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Development of a polymer-modified
bitumen specification based on
empirical tests – case study for Sri
Lanka
Hansinee Sakunthala Sitinamaluwaa & Wasantha Kumara
Mampearachchia
a Department of Civil Engineering, University of Moratuwa,
Katubedda, Moratuwa 10400, Sri Lanka
Published online: 29 Apr 2014.
To cite this article: Hansinee Sakunthala Sitinamaluwa & Wasantha Kumara Mampearachchi (2014)
Development of a polymer-modified bitumen specification based on empirical tests – case study for
Sri Lanka, Road Materials and Pavement Design, 15:3, 712-720, DOI: 10.1080/14680629.2014.909873
To link to this article: http://dx.doi.org/10.1080/14680629.2014.909873
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Road Materials and Pavement Design, 2014
Vol. 15, No. 3, 712–720, http://dx.doi.org/10.1080/14680629.2014.909873
Development of a polymer-modified bitumen specification based on
empirical tests – case study for Sri Lanka
Hansinee Sakunthala Sitinamaluwa∗and Wasantha Kumara Mampearachchi
Department of Civil Engineering, University of Moratuwa, Katubedda, Moratuwa 10400, Sri Lanka
(Received 28 June 2013; accepted 25 March 2014 )
Pavements with polymer modification exhibit greater resistance to permanent deformation,
less thermal cracking, less fatigue damage and less temperature susceptibility. Implementation
of polymer-modified bitumen (PMB) is currently taking place in developing countries and
the absence of PMB specification has always been a constraint. This research was aimed at
developing a testing procedure for PMB, based on test methods that are currently available in
Sri Lankan laboratories. The test methods were selected considering the adequate control of
binder properties during application and usage. Penetration test is included to control the inter-
mediate temperature properties and identify binder grades. Softening point test controls the
high-temperature properties while viscosity test controls the mixing and compaction tempera-
tures. Elastic recovery test and solubility test were employed in order to identify the presence
of polymer in PMB. Storage stability test determines the separation tendency of polymer from
bitumen. Flash point limits are set for the application safety. Thus all the essential parameters
of bitumen are controlled by the proposed specification. The acceptance limits are determined
considering different PMB specifications of several other countries, past research outcomes
and laboratory test results. The proposed specification which is based on empirical test meth-
ods facilitates adequate quality control of PMB and it would be a useful guideline for the
implementation of PMB for hot mix asphalt in Sri Lanka.
Keywords: polymer-modified bitumen; PMB; specification
1. Introduction
Bitumen is one of the main materials used in the construction of hot mix asphalt (HMA) pave-
ments, which has a critical influence on pavement performance (Shell Bitumen, 2003). Though
the conventional bitumen performed satisfactorily over many years, today better performance
is expected from the HMA pavements, including less maintenance and longer service life. The
problems related to deficiencies of bitumen properties can be addressed by enhancing the binder
properties using necessary modifiers (Awwad & Shbeeb, 2007;Becker, Mendez, & Rodriguez,
2001;RTANSW, 2010;Shell Bitumen, 2003). However, careful attention must be given for
the control of polymer-modified bitumen (PMB) properties during application. Due to the less
compatibility of bitumen and polymer, many problems can occur resulting in poorly constructed
pavements (AAPA, 2004;RTANSW, 2010). Therefore, strict quality control measures are needed
to be employed when using PMB in pavement construction.
Testing and quality control of PMB has always been a challenging task due to complex thermo-
rheological nature of the modified bitumen. Even the latest and most sophisticated technologies
∗Corresponding author. Email: hansiuom@gmail.com
© 2014 Taylor & Francis
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Road Materials and Pavement Design 713
currently in existence exhibit certain inadequacies in assessing PMB properties (Bahia et al.,
2001). The U.S. performance graded (PG) binder specification was developed to handle modified
binders, but has serious shortcomings in this area. As a result, most of U.S. states have introduced
extra specifications for polymer-modified binders (Austroads, 2006). However, for a developing
country such as Sri Lanka, the most feasible solution is to come up with PMB specification with
available empirical test methods, which gives adequate control on bitumen properties. Particu-
larly in Europe, empirical tests have always been considered as indirectly related to pavement
performance (van de Ven, Jenkins, & Bahia, 2004).
