Danish Road InstituteReport 1092001

Ministry of Transport - Denmark

Microstructure of polymermodified binders inbituminous mixtures

Title: Microstructure of polymer modified binders in bituminous mixturesAuthor: Vibeke Wegan, Carsten Bredahl NielsenDated: September 2001Copyright: Road Directorate, All rights reservedPublished by: Road Directorate, Danish Road InstituteISBN: 87-90145-82-8ISSN: 0909-1386

Road DirectorateDanish Road InstituteElisagaardsvej 5P.O. Box 235DK-4000 RoskildeDenmarkTelephone: +45 46 30 70 00Telefax: +45 46 30 71 05e-mail: vd@vd.dkweb: www.vd.dk

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Danish Road InstituteReport 1092001

Microstructure of polymermodified binders inbituminous mixtures

Vibeke Wegan

Carsten Bredahl Nielsen

5

Contents

Preface ........................................................................................................... 6Abstract.......................................................................................................... 71. Introduction ............................................................................................... 82. Experimental Procedures ........................................................................ 10

2.1 Preparation of thin sections ................................................................. 102.2 Analyses of thin section by microscopy ................................................ 102.3 Analysis of bituminous specimen by Infrared Fourier Transform

Spectrometer ...................................................................................... 112.4 Dynamic Creep test............................................................................. 11

3. Results ...................................................................................................... 123.1 Polymer modified binder ..................................................................... 123.2 Microscopy ......................................................................................... 123.3 Infrared Fourier Transform Spectrometer.............................................. 133.4 Creep test........................................................................................... 15

4. Discussion ................................................................................................ 165. Conclusions .............................................................................................. 186. References................................................................................................ 19

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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 pre-sented by Vibeke Wegan in session 1: “Performance Testing and Specifications forBinder and Mix”, having Bernard Brûlé as chairman.

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Abstract

Examination of the structure of the polymer modified binder in a bituminous mixturehas been performed together with infrared analysis and creep test to evaluate the per-formance of a polymer modified binder course.

In a recent paving job, a random sample control of the polymer modified bindershowed that the binder had a large tendency towards separation (storage stability).The binder was modified with Styrene-Butadiene-Styrene (SBS). To examine whetherthis separation tendency of the polymer modified binder also was found in the asphaltmixture, asphalt cores were taken from the job site. These cores were examined tocharacterise the structure of the polymer modified binder, to estimate the polymercontent and to evaluate the creep performance of the asphalt mix.

The structure of the polymer modified binder has been determined by examination ofcut and ground bituminous specimens prepared as thin sections. The surface of thesethin sections was illuminated with incident UV-light whereby the polymer phase isvisible.

Thin sections and estimation of the polymer content by infrared analysis showed thatthe 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 nopolymer 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 resistancecompared to other Danish asphalt courses.

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1. Introduction

In 1999, a 4-km bypass was constructed in Denmark connecting a motorway with alarge cargo terminal. The bypass was designed for a traffic load of 6.4 millionEASELs in ten years. The unbound pavement consists of 350 mm gravel subbase and200 mm gravel roadbase. The specified asphalt pavement is 150 mm asphalt concretebasecourse, 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 poly-mer modified binder to improve the stability and rut resistance of the asphalt pave-ment.

During the paving job, a random sample control of the polymer modified bindershowed a large tendency towards separation of the polymer in the binder. It was oflarge interest whether this separation also was found in the asphalt mix with the poly-mer phase separated to the top or with inhomogeneous distribution of the polymerphase. It was also of interest to analyse whether the asphalt pavement performed asexpected or the separation tendency of the binder had resulted in an asphalt pavementwith decreased stability.

To analyse the quality of the binder course in the pavement, asphalt cores were takenat ten positions evenly distributed over the length of the bypass. The binder course inthe 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 theasphalt 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 bitu-minous material (30 x 40 mm and 20�m in thickness), have been used at the DanishRoad Institute since 1995 to characterise the structure of the polymer modified binderdirectly in a bituminous mixture by fluorescent microscopy [1]. Another type of thinsections can be produced to characterise microscopical features in the bituminousmixture using polarising and fluorescent microscopy. From these sections, informationcan be obtained about the bituminous mixture, such as filler distribution, aggregatedegradation, adhesion between aggregates and binder, signs of stripping, binder intru-sion in porous aggregate particles, location and size of micro-cracks, aggregate andfiller mineralogy etc. Finally, larger specimens (10 x 10 cm and 1 cm in thickness) areprepared as plane sections, which primarily are used to characterise the air void struc-ture in the compacted bituminous mixtures not only by content but also by size, formand distribution, which are parameters very important for the performance of a bitu-minous mixture.

