Science of potholes
by Mike Kroll
The Galesburg Zephyr
Nobody likes them (except maybe
people who sell tires or repair cars) but potholes are an unavoidable fact of
life. Furthermore, it doesn't take great knowledge or wisdom to discern that
this winter has been very, very good for potholes in Galesburg. Driving
anywhere in town and you can experience potholes of varying sizes repeatedly
for yourself. And yes, even Galesburg public works director Larry Cox admits
that there are definitely more potholes in our city streets this winter than we
have seen in recent past winters.
Where do potholes come from and why
are they more common this winter? I asked Cox to explain the answer to that
question. The normally quiet and reserved public works director actually seemed
to brighten up and enjoy discussing this issue even if he too doesn't
appreciate potholes themselves.
ÒPotholes are a function of moisture
combined with frequently alternate cycles of freezing and thawing,Ó explained
Cox. ÒAs everyone knows water expands when it is frozen and contracts as it
returns to liquid form. When we have lots of moisture during the winter some of
it gets into cracks and crevices in the street and finds its way beneath the
pavement. When our temperatures drop below freezing this water beneath the
pavement expands and pushes up on the road surface expanding the existing
cracks and often creating new ones. As it snows more moisture sits on the
surface until the temperature goes above freezing and it too melts and finds
its way beneath the road surface. This is a cycle that is repeated frequently
throughout the winter, more so when we have a long variable temperature winter
such as this one.Ó
Just as a can of pop left too long in
your freezer is prone to expansion and even explosion the water that seeps
under our streets can cause all kinds of damage to the road surface and
subsurface alike. Around manholes or joints between street and curb or along
structures is is not uncommon for the water to seep in, expand with freezing
pushing the street surface up and then melt with warming temperatures and
actually cause erosion in the street base. Between the upward pressure and
eroded base a void is created and eventually the street surface will fail.
Cox continued, ÒActually winters in
central Illinois are worse for the creation of potholes than those in much
colder climates such as Minnesota because those colder climates spend more time
below freezing temperatures and experience fewer freezing-thawing cycles per
year. Some of the worse pothole conditions in the United States is in
Flagstaff, Arizona where they experience even longer periods of nighttime
freezes and daytime thawing despite their comparatively milder climate.Ó
Southern states that rarely
experience below freezing temperatures are far less likely to have pothole
problems but their consistently hotter and longer summers create a different
set of problems for roadway maintenance. Wintertime potholes are doubly
troublesome due to the difficulty of repairing or maintaining the road surface
under winter conditions.
ÒMost streets in Galesburg are
composed of multiple layers including a compacted base of crushed rock, sand
and/or soil with multiple layers of concrete and/or asphalt laid above in
varying thicknesses,Ó explained Cox. ÒWhen moisture gets under asphalt and
freezes it creates voids between the layers and an uneven surface with cracks
above. When this happens beneath concrete an entire slab of concrete can move
or buckle. As we plow snow on the surface of affected streets the plows themselves
will catch on raised portions of the road surface and inadvertently do more
damage.Ó
To make matters worse cold
temperatures, even above freezing, make it impossible to properly repair either
an asphalt or concrete surface. Both regular asphalt and concrete require
warmer temperatures to permit proper application. In the past Galesburg and
many similar cities have used a specially designed asphalt substitute known as
Òcold-patchÓ to repair potholes in the winter.
Cox explained the city's historical
approach to pothole maintenance. ÒWhen our street crews aren't plowing snow in
the winter months we would send them out in their trucks with cold-patch and
some rakes and shovels to repair potholes. They would do their best to clean
and dry the hole and then shovel in the cold-patch material and tamp it down.
We encouraged them to use the wheels and weight of the truck to really push
down the cold-patch. But cold-patch is only a temporary solution and left alone
will eventually fail itself or simply be worn away.Ó
The city street department depends
upon reports of potholes or other pavement problems to assign crews to repair
them and tries to get to the most serious problems as soon as weather and
resources permit but Cox admits that they simply can't know about or fix every
single pothole in the city. Even when potholes are fixed the old cold-patch
material frequently had a short lifespan on heavily traveled roads.
ÒWe now use a new product called UPM
which stands for 'unique paving material' that is designed specifically to
address many of the weaknesses of traditional cold-patch materials,Ó said Cox.
ÒUPM is much more expensive than cold-patch, at least twice the cost, but it is
designed to adhere better to the damaged road surface and be less sensitive to moisture
and dirt on the surface of the repaired hole. The sales people for this product
even demonstrate it by applying it to a pothole literally filled with water.
Now we would never use it that way but our experience is that this product does
have a much longer lifespan than cold-patch. The sales people say it will last
as long as the surrounding road surface but I'm not yet convinced of that. UPM
is worth the extra cost if it just survives until warmer weather permits us to
get around to the worst damage and properly repair the surface during the
spring and summer months.Ó
Years ago Galesburg initiated a
street maintenance program called the paver program that was predicated on as
assumption that it is cost-beneficial to do ongoing street maintenance such that you maximize the useful
life of a street before it must be resurfaced entirely. While this program
still exists it has been greatly scaled back in recent years. As Cox puts it
the Paver program is Òa shadow of its former selfÓ due to reduced resources
within the street department. Public works was among the most affected city
departments when the early retirement program was implemented and savings was
realized by not replacing many of those who retired resulting in less manpower.
ÒAnytime you can prevent the street
surface from getting into such bad condition and replacement is the only option
you extend the useful life of that street,Ó explained Cox. ÒMany of the streets
in Galesburg were originally constructed in a less than optimum fashion and are
therefore more prone to fail and require replacement or resurfacing. Today we
have adopted the IDOT road surface standards and require that all new
development adhere to them as new roads are created such as in the new Seminary
Square shopping development. Better built roads should last longer and cost
less to maintain but available resources still determines just how much street
maintenance we can accomplish.Ó
2/14/08