolling bottle test is used to evaluate

aggregate binder adhesion. However, the addition of rubber to the

binder can cause thicker film thicknesses to occur in some spots and

lead to increased coverage.

• The stripping of blends using ASAs followed a linearly decreasing

model with respect to rolling time. The stripping of mixes without

ASAs followed an exponentially decreasing model with respect to

rolling time. The blends incorporating rubber had higher goodness of

fit values than the blends with no rubber, regardless of ASA.

• Visual examination of the aggregate samples suggests that large parts

of the stripping occurring in rolling bottles test for ASA samples occur

due to aggregate abrasion, rather than due to the fact that water has

penetrated the binder film.

• Crumb rubber modification is more compatible with hydrated lime as

binder coverage levels remained stable with increasing rubber contents,

while amine modification yielded slight decreases in binder coverage

with increasing rubber contents.


6. Acknowledgements

The authors wish to acknowledge the NPRA for funding portions of this research.

The authors also wish to thank Lisbeth Johansen, Stein Hoseth, Eirik Ohma

Solberg, Bruck Haile, and Brhane Yzgaw for their work on the project.

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