Asphalt boasts a performance profile unique among paving materials. On the one hand, it offers the strength and durability often associated with harder substances like concrete. On the other hand, asphalt possesses a natural degree of flexibility that allows it to more easily withstand stresses and common forms of damage.
Sometimes asphalt’s flexibility can work against it, however, leading to the formation of ruts. Such rutting usually occurs along wheel paths, since those areas have to bear the greatest amounts of pressure from above. This article takes a closer look at three of the most common causes of asphalt rutting, as well as what can be done to prevent them.
1. Unstable Subgrade
An asphalt surface can only be as strong and stable as its subgrade — in other words, the ground beneath the pavement. If this dirt and soil should shift or compact over time, your asphalt may end up without the support necessary to prevent rutting. The soil in certain regions may be naturally more unstable than that in others.
Fortunately, experienced contractors know that subgrade stability can be improved by compacting it before proceeding with the asphalt installation. Vibrators and compactors press the soil particles into as tight a configuration as possible. This compacted soil provides a much greater degree of stiffness and stability for the asphalt.
2. Insufficient Subbase Thickness
Yet asphalt cannot be installed directly on top of the subgrade, no matter how well compacted. Soil simply has too much inherent instability, especially when it becomes saturated with moisture. To ensure your asphalt remains in place for years to come, it must be installed atop a layer of crushed gravel, known alternatively as either the subbase or the aggregate base.
A subbase requires two things in order to provide effective results. First, just like the subgrade (and the asphalt itself) it has to be thoroughly compacted. Second, the subbase must be of a sufficient thickness to support the asphalt.
Subbase thickness varies from project to project, depending largely on the particular application of the asphalt. Asphalt that will bear the weight of vehicles requires a thicker subbase. For instance, most driveways contain subbases between 4 and 6 inches thick. Roadways, on the other hand, need even thicker subbases of between 6 and 9 inches.
3. Poor Mix Design
Even an asphalt surface with an adequate subgrade and subbase may experience rutting. In such cases, the issue generally stems from problems with the composition of the asphalt itself. In technical terms, an asphalt must possess enough structural integrity to overcome the shear forces exerted on it by the tremendous weight of vehicles.
Put more simply, the asphalt has to have enough internal strength not to compress beneath the weight of traffic. Here the asphalt composition has to be tailored for the specific sorts of traffic exposure it will experience. In other words, asphalt that will regularly be exposed to heavy vehicles traveling at high speeds must have a much stiffer performance profile.
Two factors affect the overall stiffness of asphalt. The first involves the particular sizes and proportions of the gravel aggregate used to give the asphalt its body. An evenly-grade mix — one that contains roughly equal amounts of small, medium, and large sized aggregate — tends to resist stress best. Such a mix minimizes the amount of void spaces between aggregate particles.
The second factor has to do with the asphalt binder used to hold the pavement together. Manufacturers grade asphalt binders in terms of stiffness. Here a compromise must be struck between flexibility and stiffness. Flexible binders resist cracking better, while stiff binders resist rutting.
For more information about what it takes to develop the best possible asphalt mix for a paving job, please contact the industry experts at JR Paving & Construction Co., Inc.