Indian Journal of Engineering & Materials Sciences
Vol. 20, April 2013, pp. 132-138
Study of physical and rheological properties of wax modified binders using
classic and SHRP testing methods
Shengjie Liu*
School of Highway, Chang’an University, Xi’an 710064, China
Received 22 October 2012; accepted 30 January 2013
Considerable effect of Fischer-Tropsch (FT) paraffin on improving the original asphalt properties along with reducing
the emissions and saving energy, encouraged using this additive as an effective modifier. In this study, extensive laboratory
investigations have been carried out using classic and SHRP testing methods. The results of both test methods show that
adding FT paraffin into original asphalt reduce penetration, ductility at 15°C, and m-value, while enhance penetration index,
softening point and creep stiffness. Results indicate that there is an inflection point or inflection interval in ductility at 5°C.
Rheological properties of modified specimens are investigated with DSR in temperature sweep and creep tests. Results
indicate that the aforementioned parameters increase by increasing the FT paraffin content. Furthermore, at higher
temperatures , increasing the FT paraffin additives content can reduce the binder viscosity and at lower temperatures less
than the melting point of FT paraffin the reverse is true.
Keywords: Wax modified binders, FT paraffin, Classic tests, Rheological property, Creep property, DSR
In recent years, environmental awareness such as
global warming and emissions has been increasing
rapidly. One of the main sources of energy
consumption and environmental emissions stemming
from industries related to transport infrastructures,
resides in the manufacturing, spreading, and
conservation of asphalt mixes1. Meanwhile, the
energy consumed is a major component of pavement
construction that significantly contributes to the total
cost2. In order to reduce the emissions and save
energy, the asphalt industry is constantly trying to
lower the mixing and compaction temperatures of the
mixes by using new energy-saving technologies,
while maintaining or improving the pavement
performance.
The warm mix asphalt (WMA) refers to
technologies that allow reducing the asphalt binders’
mixing and compaction temperature by reducing the
binders’ viscosity at a given temperature. Typically,
the mixing temperatures of warm mix asphalt
mixtures range from 100°C to 140°C (212°F-280°F )
compared to the mixing temperatures of 150°C-180°C
(300°F-350°F) for hot mix asphalt3-5.
From an environmental and economic standpoint,
use of WMA technologies may contribute to reduce
mixing and compaction temperature, while other
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*Email: [email protected]
pavement properties such as crack susceptibility at
low temperatures, fatigue resistance and adhesion are
not dramatically affected by the use of flow
improvers5,15.
Different WMA technologies were used to reduce
mixing and compaction temperature6, although these
technologies are quite different, they all target the
same goals, namely, lower bitumen viscosity, better
mat workability, and improved workability and
emissions conditions. During the several additives,
FT paraffin has been used widely as an commercial
wax7,8, because of its excellent insulating
characteristics. Previous research studies have taken
to prove the FT paraffin have significant effects on
asphalt performance9-11.
Below the laying and compaction temperatures,
there may be an increase in viscosity due to wax
crystallization of FT paraffin, which in turn could
increase the asphalt pavement resistance to plastic
deformation12. Decreasing temperature in the mixing
and compaction will reduce odor emission from plants
such as traditional gaseous pollutants (CO, NOx and
SO2), greenhouse gases (CO2); and improve the
working conditions at plants and paving sites13. So the
asphalt plant can be located at regions of strict air
pollution regulations. The lower mixing and
compaction temperature will also allow for longer
haul distance and help to extend the paving season14.
LIU: WAX MODIFIED BINDER
Numbers of wax modified binders trial projects
have been implemented in many states. In spite of
several researches conducted in the field of wax
modified binders, potential problems and unknowns
still exist. The short life of wax modified binders
tends to raise logical doubts and uncertainties
about their use, for it is still at a stage of
standardization16，especially the current binder
specifications in China are based on penetration
testing, which could not properly account for
pavement
performance.
