**Introduction**

In general, this statement is true. This fact is a common rule of thumb which can be used in most situations involving concrete pavement thickness design for street and highway pavements (6-12 inches).

**Example**

In particular, the following example shows the result of adding one inch of thickness using ACPA's WinPAS program, which is based on the 1993 AASHTO Pavement Design Guide Equations. These numbers represent average values for an Interstate highway design situation:

- 90% reliability
- concrete modulus of rupture (MR) = 700 psi
- concrete modulus of elasticity (E) = 4.73 million psi
- load transfer coefficient (J) = 2.70
- modulus of subgrade reaction (k) = 200 pci
- drainage coefficient = 1.15

Start with the above values and an estimated thickness of 10 inches. Solve for ESALs (Equivalent Single Axle Loads), which is the result that the AASHTO equation uses to convert all types of traffic (cars, buses, trucks) to equivalent loads on a pavement. This number is proportional to the life of the pavement. The result for the example is 46.9 million ESALs.

Next, run the same input values with a thickness of 11 inches. The result is 86.6 million ESALs, which is an increase from the original (10 inch) of 85%, or slightly less than two times the capacity. If the inputs are tweaked slightly, for example, going from an 8-inch to a 9-inch thick concrete pavement, it will result in an increase of 105% the ESALs (11.7 million to 24.1 million), or slightly more than two times the capacity.

**Rule of Thumb**

The specific results can be varied based on the particular inputs used, but the average increase in life is about two times when adding one additional inch of concrete pavement thickness.