Bitumen performance in hot and arid climates

 

By: Anil Srivastava[1] and Ronald van Rooijen [2]

  

Prepared for

 

Pavement Seminar for the Middle East and North Africa Region

Innovative Road Rehabilitation and Recycling Technologies

New Policies and Practices in Pavement Design and Execution

 

24 – 26 October 2000 Amman, Jordan

 

SUMMARY

 

From a performance point of view, bitumen is one of the most important constituents of an asphalt mixture. The quality and properties of bitumen depend largely on the chemical composition of the bitumen, which is mainly controlled by the crude oil and production process. This paper covers the life cycle of bitumen. It starts with a brief discussion about the influence of crude oil and production process on the quality and properties of the bitumen. Also, some attention is paid to models, which describe the bitumen structure.

 

Usually the contractor or asphalt producer selects the bitumen. Ideally, the selection is based on the performance requirements for the asphalt mixture/ asphalt layer. To be able to do so, they should know about the asphalt pavement performance requirements, the significance of bitumen with respect to the performance requirements for asphalt mixtures/asphalt layers and the performance/properties of the available bitumen. These aspects are thoroughly discussed in this paper. Special attention is paid to the asphalt and bitumen performance requirements for hot and arid regions like the Middle East and North Africa, and ageing of bitumen, i.e. the ageing mechanisms, determination of ageing resistance and changes in bitumen composition and performance/properties due to ageing. Finally, some ways to improve the performance/properties of bitumen are also discussed.

 

In this paper relevant test data from international studies as well as studies performed by Ooms Avenhorn Holding are presented. Included are many examples of well performing bitumen and some examples of unsuitable bitumen.


INTRODUCTION

 

For areas with hot temperatures the most important performance requirements for asphalt mixtures and asphalt layers are resistance to permanent deformation (rutting) and resistance to surface cracking induced by ageing. The bitumen has a great influence on these performance requirements.

 

In this paper all major aspects related to quality and properties of bitumen are discussed. Included are bitumen composition and structure, bitumen production, physical characterization, specifications, ageing, upgrading and modification of bitumen. Special attention is given to the performance requirements for bitumen used in areas with hot temperatures like the Middle East and North Africa.

 

CHEMICAL CHARACTERISATION OF BITUMEN

 

Elemental composition

 

Bitumen is a complex mixture of molecules of a predominantly hydrocarbon nature, which vary widely in their composition. They contain amongst others minor amounts of heteroatoms containing sulphur, nitrogen and oxygen and trace quantities of metals such as vandium, nickel, iron, magnesium and calcium, which occur in the form of inorganic salts and oxides. The chemical composition of bitumen depends on the origin of the crude oil and the processes used during bitumen manufacture. Since the chemical composition of bitumen is extremely complex with the number of molecules with different chemical structures being astronomically large, it is not feasible to attempt a complete analysis of bitumen. Besides, the elemental composition of bitumen provides little information of what types of molecular structure are present in the bitumen. This knowledge is necessary for a fundamental understanding of how the composition of the bitumen affects the physical properties and chemical reactivity.

 

Fractional composition

 

There are three principal types of molecules found in bitumen: aliphatics (or paraffinics), naphthenics (or cyclics) and aromatics. The physical and chemical behaviour of bitumen is affected by the various ways in which these compounds interact with one another. The molecules are held together through chemical bonds that are relatively weak and can be broken by heat and/or shear forces.

 

In general bitumen can be divided into two broad chemical groups: asphaltenes and maltenes. The maltenes can be further subdivided into saturates, aromatics and resins. Although these groups are not completely defined and have some overlap, they enable to compare bitumen properties with broad chemical composition.