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As support for the investigation in the microscope, infrared analysis has been made onrecovered binder from the top and bottom of the asphalt layer to estimate the polymercontent. The polymer content is estimated since the polymer content has been calcu-lated based on a standard calibration curve and not on a calibration curve producedfrom the exact polymer and bitumen type used at the actual paving job. To evaluate the performance, dynamic creep test were performed on asphalt corestaken from the pavement.

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2. Experimental Procedures

2.1 Preparation of thin sectionsA small bituminous mixture specimen is cut, approximately 30 x 45 mm and10–20 mm in thickness, by means of a thin saw (1.3 mm blade thickness) using a thinsection apparatus. The specimen is glued onto a plane glass slide, which helps to at-tach the specimen during the preparation procedure and impregnated under vacuumwith a colourless epoxy resin, which after curing helps to stabilise the specimen. Thesurface is then sawed very close to the impregnated surface, ground by diamondcoated rollers and polished by a rotating pellet disc.

An object glass is glued to this surface of the specimen, which is the first finished sideof the thin section. The specimen is cut once more close to the object glass, ground bydiamond coated rollers and polished to a final thickness of 20 �m.

Throughout the preparation procedures the specimen and equipment is constantlycooled to approximately -5ºC and ice cooled water is sprayed on the specimen duringall sawing, grinding and polishing to avoid smearing the polymer phase. The prepara-tion of a thin section is described in details in test procedure 30-16 [2 and 3].

2.2 Analyses of thin section by microscopyThe structure of the polymer modified binder in thin sections was investigated undera Leitz Medilux microscope with incident UV-light. The light source comes from ahigh-pressure Xenon lamp, 75 W. The microscope was equipped with a three filtersystem; an excitation filter (BP 420/490), a beam splitter filter (RKP 510) and a bar-rier filter (LP 515).

When thin sections are illuminated with the UV-light, the polymer phase, swollen bya part of the maltenes from the bitumen, emits yellow light. The fine and coarseaggregates often appear green and the bitumen phase is black. Air voids or cracks ap-pear with a yellow-green colour. The different phases can appear with other colours ifthe thin section is analysed under a polarisation microscope with transmitted light andparallel or crossed nicols. Examination with transmitted light can be a useful tool, if itis difficult to distinguish between the different phases.

Attention should be drawn to the fact that the binder in an unmodified bituminousmixture in some cases can be seen with a slightly yellow fluorescent colour when thethin sections are illuminated with incident UV-light. This is due to the poly-aromaticstructures in the maltenes in bitumen, when the bitumen has a low content of asphalte-nes.

The specimens in this study were examined with magnifications of 100 – 250.

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Figure 1 Example of the different phases when a thin section of a bituminous mixture isilluminated 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 FourierTransform SpectrometerTo estimate the polymer content on the top and bottom of the asphalt layer in each as-phalt core, a bituminous specimen was cut from the top or the bottom with a height ofapproximately 1 cm, 1 cm wide and a length of 3 cm. The bituminous specimen wasafter division recovered in Thrichloroethylene (p.a. C2HCl3). Five times, with an inter-val of approximately 20 minutes, the specimen was shaken, after which the extractwas decanted and saved in a preparation glass. The aggregate and filler were recov-ered 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 extractwas taken with a pipette without disturbing the extract. Six to ten droplets of the ex-tract were then transferred to a KBr-window in a smooth film. After evaporation of thesolvent, the KBr-window was placed in an Infrared Fourier Transform Spectrometer(Perkin-Elmer, model 1710) and the spectra recorded after 5 scannings from4000 - 400 cm-1 with a resolution of 2 cm-1. The spectra was analysed in a computerprogramme (Spectrafile-IR, version 2.0), and the SBS content calculated based on acalibration curve using the peaks at 970 and 702 cm-1 as described by Choquet and Ista[4].

