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. Today, we’re talking about asphalt concrete for roadways.

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. 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. When viewing 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.