Traffic Loading and Strength of the Subgrade are critical to determining the design thickness of the pavement
By following a few basic principles of pavement design, one can ensure that a pavement will be strong and built to last. Asphalt pavements are made of an aggregate base layer and an asphalt pavement layer. The thickness of each layer is critical to resist the expected traffic loads and protect the subgrade. Therefore, asphalt pavement design is the process of determining the proper thicknesses of the aggregate base and asphalt pavement layers by defining the expected traffic loading and strength of the subgrade materials.
As vehicles use a pavement, the weight of the vehicle causes asphalt pavements to flex underneath the wheels. Each time the pavement flexes, a very small amount of damage is caused. Heavier vehicles cause slightly more flexing and, therefore, more damage to the pavement. The size and frequency of vehicles using the pavement being designed should be considered when selecting the traffic loading category.
Trails – Pedestrian walking paths
Parking Lots – Car parking lots with some light trucks
Medium Duty – Some large trucks (ex: Loading Dock Area)
Heavy Duty – Frequent large trucks (ex: Distribution Center)
The strength of the subgrade can be measured or it can be estimated from the type of subgrade material. The DCP is a tool used to measure the strength of the subgrade by driving a metal rod into the ground and measuring the depth, which indicates resistance and can be used to determine strength.
- Soft – Clays (DCP: < 1 blow/inch)
- Medium – Silty loams (DCP: 1-2 blows/inch)
- Firm – Silty sands (DCP: 3-4 blows/inch)
Granular Equivalent (GE) is a concept for relating the strengths of different materials. 1 inch of Class 5 Aggregate is equal to 1 GE. Yet, 1 inch of asphalt pavement (which is much stronger than Class 5 Aggregate) is equal to 2.25 GE. Therefore, to design your pavement, you must select and modify the thicknesses of your Asphalt and Class 5 Aggregate layers until the sum total GE of your pavement design is greater than Minimum GE from the table below.
Pavement Design Example
A loading dock area (Medium Duty) with a Clay Subgrade (Soft Subgrade).
By using the graph above, with a Medium traffic loading category and a soft subgrade, the minimum Granular Equivalent will be 23.0 for this pavement example.
4 inches of asphalt = 4 x 2.25 = 9.0 GE
8 inches of Class 5 = 8.0 GE
Design GE = 9.0 + 8.0 = 17.0 GE < 23 GE Required
5 inches of asphalt = 5 x 2.25 = 11.25 GE
12 inches of Class 5 = 12.0 GE
Design GE = 11.25 + 12.0 = 23.25 GE > 23 Required GE
Pavement Design #2 will Support the Traffic Loading!