Asphalt Composition

Conventional chemical analysis shows that bitumens contain mainly carbon and hydrogen with small amounts of oxygen, nitrogen and sulphur and trace amounts of metals. A typical analysis is 83% carbon, 10% hydrogen, 7% oxygen, nitrogen and sulphur and trace amounts of vanadium, nickel, aluminium and silicon.

More complex methods of analysis, for example, infrared, ultraviolet and nuclear magnetic resonance, identify classical chemical groupings and confirm that bitumens are complex mixtures, mainly of high molecular weight hydrocarbons.
Using a selective solvent such as normal heptane, bitumen may be separated into asphaltenes (which are precipitated) and an oily fraction (maltenes). With adsorption chromatography the maltene fraction may be separated further into resins, aromatic oils and saturated oils. These four groups of constituents differ in nature:

- Asphaltenes
Are brittle brown to black amorphous solids. They contain mainly carbon and hydrogen but also oxygen, nitrogen and sulphur. Chemically, they consist of highly condensed aromatic compounds of high molecular weight. The concentration of asphaltenes varies with a higher proportion in the harder bitumens.

- Resins
Are brown to black, adhesive, shiny solids or semi-solids. They contain mainly carbon and hydrogen but also small amounts of oxygen, nitrogen and sulphur. Chemically they stand between the asphaltenes and the aromatics.

- Aromatic Oils
Are viscous dark brown liquids comprising mainly carbon, hydrogen and sulphur with minor amounts of oxygen and nitrogen. They contain numerous naphthenic-aromatic ring compounds.

- Saturated Oils
Are viscous liquids or solids which range from straw to white colour. They consist mainly of long chain saturated hydrocarbons with some branched chain compounds, alkyl aromatics with long side chains, and cyclic paraffins (naphthenes).

Average molecular weights cover a continuous range from saturated and aromatic oils (500 to 10001 through resins (1000 to 2000) to asphaltenes (greater than 2000).

Bitumens have a colloidal nature in which large structures (the asphaltenes) are dispersed in the form of micelles in an oily liquid phase (the maltenesl. Depending on the relative proportions of the four groups described above the structure will vary between "so/" in which the micelles are dispersed and a "gel" in which micelles are organized to more network-type structures. Thus, saturated oils which have little solvency power for asphaltenes, promote a predominantly gel character; aromatic oils have greater solvency power and promote a predominantly sol structure.

Composition, structure and behaviour are related. For example, air blowing changes aromatic oils to resins and resins to asphaltenes. Heavily blown bitumens have a predominantly gel character which has reduced temperature susceptibility. Deeper distillation will preferentially reduce the saturated oil content and give a bitumen which is more sol in character and has greater temperature susceptibility. Thus, an understanding of composition and structure assists in interpreting the rheological behaviour of bitumen and the effects of changes in temperature.

Polycyclic Aromatic Hydrocarbon (PAH) Content of Bitumen and its Fumes

Although PAHs exist in crude oils, they are generally present in more limited amounts in bitumens. This is because the principal refinery processes used for the manufacture of bitumens, contain a vacuum distillation step which removes materials with low to moderately high molecular weight, including most PAHs with 3-7 fused rings. The temperatures involved in the vacuum distillation process are not high enough to result in any substantial PAH generation.

The levels of PAH found in penetration and oxidized bitumens are shown in Table 1 together with the sources of the data. One of the penetration grades analysed by was excluded from the table as its PAH levels were in the order of 10x greater than the rest (interestingly this sample did not have clearly greater biological activity. The other bitumens of penetration or oxidized grades had only low levels of PAHs. Compared with coal tar itch, PAH levels in bitumen ranged from 4-5 orders of magnitude less (10-4-10-5 ). 

When bitumen is heated to allow application, fumes are given off and these have been condensed and analysed. Invariably the PAH content of the condensed fume is greater than that of the parent bitumen (see Table 1). NO substantial differences exist between the PAH contents of condensed fumes generated from penetration or oxidized bitumens heated to similar temperatures. The temperature of fume generation affects both the relative proportions of individual PAHs in the fume and amounts of fume generation. Comparing the condensed fumes obtained from generation temperatures of 160C and 250C the lower temperature fume contained higher levels of 3-4 ring PAHs whereas the higher temperature fume contained slightly greater levels of 5 + ring PAHs.

At higher temperatures (316C) the situation becomes more confused probably because non-PAH components increased in amount. Hence no clear conclusions can be reached on the relative concentration of carcinogenic PAHs in the condensed fumes from different generation temperatures. However the amounts of fumes generated at different temperatures are much more relevant to human PAH exposure. It has been reported that eighty-fold more fume is given off at 250C than at 160C, hence temperature control will considerably reduce emissions of PAHs from bitumens. In comparison, the condensed fumes from heated coal-tar pitch contain approximately 500 times more PAHs than condensed bitumen fumes and emission levels of PAHs from heated coal-tar pitch are approximately three or four orders of magnitude greater.

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