Thus,
a
thorough
understanding of the properties and performance of
wax modified binders is necessary in order to
implement it successfully.
The objective of this study was to investigate the
engineering properties of wax modified binders
incorporating the FT paraffin technologies.
Laboratory experimental tests including classic tests
and SHRP test methods were conducted to simulate
the physical and rheological properties of wax
modified binders. In this regard, the following
objectives were supposed: (i) Effect of FT paraffin
on the basic rheological characteristics of studied
bitumen such as penetration, adhesion, and
elastic properties, (ii) Investigating the temperature
susceptibility of FT paraffin modified bitumen, Effect
of FT paraffin on stiffness and cracking potential of
the original bitumen using classic and Strategic
highway research program (SHRP) testing methods
and (iv) Comparison between the results of classic
and SHRP testing methods and investigating the
correlation between results.
133
The additive used in this study was a product
of wax-FT, which is a long chain of aliphatic
hydrocarbons obtained from coal gasification
using the Fischer-Tropsch (FT) process. After
crystallization, it forms a lattice structure in the binder
which is the basis of the structural stability of the
binder containing FT paraffin. It shows high viscosity
at lower temperatures, and low viscosity at higher
temperatures. Table 2 shows the relevant performance
indexes of this FT paraffin.
For all specimens, wax modified binders in this
study was prepared in the laboratory using a shear
mixer, which was made by Yaxing Maching Co. Ltd.,
China. Firstly, the asphalt (about 400 g) was heated
until became fluid in an iron container, and then
the Wax-FT pellets were added into hot binders
respectively when the temperature above 120°C. In
order to ensure the wax-FT was adequately soluble in
the asphalt, the shear speed was adjusted to 400 rpm
for 30 min. And then the asphalt was immediately
used for tests. Five contents (0%, 1%, 2%, 3%, 4%,
5% by weight of binder) were selected based on
previous studies and field trial experiences.
Then, specimens were tested by classic and SHRP
testing methods. Classic tests consisted of penetration,
softening point, ductility, viscosity which were
carried out using related China standards. Also,
dynamic shear rheometer (DSR) and bending beam
rheometer test （BBR）tests were performed and
obtained results were compared with classic tests
results.
Results and Discussion
Experimental Procedure
The asphalt binder selected for this investigation
was a 60/80 penetration grade (PG70-22) base
bitumen, which was provided by Sinopec Asphalt Co.
Ltd. 60/80 bitumen has the maximum application in
most regions in China. The basic properties of binders
as per (JTJ052-2006) are shown in Table 1.
Classic tests results and analysis
Penetration
Penetration is a performance index that evaluates
hardness and consistency of asphalt, since 1889
it has been used as a standardized phenomenological
test in the highway industry and it is also the most
Table 1—The properties of asphalt binder
Test
items
Penetration
(25°C, 0.01 cm)
Ductility
(5°C, cm)
Softening
point (°C）
Viscosity
(135°C, Pa.s）
Penetration
index (PI)
Density
(15°C, g/cm)
Wax
content (%）
Value
70.2
1.5
48
0.434
-0.55
1.032
2.02
Table 2—The basic properties of wax-FT
Test
Items
Congealing point
(°C)
Penetration
(25°C, cm)
Penetration
(65°C, cm)
Viscosity
(135°C, cP）
Melting
temperature (°C)
Density
(25°C, g/cm3）
Value
98
＜0.7
7
12
100
0.94
134
INDIAN J. ENG. MATER. SCI., APRIL 2013
common control test for penetration grade asphalt.
Figure 1 illustrates the Penetrations at three
temperature (15°C, 25°C, 30°C) of wax modified
binders based on results obtained from lab tests.
As shown in Fig. 1, compared with the control
binder (with 0% FT paraffin content), the addition of
wax-FT into asphalt binder did affect the penetrations
of binders. The penetrations decreased gradually
by increasing wax-FT content, indicating wax-FT
make the asphalt binder harder and more viscous.
