1 Wearing Course
This is the top layer that’s designed to provide a smooth riding and driving surface with enough abrasion to provide friction to our vehicle’s wheels for good traction, steering, braking ability. As the name suggests, this top layer is expected to wear and but can be restored many times over the lifetime of the road.
2 Base Course
This is a layer of asphalt that gives the top layer a firm base and takes most of the stresses given to the road by the traffic above. This layer’s thickness varies depending on the location and purpose of the road and the stresses it’s been designed to take. Some road failures begin here when anticipated traffic and loads are higher than those predicted.
This is a layer of granular material, usually compacted gravel, that divides the softer subgrade from the rigid base course. It serves several functions including acting as a drainage layer and general water manager, as well as a shock-absorber for the continuous stresses on the asphalt layers above. Serious road surface problems begin when cracks in the top layers allow too much water to get to this layer.
The first layer of a road that sits on raw earth and allows for some movement to help keep the layers solid above. This is the most important layer to the long-term success of the road. If it fails, the entire road will need to be rebuilt at substantial cost to taxpayers.