There is a common belief that concrete pavements are stronger than asphalt pavements. The reason for this misconception is that comparisons are not made on equivalent designs. The traffic-carrying ability of asphalt or concrete pavement is approximately the same for each inch of pavement thickness.
To illustrate how this relates to pavement design, the concept of Structural Number must be considered. The Structural Number (SN) is an empirical strength value assigned to a particular material. The SN has no units. It is a relative number that is used to compare different materials.
To establish a standard with which to compare the structural numbers, the strength of a graded aggregate base is assigned a value of 1.0 for each inch of thickness. On this scale a 6-inch thick base would have a Structural Number of 6.0 and a 10-inch thick base would have a SN of 10.
A dense-graded asphalt pavement is assigned a Structural Number of 3.0 or three times as strong per inch of thickness as the graded aggregate base. For example, a three-inch asphalt base could be substituted for a nine-inch aggregate base and provide the same support for the traffic. Once you know the Structural Numbers of the materials, comparisons can be made when considering alternative materials or different designs.
A typical 3,000-psi concrete slab would also be assigned a structural number of 3.0, per inch of thickness. Therefore, a 4-inch concrete pavement and a 4-inch asphalt pavement have the same load-carrying capabilities. This answers the question.
Asphalt and concrete will provide the same traffic-carrying strength when the same thickness is used. A pavement with a SN of 10.0 will always be twice as strong as pavement with a SN of 5.0, regardless of the material used.
Most people have a general idea of the concrete thickness necessary to make an acceptable pavement: a sidewalk, 4 inches; a driveway, 6 inches; a residential street, 8 inches and expressways up to 12 inches thick depending on the amount of heavy traffic.
Let's analyze the Structural Numbers of the above designs and compare them to often-used asphalt pavements. The strength of a 4-inch-thick concrete sidewalk would be four times 3.0 or a total SN of 12. A 6-inch-thick driveway would be six times 3.0 or a total SN of 18. The residential roadway at 8 inches would yield a SN of 24 and a major highway at 12 inches would yield a SN of 36.
Let's consider typical asphalt pavements used in these applications for comparison, to see if the designer is asking for a pavement with the same relative strength. A sidewalk or cart path is often constructed with a 4-inch rock base and a 1-inch layer of asphalt. The structural number of this combination is 4 inches of rock times a SN of 1.0, plus 1 inch of asphalt times a SN of 3.0, for a total SN of 7. This is about one-half of the design strength of the 4-inch concrete option.
When considering pavement designs for durability and life expectancy, designs of equal strength i.e., equal Structural Numbers must be used. No one would expect a 2x4 board of lumber to have the same load-carrying capacity as a 4x4. We do not want to make the same strength mistake with pavements.
The analysis also works in reverse. The above sidewalk asphalt design with a SN of 7.0 should offer the same performance as a concrete design of SN 7.0. To determine the relative concrete thickness, divide the total SN of 7 by the SN per inch of 3.0 for a slab thickness of 2.3 inches. It is obvious a 2.3-inch concrete slab would be very fragile.
Click here to view pdf version of the Typical Concrete Designs table.
Click here to view pdf version of the Typical Asphalt Designs table.
The sidewalk comparison is used because it is easy to visualize the relative impact of the designs, since they are both inherently weak. If a loaded dump truck drives across a 4-inch-thick concrete sidewalk, what is the chance of the sidewalk breaking? The chances are rather high; because a rigid pavement fails when the load applied exceeds the pavement's total strength.
A flexible pavement of full depth asphalt or rock and asphalt will often flex and rebound rather than break under a single heavy load. However, an asphalt pavement will fail relatively quickly under a series of loads that exceed its design capacity. The proper way to determine the design is to calculate the correct SN for the traffic and convert to the required layer thickness that meets the need.
Click here to view pdf version of the Equivalent Asphalt Designs table.
Another dilemma with asphalt pavement is that it is easy to under-design and put into service. A concrete pavement that is under-designed will quickly crack and fail. The inadequacy will often be immediately apparent.
An asphalt pavement, being flexible, will have a slower deterioration rate, but will also eventually fail if overstressed. These failures are often blamed on poor construction or sub-standard materials instead of the actual cause: insufficient SN to support the loads. This lack of a clear "cause/effect" relationship enables many pavement problems to be misdiagnosed.
Often contractors are asked to repair asphalt pavements when the basic problem is sub-standard design. When the pavement is repaired to the original thickness, it will soon exhibit the same type of failure. Asphalt has numerous patching and repair methods available to the contractor. Concrete, once it is broken, has only one option: remove and replace.
With an understanding of the Structural Number concept and the component values of Base Rock = 1.0, Asphalt = 3.0, Concrete = 3.0 comparisons of multiple pavements are possible. Also, existing or proposed pavement options can be evaluated. The durability of pavements with the same total SN value will be similar, given uniform subgrade conditions.
The above pavement designs are examples only. Actual designs should reflect actual traffic loading and subgrade conditions. The reference for this article is NAPA publication IS-20 "A Guide to Thickness Equivalencies for the Design of Asphalt Pavements."
Jon Chellgren, a professional engineer with more than 30 years of experience managing paving and grading companies, is a consultant in Boca Raton, FL. You can contact him at [email protected].
