How thick should asphalt be after compaction?

Installing the proper layers of asphalt paving for all courses above the soil sub-grade creates a full-depth asphalt pavement. Customers typically ask how thick the asphalt pavement should be when considering full-depth asphalt. Sadly, there isn't a simple answer to that question. Before determining the optimum thickness, asphalt paving professionals must consider several factors:

The intended use of the pavement

The weight of the vehicles that will be utilizing the pavement, their speed, and the number of vehicles using the pavement on a typical day are all considerations to consider. For example, the pavement on a residential driveway that will be used by three or four-passenger cars each day at a low pace does not need to be as thick as the pavement on an interstate highway that carries hundreds of tractor-trailers and thousands of cars each day. A home driveway with a compacted asphalt thickness of 2.5 to 3 inches may be sufficient, while a busy mixed-use property may require a compacted asphalt thickness of 4 inches or more.

More and more roads today get damaged due to the volume of vehicles and traffic frequency. Public works use a road dilapidation report to determine the condition of roads so that the proper thickness of asphalt can be applied. It uses digital scanning and surveying to come up with accurate computations.

The type of soil

For the finished pavement, the soil must offer a solid, sturdy basis. Contractors create the subgrade by grading and compacting the soil. If the soil is rocky, sandy, or gravelly, it should be compacted and drain well. It will be more difficult to accomplish correct compaction if the soil is loam, clay, or another soft soil. Clay soils, in particular, do not drain well and tend to expand and contract in reaction to moisture levels. If adequate compaction cannot be accomplished, some soil may need to be removed and replaced, or an aggregate sub-base may be used. The depth of the asphalt needed to get excellent results can be influenced by the depth to which the soil can be compacted or the usage of an aggregate sub-base.

Soil is extremely important in determining the load-bearing capacity of the finished pavement. Improving the load-bearing of the subgrade soil will increase the tenacity of the structural support of the pavement and therefore means a sturdier road. This can be more economical in the long run.

The type of asphalt mix

There are several varieties of asphalt mixes, including open-graded and standard asphalt pavement. The aggregate size has an impact on the asphalt pavement thickness. For example, the typical rule is that each layer's thickness must be at least three times bigger than the aggregates nominal maximum size. The aggregate size in open-graded mixes, on the other hand, can be up to 80% of the layer thickness. Furthermore, numerous sorts of mixes can be used to construct the pavement. The lower layer, for example, could be a typical asphalt mix. Still, the top layer could be a thinner layer of a mix with smaller aggregates to provide visual appeal without compromising strength.

Hot asphalt is the easiest and most economical of all types of asphalt mixes. Though what they need to remember is it gets difficult to manipulate once it cools down so speed in layering is crucial. 

MC Cold mix asphalt, meanwhile is used in cooler climates though it takes quite a long time to cure and is only best to use on roads with very minimal traffic. You can imagine the downside of using this, there could be substantial economic implications, so deciding on this type needs to be thoroughly studied.

Driveways and roads are different

Asphalt paving is done with paving machines that apply the asphalt paving material in an equal thickness. Pavers refer to this application as a "lift". Each lift is compacted following an application by a powerful roller compactor, ranging in thickness from 2 12” to 3′′. Paving is frequently performed in two “lifts” of 2 12” for heavily-traveled roads that must sustain the steady weight of hundreds of vehicles, including big trucks travelling over it at high speeds. To generate the ultimate thickness, the two layers are compacted to a thickness of 4′′ after paving. Parking lots of supermarkets and big-box retailers, for example, are paved in the same fashion, with two "lifts".

On the other hand, residential driveways are only used by a few vehicles going at a slow speed. Richfield Blacktop typically paves home driveways with a single 3′′ thick lift of asphalt. The material is subsequently crushed to a thickness of 2 12”. This, paired with the properly compacted sub-base, is the optimal thickness for a residential driveway to ensure maximum life and endurance. If 3-ton asphalt roller is laid too thickly, it will not be adequately compacted, resulting in depressions where traffic is heavy over time.

Examples of how thickness can vary

Assume you need to construct a parking lot using full-depth asphalt pavement to connect the components. In terms of volume and car weights, the parking lot will be extremely busy. If the soil sub-grade is of good quality, the asphalt thickness can be as low as 6.5 inches, but the thickness may need to be increased to 11.5 inches if it is of poor quality. If the soil sub-grade is good, a 4-inch depth of asphalt may be sufficient for a parking lot with modest traffic, but if the sub-grade is bad, the asphalt depth may need to be extended to 6 inches.

Keep in mind that the asphalt thicknesses relate to the total thickness of all asphalt layers after being compacted. For a 4-inch thickness, the exact composition might include laying down a 3- inch-thick course of asphalt and compacting it to 2.5 inches before laying down a 2-inch-thick course and compacting it to 1.5 inches.

Doing a project like this can be daunting, if you don’t have the expertise, it would be best to rely on experienced contractors

The key to a long life and durability for your driveway is the appropriate installation and compaction of a good sub-base. Different soil types require different sub-bases because the sub-base must be correctly compacted and prevent penetration by the underlying soil.