the colloidal structure of bitumen: consequences on the rheology and on the mechanisms of bitumen...

Advances in Colloid and Interface Science 145 (2009) 42–82

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Advances in Colloid and Interface Science

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The colloidal structure of bitumen: Consequences on the rheology and on themechanisms of bitumen modification

Didier LesueurEurovia España, Pol. Ind. Villapark - Avda Quitapesares, 48, 28670 Villaviciosa de Odón (Madrid), Spain

E-mail address: [email protected].

0001-8686/$ – see front matter © 2008 Elsevier B.V. Adoi:10.1016/j.cis.2008.08.011

a b s t r a c t
a r t i c l e i n f o
Available online 9 September 2008

Keywords:

The use of bitumen as a connaturally occurring in contrThis article reviews the cu

BitumenMineral fillersPolymer-modified bitumenRheologyMultiphase materials

rrent understanding of bitumen structure and the consequences in terms ofproperties, with a strong emphasis on the rheological properties. The links between chemistry, structure andmechanical properties are highlighted in the framework of an updated colloidal picture of bitumen. It showsthat a simple solvation parameter allows quantifying the effect of the asphaltenes on the rheologicalproperties of bitumen. This appears as a promising approach in order to understand more complexphenomena such as bitumen ageing or the diffusion of rejuvenating oils into an older bitumen.From this structural modelling, the effect of several modifiers, such as polymers, acids or mineral fillers, is

struction material dates back to antiquity. The materials in use then were mostlyast to modern bitumens which have become highly technical artificial materials.

explained using fundamental results from the mechanics of colloidal suspensions and multiphase materialsthrough the Palierne model. Thus, relevant parameters describing polymer-bitumen or mineral fillers-bitumeninteractions can be extracted, as detailed from literature data. In the case of mineral filler, volume fraction is thekey parameter but particle size comes also into play when fine fillers are considered. In the case of polymer-modified bitumens, the swelling extent of the polymer controls all other parameters of importance: volumefraction of dispersed phase and mechanical properties of both dispersed and continuous phases. In addition,interesting rheological features due to droplet shape relaxations are described in polymer-modified bitumens.Although a general picture of bitumen structure is shown to emerge, themany fundamental points that remain tobe addressed are discussed throughout the paper.

© 2008 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432. Bitumen chemistry and structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

2.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442.2. Bitumen manufacture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 452.3. Bitumen overall physical and chemical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 462.4. SARA fractions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

2.4.1. Saturates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 482.4.2. Aromatics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 492.4.3. Resins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 492.4.4. Asphaltenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

2.5. Ion exchange chromatography fractions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512.6. Natural surfactants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 522.7. Early model of bitumen structure: sol and gel bitumens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 522.8. The dispersed polar fluid and other homogeneous models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 532.9. A modern colloidal picture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

2.9.1. Asphaltenes micelles in bitumen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 542.9.2. Asphaltenes micelles aggregation in bitumen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 552.9.3. Use of a solvation parameter to describe asphaltenes structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

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43D. Lesueur / Advances in Colloid and Interface Science 145 (2009) 42–82

2.10. Wax crystallization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 562.11. Chemical ageing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

2.11.1. Effects on the chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 582.11.2. Effects on the rheology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

2.12. Steric hardening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 603. The rheology of paving grade bitumens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

3.1. Early evaluation of bitumen rheology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 603.1.1. Needle penetration test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 613.1.2. Ring and ball softening temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 613.1.3. Penetration index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

3.2. General features of bitumen rheology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 613.3. Performance grading (PG) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 623.4. Time-Temperature Superposition Principle (TTSP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 633.5. Modelling the linear viscoelastic behaviour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 643.6. Modelling the non-linear behaviour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 653.7. Temperature dependence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 653.8. Structural modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

3.8.1. Newtonian behaviour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 663.8.2. Newtonian/viscoelastic transition: α-relaxation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 673.8.3. Viscoelastic/elastic transition: β-relaxation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

3.9. Practical consequences of the structural modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 683.9.1. Effect of the bitumen manufacture process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 683.9.2. Bitumen distribution of relaxation times and the origin of its viscoelastic behaviour . . . . . . . . . . . . . . . . . . . . . 683.9.3. Ageing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 693.9.4. Blending of bitumens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 693.9.5. Bitumen rejuvenation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 693.9.6. Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

4. Principles of bitumen modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 704.1. Acid modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 704.2. Rheology of multiphase viscoelastic materials: the Palierne model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 704.3. Application to mastics: the suspension limit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 714.4. Application to polymers modified bitumens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

4.4.1. Polymer/bitumen compatibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 734.4.2. The structure of polymer-modified bitumens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 744.4.3. Polymer-rich phase rheology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 754.4.4. Mechanical modelling of polymer-modified bitumens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 754.4.5. Practical consequences: parameters governing the rheology of polymer-modified bitumens . . . . . . . . . . . . . . . . . . 764.4.6. Practical consequences: combined modification by acid and polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

4.5. Synthetic bitumens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 775. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

1. Introduction

The earliest record of human use of bitumen to date, is as a haftingmaterial 180,000 years ago in the El KowmBasin in Syria, where it wasapplied to stick flint implements to the handles of various tools in away that persisted until Neolithic time [1]. At first, its adhesive andwaterproofing properties were generally emphasized. Even the Biblecites examples such as the waterproofing of Noah's arch, of the Babeltower or of the cradle of Moses [1,2]. Medical uses were also reported,with bitumen acting as a remedy for various illnesses (trachoma,leprosy, gout, eczema, asthma ...), as a disinfectant or as an insecticide[1–3]. Another well studied historical application was for theembalming of mummies by the Egyptians [2–4].

The first mention of the use of bitumen in road constructiondates back to Nabopolassar, King of Babylon (625–604 BC): abitumen-containing mortar cemented both the foundation made ofthree or more courses of burnt bricks and the stone slabs put on top[2]. However, bitumen essentially disappeared from the pavementsuntil the early 19th century, when the recently rediscoveredEuropean sources of natural bitumen led to the development ofthe modern applications for this material [5]. The use of naturalbitumen in road construction started to decay in the 1910s with theadvent of vacuum distillation which made it possible to obtainartificial bitumen from crude oil [6] and nowadays, paving gradebitumen is almost exclusively obtained as the vacuum residue ofpetroleum distillation.

At the present time, 95% of the almost 100 Mt of bitumen that areproduced worldwide each year are applied in the paving industrywhere they essentially act as a binder for mineral aggregates to formasphalt mixes, also called bituminous mixes, asphalt concrete or bitu-minous concrete. Asphalt mixes are generally fabricated by firstheating typically 5 wt.% bitumen up to around 160 °C in order todecrease its viscosity and then blend it with 95 wt.% aggregates. Thisprocess is referred to as hot mixes, and the specifications on bitumenfor pavements are mostly based on this application. Other techniquesto manufacture asphalt mixes using bitumen emulsions or foamedbitumen are also available, but they amount to less than 5% of the totalasphalt mix production.

In order that the mixes resist climate and traffic, specifications onpaving grade bitumens have become quite severe. The properties thatare needed to obtain suitable bitumen aremostly rheological. First, thebitumen has to be fluid enough at high temperature (around 160 °C) tobe pumpable and workable to allow for a homogeneous coating of theaggregates upon mixing. Second, it has to become stiff enough at thehighest pavement temperature to resist rutting (around 60 °C,depending on local climate). Third, it must remain soft enough atthe lowest pavement temperature to resist cracking (down to around−20 °C, depending on local climate). All these properties are quiteopposite, and it is therefore difficult to obtain bitumen that wouldwork under all possible climates. As a consequence, different pavinggrades exist, the softer being generally suitable for cold climates andthe harder, for hotter regions. In order towiden the temperature range

44 D. Lesueur / Advances in Colloid and Interface Science 145 (2009) 42–82

of bitumen, additives such as polymers and/or acids are increasinglyused.

The purpose of this article is therefore to explain the current statusof bitumen science, with special emphasis on the relationshipsbetween the structure and the rheological properties. It intends togive a simple physical picture of bitumen that helps understandknown results on the effect of different modifiers. Clearly, theemphasis is on paving grade bitumens but many of the featuresshould equally apply to heavy oil or to industrial grades of bitumen.

In a first section, the structure of bitumen is presented using anupdated colloidal model and its consequences on the mechanicalproperties are described. The limits to our current understanding arealso highlighted.

Next, three kinds of modifications are discussed. The first one isthe modification of bitumen by an acid. This process has become ofincreasing importance in recent years, especially for the productionof harder grades of bitumen. The effect of the acid can directly beunderstood in the light of the colloidal model. Then, the modificationof bitumen by mineral fillers is addressed. Bitumen andmineral fillerblends are usually referred to as mastics. Mineral fillers are generallynot commercial modifiers as such. Since they are always present aspart of the mineral aggregates, a mastic is indeed formed in-situwhen manufacturing a hot mix. Therefore, it is in this mastic formthat the bitumen really acts within the final mix, hence its criticalimportance. Finally, the last modification presented is that by apolymer. It is the most important commercial modifier at themoment, with an estimate of 4 Mt of Polymer-Modified Bitumen(PMB) produced in the United States [7]. This represents almost 10%of the total road binder market.

The effects of fillers and polymers are explained in the light ofknown results on model viscoelastic systems, i.e., suspensions andemulsions, using the Palierne model. This clarifies how and why theseadditives modify the properties of the bitumens, highlighting therelevant physico-chemical parameters governing the process.

2. Bitumen chemistry and structure

2.1. Definitions

Many definitions were proposed for bitumen, asphalt and relatedsubstances, some of them quite opposite and sometimes scientificallyincorrect [2,8]. Incorrect definitions are those presenting bitumen as apasty or semi-solid material [3], when the correct description wouldbe that of a viscous viscoelastic liquid (at room temperature), as will bedescribed in more details in Section 3.

In 1728, Chambers, in the first modern dictionary, defined bitumenas a generic term encompassing naphtha, petroleum, pitch and mostmineral hydrocarbons forms, whether hard, soft or liquid [9]. Nowa-days, bitumen is still the viscous mostly-hydrocarbon component ofnatural asphalts, and can be thought of as an extra heavy crude oil[2,10]. This definition was however extended to include the viscouspetroleum distillate (or residue) containing a low percentage ofvolatile compounds.

More generally, bitumen is now defined as a “virtually involatile,adhesive and waterproofing material derived from crude petroleum, orpresent in natural asphalt, which is completely or nearly completelysoluble in toluene, and very viscous or nearly solid at ambienttemperatures” in the current European specifications [11]. Solubilityin toluene superior to 99% is asked for in the paving specifications [12].We will stick to this definition, which can be thought of as theEuropean definition, the American synonym for bitumen being asphaltcement or simply asphalt [3].

In the European termonology, asphalt or native/natural asphaltrepresents the natural deposits consisting of natural bitumen andmineral matter. Some of these deposits are of high historicimportance. As an example, the asphalt from the Dead Sea, also called

Lake Asphaltite, is the main source of asphalt described in theliterature from ancient times until the modern era. It exists there inthe form of an asphaltic limestone, containing typically 5% bitumenimpregnating a porous limestone [2]. Still, Dead Sea bitumen could befound in an almost mineral-free form through seepages floating abovethe Dead Sea. The sources, from which the modern use of bitumenevolved, especially in Europe, were the deposits of Seyssel in Franceand Val de Travers in Switzerland. Both were asphaltic limestones too,the Seyssel one containing 8% bitumen, and the Val de Travers one,between 8 and 12% bitumen [2,5]. The reference material for pavingapplications in the US at the beginning of the 20th century was theTrinidad Lake asphalt, which is one of the largest asphalt deposits inthe World. Its composition is somewhat different from the other just-mentioned sources, as it contains 39% bitumen, 33% emulsified waterand 27% mineral matter in the form of colloidal clay [2,5]. Anothersource of great importance for the US market at the beginning of the20th century was the Bermudez Pitch Lake asphalt from Venezuela,another of the largest deposits in the World. It contains 64% bitumen,30% water, 4% insoluble organic matter and 2% dispersed mineralmatter [2]. In some cases, the words native/natural bitumen is directlyused for asphalts whose main component is bitumen, such as theTrinidad Lake asphalt.

To avoid confusion, we will use the word bitumen in the Europeansense throughout this paper and leave the word asphalt to describethe natural deposits such as those just-described.

For the geochemists, bitumen is widely used to describe extraheavy petroleum, in conceptual agreement with the above definition.More precisely, UNITAR (United Nations Institute for Training andResearch - [13]) separates between a heavy crude, defined as onehaving a density at 15.6 °C (60°F) between 0.934 and 1 g/cm3

(between 10 and 20° in the American Petroleum Institute API gravityscale) and a viscosity at 15.6 °C inferior to 10,000 cP, an extra-heavycrude defined as one having a density superior to 1 g/cm3 (inferior to10° API) and a viscosity inferior to 10,000 cP and a tar (used as asynonym for bitumen) as one having a density superior to 1 g/cm3

(inferior to 10° API) but a viscosity superior to 10,000 cP.Note that the word tar generates a somewhat confusing nomen-

clature. As exemplified by the UNITAR definition, a sandstonereservoir impregnated by viscous extra heavy crude oil is commonlycalled tar sand, when a more appropriate labelling would be bitumi-nous sand [10]. This choice of words is necessary to avoid confusionbetween bitumen and coal tar. This latter is the residue of thepyrolysis of coal, with very similar uses and properties than bitumen.Unfortunately, the name coal tar remains widely used in every daylanguage as a general term for a black pavingmaterial when its use hasalmost disappeared due to economic reasons such as the decline of thecoal industry and the rise of the all-petroleum era in the 1950s. Safetyreasons finally ended coal tar use, as a consequence of its high contentof carcinogenic PolyAromatics Hydrocarbons (PAH), which are onlypresent as traces in bitumen [14]. Preferably, and in a more rigoroussense, the word tar should be restricted to the products of thedestructive distillation (pyrogenation) of various organic sources(coal, wood …).

Note that bitumen for the geochemist sometimes takes on anothersomewhat different meaning, when it is opposed to kerogen, theinsoluble organic matter present in sedimentary rocks [10,15]. Thislast definition encompasses materials of different composition andproperties, and bitumen, as described in the rest of this article,probably does not apply to this kind of product.

Finally, and since bitumen is essentially defined by its solubility intoluene, it is worth mentioning that other types of natural bitumen-like materials exist. They are very hard bitumens, partly insolubles intoluene (or in other solvents) and are generally further separated intoasphaltites and (asphaltic) pyrobitumens [2,10]. They have howeverdifferent physical properties and it is not clear at themoment whetherwhat is known of bitumen could apply to these classes of materials.


They are sometimes used as an additive to bitumen in order to obtain aharder grade [18]. It remains to be demonstrated whether hardbitumens obtained by these means behave like any other hard gradebitumen.

2.2. Bitumen manufacture

Until modern refining technologies became available at the turn ofthe 20th century, bitumen was only recovered from natural sources[5]. Currently, bitumen is essentially obtained by the distillation ofcrude oil [16,17]. Not every crude oil yields sufficient amounts ofbitumen and about one tenth of the available petroleum is foundsuitable. As a rule of thumb, the heavier the crude oil, the higher itsbitumen yield [16,17]. Native bitumen is still in use in the pavingindustry but for small and very specific markets, generally as anadditive for straight-run bitumen [18].

The typical distillation process (Fig. 1) consists in first separatingthe light components from the crude oil by atmospheric distillation attypically 350 °C. In a second step, the residue from the atmosphericdistillation is further refined at a slightly higher temperature (350–425 °C) but under vacuum with pressures of order 1–10 kPa (0.01–0.1 bars) [10,17]. Bitumen is then the vacuum residue of the crude oil,corresponding to an equivalent atmospheric cut-point of typically500 °C [10]. The properties of the resulting material, sometimes calledstraight-run bitumen, depend on the crude origin and actual operatingconditions. Generally, refiners manufacture only two grades ofbitumen, a soft one and a hard one, and the intermediate grades areobtained by blending [16]. With the advent of new vacuum distillationcolumns with structured packing internals, harder grades can now be

Fig. 1. The distillation of bitum

obtained by distillation, sometimes referred to as special bitumens[18]. This wording encompasses however another product known asmultigrade bitumen, which is not necessarily a hard grade but has awider operating temperature range than normal bitumen and can beobtained not only through new refining processes but also throughacid modification.

Although of less commercial importance, other techniques stillexist to obtain bitumen from petroleum. Air-blowing developed in thelate 19th century and was the first process that led to the manufactureof bitumen from an otherwise too fluid crude source [6,10]. It consistsin air-oxidizing the oil (or soft bitumen) for a few hours at 200–275 °C,sometimes in the presence of a catalyst, such as copper sulphate, zinc,ferric or aluminium chloride, boric acid or phosphorous pentoxide[16]. The properties of the resulting material, called air-blown or oxi-dized bitumen, depend on the crude origin (or the soft bitumen) andthe operating conditions, especially blowing temperature and time. Itremains the best process to make very hard bitumens, especially forwaterproofing. For paving applications, however, air-blown bitumenswere discarded because they were too susceptible to cracking [18].When a soft bitumen is gently blown to pass paving specifications, thecorresponding bitumen is sometimes called semi-blown bitumen.

Another process of less commercial importance is solventdeasphalting, the typical solvent for recovering bitumen beingpropane [16]. The heavy fraction of a crude oil (or soft bitumen) ismixedwith 3–10 times its volume of liquefied propane and then left tosettle [10]. Typical conditions are a temperature in the 25–70 °C rangeand a pressure of 120 kPa (12 bars) to maintain propane in the liquidstate. Bitumen is then the propane-free precipitate obtained aftersettling. This process developed in the 1930s as an alternative for

en (courtesy from Nynas).

Table 1Elemental analysis for the core SHRP bitumens

Origin AAA-1 AAB-1 AAC-1 AAD-1 AAF-1 AAG-1 AAK-1 AAM-1

Canada USA Canada USA USA USA Venezuela USA

C wt.% 83.9 82.3 86.5 81.6 84.5 85.6 83.7 86.8H wt.% 10.0 10.6 11.3 10.8 10.4 10.5 10.2 11.2H+C wt.% 93.9 92.9 97.8 92.4 94.9 96.1 93.9 98.0H/C Molar 1.43 1.55 1.57 1.59 1.48 1.47 1.46 1.55O wt.% 0.6 0.8 0.9 0.9 1.1 1.1 0.8 0.5N wt.% 0.5 0.5 0.7 0.8 0.6 1.1 0.7 0.6S wt.% 5.5 4.7 1.9 6.9 3.4 1.3 6.4 1.2V ppm 174 220 146 310 87 37 1480 58Ni ppm 86 56 63 145 35 95 142 36Mn g/mol 790 840 870 700 840 710 860 1300

Data from [24].

Fig. 2. Functional groups present in bitumen reprinted from [26] with permission byNational Academy of Science.


vacuum distillation but proved less industrially successful although itremains the preferred method for recovering lubricants, bitumenbeing then a by-product [6,10].

Yet another process for obtaining bitumen is as the residue from avisbreaking unit [10]. Visbreaking is a mild thermal cracking processoperated at 455–510 °C and 0.3–2 MPa (3–20 bars) for a shortresidence time (1–3 min), which allows for reducing the viscosity ofresidua or heavy oils, which can then enter the distillation units [10].

2.3. Bitumen overall physical and chemical properties

Bitumen density at room temperature lies typically between 1.01and 1.04 g/cm3, depending on the crude source and paving grade [17].As a rule of thumb, the harder it is, the denser the bitumen.

When referring to a natural source, a heavy crude oil is generallytermed bitumenwhen its density overpasses 1 g/cm3 at 15.6 °C (that isbelow 10° in the API scale), although a definite classification remainsto be published [10].

Bitumen exhibits a glass transition around −20 °C, although itvaries in a very wide range from +5 °C down to −40 °C dependingessentially on the crude origin and somewhat less on the process[19–23]. The transition range typically spans 30 to 45 °C and −20 °Ccorresponds to the typical midpoint value [23]. Therefore, on a ther-modynamical standpoint, bitumen is a very viscous liquid at roomtemperature.

The complexity of bitumen chemistry lies in the fact that manydifferent chemicals are present. As an overall descriptor, thechemical nature of the crude oil is generally described as paraffinic,naphthenic or aromatic if a majority of saturate, cyclic or aromaticstructures, respectively, are present [10]. This classification of thepetroleum is sometimes applied to the corresponding bitumen. Forexample, Venezuelan bitumens are generally known as naphthenicbitumens.

The elemental composition of a bitumen depends primarily on itscrude source and it is difficult to give geographical generalization[17,24,26]. The data in Table 1 illustrate this fact, showing that variousmaterials from the USA or Canada can have quite different composi-tions. The data in Table 1 come from the impressive work on bitumenchemistry, structure and properties realized in the USA as part of theStrategic Highway Research Program (SHRP) in the late 1980s [25].Since eight bitumens of extreme properties (the SHRP core bitumens)were fully characterized, they will be used throughout the paper toillustrate the key features of bitumen properties. The coding (AAA-1…) refers to the one used in the SHRP materials library [24].

As described in Table 1, bitumenmainly consists in carbon (typically80–88 wt.%) and hydrogen atoms (8–12 wt.%). This gives a hydrocarboncontentgenerally superior to90wt.%with anhydrogen-to-carbonmolarratio H/C around 1.5. This H/C ratio is therefore intermediate betweenthat of aromatic structures (benzene has H/C=1) and that of saturatealkanes (H/C ∼2) [17,26] (Table 1).

In addition, heteroatoms such as sulphur (0–9 wt.%), nitrogen (0–2 wt.%) and oxygen (0–2 wt.%) are generally present. Traces of metalsare also found, the most numerous being typically vanadium, up to2000 parts per million (ppm), and nickel (up to 200 ppm) (Table 1 -[10,17,22,26]).

Sulfur is generally themost present polar atom (Table 1). It appearsin the form of sulfides, thiols and, to a lesser extent, sulfoxides (Fig. 2).Oxygen is typically present in the form of ketones, phenols and, to alesser extent, carboxylic acids (Fig. 2). Nitrogen exists typically inpyrrolic and pyridinic structures and also forms amphoteric speciessuch as 2-quinolones [10,26] (Fig. 2). Most of the metals formcomplexes such as metalloporphyrins [10].

Given the concentration of polar atoms, functional groupsgenerally do not amount for more than a few 0.1 mol/l for straight-run bitumens [26]. Their concentration can however increase uponageing, as will be detailed later on (§ 2.10).

