Composition (a) Initial conditions

Bitumen is a mixture whose parts are composed mainly of hydrocarbons (Section 33.1) and their derivatives. By weight, it is about 85 percent carbon. The precise composition of bitumen depends on its source, and can vary from oil well to oil well and from time to time at a particular oil well.

The four main parts of bitumen are:

(1) Maltene (or malthene or petrolene). This part is the main component of the mix — at about 55 percent — and supplies the mix with its visco-elasticity (Section 33.3.3). Maltene is an aliphatic non-polar, hydrocarbon oil composed of products such as acidaffins and paraffins. In the laboratory, it can be subjected to further fractionation. It is a solvent and is soluble in petroleum ether. Maltene has an atomic carbon-to-hydrogen ratio of less than 0.8 and a relatively low molecular weight. The molecular structure tends to be chain-like.

(2) Asphaltenes. This part is probably a hydrocarbon oxidation product and represents about 20 percent of bitumen by weight. It supplies the hard ‘body’ of a bitumen. Asphaltenes are in colloidal suspension in maltene, usually as a sol (particles dispersed) rather than a gel (particles in contact). They can be separated by precipitation in low molecular weight hydrocarbon solvents. They are polar and this means that they control the adhesion properties of the bitumen (Section 12.3.2). Due to their polarity, they form into hydrocarbon micelles (elementary lamella a few molecules thick) held together loosely by hydrogen bonding, creating

a series of polynuclear sheets which are brown-black in colour, aromatic, amorphous, hard, and relatively inert. On their own, they are solid but powdery materials. Heating bitumen breaks down the asphaltene adhesion and therefore reduces the viscosity of the bitumen.

Asphaltenes are soluble in carbon disulfide but not in petroleum and are thus easily distinguished from maltene. The molecules are relatively large and have an atomic carbon-to-hydrogen ratio of more than 0.8, a high molecular weight (c. 1000-20 000), and are more aromatic than maltene. The molecular structure tends to be ring-like.

(3) In between the maltene and asphaltenes phases are resin-like intermediate molecular-weight hydrocarbons. These comprise about 20 percent of the mixture, and usually exist as a sheath of adsorbed material covering the asphaltene molecules. They assist in maintaining the colloidal stability of the sol suspension of the asphaltenes in maltene and provide ductility and adhesion.

(4) The remainder of the mixture — usually about 5 percent — consists of atoms of sulfur, nitrogen, and oxygen that are mainly attached to the various hydrocarbons and give them a polar character.

The resulting mixture of the four parts — bitumen — is consequently a black or brown viscous liquid, although at its lower operating temperatures it is almost a solid. It is soluble in carbon disulfide, benzene, and trichloroethylene. It is largely non-volatile and is resistant to most acids, alkalis, and salts. Its deformation response is dependent on both its temperature and the rate at which it is loaded. Its relevant roadmaking properties are its adhesion, cheapness, workability, strength, durability, and imperviousness. Indeed its main virtue is that it is currently the cheapest durable glue (or binder) available and in asphalt is primarily used as a glue to hold particles of aggregate together and thus form a stiff and impervious composite material (Section 12.2).

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A comprehensive review of bitumen properties and applications is given in Dickinson (1984). A formal description and definition of bitumen is given in WHO (2004).

(b) Changes with time

Over time, bitumens:

* harden, stiffen and become more viscous, * become brittle and lose their ductility, and * lose their adhesiveness.

The assessment and management of bitumen durability is discussed in Section 8.7.7. The ageing process that causes these effects results from a combination of:

# volatilisation, # oxidation,

# steric hardening (molecular restructuring over time and related thixotropic effects), and

# actinic light (ultraviolet light effects).

Although the maltene and other volatile, ‘lighter’ components of bitumen provide much of its plasticity and fluidity, hardening with age due to the direct loss of such components by volatilisation is not as common a problem as is oxidation.

Oxidation occurs when hydrogen in the maltene combines with oxygen and is removed via water molecules. It therefore requires the presence of oxygen, which must be able to diffuse into the bitumen for this effect to occur (Dickinson, 1982). The presence of light speeds up this oxidation reaction but, because bitumen is a good light-absorber, this is confined to the top 5 µm of the exposed bitumen. Oxidation in the absence of light is much slower and is temperature-dependent, with the rate doubling for every 10 C temperature rise. A similar oxidation process occurs with air blowing during the manufacture of bitumen (see Section 8.7.2b). The speed of reaction is thus partly controlled by temperature, partly by light, and partly by the rate at which the oxygen can diffuse into the bitumen. As oxidation progresses, the asphaltene proportion in the bitumen increases and it plays a more dominant role; and so the bitumen itself hardens and becomes less durable. A colour change from black to grey is also common.

Asphalt production is discussed in Sections 12.2.1 and 25.6. It will be evident from comparing those descriptions with the above causes of bitumen hardening, that a considerable amount of bitumen hardening occurs during asphalt production.

