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.
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 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 (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 (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 (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 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.
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.
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 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.
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.
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.
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 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 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.
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 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
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 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.
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 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.
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.
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.
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) |
||
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.
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[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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