The number-average molecular weight of bitumen falls typically inthe range 600–1500 g/mol (Table 1 - [26,27,28]). Still, the distributionextends tomolecular weights up to 15,000 g/mol and the values foundin the literature can vary somewhat depending on the experimentalset-up [29]. The values given in Table 1 were measured by VaporPressure Osmometry in toluene and pyridine at 60 °C (ASTM D2503).

Average molecular structures for bitumen have been proposed(Fig. 3 - [30]). Themolecular weight of the average structure equals theaverage molecular weight of the studied bitumen and the atoms aredistributed in order to give a similar NMR spectrum as the oneobtained for the corresponding bitumen [30]. Note that bitumen

Fig. 3. Average molecular structure for two bitumens of extreme compositions:California Coastal (AAD-1 - A) and West Texas Intermediate (AAM-1 - B) (after [30]).

Fig. 4. Separation into SARA fractions for the core SHRP bitumens (after [24]).


molecules are thereforenotmacromolecules in thepolymeric sense.As aconsequence, caremust be takenwhen trying to compare the propertiesof polymers to that of bitumen, especially when it comes to themodelling of the viscoelastic properties based on amolecular approach.

Given these molecular weights and the proportion of polar atoms,it is clear that only 1–3 polar atoms are present on average in eachbitumen molecule, as pictured in Fig. 3.

Still, approaching bitumen chemistry on a global basis is notsufficient when one tries to understand the properties of bitumen.Thus, the molecules are generally separated into different chemicalfamilies, depending on their size and solubility in polar, aromatic ornon-polar solvents.

2.4. SARA fractions

In 1836, Boussingault separated by distillation (at 230 °C for a fewdays) two components of a native bitumen from the city ofBechelbronn (meaning “pitch fountain”, nowMerkwiller-Pechelbronnin Alsace, France): 85 wt.% of a distillable fraction he called petroleneand of 15 wt.% of a solid fraction, he called asphaltene [31,32].Petrolene was a yellow liquid of density 0.891 at 21 °C, consisting of apure hydrocarbon with H/C=1.56. Asphaltene was found to be a solidwith H/C=1.58 and contained 14.8 wt.% of oxygen. Asphaltene was acompound very similar in composition and properties to naturalasphalt, such as that from Coxitambo (Peru), hence the name. For the

record, the Coxitambo asphalt would now be classified as an asphalticpyrobitumen [2]. Given their similar H/C ratio, Boussingault suggestedthat asphaltene originated from the oxidation of petrolene [31].

Richardson defined a few decades later the asphaltenes as theinsoluble part of bitumen in naphtha from paraffin petroleum of 62 or88° Baumé [33]. This corresponds to absolute densities of 0.729 and0.642 respectively. As a comparison, linear n-alkanes of same densitywould be decane and a 50/50 hexane/pentane blend respectively. Thesoluble part was called the malthenes, because of their resemblance tomaltha, a soft kind of native bitumen [33]. For some unknown reason,malthenes lost their “h” in the course of the 20th century, and the mostcommon spelling is now maltenes (although the original orthographsometimes reappears), which will be adopted in the rest of this article.

In a strict sense, and followingRichardson's definition [33], petrolenewould be best limited to the distillable fraction of bitumen whenmalteneswould be thebitumen fraction soluble in n-heptane. Still,mostauthors use them interchangeably as the non-asphaltenes part ofbitumen andwewill use thewordmaltene in the restof this paper as theoily constituent of bitumen as opposed to the asphaltenes.

Richardson defined the asphaltenes as insoluble in naphtha butsoluble in carbon tetrachloride and also introduced the terms carbenesfor the fraction insoluble in carbon tetrachloride but soluble in carbondisulfide [33]. The word carboids for the insoluble part in carbondisulfide is also seldom used [10] (although not used by Richardson[33]). In all cases, these two additional fractions are present in verylimited amounts in paving grade bitumens [34] and are generally notmentioned in the road industry.

Interestingly, Richardson recognized that asphaltenes and mal-tenes were complex blends of hydrocarbons and suggested to use theplural forms, instead of the singular used by Boussingault [33].

Since maltenes are by far the most numerous components,separating bitumen into only two fractions rapidly proved unsatisfy-ing. Initially, solvent extractionwas the only way to refine the analysis.In fact, as early as 1879, Kayser separated bitumen into three fractions,α, β and γ bitumens based on their solubility in alcohol, ether andchloroform [3,35], but this method did not persist. A three fractionssolvent extraction method of more success was that by Hoiberg andco-workers in 1939. They further separated the n-hexane maltenesinto resins and oils, defining the resins as the pentane precipitate andthe oils as the soluble part [36].

The separation of the maltenes into more fractions really improvedwith the advent of modern chromatographic methods. In fact, suchtechniques were used on bitumen as early as 1908 [36]. The referencemethod is now elution-adsorption liquid chromatography on activealumina with solvents of increasing polarity and aromaticity asproposed by Corbett [37]. His set-up is now the reference methodfor the separation of the maltenes into saturates, aromatics and resins,

Fig. 5. Separation of bitumen into its various fractions, highlighting the SARA fractions after [43].


according to their elution by, respectively, n-heptane, benzene and afinal two-steps elution by first a 50/50 blend of benzene andmethanolfollowed by trichloroethylene. Thanks to this method, the composi-tion of bitumen is usually given in terms of the relative quantity of itsso-called SARA fractions for Saturates, Aromatics, Resins and Asphal-tenes (Fig. 4). Note that Corbett did not use the words aromatics andresins but naphthene aromatics and polar aromatics instead [37] andwas not the first to use these fraction names [36].

Nowadays, other solvents if not experimental set-ups are preferredfor safety reasons and convenience (Fig. 5 - [10]). The referencemethod is the ASTM D-4124 [38], also in use for crude oil [10], andquite similar to the Corbett procedure. The first step consists inseparating the asphaltenes by n-heptane precipitation. This is done byblending typically 12 g of bitumen in 1.2 L of n-heptane and recoveringthe insoluble part after stirring 1 h near the boiling point of n-heptane(98 °C). Note that the deasphaltened bitumen is called “petrolenes” inthe standard but, as discussed before, we would preferably call itmaltenes as it is the more generally accepted name.

In a second step, the maltenes in solution at typically 10 g in 50 mlof n-heptane, are passed through a chromatographic column contain-ing CG-20 chromatographic grade alumina. Saturates are firstseparated using n-heptane as an eluant. Then, pure toluene followedby a blend 50/50 with methanol, are used to separate the aromatics.Finally, trichloroethylene is used to separate the resins. Fig. 5 describesthe separation method although with somewhat different solvents(toluene only for the aromatics and pyridine for the resins).

Another widely used experimental set-up is the coupling betweenthin layer liquid chromatography on silica gel and a flame ionisationdetector (IATROSCAN), usingeither the set of ASTMsolvents [39] orothersuccessive eluants. For example, Leroy used cyclohexane, dichloro-methane and a 70/25/5 blend of dichloromethane/methanol/isopropa-

Table 2Chemical properties of bitumen and the SARA fractions: typical H/C, elemental analysis, nu

H/C C H O N

– % % % %

Bitumen 1.5 80–88 8–12 0–2 0–2Saturates 1.9 78–84 12–14 b0.1 b0.1Aromatics 1.5 80–86 9–13 0.2 0.4Resins 1.4 67–88 9–12 0.3–2 0.2–Asphaltenes 1.1 78–88 7–9 0.3–5 0.6–

Data from [26,37,39,42,43,57].

nol for saturates, aromatics and resins respectively [40]. Anotherexample, closer to the ASTM approach, consists in using n-heptane, a80/20 toluene/n-heptane blend and a 95/5 blend of methylenedichlor-ide/methanol for the saturates, aromatics and resins respectively [39].

The changes in the experiment set-up, mainly eluant nature butsometimes also the stationary phase, significantly affect the relativeproportion of each fraction [26,36]. It is therefore crucial to state theexperimental conditions when comparing the chemical compositionof various bitumens. Even though the exact composition of each ofthese chemical families depends on the crude origin and on theexperimental set-up, they show some common features and overallproperties that remain pretty much unchanged, as described in thenext sections. This leads to the conclusion that bitumen must bethought of as a chemical continuum with a gradual increase of molarmass, aromatic content and polarity from saturates to asphaltenes.

2.4.1. SaturatesSaturates usually amount for 5–15 wt.% of a paving grade bitumen

([37] - Fig. 4). They form a colourless or lightly coloured liquid at roomtemperature [37] because of a very low glass transition temperaturearound −70 °C, that is typically 40 °C below the glass transition of theirparent bitumen.

Their H/C ratio is close to 2, with only traces of hetero atoms. Theycontain a few crystalline linear n-alkanes, typically in amounts of 0–15 wt.% of the overall fraction [20]. Their number-average molecularweight is around 600 g/mol (Table 2) and they are mainly aliphatic:Fourier-Transform Infra-Red spectroscopy (FTIR) reveals differentbranching structures and some long aliphatic chains. Very few polaratoms or aromatic rings are present [41].

Their solubility parameter lies between 15 and 17 MPa0.5 [10] andtheir density at 20 °C, around 0.9 g/cm3 [37].

mber average molar mass and solvent used in ASTM D-4124

S Mn Solvent in ASTM D4124

% g/mol

0–9 600–1500 –

b0.1 470–880 n-heptane0–4 570–980 toluene and toluene/methanol 50/50

1 0.4–5 780–1400 trichloroethylene4 0.3–11 800–3500 n-heptane insoluble


2.4.2. AromaticsAromatics, also called naphthene aromatics, are the most abundant

constituents of a bitumen together with the resins, since they amountfor 30–45 wt.% of the total bitumen ([37] - Fig. 4). They form a yellowto red liquid at room temperature [37]. They are somewhat moreviscous than the saturates at the same temperature, because of ahigher glass transition temperature around −20 °C, which is similar tothat of the parent bitumen [20,21].

Their carbon skeleton is slightly aliphatic with lightly condensedaromatic rings [41] and a number-average molecular weight of order800 g/mol (Table 2).

Their solubility parameter is between 17 and 18.5 MPa0.5 [10] andtheir density at 20 °C close but inferior to 1 g/cm3 [37].

2.4.3. ResinsResins, also called polar aromatics, can be numerous (30–45 wt.%

[37] - Fig. 4). Depending on the solvent used, they can outnumberaromatics. As a matter of fact, the Corbett resins incorporated thefraction eluted by the benzene/methanol blend. On the contrary, ASTMD4124 considers the toluene/methanol eluted fraction as part of thearomatics.

If saturates and aromatics are oily liquids at room temperature,resins form a black solid at room temperature [37] and it is not clearwhether they exhibit a glass transition [20,21].

Koots and Speight showed that their composition is close to that ofasphaltenes except for their lower molar mass, of circa 1100 g/mol, asomewhat higher H/C ratio between 1.38 and 1.69 andmost of all a lesscomplex aromatic structure ([42] - Table 2). Piéri showed that they cansometimes be more polar than asphaltenes, but again with lesscondensed aromatic rings [41]. They typically contain fused aromaticrings, with amost probable structure corresponding to 2–4 fused rings[41].

Their solubility parameter lies between 18.5 and 20MPa0.5 [10] andtheir density at 20 °C, close to 1.07 g/cm3 [37].

They play a crucial role in the stability of bitumen, since they act asa stabilizer for the asphaltenes as will be detailed later (§ 2.9).

Fig. 6. Average molecular structures for asphaltenes from oilsands: Athabasca (Canada) asphasphaltenes (N) and Sunnyside (Utah, USA) asphaltenes. The numbers are dimensions in Å

2.4.4. AsphaltenesAsphaltenes represent generally between 5 and 20wt.% of a paving

grade bitumen ([37,43] - Fig. 4) and are by far the more studiedbitumen fractions because of their viscosity building role [26]. Theyare also of great interest to petroleum chemists for their importance inthe processing of crude oil [44]. As they represent the low solubilityclass of bitumen molecules, geochemists believe they hold somesimilarities with kerogen [45,46].

Asphaltenes now are defined as the insoluble part of a bitumen (or acrude oil) in n-heptane (ASTM D3279) but soluble in toluene. Note thatsolubilitymust be understood here as “not generating a precipitate" andnot as molecular solubility, since asphaltenes are known to formmicelles in toluene, as detailed later on. Sometimes, the insolubility isdefined in other solvents such as n-pentane or n-hexane. Generally, thelower the carbon number of the alkane, the higher the asphaltenescontent [26,34,36,47,48]. Note thatwhen dealingwith highwax contentmaterials, the procedure might generate a joint-precipitation ofpolycristalline waxy materials as observed with crude oils [49].

Asphaltenes form a black powder at room temperature [37] andare largely responsible for the black colour of the bitumen. They do notdisplay any thermal transition in the normally investigated tempera-ture range (up to 200 °C [20]).

Their solubility parameter lies between 17.6 and 21.7 MPa0.5

[10,50,51] and their density at 20 °C close to 1.15 g/cm3 [37]. It washowever shown by Rogel that the solubility parameter can vary withthe aggregation state and therefore these values must be used withcare [52].

Their number-average molecular weight is estimated to be 800–3500 g/mol by Vapor Pressure Osmometry (Table 2), one of the veryfew techniques allowing dissociating the micelles [10,48,53].

Their elemental analysis is stable fromone asphalt to the otherwithaH/C ratio between0.98 and1.56 ([42] -Table 2). As amatter of fact, then-pentane asphaltenes from petroleum exhibit almost a constant H/Cof 1.15 [43]. They usually gather the traces of transition metals of thewhole bitumen, froma fewppmup to a few tenth ofwt.% of Ni, Va, Fe,...,in the form of complexes such as metallo-porphyrins [26].

altenes recovered using distinct procedures (AM-W, AM-T and AS), Dahomey (Nigeria)(reprinted from [61] with permission by Elsevier).

Fig. 7. Formation of dimer, trimer and tetramer for a Venezuelian asphaltene molecule (reprinted from [53] with permission by American Chemical Society).

Fig. 8. The Yen model of “pure” solid asphaltenes (reprinted from [53] with permissionby American Chemical Society).


Ultra-Violet fluorescence [41,54], FTIR [41,55], X-Ray Ramanspectroscopy [56] and Nuclear Magnetic Resonance [55,57,58] showthat they contain fused aromatic rings, with a most probable structurecorresponding to 4–10 fused rings, causing a maximum UV-fluores-cence around 450 nm [54], together with some pending aliphaticchains. Average molecular structures for asphaltenes have beenpublished [46,52,59,60], examples of which are reproduced in Fig. 6.In comparison to other bitumen molecules, asphaltenes contain morecondensed aromatic rings andmore polar groups (Fig. 6 - [61]) but thepresence of fused aromatic structures seems to be the fundamentalfeature differentiating the asphaltenes from the rest of the bitumenmolecules.

Because of the many condensed aromatic rings, asphaltenes formalmost planar molecules that can associate through π–π bonding toform graphite-like stacks (Fig. 7 - [44,55]). Computer simulationsshow that the association energy difference between two monomersand a dimer in vaccuum lies around 200–400 kJ/mol [52,62]. Theenergy difference decreases to 40–200 kJ/mol when the association isstudied in a solvent such as toluene or n-heptane [52]. Interestingly,the solubility parameter of the asphaltenes aggregate decreased as thenumber of molecules in the aggregate increased: values between 14and 18 MPa0.5 were found for the monomer while values of 13–14MPa0.5 were found for the trimer [52]. This comes from the fact thatthe aliphatic structures of themolecules are found on the external partof the aggregates (Fig. 7). Note that multimer formation may beenhanced by the presence of polar interactions between oxygenfunctional groups as observed on model pyrene compounds [63].

As a consequence of this association behaviour, X-Ray diffractionspectra of “pure” solid asphaltenes highlight two main features[48,64,65]: an amorphous peak at 2θ=19° together with a diffractionpeak at 2θ=26°, corresponding to the Bragg angle for (0 0 2) planes ofgraphite. The amorphous peak mainly comes from the aliphaticstructures of the molecules. The 26° peak is very wide, meaning thatthe graphite-like crystals are very small. The half-width of the peak was

used to estimate that their size is of order of 2–5 nm; therefore, thesegraphite-like crystals contain an average of 5 unit sheets [64]. Usingother experimental techniques, it was deduced that aggregates withmore than 8 sheets are quite unlikely [45]. The often-cited Yenmodel ofasphaltenes which essentially derives from X-Ray diffraction studies,pictures the stacking behaviour of solid asphaltenes (Fig. 8 - [53]).

When put in a solvent, the asphaltenes still associate and theaggregation process described in Fig. 7 leads to what is generallycalled “micelles” [10,66–69]. Micelle size depends primarily on thesolvent nature, on the asphaltenes content and on temperature, asshown by their wide range of measured apparent molar masses[48,53,69]. The aggregation process persists inside the bitumen and

Fig. 9. Separation of bitumen into strong and weak acids, strong and weak bases andneutrals through IEC (after [83]).


form the basis of the colloidal structure of bitumen as will be detailedlater (§ 2.8).

Also, asphaltenes carry permanent electrical charges [70,71] andtheir dielectric constant lies between 5 and 7 [72,73,74]. The electrical

Fig. 10. Separation of bitumen into acids, bases, am

charge is thought to arise as a consequence of the high degree ofelectron delocalization due to the graphite-like stacking of themolecules, giving an overall negative charge [71,44]. In consequence,asphaltenes micelles may behave like ions in solution [70,75].

Thanks to the presence of polar groups, together with polycyclicaromatics and metal-complexes, asphaltenes greatly contribute to thesurface activity and the adhesion of bitumen onto mineral aggregates[76–78].

2.5. Ion exchange chromatography fractions

Even if SARA fractionation is the most popular separation method,Ion Exchange Chromatography (IEC) gives a complementary picture ofbitumen chemistry and has found increasing interest in the past20 years. It separates bitumen into five fractions corresponding tostrong and weak acids and bases, and neutral components [26], butthe recovered fractions do not exhibit the progressive range ofproperties found in the SARA fractions. It therefore proves morecomplicated to understand bitumen structure based on these IECfractions. They are however of great interest when the chemical andinterfacial properties of bitumen are studied.

Bitumen components were separated into IEC fractions startingfrom the pioneering work of Boduszynski and coworkers in 1977[79,80]. The most used set-up however is the one proposed by Greenet al. [81] and further developed by SHRP researchers [26]. It allows toseparate bitumen components into strong and weak acids, strong andweak bases and neutral fractions (Fig. 9).

The SHRP protocol [82,83] consisted in first dissolving 16 g ofbitumen into 64 g of a blend of 45/45/10 vol.% benzene, tetrahy-drofuran and ethanol respectively, and thenpassed through an anionicion-exchange resin (Fig. 9). The anionic resinwas based on quaternaryammonium functional groups attached to a styrene divinylbenzenecopolymer lattice (macroporous anion resin AG MP-1, 75–150 μ,chloride form from Bio Rad Laboratories). The second ion-exchangeresin is connected in series with the first one and is a cationic one,based on sulfonic acid functional groups attached to a styrenedivinylbenzene copolymer lattice (macroporous cation resin AG MP-50, 75–150 μ, hydrogen form from Bio Rad Laboratories). The acids areeluted using a blend of benzene and formic acid while the bases areeluted using a blend of benzene with 1-propyl amine (Fig. 9).

In order to separate amphoteric species, a second set-up wasproposed by SHRP researchers, as pictured in Fig. 10. It consists in

photerics and neutrals through IEC (after [83]).

Fig. 11. Separation into IEC fractions for some of the same SHRP bitumens as thosepictured in Fig. 4 (after [24]). The amphoterics were quantified for only 4 of the studiedbitumens.


using only cyclohexane as a solvent for bitumen. With cyclohexane asa solvent, both amphoterics and bases adsorb onto the cation resin[82]. Then, amphoterics and bases are separated through the anionresin (Fig. 10).

Combining the two above set-up, it was possible to characterizethe same bitumens as those already described in Fig. 3, by IEC (Fig. 11).Given that the quantity of neutral compounds amounts to typically60 wt.% of the total bitumen, this strongly suggests that the acidic,basic or amphoteric species are distributed mostly between the resinsand the asphaltenes.

The strong acids contain most of the carboxylic acids, phenols and2-quinolones of the parent bitumen [26]. When amphoterics areisolated, they have high amounts of 2-quinolones [26]. Pyrroles areconcentrated within the weak acid fraction. The bases are enriched insulfoxides and ketones. The neutral fraction is almost free of polargroups [26].

Neutral fractions are liquid. The amphoteric fraction is a blackfriable solid and therefore resembles asphaltenes. Base fractions forma tacky solid resembling resins [26].

2.6. Natural surfactants

Although this article mainly focuses on the bulk properties ofbitumen, it is worthmentioning that some of the molecules present inthe bitumen have surfactant behaviour that can affect significantly itsinterfacial properties (wetting, emulsification …).

The surfactant-like behaviour of the resins was already mentioned,describing them as a stabilizer for the asphaltenes. This fact isdiscussed more thoroughly in a later section (§ 2.9). The surfaceactivity of the asphaltenes was also cited (§ 2.4.4 - [76–78]). Whenbitumen is in contact with water, these species might migrate to theinterface thus modifying the interfacial properties. As a rule of thumb,the lower the resin to asphaltenes ratio, the higher the interfacialstability with water [84]. This means that the asphaltenes, when fullystabilized by the resins, accumulate less at the interface.

Some molecules of the initial crude oil that finally end up in thebitumen are known to have interfacial behaviour. Examples arenaphthenic acids, porphyrins and wax crystals [85,86]. Just like for thechemical composition, the amount of natural surfactants highlydepends on the crude source. In particular, most Venezuelianbitumens used in the road industry have particularly high amountsof endogenous surfactants.

To our knowledge, the more thorough study of the native (orendogenous) bitumen surfactants was done in the late 1970s by

F. Durrieu [87]. She studied six bitumens from various geographicallocations worldwide and all held natural surfactants, as confirmed bya more recent study [88]. The interfacial behaviour of a bitumensolution in benzene was similar to that of its asphaltenes in benzene.She isolated naturally occurring surfactants using ion-exchangechromatography. Acidic surfactants were separated using a quatern-ary ammonium column (Amberlyst A29) and basic surfactants, with asulfonic acid column (Amberlyst A15).

