Microstructure of polymer
modified binders in
bituminous mixtures
Danish Road Institute
Report 109
2001
Ministry of Transport - Denmark
Road Directorate
Danish Road Institute
Elisagaardsvej 5
P.O. Box 235
DK-4000 Roskilde
Denmark
Telephone: +45 46 30 70 00
Telefax:
+45 46 30 71 05
e-mail:
[email protected]
web:
www.vd.dk
Title:
Author:
Dated:
Copyright:
Published by:
ISBN:
ISSN:
Microstructure of polymer modified binders in bituminous mixtures
Vibeke Wegan, Carsten Bredahl Nielsen
September 2001
Road Directorate, All rights reserved
Road Directorate, Danish Road Institute
87-90145-82-8
0909-1386
Microstructure of polymer
modified binders in
bituminous mixtures
Vibeke Wegan
Carsten Bredahl Nielsen
Danish Road Institute
Report 109
2001
1
1
Contents
Preface ........................................................................................................... 6
Abstract.......................................................................................................... 7
1. Introduction ............................................................................................... 8
2. Experimental Procedures ........................................................................ 10
2.1 Preparation of thin sections ................................................................. 10
2.2 Analyses of thin section by microscopy ................................................ 10
2.3 Analysis of bituminous specimen by Infrared Fourier Transform
Spectrometer ...................................................................................... 11
2.4 Dynamic Creep test............................................................................. 11
3. Results...................................................................................................... 12
3.1 Polymer modified binder ..................................................................... 12
3.2 Microscopy ......................................................................................... 12
3.3 Infrared Fourier Transform Spectrometer.............................................. 13
3.4 Creep test........................................................................................... 15
4. Discussion ................................................................................................ 16
5. Conclusions .............................................................................................. 18
6. References................................................................................................ 19
5
Preface
This report contains a reprint of a paper written and presented at the 2nd Euroasphalt &
Eurobitume Congress held in Barcelona, September 20-22, 2000. The paper was presented by Vibeke Wegan in session 1: “Performance Testing and Specifications for
Binder and Mix”, having Bernard Brûlé as chairman.
6
Abstract
Examination of the structure of the polymer modified binder in a bituminous mixture
has been performed together with infrared analysis and creep test to evaluate the performance of a polymer modified binder course.
In a recent paving job, a random sample control of the polymer modified binder
showed that the binder had a large tendency towards separation (storage stability).
The binder was modified with Styrene-Butadiene-Styrene (SBS). To examine whether
this separation tendency of the polymer modified binder also was found in the asphalt
mixture, asphalt cores were taken from the job site. These cores were examined to
characterise the structure of the polymer modified binder, to estimate the polymer
content and to evaluate the creep performance of the asphalt mix.
The structure of the polymer modified binder has been determined by examination of
cut and ground bituminous specimens prepared as thin sections. The surface of these
thin sections was illuminated with incident UV-light whereby the polymer phase is
visible.
Thin sections and estimation of the polymer content by infrared analysis showed that
the polymer phase was very inhomogeneously distributed in the asphalt binder course.
The binder phase in the binder course varied from a bitumen phase with nearly no
polymer phase to a continuous bitumen phase with small areas of polymer phase.
The dynamic creep test indicated that the binder course have no superior rut resistance
compared to other Danish asphalt courses.
7
1. Introduction
In 1999, a 4-km bypass was constructed in Denmark connecting a motorway with a
large cargo terminal. The bypass was designed for a traffic load of 6.4 million
EASELs in ten years. The unbound pavement consists of 350 mm gravel subbase and
200 mm gravel roadbase. The specified asphalt pavement is 150 mm asphalt concrete
basecourse, 60 mm binder course and 40 mm stone mastic asphalt wearing course.
Due to the expected heavy traffic loads, the binder course was designed with a polymer modified binder to improve the stability and rut resistance of the asphalt pavement.
During the paving job, a random sample control of the polymer modified binder
showed a large tendency towards separation of the polymer in the binder. It was of
large interest whether this separation also was found in the asphalt mix with the polymer phase separated to the top or with inhomogeneous distribution of the polymer
phase. It was also of interest to analyse whether the asphalt pavement performed as
expected or the separation tendency of the binder had resulted in an asphalt pavement
with decreased stability.
