he amount
and severity of distress and extend the service life of hot mix asphalt (HMA)
pavements and overlays. Particular types of PMA can be selected to increase
the mixtures resistance to rutting as well as for increased durability and
resistance to thermal and fatigue cracking. Most transportation agencies use
PMA only in the wearing surface or the top 2 to 4 inches of pavement when the
PG binder specification dictates the need. However, some use PMA in all the
HMA mixtures of the pavement including the binder layers. Additionally there are
also proprietary PMAs that can be designed for specific HMA pavement
performance requirements.

In sprayed seal and interlayer (membrane) applications, polymers can greatly
prolong pavement life by inhibiting reflective cracking. In dense-graded asphalt
applications, PMAs are effective in reducing rutting and improving fatigue and
thermal crack resistance. The higher shear resistance provided by PMAs can
provide beneficial effects at intersections, tight corners and other high stress
areas. Polymers, particularly elastomeric polymers, have also demonstrated the
ability to prolong the life of open-graded surfacings by improving HMA binder
drainage properties and thus allowing thicker binder films, which are less prone
to oxidation. Greater adhesive and cohesive properties imparted by elastomeric
PMAs vastly improve the surfacings resistance to aggregate loss.and tougher
films that resist collection of foreign matter and dust, thus maintaining the
desirable water drainage capacity of these mixes.

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A recent study sponsored by the Asphalt Institute, Quantification of the Effects of
Polymer-Modified Asphalt, quantifies the enhanced performance characteristics
of PMA mixtures. A comparison of PMA test sections and their companion
sections showed that the use of an appropriate PMA will definitely extend the
service life of flexible pavements and HMA overlays. The PMA mixtures that
were studied were found to have lower amounts of fatigue cracking, transverse
cracking and rutting. On average, the amount of fatigue cracking exhibited on
flexible pavements that include the use of PMA wearing and binder mixtures was
about half of the cracking exhibited on companion projects.

The amount of rutting with PMA mixtures was about 40% of that on companion
projects. Average service life increased by 25% using PMA mixtures or by 2 to
10 years compared to conventional unmodified HMA mixtures. Use of PMA
mixtures can also decrease the cost of maintenance along with the associated
traffic delays and safety concerns.

What is a polymer?

'Polymer' is a derived word meaning "of many parts". Polymers can be thought
of as long chemical strands that are made up of many smaller chemicals
(monomers) that are joined together. Natural polymers, such as cotton, starch,
proteins, wool and natural rubber, have been known for centuries. Early in the
20
th
Century, the first synthetic polymers, such as bakelite, nylon and synthetic
rubber, were discovered. Since then, then the use of polymers has grown
tremendously.

Polymers are made by chemically
connecting small molecules. The
same or different small molecules or
monomers can be joined together to
form polymers. Names of polymers
are usually based on the names of
the monomers used to make the
specific polymer. Thus, the polymer
that comes from polymerizing
ethylene monomers is polyethylene.


Most often when monomers are polymerized, they form long chains. This type of
polymer, called a thermoplastic, can be melted and reformed. Nylon,
polyethylene, and polyethylene terephthalate (PET) used in clear bottles are
examples of thermoplastics. If, on the other hand, the small molecules are
connected to form a three-dimensional network, the polymer is called a
thermoset. After polymerization, the shape of a thermoset polymer is permanent.
Epoxy resins and vulcanized rubber are examples of thermosets.

© 1999-2006 BP p.l.c.
3
Polymers can be made up of different numbers of the monomers and therefore
they can have different 'chain lengths'. Only certain chain lengths may be
suitable for a particular polymer type when used in asphalt. For example, the
polymer 'polystyrene' is made up of many styrene molecules linked together one
after the other. A copolymer has two different sorts of repeating molecular units.
The most common co-monomer used with styrene is butadiene. Block
copolymers, such as styrene-butadiene-styrene (SBS), have these repeating
molecular units in a regularly occurring block pattern. When styrene and
butadiene are polymerized in a random arrangement, the polymer is called
styrene-butadiene rubber or SBR. The physical and chemical properties of a
polymer will depend on the nature of the individual molecular units, the number of
units in each polymer chain and their combination with other molecular types.
Polymers can also have either a linear structure or a radial structure.
Consequently, the different polymers behave in different ways and generally the
different PMAs have to be tried out in asphalt applications before they can be
considered suitable.