Penetration index (PI)
Penetration index (PI) is one of the important
indexes to reflect asphalt temperature sensibility,
With respect to the effect of wax-FT additive，the
penetration of each binder were obtained at
three temperatures（15°C, 25°C, 30°C)，and then
penetration index (PI) is calculated as follows:
PI = (20 − 500 A) /(1 + 50 A)
where A = (log( penT1 ) − log( penT2 )) /(T1 − T2 )
Here pen is the abbreviation of penetration, T1 and
T2 are different temperatures, and T1 > T2. A is
penetration temperature index.
Figure 2 shows the statistical results of the
change in the penetration index as a function
of the warm asphalt additive. A general trend is
found from the results that the addition of wax-FT
increase the binder’s penetration index, compared
to the control asphalt binder. With wax-FT added
into the base asphalt, this was an inflection point or
inflection interval in penetration index, PI first
increased with increasing wax-FT content up to 4%
and then started to decrease. This phenomenon
Fig. 1 – Relationship lines between penetration and wax-FT content
indicated that the wax-FT content is statistically
insignificant on the temperature susceptibility of
wax modified binders.
Softening point
The softening point is the temperature at which the
binder begins to show fluidity. It is defined as the
temperature at which a bitumen sample can no longer
support the weight of a 3.5 g steel ball. The test
results are shown in Fig. 3. Through the lab tests, it
can be seen that the softening point is further
improved by increasing the wax-FT contents.
The increasing nature of the softening point
indicates that wax-FT can improve the high
temperature performance of asphalt, and which will
also increase the high temperature stability of asphalt
mixture. Furthermore, the binder and mixture have
more resistance against rutting at high temperatures
Ductility
The results of ductility test for specimens are
shown in Fig. 4. As shown in this figure, ductility
Fig. 2 – Relationship lines between PI and wax-FT content
Fig. 3 – Relationship lines between soft point and wax-FT content
LIU: WAX MODIFIED BINDER
135
Fig. 4 – Relationship lines between ductility and wax-FT content
at two temperatures
Fig. 5 – Relationship lines between viscosity and wax-FT content
at different temperatures
(at 5°C) of the specimens showed an inflection point
or inflection interval between 2% and 4%.
Ductility (at 5°C) first increased with increasing
wax-FT content up to 3% and then started to decrease.
While ductility (at 15°C) sustainly decreased by
increasing wax-FT content from 0% to 5%. It is noted
that when the content of wax-FT increased from 3%
to 4%, the ductility (15°C) decreased sharply from
125 cm to 79 cm.
The results indicated that the addition of
wax-FT would be likely to affect low temperature
performance of the binder.
viscosity. It can be found that the temperature of
100°C is the turning point, when the temperature
below it, the viscosity of asphalt increased with
increase of content; whereas at the temperature above
100°C, the viscosity of asphalt decreased as the
content increased. This can be explained as wax-FT is
a material with different organic structures within
different temperatures, and its melting point is about
100°C, when the temperature is higher than its
melting point, it could dissolve into base asphalt and
keep in liquid state entirely, so it could enhance the
rheological properties of asphalt, however when the
temperature is lower than its melting point, it will
keep the crystal status and disturb molecule
movement, and then the asphalt viscoelastic will
strengthen.
Viscosity is a generally accepted measuring factor
for determine, lower viscosity indicates asphalt
mixtures can be easily mixing and compacting.
The viscosity of asphalt binder decreased with
wax-FT content increasing at the high temperature
indicating that wax-FT addition contribute to
drop down the mixing and compacting temperatures
of asphalt mixtures.
Viscosity
Viscosity at high temperature is considered to be an
important property because it represents the binder’s
ability to be pumped through an asphalt plant,
thoroughly coat aggregate in asphalt concrete mix,
and be placed and compacted to form a new pavement
surface.