To analyse the quality of the binder course in the pavement, asphalt cores were taken
at ten positions evenly distributed over the length of the bypass. The binder course in
the asphalt cores were analysed in the laboratory by:
·
·
·
Thin sections
Infrared Fourier Transform Spectrometer (FTIR)
Dynamic creep test
Thin sections were produced to investigate the distribution of the polymer phase in the
asphalt pavement and to evaluate the polymer content in the different asphalt cores.
Thin sections, which are a cut and ground epoxy impregnated specimen from a bituminous material (30 x 40 mm and 20mm in thickness), have been used at the Danish
Road Institute since 1995 to characterise the structure of the polymer modified binder
directly in a bituminous mixture by fluorescent microscopy [1]. Another type of thin
sections can be produced to characterise microscopical features in the bituminous
mixture using polarising and fluorescent microscopy. From these sections, information
can be obtained about the bituminous mixture, such as filler distribution, aggregate
degradation, adhesion between aggregates and binder, signs of stripping, binder intrusion in porous aggregate particles, location and size of micro-cracks, aggregate and
filler mineralogy etc. Finally, larger specimens (10 x 10 cm and 1 cm in thickness) are
prepared as plane sections, which primarily are used to characterise the air void structure in the compacted bituminous mixtures not only by content but also by size, form
and distribution, which are parameters very important for the performance of a bituminous mixture.
8
As support for the investigation in the microscope, infrared analysis has been made on
recovered binder from the top and bottom of the asphalt layer to estimate the polymer
content. The polymer content is estimated since the polymer content has been calculated based on a standard calibration curve and not on a calibration curve produced
from the exact polymer and bitumen type used at the actual paving job.
To evaluate the performance, dynamic creep test were performed on asphalt cores
taken from the pavement.
9
2. Experimental Procedures
2.1 Preparation of thin sections
A small bituminous mixture specimen is cut, approximately 30 x 45 mm and
10–20 mm in thickness, by means of a thin saw (1.3 mm blade thickness) using a thin
section apparatus. The specimen is glued onto a plane glass slide, which helps to attach the specimen during the preparation procedure and impregnated under vacuum
with a colourless epoxy resin, which after curing helps to stabilise the specimen. The
surface is then sawed very close to the impregnated surface, ground by diamond
coated rollers and polished by a rotating pellet disc.
An object glass is glued to this surface of the specimen, which is the first finished side
of the thin section. The specimen is cut once more close to the object glass, ground by
diamond coated rollers and polished to a final thickness of 20 mm.
Throughout the preparation procedures the specimen and equipment is constantly
cooled to approximately -5ºC and ice cooled water is sprayed on the specimen during
all sawing, grinding and polishing to avoid smearing the polymer phase. The preparation of a thin section is described in details in test procedure 30-16 [2 and 3].
2.2 Analyses of thin section by microscopy
The structure of the polymer modified binder in thin sections was investigated under
a Leitz Medilux microscope with incident UV-light. The light source comes from a
high-pressure Xenon lamp, 75 W. The microscope was equipped with a three filter
system; an excitation filter (BP 420/490), a beam splitter filter (RKP 510) and a barrier filter (LP 515).
When thin sections are illuminated with the UV-light, the polymer phase, swollen by
a part of the maltenes from the bitumen, emits yellow light. The fine and coarse
aggregates often appear green and the bitumen phase is black. Air voids or cracks appear with a yellow-green colour. The different phases can appear with other colours if
the thin section is analysed under a polarisation microscope with transmitted light and
parallel or crossed nicols. Examination with transmitted light can be a useful tool, if it
is difficult to distinguish between the different phases.
Attention should be drawn to the fact that the binder in an unmodified bituminous
mixture in some cases can be seen with a slightly yellow fluorescent colour when the
thin sections are illuminated with incident UV-light. This is due to the poly-aromatic
structures in the maltenes in bitumen, when the bitumen has a low content of asphaltenes.
The specimens in this study were examined with magnifications of 100 – 250.
