The qualities that make roadways strong and safe are more than skin deep. Although asphalt surfaces appear to be simple, there’s a lot going on underneath what we can see that contributes to the durability, performance, and sustainability of roads and highways. A road actually consists of multiple layers, each one of which serves a critical role in ensuring the integrity of the surface.
Simply laying down a layer of hot mix asphalt on top of the ground is far from enough to build a road, as there wouldn’t be enough to support it, hold it together, and prevent it from crumbling under the weight of even the lightest traffic. From the subgrade all the way up to the sealing coat applied to the asphalt, a roadway is like a sandwich piled high with toppings. But how many layers of asphalt for road construction are necessary? How many layers is the typical roadway made of, and what purpose does each serve? Here at Rock Solid Stabilization & Reclamation, Inc., we have helped complete countless successful road construction projects over the years, giving us first-hand knowledge of everything that goes into building a strong, stable roadway beyond just what you see.
What is Asphalt Pavement?
Consisting of aggregates coated with bitumen, asphalt is one of the most commonly used materials used for road construction. Bitumen is a petroleum product made from crude oil that helps the aggregate stick together and cures to a relatively solid consistency. Unlike concrete, asphalt is what’s known as a flexible pavement. This means that it gives slightly when under pressure, making it more effective at supporting heavy traffic without cracking or settling. This is because the surface’s relative flexibility means pressure is distributed more evenly than with a rigid material such as concrete.
Peeling Back the Layers of Asphalt Paving
A typical road made with an asphalt surface actually consists of several different layers of materials underneath it, as well as some on top of it that aren’t immediately noticeable. Here is a rundown of each layer in asphalt pavement, what they’re made of, and the function they serve, from the bottom all the way up to the top:
Subgrade
The subgrade is the very bottom layer of a roadway, and it consists of compacted natural soil to provide a stable and level foundation for all the layers to follow. Compacting the soil for the subgrade layer is especially important because without that step, there’s a much higher risk of the ground shifting, settling, or buckling during freeze-thaw cycles. That can lead to cracks, potholes, and other defects in the surface of the road over time.
Sub-Base Course
Made from crushed stone and gravel primarily, the sub-base course is the portion of the roadway that bears much of the weight of traffic. It also serves a critical purpose as a barrier between the top and bottom layers to prevent groundwater from seeping through to the surface of the road, as well as allowing water that pools on the road to seep down past the surface before being absorbed into the subgrade.
Base Course
On top of the sub-base course goes the base course, which is also made of crushed stone but is built to be stronger than the layer beneath it. Its role is to bear heavy loads and distribute them equally before they reach the bottom layers. Without this layer, roads may become cracked and uneven.
Prime Coat
A critical step before the asphalt is actually applied to the roadway is the addition of a prime coat. This is a thin layer of oil or other liquid such as watered-down bitumen that serves the purpose of helping the asphalt layer adhere to the base course underneath it. This is critical because otherwise the top asphalt layers may be prone to shifting or coming loose under heavy wear and tear.
Binder Coat
This layer typically takes the form of a mixture of thick tar and stone. The purpose of the binder layers is to provide an added layer or protection to help everything hold together more effectively and prevent cracking or shifting.
Tack Coat
The tack coat is a thin layer of asphalt mixtures applied on top of the binder course. This is done to help hold the surface course onto the layers beneath so it doesn’t move as force is applied to it.
Surface Course
Once all those underlying layers are applied, it’s now time for the final layer of asphalt to be laid down. This is what’s known as the surface course, and it’s the layer that we see on the road. The asphalt used for this layer features smaller stones mixed with bitumen to provide the smoothest possible surface. Because it is porous, it allows rainwater and melting snow to seep down into the underlying layers without pooling on the surface layer long enough to create hazardous conditions.
Seal Coat
The last thing to be applied onto the surface of an asphalt road is the seal coat, which is exactly what it sounds like. This is a very thin layer of bitumen that is sprayed over the surface course to prevent cracks from developing. In some cases, this coat may consist of aggregate as well as bitumen.
Determining How Thick Asphalt Layers Should Be
It should stand to reason that the more weight a road is expected to support, the thicker the pavement layers of asphalt that constitute it should be. The exact measurements may vary depending on the typical traffic or other conditions, but there are some general rules of thumb that apply to asphalt pavement. In terms of the asphalt layers themselves, these can be anywhere between four to eight inches altogether. Specifically, the road surface course that traffic drives upon is usually no more than two inches thick, and no less than 1.5 inches thick. The base layers on top of the compacted subgrade generally are between six and 12 inches in depth.
Work with Rock Solid for Stable Subgrades
No matter how many layers of asphalt you put down, they won’t stand much of a chance without a stable subgrade supporting them. That’s why so many builders work with Rock Solid Stabilization & Reclamation, Inc., to provide them with expert services. Our expertise when it comes to soil improvement and stabilization gets road construction projects off on the right foot, with strong lower layers that are capable of holding up under heavy loads and ensuring asphalt pavement lasts for as long as possible.
Our experience with value engineering makes us a valuable partner who can be brought on early in the planning process. With our experts working alongside other members of the project team, we can help them root out potential concerns that could have a negative impact on the longevity and/or stability of the finished roadway in the future. We employ the latest and most effective methods and technologies, ensuring that our clients achieve the strongest possible results.
If you’d like to learn more about everything Rock Solid brings to the table, check out some of our past projects. To get started with adding our expertise and know-how to your next road construction job, reach out and speak with one of our knowledgeable team members today to discuss your needs and get a quote.