What are roads made of? A pavement materials engineer explains the science behind the asphalt you drive on

What Are Roads Made Of? A Pavement Materials Engineer Explains the ...

While on the road, you’re probably thinking more about your destination than the pavement you’re driving over. But building roads requires a host of engineering feats, from developing the right pavement materials to using heavy equipment to lay them down. The better they’re built, the longer roads last and the fewer construction delays drivers have to endure.

I am an engineer who does research on materials used in roads. Scholars in my field are working to develop materials that can make roads stronger and last longer.

Road materials

So, what are roads really made of? The simple answer is that they are made of typical construction materials such as aggregates – soils and rocks – as well as asphalt binder and Portland cement, which act like glue to bond it all together.

Asphalt binder is refined from crude oil. From crude oil, refiners first extract gasoline, kerosene and oil, and what remains at the bottom becomes the asphalt. Portland cement is manufactured using several different ingredients, including limestone, sand, clay, silica and alumina.

Engineers compact the mixture of asphalt binder and aggregates together at an elevated temperature, about 300 degrees Fahrenheit (150 degrees Celsius), which glues the aggregates together into the final product, called asphalt concrete.

If they’re using Portland cement rather than asphalt binder to glue the aggregates together, the engineers cure the mixture of the cement and aggregates with water through a process called hydration.

Hydration bonds the cement to the aggregates to make the product, called Portland cement concrete, stronger. With this process, there’s no external heating involved.

Pavement structure

Asphalt concrete’s pavement structure typically has three main layers: the base layer, the intermediate layer and the surface layer.

A diagram showing five distinct pavement layers, including the surface, intermediate and base layers of the concrete, and then the sub-base and subgrade.

The layers that make up pavement.
Mansour Solaimanian

Engineers call the existing ground where the pavement goes the subgrade. On top of the subgrade goes a new layer of unbound soil and stone, where the aggregates aren’t glued together. This is called the subbase, or unbound aggregate base.

The base layer can be either stones packed together without any binding agent or a combination of stone and asphalt binder.

Once road builders make the base, it is time to build the asphalt concrete layers: the base layer, the intermediate layer and the surface layer. All these layers contain the aggregates – the pieces of rock and sand – glued together with the asphalt binder in some way.

Engineers determine how many layers to build and how thick to make each layer by figuring out how much traffic will drive over the road. The more traffic, the thicker the pavement needs to be. For example, on interstate highways, the depth of the layers combined could be 20 inches (51 centimeters) or more.

A machine drives over dark pavement.

The asphalt concrete base layer is placed and compacted by a paver.

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