10
Aggregate
Polymer
phase
Binder/
filler phase
Figure 1 Example of the different phases when a thin section of a bituminous mixture is
illuminated with incident UV-light. The polymer phase (yellow/white) can be seen as spots
in a continuous bitumen phase (black).
2.3 Analysis of bituminous specimen by Infrared Fourier
Transform Spectrometer
To estimate the polymer content on the top and bottom of the asphalt layer in each asphalt core, a bituminous specimen was cut from the top or the bottom with a height of
approximately 1 cm, 1 cm wide and a length of 3 cm. The bituminous specimen was
after division recovered in Thrichloroethylene (p.a. C2HCl3). Five times, with an interval of approximately 20 minutes, the specimen was shaken, after which the extract
was decanted and saved in a preparation glass. The aggregate and filler were recovered once more and the extract was added to the first extract in the preparation glass.
The extract was allowed to precipitate minimum 20 hours, after which 1 ml extract
was taken with a pipette without disturbing the extract. Six to ten droplets of the extract were then transferred to a KBr-window in a smooth film. After evaporation of the
solvent, the KBr-window was placed in an Infrared Fourier Transform Spectrometer
(Perkin-Elmer, model 1710) and the spectra recorded after 5 scannings from
4000 - 400 cm-1 with a resolution of 2 cm-1. The spectra was analysed in a computer
programme (Spectrafile-IR, version 2.0), and the SBS content calculated based on a
calibration curve using the peaks at 970 and 702 cm-1 as described by Choquet and Ista
[4].
2.4 Dynamic Creep test
The dynamic creep test was performed according to a Swedish test procedure [5],
using the Nottingham Asphalt Tester (NAT). The test procedure is similar to
BS DD 185 performed on 150-mm diameter cores with a 100-mm loading disc.
The tests were performed at 40°C.
11
3. Results
3.1 Polymer modified binder
Due to the pure result of the storage stability test, the binder was not analysed after the
entire test programme planned for the random spot test. Results after analysis of the
binder are given in Table 1.
Table 1. Physical Properties of the SBS-modified binder taken as a random spot test.
Properties
Softening Point, Ring and Ball
Penetration, 25°C, 100 g, 5 sec.
Penetration Index
Elastic Recovery, 10°C
Storage stability
D Softening Point, Ring and Ball
D Penetration, 25°C, 100 g, 5 sec.
D Penetration Index
D Elastic Recovery, 10°C
Structure of the polymer phase
Unit
°C
1/10 mm
%
°C
1/10 mm
%
0.8 x 0.5 mm
Data
52
72
0.2
71
35
54
7.9
45
3.2 Microscopy
Microscopy of ten thin sections prepared from the asphalt layer showed that the polymer phase was very inhomogeneously distributed over the thickness of the layer. The
binder phase varied from a continuous bitumen phase with nearly no polymer phase to
a continuous bitumen phase with small areas of polymer phase. This variation was not
only seen between the ten asphalt cores taken from 4-km of the asphalt layer but also
between the top and bottom in the asphalt layer in some of the asphalt cores. A consistent tendency for the polymer phase to separate towards the top was not found.
Examples from the microscopy of the thin sections are given in Figure 2 and a visual
evaluation of the content of polymer phase is given in Table 2. The photos correspond
to 0.35 x 0.53 mm.
12
Top of thin section
Bottom of thin section
Asphalt Core No. 2
Asphalt Core No. 3
Asphalt Core No. 4
Asphalt Core No. 6
Figure 2. Examples from the microscopy of four selected asphalt cores. The polymer phase can be seen as spots
of varied size and density in a homogeneous bitumen phase.
Table 2. Visual evaluation of the content of polymer phase after examination of thin sections.
Asphalt core
1
2
3
4
5
6
7
8
9
10
Top
3
1
3
3
3
1
2
1
2
1
Bottom
3
1
3
1
2
3
1
2
1
1
1 = nearly no visual polymer phase 2 = small spots of polymer phase in a continuous bitumen
phase and 3 = spots of polymer phase in a continuous bitumen phase.