During the last three decades, the use of PMB was widespread and PMB specifications have
been designed in many countries. New specifications have been designed and pre-existing ones
have been modified to capture the rheological properties of PMB. Tests such as elastic recovery
and torsional recovery were developed, but many researchers claim that most of these test methods
only identify the presence of polymer modification but does not give an acceptable indication of
pavement performance. Therefore, the test methods to capture the performance-related properties
of PMB are continually being developed.
In this study, the PMB specifications of several countries/regions were selected and reviewed.
And also bitumen grading systems and the test methods were studied. Furthermore, the indirect
relationships between the test methods and pavement performance were studied and results from
this study together with laboratory test results and past research outcomes are used to develop the
Sri Lankan specification for PMB.
2. Review of PMB specifications
The reviewed specifications include superpave (USDOT, n.d.), European, Australian (Austroads,
2006), Indian, Chinese, Japanese, Brazil and Russian PMB specifications. Table 1represents the
summary of test methods used in these specifications. All the reviewed PMB specifications except
superpave use mostly traditional test methods for testing of PMB.
The superpave specification, which was developed in the 1990s in USA, used a completely
new approach in bitumen testing where the fundamental rheological properties of bitumen were
linked with the pavement performance (Austroads, 2008;TRB, 2010;USDOT, n.d.). In that
system, the actual pavement conditions, i.e. pavement temperature, traffic load and traffic speed
were linked with fundamental properties of bitumen. This resulted in the development of binder-
blind specification, where the specification could be used regardless of whether binder is modified
or not.
Even though superpave system can be considered as the most successful bitumen testing system
developed so far, it also caused some complexities with the use of PMB. In a survey carried out
among USA state highway agencies by National Corporative Highway Research Program in 2001
(Bahia et al., 2001), these problems were highlighted. Majority of selected population reported
several problematic areas related to binder testing, which included compatibility/separation prob-
lems, short- and long-term ageing, determination of mixing and compaction temperatures, etc.
(Bahia et al., 2001).
On the other hand, the summary in Table 1presents that penetration, softening point, viscosity,
rolling thin film oven-short-term ageing and storage stability test are most common among the
specifications. Moreover, other than superpave system, almost all other specifications are based
on the conventional test methods. To use these test methods in PMB testing, it is important to study
the relationships between the tested properties and field performance. These empirical tests do not
measure fundamental rheological properties, which are directly related to pavement performance.
However, empirical test methods have been successfully used in binder specifications, implying
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714 H.S. Sitinamaluwa and W.K. Mampearachchi
Table 1. Summary of test methods used in different PMB specifications.
Test method Superpave Europe Australia India China Japan Russia Brazil
Penetration at 25◦C
Viscosity at 60◦C
Softening point (ring and ball)
Viscosity at high temperatures
(≥135◦C)
Flash point
Elastic recovery
Storage stability of PMB
Dynamic shear rheometer
Bending beam rheometer
RTFO/TFO (short-term ageing)
PAV (long-term ageing)
Direct tension test
Ductility
Toughness
FRAASS breaking point
Stiffness at intermediate temperature
Stiffness at low temperature
Torsional recovery
Embrittlement point
Notes: RTFO, rolling thin film oven; TFO, thin film oven; PAV, pressure ageing vessel.
Table 2. Summary of tested bitumen properties in different PMB specifications.
Tested properties Superpave Europe Australia India China Japan Russia Brazil
Consistency at intermediate
temperature (25◦C)
Consistency at the maximum road
temperature
Consistency at high service
temperatures
Consistency at low service
temperatures
Tendency to separation of polymer
Strain recovery of bitumen
Adhesion and cohesion properties
Durability (short term)
Durability (long term)
Rutting potential
Fatigue potential
Low temperature cracking potential
Consistency at mixing/compaction
temperatures
Hardening potential of
bitumen/safety
that the empirical properties can be indirectly linked to pavement performance. Table 2gives a
summary of tested properties by the test methods used in each specification, which includes both
rheological and empirical tests.
Several models have been presented in the past to link the field performance with the empirical
properties. One such approach is bitumen test data chart (Shell Bitumen, 2003), which permitted
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Road Materials and Pavement Design 715
penetration, softening point, FRAASS breaking point and viscosity data to be described as a
function of temperature in one chart. FRAASS breaking point was considered as a measure of
low temperature cracking, penetration test to monitor properties at intermediate temperatures,
softening point to describe properties at hot summer (or maximum pavement temperature) and
viscosity at 135◦C describes the properties at mixing and compaction stage.