2.4 Dynamic Creep testThe dynamic creep test was performed according to a Swedish test procedure [5],using the Nottingham Asphalt Tester (NAT). The test procedure is similar toBS DD 185 performed on 150-mm diameter cores with a 100-mm loading disc.The tests were performed at 40°C.

Polymerphase

Aggregate

Binder/filler phase

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3. Results

3.1 Polymer modified binderDue to the pure result of the storage stability test, the binder was not analysed after theentire test programme planned for the random spot test. Results after analysis of thebinder are given in Table 1.

Table 1. Physical Properties of the SBS-modified binder taken as a random spot test.

Properties Unit DataSoftening Point, Ring and Ball °C 52Penetration, 25°C, 100 g, 5 sec. 1/10 mm 72Penetration Index 0.2Elastic Recovery, 10°C % 71Storage stability � Softening Point, Ring and Ball� Penetration, 25°C, 100 g, 5 sec.� Penetration Index� Elastic Recovery, 10°C

°C1/10 mm

%

35547.945

Structure of the polymer phase 0.8 x 0.5 mm

3.2 MicroscopyMicroscopy of ten thin sections prepared from the asphalt layer showed that the poly-mer phase was very inhomogeneously distributed over the thickness of the layer. Thebinder phase varied from a continuous bitumen phase with nearly no polymer phase toa continuous bitumen phase with small areas of polymer phase. This variation was notonly seen between the ten asphalt cores taken from 4-km of the asphalt layer but alsobetween the top and bottom in the asphalt layer in some of the asphalt cores. A con-sistent 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 visualevaluation of the content of polymer phase is given in Table 2. The photos correspondto 0.35 x 0.53 mm.

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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 spotsof 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 10Top 3 1 3 3 3 1 2 1 2 1Bottom 3 1 3 1 2 3 1 2 1 11 = nearly no visual polymer phase 2 = small spots of polymer phase in a continuous bitumenphase and 3 = spots of polymer phase in a continuous bitumen phase.

3.3 Infrared Fourier Transform SpectrometerThe polymer content estimated based on a standard calibration curve on top and bot-tom of the binder course in each asphalt core is illustrated in Figure 3. The estimatedpolymer content in the pure binder is also given.

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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 thepeak at 970 cm-1 represents the butadiene part and 702 cm-1 represent the styrene partof the polymer. The ratio between the two peaks is normally constant for the samepolymer blended with the same bitumen. The ratio between the styrene peak and thebutadiene peak is illustrated in Figure 4 for the pure binder and for the top and thebottom of the asphalt layer in the ten asphalt cores.

Figure 4 The ratio between the peak representing the styrene part and the peak representingthe 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.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

PB AC 1 AC 2 AC 3 AC 4 AC 5 AC 6 AC 7 AC 8 AC 9 AC 10

Asphalt Core No.

SBS

cont

ent [

%]

PB TOP BOTTOM

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

PB AC 1 AC 2 AC 3 AC 4 AC 5 AC 6 AC 7 AC 8 AC 9 AC 10

Asphalt Core No.

Rat

io 9

70-1

/702

cm

-1

PB Top Bottom

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3.4 Creep testThe 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 materialstested by the Danish Road Institute are also stated for a comparison of the obtainedresults. All specimens are cores drilled from a pavement and tested at 40°C. Thethickness 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. InTable 3 the total creep after 3,600 loads and the creep rate from 2,500 to 3,600 loadsare given.

Table 3. Dynamic creep results for the bypass pavement (1) and four other pavements (2-5).

Thick-ness

Cores Total creepmm/m

Creep ratemicrostrain

Pavement layers

mm No. Diametermm

Mean Std. Mean Std.

1. AC binder course (bypass) 43 6 150 17 4 0.7 0.22. HRA 30 4 150 24 5 2.4 0.63. HRA 30 4 100 37 7 3.9 0.84. SMA 45 4 100 22 7 0.4 0.15. AC gap graded

AC binder courseAC basecourse

3250

1095 150 9 0.7 1.0 0.2

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4. Discussion

The results obtained from the storage stability test shows that the polymer phase has alarge tendency for separation towards the top. The difference in softening point, Ringand Ball between top and bottom is found to be 35°C. This value is significantlyhigher than the specified value that only allows a maximum difference of 3°C. Theother binder data measured after the storage stability test also shows an unacceptabledifference between data in top and bottom, all evidence that the polymer phase isseparated to the top. It is not known whether this separation is due to insufficientblending of the polymer and the bitumen, incompatibility between the polymer and thebitumen or a third reason.