Various techniques have been developed to separate bitumen into fractions. These techniques are based on differences in molecular size, chemical reactivity and/or polarity. Chromatographic techniques are the most common methods.  They are based on differences in chemical reactivity and polarity. The basis of the chromatographic techniques is to initially precipitate the asphaltenes with a n-alkane (usually n-pentane), followed by chromatographic separation of the remaining maltene material. Using this technique, bitumens can be separated into the four groups: asphaltenes, resins, aromatics and saturates. These groups are called SARA fractions (Saturates, Aromatics, Resins and Asphaltenes). Their main characteristics are as follows:

 

Asphaltenes

 

Asphaltenes are considered as highly polar, complex aromatic materials with a tendency to interact and associate. They have fairly high molecular weights ranging from about 1,000 to 100,000. The asphaltene content has a large effect on the rheological characteristics. Increasing the asphaltene content produces harder bitumen with a lower Penetration, higher Softening Point and consequently higher viscosity. Generally, bitumen contains 10 to 20% asphaltenes.

 

Resins

 

Resins (polar aromatics) are very polar in nature, which make them strongly adhesive. They are dispersing agents for the asphaltenes. Resins have molecular weights ranging from 500 to 50,000. Generally, bitumen contains 10 to 25% resins.

 

Aromatics

 

Aromatics (naphthene aromatics) are weakly polar. They serve as the dispersion medium for the peptised asphaltenes and constitute 55 to 70% of the total bitumen. The average molecular weight ranges from 300 to 2,000.

 

Saturates

 

Saturates (aliphatics) are non-polar viscous oils with a similar molecular weight range to aromatics. The components include both waxy and non-waxy saturates. Saturates form 5 to 15% of the bitumen.

 

Bitumen structure

 

Colloidal Model

 

Bitumen is traditionally regarded as a colloidal system consisting of high molecular weight asphaltene micelles dispersed or dissolved in a lower molecular weight oily medium (maltenes). The micelles are considered to be asphaltenes together with an absorbed sheath of high molecular weight aromatic resins, which act as a stabilising solvating layer and peptise the asphaltenes within the solvent maltenes phase. Away from the centre of the micelle there is a gradual transition to less polar aromatic resins and, finally, to less aromatic oily dispersion medium.

 

In bitumens with sufficient quantities of resins and aromatics of adequate solvating power, the asphaltenes are fully peptised and the micelles have good mobility within the bitumen. These bitumens are known as solution or ‘SOL’ type bitumens. If the quantity of the aromatic/resin fraction is insufficient to peptise the micelles or has insufficient solvating power, the asphaltenes can associate to form large agglomerations or even a continious network throughout the bitumen. These bitumens are known as gelatinous or ‘GEL’ type bitumens. In practice most bitumens are of intermediate character.

 

The Index of Colloidal Instability (CI), which is defined as the ratio of the amount of asphaltenes and saturates to the amount of resins and aromatics, is sometimes used to describe the stability of the colloidal structure. The higher CI, the more the bitumen is regarded as ‘GEL’ type bitumen. The lower CI, the more stable the colloidal structure.

 

The degree to which asphaltenes are peptised will considerably influence the viscosity of the bitumen. The viscosity of the saturates, aromatics and resins depend on their molecular weight distributions. The higher the molecular weight the higher the viscosity. The viscosity of the maltenes imparts an inherent viscosity to the bitumen, which is increased by the presence of the dispersed asphaltenes. Saturates decrease the ability of the maltenes to solvate the asphaltenes.

 

SHRP Model 

 

Under the Strategic Highways Research Program (SHRP) a microstructural model was developed. The model states that the bitumen structure consists of microstructures (comprised of polar, aromatic, asphaltenelike molecules that tend to form associations) dispersed in a bulk solvent moiety consisting of relatively non-polar, aliphatic molecules. Many of the molecules comprising the dispersed phase are assumed to be polyfunctional and capable of associating through hydrogen bonds, dipole interactions and B-B interactions to form primary microstructures. Under proper conditions, the primary microstructures can associate into three-dimensional networks, which may be broken, together with the microstructures, by heat and shear stress. According to the model, bitumen physical properties are described by the effectiveness with which the polar, associated materials are dispersed by the solvent moiety rather than being described by global chemical parameters such as elemental composition.

