How thick should your asphalt pavement be? The answer to that question depends on the anticipated traffic load the pavement will handle once it’s installed. But, to determine an ideal thickness of the actual asphalt layer, there are a few variables you’ll need to consider in addition to the pavement’s intended use.
The strength of the subgrade layer, as well as the thickness of the aggregate base layer, are critical considerations in the overall pavement design process. These two factors, along with your anticipated traffic load, will ultimately determine just how thick your asphalt should be.
Read on to learn what you need to know to determine an appropriate thickness for your pavement.
Traffic Loading and Your Pavement
When vehicles drive on your asphalt pavement, they cause a small amount of damage to the asphalt since their wheels force the pavement to flex under their weight. Whenever the pavement flexes, it incurs damage (however slight) because it can’t handle traffic load all on its own.
To mitigate that damage and provide strength to ensure the pavement lasts, the asphalt layer needs an aggregate layer beneath it to help support the weight of the vehicles that will ultimately drive on it. Having a solid understanding of the pavement’s anticipated traffic load is critical for determining appropriate asphalt thickness.
Here are the traffic use categories you should be familiar with:
- Trails – pedestrian walking paths, golf cart paths, and playgrounds fall into this category.
- Light Duty – Less than 200 vehicles per day, this category typically includes parking lots with less than 500 spaces and residential roadways.
- Medium Duty – This category could have up to 4,500 vehicles per day and include parking lots with more than 500 spaces, areas where heavy vehicles are moving around, and loading docks.
- Heavy Duty – Included in this category would be more than 9,500 vehicles per day, many heavy vehicles are using the space and include distribution centers, industrial roads and airports.
These classifications are based on an average pavement lifetime of 20 years, and “heavy trucks” refers to vehicles that have two or more axels and six or more tires.
The Importance of Subgrade Strength
The load-supporting capacity of the subgrade — the layer beneath the aggregate and asphalt layers — is one of the most important factors that helps determine the appropriate thickness of the asphalt itself.
This layer must support construction equipment during the pavement installation, and it must also serve as the load-distributing foundation of the pavement after the asphalt installation is complete. As such, the strength of the subgrade must be evaluated thoroughly before the structural pavement design process can begin.
To determine subgrade strength, you can either estimate it from the type of subgrade material (soil) that’s there or use a DCP (Dynamic Cone Penetrometer) tool to get a more accurate measurement. This tool measures subgrade strength by assessing soil resistance. The tool is designed to penetrate the ground and the depth of the penetration indicated resistance and is then correlated to soil strength.
Subgrade comprised of sandy soil will generally be capable of supporting greater loads than clay soils, for example. If the subgrade material is unsuitable for handling the anticipated traffic load of the pavement, the asphalt layer will need to be thicker to accommodate the load.
Aggregate Base Layer
The thickness of the aggregate base that lies between the subgrade layer and the asphalt layer is another critical factor in determining the appropriate thickness of the asphalt. This layer is composed of crushed rock and serves to distribute load stresses through the full depth of the pavement.
The appropriate thickness of this base layer is directly related to the strength of the soft subgrade beneath it. The softer (and therefore, less stable) the subgrade layer is, the more aggregate will be necessary to help the asphalt support the load.
Subgrade, Aggregate Base, & Asphalt: Tying it All Together
Once you’ve figured out the strength of your subgrade, you can start working out the appropriate thickness of the aggregate and then the thickness of the asphalt itself. But to work those figures out, it’s important to consider the strength of the aggregate layer, as well as the strength of the asphalt, which is measured in granular equivalence, or GE.
When the granular equivalents of the aggregate base layer and the asphalt layer are added together, the resulting sum must be greater than the minimum granular equivalent required for the pavement’s anticipated traffic load.
So, what’s the GE of aggregate and asphalt? One inch of Class 5 aggregate is equal to one GE, while one inch of asphalt pavement is equal to 2.25 GE. Given these figures, you can begin selecting various thicknesses of aggregate and asphalt and modifying the numbers until you reach your target GE.
To get an idea of the minimum GE of various traffic loading categories, take a look at the table about halfway down our Pavement Design page.
Need Asphalt Paving Services in the Twin Cities? Contact Bituminous Roadways
At Bituminous Roadways, we’ve proudly served the Twin Cities metro with premium-quality asphalt paving services for more than 75 years. Whether you’re in the market for new asphalt pavement or you’re looking for asphalt repairs, we’ve got you covered. To get started or learn more about how we can help you, give us a call today at 651-686-7001 or send us a message with any questions or concerns, and we’ll get in touch!