Binder (bitumen) modification

Binder (bitumen) modification is done with the help of additives which may or may not react chemically with bitumen. Table 1 presents a partial list of various types of binder modifiers, their purpose and examples. Binder modification results improvement of one or more properties of the binder (and hence the mix) viz. fatigue resistance, stiffness modulus, rutting resistance, stripping potential, temperature susceptibility, oxidation potential etc.

Table 1. Some examples of binder (bitumen) modifiers

(RILEM 1998; ETM 1999; Asphalt Handbook 2000; Widyatmoko 2002, SEAM 2004 )              

	Purpose	Examples
Non Polymers
Fillers	to improve bitumen durability and check rutting	Lime, carbon black, fly ash
Anti-oxidants	to check oxidative hardening	Zinc anti-oxidants, lead anti-oxidant, phenolics, amines
Anti-stripping additives	to achieve better adhesion of bitumen to aggregates	Organic compounds (like arnines, andamides)
Extenders	to act as bitumen substitute and to improve fatigue resistance	Lignin, sulphur
Polymers
Fibers	to reduce viscosity, as filler material,	Polyester fibers, Polypropylene fibers
Plastics -Thermoplastics     -Thermosets	to increase the viscosity and stiffness of bitumen at normal service temperatures without compromising with fatigue performance to obtain insoluble, infusible material that do not flow on heating	Polyethylene (PE), Polypropylene (PP), Polyvinyl chloride (PVC), Ethylene vinyl acetate (EVA). Epoxy resins
3. Elastomers - Natural - Synthetic - Reclaimed rubbers	to reduce temperature susceptibility and temperature distresses, age-hardening, bleeding and binder-aggregate stripping.	Rubber Styrene-butadiene copolymer (SBR), Styrene-butadiene-styrene copolymer (SBS), Isobutene-isoprene copolymer (IIR)

For conventional binders, it is generally observed that the mixes with high stiffness modulus (E) show low fatigue life, and vice versa. However, for an economical pavement design, both high elastic modulus as well as high fatigue life is desirable. Through binder modification, this particular disadvantage can be avoided. Figure 5 presents this concept schematically.

Figure 5: Schematic diagram explaining the advantage of binder modification.

 

As can be seen in Figure 5, for mixes with ordinary binder, although elastic modulus E value is higher initially at low temperatures, at high E value the fatigue performance generally becomes poor. On the other hand, at high temperature the E value becomes too low and the mix becomes soft. The bituminous mixes with modified binder does not allow the mix to be too hard (high E value) or too soft (low E value) at low and high temperatures respectively. Thus the stiffness versus temperature curve takes a 'S-shape' as shown in Figure 5.

Aggregate modification

• The marginal or poor quality aggregates can be improved by using some cementing material such as cement, lime, pozzolanic substance etc.
• The proportions of the cementing material and other ingredients (like water) can be suitably estimated in the laboratory.