Of all the ubiquitous things in our environment,
roads are probably one of the least noticed. They’re pretty hard to get away from, and
yet, most of us don’t give much consideration for how they’re made. Turns out, there are a lot of ways to make
a road. Not to get too philosophical, but there’s
really no right answer to what a road even is. How much improvement of the ground is needed
before it stops being just the ground and becomes a road? Depending on the capabilities of your vehicle,
sometimes not much. Over the years, the demands on roadways have
increased as more people and goods are on the move. So, the designs have evolved alongside. The Romans were famous for their stone-paved
roads, many of which still exist a couple of thousand years later. In modern times, the design of pavement has
converged significantly. The vast majority of roadways worldwide, if
they’re paved at all, are paved with one material. Hey, I’m Grady, and this is Practical Engineering. On today’s episode, we’re talking about
asphalt concrete for roadways. This video is sponsored by HelloFresh, America’s
#1 Meal Kit. More on that later. When you hear the word concrete, asphalt isn’t
the first thing you think of. In fact, in some ways, it’s the opposite
of what we traditionally know as concrete. But we engineers can be pedantic, especially
when our designs can affect public safety. When the cost of making a mistake is severe,
it’s super important that communication is crystal clear. The strict definition of concrete is essentially
rocks plus a binder material. For the hard grey concrete, we’re all familiar
with, that binder is portland cement. I’ve done a whole series about this kind
of concrete, so take a look in the back catalog after this if you want to learn more about
it. And in fact, we do use cement concrete as
pavement for roadways. It is really hard and really durable, akin
to those Roman roads I mentioned in the intro. You’ll mostly see concrete used for pavement
on highways with lots of truck traffic because it can withstand these forces much better,
and it lasts a lot longer than other types of pavements. But, concrete isn’t the ultimate solution
for roadway surfaces. It’s harder to repair because it takes a
long time to cure, extending the duration of road and lane closures. It’s not as grippy, so it has to be grooved
for traction with tires. It’s not flexible, so it cracks if the ground
settles or shifts. And most importantly it’s expensive. Even when you compare lifecycle costs, which
include the fact that concrete lasts longer and requires less maintenance over time, it
often still comes out less cost-effective. So, luckily other materials can bind rocks
together, the most prevalent by far of those being asphalt. Asphalt concrete just ticks so many of the
boxes needed for modern roadways: It’s easy to construct.The materials are readily available. It provides excellent traction with tires
without needing grooves. That means it’s relatively quiet, which
can matter a lot depending on the location. It’s flexible, so it can accommodate some
movement of the subgrade without failure. It’s also easy to fix and ready to drive
on almost right after it’s placed. This is why so many of our roadways use asphalt
concrete for pavement. But what is it? On the one hand, it’s a straightforward
question to answer because asphalt concrete really only has two ingredients: rocks (known
as aggregate in the industry) and asphalt, also sometimes called bitumen. The asphalt is a thick, sticky binder material
that is occasionally found naturally occurring but most often comes from the refinement of
crude oil. On the other hand, the answer to the question
of what is asphalt pavement is much more complicated. The science of pavement is huge because the
pavement industry is huge. The average person makes several trips to
various places on a given day by car, bike, or public transportation, and all those vehicles
need roads. We collectively spend tremendous amounts of
money on building and maintaining roadways each year. It might not seem like it, but we ask a lot
of our roads: we want them to be stable and durable, resistant to skidding, impermeable
to water intrusion, and we’d like it if they were quiet to boot. Accomplishing all this in various geographic
regions with different material availability, varied climates and weather patterns, and
different types of traffic is next to impossible. That’s why, just like cement concrete, the
mix design of asphalt can be pretty complicated. You might think rock is rock, and asphalt
is the same as any other refined residue from the crude oil refinement process. But you’d be wrong, and if you go to just
mixing any old aggregate with any old bitumen, you could end up with a pavement that doesn’t
work very well as a roadway surface. The only way to know for sure is either to
mix the same materials in the same proportions as some previous mixture that you know was
successful or by testing a bunch of small batches with different blends of materials. In the U.S., we’ve combined both of those
processes into a system called Superpave, which provides guidelines for the qualities
of materials and various testing needed to mix up a successful and high-performance batch
of asphalt concrete. But, even once you get the rocks and binder
right, there’s more to the mix. We include a wide variety of additives that
can extend the life of pavement by improving various properties of the asphalt. Polymers, hydrocarbons, and even recycled
tires get added to the mix to help with fatigue resistance, reduce sensitivity to moisture,
and, most importantly, help a pavement perform better at extreme temperatures. This is because, unlike cement concrete that
goes through a chemical process to cure and harden, asphalt is the same stuff when you’re
installing it as it is when you’re driving over it. The only difference is its temperature. This is a graph of the viscosity (or stiffness)
of asphalt over a range of temperatures. You can see that the hotter it gets, the less
stiffness it has. Most asphalts used in roadways are known as
“hot mix” because you have to get it hot for it to be workable enough to mix, transport,
place, and compact. As it cools down, the asphalt gains stiffness