 

BITUMEN PRODUCTION

 

Most of the bitumen used in asphalt pavements is produced during the distillation processes of crude oil. Only a small amount comes from natural resources, like Trinidad Lake Asphalt.

 

Crude oils

 

Crude oils differ in both their physical and chemical properties. Physically they range from allmost solid to free flowing at room temperature. The physical state can be described with the API gravity, which is directly related to the density of the crude oil. The API gravity varies from 0.0 (e.g. Sesmaria crude oil from Brazil) to more than 70 (e.g. San Roque crude oil from Bolivia). Crude oils with a low API gravity are viscous and generally contain a high percentage of bitumen.  Bitumen has an API number of 2 to 4. Some examples of crude oils with their API gravity, density and percentage bitumen are given in table 1.

 

Parameter

Boscan

(Venezuela)

Arabian Heavy (*1)

Nigeria Light

API gravity

10.1

28.2

38.1

Density

0.999

0.886

0.834

Bitumen

58%

27%

1%

*1: blend of several crude oils

Table 1 Details of some crude oils

 

Chemically, crude oils may be predominantly paraffinic, naphthenic or aromatic. The K factor indicates whether the crude oil is paraffinic (K factor: 12.5–13.0) or naphthenic-aromatic (K factor: 10.5–12.5). Paraffinic crude oils are not suited for bitumen production. The K factor is calculated from the average boiling point and the density of the crude oil. Other important parameters are the paraffin or wax content and the Bromine number. The paraffin or wax content is important with respect to the Rheological and adhesive properties of the bitumen. It should be lower than 0.5%. The Bromine number is an indication for the presence of reactive compounds (olefines) which have a large influence on the ageing behaviour of bitumen. The amount of Vanadium and Nickel is unique for each crude oil and can therefore serve as a fingerprint of the crude oil.


Production processes

 

Distillation

 

Bitumen is produced by fractional distillation of crude oil. Usually, distillation is done in two steps.

 

First the crude oil is heated up to 300-350°C and introduced into an atmospheric distillation column. Lighter fractions like naphtha, kerosene and gas oil are separated from the crude oil at different heights in the column. The heaviest fractions left at the bottom of the column are called long residue.

 

The long residue is heated up to 350-400°C and introduced into a distillation column with reduced pressure (vacuum column). By using reduced pressure it is possible to further distillate lighter products from the residue because the equivalent temperature (temperature under atmospheric conditions) is much higher. If second distillation were carried out under atmospheric conditions and by increasing the temperature above 400°C, thermal decomposition of the long residue would occur. The residue at the bottom of the column is called short residue and is the feedstock for the manufacture of bitumen.

 

The viscosity of the short residue depends on the origin of the crude oil, the temperature of the long residue, the temperature and pressure in the vacuum column and the residence time. Usually, the conditions are such that short residue is produced with a Penetration between 100 and 300 dmm. The amount of short residue decreases and the relative amount of asphaltenes increases with increasing viscosity of the short residue.

 

Bitumen manufactured from the short residue is called straight run bitumen. The differences in properties between high and low penetration grade bitumen are mainly caused by different amounts of molecule structures with strong interactions. Low penetration grade bitumen contains more of these molecule structures. This is the main reason why their viscosity, Fraaß Breaking Point, Softening Point, etc., is so much higher than for high penetration grade bitumen. The fact that they contain less low viscosity products is of less significance.

 

Blowing

 

One way to make bitumen harder is to blow air through it. This process is called blowing. Air is heated up to 150–250°C and introduced at the bottom of a blowing column. It then migrates through the bitumen to the top of the column. The chemical reactions result in bitumen with a different mixture of molecular structures. Catalysts can influence this process.

 

Blown bitumen has more and stronger molecular interactions than the original bitumen and is therefore more cohesive. Blowing causes the Softening Point to increase and the Penetration to decrease. However, the increase in softening Point is usually more than the decrease in Penetration. This means that blowing reduces the temperature susceptibility of bitumen. The effectiveness of blowing depends largely on the original bitumen (i.e. the original mixture of molecular structures).