The acidic surfactants displayed a strong interfacial activity asobserved by the lowering of the interfacial tension of their benzenesolutions with basic water. The effect was strongly crude sourcedependent and could be so strong for certain bitumen sources that theinterfacial tension could decrease to near zero value for pH values oforder 12. This fact was already known to bitumen emulsionmanufacturer for several decades: some bitumen sources can beemulsified without adding any emulsifier when put in contact withalkaline solutions [86,89].

The cationic surfactants displayed a lower interfacial activity asobserved by a gentler lowering of the interfacial tension of their toluenesolutionswith acidicwater. The effect never yielded near zero interfacialtension as was observed for the anionic surfactants. An interestingfinding was that the cationic surfactant also displayed anionic activity,showing that they were in fact amphoterics [87]. Also, all studiedbitumens displayed an almost similar interfacial tension–pH curve.

As seen in the Durrieu study [87], there is evidence that the mostefficient endogenous surfactants are part of the asphaltenes. Thisagreeswith the very slowdynamics of stabilization generally observedfor bitumen-water interfaces [88,90]. This also agrees with the alreadymentioned observation that amphoterics species isolated by IEC aresimilar in physical aspect to n-heptane asphaltenes (see § 2.5). Therecent study by Chaverot and co-workers [88] proposed that they are aspecial class of asphaltenes amounting to less than 0.015% of the totalasphaltenes content. Still, there exist a significant non-asphaltenic partof the endogenous surfactants as demonstrated by Gu and co-workers[91]. They observed an increase in emulsion stability after extractingthe water-soluble surface active components of the bitumen. Anyway,it remains to be shown if themost effective surface active species of thebitumen, especially when it comes to bitumen-water emulsionstability, are asphaltenes, as occurs for heavy oil [85].

In all cases, itmust be remembered that bitumenholds endogenoussurfactants in amounts depending on the crude source. In the case ofbitumen-water interfaces, the pH of the water phase would be ofcritical importance since an acidic pH would promote the cationicsurface active species while a basic pH would favour the anionic ones.

2.7. Early model of bitumen structure: sol and gel bitumens

Rosinger suggested a colloidal structure for bitumen as early as 1914[92]. However, the first description of the colloidal structure of bitumenis generally attributed to Nellensteyn in 1923 [93,94] even if Errera alsopublished a discussion on the subject the same year [35]. Nellensteynargued that asphaltenes are very close in structure to free carbon andform a colloidal suspension within the maltene phase. This wassupported by the Tyndall effect of asphaltenes solutions, ultramicro-scopic observation of the Brownian motion of asphaltenes in suchsolutions and the absence of diffusion through membranes [94].

The colloidal model was further developed by Pfeiffer and co-workers to explain the difference in rheological properties betweenwhat they called sol and gel bitumens [95,96]. Sol bitumens exhibitedNewtonian behaviour, whereas gel bitumens (generally blown ones)were highly non-Newtonian. Between these two extremes, a majorityof bitumens was found to have an intermediate behaviour due to amixed “sol–gel” structure (called in fact “gel–sols” or “elastic sols” bySaal and coworkers). In modern terms, the non-Newtonian behaviourwould be described as delayed elasticity together with some non-linearity in the viscoelastic properties [97].

Fig. 12. The original colloidal model: sol and gel bitumens (reprinted from [17] withpermission by Shell UK).


To describe a little bit more these experimental observations, atypical sol–gel bitumen (penetration at 25 °C of 17 1/10 mm, ring andball temperature of 62.5 °C, IP=−0.7 — see § 3.1 for a definition ofthese parameters) was found to have a slight elasticity in a creep testwith the Couette device at 35 °C, as materialized by a slope slightlyinferior to 1 in a log–log plot of deformation vs time. Some non-linearity was observed for deformations superior to 10 (appliedstresses ranging from 800 to 7000 Pa) [97]. On the other side, a typicalgel bitumen (penetration at 25 °C of 64 1/10 mm, ring and balltemperature of 68 °C, IP=3.2) was found to have a stronger elasticityin the same creep test, as materialized by a slope clearly inferior to 1 ina log–log plot of deformation vs time. A strong non-linearity wasobserved for deformations superior to 1 (applied stresses rangingfrom 700 to 6000 Pa) [97].

In structural terms, the sol type was thought to occur when theasphaltenes micelles were fully dispersed and non-interacting(Fig. 12A - [95]). The non-Newtonian behaviour was thought tooriginate from a gel structure due to fully interconnecting asphaltenesmicelles (Fig. 12B - [95]). The sol–gel structure consisted in thecoexistence of sol-type micelles and a gel structure. Still, thisdescription proved somewhat incorrect since a gel structure wouldresult in a yield stress or a plateau of modulus versus temperature andfrequency [98–100] which have never been observed for paving gradebitumens [101–103].

Anyway, this interpretationwas in line with the well-known resultthat a soft grade differs from a harder grade from the same crude oil byhigher asphaltenes content and lower aromatics content with almostunchanged resin and saturate contents [17].

As a consequence, an instability or colloidal index IC wasintroduced by Gaestel and co-workers [104] as:

IC ¼ xasph þ xflocxsurf

ð1Þ

wherein xi was the weight content of the asphaltenes (i=asph), of the“flocculants” (i=floc), that is the part of the maltene generatingasphaltenes flocculation and of the “peptidizing agents” (i=surf), i.e.,the molecules acting as a surfactant for asphaltenes dispersion. Whenderiving this relation, Gaestel separated pentane asphaltenes. The“flocculants” and the surfactants were separated by pentane elutionon alumina. The surfactants were the polar and aromatic moleculesadsorbed on the alumina. In terms of SARA fractions, the “flocculants”would therefore be a mix of saturates and aromatics, while thesurfactants would be amix of aromatics and resins. The colloidal indextypically ranged from 0.5 and 2.7 for current road bitumens. A markedgel character was observed for IcN1.2 and a typical sol behaviour wasfound for Icb0.7 [104].

2.8. The dispersed polar fluid and other homogeneous models

Some researchers, especially most of the ones implied in the SHRP,concluded that asphalt is a simple homogeneous fluid [105]. Some ofthem called this model the Dispersed Polar Fluid (DPF) [106].

They discarded the colloidal hypothesis based on a series ofarguments, some valid such as the inexistence of a elastic plateau forgel bitumens, but some others of a more philosophical nature such as:An important question here is whether amphipathic molecules arepresent in asphalt cement in large enough amounts to produce enoughmicelles to account for the level of structure present in asphalt.Additionally, there is no evidence that the micelles in micellar solutionscan coalesce into a three-dimensional network. Such coalescence wouldprobably be necessary to explain the steric hardening and oxidativehardening seen in most asphalts. [106, pp.24–25]. In addition, themonotonic time dependence and the unimodal relaxation spectrumfound for bitumen viscoelasticity, was believed to be a key argumentin favour of an homogeneous model [105,106].

More recently, other authors claim that asphaltenes in bitumenform a molecular solution based on their solubility parameters [107].

Clearly, these arguments discarding the colloidal structure ofbitumen can not withstand a critical scientific analysis. First, thediscussion about whether known colloidal models can be applied tobitumen microstructure lacks basic knowledge about colloidalphysico-chemistry. For example, the above excerpt simply ignoresknown results on micellar solutions, with the existence of a widerange of structures that can generate viscoelastic effects [108]. The restof this article demonstrates how modern colloid science can be usedto understand bitumen microstructure and rheology.

Second, the argument based on the rheological properties ofbitumen simply ignores that some heterogeneous materials can stilldisplay a monotonic time-dependence, a unimodal relaxation spec-trum and thermo-rheological simplicity [103]. This is all-the-moresurprising that such an argument was ever raised in bitumen science,because mastics and asphalt mixes, which are by fabricationheterogeneous blends of mineral aggregates and bitumen, aregenerally believed to display a monotonic time-dependence, unim-odal relaxation spectrum and thermo-rheological simplicity [109]!

Third, thematching of solubility parameters is a necessary conditionfor molecular solubility but by no means a sufficient one, especiallywhen it comes to large molecules [110]. Also, solubilization through amicellar solution is one special form of solubility that arises withamphiphilic molecules such as surfactants [111]. In this latter case, theconcept of solubility parameter is not adequate to explain their peculiarbehaviour in solution. Since asphaltenes exhibit surface active behaviour[69], their behaviour in solution can not be fully captured by thesolubility parameter approach only. This was clearly demonstrated bythe computer simulation of asphaltenes aggregation of Rogel [52], whoobserved a lower solubility parameter for the asphaltenes molecules inthe form of molecular aggregates (Fig. 7 - [52]).

Moreover, the proponents of the homogeneous model of bitumencan not explainwhy direct investigation of bitumen structure using X-

Fig. 14. Another model of asphaltenes aggregation: (A) an asphaltene molecule, (B) anasphaltenes micelle with size ca. 3–4 nm and (C) an asphaltenes aggregate with size ca.12–15 nm and apparent fractal dimension between 2–2.5 (reprinted from [125] withpermission by Francis and Taylor).

Fig. 13. SAXS pattern for straight-run bitumen (1), asphaltenes (2), resins (3), oxidizedbitumen (4) and maltenes (5) (after [112].).


ray or neutron scattering, as detailed hereafter, confirm its hetero-geneous nature [46,112–114]. Also, and as discussed thoroughly in thenext sections, many experimental results can not be interpretedsimply without introducing the colloidal hypothesis. For all thesereasons, the colloidal model is the only one at the moment that canreasonably explain the peculiar features of bitumen properties. This iswhy it is used throughout this article.

2.9. A modern colloidal picture

As just shown, bitumen was early described as a colloidaldispersion of asphaltenes micelles in the maltenes. The resins, thatis the polar components of the maltenes, were thought to stabilize theasphaltene micelles. A more precise description of this process isstarting to arise.

2.9.1. Asphaltenes micelles in bitumenSmall Angle X-Ray Scattering (SAXS) and Small Angles Neutrons

Scattering (SANS) confirm that asphaltenes form micelles in organicsolvents [115–118], in crude oil [115,119,120] and in bitumen[46,112,121]. It was also shown that the diffusion pattern observedin SAXS or SANS experiments, disappeared once the asphaltenes wereremoved from the bitumen [112] or the crude oil [119] (Fig. 13).

As detailed later on, viscoelastic studies also suggest thatasphaltenes micelles exist inside bitumen and they experienceBrownian motion at high-enough temperature [103,122]. This Brow-nian diffusion explains the increasing electrical conductivities ofbitumenwith temperature (to values above 10−12 Ω−1 cm−1 [123,124])as a consequence of asphaltenes being charge carriers.

The colloidal model is also consistent with results obtained bythermal analysis. As previously described, bitumen undergoes a glasstransition at a temperature very close to that of its aromatics. Thisstrongly suggests that the asphaltenes exist as dispersed solid particlesand do not directly participate to the glass transition. Still, some resultssuggest that asphaltenes extend the span of the glass transition range,implying that at least somemolecules of this familymight participate tothe glass transition when mixed with maltenes [23].

All these evidences make it difficult to deny the colloidal nature ofbitumen. The most convincing evidence is the one coming fordiffusion experiments.

In all diffusion studies, the scattering techniques highlighted anelementary structure consisting in diffusive particles of radius 2−8 nm.Such a particle size is reminiscent of the crystal size of “pure”asphaltenes [64,121] and concurs to the earlier description ofasphaltenes micelles as being made of a few individual molecules(Fig. 7).

Still, the exact description of the asphaltenes micelles needs someclarifying. Although it is easily pictured as a somewhat sphericalobject, as proposed earlier on in Fig. 7, the asphaltenes micelle couldbe a more open molecular assembly of low fractal dimensionreminiscent in concept of polymeric coils (Fig. 14B - [125]). Variousgeometrical shapes were indeed proposed for the asphaltenesmicelles: if Storm suggested they were spherical [113,114], Raveyproposed they formed flat sheets [116] and others believed they madecylinders [117,126]. Therefore, it must be kept in mind that thespherical description in Fig. 7 remains a crude approximation that willhowever be used throughout this review as a pedagogical aid.

The role of the resins in asphaltenes micelles stabilization remainsto be fully understood. Swanson showed that asphaltenes precipitatewhen mixed with only the oily components of their parent bitumen[72]. He further showed that the resins could help disperseasphaltenes in benzene, highlighting their key role in asphaltenesstabilization inside bitumen [72]. More recently, Lian et al. showedthat the resins could better stabilize asphaltenes in toluene than anon-ionic surfactant such as ethoxylated nonylphenol [50].

The role of the resins as a stabilizer for the asphaltenes was furtherstudied by Koots and Speight who confirmed that the asphalteneswould precipitate from the oily bitumen components without theresins [42]. They quantified that at least 75% of the original resincontent were necessary in order to prevent asphaltenes precipitation[42]. Interestingly, they also observed that the resins from one crudeoil could not stabilize the asphaltenes from another crude oil. This wasfurther studied by Murgich et al. who proposed that the shape of theasphaltene molecules allows the aggregation of only those resins that fitits aromatic regions and show the lowest steric interference with its alkylgroups [127]. There is therefore somemolecular recognition effect that

Fig. 16. Observation of a “gel” bitumen by AFM. Note the “bee” structure with typicalsize 1 μ (reprinted from [133] with permission by Taylor and Francis).


permits the resin to enter the asphaltenesmicelles. This helps smotherthe differences in polarity between the asphaltenes and the maltenes,and therefore creating this surfactant-like behaviour.

2.9.2. Asphaltenes micelles aggregation in bitumenIf a reasonable agreement exists concerning the size of the

asphaltenes micelles, their organisation at a higher scale remainsquite hypothetical due to experimental difficulties [125].

For example, the often-cited bitumen model by Yen (not to beconfused with the other Yen model proposed for solid asphaltenes inFig. 8), lacks strong experimental backing. It considers the existence ofmicelles aggregate called supermicelles which then generate biggersupermicelles (or flocs) until the structure reaches a critical size thatyields to its precipitation. This entire superstructure remains thereforehighly hypothetical.

Gel permeation chromatography is often cited as an evidence ofthe presence of asphaltenes micelles in bitumen [128]. It highlightsthe presence of large objects but the presence of an additional solventmakes it difficult to interpret in terms of bitumen structure.

A more precise picture of asphaltenes micelles aggregation arisesfrom SAXS or SANS experiments, even if the interpretation of thespectra remains controversial [126,129,130]. These results suggest thatthe aggregation state of asphaltenes in solvents like toluene [67,129]and in bitumen [46] is consistent with that of fractal aggregates offractal dimension 2. A possible schematic description of such anasphaltenes aggregate is given in Fig. 14C.

Other structures with a fractal dimension of 2 originate from theso-called “Reaction-Limited Cluster Aggregation”, which are observedwhen Brownian clusters of particles interact at random and manyencounters are necessary before aggregation occurs [131]. This wasobserved for example with clusters made of gold colloids destabilizedby small amounts of pyridine [132]. This means that, regardless of thedetails of the asphaltenesmicelles, their interaction through Brownianmotion could generate such fractal aggregates (Fig. 15).

In parallel, observations of bitumenwith Atomic Force Microscopy(AFM), Scanning Electron Microscopy (SEM) or Confocal Laser-Scanning Microscopy (CLSM) have been published.

The AFM shows a peculiar “bee” structure that was initially onlyfound for a “gel” bitumen (Fig. 16 - [133]). The same “bee” structure(also called catana phase) was repeatedly observed in other workswith an average height between 22 and 85 nm and a typical distancebetween strips of order 150 nm [134,135]. The link between the extentof “bee” phase and the asphaltenes has been confirmed in one study[136] when it was discarded and a correlation was instead proposedwith the metal content of the bitumen in another one [134]. Apartfrom the bee structure, some bitumens displayed a rather flat

Fig. 15. A gold particle aggregate with fractal dimension 2 observed by transmissionelectronic microscope (reprinted from [132] with permission by American PhysicalSociety).

structure and some others displayed “flake”-like domains of diameter1 μ [134]. The additional use of AFM in the Phase DetectionMicroscopymode (PDM) suggest that at least three phases are present andtemperature-dependent [134–136].

Clearly, a full interpretation of these features remains to bepublished. As a result, it is not clear how these surface observationsrelate to the bulk structure of the bitumen and how they relate to theliquid–liquid phase-separation and the wax crystals observed byClaudy et al. [20–21] and described later on (§ 2.10).

The SEM observation of the same “gel” bitumen that gave “bee”structure in AFM (Fig. 16), showed connecting aggregates of what wasbelieved to be asphaltene particles of diameter around 100 nm (Fig. 17 -[133]). Here, it is not clear how the preparation procedure affects theobserved morphology and whether it is representative of the structureinside the bitumen. Almost similar structures were observed onprecipitated asphaltenes [137].

Confocal Laser-Scanning Microscopy (CLSM) [138] has also beenused in order to better understand bitumen structure. Micron-sizeheterogeneities were detected. However, the observed features looksremarkably similar in shape and size to the paraffin crystalsunmistakably observed with the exact same technique in [139].Therefore, CLSM seems to be a tool more appropriate for the study ofwaxes rather than the asphaltenes.

In all cases, modern observation techniques could allow for a betterunderstanding of bitumen structure. At the moment, more work isneeded in order to understand the origin of all the observed features,especially the “bee” phase revealed by AFM. Combined AFM, SEM andSAXS/SANS experiments might give an answer, from which a fulland definite description of bitumen structure at all scales could beobtained.

2.9.3. Use of a solvation parameter to describe asphaltenes structureAs already described earlier, the resins are known to stabilize the

asphaltenes. This was indeed demonstrated on bitumens [42,50,72,140]and crude oils [141–143]. They act as somewhat like a surfactant,creating a so-called solvation layer, and help maintain them insuspension. Early vocabulary for that was the word peptiser, still in usetoday, as a synonym for the current surfactant. However, it must beremembered that the asphaltenes themselves also act as their own

Fig. 17. Observation of a “gel” bitumen by SEM. The particle size inside the aggregatesmeasured 100 nm (reprinted from [133] with permission by Taylor and Francis).

Fig. 18. A simplified view of the colloidal structure of bitumen: the asphaltenes micellesare pictured as spherical (see text for details) to illustrate the concepts of solvation layer(resin shell) and effective volume. The oily dispersion medium is called the maltenes.

Table 3Typical values for the solvation constant K/ϕm for various bitumen sources

Bitumen source Temperature for K/ϕm K/ϕm Ea Temp. range Reference

°C – kJ/mol °C–°C

Ratawi 65 3.1 7.5 25–400 [114]150 1.7 [114]

Venezuela 1 60 5.1 4.0 35–100 [122]Saudi Arabia 60 5.6 2.7 35–100 [122]

60 5.6 4.0 65–135 [144]France 60 6.5 3.0 65–135 [144]Venezuela 2 60 7.8 3.0 65–135 [144]Mexico 60 5.2 2.0 65–135 [144]Kuwait 60 8.0 6.0 65–135 [144]Black Sea 60 12.8 4.1 65–135 [144]


surfactant: as described before, the aggregation of asphaltenes yields tothe formationofmicelleswith a lower solubilityparameter (Fig. 7 - [52]).

In all cases, it is believed that the extent of the solvation layer istemperature dependent and therefore, modelling bitumen as a colloidnecessitates considering that the amount of total solid phase in abitumen (asphaltenes and adsorbed resins) is temperature dependent,as was recognized as early as 1932 byMack [140]. Given that the resinsmay amount to almost 50% of a given bitumen and that Koots andSpeight showed that at least 75% of them were needed to stabilizeasphaltene dispersion in the remaining maltenes [42], the amount ofsolid phase (that is asphaltenes+stabilizing resins) within a givenbitumen must be significantly higher than its asphaltene content.

This phenomenon was described by Storm and co-workers bymeans of a solvation parameter K that quantifies the increase involume fraction of solid phase due to the adsorbed resins [113–115]. Ifxasph is the mass fraction of asphaltenes, Kxasph represents theeffective volume fraction of solid phase ϕeff (Fig. 18), whoseimportance for the rheological properties will be discussed later on:

�eff ¼ Kxasph: ð2Þ

In fact, the physical interpretation in terms of solvation layer mustbe considered with care, and clearly, the solvation parameter does notonly take into account the solvation layer:

a) The true asphaltene content in bitumen, in the sense of solid particles,is very likely to be different than that precipitated by n-heptane,becausen-heptane is not exactlya solvent-equivalent to themaltenes.

b) The effective volume fraction is usually determined by means ofrheological measurements and must thus take into account thesolvent entrapped in-between the asphaltenes micelles (Fig. 18),

c) Also, the possibility to have a solvated asphaltenes cluster insteadof a cluster of solvated asphaltenes micelles must be taken intoaccount,

d) In the same line of argument, the effective volume fraction mustalso take into account the solvent entrapped inside the openasphaltenes micelles as pictured in Fig. 14.

e) The solvation constant alone can not be determined by rheologicalmeans, but rather the ratio K/ϕm where ϕm, is the volume fraction atmaximumpackingof theasphaltenesmicelles, aswill bedetailednext,

f) This simple picture does not take into account the possibility thatasphaltenes aggregation and conformation might also change withtemperature and concentration, as was proposed for heavy crudeoils [119].

g) The transient effects are not taken into account. However, it ishighly probable that the asphaltenes micelles have definite life-

time, but of coursemuch shorter than the observation time in SAXSor SANS experiments. If there exists only one micelle size indynamic equilibriumwith free asphaltene molecules, as occurs forsurfactant micelles at low concentration [111], an increase inmicelle life-time without changing the overall number of aggre-gated and free molecules, would generate in turn an increase inmean micelle size. The solvation layer would then be an artefact ofan increased life-time of the asphaltenes micelles.

Hence, the contribution from each of these different phenomena inthe solvation constant is difficult to separate at the present time. Still, itsuffices to note that the asphaltene content as measured by n-heptaneprecipitation is assumed to be proportional to the effective volume ofsolid phase in bitumen.

The proportionality constant, called solvation constant, is writtenas K/ϕm and ranges from 3 to 8 at 60 °C, with a typical value of 5.5(Table 3). This parameter is temperature dependent, with activationenergy of order of a few kJ/mol (Table 3). Note that one of the bitumenfamily showed a very high value of the solvation parameter (Black Seabitumen in Table 3). These materials had the lowest n-heptaneasphaltenes content (5% - [144]). Therefore, the typical value of 5.5 at60 °C for the solvation parameter seems to only apply to bitumenswith n-heptane asphaltenes content in the 7–25% range. For lowerasphaltene contents, the solvation parameter increases significantly.

