Ageing part 2

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.