Brookfield rotational viscometer was used to test
the viscosity of these virgin and wax-FT binders at
seven different temperatures at a fixed frequency
of 6.8 rad/s. Figure 5 illustrates the viscosity of
asphalt binder decreases rapidly by increasing the
temperature from 80°C to 140°C as expected. In
addition, with respect to the effect of wax-FT,
large variability of viscosity was observed with the
change of wax-FT content at different temperatures, it
can be observed that, at the temperature of 80°C, 5%
wax-FT addition contributed to the highest viscosity,
whereas at a temperature of 150°C, the mix with 0%
wax-FT content had the highest viscosity.
Further analysis was performed to investigate the
significance of the effect of wax-FT content on
SHRP result and analysis
Rheological measurement results and analysis
The DSR was suggested as a means to characterize
asphalts’ viscoelastic properties during the Strategic
Highway Research Program (SHRP), a 5-year
$150 million United States research effort established
and funded in 1987. The DSR is a classic rotational
rheometer that applies oscillatory shear to asphalt
using the parallel-plate configuration and assesses its
136
INDIAN J. ENG. MATER. SCI., APRIL 2013
rheological behavior through the response of the
material to the imposed stresses or deformations17.
Two measured properties (complex modulus
G* and phase angle δ) were transformed into two
performances based properties G*sinδ and G*/sinδ
to reflect the dissipated energy by the nonelastic
components of the material response. The parameter
G*sinδ measures the damage or the dissipated energy
for a linear viscoelastic material subjected to a strain
controlled load, which is an indicator of fatigue
cracking for thin pavements at intermediate pavement
temperatures. For a stress controlled mode of loading,
the dissipated energy is G*/sinδ, which is a measure of
rutting potential at high temperatures. This parameter is
also used as an indicator of fatigue cracking for thick
pavements at intermediate temperatures. It is
imperative both parameters are chosen that best relates
to the material’s rheological behavior.
Based on the lab tests, the results of G*/sinδ for
warm asphalt binders containing different content of
wax-FT are shown in Fig. 6. For different temperature
conditions (at 70°C, 76°C, 82°C), the rutting factor
increased by increasing wax-FT content, and the
increment became more significant at a higher
percentage of wax-FT, indicating content increasing
contributed an promotion of rutting resistance at a
high temperature.
Figure 7 shows the results of the fatigue factor,
G*sinδ, as a function of temperature at 25°C, 28°C
and 31°C. It can be seen that G*sinδ increased by
increasing wax-FT content, and the additions of 1%
wax-FT showed a very rapid increase compared
to the control binder, which implied that wax-FT
contributed to adverse effects on fatigue resistance.
As the wax-FT added into asphalt, the elastic of
binder would be greater than the base asphalt,
which weaken the capability of asphalt to relieve
stress and reduced the ability of fatigue resistance.
According to the AASHTO M 320, the G*/sinδ
should be less than 1.0 kPa for the unaged asphalt,
through the DSR test, it is found that the control
PG70-22 binder does not meet the requirement
at 76°C and 80°C, but with the addition of wax-FT
is 1%, the binder can meet the criteria at 76°C,
while the content is 5%, the high temperature grade
will near the criteria at 80°C, showing the addition
of wax-FT could improve the high temperature
grade from 70°C to 76°C, even to 80°C. Results
indicate that the high temperature performance
and rheological behavior has increased by increasing
the wax-FT content.
The BBR is a three-point bending-beam experimental
set-up, designed to characterize the low-temperature
viscous behavior of bitumen. It can be used to
evaluate how much a binder deflects or creeps under a
constant load at low temperature. The creep stiffness
is the ratio of a certain load applied to the variation in
strain or displacement in the prepared asphalt beam
and the m-value is the slope of the stress against strain
curve relationship in a log scale. Lower creep stiffness
and higher m-value of PAV aged binder at a low
temperature usually mean a higher resistance to low
temperature cracking of asphalt binders.
