Presently, our highways are filled with trucks that apply heavy radial truck tyre pressure of about 861.845 kPa (125 psi) to today’s pavement. Due to this increasing wheel load, conventional bituminous roads do not stand the test of time and polymer modification of bitumen studies have been on the rise as a solution to the challenge. Modified binders or asphalt are used to reduce the amount and severity of pavement distresses and to increase service life (about 4 – 6 years of additional service life or an average of 25% increase in service life). They also help improve pavement rutting resistance with less thermal (cold-temperature) cracking and overall improved moisture durability being secondary benefits. Some of them also provide improved stripping (moisture damage) resistance. Polymer modification of bitumen in essence means the incorporation of polymers by mechanical mixing or chemical reaction to the pavement binders. To know whether a binder is modified or not, the useful temperature interval (UTI) is examined. If the UTI is less than 92oC (UTI ˂ 92oC), the binder is not modified.
The materials commonly used for polymer modification of binders are:
Polymers: Polyethylene (PE), polypropylene (PP), ethylene-vinyl acetate (EVA), ethylene-butyl acrylate (EBA), styrene-butadiene-styrene (SBS) – most widely used, styrene-isoprene-styrene (SIS) and styrene-ethylene/butylene-styrene (SEBS). SBS, SIS, and SEBS are elastomers while others are plastomers.
Chemical modifiers: phosphoric acid (PPA). It may be used together with polymers to increase the high-temperature stiffness.
Asphalt binder extenders: Sulphur
Oxidants and antioxidants
Hydrocarbons and anti-stripping additives
Expected quality of modifiers
1. Adhesion with aggregate
2. Long-term performance
3. Recyclability
Challenges of modifiers
1. High cost
2. Low ageing resistance
3. Poor storage quality
Justification for the use of modifiers
1. High-traffic loading
2. Anticipated improved service life
3. Environmental conditions
4. Desired performance
Expected quality of bitumen after modification
1. High stiffness at high temperatures
2. High cracking resistance at low temperature
3. Better moisture resistance or longer fatigue life
For polymer modification of bitumen, 2-6% by weight of the material is usually added. However, within Europe, a typical SBS polymer content is around 3.5% by final product weight.
SBS is the most widely accepted modifier because of
1. Good dispersibility (or solubility) in bitumen
2. Excellent properties
3. Acceptable cost of SBS modified bitumen
In Nigeria, Axion Bitumen Booster (a polymer-modified bitumen) that is commercially available is attributed to enable asphalt to withstand temperatures of up to -24oC to 82oC, enable up to 79% elastic recovery, ensure 100% waterproof with nylon tar properties and increase the volume of bitumen by 30%.
For a given polymer modifier, the selection of base bitumen which usually composes over 90% of the polymer-modified bitumen (PMB) by weight is vital because each bitumen has its own particular chemical composition and structure that introduce overriding influences on the final product of the PMB.
Table 1; Advantages and disadvantages of thermoplastic and elastomers
