Constituents of Bituminous Materials

Bituminous materials consist of a graded aggregate bound together with bitumen. In addition, the mixture contains a small proportion of air. Thus bituminous materials are three-phase materials and their properties depend upon the properties of the individual phases as well as the mixture proportions.

The two solid phases are quite different in nature. While the aggregate is stiff and hard, the bitumen is flexible and soft and is particularly susceptible to temperature change. Therefore the proportion of bitumen in the asphalt mixture has a great influence on the mixture’s properties and is crucial in determining the performance of the material.

Bitumen may be supplied in a number of forms either to facilitate the mixing and laying process or to provide a particular performance. Aggregates may come from a wide range of rock types or from artificial sources such as slag. The grading of the aggregate is important and ranges from continuous grading for mixture types known as asphalt concretes (previously known in the UK as ‘macadams’) through to gap grading for mixtures known as hot rolled asphalts or stone mastic asphalts (previously known in the UK as ‘asphalts’).

The very fine component of the aggregate (passing 63 microns) is called filler. Although the graded aggregate will normally contain some material of this size it is usually necessary to provide additional filler in the form of limestone dust, pulverised fuel ash, hydrated lime or ordinary Portland cement.

Sources of Bitumen

There are two sources of bitumen: natural deposits and refinery bitumen.

Natural asphalts: Bitumen occurs naturally, formed from petroleum by geological forces, and always in intimate association with mineral aggregate. Types of deposit range from almost pure bitumen to bitumen-impregnated rocks and bituminous sands with only a few per cent bitumen.

Rock asphalt consists of porous limestone or sandstone impregnated with bitumen with a typical bitumen content of 10%. Notable deposits are in the Val de Travers region of Switzerland and the ‘tar sands’ of North America. Lake asphalt consists of a bitumen ‘lake’ with finely divided mineral matter dispersed throughout the bitumen.

The most important deposit of this type, and the only one used as a source of road bitumen in the UK, is the Trinidad Lake. The lake consists of an area of some 35 ha and extends to a depth of 100 m. Asphalt is dug from the lake, partially refined by heating to 160°C in open stills to drive off water, then filtered, barrelled and shipped. The material consists of 55% bitumen, 35% mineral matter and 10% organic matter. It is too hard in this form to use directly on roads and is usually blended with refinery bitumen.

Refinery bitumen: This is the major source of bitumen in the UK. In essence, bitumen is the residual material left after the fractional distillation of crude oil. Crudes vary in their bitumen content. The lighter paraffinic crudes, such as those from the Middle East and North Sea, have a low bitumen content, which must be obtained by further processes after distillation.

Heavier crudes, known as asphaltic crudes, such as those from the countries around the Caribbean, contain more bitumen, which is more easily extracted.

Manufacture of Bitumen

The process of refining crude oil yields a range of products, as shown in Fig. 1. These products are released at different temperatures, with the volatility decreasing and viscosity increasing as the temperature rises.

Bitumen is the residual material but its nature will depend on the distillation process and, in particular, on the extent to which the heavier oils have been removed. If the residual material contains significant amounts of heavy oils, it will be softer than if the heavy oils had been more thoroughly extracted.

Modern refinery plant is capable of very precise control, which enables bitumen to be produced consistently at a required viscosity.

Chemistry and Molecular Structure

Bitumen is a complex colloidal system of hydrocarbons and their derivatives which is soluble in trichloroethylene. The usual approach to the determination of the constituents of a bitumen is through the use of solvents. It may be subdivided into the following main fractions:

• Asphaltenes – fraction insoluble in light aliphatic hydrocarbon solvent, e.g. n-heptane.
• Maltenes – fraction soluble in n-heptane.

The maltenes may be further subdivided into resins (highly polar hydrocarbons) and oils (subdivided into aromatics and saturates). The asphaltenes have the highest molecular weight but their exact nature is dependent on the type of solvent and the volume ratio of solvent to bitumen. If small amounts of solvent are used, resins, which form part of the maltenes fraction, may be adsorbed on to the asphaltene surfaces, yielding a higher percentage of asphaltenes.

Although they may vary according to the method of extraction, the appearance of asphaltenes is always of a dark brown to black solid that is brittle at room temperature. They have a complex chemical composition but consist chiefly of condensed aromatic hydrocarbons and include complexes with nitrogen, oxygen, sulphur and metals such as nickel and vanadium. The structure of asphaltenes is not known with certainty. One suggestion is of two-dimensional condensed aromatic rings, short aliphatic chains and naphthenic rings combined in a three-dimensional network (Dickie and Yen, 1967).

Another suggestion is that there are two different molecular types, one being a simple condensed aromatic unit and the other consisting of collections of these simple units (Speight and Moschopedis, 1979). It is likely that all of these may exist in bitumens from different sources since the nature of the molecules present in a crude oil will vary according to the organic material from which the crude was formed, and to the type of surrounding geology.

Maltenes contain lower molecular weight versions of asphaltenes, called resins, and a range of hydrocarbon compounds known as ‘oils’ including olefins, naphthenes and paraffins. The aromatic oils are oily and dark brown in appearance and include naphthenoaromatic type rings. The saturated oils are made up mainly of long straight saturated chains and appear as highly viscous whitish oil.

Bitumen is normally described as a colloidal system in which the asphaltenes are solid particles in the form of a cluster of molecules or micelles in a continuum of maltenes (Girdler, 1965). Depending on the degree of dispersion of the micelles in the continuous phase, the bitumen may be either a sol, where there is complete dispersal, or a gel, where the micelles are flocculated into flakes.

Bitumens with more saturated oils of low molecular weight have a predominantly gel character. Those with more aromatic oils, which are more like asphaltenes, have a predominantly sol character.

In terms of their influence on the properties of bitumen, asphaltenes constitute the body of the material, the resins provide the adhesive and ductile properties, and the oils determine the viscosity and rheology.

Although bitumens are largely complex mixtures of hydrocarbons, there are other elements present. The high-molecular-weight fraction contains significant amounts of sulphur, the amount of which influences the stiffness of the material. Oxygen is also present, and some complexes with oxygen determine the acidity of the bitumen. This is important in determining the ability of the bitumen to adhere to aggregate particles.

Physical and Rheological Properties

Bitumen is a thermoplastic, viscoelastic material and as such its physical and rheological (flow) properties are a function of temperature, load (stress) level and load duration (time of loading). Under extreme conditions, such as low temperatures (< 0°C) and short loading durations (< 0.1 seconds) or high temperatures (> 60°C) and long loading times (> 1 second) it shows elastic and brittle behaviour or viscous and fluid-like behaviour, respectively.

At intermediate temperatures and loading times, bitumen possesses both elastic and viscous properties (viscoelastic response), the relative proportions of these two responses depending on temperature, loading rate and applied stress level.

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