Why Polymer Modified Asphalt?
Asphalt pavements are flexible by design.
Asphalt is a plastic material
which melts and flows at high temperatures and becomes hard and glassy
at low temperatures. Therefore, it's ability to perform successfully as
a binder is limited by temperatures and stresses applied to the pavement.
The Federal government's Strategic Highway Research Program has identified
the limitations of asphalt as a binder by developing new test equipment
and methods to better classify asphalt. As the result, a new grading system
for asphalt has become accepted across the nation. This system, known as
Performance Grading, classifies an asphalt's ability to adequately perform
at both high and low temperatures. Under this system, asphalts are given
a high and low temperature identification in their name, such as PG 76-22.
Many of these new grades of asphalt can only be made with the addition
of polymers.
Polymers impart significant
improvements in an asphalt's ability to withstand temperatures extremes.
Polymer modified asphalt is more viscous at high temperatures which improves
a pavement's resistance to rutting and shoving. At low temperatures, polymer
modified asphalt is more ductile, less glassy. As a result, pavements constructed
with polymer modified asphalt are less likely to ravel or become brittle
at low temperatures. The elastic component that polymers add to an asphalt
makes the pavement tougher and more resistant to fatigue cracking as well
as provide a more adhesive binder that adheres to aggregates better even
under damp conditions found on tree covered streets.
But what are polymers? Are they all the same?
Polymers are simply very
large molecules that are made by connecting smaller molecules called monomers
together to form long chains. The physical properties of these polymer
chains is dependent upon properties and structure of the monomers and how
the monomers are connected. Polymers used for asphalt generally fall into
two types being either elastomers or plastomers. Plastomers merely stiffen
the asphalt while elastomers make asphalt more flexible at low temperatures
as well as stiffer at high temperatures. Polymers of choice for asphalt
modification are generally elastic in nature.
For a polymer to be effective
it must disperse to form a swollen polymer network in the asphalt. The
most economically effective polymers used to modify asphalt are Styrene
Butadine polymers in either solid pellet form or liquid latex form. Within
the family of styrene butadiene polymers are many types of which only a
few are useful and swell appropriately in asphalt to form an effective
network. Solid styrene butadiene polymers such as SB and SBS materials
must be melted and sheered into hot asphalt at a refinery or terminal.
SBR latex, the liquid form of styrene butadiene polymer, is a microscopic
dispersion of polymer in water that can be added directly in the production
of hot mix at the contractor's plant. Due to the microscopic particle size,
the polymer disperses rapidly into asphalt without the need to be melted
or sheered in with costly equipment. SBR latex offers many other advantages
to both the contractor and engineer to produce higher quality pavements
at the lowest cost.
