Two ageing conditions can be
distinguished: short-term ageing and long-term ageing.
The largest contribution to short-term
ageing is in the production of asphalt. During the intermixing of bitumen and
mineral compounds two loss flows can be identified. One flow is a material loss
by particle emission as aerosols. These particles consist nearly to 100% of
organic hydrocarbons. The other flow is the evaporation of oxidised and
decomposed compounds. Saturates and aromatics evaporate at a temperature of 100
to 350 °C under nitrogen atmosphere. Resins have an initial weight loss
from 50 to 150 °C and continue till 350 °C. Mass loss of bitumen occurs
in the temperature range from 100 to 350 °C under nitrogen atmosphere and
above 350 °C cracking of heavy fractions of bitumen or bitumen fractions begins. Temperatures above 100 °C are typical for hot and partially
for warm mix asphalt concrete. Additionally the higher temperature until placement is concluded
might be responsible for evaporation. Particles (PM10 – particulate matter with
diameter of 10 µm or less) measured during a laboratory mixing experiment
revealed that the emitted particles consisted to a high part of organic carbon,
which decreases with increasing distance to the blender. Analysed volatile
organic compounds from hot bitumen identified ketones, alkenes, aldehydes and
aromatics to be the main functional groups in emitted volatile compounds.
Under service the surface of asphalt
roads rarely exceeds 75 °C (Austria), therefore evaporation of compounds is less favoured until they are
fragmented by chemical reactions or radiation.
The sensitivity to chemical ageing varies
widely with the bitumen origin (chemical composition). Oxidation introduces
polar, oxygen-containing functionalities and may cause aromatization. Often the
assessment of chemical ageing is done on fraction level or by studying the
increase of functionalities. So it is generally accepted that the aromatic content
decreases and the aphaltene content increases, thereby the asphaltene content
can give hints of the state of ageing. The resin content varies, depending on
formation of asphaltenes out of resins or aromatics and formation of resins out
of aromatics. The saturate content remains essentially constant, due to low
chemical reactivity. However, this is an interpretation in the case of a polarity and
solubility concept and may not be understood from a chemical and structural
point of view. The “new” formed asphaltenes during ageing may differ from the
“original” asphaltenes in their chemical structure. Isolated asphaltenes are
fairly unreactive to atmospheric oxygen at ambient temperature, but when melted
or in solution they are highly reactive. The state in which asphaltenes occur in bitumen can be notable.
The same applies for structural features, which change during ageing.
The major chemical functional groups containing
oxygen, which increase and form through ageing, are ketones, anhydrides,
carboxylic acids and sulphur oxides. Ketones and sulphur oxides are rapidly
formed during the shot-term ageing with sulphur oxides as the easiest formed
oxidation products. During long-term ageing their formation proceeds at a much
slower rate (oxidation of bitumen films showed that the oxygen uptake rate
decreases until the rate becomes linear) and the oxidation reaction usually
stops with ketone formation. The good correlation between ketone formation and
viscosity implies that ketones are responsible for the formation of additional
asphaltenes. Sulphur oxides generally reach a steady state, depending on
sulphur content and oxygen diffusion.
The mineral aggregates in asphalt can act
as a catalyst, especially to oxidise the more non-polar fractions, but it was
proposed that polar functional groups of the more polar fractions can be
adsorbed onto the catalytically active sites and so prevent further activity.
Thus the catalytic effect of mineral aggregates on bitumen oxidation is
negligible, relative to the amount of oxidation that occurs in the bitumen
bulk. The same applies to metals, metal salts and some metal-organic complexes,
which can have catalytic effects.
Core samples from road constructions
showed that general bitumen from the wearing course (upper part) is oxidised stronger
than bitumen from the base course (lower part). The development of the
oxidation gradient over the vertical cross section depends on the variability
in permeability, which itself depends on void content, peroxidation of bitumen
until paved and environmental conditions. The gradient within bitumen is limited
to a significant degree by the diffusion rate of oxygen from the surface into
the bulk, where in asphalt the mineral aggregation matrix presents a hindrance.
The effect of radiation on bitumen
becomes important during service life, dropping the life span of road
construction. High energy radiation penetrates through bitumen and introduces bond breaking in the main chain or in
side chains (scission; even for polymeric modifier) and crosslinking through
polymerization (enhancing elasticity). Ultra violet radiation (UV radiation)
affects mainly the surface, causing lesser scission and polymerisation
reaction. Radiation increases the concentration of carbonyl compounds (like
ketones or carboxylic acids) and sulphur oxides. Scission produces small
molecular fragments, which cause a decrease in creep compliance and increased
compliance.
The majority of publications worked with
artificially aged bitumen, aged at higher temperature (like RTFOT and PAV), to
reduce time effort. This is not a problem for the investigation of short-term
aged bitumen, but for long-term aged bitumen under the exposure of
environmental condition it can lead to interpretations, which are only correct
for the investigated method. As an example: The rolling thin film oven method (RTFOT) reproduces the rheological values of short-termed aged bitumen well, the same
values can be produced with high energy radiation,
but both artificially aged bitumen differ in their chemical composition.
It is necessary to understand the chemical reaction, which take place inside the construction. So environmental processes should be includes in ageing investigations. Many atmospheric radicals are produced by the traffic and affect the road construction.
It is necessary to understand the chemical reaction, which take place inside the construction. So environmental processes should be includes in ageing investigations. Many atmospheric radicals are produced by the traffic and affect the road construction.