The results indicate that the bituminous mixture had a similar problem with separationor an inhomogeneous distribution of the polymer phase in the binder. Separation or aninhomogeneous distribution of the polymer phase in the binder in the mix could meanthat the modification has no effect and hence no benefit in the way of a more rut re-sistant pavement is gained from the modification and the more expensive asphaltlayer.

Microscopy of the asphalt cores shows that the polymer phase is inhomogeneous dis-tributed in the bitumen phase. Some asphalt cores have the same content of visualpolymer phase in top and in bottom, while others have a visual difference in the poly-mer content (Figure 2, Table 2). Some asphalt cores have the largest visual content ofpolymer phase in the top, while others have it in the bottom.

Earlier studies have shown reasons for variations in the distribution of the polymerphase. In a bituminous materials produced with the same polymer modified binder, avariation in the polymer distribution is only expected if the binder is used in asphaltlayers with a significantly different thicknesses, in asphalt layers made with differentcontent of aggregates or filler or different mineralogy. Variations are also expected ifthe thermal history of two asphalt materials with the same polymer modified binder issignificantly different (mixing temperature, mixing time or mixing technique). Allthese parameters are presumed to be nearly the same for the ten asphalt cores taken thepavement 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 coresbut also between top and bottom of some of the asphalt cores. It should be noted thatthe estimated polymer content is higher in the pure binder compared to the estimatedpolymer content in the recovered binder from all asphalt cores. Whether the polymeris degraded or altered in the asphalt layer or moved to another part in the asphalt is notknown.

The FTIR analysis gives another remarkable result. Figure 4 shows that the purebinder has a larger butadiene/styrene ratio compared to the recovered binders from

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both top and bottom of all the asphalt cores. Further, it is surprising that the ratio be-tween the butadiene part and the styrene part is varying not only between the differentasphalt cores but also between top and bottom of the asphalt layer. This indicates thatthe chains of butadiene are degraded or altered with a varied extend in the bituminousmixture.

The inhomogeneous distribution of the polymer phase in the asphalt concrete bindercourse 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 coursewas designed to be rut resistant using a gap graded aggregate and a polymer modifiedbinder. The dynamic creep and the creep rate are therefore expected to be smaller thanfor other Danish asphalt courses. Compared to hot rolled asphalt (HRA) this is clearlythe case, but HRA is known not always to be very rut resistant. The creep rate of thebinder course is larger than of a SMA, even though the measurement of the creep rateof the SMA is performed on 100-mm diameter cores, which from other experience isknown to give lager creep rates than when performed on 150-mm diameter cores. Thisis seen from the creep properties given in Table 3 for the same HRA performed on100-mm and 150-diameter cores.

The creep properties of the binder course are of the same order of magnitude as thecreep properties of three courses tested as a whole including a binder course with amodified binder. It was expected that the total creep and the creep rate were signifi-cantly larger for three layers including a wearing course and a basecourse. This indi-cates that the binder course in the bypass pavement have no superior rut resistance.

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5. Conclusions

During a paving job in Denmark, a random sample control of the polymer modifiedbinder showed a large tendency towards separation of the polymer in the binder. Itwas feared that the polymer was inhomogeneously distributed in the asphalt mixperformed with this binder and hence the expected benefit of the modification was notobtained. Microscopy of thin sections made from ten cores sampled from the asphaltpavement concluded that the polymer phase was inhomogeneous distributed. Estima-tion of the polymer content by FTIR supported this conclusion and indicated that thebutadiene part of the polymer was degraded or altered. Measured dynamic creep prop-erties indicates that the binder course in the bypass pavement have no superior rutresistance. This might be caused by the inhomogeneous distribution of the polymer inthe binder.

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6. References

[1] V. Wegan, B. Brûlé (1999): "The structure of Polymer Modified Binders andCorresponding Asphalt Mixtures". Proceedings of the Association of AsphaltPaving Technologists, pp 41-64, Chicago.

