Bitumen types (a) Conventional bitumen

The refinery production of undiluted straight-run bitumen is discussed in Section 8.7.2. This material must be heated to become of low enough viscosity to be workable and to properly wet aggregate surfaces.

(b) Cutback bitumen

As an alternative to heating bitumen, its viscosity can be temporarily reduced by the addition of a suitable volatile diluent: the diluent is called a cutter and the new compound is referred to as a cutback bitumen. Typical cutters are:

* the relatively volatile petrol and naphtha, used to produce very short-term effects; * the lighter petroleum oils such as kerosene and turbine fuel which are the most

common materials used and provide short- to medium-term cutting. They are often called cutter oils.

* furnace and diesel (or automotive gas) oils which are used as long-term cutters and create a fluxed (i.e. ‘permanently’ softened) bitumen. They are sometimes called flux oil.

* most adhesion additives (Section 12.3.2) are also diluents.

These materials may cause skin and respiratory irritation (Section 8.7.3), pollution and fire-risk problems during preparation and placement and while the cutter is evaporating.

Cutback bitumens retain their low viscosity for a much longer period and at lower temperatures than is practical with a heated straight-run bitumen. The effects of the diluents diminish with time. Relatively rapid evaporation is usually required to permit the bitumen to become stiff enough to carry construction traffic. This stiffening process caused by cutter evaporation is a form of curing.

As with conventional bitumen, cutback bitumens can be classified according to their viscosity at 60 C — as shown in Table 8.8.

Table 8.8 Cutback bitumen types.

Surface treatment application (Section 12.1.4) 60ºC viscosity range, (Pa.s) Application temperature, (ºC) 0.008–0.016 10–30 Priming 0.025–0.050 35–55 0.060–0.120 60–80 Primer-seal and 0.22–0.44 75–100 premix 0.55–1.1 95–110 2.0–4.0 110–135 5.5–11 120–150 Sealing 3–26 135–160 43–86 150–175 (c) Emulsified bitumen

A second alternative to heating bitumen to reduce its viscosity is to emulsify it. An emulsion is the intimate dispersion of one immiscible fluid into another. A bitumen emulsion is a finely divided dispersion of minute (1–10 µm) droplets of bitumen in water. Preparation of the emulsion requires the use of a high-speed, high-shear mechanical device such as a colloid mill. The emulsified state is then preserved by the use of a water- soluble emulsifying agent — or emulsifier — which creates surface charges on the droplets and thus causes mutual repulsion between them.

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A range of emulsifiers is available; some can be used cold but others must be heated. They must be water-soluble and are commonly surfactant molecules (such as proteins and soaps) with a long hydrophobic hydrocarbon tail that seeks a bitumen droplet, and an ionised hydrophilic head that seeks water. They thus collect on droplet surfaces in an oriented fashion. Emulsifiers may be either anionic or cationic:

(i) An anionic emulsifier places negatively charged anions on the surfaces of the bitumen droplets. The ‘water’ is usually alkaline. The emulsion breaks as the water evaporates and the emulsion changes in colour from black to brown. This is usually a very slow process and anionic emulsions remain fluid for long periods. They are therefore liable to flow if placed on slopes or to be affected by traffic using the road within a few hours of application. They can assist with adhesion (Section 12.3.2) when aggregate surfaces are positively charged, as is commonly the case with basalt, dolomite, and limestone. Anionic emulsions were first developed in the 1920s.

(ii) A cationic emulsifier places positively charged cations on the surfaces of the bitumen droplets. The ‘water’ is usually acid. Cationic emulsions do not rely significantly on water evaporation in order to break and so are not as fluid as anionic ones. The break is irreversible and shorter than the break with anionic emulsions. Nevertheless, cationic emulsions can still take a significant time to break and become stiff enough to carry traffic. They can assist with adhesion (Section 12.3.2) when aggregate surfaces are negatively charged, as is commonly the case with quartz, quartzite, granite, and sandstone gravel. Cationic emulsions were first developed in the 1950s.

Typically, the emulsion will be 60 percent bitumen, 40 percent water, and a small amount of emulsifier. Emulsions therefore avoid the pollution and safety problems caused by the use of volatile and flammable hydrocarbons in cutback bitumen. They are also often cheaper than cutback bitumens. Care must be taken to ensure that the additives used do not become active carcinogens.

