To understand asphalt pavement maintenance procedures one should first
understand what an asphalt pavement is - how it is made, how it is installed
and how it ages. Asphalt paving mixtures are all basically similar. They are
comprised of aggregate (rock), fines (sand) and a small amount, typically five
to six percent, of asphalt oil (AC).
Various quantities of these components are blended at high temperatures to
manufacture the paving material. They are then transported to the job site, run
through the paving machine and compacted to provide a smooth wear surface.
Left alone and not properly maintained, the aggregate and sand are not affected
appreciably by aging. But the oil or asphalt cement that binds the mixture
together starts to break down or age as soon as this oil is exposed to heat
when it’s being mixed. Further exposure to air, water and sunlight combine to
cause oxidation, the aging process of asphalt. More importantly, oxidation ages
the oil that holds the aggregate and sands together to form a flexible asphalt
pavement.
The asphalt oil is made up of asphaltenes and maltenes. The asphaltenes are
fibrous, graphite-like materials in the oil that do no break down or oxidize.
The maltenes are the tacky, glue-like materials in the oil that effectively
bond together the aggregates and sands. The maltenes are the portion of the oil
that degrade and oxidize from the pavement.
Sunlight (UV) fractures the maltene molecules and they discharge from the
pavement. This is represented in the graying out of the pavement surface.
During this process, the oil is losing its ability to bind the surface fines or
sand materials that provide the smooth texture of a pavement. These fines will
erode from the surface as it is exposed to water. The next step is that the
aggregate itself begins to pop out of the pavement leaving you with a gray,
pitted and brittle pavement surface.
Water-based coal tar emulsion coatings, or “sealcoats”, are commonly
placed as a pavement maintenance procedure. Test studies validate the ability
of Pavement Dressing Conditioner (PDC) to rejuvenate a pavement, no
matter how old it is, and offer a longer lasting, black coal tar protective
finish. This has been a standard practice and it was thought that sealcoating
would protect a pavement from oxidation and fuel spill or water damage. These
claims of protection are simply not true.
Although aesthetically pleasing when initially placed, these coatings are very
short lived. They wear off under traffic and it has been determined through
government that these sealcoats will crack. Cracking in the coating
allows contaminates as well as the oxidizing factors to permeate the seal and
damage the pavement.
A sealcoat will do nothing to replace low maltenes or restore flexibility
to an aging asphalt. Simply put, placing a coating on your pavement will not
extend the life of that pavement, or postpone overlays or reconstruction.
Let us now introduce you to a treatment that provides unparalleled protection to
a pavement from oxidation and water or fuel-spill damage. This treatment will
also put back maltenes lost through oxidation. Better put, this treatment will
not only stop oxidation (the aging of asphalt), but will reverse it, putting
life back into the pavement, therefore extending service and postponing
overlays or reconstruction.
The product that does this is Pavement Dressing Conditioner (PDC). PDC is
specified by the Federal Department of Transportation as a coal tar sealer/rejuvenating
agent. This material is designed to penetrate into an asphalt oil, providing a
fuel and water resistant surface. Now the source of protection for your coal
tar is in the pavement, not just on it.
PDC will restore flexibility and durability. Rejuvenation is the
softening of a pavement surface and the revitalization of the oil’s ability to
retain the fines and aggregate. The rejuvenation process ages much
slower through evaporation of the coal tar rather than the rapid aging of
pavement through oxidation.
Test studies validate the ability of PDC to rejuvenate a pavement,
no matter how old it is, and offer a long-lasting, black coal tar protective
finish.
Article written by Bob Hegerman who was President of Asphalt Restoration
Technology of Florida from 1993 to 1999.