3.3 Infrared Fourier Transform Spectrometer
The polymer content estimated based on a standard calibration curve on top and bottom of the binder course in each asphalt core is illustrated in Figure 3. The estimated
polymer content in the pure binder is also given.
13
PB
TOP
BOTTOM
3.5
SBS content [%]
3.0
2.5
2.0
1.5
1.0
0.5
0.0
PB
AC 1
AC 2
AC 3
AC 4
AC 5
AC 6
AC 7
AC 8
AC 9 AC 10
Asphalt Core No.
Figure 3. Variation in the estimated SBS-content in top and in bottom of an
asphalt layer in 10 asphalt cores. PB = Pure binder.
SBS polymer types are characterised by a peek around 970 and 702 cm-1 where the
peak at 970 cm-1 represents the butadiene part and 702 cm-1 represent the styrene part
of the polymer. The ratio between the two peaks is normally constant for the same
polymer blended with the same bitumen. The ratio between the styrene peak and the
butadiene peak is illustrated in Figure 4 for the pure binder and for the top and the
bottom of the asphalt layer in the ten asphalt cores.
PB
Top
Bottom
1.4
1.0
0.8
-1
Ratio 970 /702 cm
-1
1.2
0.6
0.4
0.2
0.0
PB
AC 1
AC 2
AC 3
AC 4
AC 5
AC 6
AC 7
AC 8
AC 9 AC 10
Asphalt Core No.
Figure 4 The ratio between the peak representing the styrene part and the peak representing
the butadiene part of the polymer for the pure binder and for the top and the bottom of
an asphalt layer in ten asphalt cores. PB= Pure binder.
14
3.4 Creep test
The results of the creep tests of the asphalt binder course of the bypass pavement (1)
are given in Table 3 for. Some results from dynamic creep tests of other materials
tested by the Danish Road Institute are also stated for a comparison of the obtained
results. All specimens are cores drilled from a pavement and tested at 40°C. The
thickness of the layer, the number of cores and the diameter are also given in Table 3.
In test series number 5 the specimens consist of three layers tested as a whole. In
Table 3 the total creep after 3,600 loads and the creep rate from 2,500 to 3,600 loads
are given.
Table 3. Dynamic creep results for the bypass pavement (1) and four other pavements (2-5).
Pavement layers
1.
2.
3.
4.
5.
AC binder course (bypass)
HRA
HRA
SMA
AC gap graded
AC binder course
AC basecourse
Thickness
mm
43
30
30
45
32
50
109
Cores
No.
6
4
4
4
Diameter
mm
150
150
100
100
5
150
Total creep
mm/m
Mean
Std.
Creep rate
microstrain
Mean
Std.
17
24
37
22
4
5
7
7
0.7
2.4
3.9
0.4
0.2
0.6
0.8
0.1
9
0.7
1.0
0.2
15
4. Discussion
The results obtained from the storage stability test shows that the polymer phase has a
large tendency for separation towards the top. The difference in softening point, Ring
and Ball between top and bottom is found to be 35°C. This value is significantly
higher than the specified value that only allows a maximum difference of 3°C. The
other binder data measured after the storage stability test also shows an unacceptable
difference between data in top and bottom, all evidence that the polymer phase is
separated to the top. It is not known whether this separation is due to insufficient
blending of the polymer and the bitumen, incompatibility between the polymer and the
bitumen or a third reason.
The results indicate that the bituminous mixture had a similar problem with separation
or an inhomogeneous distribution of the polymer phase in the binder. Separation or an
inhomogeneous distribution of the polymer phase in the binder in the mix could mean
that the modification has no effect and hence no benefit in the way of a more rut resistant pavement is gained from the modification and the more expensive asphalt
layer.
Microscopy of the asphalt cores shows that the polymer phase is inhomogeneous distributed in the bitumen phase. Some asphalt cores have the same content of visual
polymer phase in top and in bottom, while others have a visual difference in the polymer content (Figure 2, Table 2). Some asphalt cores have the largest visual content of
polymer phase in the top, while others have it in the bottom.