When the repulsion disappears the emulsion is said to break. The separation of the bitumen particles can then no longer be sustained and the bitumen droplets recombine or coalesce. This can occur when the water is evaporated or absorbed by another agent, when certain ions or salts are introduced into the water, or when agitation in the presence of rock particles occurs. The first stage of breaking is called the cheesy or curing state. Breaking is called instability when it occurs earlier than intended and setting when it occurs as intended. The control of breaking is critical to emulsion use and can be managed through either chemical or physical means. Common means are increasing the pH, increasing bitumen content, and water evaporation.

The common emulsion grades are:

(a) ARS — anionic rapid-setting. This emulsion breaks rapidly (in less than 3 minutes) and so in warm climates it cannot be mixed with aggregate. It is useful for tack and seal coats (Sections 12.1.3&4).

(b) ASS — anionic slow-setting. This emulsion has sufficient stability to last for at least 8 minutes and therefore can be used for all operations involving mixing. (c) HF — high float. This is a medium-setting anionic emulsion that is useful in low

temperature applications.

(d) CSS — cationic slow-setting. This is used for soil stabilisation (Section 10.4), surface enrichment (Section 12.1.5), and laying dust (Section 12.1.2).

(e) CAM — cationic special-grade. This is used for making asphalt mixes intended to be stockpiled at ambient temperatures (coldmixes, Section 12.2.7).

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(f) CRS — cationic rapid-setting. This is the cationic form of ARS.

They are designated by the relevant acronym, followed by the name of the bitumen grade used.

Cutters and emulsions can be more expensive than bitumen but their ability to be used when cold makes them particularly appropriate for maintenance operations. Emulsions are also used for filling cracks in pavements.

8.7.7 Durability

The durability of bitumen is mainly limited by oxidation in the presence of sunlight. The process is described in Section 8.7.1(b) where it is seen that bitumen is most susceptible to chemical decomposition in hot, exposed conditions, in porous mixes, and/or when used in thin films to coat aggregate. The thin film condition is also the most critical condition for the physical properties of bitumen. However, in all applications an ageing bitumen becomes more brittle and crack-susceptible.

To imitate the stone-coating situation and to simulate and accelerate the bitumen hardening to be expected during the manufacture and placement of asphalt (Section 8.7.1), the Californian State Highways Department in 1963 developed the Rolling Thin Film Oven (RTFO) test (ASTM D2872, AS 2341.10 and Dickinson, 1984). 35 g of bitumen is placed in a bottle that is then rotated in a convection oven. The sample viscosity is then measured. The RFTO test has been modified to also represent long-term hardening of asphalt in the field, as in the SHRP-PAV test.

To reproduce the demanding spray and chip seal conditions, the RFTO was modified to further test thin films of bitumen in order to assess their relative hardening rates under long-term exposure at the pavement surface and hence permit the selection of bitumens least likely to lose durability in service. This test is called the ARRB Durability test (Witt 1976, SAA 1980). Its object is to find how long a bitumen must be exposed to a temperature of 100 C in order to reach an apparent viscosity of 5.67 log Pa.s which is the viscosity that had been found to be associated with the onset of cracking in spray and chip seals in the winter in the southern portion of Australia. The data suggest that the hardening viscosity for distress is lower in cold conditions whereas the rate of hardening is higher in hot conditions. To find this exposure time, the test is done at a range of times and interpolation used to give the appropriate one. A test result of at least ten days is considered desirable (Dickinson, 1984). For service conditions, the following empirical relationship has been established:

log(viscosity, Pa.s) = 3.59 + 0.0476Y√t − 0.0227D√t

where Y is the yearly mean of the maximum daily air temperature (C) at the site, t is the seal service life in years, and D is the Durability test result in days. The equation indicates the improvements likely from selecting high-durability bitumens (Oliver, 1984). The use of the 100 C test temperature can be questioned as it represents only one set of reaction conditions and processes, and as it is well above most service temperatures.

There is disputed evidence that the addition of some hydrocarbons can reverse the oxidation of bitumen. Further, the addition of oils to oxidised bitumen may lower its viscosity and hence extend the life of a surfacing. However, rapid penetration of oil into non-porous surfacings is not easily achieved. Thus diffusion times are very long and the process may only be practical for porous surfacings.

Pavement Materials 163 8.7.8 Rubber and other additives

As seen in the preceding sections, bitumen has a number of mechanical disadvantages – such as poor creep resistance as temperatures increase and brittleness as temperatures decrease. A number of additives have been introduced to minimise these deficiencies. There are five key requirements for any bitumen additive. It must:

(a) significantly improve important properties, (b) maintain the properties in service,

(c) be able to be processed by the range of manufacturing and construction equipment, (d) not detrimentally affect basic bitumen properties, and

(e) be cost-effective. Additives are typically about ten times the cost of bitumen and so must lead to significant improvements.