8.7.2 Manufacture

Bitumen exists in solution as a natural constituent of most crude petroleums, and may constitute from zero to 60 percent of the crude. The higher percentages are usually associated with crudes having a relatively high specific gravity. The bitumen is usually obtained by treating the heavy-end residue from the distillation process. This product is therefore known as residual bitumen. The other product obtained from the heavy-end residue is residual (or heavy) furnace oil. Section 8.7.5 shows how bitumen is sometimes recombined with the lighter distillation products to produce cutback bitumens. In all practical cases, bitumen is produced to meet a performance specification, rather than to achieve a specified chemical composition.

Bitumen manufacture is usually a multi-stage process (Figure 8.13). Three main production processes are used:

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(a) distillation. Distillation of crude oil at atmospheric pressure removes lighter fractionation products such as petrol and kerosene, leaving a ‘long’, higher boiling point, residue. Atmospheric distillation of a crude oil must occur at restricted temperatures to prevent cracking, which is a decomposition of the chemical structure of the bitumen during which the higher molecular-weight hydrocarbons are split into lighter oils and free carbon. Thus the long residue is usually treated by vacuum distillation, reduced pressure, or steam distillation, to produce a ‘short’ residue that provides both fuel oils and the various bitumen grades. These processes permit temperatures as high as 400 C to be attained during processing without causing cracking. However, the bitumen produced becomes harder as either the vacuum or the temperature is increased. The resulting products are called straight-run bitumens. As the above processes are physical rather than chemical, the products can be later recombined to produce a homogeneous material.

(b) distillation plus air blowing. Blowing air into the hot, liquid distillation residue causes some oxidation to occur and higher molecular-weight hydrocarbons to form. These effects raise the viscosity of the straight-run bitumen and reduce its temperature susceptibility (Sections 8.7.4 and 33.3.3). However, air blowing also increases hardness (Sections 8.7.1 & 33.3.4) and colloidal instability (the tendency to change from a sol to a gel, Section 8.7.1(a)), and lowers ductility. The amount of air blowing is therefore a compromise between these two sets of effects.

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(c) distillation followed by blending with solvent-precipitated residue. The solvent is usually propane and the residue is usually a pitch. The resulting harder bitumen is therefore called a propane precipitated (PPA) bitumen.

Historically, a significant quantity of bitumen was also obtained from the naturally- occurring native asphalts (see Sections 3.6.2 and 12.2.1), particularly Trinidad Lake asphalt. The Lake asphalt contains about 55 percent soluble bitumen that was often fluxed with oil to raise the bitumen percentage to about 65 percent. More recently, the natural product was mixed with residual bitumen to raise the percentage to 75 percent. Some native asphalt is still used in the U.K. for surface courses, due to its good weathering properties (Section 12.5.2).

Very hard bitumens cannot be obtained from the oil-refining process, but may be found in naturally-occurring deposits. Some, like Gilsabind/Gilsonite, are useful in treating existing bitumen surfaces to increase their viscosity.

8.7.3 Performance

Temperature plays a major role in determining bitumen performance. The highest handling temperature for bitumen is about 150 C and the highest pavement temperature in service is almost 70 C. At temperatures between 70 and 150 C bitumen is predominantly viscous and can be modelled as a viscous fluid (Section 33.3.3). Bitumen is therefore made workable by heating. This also means that the transport and storage of bitumen requires sophisticated equipment and a great deal of energy.

When bitumen is heated, some fumes are produced. Their organic component comprises benzene-soluble matter that in turn contains polycyclic aromatic hydrocarbons. These are potential carcinogens (WHO, 2004) and so breathing the fumes from heated bitumen should be avoided. At higher temperatures, the fumes increase and may also contain semi-volatiles that may irritate the skin and respiratory system. Thus temperatures should be kept as low as possible, in the context of proper handling and application procedures. It has been suggested that an acceptable level of total particulate matter might be 5 mg/m3.

At pavement temperatures below 70 C, the temperature and loading rate effects become important and behaviour can be described as linearly visco-elastic or pseudo- plastic. Thus the bitumen behaves in a predominantly elastic manner when cold or subjected to rapid loading, and tends towards viscous flow when hot or subjected to long- term loading. True viscosity measurements are no longer possible. At temperatures of around 0 C or less, most bitumens begin to fracture without any prior plastic deformation. At the bottom of the range of service temperatures, bitumen reaches the glass transition temperature and becomes a weak and brittle elastic solid. This is thus an important temperature for roads in cold climates and its value for many bitumens is between 0 and −40 C (Dickinson, 1984). Generally, the viscous properties of a bitumen should be such that it is:

(a) sufficiently fluid at high temperatures to permit it to be handled during construction and to coat the entire surface of pieces of aggregate;

(b) sufficiently viscous at high pavement temperatures to ensure that it will not permanently deform under traffic; and

(c) sufficiently plastic at low temperatures to avoid fracture and cracking. Clearly, there is conflict between these requirements and some compromise must be reached. A performance specification for bitumen would control its:

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(1) deformation response over the whole service range of loading rates and temperatures, and

(2) tendency to harden during handling and service (Section 8.7.1).

