by Rick Stryker, P.E.
Several years ago (March/April 2003), Building Principles took a broad
look at camp roads. Recently, we've had a number of calls about
road issues so we thought that this might be the perfect time to expand
on the roadway topic a bit, and talk about a few
products that have really come of age in the past four years.
Bituminous concrete ("asphalt" or "blacktop"),
known as "flexible pavement" in the industry, is probably
the most common roadway surface you'll find, not just at camp,
but everywhere. That's true for a host of reasons but mostly because
it's relatively inexpensive (though prices have skyrocketed with
the increase in petroleum costs) to buy and install. Some people even
think that it's cheap and easy to repair, and when it's done
badly, they will correct it . . . at least for a short while. Let's
look at why asphalt fails, and then it will be clearer why a properly
done repair may not be as inexpensive as we thought.
We all have experienced the "flexible" part of "flexible
pavement" at the height of summer, when the parking lot feels soft
under our feet. The opposite effect happens in the winter: asphalt roadways
get very hard and brittle when cold, and being exposed to the weather
it's the first layer to freeze and the first to warm. If the asphalt
and the layers beneath are frozen, the stacks of material act together
to support any loads applied. When the road bed is frozen, but the asphalt
is soft (like on a south facing hill during a sunny day in January),
the layers can separate ("delaminate") and allow seepage
through surface cracks. When the water freezes again, it expands and
makes the condition worse. Sheets of road surface lift, are stripped
by the snow plow, and deposited on the shoulder or ditch. Large, relatively
shallow potholes usually result from this particular cycle. Also, heavy
loads on pavement can add to and accelerate the deterioration (e.g.,
a plow truck full of cinders or salt). Finally, what about in the spring,
when both the pavement and the road bed below are above freezing, but
the bed is saturated with snow melt and ground water? Vehicles traveling
over the "raft" of asphalt have little to no support beneath,
the asphalt yields, and deep potholes develop.
This path to failure is no secret to the folks who design, build, and
maintain roads all the time, so how did we come to this place? Think
back to a time before the asphalt. Who remembers that the road was gravel
or dirt and it was always full of potholes, ruts, and washes? Someone
called the local paving company and told them to "blacktop the
road." They did, and it looked great for a while. But now the potholes
are more abrupt than they were in the gravel road, and the repairs are
more expensive. No need for a show of hands, you know who you are. Take
heart, though, because just about everyone else is in that boat too!
Road Construction 101
Because climate, conditions, and local practices vary widely, every
state Department of Transportation (DoT) has developed a list of criteria
("specifications") for road building materials from the stone base through
the asphalt layers. Learn what those designations are and what they represent
so that you understand what the contractor is talking about. From the
bottom to the top, the materials get gradually more fine with fewer and
fewer spaces or voids between the particles. The last layer, just under
the asphalt, will likely be a crushed stone and dust combination so that
the voids between the individual stones are completely filled with stone
dust.
Beginning with the native soil, each layer should be as tightly compacted
as possible. When the lowest elevation for the road has been excavated,
the native earth is compacted. Sandy soils are best compacted using vibration
and knobby steel rollers (sheep's foot). Rubber-tired rollers are
used for other soils and rely on their sheer mass to compress the material
beneath. Back and forth, the equipment rolls and pounds the material
until it is "tight enough." But how do you know? There are
laboratory tests that can confirm the compaction, but these take much
time and money. Instead a field test called a "proof roll" is
conducted like this: Before a new layer (or lift) is added, a fully loaded
dump truck of stone drives slowly down the compacted area with a checker
walking behind watching the impression made by the wheels. Any location
where the material appears to move under the load is marked with spray
paint for rework. Sometimes it just needs more compaction. Other times,
the area has to be dug out, replaced with new material, and then retested.
Only when the entire run has been tested and passed "proof roll" is
the next layer allowed to be placed. By ensuring that each successive
layer is packed as tightly as possible, there is more and more certainty
that the road will resist water intrusion from beneath. It should be
more clear now why "blacktopping" that existing gravel road
without compacting the material beneath is a sure recipe for pavement
failure, sooner rather than later. This may seem time consuming, and
perhaps it is. But it is a world cheaper than a failed roadway.
Through proper material selection and thorough compaction, we've
worked really hard to make sure that water is excluded from our road
base. We're ready for the asphalt. This too has specifications
that your DoT has set up for your conditions, and the same homework is
required to research the details. Too much bitumen ("black stuff")
will make the mix too soft. Given the current cost, that won't
probably be a problem, however not enough bitumen will make the pavement
brittle. There simply won't be enough glue to hold the aggregate
together in the proper matrix. In addition to mix recommendations, the
regional office of the DoT will probably be glad to offer suggestions
about how many inches of asphalt binder (bottom layer) and wearing course
will serve your traffic load economically.
One other detail that shouldn't be overlooked is known as the "tack
coat." This is a coating of raw bitumen emulsion that glues the
layers together to prevent delamination and to help to exclude water
infiltration from the surface. It's expensive because it's
all petroleum product (with some sand), but it will pay for itself. Don't
skip it.
Most roadways are constructed to direct water away from the centerline.
That is, they slope toward both shoulders. This is called "crowning,"
and while it's more prevalent in the northern climes, it's not
unheard of in the rest of the country. One-quarter of an inch per foot
(2 percent) is pretty common, and normally it's built in to the
placement of the asphalt by an adjustment in the machine that lays the
coat of material down (called the "screed"). It's then
compacted and shaped with a hot roller to set the final cross-slope or
crown.
Making It All Go Away
We've now built a road for the ages. The Roman Empire would be
proud. But what will happen to the rain and snow melt? We got it to the
edge of the pavement with the crown, but where does it go from there?
Where there is no curbing (who has curbs along camp roads?!?), the shoulder
should be wide enough to accommodate vehicles, compacted to withstand
wheel loads, and sloped steeper than the roadway. It's likely that
the shoulder material will be rougher than the smooth pavement, so water
will tend to slow here. For example, where the normal crown is 2 percent,
shoulders are often sloped at 4 percent to shed the water into the ditch.
There should be about a half inch of drop. More than that, and water
will tend to channelize along the pavement edge and erode the soil. Less
than that and the grass will tend to grow over the pavement and create
a dam for the run off. The best way to go is with a gravel or crushed
stone shoulder (properly compacted of course!) which has no drop off
at all.
Who Can Afford That?!?
We're fond of telling folks that you can pick any two of the following
for each project you undertake: You can have it right, fast, or cheap.
When you're thinking about rehabilitating thousands of feet, sometimes
miles of roadways, costs sure can add up in a hurry. Nobody that we work
for can afford to peel up their roads and start over, so instead we recommend
that roadway rehab be part of an annual improvement investment program.
Commit to (and follow through with) budgeting a certain amount of money
or a section of road each and every year, and rebuild it right. Start
with the areas that are in the most distress or which represent the greatest
risk to your guests, and do what you can this year. Next year, take on
the next spot on the priority list. Before long your roads will have
a somewhat patchwork look, but properly built and managed, you'll
be in great shape to top off the whole thing for your final roadway construction
installment.
As usual, an ounce of prevention is worth a pound of cure (thank you,
Ben Franklin). Proper drainage of the road bed means that it is largely
without water, and therefore not inclined to freeze and thaw as dramatically.
Unfortunately, most, if not all, camps have roads that didn't receive
enough attention and care when they were built in the first place, and
now it's come home to haunt your organization today. Stop the madness!
Spend the money well, and your investment will benefit your successors
for years to come.
Originally published in the 2007 July/August
issue of Camping Magazine. |
