Roads seem to be just a flat, black sheet of asphalt, but they are a very complicated structure that is engineered. There is a reason for every layer of the road, and all of them serve purposes for supporting heavy vehicles and lasting through years of weather. Engineers and specialists in asphalt and subbase construction don’t you in the way of thinking about layers so that a road can last for decades with little or no repairs.
Knowing about these layers can help understand why good road construction a science and an art is, as it takes a craftsman to combine materials and methods that will work when they are used together.
Subgrade Explained
The lowest of the layers of a road is called subgrade. This is a natural earth, or sort of material that the whole road will sit on and becomes more critical than most individuals think. Before any asphalt or gravel can be put on to the “finished” roadway, the soil has to be tested for strength and compacted to the optimal point it can take heavy traffic. If the soil is not strong enough, it can shift or sink creating cracks and potholes in the finished roadway. For deeper understanding, click this page.
In some situations, contractors will use different stabilizing materials to the subgrade, such as crushed stone, lime or cement, to provide and stabilize the earth. This is especially useful to stop low levels of settling, especially if there is clay or sandy soil. Good conditioning of the subgrade when it is a proper full first line of defense for protecting the ground from acting as a firm base for everything that goes above it. Any road built, will not last long term in any condition without a proper subgrade, thus it will not matter how well the upper layers were built up.
- Compact and tested soil in its natural state
- Can be lime, cement, or gravel stabilized
- The foundation for every other layer
Base and Binder Layers
Once the subgrade is established, the base layer is established on top of it, usually consisting of crushed rock or gravel. The base layer is thicker than most people think, and it plays a big part in strengthening the road. The base layer is also responsible for moving water away from the road to help with stability against the freeze-and-thaw cycles. Establishing a solid base is critical to highways, industrial parks and any location used every day by heavy trucks or buses.
The binder layers goes on top of the base layer and consists of coarse asphalt mixed with more coarse aggregate. The binder layer provides a cushion for spreading vehicle weight and adds flexibility in the overall pavement system. The binder layer connects the stable base to smooth surface layer and acts as a proper conduit for ensuring the complete structure acts as one element. Many projects by the road construction company Nashville include these critical steps for performance and durability. If the base layer and binder layer aren’t installed properly, the difference could be a road that last thirty years or one that needed repairs in three years.
The Asphalt Surface
The asphalt surface is the final layer. This is the actual surface that you see and drive on. It consists of fine aggregate, sand, and asphalt cement, all cooked together at high temperatures to form a sturdy material. The top layer is designed to be smooth, waterproof, and to provide traction for the vehicles that drive over it. This layer is also able to withstand the daily loads of braking, starting, stopping, and turning; some other materials would fail under such stress.
Check out https://www.sciencedirect.com/topics/engineering/asphalt-surface to learn more.
Some roads get special treatments such as sealcoating. Sealcoating protects the underlying asphalt from sun, water, and oil exposure. Sealcoating enhances both durability and the dark black color of fresh asphalt. Any painted markings, such as striping in parking lots or lane dividers, also get painted at this stage in the construction process. These painted markings are there to help direct traffic safely. Without the top layer, the even the subgrade would fail in a matter of months due to water, sun, and constant loading and unloading of traffic.
How Layers Work Together
Each layer of a road performs a job and contributes to the overall system. The subgrade is the foundation. The base holds the weight of the traffic and drains water. The binder is able to absorb the stresses of the weight of the vehicles driving over, and the surface provides a smooth driving experience. Any single layer failing can create rapid deterioration of the whole structure. That is why paving contractors will put a ton of focus on performing each step correctly.
If you think of each layer being part of a building, you can visualize the system:
- Subgrade = foundation
- Base layer = beams and flooring
- Binder = insulation and supportive structure
- Asphalt surface = roof and outside finish
The system of layers also allows for the expansion and contraction of the materials due to changing weather. Each layer can expand and contract during freezing winters or hot summers without separating. Engineers take great care in the design of road layers to balance strength, drainage, and some level of flexibility in the system.
Why Roads Last for Decades
Quality roads can last 20 to 30 years before major maintenance is needed, given it some minor and major maintenance along the way. Road’s strength is from the layer of each section that creates a system that resists cracking and rutting. Regular maintenance = no cracking = much longer life service of roads! Roads may be surfaced, cracks filled, sealed, or stripes painted to prolong service life.
Roadways include more than just asphalt; they are engineered systems that balance strength, drainage, and durability for many years. From small subdivision driveways to large highways, the science behind the layers of a roadway system keeps traffic moving smoothly for decades. The experience planning and permitted by paving professionals continues to work for communities for years to come, as quality construction is a worthwhile investment that pays off year after year.