In this context, the modern colloidal picture essentially differsfrom the earlier one (Fig. 12) in the sense than all bitumens arethought to have the same structure and that the only differencebetween former sol and gel bitumens would be the amount ofsolvated asphaltenes, i.e. the volume fraction of dispersed solid phase.

2.10. Wax crystallization

In addition to the nanostructure arising from the asphaltenes,linear alkanes naturally present within bitumen can crystallize. Thesemolecules typically have between 24–40 carbons [20,145] which

Fig. 19. Various geometries of wax crystals inside bitumen as observed by CLSM. The scale marks 10 μ (reprinted from [139] with permission by Springer).


would give a melting temperature for the pure components in the 50–80 °C range.

The consequences of wax crystallizations on the structure ofbitumen are not clear at the present time [20]. As observed byPolarized Light Microscopy (PLM) and Confocal Laser-ScanningMicroscopy (CLSM), wax crystals display different geometriesdepending on the crude source and crystallization conditions(elongated, crescent-like, flakes,…) with typical sizes of order of 1–10 μ (Fig. 19 - [20,139]). As discussed earlier, it is highly likely that thefeatures observed with CLSM in [138] and attributed to asphaltenes,were in fact wax crystals.

In parallel to the crystallization, it was observed a liquid–liquiddemixtion by phase contrast microscopy [20,21]. The interconnectedphases had domain size of order 1–3 μ, that is significantly smallerthan crystal size. The phase separation was all the more pronounced

Fig. 20. DSC thermograms obtained at a heating rate of 5aC/min for various bitumens. The gallowing for crystalline wax-content integration (CF) is also displayed. Values for Tg and FC

that the amount of crystalline wax was important [20,21]. It could bethe reason why waxy bitumens exhibit in fact two glass transitionswhen carefully analyzed by Modulated-DSC (MDSC) in the lowtemperature range [146].

Waxes typically crystallize in bitumen at temperatures startingfrom 90 °C down to the glass transition temperature. Since thiscrystallization is an exothermic process, Differential Scanning Calori-metry (DSC) is the preferred technique for quantifying the amountof paraffin-like crystalline materials within bitumen. It highlightsone or more endothermic peaks upon heating for bitumen specimens(Fig. 20 - [20,21]).

Apart from the polydispersity of crystallizing waxes and possiblesolid-solid transitions typical of paraffinic compounds, the origin ofthese two or more peaks could mostly lie in the liquid–liquiddemixion [20,21], one peak coming from the dissolution in the first

lass transition temperature Tg at the midpoint is marked on each curve. The base lineare given (reprinted from [20] with permission by LCPC).


liquid and the other from the dissolution in the second liquid (Fig. 21).A full description of these features therefore remains to be published.

MDSC might give a better description of the structural changes atstakes, but the available interpretations are not conclusive [147–149]since they fail to explain two fundamental findings. First, theendothermic behaviour observed on a given bitumen upon heatingis essentially retrieved on the DSC curve of its saturates [103]. Second,the doping of a non-waxy bitumen with linear n-alkanes is shown togenerate this endothermic feature [146]. Both elements stronglysuggest that the endothermic effects observed between −30 °C and100 °C upon heating a bitumen must be interpreted as a consequenceof wax dissolution. This also shows that a correct interpretation ofthese very complex phenomena will only come by comparing resultsfrom various analytical techniques such as MDSC, rheology, X-Rayscattering and microscopic imaging.

The wax-crystals generate X-Ray diffraction peaks [28] at Braggangles of 2θ=21.4 and 23.8° [144], the first one being 3 times moreintense than the second one. This pattern is typical of the (110) and (20 0) planes of the orthorhombic crystals obtained for large paraffinsand polyethylene [150].

Their effect on the properties was only carefully studied throughphysical hardening, although it is documented that they affect the hightemperatures too [151]. Theymight also be responsible, at least partly, ofthe steric hardening described in a forthcoming section (§ 2.12).

Most paving grade bitumens have amounts of paraffin-like crystallinematter less than 5 wt.%, as measured by DSC, whose effect on therheological properties seemessentially to be the low temperaturephysicalhardening [152] attributed to wax crystallizations [153] (see § 2.12).

For specific bitumens with significative amounts of paraffin-likemolecules, the modelling presented in the next sections should alsoincorporate the contribution of the paraffin-like components on thestructure. In particular, asphaltene/wax interactions give very peculiarbehaviour [154]. Since this influence remains barely known, it willhowever not be taken into account and leave it to later refinements.

Given the quest for lowermanufacturing temperatures for hotmixesand the adventofwarmmixes [155], highmeltingpointwaxes have gainrenewed interest [156,157]. They are among the additives that arecurrently available in order to decrease the viscosity of bitumen at hightemperature without deteriorating the service properties. In general,they can be thought to act through a simple basic principle: attemperature above their dissolution temperature inside the hostbitumen, they act as a plastifier [158]. At temperatures below, theycrystallize and act as a filler, increasing the viscosity [158]. This isreminiscent of the polyethylene effect described later on. The transitiontemperature is the key and high melting point waxes can displace thedissolution temperature to above the normal operating range of a road

Fig. 21. Correlation between the DSC thermograms and the corresponding phasecontrast microscopic observation (picture width is 100 μ) of the structure of a bitumenwith 14.6% naturally occurring waxes reprinted from [20] with permission by LCPC.

bitumen, therefore increasing the plastifying effect up to the manu-facturing temperatures. Thisway, the risk tohave theplastification effectin the operating range of the bitumen is prevented.

2.11. Chemical ageing

From the above picture, bitumen structure is quite complex withslow temperature-dependent evolutions arising from molecularorganisation processes that are far from being fully understood. Inaddition to that, the molecules may irreversibly evolve throughchemical ageing which is generally thought to be a sum of oxidationreactions and polymerisation, and to a lesser extent, lighter compo-nents evaporation [17,159,160]. As a result, chemical ageing leads to aglobal hardening of the material [17,161], which in turns increases thecracking probability [162]. In fact, this chemical hardening is usedindustrially, since oxidizing soft vacuum residues constitute a way toobtain harder bitumens (air-blowing - see § 2.2). However, air-blowing occurs at high temperature and the chemical processes atstake are not necessarily the same as the one occurring for moregentle conditions. In all cases, it must be kept in mind that thesusceptibility of a given bitumen to chemical ageing depends on itscrude source and manufacturing process.

Apart from the manufacturing process, two ageing conditions aregenerally separated for bitumen. First, there is a rapid chemical ageingupon mixing the hot bitumen in thin film with the hot aggregate.Second, there occurs an in-situ ageing during the service life of thepavement.

The first one, sometimes called short-term ageing, occurs for ashort time at a high temperature of typically 160 °C and is wellsimulated by the Rolling Thin Film Oven Test (RTFOT - ASTM D2872 -EN 12607) which “cooks” the bitumen in 1.25-mm thick moving filmsat 163 °C for 75 min. Under average processing conditions, this leadstypically to a doubling of the viscosity, although the extent ofhardening is bitumen-dependent and ranges typically between 1.5and 4 for the viscosity at 60 °C [163], although higher values aresometimes found [160]. In the mean time, the asphaltenes contenttypically increases by 1–4 wt.% [170]. Current specifications eliminatebitumens whose ageing upon mixing would be too fast [12].

The second one, sometimes called long-term ageing, occurs for amuch longer period of time since service life of the pavement canextend to several decades. It depends of course on the position of thebitumen inside the pavement, the top layers being more exposed thanthe base course. Mix formulation also comes into play, the bitumenthickness and the mix porosity being important parameters. It finallydepends on the local climate and all these involved parameters makeit quite complicated to accurately describe in-situ ageing.

Simulation of in-situ ageing is thereforemore difficult and awidelyused testing procedure, the Pressure Ageing Vessel Test (PAV - ASTMD6521 - EN 14769), was shown to reproduce satisfyingly the in-situageing of bitumen in surface courses for approximately 4 to 8 years inlocations such as Wyoming and Florida [101].

2.11.1. Effects on the chemistryIn chemical terms, ageing leads first to a decrease in aromatic

content and subsequent increase in resin content, together with ahigher asphaltene content (Fig. 22). Therefore, it is generally acceptedthat the aromatics generate resins which in turn generate asphaltenes[17,161,164–166]. The saturates remain essentially unchanged, ascould be assessed from their low chemical reactivity [17,161]. Allthese changes result in a slightly higher but almost unchanged glasstransition temperature [19,170].

Asphaltenes produced upon aging may be somewhat differentthan the initial ones. Corbett reported increased molecular weights ofthe asphaltenes upon air-blowing, suggesting the presence ofpolymerisation reactions [167]. This was confirmed by Kats andcoworkers who observed the formation of carbenes and carboids from

Fig. 22. SARA fractions for various bitumens before and after 85 min and 340 min RTFOT ageing (after [166]).


asphaltenes upon photooxidation, but only for one of the two studiedbitumens [168].

A detailed study of bitumen oxidation was performed byMoschopedis and Speight [169]. In parallel to the usual increase inasphaltene content, they measured an increase in asphaltenesmolecular weight from 5600 g/mol to 8000 g/mol upon blowing anAthabasca bitumen a few hours at 260 °C. Interestingly, the molecularweight only increased to 7000 g/mol when blowing temperature wasincreased to 290 °C and was found unchanged when the blowingtemperature was further increased to 320 °C. In parallel, the H/C, N/Cand S/C ratio were observed to decrease while the O/C ratio toincrease. This indicated that the oxygen uptake of the asphaltenes wasdeveloping in parallel with sulfur and nitrogen release [169]. Thealmost same trend was observed on the resins, except that their H/Cincreased. This was interpreted as a consequence of the formation ofresins from aromatics [169].

The rate of asphaltenes production was found to be essentiallylinear with time in RTFOT laboratory experiments at 163 °C with 6–7 wt.% asphaltenes produced in 340 min [165]. This linear increasewas also observed for in-situ aging of bitumen recovered from realpavement sites, with increases between 2 and 10 wt.% in a 90 monthsperiod under French southern climate (near Marseilles) [170].

More precisely, ageing was seen to result in the formation of firstsulfoxides, and, although at a somewhat slower rate, ketones thatfinally yields anhydrids and carboxylic acids (Fig. 2 - [166,171]).Sulfoxides are rapidly formed but are thermally unstable. Therefore,they reach a steady-state level that not only depends on the initialsulphur content of the bitumen, but also on the oxygen diffusion intothe bitumen [171]. Ketones and carboxylic acids are more stable anddo not reach an asymptotic value in laboratory ageing experiments[171]. Anhydrides form once a significant amount of ketones hasappeared. They are thought to derive from the oxidation of benzylic

carbons at the 1,8 bridgehead position of a naphtalene ring [171]. Thein-situ ageing seems to yield a steady-state level not only of sulfoxidesbut also of carboxylic acids, after 2 years of service life in SouthernFrance [172]. As a result, the amount of functional groups after ageingmay rise up to more than 1 mol/l [26].

Based on oxygen up-take, the ageing kinetics is initially linear andtends to slow down as the reaction proceeds [161,171]. Bitumenoxidation is generally said to proceed in an auto-catalytic way [159].This summarizes the observation that air-blown bitumens display afaster ageing kinetics than straight-run ones [171,173]. Still, this factdoes not rely on chemical indicators but rather on the viscosityincrease. We will discuss this matter in the next section.

Typical antioxidants for lubricating oil or rubber, such asalkylphenylenediamine or phenothiazene at 0.5 wt.%, were observedto be essentially useless to prevent bitumen hardening [174]. Free-radical inhibitors such as phenols were also observed to be essentiallyuseless as bitumen antioxidant [26]. In fact, the naturally presentphenols seem to regulate bitumen oxidation [26]. Among the fewchemicals having a demonstrated antioxidant effect, lime [175],phenothiazine [161], amines [176] and phosphate such as zincdialkyldithiophosphate [177] are generally mentioned.

In addition, ultra-violet light is known to increase the oxidationprocess by activating photo-oxidation reactions [160,161]. This effectwas known to Niepce since he used the formation of insolublecompounds in Dead Sea bitumen after light exposure, tomake some ofthe first recorded photograph in the early 1820s [35,178]. Photo-oxidation is believed to generate polymerisation reactions, not onlyfor the asphaltenes [168], but also for the lower fractions down to thesaturates [179]. Photo-oxidation is strongly radiation intensitydependent [180]. However, it is almost temperature-independentwhereas dark thermal oxidation is highly temperature dependent[180].

Fig. 23. The Petersen model of asphalt oxidation. A: Ageing kinetics, highlighting thedifferences in behaviour for AAG-1 (same kinetics regardless of temperature) and AAD-1(slower kinetics at lower temperature). B: The differences in ageing kinetics are believed tooccur as a consequence of asphaltenes aggregation at low temperatures, which limits thenumber of accessible oxidation sites (reprinted from [171] with permission by Taylor andFrancis).


2.11.2. Effects on the rheologyAgeing is highly studied because of its impact on the mechanical

properties of the binder (hardening). Therefore, all rheologicalindicators are used to quantify its importance [160,170]. In general,viscosity is mostly used as an indicator for ageing, an ageing indexbeing usually defined either directly as the viscosity ratio or as therelative increase in viscosity versus time [160].

Room temperature viscosity was observed to increase linearlyversus aging time up to 15 h in thin film (3.2 mm) at 163 °C by Traxler[159]. Depending on bitumen nature, the viscosity was multiplied by afactor of 5 to 20 in 15 h. The viscosity increase rate was observed to behigher for the more structured bitumens (“gel type”), suggesting anauto-catalytic reaction as mentioned in the previous section [159]. Forlonger times, the viscosity was seen to slow down for the fastesthardening bitumen [159], therefore more universal models of theviscosity versus time curve involve the use of hyperbolic fits [160].

More recently, it was observed that some asphaltenic bitumensdisplayed a different kinetics at high and low temperature whereas“normal” bitumens only showed a single kinetics (Fig. 23A - [171]). Anexplanation was proposed by Petersen based on a structural modelvery similar to the colloidal one. For asphaltenic bitumens, asphal-tenes were believed to strongly aggregate at low temperature (60 °C),thus preventing oxygen to diffuse inside the aggregates which in turnlowered the oxidation rate. At high temperature however (130 °C), thedispersion was better therefore allowing for a better oxygen diffusionand higher oxidation rate (Fig. 23B). For “normal” bitumens, thedispersion state was always good enough so that the kinetics wasunchanged in this temperature range.

This interpretation can however not explain the two differentoxidation rates observed as a function of time by Campbell and co-workers at low temperature [181]. Another interpretation could as welllie in the differences in asphaltenes micelles dynamics at long and shorttimes, as a consequence of the freezing of the Brownian diffusion. A fullcharacterization of the bitumen structure and rheology at the ageingtemperature would be needed in order to conclude on this issue.

2.12. Steric hardening

When monitoring the viscosity (or any mechanical property) of abitumen versus time around room temperatures, a very slow heat-reversible hardening is indeed observed and generally referred to assteric hardening [182]. The viscosity increase follows a power lawversus time with a slope between 0.017 and 0.183 [183], correspond-ing to a viscosity increase between 10 and 200% in 2000 h at 25 °C [27],depending on bitumen type.

This effect was first reported by Traxler and Schweyer in 1936 andthey showed that it is not a consequence of chemical ageing asdescribed before, especially because of its reversible nature [184].

The proposed interpretation generally relies on the sol-gel picturewith the build-up of a stronger asphaltenes network with time [182],as suggested by the correlation between the rate of viscosity increaseand the degree of complex flow, defined as the slope of the flow curve(stress versus rate of strain in a log–log scale) [183].

More recently, it was observed that steric hardening occurs with aslight volume contraction, of order of 0.1% in 5 h [185], with the fastestrate between 20 and 80 °C occurring at 50 °C. The kinetics of volumecontraction followed an Avrami lawwith exponent between 1 and 1.2,which is generally interpreted as the growth of linear crystals [185].

If this experimental fact were to be confirmed on fully character-izedmaterials, the presence of a fastest contraction rate at 50 °Cwouldbe difficult to interpret within the colloidal framework, since a gradualslowing down of the processes with a decrease in temperature wouldbe expected. However, this effect might be consistent with crystal-lisation kinetics, since 50 °C corresponds, for most paraffinic bitu-mens, to the maximum of one of the exothermic peaks in the DSCthermograms [20,21]. In this sense, steric hardening would be, at least

partly, a high temperature manifestation of what was named physicalhardening, which was attributed to wax crystallisation (§ 2.10).

As a conclusion, a complete description of this effect remains to bepublished before a full understanding of this phenomenon can beobtained.

3. The rheology of paving grade bitumens

3.1. Early evaluation of bitumen rheology

The use of bitumen in paving applications has generated a lot ofinterests in its rheological properties, because of their importance inthe manufacture and quality of bituminous pavements. As a matter offact, the development of the early colloidal model was based onrheological observations (§ 2.7). Long before that, ancient users ofbitumen observed the strong effect of temperature on its consistency


[2,3]. But due to its highly viscous character at room temperature,giving rise to a confusing and somewhat imprecise description such aspasty or semi-solid [3], bitumen rheological behaviour remained hardto quantify.

3.1.1. Needle penetration testIn the absence of simple test procedures, early evaluation of

bitumen rheology for specification purposes was based on tactileobservations or even chewing, until Bowen proposed to quantify it bythe needle penetration test in 1888 [186].

Dow refined the penetrometre in 1903, using a set-up very similar tothe current one [186] and nowadays, the penetration test (ASTMD5 - EN1426) consists in measuring the depth (expressed in tenth ofmillimetres) at which a standard needle penetrates after 5 s loadingtime and with a 100 g load. Under these conditions, typical values forpaving grade bitumen range between 15 and 200 1/10 mm. Forpenetration less than 30 1/10 mm, the bitumen is generally said to behard. On the contrary, penetration values higher than 100 1/10 mmcorrespond to soft bitumens.

Penetration remains the basic test for paving grade specificationsin Europe and European paving grades are labelled by theirpenetration grade. For example, a 70/100 penetration grade bitumenhas a penetration value at 25 °C ranging from 70 to 100 in units 1/10 mm [17].

It was early recognized that the penetration test was toocomplicated to interpret in rational terms and that only correlationscould be found between penetration and intrinsic properties such asviscosity [182]. For example, penetrationwas shown to be a somewhatequiviscous temperature and Saal and Koens published equivalencebetween viscosity η (in Pa s) and penetration P (in 1/10 mm) usingstandard loading conditions (5 s and 100 g) at the same temperaturein the form of the general relationship [187]:

η ¼ AP−b: ð3Þ

The initially proposed values were A=5.3 108 and b=1.93 [187].This applied reasonably well for the studied binders but proveddifficult to extrapolate to all materials [182]. Therefore, othercoefficients were later proposed and a good approximation wasgiven for example by Duriez and Arrambide with b=2 in all cases, andA=8 108 for soft bitumens (penetration grade 180/200) and A=109 forharder ones (penetration grade 40/50) [188].

Later on, Van der Poel observed the equivalence betweenpenetration P (in 1/10 mm) using standard loading conditions (5 sand 100 g) and the relaxation modulus E (in Pa) in tension after 0.4 sloading time at the same temperature. Although he did not state itsmathematical formula, the correlation based on his nomograph [195]would be equivalent to the following relation:

E 0:4sð Þ ¼ 1:62 � 109P−1:85: ð4Þ

3.1.2. Ring and ball softening temperatureAnother consistency test that is still being used in the European

specifications is the Ring & Ball softening temperature (TR&B). It wasalready proposed as a standard for ASTM in 1915 [186]. Theexperimental set-up for the current procedure (ASTM D36 - EN1427) consists in preparing a 8-mm thick bitumen film inside a metalring (average diameter close to 16 mm). The bitumen film is put insidea water bath at an initial temperature of 5 °C and a normalized 3.5 gsteel-ball (9.5 mm diameter) is placed onto the bitumen. Thetemperature is then increased at a rate of 5 °C/min. The softeningtemperature is defined as the temperature at which the steel balldeforms the bitumen film to such an extent that it contacts the bottomof the vessel 25 mm below.

Under these conditions, typical values for paving grade bitumenrange between 35 and 65 °C. A hard bitumen generally has a softening

temperature close to 60 °C while a softer grade will typically have asoftening temperature around 40 °C.

As for penetration, it was early recognized that the Ring & Ballsoftening temperature was equally difficult to interpret in rationalterms [182]. As a result, the viscosity of bitumen at the Ring & Ballsoftening temperature was initially thought to be an equiviscoustemperature corresponding to a viscosity between 800 and 3000 Pa s[182], with an average value of 1200 Pa s [189]. Later work, based onmore precise rheological testing, proposed instead a value of theapparent viscosity at the Ring & Ball softening temperature of5000 Pa s, corresponding to an average shear stress of 15 kPa or anaverage rate of shear of 0.3 s−1 [190].

3.1.3. Penetration indexGiven the two above testmethods, Pfeiffer and Van Doormaal [191]

defined a Penetration Index PI as:

log800−logPTR&B−T

¼ 150

� 20−PI10þ PI

ð5Þ

where all symbols are defined as above (with the same units), T beingthe temperature at which the penetration is measured, typically 25 °C.The values for the coefficients where chosen so that a referenceMexican bitumen with P=200 1/10 mm and TR&B=40 °C gave PI=0.The relation made the assumption that the penetration at TR&B is 8001/10 mm, when the real value would probably range between 750 and1000 1/10 mm [188]. Measuring such high values of penetration is nottechnically possible and these numbers are therefore estimates [188].

The PI proved a rather powerful indicator for classifying bitumenrheological behaviour, and it was observed that blown bitumens hadtypically PIN1 when straight-run bitumens had −1bPIb+1. Onlyvery susceptible materials such as coal tar gave PIb−1 [191].Therefore, PI was thought to be a good indicator of bitumen type,with PIN2 being indicative of a gel bitumen whereas PIb0 beingtypical of a sol [95]. In general, PI varies between −2.6 and 8 whenincluding all types of bitumen (including blown ones), but typicallylies between −2 and +2 for paving grades bitumens [195]. Solvent-deasphalted bitumen could give PI values between −2 and 2, basedon the exact process used (especially type of solvent and yield –

[28]). Still, negative values were more frequent and deasphaltedbitumens are generally considered more susceptible than straight-run ones.

Van der Poel showed that bitumens with the same PI had similarrheological master curves [195]. Since a mathematical model turnedout to be quite difficult to propose, he instead developed hiscelebrated nomograph from which the complex (or creep) modulusat whatever temperature and frequency (or time) can be predicted fora given bitumen, knowing only its PI and Ring and Ball softeningtemperature. This device still gives acceptable values [101].