In this study, the creep stiffness and m-value of
PAV aged binders were conducted by BBR tests at
T = -6°C, -12°C and -18°C and the average value
was determined. Figure 8 shows the creep stiffness
of WMA with different wax-FT contents at three
Fig. 6 – Relationship lines between Gsinδ and wax-FT content at
different temperatures
Fig. 7 – Relationship lines between Gsinδ and wax-FT content at
different temperatures
LIU: WAX MODIFIED BINDER
137
With the wax-FT content increased to 2%, the
WMA failed the requirement at -12°C, and further,
when the content was 5%, it failed the criteria
at -6°C, showing that addition of wax-FT could drop
the low temperature grade from -22°C to -16°C,
which means the low-temperature performance of
asphalt have been weakened .
Comparison between the results of classic and SHRP tests
Fig. 8 – Relationship lines between stiffness and wax-FT content
at different temperatures
Fig. 9 – Relationship lines between m value and wax-FT content
at different temperatures
temperatures. Addition wax-FT into asphalt greatly
increased the low temperature stiffness of wax
modified binders, which will decrease the toughness
of wax modified binders mixtures and increased the
occurring possibility of the asphalt binder at low
temperature. Similar to what was discussed above, the
change trend of m-value also proved this, as shown
in Fig. 9, the increasing of wax-FT content lead to
decrease of m-value at three temperature.
AASHTO M 320 requires the creep stiffness
should be less than 300 MPa and the m-value
should be greater than 0.300 at the test temperature
during the performance grading of the asphalt.
It is noted that, for the control asphalt binder of
PG70-22, it meets this requirement at -6°C and -12°C.
For the asphalt specimens, penetration steadily
reduced by increasing the wax-FT content. In contrast,
obtained results for G*sinδ at three temperatures
increased by reduced wax-FT content. Negative
correlation exists between penetration and G*sinδ.
PI parameter increased by increasing the wax-FT
content, whereas G*/sinδ parameter in three
temperature reduced by increased wax-FT content.
There was a same trend in diagrams. Therefore, PI
could be used as an indicator for temperature
susceptibility evaluation.
The results of softening point, G*sinδ, G*/sinδ and
the differences between these parameters have been
discussed. The effect of wax-FT content on both
parameters was to some extent similar. Therefore, in
overall, it could be concluded that for studied
specimens there is a positive correlation among
G*sinδ, G*/sinδ and softening point.
The differences among creep test at 25°C, ductility
at 5°C and 15°C results have been discussed. By
increasing the wax-FT content, ductility at 15°C of
specimens reduced whereas stiffness increased upon
increasing the wax-FT content. Ductility at 5°C first
increased and then decreased by increasing the
wax-FT content. Obtained results imply that there
is poor relationship among creep, ductility at 5°C
and 15°C.
Conclusions
Based on laboratory investigations and obtained
results in this study, the following conclusions can be
drawn:
(i) The penetration, ductility at 15°C, m-value
reduced gradually as the content of the wax-FT
increased, while the penetration index (PI),
softening point and creep stiffness results
decreased.
(ii) Wax-FT additives could reduce binder viscosity
at high temperatures and thus allow lower
mixing and laying temperatures. But when the
asphalt temperatures below the wax-FT ’s
INDIAN J. ENG. MATER. SCI., APRIL 2013
138
4
melting point this reduced viscosity is offset
again or the effect is even reversed.
(iii) Both classic tests and SHRP tests arrived the
same results that wax-FT can enhance the
consistency of asphalt, and reduce the
temperature sensitivity of asphalt through
improving the high temperature performance of
asphalt. However, it has negative effect on low
temperature performance and fatigue resistance
performance of wax-FT modified asphalt.
(iv) Based on analysis between the results of classic
and SHRP tests. There is a meaningful
relationship between (penetration-G*sinδ),
(PI-G*/sinδ), (softening point-G*sinδ), (softening
point -G*/sinδ). In contrast there is no
relationship between (ductility-stiffness/ m value)
parameters.
13
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