The purpose of any pavement, including asphalt pavement, is to transfer loads from the top surface of the pavement to the subgrade below. A proper asphalt pavement design should also provide a smooth surface for traffic and protect the subgrade from moisture intrusion and deformation. Asphalt pavement is typically constructed in layers, which include:
- Subgrade Soils: Subgrade soils are the foundation for asphalt pavement and must be prepared to receive the layers above. Considerations for the subgrade include proper compaction, moisture control, and consistency of the soil characteristics.
- Base Materials: Above the subgrade are one or more layers of “base” materials, usually compacted gravel or crushed stone, which may or may not be treated with various soil stabilizers including small amounts of asphalt or Portland cement. If two layers of base materials are used in a particular pavement cross-section, they may be referred to as the base and sub-base.
- Courses: On top of the base material layers are courses of asphalt concrete, usually applied in two layers. In modern roadway paving, these are referred to as the wearing (or surface) course above a binder (or base) course. The wearing course usually consists of a higher asphalt content and finer aggregates than the binder course. The wearing course is then prepared with a sealant to further prevent water intrusion, and may be painted with any required surface markings.
The amount or depth of these layers depends on the loads that need to be carried as well as on specific site conditions; depths range from a couple of inches to several feet. Proper drainage is an important part of the design since water intrusion can compromise any of these pavement layers over time. If all layers above the subgrade are treated with, or contain asphaltic cement to any degree, the design is referred to as full depth asphalt pavement.
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The YouTube video above shows asphalt pavement being installed