PI determination is now standardized and its calculation isdescribed in the annex of the European specifications on pavinggrade bitumens [12].

3.2. General features of bitumen rheology

Although the above empirical tests proved sufficient for specifica-tion purposes until the development of modern polymer-modifiedbitumens, a lot of research work was devoted in the past 130 years tobetter describe and understand bitumen rheology. As a consequence,the most important features of bitumen rheology had already beenhighlighted at the turn of the 20th century: the susceptibility toloading stress, loading time and temperature.

The first attempt to measure the viscosity of a bituminous-likesubstance was probably that by Von Obermayer, who used threerheometers (sliding plate, parallel plate and torsion plate) tocharacterize a tar (Schwarzpech) as early as 1877 [192]. Trouton

Fig. 24. Complex shear modulus and phase angle versus frequency at varioustemperatures for a typical road bitumen and corresponding master curve at 25 °C(reprinted from [197] with permission by Springer).


started his experiments on the subject in 1904, and derived hiscelebrated ratio between tensile and shear viscosity using pitch. Heseems to be the first author to describe the non-Newtonian behaviourof pitch, based on the departure from linearity of the earlydeformation rate in tensile creep and on its non-proportionality tostress [193].

From then on, many experimental set-ups were proposed[182,194] and a general agreement was found on the marked non-Newtonian behaviour at room temperature of most bitumens in the1950s [182]. As a result, Pfeiffer and co-workers correctly attributedthe non-Newtonian effects to viscoelasticity, and interpreted it as aconsequence of the gel structure of the materials [95–97]. In modernterms, their measurements would account for the presence of delayedelasticity and some non-linearity, a behaviour now known to bepresent in every bitumen [101].

The first modern description of bitumen viscoelastic propertieswas probably that by Van der Poel [195], who combined static creepmeasurements with dynamic experiments at various temperatures.The results were used to build his celebrated nomograph allowing forpredicting the modulus of a bitumen from routine test data(penetration and ring and ball softening temperature - [195]). Theconstruction was based in fact on master-curves using the sameshifting procedure that made famous Williams, Landel and Ferry ayear later [196].

From then on, many results were accumulated using a procedurevery similar to that by Van der Poel, based on the use of dynamicexperiments quantifying the rigidity of bitumen by means of itscomplex modulus (Fig. 24 - [197]).

At low temperatures (typically below −20 °C), the shear modulusreaches a constant value of order 1 GPa independent of temperatureand frequency corresponding to the glassy state [122,195,198,199].This value is typical of amorphous organic materials such as polymers[99]. In parallel, the phase angle diminishes down to 0° (Fig. 24).

At high temperatures (typically above its softening temperature,that is above 60 °C), bitumen is a viscous Newtonian liquidcharacterized by a temperature-dependent viscosity η0 related tothe complex modulus G⁎(ω) by:

η0ω ¼ G⁎ ωð Þ ð6Þ

wherein ω is the testing frequency in rad/s. In this region, the phaseangle increases up to 90° (Fig. 24).

In-between these two asymptotic behaviours, the mechanicalresponse of a bitumen is intermediate between that of an elastic solidand a viscous liquid and is thus said to be viscoelastic. In all cases,bitumen at room temperature is a highly viscous viscoelastic liquidand the often found confusing terms such as pasty solid or a semi-solid [3] should be avoided.

For the record, the celebrated “pitch drop experiment” set-up byPr. Parnell at the University of Queensland in 1927 illustrates theviscous nature of bitumen. It is regarded by the Guiness Book ofRecords as the world's longest continuously running laboratoryexperiment [200]. The experiment consists of a bituminous-likepitch that slowly drips out of a funnel at room temperature. Up to thepresent time, only eight drops have fallen since 1930! This gives anestimated viscosity of around 230 MPa s.

3.3. Performance grading (PG)

If European paving grade bitumens are still defined on empiricaltests such as penetration and ring and ball softening temperature, theUS adopted rheological specifications at the end of the SHRP programin the early 1990s [25]. The current American specifications forbituminous binders [201] are known as performace grades (PG).Research is still going on in order to improve the current specifications[202].

The PG system relies essentially on two test methods: onecharacterizing the binder in the high temperature range, the otherone for the low temperature range. The binder is said to be PG H-L,where H is the limiting high temperature and -L, the limiting lowtemperature. For example, a binder with PG 58-28 means that itslimiting high temperature is 58 °C while its limiting low temperatureis −28 °C [201].

The limiting high temperature corresponds to the temperature atwhich the inverse viscous compliance of the binder becomes inferiorto 1 kPa when measured at a frequency of 10 rad/s (AASHTO T315 -[203]). The test is typically performed on the unaged neat binder.Grades are defined by 6 °C steps starting from 46 °C up to 82 °C. Theidea behind this criterion is that too-fluid a binder generates a risk ofrutting for the pavement. Thus, the limiting high temperature can bethought of as the temperature above which the risk of rut formationbecomes important for a hotmixmadewith the corresponding binder.

The limiting low temperature corresponds to the temperature atwhich the flexural creep modulus of the binder becomes superior to300 MPa when measured at a loading time of 60 s (AASHTO T313 -[204]). Grades are defined by 6 °C steps starting from −10 °C down to−46 °C. The limiting temperature stated in the paving grade is in fact10 °C lower than that of the creep test. In other words, if the binder hasa creep modulus of 300 MPa at −12 °C and 60 s, the limiting lowtemperature will be −22 °C. The idea behind this criterion is that too-rigid a binder generates a risk of cracking for the pavement. Thus, thelimiting low temperature can be thought of as the temperature belowwhich the risk of cracking becomes important for a hotmixmadewith

Fig. 25. Failure of TTSP for a non-waxy bitumen after PAV aging: A/ master curve interms of modulus, B/ same master curves but in terms of phase angle and C/ Blackdiagrams (adapted from [103]).


the corresponding binder. Since the risk increases with binder ageing,the test is typically performed on the RTFOTand then PAV-aged binder(§ 2.11).

3.4. Time-Temperature Superposition Principle (TTSP)

The Time-Temperature Superposition Principle (TTSP) states thatthe effect of increasing the loading time (or decreasing the frequency)on the mechanical properties of a material is equivalent to that ofraising the temperature [205]. This implies that the relaxationfunction is only shifted towards higher or lower times by a changeof temperature and a material behaving this way is said to be ther-morheologically simple [205].

As an example, amorphous linear polymers are generally thermo-rheologically simplematerials [98,99,206]. A physical reason for this isthat temperature does not affect the structure. Therefore, temperatureonly alters the Brownian dynamics of the molecules and molecularsegments, hence changing only the absolute value of the relaxationtimes and not the overall relaxation function [98,206]. Given theinterdependence between viscosity and relaxation function [207], thetemperature-dependence of the relaxation function is in consequencethe same as that of the viscosity.

Whether this holds for bitumen is still highly debated [124,199,208].High asphaltenes and high paraffin content asphalts do not conform toTTSP, as a consequence of the temperature-dependence of the structuredescribed above [103,154,199]. Still, these effects are limited for mostpaving grade bitumens in the frequency range available with currentcommercial rheometers, and they are usually negligible for straight-runnon-waxy bitumens (Fig. 24). They become significant only withrheometers with higher frequencies [199,209] or for highly asphaltenic(such as aged or air-blownmaterials) or waxy bitumens, whose peculiartemperature dependence was already recognized by Heukelom [151].

Fig. 25 illustrate the non-applicability of TTSP for bitumen. Theconsidered bitumen is a non-waxy one [103]. The bitumenwas testedbefore and after aging. The aging procedure consisted in both RTFOTand then PAV aging (§ 2.11). Master curves were plotted from raw dataat various temperatures and frequencies in order to get the bestpossible superposition of both the G′ and G″ curves. The resultingcurves are shown in Fig. 25A and look quite decent. If the same dataare looked upon in terms of phase angle, then the superposition in thelow frequency range for the aged bitumen does not look so good(Fig. 25B). If now the phase angle is plotted versus the norm of thecomplex modulus (Black diagram - Fig. 25C), the poor superposition isquite apparent for the aged bitumen. Since the Black diagram isobtained without any shifting procedure, it is a better indicator ofTTSP failure or application. As seen in Fig. 25, it acts as a magnifyingglass since it focuses on the phase angle which is the most sensitiveparameter.

The non-applicability of TTSP to bitumen is limited to hightemperatures (say above room temperature), because at lowertemperatures, the glass transition slows down the structure changesand no more evolutions are observed in the time scale whererheological tests are performed. Still, TTSP could be found not toapply if longer storage time were to be used. The applicability of TTSPat low temperature is readily observed in Fig. 25 where thesuperposition is very good for all materials for moduli higher than1 MPa.

In fact, the American specifications make use of TTSP. As rapidlysaid in the former section, the limiting low temperature in the PGsystem is 10 °C lower than that at of the test temperature. This isbecause the risk at stake is thermal cracking, with loading times due todaily temperature change of order 24 h. Typical TTSP constants forbitumen show that a 24 h loading time is equivalent to 60 s loadingtime at a temperature 10 °C higher [101].

Still, some authors claim that TTSP does apply to every bitumen[208]. This is however a consequence of a limited choice of materials

and the use of an experimental procedure which proceeds throughmeasurement upon a cooling procedure that does not allow forequilibrium structures to develop. Rather, a general continuousstructural change is obtained that is barely perceptible. In order tostate that TTSP applies from such data, it should be first made clearthat the cooling rate does not affect the data and for example that thesame data measured upon heating superpose with that upon cooling.This was however shown not to be always the case by Garin [210].Thus, the most suitable way to test bitumen rheological properties isthrough measurements at constant temperature after some thermalequilibrium period has elapsed, typically 20 min [122,101].

In any case, the rise of high frequency rheology [211] will certainlygive a definite answer to this problem and the scarcely availableresults on the subject tend to confirm the non-applicability of TTSP tobitumen [212]. More precisely, recent work showed the existence of ahigh-shear viscosity plateau for the real part of the complex viscosityof bitumen in the kHz range [212]. On the contrary, master curvesbased on the shifting procedure do not predict any plateau but rather acontinuously decreasing slope. Note that the existence of such aplateau would be expected for high-volume fraction suspensions[213–214].


3.5. Modelling the linear viscoelastic behaviour

The first attempts at modelling the viscoelastic behaviour ofbitumen were the works of Saal and Labout [97] and Lethersich [215].They proposed analogical models with just a few springs and dashpotsto describe bitumen viscoelasticity, which rapidly showed not preciseenough when various temperatures and loading times were laterused. Interestingly, Saal and Labout based their modelling on thestructure of bitumen: The elasticity came both from the gel structureand from micelle elastic deformation while the viscous componentwas associated to the intermicellar fluid [97].

Van der Poel found it more useful to develop a nomograph ratherthan giving a mathematical equation to describe the master curves[195], from which it is generally thought that he did not try to modelthe obtained curves [101].

From then on and although TTSP does not strictly apply tobitumen, most authors starting by Van der Poel [195], plotted mastercurves using the shifting procedure developed by Williams, Landeland Ferry [99,196], as illustrated in Fig. 24. Then, mathematical modelscan be proposed to describe the frequency and temperaturedependence of bitumens (Table 4 - [163,197,198,216,217,219,223]). Alimited number of studies also proposed to describe bitumenviscoelastic behaviour by means of phenomenological models ateach temperature, i.e. without constructing master curves (Table 4 -[122,218]).

Most models rely on a very limited number of parameters, at leastthree, including either the zero-shear viscosity and/or the glassymodulus, togetherwith a parameter describing the shape of themastercurve with almost similar definitions (R, b, βJK and βDW in Table 4).

Table 4Models proposed in the literature to describe the linear viscoelastic properties of bitumens

Authors Experimental Range Equation

Huet [216] −25 to 25 °CGr ¼ 1

1þ jωτmð Þ−hþδ jωðSayegh [217] 6.10−2 to 630 rad/sbiparabolic model

Dobson [198] −10 to 70 °C ωrN10−1/b: −blogωr ¼ l10 to 103 rad/s ωrb10−1/b: logωr ¼ logG

in all cases: log 1þ tanδð

Jongepier and Kuilman [197] −20 to 160 °C H τð Þ ¼ Gg

βJKffiffiffiπ

p exp−ln�β

8<:

3.10−3 to 32 rad/s

Dickinson and Witt [219] 2 to 52 °ClogGr ¼ 0:5 logωr− loð

h�

hyperbolic model 12 to 750 rad/sδ ωrð Þ ¼ 45 1−logωr lð

h�

Verney et al. [218] 21.5 to 60 °C Tb40 °C: η⁎ ωð Þ ¼1þ jωð

0.01 to 100 rad/sTN40 °C: η⁎ ωð Þ ¼ η∞ þ

Christensen and Anderson [163] −35 to 60 °C Gr ¼ 1þ ωrð Þ−log2R

−Rlog2

0.1 to 100 rad/sδ ωrð Þ ¼ 90

1þ ωrð Þlog2R

Stastna and Zanzotto [223] not given

η⁎ ωð Þ ¼ η0

∏m

k¼11þ jωð

∏n

k¼11þ jωð

26664

fractional model

Lesueur et al. [103] −20 to 100 °CTNRoom Tp: η⁎ ωð Þ ¼

1½0.1 to 100 rad/s TbRoom Tp: Christense

The equations are sometimes given for the complex modulus G⁎(ω) or for the modulus of thj (with j2=−1) appears in the first case and the phase angle δ is also given in the second casviscosity at Tr.

A most convenient definition is that by Dickinson and Witt [219],later used by Christensen and Anderson [163] although with adifferent symbol (Table 4): βDW=R=log Gg− log Gx where Gx is thecrossover modulus defined as the modulus at which frequency andtemperature are such that Gx/√2=G′=G″. Typical values for thisparameter ranged between 1.24 and 1.93 for neat bitumens andbetween 1.35 and 2.21 for RTFOT aged ones (Table 5 - [163]).

Most of the listed models describe the complex modulus as afunction of frequency. However, some authors preferred to work withthe complex viscosity or the relaxation spectrum. These functions areinterrelated and the corresponding interrelationships can be found inany textbook on viscoelasticity [99,205,207]. Although the complexmodulus is obtained from the material response to sinusoidal loading,the response to any other type of solicitation (creep, stress relaxa-tion, …) can be calculated from these models using known interrela-tions, provided the deformations remain small enough so that testingis performed within the linear viscoelastic range of the materials[99,205,207]. The validity of some of the interrelations for bitumenwas successfully tested by various authors [101,195,220].

It is not the purpose of this review to compare these models andseveral articles were devoted to the subject [221,222]. In general, alllisted models gave acceptable fit of experimental data in theinvestigated range (earlier attempts to describe the viscoelasticity ofbitumen with clearly unfitting models were discarded). Because of itslimited number of parameters, a very practical one is the Christensen-Anderson model [163] for which typical parameters are given inTable 5. It is very accurate for the low temperature rheology and startsto slightly diverge from experimental values in the terminal New-tonian zone [163].

Parameters

τmÞ−kτm: mean relaxation time

h, k: exponents (b1)δ: weighing factor

og G−br −1

� �þ 20:5−G−br

230:3 ωr=η0ωaT/Gg with aT shift factorr b: shear-susceptibility indexωrð ÞÞ ¼ −blogGr

ττm

�

JK

9=;

2

H(τ): relaxation spectrumτm: mean relaxation timeβJK: width of relaxation spectrum

gωrÞ2þ 2βDWð Þ2i0:5�

log Gg=8.88+0.58βDW

ogωrÞ2þ 2βDWð Þ2i−0:5� βDW: shear susceptibility index with βDW=log Gg− log Gx

ωr=ωaT with aT shift factor

η0τh Þ1−hþ jωτkð Þ1−k η∞: high-shear viscosity

η0−η∞1þ jωτhð Þ1−h

τh, τk: relaxation timesh, k: exponents (b1)

R: rheological index with

R=log Gg− log Gx

ωr=ωτx with τx crossover time

μkÞ

λkÞ

37775

1n−m µk, λk: relaxation times

m+n: total number of relaxation times (≥7 for bitumensand ≥9 for PMBs)

η0þ jωταð Þa �b τα: α-relaxation time

n-Anderson modela, b: exponents (b1)

e complex modulus also called G⁎(ω). There should however be no confusion becausee. Other notations are: Gr=G⁎(ω)/Gg with Gg the glassy modulus and η0,the zero-shear

Table 5Typical values for the rheological parameters of the Christensen-Anderson model for several bitumens

Bitumen origin Bitumen name Penetration at 25 °C Rheological index R Mean β-relaxation time log τβ Temperature for the relaxation time Reference

– 1/10 mm – log s °C

Lloydminster AAA-1 155 1.50 −4.17 25 [163]aged AAA-1 80 1.75 −3.72 25 [163]

Wyoming High Sulfur AAB-1 90 1.76 −3.41 25 [163]aged AAB-1 56 2.06 −2.65 25 [163]

Redwater AAC-1 102 1.63 −3.55 25 [163]aged AAC-1 54 1.80 −2.56 25 [163]

California Coastal AAD-1 137 1.66 −3.95 25 [163]aged AAD-1 60 1.80 −3.33 25 [163]

West Texas Sour AAF-1 54 1.60 −2.82 25 [163]aged AAF-1 29 1.77 −2.15 25 [163]

California Valley AAG-1 55 1.24 −3.14 25 [163]aged AAG-1 35 1.35 −2.75 25 [163]

Boscan AAK-1 65 1.60 −3.42 25 [163]aged AAK-1 40 1.80 −2.78 25 [163]

West Texas Intermediate AAM-1 63 1.93 −2.65 25 [163]aged AAM-1 42 2.21 −1.92 25 [163]

Venezuela A1 40 1.8 3.78 −15 [122]A4 182 1.4 1.85 −15 [122]

Middle East B1 37 2.3 [122]B3 82 1.7 [122]

The aged materials were tested after the RTFOT aging procedure.


Note that all thesemodels were intended to describe the behaviourof neat bitumen only, and not for PMBs. Still, general models such asfractional moduli can be adapted to PMBs – provided they conform totime-temperature superposition principle – by adding new elementsand therefore increasing the number of parameters [223].

3.6. Modelling the non-linear behaviour

As said before, Trouton was possibly the first researcher toobserve non-Newtonian effects in bituminous-like substances [193].Part of this non-Newtonian behaviour is of linear viscoelastic originand was described in the former section. However, non-linearityeffects can also be observed, especially when studying bitumen withconstant stress or strain rate experiments. The first precise descrip-tion of the non-linearity was probably the work by Saal and Koens in1936 who found that bitumen had a plastic behaviour [187]. At thattime, plasticity was used as a general word for materials having thepossibility to be moulded and was only starting to be applied in itsmodern meaning as a material having a yield stress [101,224]. Inmodern rheological terms, their finding would be called shear-thinning behaviour since no yield stress was really observed [101].This was quantified later on by several researchers [101,224,225]who introduced a degree of complex flow, defined as the exponent of apower law for stress versus shear rate. Values ranging between 0.4and 1 were found for various bitumens at 5, 25 and 50 °C, the degreeof complex flow decreasing with temperature especially for airblown materials [225].

Gaskins and co-workers [190] recognized that the non-Newtonianflowwas indeed observed after a Newtonian plateau, an experimentalfact later formalized by Sisko who proposed an adequate mathema-tical model [226], sometimes called the modified Cross model[227,228]:

η γð Þ ¼ η01þ γτð Þn ð7Þ

where ηγ is the shear-rate dependent viscosity, η 0 is the zero-shearviscosity, n, an exponent close to 0.5 and τ, the materials timeconstant. This model was recently extended to a three-dimensionsstate of strain by Ossa et al. [228].

Shenoy and co-workers showed that a unique master curve ofviscosity versus shear rate could be obtained for different bitumens byusing reduced variable: the viscosity divided by its zero shear valueand the shear rate multiplied by the zero shear viscosity [229].

Attané and co-workers [230] confirmed the construction of mastercurve as proposed by Shenoy et al. and showed that the shear-thinning region could be described by a power law behaviour with atypical exponent of −0.5 for the reduced viscosity (i.e., n=0.5 in theabove equation) and an exponent of −1.5 for the first coefficient ofnormal stresses. In comparison, Sisko found n=0.7 for three differentbitumens [226].

3.7. Temperature dependence

The temperature dependence of bitumen viscosity at hightemperature (typically superior to the Ring & Ball softening tempera-ture) is generally described by the celebrated Walther equation[17,182,231], known to apply to many petroleum products:

log log 0:95þ vð Þ½ � ¼ −mlogT þ c ð8Þ

wherein ν is the kinematic viscosity in mm2/s and T the absolutetemperature. This generally gives values for c between 8.7 and 10.4, 9being a typical value andm in the 3.0–5.0 rangewith a typical value of 3.5.The constant0.95, usuallynegligible for bitumen, is sometimes cited tobe0.8 or 1 [17,182,188], although the originalWalther paper gave 0.95 [231].

Once the temperature falls well below say 100 °C, it is morecommon to use either an Arrhenius or a WLF law [99,196]:

logη Tð Þη Trð Þ

� �¼ −C1

T−TrC2 þ T−Tr

: ð9Þ

In this case, the parameters can be derived either from viscositydata or from the temperature dependence of the relaxation timeswhich can be obtained by the WLF shifting procedure [99,196]. TheWLF law is widely used to describe the temperature dependence ofbitumen and various studies [163,198,219,232–233] propose to useone set of constants for all bitumens, as given in Table 6.

