What Makes a Good Asphalt Driveway?
By John Davis and Dwight Walker, P.E.
www.asphaltmagazine.com
Delta Mangers are often asked about putting in a drive-way.
There are some key considerations in having a good asphalt driveway.
These critical points include:
• adequate foundation;
• proper drainage;
• appropriate materials;
• good construction practices; and
• timely maintenance
Don Siler, asphalt technology specialist for Marathon Petroleum
Company (MPC), concurs. “Good materials, good practices, good
workmanship and good site preparation make a good driveway,” he
says. “Driveways must be properly designed and constructed,” he
adds. “If the design procedure is wrong, then the driveway will be
faulty.”
Adequate Foundation
Preparing an adequate foundation includes having a solid subgrade
and building a strong aggregate base.
A common problem is subgrade that is not properly stabilized, says
Siler. “When there is wet, soggy clay present, you need to remove it or
put down a good stone base.” He adds that the worst failures can be
subdivision driveways.
“Bad preparation on driveways in some subdivisions will result in
construction (truck) traffic ruining the driveway. Driveway pavements
can get buckled by truck traffic.”
Buddy Prather of Prather Paving in Lexington, Kentucky, concurs.
“Soft dirt is a real issue,” he says. “You have to get rid of the soft topsoil and get to something solid. Around new homes, around the sides
of the house and around the garage, you get backfill that hasn’t been
compacted. It’s soft dirt. You have to replace it with something solid.”
“Putting down a rock base before placing the hot mix is critical,” Prather
says. “The size or thickness of the rock makes a difference. We use
2-inch (top-size) rock—as big as your fist. The 2-inch rock sinks into
the subgrade and stabilizes the earth,” adds Prather.
Proper Drainage
Another key point is proper drainage. Problems develop when water
removal is not sufficient.
“You need to drain water away from the pavement — away from the
edge of the pavement,” says Siler. He recommends using a French
drain (a trench filled with gravel or rock with a pipe designed to redirect
water) to get moisture away from the pavement below the pavement
level.
“Drainage, drainage, drainage is what my Dad always says,” said
Prather. “He ran the business before I did. You’ve got to have good
drainage. You’ve got to have slope to your pavement so the water will
run off. The water needs to run off to the side in a sheet.
“If you have good drainage, your pavement will last,” adds Prather.
“Both good surface drainage and good subgrade drainage. The rock
has to be laid right so it will allow the water to leach out. Don’t let the
water gather in low places and bust-up the pavement. Big rock will let
the water drain out. It acts like a French drain. And we use geotextile
around the drainpipes.”
MPC’s Siler says there is a definite benefit from a gravel base because
it allows the water to drain.
Appropriate Materials
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It is important to use the right asphalt mix. Unless they own their own
hot mix plant, the driveway paving contractor may get “the-mix-ofthe-day” from the local hot mix plant. This material may not be suited
for driveways. Driveways are susceptible to brittleness caused by
oxidation and weathering. Some mixing plants set aside one of their
storage bins for private mix projects.
Delta Triangle Newsletter 2014 | ISSUE 2
Opinions differ as to what makes the best driveway mix. In general,
driveway mixes should be designed with more asphalt binder and
less air voids than highway mixes. The aggregate structure is where
opinions differ.
Many experts prefer that driveway surface
mix have a finer gradation than highway mix.
This gives a finer surface texture and smooth
appearance. Siler prefers a mix composed of
good angular aggregate with stone-on-stone
contact (sometimes called aggregate interlock).
“It may not be the prettiest looking mix, but it
is the most durable. Avoid sand and rounded
particles. They don’t have the strength of
angular particles, even though they make a
pretty mix. Err on the side of the stronger mix,”
advises Siler.
The choice of gradation depends on the loading
and desired appearance. Finer gradings will
shed more water and look more uniform. Stony
mixes can carry heavier loads and require thicker placement depths for
compactibility. Prather says that they always use a 2-inch compacted
binder course (with larger top-size stone) on farm driveways because
it can take heavier loads.
Good Construction Practices
“Good workmanship is important, so the driveway will last,” says
Prather. “Compaction is critical, including the edges and joints in t he
pavement.
“You have to tamp the edges of the pavement,” adds Prather. “And
make the seams (joints) look good — tight and good.”
Prather says they always use a tack coat on residential driveways.
“It’s a binding agent. We use SS-1H (asphalt emulsion). It prevents the
surface course from slipping. If the surface course slips, it probably
wasn’t tacked properly.”
It’s important to avoid segregation of the mix because a segregated
mix can lead to potholes and pavement failures.
Maintenance
Most experts agree that the quality of the paving job is what
matters most to driveway performance — particularly using a mix
that is appropriate for the job and achieving good compaction. But
maintenance is important, too.
Recommended maintenance treatments include taking care of the
drainage features and crack-sealing. Overall pavement sealing is a
matter of choice and wear-and-tear.
Water is the enemy of pavements. Proper installation of drainage at the
time of construction is important, but it is equally important to assure
that the drainage continues to work. Water can soften the subgrade
and/or undermine the pavement.
Crack-sealing is an important part of maintaining the driveway
pavement. Water and foreign material can enter the pavement and
cause damage, if the cracks are not sealed. Some owners do not like
the appearance of sealed cracks.
“Crack-filling does maintain the driveway, but it doesn’t look good.
That’s why we seal the whole driveway, after we fill the cracks. Filling
the cracks does stop water penetration and prolongs the life of the
driveway,” says Prather.
In addition to sealing the driveway for appearance purposes, sealing
can be used to preserve the pavement. Fuel-resistant, polymermodified asphalt sealers can guard against spills. Stony mixes or
pavements with open textures benefit from sealing.
-excerpt from Asphalt Magazine, originally posted 02/24/2014