 

With respect to the composition, generally the amount of saturates do not change, the amount of aromates decreases because some oxidized aromates behave like resins, the amount of asphaltenes increases due to trans-formation of some resins and the total amount of resins stays the same. This can also be observed in figure 1, which gives the composition of a Pen 200 bitumen after different blowing times in a laboratory oxidation column.

 

When bitumen is strongly blown it becomes so cohesive that the adhesive properties become so poor that it is not suited for asphalt applications anymore. Therefore, only semi-blown bitumen is suited for asphalt applications. Semi-blown bitumen can have both improved cohesion and improved adhesion.

 

Visbreaking

 

Light products have a higher selling value than heavy products like bitumen. Visbreaking is a way to break heavy products (e.g. the residue from crude oil distillation or even very heavy crude oils) into lighter products. Hereto, the crude oil or residue is heated up to 450 °C and kept at that temperature for 1 to 20 minutes. During this period a large amount of molecular structures are broken into smaller structures. The product from the visbreaking process (VB product) is further normally distilled.

 

Bitumen produced from VB products age very fast. This is because these products contain very reactive constituents (oleofins). Even blends of straight run bitumen with bitumen from VB products have the same ageing problems. This makes them unsuitable for most asphalt applications. The properties may be somewhat improved by blowing. A comparison between two straight run bitumen and blends of straight run bitumen with bitumen from VB products is given in table 2.

 

Property

Straight run bitumen

Bitumen from VB residue

(*1)

 

 

Penetration @ 25°C [dmm]

119

139

122

169

181

Softening Point R&B [°C]

42.7

42.0

43.2

41.5

42.0

Penetration Index

-1.0

-0.7

-0.8

0.4

0.5

After laboratory ageing

Retained Penetration @ 25°C [%]

78

73

56

53

41

Increase in Softening Point R&B [°C]

3.6

1.8

5.0

7.8

7.9

Penetration Index

-0.6

-1.2

-0.9

-0.2

-0.3

*1: semi-blown

Table 2 Properties of straight run bitumen and bitumen from VB residue

 

PHYSICAL CHARACTERISATION OF BITUMEN

 

The response of bitumen to stress depends on temperature and loading time. At low temperatures and/or short loading times bitumen behaves predominantly elastic. At high temperature and/or long loading times bitumen behaves like a liquid (viscous behaviour). For typical pavement temperatures and load conditions bitumen generally exhibits both viscous and elastic behaviour.  Measurements of the physical properties of bitumen are usually associated with the characterization of the rheological (flow) behaviour of bitumen.

 

A large number of test methods have been developed to characterize bitumen. Most of these tests are empirical, i.e. the determined properties are not directly related to the performance of the bitumen. To discuss the different test methods, the bitumen properties are divided into four groups:

 

·        Performance properties;

·        Index properties;

·        Properties related to mixing and construction and 

·        Control properties.

 

Performance properties

 

Performance properties are real material properties and as such directly related to the performance of the material. Bitumen stiffness and strength are two examples of performance properties.

 

The viscoelastic behaviour of bitumen can be measured with a Dynamic Shear Rheometer (DSR). During the test, a small sample of bitumen that is placed between to parallel plates is subjected to oscillatory shear stresses or strains (figure 2). From the response stresses or strains the complex shear modulus (G*) and phase angle * are calculated. The complex shear modulus is the ratio of total shear stress to total shear strain. It consists of two components: the storage modulus G’ (elastic component) and the loss modulus G” (viscous component). The phase angle is an indicator of the relative amounts of elastic and viscous behaviour. For example, for purely elastic materials, the pase angle is 0°, while for purely viscous materials (for example water), the phase angle is 90°. By performing these tests for a wide range of temperatures and loading times (frequencies) a complete picture (fingerprint) of the rheological behaviour of the bitumen can be obtained.