In fact, the first attempts to apply the WLF law [198,219,232] usedthe constants of C1 =8.86 and C2=101.6, with the reference

Table 6Temperature-dependence of the relaxation processes of bitumen in the literature to describe the viscoelastic properties of bitumens

Authors InvestigatedTemperature Range

Reference Temperature Temperature rangefor the parameters

Ea C1 C2

°C kJ/mol – K

Brodnyan et al. [220] −20 to 300 °C Tr∼TR&B TbTr−25 bitumen dependent bitumen dependentTNTr−25 8.86 101.6

Huet [216] −25 to 25 °C – 125–258Sayegh [217]Dobson [198] −10 to 70 °C Tr=adjustable TbTr 12.5 142.5Dickinson and Witt [219] 2 to 52 °C TNTr 8.86 101.6Jongepier and Kuilman [197] −20 to 160 °C Tr=glass transition temperature 7–19 70–400

Tr=temperature at which0viscosity=2 MPa s

20–55 10–190

Vinogradov et al. [232] −60 to 60 °C Tr=TR&B TbTr 23 218TNTr 8.86 101.6

Maccarrone and Tiu [233] −10 to 80 °C Tr=adjustable 23±8 230±67Verney et al. [218] 21.5 to 60 °C – Tb40 °C 149

TN40 °C 207Christensen and Anderson [163] −35 to 60 °C Tr=adjustable TbTr 12.5 142.5

TNTr 8.86 101.6Lesueur et al. [122] −20 to 100 °C Tr=glass transition temperature T∼Tβ 17–21 58–98

T∼Tα: see text


temperature as an adjustable parameter, following the original WLFtreatment [99,196]. The constants were initially believed to beuniversal but it was rapidly observed that it was not so for polymericmaterials [99]. This procedure gives reference temperatures typically50 °C higher than the glass transition temperature [99,196], as wasequally observed for bitumen [198,219,232]. In the end, the “uni-versal” constants were thought to work out well for bitumen but onlyfor the high temperature region (Table 6).

Clearly, the use of the universal values was also questioned byother authors and the initial general agreement has disappeared(Table 6). Interestingly, Brodnyan et al. observed that all bitumenswith penetration value superior to 30 1/10 mm felt on the same curve,while harder grades exhibited distinct values [220]. Jongepier andKuilman showed that the values for the WLF coefficients of straight-run paving grade bitumens were generally almost similar but differedsignificantly from values for blown materials [197]. This suggests thatmore asphaltenic bitumens behave somewhat differently than typicalstraight-run ones, which have almost identical temperature depen-dence. For these normal paving grade materials, the slight differencesin temperature dependence can be compensated for by adjusting thevalue of the reference temperature.

A very interesting fact is that most authors showed that twotemperature region had to be separated in order to propose anacceptable fit covering the experimental range (Table 6). The switchfrom one to the other fit usually occurs at a temperature close to roomtemperature. In the same time, it is known that simple glass formingliquids usually conform to a single WLF equation over a widetemperature range from Tg to Tg+100 °C [99,196]. This stronglysuggests that bitumen is not a simple glass forming material, and thatits viscoelastic behaviour can not be understood only as theconsequence of a vitrification process.

Therefore, the existence of two regions of different temperature-dependence for bitumen might be a consequence of the existence ofanother relaxation mechanism than the glass transition in bitumen[124]. In other words, the two regions of temperature dependencehighlighted by most authors illustrate the bimodality of bitumenviscoelasticity.

On a physical stand-point, the low temperature region wouldtherefore be essentially governed by the relaxation of the maltenephase (i.e., their glass transition). However, at higher temperature, theα-relaxation also comes into play, modifying the temperature-dependence that would occur with only the maltenes, as explainedin the next section.

3.8. Structural modelling

3.8.1. Newtonian behaviourIn a approach reminiscent of that by Mack almost 60 years ago

[140] and that from Russianworks of the 1970s by Kolbanovskaya andco-workers [234], Storm and co-workers [114] used known results onsuspension rheology to show that the zero-shear viscosity η0 of abitumen is related to that of the maltene phase η0,m and to theeffective volume of solid fraction ϕeff as:

η ¼ η0m 1−�eff

�m

� �−2:5

: ð10Þ

This relationship quantifies the viscosity building role of asphal-tenes. The first mention of a concentration-viscosity relationship witha limiting compacity ϕm (although not in this form) and this exponentof −2.5 is generally attributed to Roscoe [235].

The Roscoe law was shown to apply to all tested materials usingboth reconstituted maltenes and asphaltenes blends from a givenbitumen with varying proportions [114] and bitumens from the samecrude source but different paving grades [122], as pictured in Fig. 26.In this graph, the effective volume fraction was computed using theasphaltenes content as measured by n-heptane precipitation togetherwith the solvation constant as described in Eq. (2) whose values arereported in Table 3.

Also, molecular weight of the components does not appear as aninput. It thus confirms that proportionality between viscosity andmolar mass (raised to the power 1 or 3.4), such as that found in thepolymer literature [99], does not apply to the viscosity of bitumensas already detailed in the literature [26]. It does not mean thatmolar mass has no effect on the viscosity but only through its effecton the maltenes viscosity. Then, the shape and chemical nature ofthe molecules would also be important parameters and makes ittherefore very unlikely that a unique molecular indicator such as anaverage molecular weight, would be sufficient to explain thevariability of maltene viscosity among existing bitumens. The mainparameters controlling bitumen viscosity are therefore its asphal-tenes content, its solvation constant and the maltenes viscosity.

Concerning the temperature dependence of the viscosity, theasphaltenes only have a slight influence on it. From Eq. (10),assuming Arrhenian behaviour for the viscosity and the solvationconstant, the temperature dependence of the viscosity of bitu-men, expressed in terms of activation energy Ea, is that of the

Fig. 26. Validation of the Roscoe law (Eq. (10)) for the bitumens described in Table 3.


maltenes Eaη but augmented by that of the solvation constant Eaϕas [122]:

Ea ¼ Eaη þ 2:5�eff

�m

1− �eff

�m

Ea�: ð11Þ

Mathematically, Eq. (11) implies that even if the two elementaryprocesses at stake, namely the maltenes viscosity and the solvationconstant, exhibited perfect Arrhenian behaviour, the resulting bitu-men viscosity would not. Because of that, the correction terminvolving Eaϕ is in fact temperature dependent through the ϕeff

value. It typically amounts to 10 kJ/mol and its exact value at a giventemperature increases with the asphaltenes content.

As a consequence, low asphaltenes content bitumens have atemperature-dependence close to that of their maltenes (which inturn will be close to that of lighter petroleum products) and highasphaltenes content bitumens possess higher activation energy forthe flow, meaning that they are less temperature-susceptible. Still,these effects are not of huge magnitude given the somewhatelevated values of activation energy normally found for bitumens(Table 6).

Now, the Newtonian viscosity characterizes bitumen at hightemperature (typically higher than 60 °C). At lower temperatures, itsbehaviour can still be Newtonian for very long loading times, but inmost cases, viscoelasticity shows up. The rheology of bitumens canthen be separated into two regions, corresponding to two distinctrelaxation mechanisms, each associated to one phase of the colloid[103,122]:

a) Above room temperature, a transition from Newtonian flow toviscoelastic flow occurs (α-relaxation). It is attributed to the solidphase of the bitumen.

b) In the low temperature region, a transition from viscoelastic flowto elastic glassy behaviour occurs (β-relaxation). It is due to theglass transition of the liquid phase of the bitumen.

3.8.2. Newtonian/viscoelastic transition: α-relaxationOnce the temperature is slightly decreased from the Newtonian

region, viscoelastic effects start to show up. This was termed the α-relaxation of bitumen [103].

Several models were proposed to model the α-relaxation. Themore useful for a close analysis are those by Verney [218] and Lesueur[103] (Table 4), because they allow for an extraction of the relevantparameters describing the transition, i.e., the Newtonian viscosity anda mean relaxation time.

Extracting the zero-shear viscosity η0 from such models showsthat it follows a Roscoe law (Eq. (10)).

The relaxation time τα is related to the viscosity of the material η0and to the asphaltenes micelle size a0

3 (measured by SAXS) by aStokes-Einstein equation [103,122]:

τα ¼ 6πη0a30

kTð12Þ

wherein k is the Boltzman constant (equals to 1.38 10−23 J/K) and T,the temperature. In this equation, the presence of the solvation layer,increasing the asphaltenes micelle size, could be included by usingKa0

3 instead of a03 [114]. However, this would only apply if the solvationconstant where an adsorbed layer, which is an oversimplification (seediscussion in § 2.9.3).

This equation relates the switch from Newtonian behaviour toviscoelastic flow, which occurs for loading times longer than τα, tothe disappearance of the Brownian motion of the asphaltenesmicelles. Once the viscosity of the bitumen becomes too high, theasphaltenes micelles are locked in position whereas they can freelydiffuse within the maltene matrix when the viscosity is lowenough. The elasticity in the non-diffusive case would be aconsequence of interparticle attraction as discussed later on(§ 3.9.2).

This equation also predicts that the transition from Newtonian toviscoelastic flow occurs for either a critical time τα or a critical shearrate 1/τα or a critical stress σα given by [236]:

σα ¼ η0τα

¼ kT6πKa30

: ð13Þ

This critical shear rate is essentially a function of asphaltenesmicelle size because thermal energy only changes by 20% from 20 to100 °C and can thus be considered constant. It is then an intrinsicproperty of a given bitumen, directly related to its nanostructure.

As described in a former section, measurements of the permanentflow of bitumens under constant shear rate highlight such a criticalstress proportional to the viscosity [229,230]. Taking values formicellesize between 2 and 8 nm give calculated σα ranging from 0.4 to 27 kPa,to be compared to the bitumen-dependent values in the same rangemeasured by Dobson [198], to the almost constant value of 0.3 kPaobserved for different materials by Gaskins [190], or to the typicalvalue of 1 kPa found by Cheung [227].

This is reminiscent of the classical observations by Krieger, whohighlighted a critical shear stress above which non-Newtonianeffects showed up in the permanent flow of monodisperse lattices[213].

In summary, the bitumen α-relaxation at which viscoelastic effectsstart to appear is associated to the Brownian motion of theasphaltenes micelles and the associated relaxation time is propor-tional to the bitumen viscosity and the cube of the asphaltenesmicellesize. The maltene phase is however also directly involved in thephenomenon through the bitumen viscosity and there is therefore astrong coupling between the two phases.

3.8.3. Viscoelastic/elastic transition: β-relaxationIf the temperature keeps decreasing below the α-relaxation, the

elastic character becomes predominant and the transition to elasticglassy behaviour is observed. This was termed the β-relaxation ofbitumen [103].

In this low temperature range, the Christensen-Anderson model[163] describes the frequency dependence of the complex modulusand phase angle very well (Table 4). The three needed parameters areonly the glassy modulus Gg, the β-relaxation time τβ and therheological index R. The rheological index, which describes thefrequency dependence of the material, was earlier defined as R=logGg− log Gx (§ 3.5), the crossover modulus Gx being associated to thecrossover frequency ωx=1/τβ.


The rheological index was found to be related to the effective solidfraction at 25 °C by a linear equation of the form, validated on variousgrades of bitumen from two different crude sources [122]:

R ¼ 1:63�eff

�mþ 0:43: ð14Þ

The temperature dependence of the relaxation time τβ is welldescribed by a WLF equation with a reference temperature Tβ similarto the glass transition Tg measured, for example, by DSC.

logτβτ0

� �¼ −

C1 T−Tβ �

C2 þ T−Tβ: ð15Þ

The WLF coefficients C1 and C2 are only crude source dependent,regardless of paving grade [122], and can even be considered to beconstant regardless of crude sourcewith a decent approximation if thereference temperature is used as an adjustable parameter (Table 6 andsee discussion in the corresponding section).

In summary, the β-relaxation, transition from the viscoelastic tothe elastic regime, is a consequence of the vitrification of the maltenephase and the higher the glass transition temperature, the higher themodulus (higher τβ). The asphaltenes still have an influence on therelaxation function, and the higher the asphaltenes content, thesmaller the relaxation rate (higher R value). Thus, just like for the α-relaxation, there is again a coupling between asphaltenes andmaltenes at low temperature as well.

3.9. Practical consequences of the structural modelling

In short, the structure-properties relationships of bitumens can besummarized as: the lower the Tg and/or the lower the solid fractioncontent, the softer the bitumen. These simple principles explain theeffect of paving grade on the rheology of bitumens from the samecrude and help understand polymer modification.

3.9.1. Effect of the bitumen manufacture processAs mentioned earlier (§ 3.1.3), the early literature on bitumen

rheology contains evidence of general rules linking bitumen manu-facturing process (§ 2.2) to its rheological properties.

As a result, air-blown bitumens were referred to as “gel” bitumens[95–97,182–184], which in modern rheological terminology accountsfor lower loading-time and temperature susceptibilities. Moreprecisely, data on a bitumen air-blown to various extents showedthat air-blowing had almost no effect on creep modulus at a giventemperature but had a strong effect on the slope of the creep modulusloading time curve [237]. This agrees with the above description ofbitumen structure, since air-blowing is a special class of ageing(see also § 3.9.3). Air-blown bitumens have an almost unchanged Tgafter air-blowing with increased solid fraction content hence thestrong effect on R (Eq. (14)). As for the temperature susceptibility,increasing the solid fraction content increases the activation energy(Eq. (11)) and this is exactly what was found with air-blown bitumens[151,195].

As for solvent-deasphalted bitumens, it was mentioned thatgeneral rules on their rheological behaviour were somewhat hard toderive, since the type of solvent and the yield could vary greatly givingmaterials of very different properties (§ 3.1.3 - [28]). However, whenonly propane-deasphalted bitumens are considered, they are gen-erally thought to be more loading-time and temperature susceptiblethan straight-run ones. This agrees with the fact that they generallymaterials with lower solid fraction content as a consequence ofpropane being used as a precipitant. To reach road-bitumenspecifications, this lack of solid fraction must be compensated for bya higher glass transition temperature. Their lower susceptibility cantherefore be interpreted using Eqs. (11) and (14) as well.

3.9.2. Bitumen distribution of relaxation times and the origin of itsviscoelastic behaviour

The existence of two separate relaxation processes not onlyexplains the two regimes for the temperature dependence of therelaxation processes (Table 6) but also explains the presence of severaldecades of relaxation times.

The former point was noticed by all former researchers on bitumenstructure and rheology but without giving any physical explanation[105]. It is all the more striking that most studies on bitumen rheologydistinguished two temperature regions, whereas most other glassforming liquids need only one temperature dependence over a similartemperature range [124,196]. From the aforementioned physicalexplanation, these two temperature regions highlight the tworelaxation modes present in bitumen [124].

The existence of very large relaxation times remained unclear untilthe last decade. In fact, the presence of long relaxation times forbitumens can not be associated to the vitrification alone, as would befor a simple glass forming liquid. As a result, long relaxation times arepresent way above offset Tg which typically lies 20 °C above midpointTg (see Fig. 20 [20,238]). In the light of the colloidal model, longrelaxation times occur as a consequence of the presence ofasphaltenes nanoparticles with a dynamic behaviour coupled to thevitrification of the maltenes through Eq. (12).

More precisely, the Newtonian viscosity can be expressed as aglassy terminal relaxation time as [99,207]:

τβ ¼ η0Gg

: ð16Þ

Therefore, substituting the product τβ Gg for η0 in Eq. (12) gives thefollowing proportionality between the two relaxation times:

τα ¼ 6πGga30kT

τβ : ð17Þ

From this, and using the typical values given earlier for each of theinvolved parameters, the α-relaxation time is about 6 decades higherthan the β-relaxation time, explaining the presence of very longrelaxation times above the glass transition in bitumen rheology.

Now, the origin of the elastic effects due to the freezing of theasphaltenes Brownian motion remains unclear. In all cases, theBrownian diffusion of the asphaltenes at high temperature is inagreement with results from dielectric spectroscopy [124] and couldexplain, as discussed below, rejuvenator diffusion and differences inageing kinetics.

Elasticity might therefore be a consequence of the deformation ofasphaltenes aggregates such as those pictured in Fig. 14C, the overallasphaltenes network being close in concept to supra-molecularassemblies found in other fields [239,240]. Still, a percolating networkof interconnecting asphaltene molecules would probably yield to ahigher electrical conductivity at low temperature, contrary to what isnormally observed [124]. Therefore, the existence of supra-molecularassemblies is most probably restricted to small sizes, not big enough togenerate macroscopic conductivity. Conductivity versus thicknesscurves might demonstrate this hypothesis and give an estimate ofmaximum cluster size.

The gel picture of the original colloidal model (Fig. 12B) might befinally realistic, except for the following differences:

a) It should be a rather fragile structure, weak enough not to generatean elastic plateau,

b) It should be a transient structure with a typical escape time due toBrownian motion given by the α-relaxation time (Eq. (12)),

c) It is present in all bitumens regardless of the asphaltenes contentand temperature.

In this sense, elasticity would originate from particle-particleinteractions which remain to be precisely described. In the absence of


a detailed model of asphaltenes micelles interactions inside thebitumen, the discussion becomes highly hypothetical.

Given our present knowledge of colloid stability [100,111,241], thestabilization of the asphaltenes micelles necessitates the existence of arepulsive force, an attractive interaction due to Van der Waals forcesbeing always present [100,111]. In aqueous media, the repulsive force isoften of electrostatic origin, giving rise to the celebrated Derjaguin-Landau-Verwey-Overbeek (DLVO) theory [100,111]. However, in anorganic medium of low dielectric constant (close to 2 [17,123]) such asthe maltenes, free-charges can not be in a large enough quantity toensure DLVO type stabilization. Therefore, another repulsion must be atplay, and among the possible non-DLVO repulsive interactions [241], itseems that steric stabilization can be at play in a low polar organicsolvent such as the maltenes. As a consequence, a model of asphaltenesstability shouldmost probably rely on a steric-like repulsion, originatingfrom the resins and/or the asphaltenes and/or their assemblies.

Another source of information lies in the clustering of asphaltenesmicelles as observed by scattering experiments and described in aprevious section (§ 2.9.2). If a reaction-limited cluster aggregationprocesswas confirmed, itwould be indicative of a small overall attractiveinteraction.

In any case, the origin of the elasticity at the α-relaxation ofbitumen will not be fully understood unless a detailed model ofbitumen microstructure is available.

3.9.3. AgeingThe above colloidal model of bitumen also explains the known

rheological effects of ageing (§ 2.11.2). Air-blowing being a special caseof ageing, what was said before on the subject (§ 3.9.1) can begeneralized to other types of chemical ageing.

As a result, the value of the ageing index based on viscosity isconsistent with the above model. As an example, RTFOT ageing isknown to multiply the viscosity by a factor of 1.5–4 with a typicalincrease in asphaltenes content of 1–4 wt.% [170]. Assuming thatageing barely affects the solvation content and the maltene viscosity,using Eq. (10) with a typical value of K/ϕm of 5.5 (Table 3), and a typicalvalue for the initial asphaltenes content of 10% (Fig. 4), a 2% increase inasphaltenes content would amount for a doubling of the viscosity.Therefore, the colloidal model seems to be able to correctly explain theevolution of the physical properties of bitumen upon ageing.

The analysis can be further extended. Eq. (10) suggests that theviscosity-increase upon ageing raised to the power −0.4, should be alinear function of the asphaltene increase. The slope of the obtainedstraight line is the opposite of the solvation parameter. This isillustrated in Fig. 27 with published data [165]. The obtained slopes

Fig. 27. Relative viscosity to the power −0.4 versus increase in asphaltene content forvarious bitumens at different ageing times (data after [165]).

give solvation parameters K/ϕm at 135 °C of 8.4 and 5.5 for the RasTanura and Bahrain bitumen respectively. These numbers areconsistent with published data (Table 3).

3.9.4. Blending of bitumensThe existence of two relaxation processes also explains a forgotten

fact about the blending of bitumens. Typically, mixing two bitumensof different penetration gives a bitumen of intermediate penetration[17].

However, Siegmann described exceptions to this rule [242]. Whenmixing two bitumens of different structural types (i.e. sol and gel) butsimilar penetration, he observed a penetration maximum.

For example, blending 65% of a Borneo bitumen (PI=−2) with 20 1/10 mm penetration at 25 °C and 54 °C softening temperature togetherwith 35% of an air-blown Mexican bitumen (PI=+3.2) with 20 1/10 mm penetration at 25 °C and 88.5 °C softening temperature, gave abitumen with a penetration of 52 1/10 mm (Fig. 28 - [242]). Theproposed interpretation was that the Borneo bitumen of the sol typebroke the gel structure of the Mexican bitumen, hence the increasedpenetration [242].

However, this interpretation only accounts for one side of theproblem and can not explain the increased penetration of the solbitumen when the gel bitumen is added.

The modern colloidal picture, in turn, gives a possible explanation,based on the two relaxation processes. Hence, the Mexican bitumenmust have quite a low glass transition temperature combined with ahigh asphaltenes content (34% [242]) to reach a penetration value of20 1/10 mm. On the contrary, the Borneo bitumen must have a highglass transition temperature combined with a low asphaltenescontent (not given in [242]) to reach the same penetration value.Therefore, the properties of the blend is a consequence of these twoopposite contributions: the Mexican bitumen tends to soften theBorneo bitumen due to a lower glass transition temperature but in themean time hardens it by increasing the asphaltenes content. Thecompetition between these two processes can lead to three scenarios:a minimum of the penetration if the hardening effect dominates, amaximum if the softening effect dominates, or a gradual change ifthey compensate. The latter case is the most usual one [17], becausenormal paving grade bitumens have generally quite similar asphal-tenes content and glass transition temperatures. On the contrary, thebehaviour observed in Fig. 28 would correspond to the case where thesoftening effect dominates: the Mexican bitumen essentiallydecreases the glass transition of the blend hence the softening. It isof course difficult to validate this interpretation since the neededparameters were not published.

Anyway, the unusual blending rule described in [242] should applyto any blend of bitumens having similar room temperature propertiesbut due to a very asymmetric combination of glass transitiontemperature and asphaltenes content.

3.9.5. Bitumen rejuvenationAnother experimental fact that can only be explained by a two

phase structure lies in the discrepancy observed by Karlsson in thediffusion process of a rejuvenating oil within a hard bitumen[243,244].

Rejuvenating a hard bitumen is an industrial process used torecycle road materials. It consists in trying to recover the initialproperties of the aged bitumen inside an old pavement by adding asmall amount of a viscous oil. The idea is then to dilute theasphaltenes, created as a consequence of ageing, by the addition ofnew maltenes.

Karlsson measured the diffusion kinetics of the oil within the hardbitumen by means of both Fourier Transform Infra-Red spectroscopy(FTIR) and rheological means [243,244]. However, he found adifference of one order of magnitude between the diffusioncoefficients by both methods, which he was not able to explain.

Fig. 28. Peculiar blending behaviour observed for a Mexican bitumen in another onefrom Borneo as detailed in the text (after [241]).


Given the colloidal model, the difference is a natural consequenceof the two phase nature of bitumen. First, the oil must diffuse throughthe maltenes, with a diffusion coefficient corresponding to amolecular diffusion process, in agreement with the estimates byKarlsson [243]. This is the process followed by FTIR.

