Ageing of bitumen is strongly related to proliferation of oxidative species in the chemistry of bitumen which mostly are irreversible reactions which alter engineering properties of bitumen permanently. Certainly, the previous statements seem to be far ahead of the pertinent questions which are:
- What is bitumen?
- Where does it come from? and
- Why is ageing an important aspect to its functional properties and performance as a road binder?
Let
us first answer
the pertinent
questions
before we
engage in the
ageing
chemistry of
bitumen and the
role
antioxidants
play in
reduction of
ageing. In this
article, once
we have
answered these
basic points,
we seek to
highlight
bitumen ageing
and the role of
antioxidants on
the performance
of bitumen as a
binder in
asphalt
mixtures.
The
basics
Bitumen
is a byproduct
of fractional
distillation in
the crude oil
refinery.
Normally, it is
the distillate
component at
the bottom of
the barrel and
hence it also
consists of
other
constituents
from the
refining
process. The
source of the
crude oil
determines its
chemistry in
addition to
refining
technology
which also
plays a key
role in the
final product
and the age
state of
bitumen.
Resultant
changes in
bitumen
colloidal
chemistry
affect its
performance as
a binder.
Bitumen
consists of
saturates,
aromatics,
resins and
asphaltene
fractions and
their chemical
changes as a
result of aging
are time and
temperature
dependent.
Saturates,
aromatics and
resins are
collectively
known as
maltenes.
Volatilization
of maltenes as
a result of
high
temperatures
disturbs the
colloidal
stability and
increases both
micellization
and
flocculation of
asphaltenes.
These
irreversible
and permanent
changes alter
the binder
engineering
properties.
The
ageing
process
Many
schools of
thought have
been suggested
with regards to
the mechanism
of bitumen
ageing and we
will discuss
some of them. A
dual oxidation
mechanism
involving
ketones and
sulfoxides from
hydroperoxide
precursor
leading to
viscosity
building
alcohols from
reactive
sulfides
scavenging
hydroperoxides
before
decomposition
to form
ketones.
Another school
of thought
suggests that
upon exposure
to air, the
molecular size
of bitumen
grows, polar
interactions
increase, and
aromaticity
propagates
pi-pi and
dispersive
interactions.
Some suggest
that contrary
to irreversible
changes are
thixotropic
effects, which
reflect
reversible
molecular
arrangement
over time based
of the free
volume theory.
Exposure to
temperature and
time leads to
different
mechanisms of
ageing which
occur as a
combination of
different
chemical
processes
taking place
concurrently.
Antioxidants
The
role played by
antioxidants is
critical in
slowing down
the generation
of oxidative
species in
bitumen.
Antioxidants
confer
stiffness to
the bitumen,
with marked
increase in
stiffness at
high
temperatures
and lower
stiffness at
lower
temperatures.
Chemically, a
decrease in
volatilization
and oxygen
species is
observed with
use of
antioxidants.
Examples of
antioxidants
are vast and
hence for the
purposes of
this discussion
we will focus
on hydrated
lime and carbon
black as
conventional
antioxidants
and
commercially
available
primary
antioxidant
Irganox 1010
and secondary
antioxidant
Irgafos 168.
Hydrated lime
is an active
filler that
reduces
chemical ageing
through
interaction
with acids in
the bitumen.
Its main
constituent is
calcium
hydroxide but
with a mass
component of
magnesium
hydroxide which
together they
are referred to
hydrated
dolomitic lime.
The two
mechanisms of
use of hydrated
lime in
bitumen,
consists of
dual
functionalities
as a chemical
ageing
deterrent and
stiffening
above room
temperature.
Carbon
black shows
effective
alleviation in
ageing although
it contributes
to
deterioration
of low
temperature
bitumen
performance.
Carbon black
exists as a
solid pearlized
powder known to
be heat and UV
light stable.
Irganox 1010 is
a primary
antioxidant
known to
peptize
asphaltenes,
leading to
improvement in
the colloidal
index with
improved
resistance to
thermal and UV
ageing.
Chemically,
Irganox 1010 is
a sterically
hindered
phenolic
antioxidant
normally used
in processing
and in-service
thermostability.
Secondary
antioxidant,
Irgafos 168
participates in
reactions with
hydroperoxides
during
processing,
hence
preventing
process-induced
degradation and
synergistically
extending
performance of
primary
antioxidants
such as Irganox
1010.
Examining
the different
interactions
The
relationship
between
saturates,
aromatics,
resins and
asphaltenes
changes as a
result of
ageing and to
counteract that
antioxidants
reduce the
extent to which
chemical
changes affect
the performance
of bitumen.
Antioxidants
reduce the
susceptibility
of bitumen to
oxidative
ageing and
hence improve
fracture
behaviour and
increased
resistance to
fatigue
cracking. The
fatigue
resistance of
bitumen has
been found to
fundamentally
determine and
directly
correlate to
the fatigue
resistance of
asphalt
mixtures.
Service life of
asphalt is
extended
through
modification of
bitumen with
antioxidants.
Antioxidants
reduce bitumen
hardening as
result of
decrease in
molecular size
of asphaltenes,
resins and
increase in
free volume
space for
saturates and
in return this
enhances the
fatigue
performance of
asphalt. The
correlation
between
laboratory and
in-service
ageing
mechanisms can
often be
misleading with
the laboratory
conditioning
limited in the
exposure of all
the possible
distresses
possible to the
bitumen.
However,
oxidation
remains the
major form of
chemical change
key to bitumen
aging although
more recently
studies have
started
including more
spectroscopic
analysis in
understanding
other
functional
groups that
cause changes
in saturates,
aromatics,
resins and
asphaltenes.
Importance
of
understanding
ageing
Bitumen
ageing is a
multi-step and
time dependent
phenomenon
consisting of
different
chemical
processes and
mechanisms that
are governed by
the nature of
exposure to
temperature and
environmental
conditions.
Each bitumen is
unique based on
its parent
crude source
and furthermore
on the refining
technology at
the different
refineries that
use crude oil
as a feedstock.
Ageing highly
depends on the
chemical
composition of
bitumen which
varies from
binder to
binder.
Antioxidants
reduce the
susceptibility
of bitumen to
ageing and this
depends on the
saturates,
aromatics,
resins and
asphaltene
fractions in
the bitumen
which determine
the chemistry
of the
respective
anti-aging
mechanisms
conferred by
the different
antioxidants.
The performance
of bitumen in
asphalt depends
on its ability
to render its
engineering
properties to
the ultimate
mix which
consists of all
the other mix
design
components such
as void
content, and
type of
aggregate.
Therefore,
bitumen with
good resistance
to ageing can
perform better
and increase
the longevity
of the road in
addition to
resistance to
other
distresses that
are exposed to
the road
surface. In
essence, ageing
is one of the
governing
factors that
influences the
bitumen and
asphalt mixture
properties
which is
directly
related to
pavement
performance.
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