We drive and walk over it every day, but few understand exactly what asphalt is, or how it’s laid down. It’s more than just tar and gravel. Asphalt paving takes a lot of precise calculations and experience to make sure it’s done right.
Whether it’s a brand new road or car park, or replacing one that’s seen better days, the area to be paved is prepared first. This means assessing the ground and formulating how much traffic will use the area over a set amount of time. Knowing this helps the experts calculate the right type and amount of asphalt to use, as well as how the area should be prepared.
After the math is done, the real ground work begins. For new areas the existing soil is typically graded, compacted, sloped, or otherwise altered so it can hold the asphalt. Objects intended to sit below or inside the pavement, such as drains or sewer access, are installed too.
For projects replacing old asphalt, the unwanted pavement is demolished or removed. How much is removed depends on the condition of the existing structure, with some projects only shaving off a few top layers and others removing the entire structure. The removed asphalt is usually recycled. After demolition and cleaning, the soil is prepared in much the same way as a brand new area would be.
Much like a house needs a solid foundation to stand, asphalt needs a good base too. Also referred to as the “sub-base”, this is an essential thick layer that provides support to the pavement. It also helps prevent damage from extreme temperature changes, which can cause asphalt to crack. The thickness of the foundation is determined by the stability of the underlying ground and how much traffic the area is projected to endure.
Some pavement companies do a test roll to make sure the foundation is appropriate. Testing helps assure that the incoming pavement will have a solid base. Testing can be done in numerous ways, but often involved rolling a heavy load vehicle over the foundation.
As the roll is conducted, workers watch for too much flexion in the base and look for any cracks. Repairs or changes are made afterward. Sometimes tests and alterations are done multiple times, until the base is perfectly balanced.
Paving usually requires several layers depending on the project. These layers consist of the actual asphalt and a binding layer. The binder is the “muscle” of the asphalt. It’s what sticks layers of asphalt together. Binders are typically made of a mixture of asphalt and tacky oil.
The aggregate, or the “rough stuff”, is typically larger in the binder than it is the top asphalt layer. This is because larger aggregate provides a better hold between layers. Conversely, the asphalt you actually drive or walk across contains smaller aggregate, which gives a smoother, fine feel.
As these layers are laid down by equipment called a paver. It makes sure the asphalt is the proper thickness and width. A compactor follows the paver and stamps down the layer to make sure it’s the right level of compactness.
A final roll is usually conducted with a roller truck. The truck makes sure there are no bumps, cracks, or other blemishes. If all of these steps are followed correctly, the new pavement should be smooth and long-lasting.