Polymer Modified Bitumen (PMB) :

Polymer Modified Bitumen (PMB) is one of the specially designed and engineered bitumen grades that are used in making pavement, roads for heavy duty traffic and home roofing solutions to withstand extreme weather conditions. PMB is a normal bitumen with the added polymer, which gives it extra strength, high cohesiveness and resistance to fatigue, stripping and deformations, making it a favorable material for infrastructure.

Styrene Butadiene Styrene (SBS) :

When a polymer is added to regular bitumen, it becomes more elastomeric, which provides it with additional elasticity. The polymer that is added is Styrene Butadiene Styrene (SBS), which acts as a binder modification agent. The primary objective of SBS polymer modified bitumen is to provide extra life to pavement, roads and construction designs. Some of the qualities exhibited by PMB are :

Higher Rigidity :

Increased resistance to deformation
Increased resistance to cracks and stripping
Better water resistance properties
High durability
Advantage of using polymer modified bitumen:

Rigidity :

Stronger road with increased marshall stability value and greater Rigidity.
better resistant towards rainwater and water stagnation.
no stripping and no potholes.
Better resistance to permanent deformation
reduction in pores in aggregate and hence less rutting and raveling.
Much higher durability

Common Types of Polymer Modified Bitumen
The following table lists some common asphalt cement and HMA modifiers and their general purpose/use.

Use as Needed :

While the benefits of using modified asphalts are widely acknowledged, not all asphalt mixes or treatments need to be modified. Each application should be evaluated to determine if the traffic loading, anticipated service life, environmental conditions and desired performance justify the use of modifiers. Modified asphalts can be a good investment.

Rheological Properties :

The rheological properties of conventional binders may be modified by the introduction of:

Crumb Rubber ;

The modification is costly and is normally justified when bituminous surfacing are subjected to severe conditions such as:

Steep Gradients ;

Very high road surface temperature;
High traffic loading; or
Heavily trafficked intersections.
Modification may also be advantageous for surfacing on highly flexible and cracked pavements, where an improvement in the rheological properties of the bitumen is required.
Use in such applications should be guided by expert opinion.
In addition to the primary aims above, the range of properties improved include

Durability;

Aggregate retention;
Resistance to permanent deformation;
Resistance to fatigue cracking;
Cohesion (internal strength);
Elasticity;
Viscosity less susceptible to temperature changes.
Modification agents
The primary aim of the modification of bitumen for use in structural layers is to increase the resistance of these layers to permanent deformation at high road temperatures without compromising the properties of these layers over the rest of the prevailing temperature range.
The use of polymer modified bitumen to obtain improved performance is rising as a result of increases in tire pressures, axle loads, and higher traffic volumes.

Permanent Strain :

Improved performance can be achieved in two ways, both of which are aimed at reducing the permanent strain:
An increase in the elastic component with an associated reduction in the viscous component; and
Stiffening of the bitumen to reduce the total viscoelastic response of the layer.
The modification is achieved by the introduction of polymers (including crumb rubber), aliphatic synthetic wax or naturally occurring hydrocarbons.

Polymers can be broadly categorized as “elastomers” (sometimes referred to as thermoplastic elastomers) for improving the strength and elastic properties of a binder, and “plastomers” (sometimes referred to as thermoplastic polymers) for increasing the viscosity of the bitumen.