Another such approach is the van der Poel nomograph (van der Poel, 1954), where the penetra-
tion and softening point tests have been used to make a reasonable approximation of the bitumen
stiffness. This bitumen stiffness is then related to the mix stiffness via volumetric properties of the
mix, and the mix stiffness together with mixture volumetrics is used to estimate fatigue lines for
mixes, which are indicators of pavement service life.
3. Development of PMB specification for Sri Lanka
Even though the empirical tests are not able to fully characterise the field performance of PMB, in
the absence of performance-based tests those methods have been used in many parts of the world
as the review of PMB specifications indicators. Further, it is evident that those specifications are
not limited to the test methods used in penetration or viscosity grading systems but covers a wide
range of test methods where all the essential characteristics of PMB are captured.
From the review, the most important characteristics were identified as consistency at interme-
diate, higher, and mixing and compaction temperatures, separation tendency of polymer from
PMB, hardening potential on heating/flammability and the presence of polymer. Apart from the
performance-related properties, it is important to identify the type of polymer modifier since the
empirical test-based specification can be specific according to the type of modifier. In the proposed
Sri Lankan specification, the penetration test, ring and ball softening point test, viscosity test, stor-
age stability test, flash point test and elastic recovery/solubility tests are selected to measure the
above-mentioned properties, respectively.
This section discusses how the requirements for Sri Lankan specifications are determined. There
are three major types of PMB, namely elastomer-modified, plastomer-modified and crumb rubber-
modified bitumen, and the requirements are determined separately for these three types of bitumen
(refer Tables 4–6) since depending on the polymer type the behaviour of each group differs.
Sri Lankan air temperature data were collected from the department of meteorology and pave-
ment temperatures were calculated according to Long Term Pavement Performance prediction
model. The resulting maximum temperatures for all the regions range from 48.6◦C to 62.0◦C and
the minimum temperatures range from 9.6◦C to 21.8◦C. When these temperatures are linked with
PG concept, the PG 58-16 binder grade is suitable for all the areas. Since Sri Lanka does not
include any snowbound areas or high-temperature areas, the same binder grade is applicable for
all the regions.
Traffic loads and speeds are classified as indicated in Table 3, considering the fact that the
heavy traffic loads and lower speeds contribute to pavement distresses. There are three different
binder grades introduced for normal, heavy and very heavy traffic in the proposed specification.
Table 3. Classification of traffic loads and speeds.
Normal traffic Loads less than 10 million ESAL, moving at a speed higher than 50 kmh−1
Heavy traffic Loads greater than 20 million ESAL, which is moving at a speed higher than 50 km h−1
Loads less than 10 million ESAL, which is moving at a speed range between 20–50 kmh−1
Very heavy Loads greater than 30 million ESAL which is moving at a speed higher than 20 km h−1
traffic Standing traffic conditions (<20 km h−1)where start and stop are involved
Note: ESAL, equivalent standard axle loads.
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716 H.S. Sitinamaluwa and W.K. Mampearachchi
Softening point limits are determined considering the maximum pavement temperature. Soften-
ing point is considered as an indirect control of high-temperature stiffness, and thus the acceptance
limits are set such that the minimum softening point of binder is higher than the maximum pave-
ment temperature. In Sri Lanka, the maximum pavement temperature varies from 48◦Cto62
◦C.
In Sri Lankan roads the 60–70 unmodified bitumen has been successfully used with acceptable
rutting resistance. Softening point of 60–70 binder lies in the range of 48–56. Therefore, it is
clear that a binder having softening point above this range would perform satisfactorily at high
temperatures. Modified binders usually have higher softening point values when compared with
unmodified binders. Therefore for normal traffic conditions, the minimum softening point of 60◦C
is considered as appropriate. For heavy and very heavy traffic conditions, an increase in softening
point by 5◦C for each category, i.e. up to 65◦C and 70◦C is recommended.
van de Ven et al. (2004) have studied development of bitumen specification for South Africa
for empirical test methods. South African maximum temperature values lies in between 49 and
64 and typical traffic speed considered for study is 60 km h−1. This condition can be considered
similar to Sri Lankan conditions. Their findings includes that for normal traffic conditions the
stiffness modulus of 100 kPa should be there to prevent rutting at the operating temperature at a
loading time of 0.015 s, as derived by using van der Poel nomograph. Similar stiffness modulus
is assumed to be adequate for Sri Lankan roads to operate satisfactorily (Figure 1).