The results can be presented in several forms. The most common forms are: isochronal plots (viscoelastic data versus temperature at constant frequency), isothermal plots (viscoelastic data versus frequency at constant temperature), mastercurves (several isothermal plots shifted along the frequency axis to produce a smooth curve) and black diagrams (complex shear modulus against phase angle). To produce mastercurves use is made of the time-temperature superposition principle. This principle implies that there is an equivalency between time and temperature, which is true for most straight run bitumen.

 

The Superpave Asphalt Binder Specification is the first and only bitumen specification in which performance properties are incorperated. To address resistance to permanent deformation minimum values are given for G*/sin* (rutting factor). These requirements apply to fresh and short-term aged bitumen. To address resistance to fatigue cracking maximum values are given for G*·sin* (fatigue cracking factor). These requirements apply to long-term aged bitumen. 

Index properties

 

Index properties are related to performance properties but are not real material properties. Examples are elastic recovery (only relevant for Polymer Modifidied Bitumens) and kinematic viscosity at 60°C. Both are related to the resistance to permanent deformation. A high viscosity at 60°C may entail a high resistance to permanent deformation. Some bitumen specifications are viscosity-graded specifications.

 

Mixing properties

 

The most important property related to mixing, transport and construction is the shear viscosity at high temperature. To allow selection of optimum mixing temperature and the temperature interval for compaction, the temperature-viscosity relation of the bitumen should be known. Ideally, the mixing temperature is the minimum temperature at which the viscosity allows quick and good coating of the aggregate. Higher temperatures only cause additional ageing. 

 

Control properties

 

Control properties include Penetration, Softening Point, Fraaß Breaking Point, Ductility, etc. The test conditions under which these properties are determined differ significantly from the load/temperature conditions in the pavement. Consequently, they are all empirical properties and thus not (directly) related to the performance of the bitumen. These properties are used for quality control and to grade bitumen.

 

Many bitumen specifications are Penetration-graded specifications. In some of these specifications additional properties are included (for example Softening Point, Fraaß Breaking Point, changes in Penetration and Softening Point due to ageing, etc.). However, all these bitumen specifications are only grading systems and not related to pavement performance.

 

BITUMEN AGEING

 

Ageing mechanisms

 

The rheological properties of bitumen change with time (i.e. bitumen becomes harder and more elastic). This phenomen is called ageing. The amount and rate of ageing depend on many factors like for example temperature, exposure to oxygen, chemical composition and structure of the bitumen, etc.  Basically, there are four mechanisms of bitumen hardening: oxidation, loss of volatiles, physical hardening and exudative hardening.

Oxidation

 

Oxidation is considered to be the main cause of bitumen ageing. Like many organic substances, bitumen slowly oxidises when in contact with air. Polar groups are formed which tend to associate into micelles of higher molecular weight. The increased and stronger interactions make the bitumen more viscous. However, results from studies show that not all bitumens harden (age) to the same extend. This may be explained by differences in bitumen structure. For ‘SOL’ type bitumen the polar groups are well peptized, which makes them almost inaccessible for oxygen. Therefore, oxidation of the highly reactive asphaltenes and resins is difficult. For ‘GEL’ type bitumen this is not the case. The polar groups of these bitumens have rather formed a continuous network with a large surface area, which make them easy accessible for oxygen. Besides, newly formed polar groups are probably quickly dispersed in ‘SOL’ type bitumen, while in ‘GEL’ type bitumen these groups can further react.

 

Some aggregates act as catalyst for the oxidation reactions, while others have inhibitive effects. Ultaviolet rays from the sun act also as catalyst. This is especially relevant for areas high above sea level, for areas with a lot of hot sunshine (like the Middle East) and for asphalt wearing courses with high void contents (like Drain Asphalt). Even elements present in the bitumen can act as catalyst. An example is Vanadylporphyrin. Probably the most used inhibitor is Calcium Hydroxide (Ca(OH)2). It was found that the ageing resistance of an asphalt mixture is sometimes improved when Calcium Hydroxide is used. The reason for this is not known. Besides, Calcium Hydroxide is often used to improve the adhesion properties of bitumen. Sodium Hydroxide (NaOH) can have the same positive effect on the ageing resistance but often has a negative effect on the adhesion properties.