Earlier studies have shown reasons for variations in the distribution of the polymer
phase. In a bituminous materials produced with the same polymer modified binder, a
variation in the polymer distribution is only expected if the binder is used in asphalt
layers with a significantly different thicknesses, in asphalt layers made with different
content of aggregates or filler or different mineralogy. Variations are also expected if
the thermal history of two asphalt materials with the same polymer modified binder is
significantly different (mixing temperature, mixing time or mixing technique). All
these parameters are presumed to be nearly the same for the ten asphalt cores taken the
pavement and cannot explain the variations in the polymer distribution observed.
The estimated polymer content confirms the result obtained from microscopy
(Figure 3). The estimated polymer content varies not only between the asphalt cores
but also between top and bottom of some of the asphalt cores. It should be noted that
the estimated polymer content is higher in the pure binder compared to the estimated
polymer content in the recovered binder from all asphalt cores. Whether the polymer
is degraded or altered in the asphalt layer or moved to another part in the asphalt is not
known.
The FTIR analysis gives another remarkable result. Figure 4 shows that the pure
binder has a larger butadiene/styrene ratio compared to the recovered binders from
16
both top and bottom of all the asphalt cores. Further, it is surprising that the ratio between the butadiene part and the styrene part is varying not only between the different
asphalt cores but also between top and bottom of the asphalt layer. This indicates that
the chains of butadiene are degraded or altered with a varied extend in the bituminous
mixture.
The inhomogeneous distribution of the polymer phase in the asphalt concrete binder
course is expected to influence the performance of the asphalt layer. To evaluate this,
the dynamic creep properties were measured using NAT (Table 3). The binder course
was designed to be rut resistant using a gap graded aggregate and a polymer modified
binder. The dynamic creep and the creep rate are therefore expected to be smaller than
for other Danish asphalt courses. Compared to hot rolled asphalt (HRA) this is clearly
the case, but HRA is known not always to be very rut resistant. The creep rate of the
binder course is larger than of a SMA, even though the measurement of the creep rate
of the SMA is performed on 100-mm diameter cores, which from other experience is
known to give lager creep rates than when performed on 150-mm diameter cores. This
is seen from the creep properties given in Table 3 for the same HRA performed on
100-mm and 150-diameter cores.
The creep properties of the binder course are of the same order of magnitude as the
creep properties of three courses tested as a whole including a binder course with a
modified binder. It was expected that the total creep and the creep rate were significantly larger for three layers including a wearing course and a basecourse. This indicates that the binder course in the bypass pavement have no superior rut resistance.
17
5. Conclusions
During a paving job in Denmark, a random sample control of the polymer modified
binder showed a large tendency towards separation of the polymer in the binder. It
was feared that the polymer was inhomogeneously distributed in the asphalt mix
performed with this binder and hence the expected benefit of the modification was not
obtained. Microscopy of thin sections made from ten cores sampled from the asphalt
pavement concluded that the polymer phase was inhomogeneous distributed. Estimation of the polymer content by FTIR supported this conclusion and indicated that the
butadiene part of the polymer was degraded or altered. Measured dynamic creep properties indicates that the binder course in the bypass pavement have no superior rut
resistance. This might be caused by the inhomogeneous distribution of the polymer in
the binder.
18
6. References
[1]
[2]
[3]
[4]
[5]
V. Wegan, B. Brûlé (1999): "The structure of Polymer Modified Binders and
Corresponding Asphalt Mixtures". Proceedings of the Association of Asphalt
Paving Technologists, pp 41-64, Chicago.
"Thin Sections of Polymer Modified Asphalt Mixtures". Danish Road Institute,
Test procedure 30-16, February 1998.
"Microscopic Analysis of Asphalt Concrete Mixtures. Preparation Techniques
for Plane Sections. Preparation Techniques for Thin Sections". Danish Road Institute, Information Guide, p 13.
Freddy S. Choquet and Emmanuel J. Ista (1990): "The Determination of SBS,
EVA and APP Polymers in Modified Bitumens".
FAS Method 468-97: "Determination of deformation resistance by dynamic
creep test" (in Swedish).