Assuming that bitumen stiffness is adequate, the suitable penetration index (PI) values for Sri
Lankan roads are derived using van der Poel nomograph. For normal traffic conditions, with
a softening point of 60◦C, the van der Poel nomograph results that a PI of +1.5 is required to
have 100 kPa stiffness modulus at a maximum pavement temperature of 62◦C. This is derived
assuming a loading time of 0.02 s (corresponding to traffic speed of 50 kmh−1). Then the required
penetration range was derived using PI nomograph. The penetration range is determined so that
the binder has a PI range of ±0.5 from the above-resulting PI value. The resulting penetration
range was 50–70 (Figure 2).
Figure 1. van der Poel nomograph.
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Road Materials and Pavement Design 717
Figure 2. PI nomograph.
Similarly, for heavy and very heavy traffic conditions, the maximum temperatures are consid-
ered as 65◦C and 70◦C, respectively, and the PI was derived for the determined softening point
limits, similar loading time and for 100 kPa stiffness modulus. This results in a PI of +2 for both
grades, and the penetration range was derived using PI nomograph. The resulting penetration
grade for heavy and very heavy traffic conditions were 40–60 and 30–50, respectively.
These values are derived with the assumption that van der Poel nomograph and PI nomograph
would give reasonable approximations for polymer-modified binders. vdP nomograph has been
used to make approximation for mix stiffness with PMBs (van de Ven et al., 2004). It is expected
to revise the specification with field trial data in future to correct the errors induced by this
assumption.
Tables 4–6present the requirements for three types of PMB, namely elastomer-modified,
plastomer-modified and crumb rubber-modified bitumen. Binder grades are named according
to polymer type, i.e. E for elastomer-modified bitumen, P for plastomer-modified bitumen and R
for crumb rubber-modified bitumen followed by the penetration range.
Here, the behaviour of three types of PMB (elastomer-modified, plastomer-modified and crumb
rubber-modified) can vary in mixing and application, and it is essential to identify the differences
and improve the specifications to account for those differences. But in this initial specification, all
the requirements are set as similar for all three types of binders and elastic recovery/solubility tests
are added to the specifications to identify the binder type. It should be noted that all three tables
are similar except the elastic recovery/solubility tests, which are added for only identification
purposes, which is essential in reviewing field performance data and improving the specification
in future.
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718 H.S. Sitinamaluwa and W.K. Mampearachchi
Table 4. Requirements for elastomer-modified bitumen.
Test method
Binder type E 50/70 E 40/60 E 30/50 ASTM
Penetration at 25◦C, 100 g, 5s 50–70 40–60 30–50 D 5
Softening point (R&B) (min) 60 65 70 D 36
Flash point (COC) (min) 230 230 230 D 92
Viscosity at 150◦C (Pa s) (max) 0.9 0.9 0.9 D 2171
Separation tendency Difference in
softening point (R&B) (max)
4 4 4 D 7173
Elastic recovery (%) (min) 70 70 70 D 6084
Note: COC, Cleavelend open cup.
Table 5. Requirements for plastomer-modified bitumen.
Test method
Binder type P 50/70 P 40/60 P 30/50 ASTM
Penetration at 25◦C, 100 g, 5s 50–70 40–60 30–50 D 5
Softening point (R&B) (min) 60 65 70 D 36
Flash point (COC) (min) 230 230 230 D 92
Viscosity at 150◦C (Pa s) (max) 0.9 0.9 0.9 D 2171
Separation tendency Difference in
softening point (R&B) (max)
4 4 4 D 7173
Polymer content Report Report Report D 5546
Note: COC, Cleavelend open cup.
Table 6. Requirements for crumb rubber-modified bitumen.
Test method
Binder type R 50/70 R 40/60 R 30/50 ASTM
Penetration at 25◦C, 100 g, 5s 50–70 40–60 30–50 D 5
Softening point (R&B) (min) 60 65 70 D 36
Flash point (COC) (min) 230 230 230 D 92
Viscosity at 150◦C (Pa s) (max) 0.9 0.9 0.9 D 2171
Separation tendency Difference in
softening point (R&B) (max)
4 4 4 D 7173
Elastic recovery at 25◦C 40 35 30 D 6084
Note: COC, Cleavelend open cup.