that makes it strong and durable against traffic. But, when it gets too cold, asphalt can also
get too stiff. Without the ability to flex under the weight
of traffic, it can begin to crack apart. Those cracks reduce the life of the pavement,
but they can cause worse problems by letting in water that can soften and weaken the base
and subgrade materials beneath. In that same vein, on warm sunny days, the
asphalt can get too soft, leading to ruts and deformation of the pavement. Ideally, the road surface would maintain a
single stiffness across all expected temperatures and only become soft and workable at the temperatures
used to place it. Additives and mix design help get us closer
to that ideal performance. The other way we have to improve the serviceability
of pavement is to make it thicker. Asphalt is considered a flexible pavement,
which means exactly what it sounds like. Instead of distributing loads over a large
area as a concrete slab would, it relies on the strength of the base course below it,
which is usually a layer of crushed rock that sits on top of the subgrade. Choosing the thickness of the base course
and surface pavement is mostly a question of economics. You can estimate how long a pavement will
last based on the strength of the subgrade soils and how much traffic you expect. Then it’s just a matter of balancing the
initial cost of installation vs. the costs associated with maintenance and, ultimately,
replacement. Of course, there’s a lot more that goes
into it, which is why we have transportation engineers. It’s also why we have weight limits. Roadways have to be designed to withstand
the heaviest traffic that passes through. It’s not worth all the extra cost to build
our highways for the occasional gigantic truck that might come along. So, instead, we say “sorry” and cap the
maximum weight at something that can accommodate most truck traffic without breaking the bank
to construct. It’s just like a weight limit on a bridge,
but if you break the rules, it doesn’t lead to spectacular failure, only accelerated deterioration
of the roadway. But what do we do when the road does start
to break down? There are lots of ways to rejuvenate asphalt
pavement without full-depth replacement. One option, called a chip seal, involves spreading
a thin layer of tar or asphalt onto the roadway and then rolling gravel into it. This helps seal cracks and fill in gaps for
a very low cost, but it does make the road rough and loud and can leave a mess of loose
rocks and tar if not applied well. Most pavement rehabilitation takes advantage
of asphalt’s most interesting property: it is nearly 100% recyclable. In fact, asphalt concrete is the world’s
most recycled material. As I mentioned, asphalt doesn’t go through
a chemical reaction to cure. We only use temperature as a way to transform
it from a workable mix to a stable driving surface, and that process is entirely reversible
and repeatable. Many of the roads you drive on every day probably
came, at least in part, from other nearby streets or highways that reached the end of
their life. We even have equipment that can recycle pavement
in place, minimizing interruptions of traffic and the costs of hauling all that material
to the job site. We don’t usually recognize the incredible
feat that roadway engineering is. We notice the ruts, potholes, cracks, and
endless orange cones. We see an ancient Roman roadway that lasted
over a thousand years and think “They just don’t build things like they used to.” But we also drive heavier trucks than we used
to. Our roads see tremendous volumes of traffic
and withstand considerable variations in weather and climate, and they do it on a pretty tight
budget. That’s really only possible because of all
the scientists, engineers, contractors, and public works crews keeping up with this simple
but incredible material called asphalt. If you’ve been watching my channel for a
while, you’ve seen me try to cook while my wife tries to capture that on video. “Yeah show the people what they want.” I hope you don’t mind if I take a minute
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you think.
I have irrationally hated asphalt since I was a child. Went to Engineering school and came to hate asphalt even more.
This video made me actually kinda be OK with it.
he makes such great videos
An asphalt mill might be the most impressive machine I've ever seen in person. A machine that can, with relative precision, mill/chew/crush anything in its path truckloads at a time. It can be run by pretty much any trained heavy equipment operator, it's transportable in just a few hours, serviceable in the field, and lasts for decades.
The guys from solar roadways should watch this video and learn from it.
This is also part of the reason the UK has so many disused railways (often bike tracks now).
During the war, we spent years with no way to attack Europe except from by air. The UK spent several manhattan projects worth of money on strategic bombing. So we built a load of asphalt/tarmac runways.
After the war, we had a lot of asphalt laid down and an enormous number of people trained to lay it down. So lots of this asphalt was recylced into the new motorway network.
And people thought this was the future, doc beeching closed the railways etc. etc.
source: the thread where A Blunted Sickle was posted
Since asphalt is basically long-chain hydrocarbon slime that doesn't have any other particularly good purpose after refining out all the other more useful stuff, making roads out of it seems to be more of a way to conveniently dispose of the gunk.
When it wears out, it's because the surface layer is slowly oxidizing and turning to ash, and then becomes brittle and cracks. Asphalt probably contributes to a small percentage of global warming, from all the road surfaces around the planet constantly oxidizing.
It will be interesting to see what happens if we reach a point with solar and wind power, where extracted fossil fuels are not used so much anymore, we are making "solar fuels" from excess electricity, and we don't have all this waste slime sitting around to make roads.
Every time one of these drops, its a karma race to post the link on this sub
He just casually throws in footage of a Delorean driving by. Roads? Where we're going, we don't need roads.