We need to have a new blacktop driveway
installed, and I thought it was going to be a simple process of comparing
quotes. It turns out it’s somewhat confusing. I didn’t know there were
different types of blacktop. What’s more, the contractors don’t seem to agree
on the best way to prevent cracks along the edge of the driveway. Can you
explain why there are different types of blacktop and what might be the best
for my situation? Do you have a blacktop drive, and what’s it like? — Maddie
H., Baltimore
I do have a blacktop driveway at my current
home. Before that, I had a unique driveway that was the first cousin to
blacktop. It was a driveway made with just the liquid asphalt cement and small
stone chips. Some call it seal chip and others call it tar and chip. Blacktop
is almost always an asphaltic concrete made from liquid asphalt cement, sand and
gravel.
If you know much about sand and gravel,
then you can start to get your head around how there can be different types of
blacktop. The reason there are different blacktop types is because there’s an
infinite amount of possible combinations when you consider the different sizes,
shapes and colors of sand and gravel.
Highway engineers have a much better handle
on this than folks like you and me because they have to construct roadways from
asphalt that support heavy trucks. The blacktop mixtures used on an interstate
highway may be just fine for some residential driveways, but you’d have to take
a number of things into consideration.
For example, let’s talk about my driveway.
I live in central New Hampshire, and we get lots of snow and ice in the winter.
Many driveways here are not flat. You need excellent traction in both wet and
snowy weather.
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Fortunately, the contractor who installed
my driveway (before I bought the home) used a special mix that has smaller and
larger pieces of crushed granite in the mix. The majority of the largest pieces
of stone in my driveway are about the size of a large grape. There is an
occasional piece of stone that’s about the size of my big toe.
These stones in the mix are by no means
smooth. The roughest edges are knocked down, but the stones have a marvelous
gripping texture. Over time the smaller pieces of sand and asphalt have eroded
slightly between the stones. This produces a rougher surface that provides exceptional
traction in wet or snowy weather.
But you may want a satin-smooth blacktop
drive for your mild climate. Many homeowners love smooth-as-silk blacktop. If
you’re installing a new driveway and you intend to be at the house for many
years, you may want to consider talking to your contractor about installing two
separate layers of blacktop to create a driveway that could last decades and
decades.
Keep in mind that the gravel base under the
blacktop is where the driveway gets almost all its strength. You want at least
8 inches of crushed compacted gravel under the blacktop.
You have the option to install one 2-inch
layer of blacktop like the kind I have on top of the gravel base. Your
contractor then would order a different batch of blacktop that only had small
stones in it no larger than three-eighths of an inch in size. He’d put a 1-inch
layer of this on top of the first 2 inches. When these smaller stones are
combined with medium and some coarse sand in the mix, the final surface after
it’s rolled is just about as smooth as you could imagine.
It’s important for you to see the different
types of blacktop that your contractors are proposing. Have them give you
addresses of completed jobs you can visit. Walk around and look at the finish
and the size of the stones in the mix. Decide which one you like best and be
sure to have the contractor match it.
[More Builder: Are homes of today built
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To prevent cracking of the blacktop on the
edges of the drive, you need to do something most residential contractors never
do. The crushed gravel rock base under the driveway needs to be at least 1 foot
wider than the actual finished driveway if you want to eliminate cracks.
Most driveway contractors install the
gravel base, compact it and then put the blacktop all the way to the edges of
the gravel. The soil on the sides of the driveway that’s added after the job is
done offers no structural strength to a sideways movement. When a heavy vehicle
tire is right at the edge of the driveway, the force not only goes down but
some of it also goes sideways.
When the soil can’t oppose this force, the
blacktop cracks away. A wider compacted gravel base will provide all the needed
support. Roads out in the country have this extended base. You’ll often see
wide gravel shoulders on country roads. These are there to keep the pavement in
great shape.
To have the best protection against edge
cracks, the gravel base or shoulder should extend to the same height as the
asphalt. Most homeowners would never put up with this look as they want the
grass to touch the blacktop. You can put in an attractive edging stone as a
compromise. Talk to your contractor and see what thoughts he has.
hua.tien@msa.hinet.net
http://www.hwatien.com/en/
hua.tien@msa.hinet.net
http://www.hwatien.com/en/
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