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  • 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 ashot mixbecause 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 saysorryand 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 thinkThey 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.

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  • Thank you for watching, and let me know what you think.

Of all the ubiquitous things in our environment, roads are probably one of the least noticed.

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