[2] "Thin Sections of Polymer Modified Asphalt Mixtures". Danish Road Institute,Test procedure 30-16, February 1998.

[3] "Microscopic Analysis of Asphalt Concrete Mixtures. Preparation Techniquesfor Plane Sections. Preparation Techniques for Thin Sections". Danish Road In-stitute, Information Guide, p 13.

[4] Freddy S. Choquet and Emmanuel J. Ista (1990): "The Determination of SBS,EVA and APP Polymers in Modified Bitumens".

[5] FAS Method 468-97: "Determination of deformation resistance by dynamiccreep test" (in Swedish).

Rapporter/Reports

Nr./NoÅr/Year

85/97 Subgrade Performance StudyPart I: Materials, Construction and Instrumentation(Robin Macdonald, Susanne Baltzer)

86/97 Fifth International Conference on the BearingCapacity of Roads and AirfieldsTrondheim, July 6 - 8, 1998, Papers(Robin Macdonald, Wei Zhang, Susanne Baltzer,P e r Ullidtz, Jesper L. Lund)

87/98 Pavements Subgrade Performance StudyPart II: Modeling Pavement Response andPredicting Pavement Performance(Wei Zhang, Per Ullidtz, Robin Macdonald)

88/98 Road UnevennessPaper presented at the 1998 FISITA WorldAutomobile Congress, Paris(Bjarne Schmidt)

89/99 Development of improved mechanistic deteriorationmodels for flexible pavements(Hans Ertman Larsen, Per Ullidtz)(Electronic edition)

90/99 FriktionsmålingerSammenlignende målinger mellem ROAR ogStradograf(Bjarne Schmidt)

91/99 Grundere til broisolering- typegodkendelse- materialevalg(Jeanne Rosenberg)

92/99 The Structure of Polymer Modified Binders andCorresponding Asphalt Mixtures(Vibeke Wegan, Bernard Brûlé)

93/99 PIARC World Road AssociationInternational Experiment to HarmoniseLongitudinal and Transverse Profile Measurementand Reporting Procedures, Draft Report(Bjarne Schmidt, Jim Wambold, Akira Kawamura,Guy Descornet)(Electronic edition)

94/99 Evolution and Harmonization of EvennessEvaluation Techniques(Bjarne Schmidt)(Electronic edition)

95/99 Investigation of Gyratory Compaction used forAsphalt Mix Design(Jørn Raaberg)(Electronic edition)

96/99 Development of Models for Economic Evaluationof Pavement Maintenance: the PAV-ECO ProjectProviding an Efficient and Socially Acceptable RoadTransport Network(Gregers Hildebrand, Philippe Lepert)(Electronic edition)

97/99 Development of a Laser-Based High SpeedDeflectograph(Gregers Hildebrand, Søren Rasmussen, Raúl Andrés)(Electronic edition)

98/99 Accelerated Pavement Testing1999 International ConferenceOctober 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 totwo Freeze - Thaw Cycles,Danish Road Testing Machine RTM2: 1998(Wei Zhang, Robin Macdonald)(Electronic edition)

101/00 Responses and Performance of a Rehabilitated TestPavement to Accelerated Load TestingDanish Road Testing Machine RTM3: 1999(Wei Zhang, Robin Macdonald)(Electronic edition)

102/00 Responses and Performance of a Rehabilitated TestPavement to one Freeze - Thaw CycleDanish 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 alongroads caused by traffic and the road pavement(Knud A. Pihl, Jørn Raaberg)(Electronic edition)

105/00 Thin pavements with synthetic binder used inDenmark(Jeanne Rosenberg)(Electronic edition)

106/00 Surfacing of concrete bridges(Vibeke Wegan)(Electronic edition)

107/01 Danske asfaltbelægningers sporkøringsmodstand(Carsten Bredal Nielsen)(Electronic edition)

108/01 Effect of Design Parameters on Polymer ModifiedBituminous Mixtures(Vibeke Wegan)(Electronic edition)

109/01 Microstructure of polymer modified binders inbituminous mixtures.(Vibeke Wegan, Carsten Bredal Nielsen)(Electronic edition)