Second, the asphaltenes must diffuse through the rejuvenatedmaltenes. Since the asphaltenes diffuse at a nanoscale, the process ismuch slower, although the maltene viscosity might decrease as aconsequence of the oil (see Eq. (12)). However, and given the viscositybuilding role of the asphaltenes, rheological measurements willcapture the combined diffusion processes and the kinetics will bethat of the slower process, i.e. the asphaltenes diffusion.

Therefore, a difference in diffusion kinetics is expected betweenrheological and FTIR measurements when studying the rejuvenationof an old bitumen, as observed by Karlsson [244]. A more detailedmathematical analysis is however difficult to tackle, since it wouldneed the use of concentration-dependent diffusion coefficients.

3.9.6. PerspectivesThe existence of two relaxation processes explains the basic

features of bitumen rheology, in particular the two regimes for thetemperature dependence of the relaxation processes and theexistence of long relaxation times.

However, even if a correct physical description of bitumen mighthave been finally touched upon in the preceding lines, there still lacksa complete rheological model mathematically describing the relaxa-tion processes of bitumen as a function of its composition. This willprobably arise from a physical description of the elasticity arising fromthe asphaltenes that remains to be achieved.

Still, this description gives a general framework from which thegeneral features of bitumen ageing or blending rules of bitumens and/or bitumen with rejuvenating oil, could be precisely described.Moreover, the above picture remains sufficiently accurate to give acorrect description of the way the existing commercial modifierswork, as discussed in the next chapter.

Finally, the modelling proposed throughout this chapter does notincludewaxes and the structure changes they generate (§ 2.10). It seemshowever to apply tomostof the current pavinggradebitumens, probablybecause of their low crystalline wax content. Anyway, a completeaccount of bitumen rheology would need to incorporate a detaileddescription of the waxes and the corresponding structure changes.

4. Principles of bitumen modification

4.1. Acid modification

The modification of bitumen by chemical reactions is not a newsubject. As described before (§ 2.2), air-blowing of soft bitumens is anindustrial application of the chemical reactivity of bitumen, in use for

more than a century. However, the process was quite complicated andcould only be done in specific production units only available to refiners.

Still, it was early observed that bitumen could be reacted withother compounds such as sulphur, chlorine or various acids (sulphuric,nitric, acid sludge, fatty acids …) in normal storage tank [242]. Forexample, the reaction of a Venezuelan bitumen (initial penetration at25 °C 178 1/10 mm and softening temperature 39.5 °C) with chlorineat 200 °C gave a final bitumen with penetration 26 1/10 mm andsoftening temperature 82 °C. 7 g of chlorine per kg of bitumen werefixed and the asphaltenes content rose from 12.3% up to 26.3%. Theresulting properties were comparable to air-blowing the samebitumen [242].

However, these processes did not get industrial success because ofthe corrosion problem involved with manipulating such products andtheir reaction by-products, together with the low economical interestgiven that the same effect on properties could be obtained by thecheaper air-blowing process [242]. In parallel, air-blowing wasprogressively abandoned for paving bitumen because of the increasedfragility of the binders and higher ageing susceptibility.

In recent years, bitumen acidmodificationwas rediscovered becauseit turned out to start to make economical sense and seemed to lack thefragility problem of air-blown bitumens [156,245]. In particular,polyphosphoric acid (PPA) modification is currently gaining industrialimportance since it permits to significantly harden bitumen in an easilycontrollable way. As a result, the reaction of 1% of PPA to bitumentypically allows for a change of one class of paving grade [246].

Although the oxidative effect of acids was long recognized as wellas its similarities with air-blowing [242], the reactivity of bitumentowards acids is still not completely understood. It is known that notall bitumens show the same reactivity, depending primarily on theircrude source [246,247,248].

Recent work proposed that PPA acts through the neutralisation ofpolar interactions between the stacked asphaltenes molecules, eitherby protonation of basic sites or by esterification. The overall effect is toincrease the solvation of the asphaltenes, increasing in turn the solidfraction and hence, the viscosity [246].

Other researchers [248] proposed various bitumen-dependentmechanism of PPA modification which also affect the lower weightcomponents of the bitumen: co-polymerisation of the saturates, alkylaromatisation of the saturates, cross-linking of neighboring bitumensegments, the formation of ionic clusters and the cyclisation of alkyl-aromatics.

Regardless of the chemical mechanism, and based on the limitedavailable evidence, acid modification is seen to increase the effectivevolume of asphaltenes by changing the solvation constant (Eq. (2)),thus raising the viscosity according to Eq. (10). Once validated amechanism for PPA, the most studied acid of the day, it will have to beshown whether this mechanism applies to other acids as well.

4.2. Rheology of multiphase viscoelastic materials: the Palierne model

To understand the effect on bitumen rheology of modifiers such aspolymers and mineral fillers, the multiphase structure they generatemust be taken into account and an appropriate general framework todo so is by using known results on emulsion rheology.

Viscoelastic emulsions constitute model systems for multiphasematerials such as polymer blends [249–251] or polymer-modifiedasphalts [252]. Besides, suspensions can be thought of as special casesof emulsions. In the general sense, emulsions are two phase systemswhere one phase is dispersed within the other (the matrix):

1/ If the matrix is liquid and the dispersed phase consists of non-deformable particles (hard spheres), the system is called asuspension,

2/ If the matrix is a liquid and the dispersed phase is another liquid,the system is called an emulsion in the classic sense.


The viscoelastic properties of emulsions weremodelled by Paliernein the general case of viscoelastic spherical inclusions in a viscoelasticmatrix [253]. The Palierne model derives from earlier treatments ofthe subject, starting by the work of Einstein [254] who showed thatthe apparent viscosity of a suspension is higher than that of thesolvent (η0) due to stress concentration effects in the vicinity of theparticles supposed to be undeformable and spherical. The total stressin the suspension is the sum of the stress in the absence of theparticles plus the excess stress associated with the presence of theparticles. Thus, the viscosity (i.e., shear stress divided by the shearrate) η of the suspension is simply given by:

η ¼ η0 1þ 2:5�ð Þ ð18Þ

wherein ϕ is the volume fraction of dispersed phase. The stressconcentration effects are thus taken into account by the term 2.5 η0ϕ.The factor 2.5 is sometimes called the intrinsic viscosity [η] and isequal to 2.5 for hard spheres. This equation is only valid for the diluteregime (a few percents of dispersed phase) when interactionsbetween neighbouring particles vanish. Refinements of Einstein'stheory in the non-dilute regime have led to many theoretical andsemi-empirical equations [214,255], like the Roscoe law used by Stormand co-workers for bitumens (Eq. (10)).

The case of non-spherical hard particles has also been studied and theinfluence of particle shape on the intrinsic viscosity is given inTable 7 forellipsoidal particles characterized by their aspect ratioa/b, i.e., the ratio ofthe long to small axe [256]. It is clear that elongated particles have ahigher intrinsic viscosity, and have amore pronounced thickening effectthan spheres.

Einstein's theory was extended to elastic particles that deform as aconsequence of the shear stress when suspended in a viscous liquid[257–259]. Deformation is then delayed because of the rate depen-dency of the matrix and non-Newtonian effects show up even whenthe matrix is a Newtonian liquid.

In the case of emulsions of two liquids, dispersed particles can stilldeform but in a viscous way. Taylor showed that the flow inducedinside the inclusion is a function of the viscosity ratio λ of the twoliquids [260]. When the dispersed liquid viscosity is much higher thanthat of the matrix, the Einstein relation is recovered with [η]=2.5. Onthe opposite, when the matrix is much more viscous, the intrinsicviscosity goes down to 1.

Oldroyd completed Taylor's work by adding the contribution ofinterfacial tension γ [263]. Interfacial tension, or more precisely theYoung-Laplace pressure γ/2a, is the force giving a droplet its sphericalshape of radius a. Thus, when stresses are applied to the disperseddroplet, interfacial tension tends to prevent deformation. Interest-ingly, Oldroyd showed that non-Newtonian effects could show upwhen two Newtonian liquids are blended in an emulsion as aconsequence of interfacial deformation.

More recently, Palierne proposed a very general approach of whichall the above cases are special cases [253]. He showed that thecomplex modulus G⁎ of an emulsion made of two viscoelasticmaterials can be written as:

G⁎ ¼ G⁎mP⁎ λ⁎;Ca⁎; �ð Þ ð19Þ

with G⁎m, the complex modulus of the matrix and P⁎, a complexfunction of λ⁎, the complex ratio of the complex moduli (which is a

Table 7The effect of shape on the intrinsic viscosity [η] of ellipsoidal particles with differentaspect ratio (ratio of the small a to the long axe b - [256])

a/b 0.1 0.2 0.5 1 2 5 10

[η] 8.04 4.71 2.85 2.5 2.91 5.81 13.6

generalization of the viscosity ratio λ introduced by Taylor) and Ca⁎,the complex capillary number, respectively defined as:

λ⁎ ¼ G⁎iG⁎m

ð20Þ

Ca⁎ ¼ G⁎maγ

ð21Þ

wherein γ is the interfacial tension and a, the radius of an inclusion.In the semi-dilute case (more than 10% vol. of dispersed phase but

less than the maximum packing concentration), Palierne showed thatP⁎ is given by:

P⁎ ¼ 1þ 1:5 E⁎D⁎�

1− E⁎D⁎ �

ð22Þ

E⁎ ¼ 2 λ⁎−1ð Þ 19λ⁎þ 16ð Þ þ 85λ⁎2Ca⁎

ð23Þ

D⁎ ¼ 2λ⁎þ 3ð Þ 19λ⁎þ 16ð Þ þ 40λ⁎þ 1Ca⁎

: ð24Þ

As a consequence of the Palierne model, relaxations occurringwithin either phase will show up as relaxations of the emulsion. Inaddition, two more relaxations can occur:

a) one related to the interfacial deformation of the droplets,corresponding to the case studied by Oldroyd [261] and calleddroplet relaxation [250],

b) and the other one related to the delayed elastic deformation of thedroplets, corresponding to the case studied by Fröhlich and Sack[257] and called hard/soft relaxation [262].

4.3. Application to mastics: the suspension limit

Mineral filler is the fine part of the granular fraction, generallybeing defined as the material passing the 200 sieve in the US(0.075 mm) or the 0.063 mm sieve in Europe, with a typical averageparticle size in the 10–50 µm range [263–265]. Since mineral fillersenter at typically 2 to 12 wt.% of the total mineral matter in abituminous mix, the bitumen/filler blend, sometimes called mastic,naturally forms when bitumen is mixed with aggregates. It isgenerally believed that the properties of the mastic are indeed morerelevant than that of the bitumen in order to obtain sustainablepavement materials [263,264].

Mastics are suspensions, as established early in 1939 by Mitchelland Lee [266] and later confirmed by Rigden [267], who highlightedthat the relevant parameter governing the stiffening effect of mineralfillers in bituminous materials is the volume fraction of additive.

Heukelom and Wijga thoroughly studied mastics and derived thefollowing viscosity law describing the increase in viscosity as afunction of filler volume fraction [258]:

η ¼ η0 1−�

�m

� �−2

ð25Þ

wherein η is the viscosity of the mastic and η0, that of the neatbitumen. This law is very similar to that proposed by Storm and co-workers to describe the influence of asphaltenes on the viscosity of abitumen (Eq. (10)), except for the exponent −2 instead of −2.5. Laws ofthis type are widely used in suspension rheology [214,255] and can begeneralized to the moduli ratio as well because of the essentiallyundeformable nature of mineral fillers as compared to bitumen


[269,270], at least for the range of concentrations found in mastics.Other relationships were proposed in the literature [269,271,272], butthe improvement in fitting quality, if any, does not give a better insightinto the physico-chemical parameters at stakes.

Rigden tried to correlate the maximum packing fraction insidebitumen [ϕm in the above equation] to the dry packing of the samefiller obtained after a fixed compacting procedure [267]. The order ofmagnitude is preserved but exact coincidence is not obtained, becausethe overall packing geometry is different in both processes.

Also, the dispersion quality within bitumen logically affects thestiffening power of a given filler and it is highly probable that masticsprepared in the laboratory are not representative of the real masticsproduced in industrial mixing plants. Therefore, the relevance of masticproperties in order to describe mix properties remain an open issue.

The fact that mastics can be described as suspensions whenbitumen is itself a suspension may sound quite surprising. In fact, theparticle size of the asphaltenes micelles (a few nm) is so smallcompared to the size of mineral fillers (a few µm), that the bitumenmatrix can be considered homogeneous at the filler scale. However, inthe case of the so-called active fillers, specific bitumen/filler interac-tions come into play giving rise to quite different behaviour, asdescribed below.

In the limit of low volume fraction, Eq. (25) reduces to the Einsteinlaw (Eq. (18)) with:

η½ � ¼ 2�m

: ð26Þ

Data on the intrinsic viscosity of mineral fillers in bitumen can befound in the literature in different ways (Table 8): under the nameEinstein constant [273], by means of Eq. (26) using published data orby recalculating from experimental data. Obviously, most mineralfiller have values for the intrinsic viscosity in the range of 2.4–4.9regardless of the chosen bitumen, except for lime (3.2–10), fly ash(10.2–14.1), asbestos (16.5), fibers (26–34) and to a lesser extent kaolin(6.7), for which the stiffening effect can be much stronger.

Table 8Values for the intrinsic viscosity [η] of different fillers in bitumen

Filler type [η] Reference

Limestone 3.8 [268]Limestone 2.6–3.9 (25 °C) [273]

3.0–3.7 (70 °C)Limestone 2.5 (65 °C) [270]

2.4 (135 °C)Dolomitic limestone 4.9 (25 °C) [273]

4.4 (70 °C)Hydrated Lime 3.2–10 [277]Lime 7 [274]Sandstone 2.8 (25 °C) [273]

4.0 (70 °C)Siliceous filler 2.4 (65 °C) [270]

2.4 (135 °C)Granite 2.7–4.2 (25 °C) [273]

3.5–4.1 (70 °C)Fly ash 10.2 (25 °C) [273]

14.1 (70 °C)Slate dust 4.2 [268]Ball clay 3.2 [270]Kaolin 6.7 [268]Carbon black 2.6 (65 °C) [270]

3.9 (135 °C)Asbestos 16.5 [274]Polyester fibers 26–34 [281]Mineral fibers 26 [281]

Note that some authors did not explicitly give the value of [η] and it was then calculatedfrom the published data. The range found for different filler/bitumen combinations isgivenwhen appropriate, corresponding to different binder or filler origin, or measuringtemperature.

Lime has been described in the literature as an example of activefiller [274]. Specific interactions between asphaltenes and lime occur[175,275–277], probably by means of the combined effect of anadsorption layer of asphaltenes around the mineral particles (similarin concept to the adsorption of resins around the asphaltenes micellespictured in Fig. 18A) and the high porosity of lime.

In fact, the layer of asphaltenes around mineral particles probablyalways exists regardless of filler type [77,278,279], but it is usuallynegligible in size as compared to particle dimensions. For lime,particle size would be small enough to make the adsorption layersignificant as compared to particle size, therefore increasing theeffective volume fraction.

The case of fly ash is probably equivalent to that of lime, since it isreported that fly ash can be very fine [280].

The case of asbestos and kaolin can probably be explained bygeometry. Kaolin consists of platelets and asbestos of fibres, i.e.,elongated particles and thus have a high intrinsic viscosity (Table 7).Other types of fibers, mineral or organic, were shown to be quiteeffective to modify bitumen with very intrinsic viscosities, a value of30 giving a 16-fold viscosity increase for only a 0.5% fiber addition[281]. In fact, this is a very specific modification which tries to gel thebitumen film inside the mix in order to prevent drainage when highbitumen contents are used, and which is generally obtained by addingthe fibre directly to the mix and not inside the bitumen.

The above results on intrinsic viscosity were meant to apply to thehigh temperature range only. Still, the same law that applies to theviscosity (Eq. (25)) was shown to also apply to the modulus of themastics as well [273,277,270], hence the data at 25 °C in Table 8.

In most cases, ϕm is barely temperature-dependent but hydratedlime behaves again differently, since the stiffening effect is highlytemperature dependent and is more pronounced in the hightemperature region [276,277]. A more rigourous analysis [282]shows that there is indeed some temperature dependence of thestiffening effect even with “conventional” mineral fillers.

As a result, suspension laws describe very well the behaviour ofmastics, as long as particles do not come in close contact (dilute andsemi-dilute regimes), which means that the volume fraction mustremain well below ϕm. If this is not the case, the properties of theconcentrated mastic become quite different from that of the parentbitumen [282] because of the presence of numerous aggregate-aggregate contacts, that generate an asymptotic elastic behaviour suchas found for bituminous mixes [109]. This situation remains, at themoment, quite difficult to mechanically model [283].

4.4. Application to polymers modified bitumens

Polymer additives to reinforce bitumens are nowwidely used in theindustry as commercial products [18]. The idea to mix bitumen andrubber dates back to 1843 [284], but the objective was more to find asubstitute for rubber than to modify bitumen. At the turn of the 20thcentury, field trials with rubber-modified bitumens were performedand continued for several decades until polymer-modified bitumensgained significant commercial interest in the late 1970s [284].

Nowadays, several polymers are known to significantly modify theproperties of bitumens with typical contents between 3 and 6 wt.%.Most of the currently used polymers are either:

a) natural or synthetic rubbers such as styrene-butadiene copolymers(random SBR, diblock SB or triblock SBS) and the likes (styrene-isoprene,…), collectively known as elastomers,

b) ethylene-vinylacetate random copolymers (EVA) or related mole-cules (ethylene-methacrylate, ethylene-butylacrylate,…), collec-tively known as plastomers,

c) polyolefins such as polyethylenes (PE) and polypropylenes (PP),d) random terpolymers comprising ethylene, glycidyl methacrylate

(GMA) and an ester group (usually methyl, ethyl, or butylacrylate).

Fig. 29. The different microstructures of an EVA-modified bitumen with increasingpolymer content observed by fluorescence microscopy. The polymer-rich domainsappear yellow while the asphaltene-rich domains remain black. 3 wt.% EVA continuousasphaltenes-rich phase (A), 5 wt.% EVA co-continuous morphology (B) and 7 wt.%continuous polymer-rich phase (reprinted from [18] with permission by LCPC).


They are generally referred to as Reactive Ethylene Terpolymers(RET) because of the chemical reactions that are thought to occurbetween asphaltenes and the polymer [285].

Many grades of each of these polymers are available [18] but it isgenerally sufficient for the bitumen industry to characterize a polymerby its monomer composition and molar mass.

Note that different processes exist that yield to modifiedbituminous mixes [18]. Polymers may be added either directly tothe bitumen prior to themixing with the aggregate (wet process) or tothemix, at the same time bitumen is blendedwith the aggregates (dryprocess). This gives mixes with different properties than thoseobtained via the wet process. Also some of the elements mightapply to the dry process, the following discussion only focuses on thewet process.

This separation also applies to the modification by crumb rubberoriginating from the recycling of scrap tyres, which is an environmen-tally friendly polymer modification technology with growing interestdue to the large amounts of old tyres to dispose of [286,287]. Providingthe rubber has been dispersed within the bitumen, the followingpicture should apply, but with the limitation that, given the coarseparticle size (at least a few 100 µm sometimes up to 1 mm),rheological testing with the tools currently used for bitumenevaluation might not be adequate [288].

Adding polymer to bitumen allows to greatly increase the hightemperature end of the paving grade, leaving the low temperatureonly slightly better.

More specifically, a rule of thumb gives that for every 1% of addedpolymer, 2 °C in high temperature PG are typically gained [237]. Onthe low temperature end, the rule becomes more 1% of addedpolymer, 1 °C in low temperature PG for SB type modifiers. Since thePG classes are based on 6 °C steps, the typical 3% polymer-contentmodification generally allows to gain one high-temperature classleaving the low temperature sometimes unchanged.

Another rule of thumb states that an operating range (limiting hightemperature minus limiting low temperature) higher than 95 °C canonly be obtained through bitumen modification.

4.4.1. Polymer/bitumen compatibilityAll commercial Polymer-Modified Bitumens (PMBs) are hetero-

geneous at the micron scale [289–292]: the polymer is swollen by thelight aromatic components from the parent bitumen (Fig. 29).Consequently, the Polymer-Rich Phase (PRP) occupies between 4 to10 times the volume of added polymer, especially for SB and EVA. Forpolyolefins, the swelling is usually smaller [293–295]. This structure isthe key to polymer-modification.

The following discussion applies to the modification of bitumen byany polymer or blend of polymers, provided the morphology remainsthat pictured in Fig. 29A or C. Although the most probable structure isthe one with continuous Asphaltene-Rich Phase (ARP - Fig. 29A), thecase of continuous PRP (Fig. 29C) can also be described by the samePalierne theory. The only casewhere it would not be appropriate is co-continuous morphology (Fig. 29B).

Completely miscible polymers were also tested at the laboratoryscale but their properties are almost those of the parent bitumen[291]. Therefore, total miscibility is not wanted and partially misciblepolymers are preferred. This partial miscibility makes it quite difficultto define compatibility. In fact, the common meaning of polymer/bitumen compatibility is a system that will be homogeneous as judgedby eyesight but heterogeneous under the microscope [18].

In consequence, even for compatible systems, the equilibriumsituation is a macroscopic phase separation of the two-phases. TheARP being the denser phase, creaming of the PRP occurs at a ratecontrolled by the Stokes sedimentation rate [296]. Thus, the larger thedensity difference and the larger the particle size, the faster thecreaming rate [297]. This confirms that polymer/bitumen compat-

ibility is indeed a dynamic concept and that compatible systems arethose with a slow creaming rate.

In order to prevent phase-separation, few stabilization mechanismswere developed. The most used one at industrial level is dynamicvulcanization [298]. This consists in adding a cross-linking agent to thePMB under agitation [18]. This allows the PRP to slightly cross-link,preventing PRP droplets to coalesce. The freezing of the equilibriumdroplet size under agitation yields to a very low particle size favouringstability.High cross-linkdensity is not sought for, because itwould resultin a lower swelling extent (§ 4.4.2). Another proposed stabilizationmechanism, although of little industrial relevance, makes use of lowmolecular weight polymers thought to act as emulsifiers [299].

Because of the above definition of compatibility, the choice of apolymer modifier for bitumen is restricted to a few chemical families[18]. Each bitumen having its own particular chemical composition,ways to predict whether a particular polymer will be compatible with agiven bitumen are not well defined and the formulator usually relies onlaboratory experiments rather than on theoretical predictions. In allcases, and even if the polymer has a potentially compatible chemistry,the formulation of PMBs requires a good selection of the initial bitumen.