 

Oxidation causes the fractional chemical composition of bitumen to change. The asphaltene content increases continuously due to oxidation of polar resins.  Part of the aromatics changes in such a way that in the composition analysis it is included with the resins. Since these ‘new resins’ do not have the natural properties of resins, an evaluation of the properties of aged bitumen on basis of the SARA fractions can be misleading.

 

Irrespective of the ageing resistance of the bitumen, the degree and rate of oxidation depend on temperature, time, exposure to oxygen and bitumen film thickness.

 

With respect to temperature the most severe conditions are found during bitumen storage, mixing and transport. When bitumen is stored at high temperature normally very little oxidation occurs. This is because the surface of the bitumen exposed to oxygen is very small in relation to the volume. However, care should be taken during heating up. When the temperature difference between the bitumen and the heating oil is large (more than 30°C), reactive constituents (oleofins) are formed, which have a detrimental influence on bitumen. During mixing at high temperature the molecular mixture of the bitumen and the viscosity change significantly. Apart from temperature, oxidation during mixing depends on mixing time, bitumen content, temperature difference between aggregate and bitumen and type of mixing plant. During storage and transport oxidation continuous, but at a slower rate. Important are duration of storage and transportation, initial temperature and the exposure to air (oxygen). Special care should be taken with transport and storage of pre-coated chippings. Because of their loose packing air has easy access to the coated surfaces, which involves a real danger for severe oxidation of the bitumen.

 

During service life oxidation depends, apart from climatic conditions and the ageing resistance of the bitumen, mainly on the amount of airvoids in the asphalt (determines the exposure to oxygen and UV radiation) and the bitumen film thickness. To minimize oxidative hardening low void contents and thick bitumen films are required. 

 

Loss of volatiles

 

Evaporation of volatile components depends mainly on temperature and the conditions of exposure. Penetration grade bitumens are relatively involatile and therefore the amount of hardening resulting from loss of volatiles is usually fairly small.

 

Physical hardening

 

Physical hardening occurs during cooling and continues at service temperature. It is attributed to reorientation of bitumen molecules and crystallization of waxes. Slow cooling speeds up the process, while instant cooling to low temperature slows the process down (especially relevant for laboratory testing of bitumen). Physical hardening is strongly influenced by aggregate-bitumen interactions. Directly after cooling asphalt sometimes appears to be soft as if it was still warm, while a few days later the asphalt seems to have matured. This phenomen is called setting and is caused by slow physical hardening. Reheating can reverse physical hardening.

 

Exudative hardening

 

If the constitution of a bitumen is unbalanced it may, when in contact with a porous aggregate, exude an oily component into the surface pores of the aggregate, resulting in a hardening of the bitumen film remaining on the aggregate surface. Exudation is primarily a function of the ratio between the amount of low molecular weight paraffinic components and the amount and type of asphaltenes. Hardening as a result of exudation can be substantional when both the exudation tendency of the bitumen and the porosity of the aggregate are high. Otherwise, exudative hardening will be negligible.

 

Determination of ageing resistance

 

Several methods are developed to simulate short-term and long-term oxidative ageing of bitumen.

 

The two most used methods to simulate ageing during mixing, transport and construction (short-term ageing) are the Thin Film Oven Test (TFOT) and Rotating Thin Film Oven Test (RTFOT). In the TFOT a certain amount of bitumen is placed on a steel sample pan with certain dimensions and stored in an oven at 163°C for 5 hours. In the RTFOT the bitumen is put into a glass cilinder of certain dimensions. The glass cilinder is fixed in a rotating shelf. During the test the bitumen flows around the inner surface of the container and is exposed to heat and air for 85 minutes. The test temperature is also 163°C. This ageing procedure is included in the Superpave Asphalt Binder specification.

