If you consider it, having paved roadways
is somewhat of a luxury.
Streets have always been around, but they
haven’t always been safe, comfortable, or
able to accommodate the enormous number and
weight of vehicles that use our present system
of roadways every day.
Whether or not you love how much roads dominate
the landscape, you have to marvel at the fact
that, in most parts of the modern world, anyone
can get in a bus, car, bike, truck, motorcycle,
or scooter, and go almost anywhere else in
relative ease and comfort.
In fact, roads make travel so convenient that
not having them - or having them be in poor
condition - is a significant source of frustration.
There are definitely times when driving does
not feel that luxurious, and one of them is
something we’ve all experienced once or
twice.
Hey, I’m Grady, and this is Practical Engineering.
On today’s episode, we’re talking about
potholes in paved roadways.
This video is sponsored by Brilliant.
More on that later.
I remember the excitement of getting my first
car as a teenager and finally being able to
drive.
Sad to say, that was probably the most joy
that driving a vehicle will ever give me.
Now, it’s kind of a chore.
And I hope I’m not out of line by saying
this, but I think for most people, driving
is a little dull.
It’s the thing we do in between where we
are and where we’re trying to be.
I don’t know about you, but I don’t wake
up in the morning excited to jump in the car
for my morning commute.
Driving is something that most of us take
for granted.
But, the only reason we’re able to do that
- to regard vehicle travel so indifferently
- is because roadways are so well designed
and constructed.
There are lots of ways to build a road.
From yellow bricks to rainbows to simple dirt
and water, the combinations of materials and
construction techniques are practically endless.
And yet, across the world, there’s really
one design that makes up the vast majority
of our roadways.
It consists of one or more layers of angular
rock called a base course and then a layer
of asphalt concrete (also called blacktop
or tarmac).
It turns out that this design strikes the
perfect balance between being cost-effective
while creating a smooth and durable road surface.
But, asphalt roadways aren’t invincible,
and they do suffer from a few common problems,
one of those being potholes.
The formation of a pothole happens in steps.
And the first of those steps is the deterioration
of the surface pavement.
Asphalt stands up to a lot of abuse.
Exposure to the constant barrage of traffic
in addition to harsh sunlight, rain, snow,
sleet, and freezing weather will eventually
wear down any material, no matter how strong.
When that happens to asphalt, the first sign
is cracking.
They might seem innocuous, but cracks are
the Achilles heel of pavement systems.
Why?
Because they let in water.
And not just let it in, but let it come back
out as well.
A hole is a lack of substance or material.
It’s the only thing that gets bigger the
more you take away.
If you started without a hole and now you
have one, that material had to go somewhere.
In the case of a pothole, the material is
the soil below the road (called the subgrade),
and where it goes has everything to do with
water.
As water finds its way into cracks and below
the pavement, it can get trapped above the
subgrade.
Eventually, these soils get waterlogged, softening
and weakening, and then the traffic shows
up.
Cars and trucks are heavy, and they pass over
the road at rapid speeds.
Because of this, traffic is just a generally
destructive environment.
It’s a lot for any road to stand up to,
let alone one that’s waterlogged and weakened.
Asphalt is called a “flexible pavement”
because it doesn’t distribute these loads
across a large area like something more rigid
would.
So, every time a tire hits this soft area,
it pushes some of the water back out of the
pavement.
That water carries particles of soil with
it.
This is a slow process at first, but every
little bit of subgrade eroded from beneath
the pavement means less support, and less
support means more free volume below the pavement
for water to be pumped in and out by traffic.
This, in turn, creates more erosion in a positive
feedback loop.
Eventually, the pavement loses enough support
that it fails, breaking off and crumbling,
and you’ve got a pothole.
Of course, this whole process is made even
worse in climates with freezing weather.
Water expands when it freezes, and it does
so with tremendous force.
Thin layers of water between pavement and
base freeze and grow into formations called
lenses.
When those lenses thaw out, all the ice that
was supporting the pavement goes away, creating
voids.
In addition, the lower layers of soil stay
frozen, trapping that meltwater between the
pavement and the subgrade and accelerating
the erosion.
Potholes exist everywhere you have asphalt
concrete roadways, but they’re worse in
areas with cold climates and much worse in
the spring as the ground begins to thaw.
They’re annoying, yes, but they’re not
just that.
Potholes cause billions of dollars of damage
to tires, shocks, and wheels of vehicles.
Even worse, they’re dangerous.
Cars swerve to miss them, sometimes at high
speeds, and if a bike, motorcycle, or scooter
hits one, it can be bad news.
So, roadway owners spend a lot of time and
money fixing them.
There is a large variety of types of pothole
fixes depending on the materials, cost, and
climate conditions.
But, they all mostly do the same thing: replace
the soil and pavement that was lost and (hopefully)
seal the area off from further intrusion of
water.
That second part is obviously critical but
much harder to do.
A pothole repair is a bandage after all, and
it doesn’t always create a perfect connection
to the rest of the roadway.
This is why, even after they’re repaired,
potholes seem to recur in the same location
over and over again.
After understanding how these annoying and
sometimes damaging defects occur, the next
logical question is, how do we prevent them
in the first place.
Obviously, we could build our roadways out
of more robust and more durable materials.
Many highways are paved with concrete for
this exact reason.
But, roads are unusual in that even a tiny
change in design has a significant overall
impact on cost.
Choosing a pavement that’s even just a centimeter
thicker could mean millions of tons of additional
asphalt because that centimeter gets multiplied
by a vast area.
So, we balance the cost of the original pavement
with the expense of maintaining it over its
lifetime.
In the case of asphalt pavement, that maintenance
primarily means sealing cracks to prevent
intrusion of water.
If you can do that and do it regularly, you
can extend the life of asphalt pavement for
many years.
Since roadways are mostly public infrastructure,
their condition (at least to a certain extent)
reflects the importance we all place on vehicle
travel.
In the broadest and most general sense, we
choose potholes by choosing how much tax we
pay, how much of those taxes we’re willing
to budget toward streets, and how large and
how many vehicles we drive over them.
Pavement is one of the highest value assets
owned by a City, County, or DOT.
It’s essential, and it’s expensive, which
means there’s an entire industry surrounding
how to design, build, and maintain roadways
as safely and cost-effectively as possible.
Politicians, government officials, engineers,
and contractors drive on the same roads as
everyone else, so they all have a vested interest
in keeping those roads as pothole-free as
possible so that we all can enjoy the luxury
of driving on paved streets in safety and
comfort.
In this video I mentioned that a lot of engineering
decisions come down to comparing costs and
benefits over the lifetime of a structure.
Interestingly enough, that type of economic
analysis isn’t a major component of traditional
engineering education.
In fact, just about every job out there requires
skills that, for whatever reason, aren’t
covered in school.
Today's sponsor, Brilliant, is a problem-solving
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