Additives which have been dispersed in minor (e.g. 3 percent) concentrations in bitumen have predominately been polymers (a chain of individual monomers or molecules). The

product is called polymer-modified bitumen (PMB). Its behaviour depends on both the physical system of the polymer and the chemical compatibility of the polymer with bitumen. Polymers used include:

1 Elastomeric polymers.

Elastomers are defined in Section 33.3.2. These bitumen enhancers are typically rubber. Natural rubber is a polymer class distinguished by the fact that it possesses molecular chains that are very long, flexible, without weak links, and cross-linked in the common vulcanised form. Its chemical formula is (C5H8)n where n is about 10 000. C5H8 is the

monomer isoprene: link ← CH₂ CH2 → link \ / C========C / \ CH_{3 H}

The additive works via its long, tangled, cross-linked, and flexible molecular chains conferring rubber-like properties on the bitumen. This enhances elasticity, crack- resistance (and thus improves an asphalt’s resistance to reflection cracking, Section 14.1B), fatigue strength, ductility at high strain rates, viscosity at low strain rates and service temperatures, and adhesion. Rutting resistance is limited by the fact that these elastic materials require large strains to develop significant resistance. One subsidiary benefit is that higher bitumen application rates are possible in spray processes (Section 12.1.5).

Rubber (usually scrap rubber broken down — comminuted — to small particles) can be added to the bitumen at the refinery, prior to mixing with aggregate (the wet process), or as a solid during the final mix (the dry process). Careful testing should be done before selecting a process as heat and oxidation can adversely affect polymers. Properties are better controlled in the wet process.

In the wet process, the rubber is digested in bitumen by heating the stirred mixture at 200 C for about 45 minutes; kerosene is often added to aid the process. The rubber remains in the mixture as a series of discrete porridge-like particles. Rubber in the dry process usually performs more like a piece of aggregate than a binder.

For concentrations of 5 percent or less, the properties of the mix of bitumen and rubber differ little from those of bitumen alone. Concentrations of up to 25 percent of scrap rubber or up to 15 percent of polymer have been used in sprayed applications.

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Many pavement crack fillers and sealants are mixtures of bitumen and about 20 percent rubber.

One disadvantage of polymer additives is that the resulting polymer-modified bitumens cool at a faster rate than unmodified bitumens and therefore have a faster increase in viscosity. This makes their field application more difficult (Section 12.4.2) and they should not be used if the air temperature is above 20 C or if there is a chance of wind chill. Another disadvantage is that they have poorer adhesion characteristics (Section 12.3.2) and so they should not be used if moisture is present.

The elastomeric polymers come in three main forms: 1.1 Natural rubber and scrap rubber.

In terms of Figure 8.15 and compared with bitumen, rubber’s modulus master- curve is more of a logistic curve (Section 33.2) and its phase angle is low except for a peak over one frequency range. Most of the rubber particles used are produced as buffings (or crumbs) during the preparation of tyres for retreading. Some particles are also made by the cryogenic shattering of tyres. Particles of natural rubber come from truck tyres, whereas car tyres produce particles of synthetic rubber. Natural rubbers are usually superior to synthetic ones. For spray and chip seals, the morphology of the rubber particles is the most important factor affecting the elastic properties of the resulting digestion. This morphology is determined by the manner in which the particles are manufactured. Fluffy particles are desirable. The morphology can be measured by a bulk density test (Section 8.1).

The useful elastic-type properties of the product are usually measured by recording the elastic strain recovery of a specimen loaded at the intended service temperature to well past the elastic limit. The recovery is linearly related to its rubber concentration.

The process does not work optimally if there is cutter (Section 8.7.6) in the bitumen. In addition, the cross-linking makes the mixture more difficult than bitumen to handle or spray during construction.

1.2 Thermoplastic synthetic rubbers such as: 1.2.1 Polybutadiene (PBD),

1.2.2 Styrene-butadiene rubber (SBR) which exists as random-copolymer (a polymer containing two or more kinds of polymer molecule) lattices and is usually added to bitumen as a water-based emulsion of polymer droplets (or latex), and

1.2.3 Styrene-butadiene-styrene (SBS). This contains about 30 percent hard polystyrene and 70 percent rubbery polybutadiene. Styrene and butadiene are both monomers which are compatible with the components of bitumen and this produces strong interactions between SBS and bitumen. SBS is an unusual block copolymer in that the two components are themselves incompatible and fight to keep apart, with the polystyrene settling as blocks at the tips of the polybutadiene. This structure aids the formation of cross- links. These links breakdown at high temperatures, thus avoiding the handling problem associated with natural rubber. Indeed, SBS is a rubbery elastomer (Section 33.3.2) at service temperatures, and only becomes a non- rubbery thermoplastic (Section 33.3.3) with bitumen-like properties at construction temperatures. A drawback of the tendency of SBS to breakdown and separate when hot, is that the material can become

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irretrievably dispersed in the bitumen prior to cooling. A petroleum-based combining agent is commonly used to aid the dispersion process.