Still, some trends were highlighted [300,301]:

a) high asphaltenes content decreases polymer/bitumen compatibility,b) the aromaticity of the maltenes needs to fall between certain

values to reach a good level of compatibility.

More precise theoretical approaches were also presented. Lavaland Quivoron showed that the compatibility between bitumen andsome epoxy monomers could be predicted from the HydrophilicLipophilic Balance (HLB) concept used in the surfactant industry [300].More precisely, the HLB of a given epoxy system needs to be [302]:

a) lower than 6.3 or 8 (depending on bitumen) for total miscibility,b) between 6.3 and 9.3 or 8 and 11.8 (depending on bitumen) for

compatibility,c) higher than9.3 or 11.8 (dependingon thebitumen) for incompatibility.

Table 9Composition of the base bitumen and of each of the phases of three polymer-modifiedbitumens

Material Saturates(wt.%)

Aromatic(wt.%)

Resins(wt.%)

Asphalt.(wt.%)

Content(wt.%)

Base bitumen 1 7 54 23 16Asphaltenes-richphase (ARP)

6 47 21 26 50

Polymer-rich phase (PRP) 9 60 25 6 50Base bitumen 2 9 68 17 6ARP 8 57 24 10 50PRP 9 77 10 2 50Base bitumen 3 7 64 16 12ARP 5 54 20 21 46PRP 9 73 13 5 54

PMB 1 and 2 were made with 10 wt.% Cariflex TR1101 (SBS triblock copolymer with 30%styrene and M=120 kg/mol) and PMB 3 with 10 wt.% EVA (33% vinyl acetate and M notcommunicated). Data from [291].

Fig. 30. Effect of polymer-modification on the colloidal structure of a bitumen: originalbitumen (A) and the corresponding PMB with increased asphaltenes content in thematrix (B).


These results are very interesting because of the predictive powerof the concept of HLB [303]. Unfortunately, the HLB concept does notapply to largemolecules such as polymers and is therefore not directlyapplicable to the current PMBs.

Other approaches were tried, based on the solubility parameter ofpolymers. This approach has also a predictive power, since solubilityparameters can also be estimated from the molecular structure. It wasobserved that the solubility parameter of a polymer needs to be close to17-18 (MPa)0.5 so that compatibility is achieved, corresponding to thesolubility parameter of the aromatic fraction of the bitumen ([304] -§ 2.4). The solubility parameters of a few polymers used in the pavingindustry are given in Table 10 [305].

4.4.2. The structure of polymer-modified bitumensWhatever the choice of polymer, providing it is compatible with the

parent bitumen, the structure of PMBs becomes that pictured inFig. 29A: because asphaltenes and polymers do not mix, a phaseseparation occurs, leaving on one side the polymer swollen by thearomatics components of the maltenes and on the other side, theasphaltenes in the remainingmaltenes. Given the lowpolymer-contentsusually found in the paving industry, the morphology is usually that ofan emulsion with PRP dispersed within an ARP matrix. The aromaticsconcentrating a high content of fluorescent benzene rings, theirincreased concentration in the polymer-rich domains explains thatthey appear yellow in fluorescent microscopy ([306] - Fig. 29).

For additive contents higher than about 6 wt.% (depending on thepolymer/bitumen couple, this concentration varies widely between 4and 10 wt.%), phase inversion occurs and the inverse morphology isfoundwith asphaltenes-rich domains dispersedwithin a polymer-richmatrix ([18,291,304] - Fig. 29C). Given their high-polymer content, thismorphology is found with many industrial grade PMBs. Near thephase inversion concentration, a co-continuous morphology is found(Fig. 29B).

As a consequence of the large quantity of aromatics required toswell the polymer, the matrix becomes depleted in maltenes andhence enriched in asphaltenes (Table 9 - Fig. 30). This was referred toas a physical distillation [252]. This increased asphaltenes concentra-tion generates a global hardening of the matrix. This hardeningmechanismwas already proposed by Arrambide and Duriez in the late1950s when studying the modification of bitumen by natural rubber[307]. In fact, physical distillation is so intense for SB polymers that itexplains most of the high-temperature modulus increase of SB-modified bitumens [252].

Note that the exact morphology of a PMB in the absence of chemicalstabilization, especially mean particle size, highly depends on themanufacturing process [308] and on thermal history of the material[309].

Factors affecting the swelling extent of the polymers are essentiallythe polymer and the bitumen chemistry since molar mass of thepolymer is not relevant for the masses found in the industry. Theswelling extent of the polymer slightly decreases as the polymercontent is increased. For example, in a given SB copolymer/bitumensystem, the swelling extent felt from 500% to 430% when PS contentincreased from 25 to 50% [291]. The swelling extent for PE is usuallyless, of order 200% [310].

On the polymer side, molar mass is known to decrease solubility[98], but molar mass barely affects the swelling extent of the polymer[18,291,304]. This might be a consequence of the small range ofmasses (typically between 50 and 300 kg/mol) usually found for EVAand SBmodifiers. Thismight as well indicate that the phase separationin PMBs is not due to the thermodynamic interactions described in[98] but rather to other mechanisms such as depletion induced phaseseparation [311,312].

The chemical nature of the polymer becomes thus the key factor,especially for copolymers. Increasing the styrene content of a SBcopolymer or the vinyl acetate content of an EVA both decrease theswelling extent of the polymer, and therefore, the efficiency of themodifier [18,291,304]. For other modifiers such as polyolefins littleinformation can be found in the literature.

More practically, Bonemazzi and coworkers proposed to use thearomatic-oil absorption test to simulate the swelling of the polymerby the bitumen components [294]. They observed that highlycrystalline materials such as HDPE did not absorb oil as much asSBS, EVA or Ethylene-Propylene rubber (EPR), in reasonable agree-ment with the observed effect on bitumen.

A simple geometric argument relates the swelling extent of thepolymer to the asphaltenes enrichment of the matrix. Assuming thatall densities are equal to 1 and assuming that all the asphaltenes arein the matrix and all the polymer in the dispersed phase [144], itcomes:

xasph ¼ xasph01−xp

1−τGxpð27Þ

wherein xasph is the mass fraction of asphaltenes in the matrix of thePMB, xasph0 that of the parent bitumen, xp the polymer content and τG,its swelling extent defined as the volume of PRP divided by the initialvolume of the added-polymer. Using this approximation into Eq. (10),viscosity can increase by a factor 10 or more as was observed with 6%polymer swollen to 500% [252].

The swelling also has an influence on the properties of the PRP. Todescribe this, the bulk properties of most of the polymers used as

Table 10Typical physicochemical and rheological features of some selected polymers

Polymer Solubilityparameter

Crystallinity Tg Tm Gg G0

(MPa)0.5 wt.% °C °C MPa MPa

HDPE 16.2 70 −120 135 2000 2.6LLDPE 16.2 50 −120 109–124 2000 2.6LDPE 16.2 20 −120 108–123 1000 2.6Atactic PP 16.2 0 −30 160 1000 0.5isotactic PP 16.2 65 −20 160 1000 0.5PS 18.0 0 100 – 1000 0.2PB 17.0 0 −80 – 1000 1.3SB (S/B 24/76) – 0 −83/+77 – 1800 9 (30 °C)SBS (S/B 30/70) – 0 −87/+100 – 1800 15 (30 °C)EVA (E/VA 82/18) – 26 −40 20–90 1000 8 (30 °C)EVA (E/VA 72/28) – 15 −40 20–90 1000 1 (30 °C)

HDPE: High density PolyEthylene, LLDPE: Linear Low density PolyEthylene, LDPE: Lowdensity PolyEthylene, PP: PolyPropylene, PS: PolyStyrene, PB: PolyButadiene, SB:diblock PS-c-PB copolymer, SBS: triblock PS-c-PB copolymer, EVA: statistic copolymer ofPoly(Ethylene-Vinyl Acetate). Data from [99,295,305,310,313–315].

Fig. 31. A schematic description of the rheological of bulk polymers properties (shearelastic modulus vs temperature).


bitumen modifiers are given in Table 10. They are summarized bysome key parameters:

a) their glass transition temperature Tg [313],b) their melting temperature Tm in the case of semi-crystalline

polymers,c) their flowing temperature Tf,d) their limiting shear moduli in the glass state (Gg) and on the

rubbery plateau (G0) [314].

The rheology of bulk polymers can be described from these key-parameters and more information is available in the literature [99].Although oversimplified, the curves in Fig. 31 show the importantrheological features of both semi-crystalline and amorphous linearpolymers.

Note that crosslinked polymers, such as vulcanized rubber, do notflow because the entanglements are chemical covalent links andtherefore have a rubbery plateau up to the decomposition tempera-ture [99].

4.4.3. Polymer-rich phase rheologyProvided dilution is limited, the typical curve shown in Fig. 31 still

applies to polymer solutions [99,315] but with different typical valuesof the parameters (transition temperatures and moduli). The case ofhigh dilution, which leads to highly different behaviour [99,315]would correspond to solutions of less than 10 wt.% polymer, which islower than the polymer concentration inside the polymer-richdomains (10 wt.% represents a swelling extent of 1000%). Hence,high polymer dilution is not relevant to the case of PMBs.

The melting temperature Tm is hardly displaced and can beconsidered constant in a first approximation providing no co-crystal-lization between polymer and solvent exists [305]. This is true for PEin bitumens andmelting temperatures are decreased by nomore than12 °C [310]. Other polyolefins behaved as well [295]. For EVA, theeffect of bitumen on the melting temperature is difficult to assessbecause bitumen crystallizes at similar temperatures and seems toinvolve some co-crystallization [316].

The glass transition temperature Tg is usually modified to an extentthat depends on that of the solvent as described by typical mixingrules [99,305]:

1Tg

¼ x11Tg1

þ x21Tg2

ð28Þ

wherein Tg is the glass transition of the blend, Tgi the glass transitionof component i of mass fraction xi. Estimates using this rule for the

PRP of a PMB, containing 20wt.% PB (Tg∼−80 °C) in bitumen aromatics(Tg∼−20 °C) gives a glass transition temperature of −36 °C for theblend, in reasonable agreement with observed values [144,316].

The Newtonian flow temperature Tf is highly affected by thepresence of a solvent for the polymer. It decreases considerably evenfor slight amounts of solvent because it lubricates inter-chain friction.In all cases, it does not have such a strong influence on PMB rheology,because it would affect only the terminal behaviour at hightemperature when the materials are almost Newtonians. Since it isquite difficult to express without introducing many details on themolecular motion of polymers [98,99,206], we will therefore nottackle this point any further.

The rubbery plateau G of the polymer solution is a decreasingfunction of volume fraction ϕ of polymer:

G ¼ G0�a ð29Þ

wherein G0 is the rubbery plateau of the bulk polymer. The exponent aexperimentally lies in the range 2–2.3 [317] and is theoretically 9/4[98,206].

4.4.4. Mechanical modelling of polymer-modified bitumensThe Palierne emulsion model applies to PMBs [252]. Therefore, the

modulus of the PMB can be calculated from the moduli of itsconstituents (Fig. 32) using Eq. (19), with the droplets Young-Laplacepressure as the only additional parameter.

Hadrzynski and Such also used the properties of the phases tocalculate the properties of a PMB using a self-consistent scheme [318].However, this approach is limited to temperatures below 40 °C,because it can not account for micro-mechanical relaxations.

A very interesting feature is the phase angle minimum observedfor PMBs in the lowmodulus range (or high temperature), showing upin the isochrones (Fig. 33 - [319]) or in the Black diagram (Fig. 34).Although it is an expected feature for PMBs with continuous polymerphase as a consequence of rubber elasticity, it still occurs even withlow polymer contents when the morphology is that of a continuousbituminous phase (Fig. 29A). In such cases, the phase angle minimumwas attributed to a hard/soft relaxation for SB modified bitumens[252,262]. Such a minimum was also observed for EVA modifiedmaterials, but would be more a consequence of a relaxation inside thedispersed phase, i.e., the melting of the PE crystals [320].

A detailed analysis of the hard-soft relaxation showed that therubbery plateau of the dispersed phase of triblock SBS copolymer-modified bitumen (41 kPa) was higher than that of a diblock SBcopolymer-modified one with a lower molar mass (1 kPa). In-situvulcanization of the diblock copolymer inside the PMB yielded to an


increased modulus of the polymer-rich inclusion up to 46 kPa [252].Such plateau values are in agreement with Eq. (29).

In order to calculate the properties of a PMB from the Palierneequations, interfacial tension γ between dispersed phase and matrix,acts as an adjustable parameter. Values of order 10−5 N/mwere foundfor SB modified bitumens [252] which are rather similar to that foundin immiscible ternary polymer/polymer/solvent blends [321,322].Thus, PMBs can simply be considered as ternary blends of copoly-mer/asphaltenes/maltenes even for their interfacial properties, aspictured in Fig. 30B.

4.4.5. Practical consequences: parameters governing the rheology ofpolymer-modified bitumens

The fact that the Palierne model applies to PMBs has severalimplications. First, a fundamental parameter is the volume fraction ofpolymer-rich inclusions. The swelling extent of the polymer has threeimportant contributions:

a) Governing the physical distillation process as expressed by Eq.(27),

b) Controlling the properties of the PRP as detailed in the section onPolymer-Rich Phase Rheology,

c) Enhancing the contribution of the polymer-rich particles to thePMB rheology.

Fig. 32. The rheological properties of each of the phases (asphaltenes-rich matrix andpolymer-rich dispersed phase) of a PMB with 6% SBS: imaginary (G″) and real (G′) of thecomplex modulus G⁎ at 10 rad/s as a function of temperature and corresponding Blackdiagram.

Fig. 33. The effect of adding 6% of various polymers to a 180/220 bitumen: storagemodulus and phase angle at 1 rad/s versus temperature for several BMP. SEBS: Styrene–ethylene–butylene–styrene copolymerwith 29wt.% styrene (Kraton G 1660 from Shell),EVA 1: ethylene–vinyl acetate copolymer with 28 wt.% vinyl acetate (Elvax 260 fromDupont), EVA 2: ethylene–vinyl acetate copolymer with 18wt.% vinyl acetate (Elvax 420from Dupont), EBA: ethylene–butyl acrylate copolymer of unknown composition(supplied by Neste Chem.) (reprinted from [319] with permission by Springer).

Second, since interfacial tension is low for the two phases of a PMB,its contribution on the rheology of the blend is only at temperaturesabove 100 °C. Therefore, particle size, which only shows up in thePalierne Eq. (19) through the Young-Laplace pressure, is not a relevantparameter when low strain rheology is studied. Thus, the exactmorphology of a PMB, which is known to be highly temperature-history dependent [309], will not affect its modulus in the tempera-ture range typical of paving applications. The relevant parameter is,again, the volume fraction of dispersed phase and not the particle size.Still, particle size has an important role on the high-strain properties,and in particular on the fracture properties of PMBs [323–325].

4.4.6. Practical consequences: combined modification by acid andpolymers

Given the above described effect of bitumen modification by acidssuch as PPA (§ 4.1), it was rapidly realized that PPA and polymer couldbe combined and it was shown that the two types of modifiers act infact in a synergetic way as far as the mechanical properties areconcerned [246]. That is, the addition of both PPA and polymer is moreefficient thanwhat would be inferred by looking at themodification ofeither of the modifiers alone and assuming the effects would sum up.

This synergetic effect can also be explained by the colloidal model:As proposed above, acid modification generates an increase ineffective asphaltenes content, of say ΔϕPPA. The viscosity increasewould then be straightforward from Eq. (10), with a viscosity ratio VRafter modification VRPPA.

Fig. 34. The rheological properties of the same PMB whose phases are described inFig. 4–6with 6% SBS: imaginary (G″) and real (G′) of the complexmodulus G⁎ at 10 rad/sas a function of temperature and corresponding Black diagram.


Polymer modification alone would be limited to the physicaldistillation generating an increase in effective asphaltenes contentΔϕPol as calculated from Eq. (27), with subsequent viscosity increaseVRPol.

Assuming the total effective asphaltenes content after combinedmodification is the sum of the two effects, then the effect of combiningboth modifier would be an effective asphaltenes content increase ofΔϕPPA+ΔϕPol. Its effect on the viscosity would be greater than VRPPA.VRPol because of the non-linear character of equation Eq. (10).

Therefore, the synergetic effect of acid and polymer modificationoccurs as a consequence of the colloidal structure of bitumen.

4.5. Synthetic bitumens

Organic binders started to be largely used as pavementmaterials atthe beginning of the 20th century. One of the most famous defendantof this technology was the celebrated Dr. Guglielminetti, also knownas “Dr. Goudron” (Dr. Coal Tar in French). He saw the advent of organicbinders as away to get rid of the dust created by the cars on dust roadswhich were the reference back then. In one of his articles advocatingthe use of coal tar as a dust preventive, he remarked:…la couleur noireque prennent les routes n'est pas très esthétique et soulève bien desprotestations. (…the resulting black colour of the pavements is quiteunaesthetic and many people complain) [326]. This statementsummarizes one of the biggest aesthetic problem arising with theuse of bitumen: it is black.

If people have gotten used to this situation, architects like topropose pavements with other colours than black, especially for urbanconstruction projects. Also, the agencies start to promote the use ofcoloured pavement materials as a way to increase safety. As a matterof fact, bicycle lanes tend to be differentiated making them green orred. Also, a clearer pavement surface can reduce pavement tempera-ture by 10 °C in the summer, increasing durability. It also reflectsbetter artificial light, allowing for important savings in energyconsumption. All these facts highlight the growing need for not-black paving materials [327].

Asphaltenes being largely responsible for the black colour ofbitumen, the solution to this problem has historically been to pigmentlow asphaltene content bitumens (sometimes called “albinos bitu-mens”). Still, theobtained colourmaintained adark toneandwasmostlyused with red pigments in order to reach a Bordeaux red product [18].

Now, bitumen substitutes have appeared on the market under thegeneral name synthetic or pigmentable or clear binders [18]. They arespecial formulations with a yellowish aspect that mimic the mechan-ical properties of bitumen.

Typical formulations consist in blending a tackifying resin of highsoftening point (colophony, hydrocarbon resin, coumaron-indeneresin,…) together with a dispersing oil [328]. This makes it possibleto manufacture “vegetal” binders by focusing on renewable rawmaterials of vegetal origin [329].

A polymer (EVA, SBS,…) can then be added if necessary in order toimprove the mechanical properties [18,330,331]. Although littlepublished results are available, the existing literature tends to showthat synthetic binders differ from straight-run bitumen by essentiallytwo rheological features:

1. The possible existence of a barely pronounced rubbery plateau forpolymer-contents higher than a concentration around 8% [330,331],

2. A higher temperature susceptibility for polymer-contents belowthe rubbery plateau [18,330–332].

This last point is interesting, because it would confirm that somekind of structuring (i.e., asphaltenes micelles) is necessary to explainthe temperature susceptibility of bitumen properties, in agreementwith the earlier discussion (§ 3.7).

In any case, this type of materials are not bituminous and thereforeinterpreting their properties in the light of the known results onbitumen science does not reallymake sense. Still, itmust bekept inmindthat these synthetic binders try to mimic bitumen rheological proper-ties. Therefore, any advance in the understanding of bitumen propertiescould in turn give ideas to the formulator of synthetic binders.

5. Conclusions

Bitumen is a complex mixture of mostly hydrocarbons whosestructure is well described by the colloidal model: solid particles (theasphaltenes) with a radius of a few nanometres dispersed in an oilyliquid matrix (the maltenes). The most important parameter describ-ing the structure of a bitumen are thus the glass transitiontemperature of its maltenes and its solid fraction content (or effectiveasphaltenes content), proportional to its asphaltenes content.

This structure generates two main mechanical transitions: reducingthe temperature from the Newtonian liquid state, viscoelastic effectsstart to appearupon freezingup theBrownianmotionof the asphaltenesmicelles. Reducingmore the temperaturewould generate a transition tothe elastic glassy state as a consequence of the vitrification of themaltenes. The coupling between the two phenomena is shown togenerate the observed large distribution of relaxation times.

This description of bitumen rheology is consistent with otherobservations, such as the hardening effect due to ageing, the interdiffu-sion of rejuvenator inside a weathered bitumen and acid modification.

Other kinds of modifications, i.e. by fillers and polymers, can alsobe interpreted based on this model of bitumen structure and known


results on the rheology of multiphase materials as established byPalierne.

Thus, the parameter governing themodification of bitumen by fillersis the filler volume fraction, as is well known from suspension rheology,which can be thought of as a special case of the Palierne model.

Polymermodification can also be explainedwithin this framework.Polymeric modifiers are characterized by their swelling extent insidethe bitumen, which in turn governs the physical distillation, i.e., theglobal hardening of the matrix due to an increase in effectiveasphaltenes content as a consequence of the swelling of the polymer.Then, the changes in properties are a complex combination of theproperties of both phases through the Palierne Eqs. (19)–(24).Therefore, the effect of polymer modification is shown to be fullyinterpretable using the colloidal model of bitumen.

The synergetic effects due to the combined modification by acidsand polymer also arise as a consequence of the colloidal model.

Finally, and although amore precise picture of bitumen structure isstarting to emerge, many points remain unclear and should fosterfurther research.

First, the origin of the elastic effects occurring upon freezing theBrownian motion of the asphaltenes micelles remains to be fullyunderstood and it is believed that a better description of theasphaltenes micelles interactions would be needed before a completemechanical description can be reached.

Then, the effect of wax crystallization on the structure of bitumenneeds to be better described and incorporated in any furtherstructure-related model of the mechanical properties. Whether theycan explain steric hardening needs also be fully studied.

Also, the application of the colloidal model to bitumen ageing andpolymer-modification has only been validated on a limited number ofmaterials and the task should be pursued to test the limits of thisapproach. In addition, the origin of phase separation in PMB still needsclarifying to check whether it occurs as a consequence of depletioninteractions.

Finally, if the colloidal model seems to agree quite well with therheological properties of bitumen, its consequences on the fractureand interfacial properties need to be studied.

From then on, and having fully understood bitumen, the next stepwould be to develop structure-related models for bituminous mixes, avery interesting class of compositematerials combining colloid science forthe bitumen part and the science of granular matter for the mineral part.

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the colloidal structure of bitumen: consequences on the rheology and on the mechanisms of bitumen...

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