19
Rapporter/Reports
Nr./No
År/Year
85/97
Subgrade Performance Study
Part I: Materials, Construction and Instrumentation
(Robin Macdonald, Susanne Baltzer)
86/97
Fifth International Conference on the Bearing
Capacity of Roads and Airfields
Trondheim, July 6 - 8, 1998, Papers
(Robin Macdonald, Wei Zhang, Susanne Baltzer,
P e r Ullidtz, Jesper L. Lund)
87/98
88/98
89/99
Pavements Subgrade Performance Study
Part II: Modeling Pavement Response and
Predicting Pavement Performance
(Wei Zhang, Per Ullidtz, Robin Macdonald)
Road Unevenness
Paper presented at the 1998 FISITA World
Automobile Congress, Paris
(Bjarne Schmidt)
Development of improved mechanistic deterioration
models for flexible pavements
(Hans Ertman Larsen, Per Ullidtz)
(Electronic edition)
90/99
Friktionsmålinger
Sammenlignende målinger mellem ROAR og
Stradograf
(Bjarne Schmidt)
91/99
Grundere til broisolering
- typegodkendelse
- materialevalg
(Jeanne Rosenberg)
92/99
The Structure of Polymer Modified Binders and
Corresponding Asphalt Mixtures
(Vibeke Wegan, Bernard Brûlé)
93/99
PIARC World Road Association
International Experiment to Harmonise
Longitudinal and Transverse Profile Measurement
and Reporting Procedures, Draft Report
(Bjarne Schmidt, Jim Wambold, Akira Kawamura,
Guy Descornet)
(Electronic edition)
94/99
Evolution and Harmonization of Evenness
Evaluation Techniques
(Bjarne Schmidt)
(Electronic edition)
98/99
Accelerated Pavement Testing
1999 International Conference
October 18-20, Reno, Nevada
(Carsten Bredahl Nielsen, Per Ullidtz, Wei Zhang,
Susanne Baltzer, Robin A. Macdonald)
(Electronic edition)
99/00
Stabilitet og holdbarhed af danske asfaltbelægninger
(Jeanne Rosenberg, Jørn Raberg)
(Electronic edition)
100/00
Responses and Performance of a Test Pavement to
two Freeze - Thaw Cycles,
Danish Road Testing Machine RTM2: 1998
(Wei Zhang, Robin Macdonald)
(Electronic edition)
101/00
Responses and Performance of a Rehabilitated Test
Pavement to Accelerated Load Testing
Danish Road Testing Machine RTM3: 1999
(Wei Zhang, Robin Macdonald)
(Electronic edition)
102/00
Responses and Performance of a Rehabilitated Test
Pavement to one Freeze - Thaw Cycle
Danish Road Testing Machine RTM3: 2000
(Wei Zhang, Robin Macdonald)
(Electronic edition)
103/01
Sensors for Pavement Instrumentation
- Application in the Danish Road Testing Machine
(edited by: Gregers Hildebrand)
(Electronic edition)
104/00
Examination of pollution in soil and water along
roads caused by traffic and the road pavement
(Knud A. Pihl, Jørn Raaberg)
(Electronic edition)
105/00
Thin pavements with synthetic binder used in
Denmark
(Jeanne Rosenberg)
(Electronic edition)
106/00
Surfacing of concrete bridges
(Vibeke Wegan)
(Electronic edition)
107/01
Danske asfaltbelægningers sporkøringsmodstand
(Carsten Bredal Nielsen)
(Electronic edition)
95/99
Investigation of Gyratory Compaction used for
Asphalt Mix Design
(Jørn Raaberg)
(Electronic edition)
108/01
Effect of Design Parameters on Polymer Modified
Bituminous Mixtures
(Vibeke Wegan)
(Electronic edition)
96/99
Development of Models for Economic Evaluation
of Pavement Maintenance: the PAV-ECO Project
Providing an Efficient and Socially Acceptable Road
Transport Network
(Gregers Hildebrand, Philippe Lepert)
(Electronic edition)
109/01
Microstructure of polymer modified binders in
bituminous mixtures.
(Vibeke Wegan, Carsten Bredal Nielsen)
(Electronic edition)
97/99
Development of a Laser-Based High Speed
Deflectograph
(Gregers Hildebrand, Søren Rasmussen, Raúl Andrés)
(Electronic edition)