Polymers Commonly Used in Asphalt

The two basic types of polymer used in modifying asphalt for road applications in
North America are elastomers and plastomers.

Elastomers

An elastomer is a polymer that has a flexible 'rubber' backbone and large side-
chains in its structure. Styrene butadiene styrene (SBS) is an example of this
type. Thermoplastic elastomers derive their strength and elasticity from a
physical cross-linking of the molecules into a three dimensional network. For
SBS, it is the polystyrene end-blocks that impart strength to the polymer and the
mid-block butadiene that gives the material its exceptional elasticity. This
combination of strength and elasticity gives SBS modified asphalts the ability to
resist permanent deformation and to minimize fatigue and low temperature
cracking. Upon heating, the polystyrene softens and will even dissociate under
stress, thus allowing easy processing. Upon cooling, the cross linked polymer
network is restored, i.e. the material is thermoplastic. SBS-based PMAs are
reasonably easy to manufacture by blending powdered SBS polymer into an
asphalt using low to medium shear mixing. Cross-linking additives such as
elemental sulfur or other proprietary chemicals, or even high temperatures, can
enhance the linking of the asphalt with the polymer structure and can minimize
the amount of polymer needed for a given level of performance.

SBS is the most common asphalt polymer used in the US. SBS is a triblock
copolymer incorporating styrene sections attached to a central butadiene section.
As with most polymers, SBS is available in many different forms. The polymer
molecules can be different lengths and can have different arrangements of the
molecules. These differences can affect the degree of modification provided by
the polymer, as well as the ease of blending and the storage stability.
© 1999-2006 BP p.l.c.
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Other common elastomers include, SB, which is a diblock copolymer of styrene-
butadiene, and PBD, which is Polybutadiene a polymer formed from the
polymerization of the monomer 1, 3-butadiene.

Styrene-butadiene rubber latex (SBR) is a random copolymer of styrene and
butadiene in a water based system. SBR is often used in asphalt emulsions for
chip sealing or slurry seals.

Ground tire rubber (GTR) or crumb rubber (CR) is produced from recycled tires.
The method of processing the recycled tires can have significant effects on the
consistency of performance of the PMA mixture.

Ethylene terpolymer, commonly known through its brand name Elvaloy
®
,
consists of an ethylene backbone with copolymers of n-butyl acrylate and glycidyl
methacrylate. (The word terpolymer refers to a polymer made from three
subunits.)

Plastomers

A plastomer is a polymer that will deform in a plastic or viscous manner at melt
temperatures and becomes hard and stiff at low temperatures, i.e. the structure
is reversibly broken down with the application of heat. Whereas elastomers can
improve the resistance to rutting as well as low temperature and fatigue cracking,
plastomers will generally only improve the resistance to rutting.

Ethylene vinyl acetate (EVA) is the most common plastomer used in asphalt and
acts by making the PMA stiffer than conventional asphalt. EVA polymers are
easily blended into asphalt by simple low shear mixing. As with most PMA
systems, there must be compatibility between the base asphalt and the EVA
polymer to ensure optimum properties are achieved. Phase separation of the
EVA polymer from the PMA can be a problem in storage.

Blending Polymers and Asphalt

The art of good asphalt modification is in trying to make the overall binder
behave more like the polymer while maintaining good workability in the end-use
application. When the polymer is blended into the asphalt it is dispersed either as
discrete particles or as a three-dimensional network in the asphalt. The challenge
is to form and maintain the desirable network consistently in different asphalt
types and for prolonged periods in hot storage. Some polymers form networks
under correct conditions due to their structure while other polymers need
c