 

Under SHRP a new procedure was developed to simulate in-service ageing (long-term ageing). The procedue involves the use of a Pressure Ageing Vessel (PAV). In the PAV the bitumen is exposed to high pressure (2.1 MPa) and high temperature for 20 hours. The test temperature depends on the high-temperature Performance Grade of the bitumen and is either 90, 100 or 110°C. The PAV ageing procedure is included in the Superpave Asphalt Binder specification and uses bitumen aged in the RTFO. The test does not account for mixture variables.

 

The ageing resistance can be evaluated by means of the ageing index, which is defined by the ratio between the value of a certain property measured on aged bitumen and the value for the same property measured on fresh bitumen.

 

Changes in bitumen composition and properties

 

Generally, (oxidative) ageing makes straight run bitumen harder and more elastic. The asphaltene content increases. These changes are discussed in more detail on the basis of results from three studies.

 

In 1990 three test sections of Stone Mastic Asphalt (SMA) with different Polymer Modified Bitumens (PMBs) were constructed on a highway in The Netherlands. Also a reference section with 80/100 bitumen was constructed. In 1990, 1992, 1993 and 1999 cores were taken from these sections and tested for some functional properties. The bitumen is recovered and tested for Penetration and Softening Point. For the 80/100 bitumen the changes in Penetration and Softening Point during mixing, transport, construction and nine years service life are shown in figure 3. In the first year the Penetration has dropped significantly (24%). This illustrates the significance of the oxidative ageing that takes place during mixing, transport and construction. During the nine years of service the Penetration continuously decreases, however at a very slow rate (approximately 2 dmm per year). The Softening Point does not change at all during these years. The bitumen ages slowly because the exposure to oxygen is limited, i.e. the void content of the asphalt is low (average 5.5%) and the bitumen films are thick, i.e. the


Above 5% the polymer usually forms the continuous phase. For these PMBs the polymer dominates the properties. Both PMBs with a continuous bitumen phase and PMBs with a continuous polymer phase are used (see figure 8).

Figure 8 Microscopic images of PMBs under fluorescent light

(left: continuous bitumen phase, right: continuous polymer phase)

 

Two recent examples of projects in the Middle East where PMBs are used, are the rehabilitation and upgrading of the runway and taxiways at Cairo International Airport in Egypt and the rehabilitation of the runway at Aden International Airport in Yemen.

 

Cairo International Airport is the busiest airport in the Middle East. The bitumen of the existing dense wearing course was severely aged (Penetration of 10 to 20 dmm and a Softening Point of 70 to 80°C). A combination of poor quality (too high wax content and low asphaltenes) bitumen, high pavement temperatures and a lot of hot sunshine (ultraviolet radiation) had caused this severe ageing. For the new wearing course jet fuel resistant PMB was required. This bitumen had to comply with the requirements for Superpave Performance Grade 76-10. The bitumen that was selected for modification was a local standard Pen 60/70 bitumen with Superpave Performance Grade 64-16. The Performance Grade of the modified bitumen (Sealoflex SFB5-JR) was 76-22. This means that the high temperature performance (i.e. resistance to permanent deformation) was improved by two grades and the low temperature performance (i.e. resistance to cracking) was improved by one grade. Construction work started at the end of 1997 and was finished eight months later. During this period approximately 260,000 tons of jet fuel resistant asphalt was applied. The production of the PMB took place in a mobile plant at the construction site.

 

The pictures of figure 9 show clearly the difference between asphalt that is resistant to jet fuel and asphalt that is not. Both specimens were immersed in jet fuel for 24 hours. The Marshall specimen with jet fuel resistant bitumen had a weight loss of less than 0.5%. The Marshall specimen with standard bitumen (Pen 45/60) had a weightloss of approximately 7%.