SBS fumes may also cause eye, ear and respiratory infection and nausea. SBS reduces permanent deformation in bitumen, and improves its temperature susceptibility (and thus reduces low-temperature cracking in surface seals and high-temperature rutting in asphalt, Chapter 12). SBS can treble the fatigue life of a bitumen. This is because the styrene domains of the polymer take-over the bitumen as the dominant phase in the mixture for breaking behaviour. For such reasons SBS is one of the most common of the bitumen additives.

1.3 Neoprene.

Neoprene was one of the first additives used, with applications in the 1950s aimed at ‘rubberising’ bitumen. It was then replaced by the use of natural rubber (see 1.1 above).

There are six other classes of bitumen additive:

2 Fibres made from polymers such as polypropylene. These act as reinforcement (see also Section 8.9.2) to improve the ductility of the material. They are added as either as random fibres or a preformed mesh.

3 Thermo-setting epoxy resins (a disadvantage with these is that it is difficult to bond future bitumen layers to an existing layer containing an epoxy additive).

4 Thermoplastics:

4.1 Polyvinyl chloride (PVC),

4.2 Polystyrene (PS, a styrene polymer which is stiff but brittle). See 1.2.3 above. 4.3 Plastomeric polymers such as:

4.3.1 Polyethylene (e.g. polyolefins). Polyethylene can be made from recycled plastic.

4.3.2 Ethyl-vinyl acetate (EVA, Elvax) copolymers. EVA is produced by copolymerisation of ethyl and vinyl acetate. The vinyl acetate content varies between zero and over 50 percent and is adjusted to suit the properties of the bitumen. EVA is relatively non-elastic, as the polymer is not cross-linked, working solely through the elasticity of each strand. Thus it lacks the elasticity of rubber and SBS at high service temperatures. It is most effective at increasing workability and raising stiffness and toughness at low temperatures. It is thus useful in a bitumen that is to be used in asphalt (Section 12.3). EVA tends to disperse and can suffer from a temperature- dependent phase change.

4.3.3 Ethylene methacrylate (EMA) copolymers.

5 Antioxidant and oxidant additives (direct chemical processes are used to increase bitumen strength and stiffness).

6 Extenders such as waste oil, lignin and sulfur, which replace the more expensive bitumen.

7 Fillers such as carbon black, fly ash and lime (Section 12.2.2). 8.8 TAR

Tars are produced commercially from the following sources: (a) carbonisation of coal at temperatures over 600 C, (b) residue from coke ovens, usually at steelworks,

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(c) residue from plants for making gas from coal, using either horizontal or vertical retorts. The latter operate at relatively low temperatures and produce tars with a high phenolic content and which are susceptible to oxidation (McGovern and Alderton, 1972).

(d) cracking petroleum at high temperatures, or

(e) the distillation of wood and the fluxing of associated pitch. These traditional processes are now rarely encountered (Section 3.6.3).

Compared with bitumen (Section 8.7.4), tars are composed of organic materials with a: * lower molecular weight,

* lower percentage of carbon in paraffins,

* higher percentage of carbon in aromatic ring structures, and

* higher percentage of polynuclear aromatic hydrocarbons with three to seven fused rings.

This last category is important for worker health, as some of its members are carcinogens. As with bitumens, tars are best characterised by their viscosity. Although tar has excellent adhesive properties, it is more susceptible to temperature than is bitumen. It is also less durable because its plasticising oils are more volatile, an effect which is heightened at high temperatures. The durability can be improved by removing low boiling-point compounds from the tar. On the other hand, compared with bitumen, chemical changes due to the effects of air and light are relatively minor. Maximum safe- handling temperatures are relatively low at about 120 C. Tars have a temperature range between brittle and soft behaviour of only 30 C, which is half the value for bitumen. This means that tars must be carefully selected.

Tar is now little used in roadmaking, with Britain one of the last countries to be major users.

8.9 CONCRETE