In talking with managers, property owners, and association boards, we are commonly asked why we’re recommending a specific depth of asphalt. Often, the age of the existing asphalt and its depth are all we need to know. Intuition tells us that an asphalt surface that is 3″ deep and has lasted 20 years is likely to last another 20 years or so if replaced at 3″, assuming conditions and traffic levels aren’t likely to change. Add an inch to make the depth 4″ and it’s likely to last even longer.
But the central problem is that contractors simply don’t know the depth of the existing asphalt. It costs both time and money to find out by taking core samples – investments many companies don’t want to make to price a project for a prospective customer. So most customers are left with getting 4″ recommendations on most repairs with 6″ recommendations for higher traffic areas. The end result is that customers often pay for far more than they need (or in some cases, far less) because they don’t know that there’s a better way to do some basic exploration and some simple design to “right size” a solution. Add to that fact that many pavement contractors don’t have basic knowledge about good pavement design and customers end up with solutions that are somehow lacking. This is one of the reasons we have made taking core samples a more common practice, particularly on larger projects where a 1″ difference in the asphalt thickness can mean big bucks.
The depth of asphalt required for the pavement to have an acceptable life expectancy varies greatly. We thought a quick crash course in pavement design methodology might be helpful to shed some light on how knowing the basics can both save money and result in a better solution.
In simplified terms, there are essentially 3 steps to designing an asphalt pavement section.
1. Determine how much strength is in the underlying native soil.
2. Determine the type and quantity of traffic and the target life of the pavement surface.
3. Determine how much strength will come from the base rock material or treated soil and how much strength will come from the asphalt (or concrete) pavement itself.
The same methodology can be slightly modified and used for new pavements (where only the native soil is the starting point), replacement (where the asphalt is removed but the soil and some portion of the base rock remain), or overlays (where the soil, base rock, and some or all of the existing asphalt remain).
Knowledgeable contractors and resourceful engineers can “value engineer” a solution that minimizes costs by building strength in each layer with the most cost effective materials and processes. Such costs can vary greatly based on prices for materials, proximity to asphalt plants and recycling sites, trucking costs, equipment access, and numerous other factors and variables.
The bottom line is that a 2″ pavement section may be ideal if it’s placed in low traffic areas with a thick base rock foundation and stable soil. On the flipside, a 6″ asphalt section may be inadequate if it’s in a higher traffic area with little to no base rock and unstable soil. The key is to make investments in the design stage to, at a minimum, make very educated guesses about proper section thicknesses based on the age of the pavement, its original thickness, and its maintenance history. On larger projects where the cost of the design is justifiable and “pays for itself,” following proper methodology with the aforementioned 3 step design/engineering process will provide the best results.