The popular spray and chip seal process (Section 12.1) relies on thin bituminous films. Two types of loading are of particular importance in relation to their performance (see Section 12.1.4). The first is the result of vehicles passing over the surfacing and is periodic and approximately sinusoidal with a duration of about 40 ms. This traffic loading can be critical at both ends of the temperature range. The second is due to thermal expansion and contraction and has a duration of about 10 ks. Thermal contraction will be critical at low temperatures when ductility is low. For a fresh bitumen, the deformation response to traffic loading at 60 C is rheologically equivalent to thermal contraction at 5 C.

8.7.4 Properties

As with most materials, the static resistance of a bitumen increases as its loading rate increases. At the other extreme, viscosity (Section 33.3.3) measures the ability of a material to resist flow at low loading rates. The key issue in determining fundamental bitumen properties is to establish its viscous properties as a function of loading rate and temperature. The sensitivity of a bitumen to loading rate is known as its shear susceptibility.

Figure 8.14 shows the deformation response of a bitumen loaded sinusoidally in shear (Dickinson, 1981 and Section 33.3.3). The absolute value of the complex shear modulus (G*, a stiffness measure) increases, at an increasing rate, with loading frequency (i.e. with strain rate) and decreases with temperature. The dynamic shear rheometer developed as part of the U.S. SHRP program measures this modulus by applying shear stress at different frequencies to a bitumen sample held between two parallel plates (as in the sliding plate viscometer described below).

Curves of the complex shear modulus against loading frequency for various temperatures can be superimposed by a loading-rate shift to give a master curve (Section 33.3.3) at a reference temperature usually taken as 25 C (Figure 8.15). The temperature susceptibility can be estimated from the shift needed. A high value means that the effect of temperature on shear modulus is high. That is, the bitumen can be readily deformed when hot and fractured when cold. Temperature susceptibility is relatively independent of bitumen composition.

The slope of the master shear-modulus vs loading-frequency curve in Figure 8.15 is used to define another shear-susceptibility parameter, ß. A high ß value means that, with increasing loading frequency, there is a slow transition from viscous to elastic behaviour. The value of ß after a Rolling Thin Film Oven (RTFO) treatment should be as high as possible, up to a maximum of 2.25 to prevent the acceptance of colloidally unstable material (Sections 8.7.2&7). The limiting (or maximum) viscosity is obtained from the reference master curve by extrapolation to the estimate of the viscous constant at zero loading rate.

The sliding plate viscometer measures viscous response by shearing a thin film of bitumen between two plates at a defined strain rate. Very large strains can be applied, but the method lacks the precision of the rotational and capillary viscometers. A typical device is the (Shell sliding plate) microviscometer (AS 2341.5). It gives an apparent (steady-state) viscosity that is calculated as the ratio of shear stress to shear strain rate at a

Pavement Materials 157 load or displace- ment time 90° out-of-phase

key: deformation response due to:

purely elastic behaviour visco-elastic behaviour purely viscous behaviour

this is also the load–time curve

Figure 8.14 Deformation response of bitumen.

defined strain rate and temperature. This quantity is sometimes termed the consistency of the bitumen. For much of its range of application, it is an ‘invention’ (hence, the use of ‘apparent’) as true bitumen viscosity is not measurable at temperatures below about 60 C. Apparent viscosity drops as the strain rate increases. The temperature susceptibility parameter increases as the ratio of the 25 C apparent viscosity to the 70 C viscosity

to elastic v alue o f ab o ut 3 0 0 M P a

to 0 º p h ase an gle

to 9 0 º

to visco u s co n stan t

lo g (lo ad in g freq u en cy) lo g (ab so lu te

v alue o f co m p lex sh ear

m o d u lus) o r p hase an g le

lo g (|co m p lex shear m o d u lu s|)

Figure 8.15 Master curve showing the influence of strain rate on the load response of bitumen (from Dickinson).

increases. The fracture stress of unconfined bitumen is about 5 MPa at a strain of between 0.01and 0.10. The value of G* also drops once strains exceed 0.01.

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The softening point of a bitumen is a simple measure of its transition from a viscous solid to a liquid. Its measurement is specified in ASTM D36 & AS 2341.9. It is conventionally set at least 10 C above the maximum anticipated air temperature. Even inherently soft bitumens would usually have a softening point of at least 30 C. The flash point of a bitumen is the temperature at which its vapour can momentarily take fire. It is therefore the maximum temperature to which bitumen can be heated safely without the danger of an instantaneous flash in the presence of an open flame. A typical value might be 250 C measured by the Pensky Martens Open Cup test. The fire point, at which the bitumen will actually burn, is usually well above the flash point. A minimum flash point is used also to safeguard against contamination by cutting and fluxing materials.