Types and Grades of Bitumen

According to Sabita Manual 2 (2007) the two main types of binders are conventional (or penetration grade) and modified (or polymer-modified) bitumen.

Other types include cutback bitumen, bitumen emulsions and bitumen rubber. These will not be discussed further since only the main two types of binders are considered for the research done.

• Conventional Binders

Conventional binders are penetration grade bitumens, which are characterized by their penetration achieved during the penetration test. Penetration grade bitumen is usually the primary binder used as base bitumen for manufacturing other binders, like modified binders, cutback bitumen or bitumen emulsions. Table 2.1 shows some of the grades of penetration grade bitumen most widely used and available.

Table 2.1 Conventional Binder Types Conventional Binder Types

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• Modified Binders

Modified binders can also be referred to as polymer–modified binders or PMB’s.

The Asphalt Institute (2007) reports through several surveys, that there are certain benefits in using PMB mixtures to extend the pavement’s service life. There are some that also state that using PMB mixtures reduces maintenance costs significantly. Three of the main reasons to use PMB’s are:

• To increase the mixture’s resistance to rutting of mixture.

• To increase resistance to thermal cracking of mixture

• To increases durability of mixture.

It is possible for modified binders to be manufactured by chemical modification, but polymer modification is the most widely used method.

Polymers consist of smaller molecules (monomers) that are chemically polymerized to form long molecular chains.

The Asphalt Institute (2007) specifies two broad classes of polymers used in the modification of binders; namely polyolefins and styrenic polymers.

As a result of polymerisation of molecules containing a simple double bond or olefin becomes polyolefins. Polyethylene, polypropylene and ethylene vinyl acetate are examples of polyolefins. Co-polymerization of polystyrene with other small molecules results in styrenic polymers, usually butadiene.

When polymers are stretched (strained or stressed) with enough force, they either exhibit plastic or elastic behaviour. Polymers that display plastic behaviour are classified as plastomers (plastics) and will yield under the force and stay stretched out when the force is released. According to Brown et al (1996) these polymers exhibit quick early strength on loading but may fracture under strain

Polymers that display elastic behaviour are classified as elastomers (elastics or rubber) and will yield, but when the force is released they will return to their original shape. Brown et al (1996) explain that these polymers add very little strength to the binder until they are stretched. However their tensile strength increases with

elongation.

Usually most polyolefins behave as plastomers and styrene-butadiene copolymers behave as elastomers. When elastomers are used to modify binders, this usually results in a more flexible and resilient pavement. Plastomers usually increase the stiffness moduli of the pavement.

However, it is important to note that these results are very dependent upon the concentration, molecular weight, molecular structure and chemical composition of the specific polymer. Other factors which are equally important are the crude source, refining process and grade of the neat bitumen. The different types of elastomers and plastomers are displayed in Figure 2.3 below. Elastomers and plastomers will now be explained in further detail.

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Figure 2.3 Different Types of Elastomers and Plastomers Available

a) Elastomers

According to the Asphalt Institute (2007), butadiene is the most widely used co-monomer to produce styrenic polymers.

Sabita Manual 2 (2007) indicates that there are three types of elastomers used in South Africa:

• Styrene-Butadiene-Rubber (SBR) latex

• Styrene-Butadiene-Styrene (SBS)

• Rubber Crumb

The Asphalt Institute (2007) explains that SBR is produced by random polymerization of styrene and butadiene. The applications of SBR in South Africa include cold

applied binder emulsion in chip seals and micro surfacing, and hot applied binder in chip seals and HMA.

SBS is produced by polymerization of styrene and butadiene in discrete, connected blocks. The Sabita Manual 2 (2007) and The Asphalt Institute (2007) indicate that SBS is the most widely used modifier in agencies. They continue by describing the potential benefits of SBS as follows:

• It helps to reduce permanent deformation or rutting of asphalt pavements.

• It helps to reduce low temperature cracking and fatigue.

Crumb rubber is reclaimed rubber primarily obtained from discarded tires. Brown et al (1996) indicate two processes by which crumb rubber is used, namely the wet and dry process. The wet process blends the crumb rubber with the binder before incorporating the binder into the project. The dry process mixes the crumb rubber with the aggregate before mixing it with the binder.

If an elastomer is used to modify the binder used in producing WMA, the resulting mixture will be more flexible and have improved strength and fatigue resistance, but

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because WMA is produced at lower temperatures the binder in the mix will age less and result in a more elastic mixture. One should take care when using elastomer modified binder with WMA technologies to produce a mixture which can be too elastic and susceptible to rutting and permanent deformation.

b) Plastomers

According to The Asphalt Institute (2007), polyolefin polymers will add to the stiffness of the binder at high temperatures but do not have elastic characteristics of styrenic polymers. Polyethylene and ethylene vinyl acetate (EVA) are most broadly used to modify binders. The main reason plastomers are used to modify binders, is to help reduce permanent deformation or rutting of asphalt pavements.

The more elastic mixture, due to less ageing of the binder when producing WMA at lower temperatures, should be alarming, since this increases the susceptibility to rutting and permanent deformation which opposes the aim of the plastomer modified binders. However, the WMA will rely more on the increased rut resistance, stiffness and viscosity achieved by the plastomer. In South Africa, EVA is most commonly used. The ethylene part of EVA improves the high temperature properties of the binder, and the vinyl acetate improves the low temperature properties.