Viscosity of a modified binder carries a higher value due to the addition of polymer. This
property is slightly unfavourable since high mixing temperatures are essential to ensure proper
mixing. A proper method is still not in existence to determine mixing and compaction tempera-
tures of PMB, and it is entirely determined by manufactures’ and users’ experience (West, Watson,
Turner, & Casola, 2010). However, a maximum mixing temperature should be limited to about
180◦C, since most of the polymers are likely to degrade beyond 200◦C. According to the values
present in the literature, styrene butadiene styrene (SBS)-modified bitumen have reported the
highest viscosities and thus the highest mixing and compaction values (Airey, 2004). Therefore,
the maximum viscosity limit was set corresponding to SBS-modified bitumen types. Two dif-
ferent SBS-modified bitumen samples, with recommended mixing temperatures of 180◦C and
185◦C, were tested at the laboratory for kinematic viscosity at 150◦C. The resulting values were
at the range of 912–913 cst, which is equivalent to 0.86 Pa s once converted by multiplying it
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Road Materials and Pavement Design 719
with bitumen density. Therefore if the bitumen viscosity is controlled in that limit, the mixing
temperature can be limited to 180◦C. Moreover, the Indian PMB standard also gives a maximum
limit for viscosity at 150◦C, which is 0.9 Pa,s. Therefore, it was concluded that this 0.9 Pa s limit
is applicable to Sri Lankan specification also (Tables 4–6).
A flash point of 230◦C is recommended for all types of bitumen (Tables 4–6), since it is the
globally accepted value for the safe handling of bitumen. Our laboratory test results on several
polyethylene- and SBS-modified bitumen samples reveal that the flash point of modified bitumen
is considerably higher than that of unmodified bitumen. Therefore, it is expected that the indicated
flash point is adequate for the safe handling of PMB.
The separation tendency is the difference of ring and ball softening point of top and bottom
parts of a bitumen sample, which was kept at a heated environment for 48 h. The maximum allow-
able difference to ensure the stability of the binder is 4◦C according to reviewed specifications;
therefore, that value is used for the Sri Lankan specification (Tables 4–6).
Elastic recovery test is used as a requirement under elastomer- and crumb rubber-modified bitu-
men. For elastomer-modified bitumen, the expected elastic recovery value is set as 70% adopting
the requirement in Indian specification. In certain PMB specifications (e.g. Europe), elastic recov-
ery test is used as an indicator of strain recovery of the binder. But recent research (Mogawer,
Austerman, Kutay, & Zhou, 2011) shows that the elastic recovery does not show any relationship
with performance properties of bitumen. But it is useful when testing elastomer-modified bitumen
to identify the presence of adequate amount of elastomeric polymer in bitumen (Table 4).
The same can be used for crumb rubber-modified bitumen also since rubber is an elastomer
material. But in crumb rubber, the recovery is limited since it contains some additives. Crumb
rubber does not show much elasticity like pure elastomer materials (Carlson and Zhu, 1999).
Specification for crumb rubber is available in Indian specification in which the recovery values
are limited to 30–40%. Therefore in the proposed specification also the elastic recovery values
are used as 30%, 35% and 40% (Table 6).
Elastic recovery is not a characteristic of plastomer-modified bitumen. Therefore to identify
the presence of plastomer material in bitumen, solubility test is recommended (Table 5).
4. Conclusions and recommendations
Sri Lanka’s experience in PMB is very limited, and the lack of the developed testing methods can be
a severe problem when it comes to quality assurance of PMB. However, a proper quality assurance
procedure is essential to implement PMB in Sri Lankan roads. The proposed specification for PMB
will fulfil this need, expressing a penetration grading system for PMB with available empirical
test methods. According to the experience of several countries, it is possible to use a set of
empirical tests to test PMB in the absence of performance-based tests. The proposed specification
requirements address all the important properties to be evaluated to ensure proper quality control
of PMB.
For the proposed specification based on empirical test methods, it is recommended to include
further tests that assess short- and long-term ageing procedure. For that, the highway laboratories in
Sri Lanka should be equipped with necessary equipment and further research work is necessary
to identify the specification limits. Furthermore, it is strongly recommended that the highway
sector in Sri Lanka should move to fundamental test methods, since accurate prediction of field
performance cannot be ensured with empirical tests. Since the proposed specification can have
several limitations in determining performance of the binder, it is further emphasised that any
particular requirement on binder recommended by suppliers should be considered. Suppliers can
specify the requirements according to their experience in handling binders and it is expected to
consider such requirements in future improvements of the specification.
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720 H.S. Sitinamaluwa and W.K. Mampearachchi
A field validation of the proposed specification was not done during this research. Therefore,
it is recommended to perform a field trial and propose necessary modifications to the proposed
specification. This specification has to be revised from time to time and the future experience of
Sri Lankan highway engineers should be included to develop this specification more effectively.
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