For Aden International Airport the PMB had to meet the requirements for the same Superpave Performance Grade (PG 76-10). It appeared that the local bitumens available for modification had a relatively high asphaltene content and low resins content (especially the Pen 60/70 bitumen). The chemical composition of the Pen 60/70 and Pen 80/100 bitumen are given in table 9. Generally, these bitumens are not very suitable for modification with polymers. For example, modification of the Pen 60/70 bitumen resulted in a PMB with a very high shear viscosity which increased during storage (up to 29 Pa·s at 135°C). Modification of the Pen 80/100 did not show this tendency (the shear viscosity at 135°C was only 2.0 Pa·s). The Performance Grade of the modified bitumen (Sealoflex SFB5-JR) was 82-16, which is three grades better than specified. Construction work was carried out in 1999/2000. During this period approximately 40,000 tons of modified asphalt was applied.

 

 

Pen 60/70 bitumen

Pen 80/100 bitumen

Saturates

2 %

10 %

Aromatics

73 %

64 %

Resins

7 %

10 %

Asphaltenes

18 %

16 %

Table 9 Fractional composition of local bitumens in Aden (Yemen)

 

Acknowledgements

 

Part of the data presented in this paper was obtained under the Brite-Euram project: “Quality Analysis of Polymer Modified Bitumens and Bitumen Products by Image Analysis with Fluorescent Light (MIAF)”. The project acknowledges the support of the European Communities, Brite-Euram II Programme, project no. P-7426/BRE2-0951 and the MIAF Consortium: Rambrll (Dansk Vejteknologi and G.M. Idorn Consult), CSTB, Jean Lefebvre, Ooms Avenhorn Holding bv, University of Nottingham and Danish Road Institute.

 

References

 

[1]    First full-scale applications of tarfree jet fuel resistant bitumen, R.C. van Rooijen and A.H. de Bondt, E&E congress, Barcelona, 2000;

[2]    Theoretical background of Sealoflex products and application in pavement design using energy dissipation concept, A. Srivastava, Sealoflex seminar, Atlanta, 2000;

[3]    Workshop briefing, Eurobitume Workshop on performance related properties for bituminous binders, Luxembourg, 1999;

[4]    Workshop proceedings, Eurobitume Workshop on performance related properties for bituminous binders, Luxembourg, 1999;

[5]    GWW Gebreken, A. Gastmans, 1998;

[6]    Gemodificeerd bitumen in asfalt, R.C. van Rooijen, 1998

[7]    Use of modified bituminous binders, special bitumens and bitumens with additives in pavement applications, International workshop modified bitumens, Rome, 1998;

[8]    Development of performance-based bitumen specifications for the Gulf countries, Hamad I. Al-Abdul Wahhab, Ibrahim M. Asi, Ibrahim A. Al-Dudabe and Mohammed Farhat Ali, Construction and Building Materials, Volume 11, 1997;

[9]    Rheological characteristics of polymer modified and aged bitumens, G.D. Airey, PhD thesis, 1997;

[10]  Performance evaluation of polymer modified asphalt at amsterdam airport Schiphol and two highways in The Netherlands, A. Rietdijk, R. van Rooijen, A. van de Streek, B. Lieshout and P. Kadar, E&E congress, Strasbourg, 1996;

[11]  Testing and appraisal of polymer modified road bitumens – state of the art, U. Isacsson and X. Lu, Materials and Structures, Volume 28, 1995;

[12]  The Shell bitumen handbook, 1990

[13]  Performance graded asphalt binder specification and testing, Asphalt Institute, Superpave series no. 1 (SP-1)

[14]  Internal reports Ooms Avenhorn Holding

 


[1] Ooms Avenhorn Holding bv, Director R&D, P.O. Box 1,1633 ZG Avenhorn, The Netherlands, Tel +31229547700, Fax +31229547701, Email asrivastava@ooms.nl

[2] Ooms Avenhorn Holding bv, Manager R&D Laboratory, P.O. Box 1, 1633 ZG Avenhorn, The Netherlands, Tel +31229547700, Fax +31229547701, Email rvrooijen@ooms.nl

 

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