If you own a property with an asphalt parking lot, or you are in charge of the maintenance, knowing the basics of asphalt can save you or your client thousands of dollars down the road. (Get it? Down the road)
The surface course is the top layer of asphalt shown in the photo to the right. The thickness will be dependent on the intended use of the asphalt.
The base course is the middle layer, typically consisting of crushed limestone that lies under the asphalt layers, or surface course. During the installation, base thickness, base stability, and compaction are essential steps. If the sub-base is not appropriately compacted, the asphalt surface on top will not provide years of durability.
The sub-grade is the soil underneath the base course. In some instances, that compacted soil may be an adequate sub base. In other cases, it may be necessary to amend the soil or install a geo-grid prior to installing the base course. This layer would be called the sub-base.