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
2
Professor,
Norwegian University of Technology and Science: Civil Engineering Department,
Lerkendalsbygget 2-051, Høgskoleringen 7a, 70347491, Trondheim, Norway
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:
Effect of Crumb Rubber Modification on Binder-Aggregate Adhesion 4
“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)
Asphalt Rubber Conference 2012 5
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.
Effect of Crumb Rubber Modification on Binder-Aggregate Adhesion 6
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
Asphalt Rubber Conference 2012 7
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.
Effect of Crumb Rubber Modification on Binder-Aggregate Adhesion 8
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
Asphalt Rubber Conference 2012 9
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
Effect of Crumb Rubber Modification on Binder-Aggregate Adhesion 10
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.
Asphalt Rubber Conference 2012 11
(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
Hydrated limey = -0,1895x + 100
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 (
%)
Rolling time (hours)
Aminey = -0,466x + 100
R² = 0,9469
Control y = 100e-0,025x
R² = 0,8687
Hydrated limey = -0,1674x + 100
R² = 0,9836
0
20
40
60
80
100
0 20 40 60 80 100 120 140
Covera
ge (
%)
Rolling time (hours)
Effect of Crumb Rubber Modification on Binder-Aggregate Adhesion 12
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.
Asphalt Rubber Conference 2012 13
(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 (
%)
Crumb rubber concentration (%)
0
50
100
0 5 10 15 20 25Covera
ge (
%)
Crumb rubber concentration (%)
0
50
100
0 5 10 15 20 25Covera
ge (
%)
Crumb rubber concentration (%)
Effect of Crumb Rubber Modification on Binder-Aggregate Adhesion 14
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.
Asphalt Rubber Conference 2012 15
0 % rubber – No ASA 0 % rubber – Hydrated
lime 0 % rubber – Amine
10 % rubber – No ASA 10 % rubber – Hydrated
lime 10 % rubber – Amine
20 % rubber – No ASA 20 % rubber – Hydrated
lime 20 % rubber – Amine
Figure 6: Asphalt binder stripping after 120 hours in rolling bottle test
Effect of Crumb Rubber Modification on Binder-Aggregate Adhesion 16
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
Asphalt Rubber Conference 2012 17
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
Crumb rubber content (%)
-2000%
0%
2000%
0 10 20
∆∆ ∆∆covera
ge d
ue
to a
dd
itiv
es
Crumb rubber content (%)
Effect of Crumb Rubber Modification on Binder-Aggregate Adhesion 18
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
Asphalt Rubber Conference 2012 19
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.
Effect of Crumb Rubber Modification on Binder-Aggregate Adhesion 20
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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