Roadway pavements are divided into two general categories: rigid and flexible. Rigid pavements are usually constructed of portland cement concrete, which behaves much like a beam over any irregularities in the underlying supporting materials. Flexible pavements are usually constructed of bituminous materials which transfer vehicle loadings directly to the underlying support materials.

The damage to roadway pavements caused by passenger cars is very limited compared with that caused by trucks. Therefore, pavements are designed to support a specified number of heavy vehicle loadings over their design life. An equivalent single-axle load (ESAL) is a standard term used in pavement design to describe the damage caused by one pass of an 18,000-pound (18-kip) axle load over the pavement surface (in SI notation, 18,000 pounds = 80 kN). Consequently, in order to design a pavement, it is necessary to express all of the traffic that will use the pavement as an equivalent number of 18,000-pound axle loads. The use of an 18,000-pound standard stems from the maximum legal axle loadings in effect at the time many pavement design methods were developed. This conversion is accomplished by first determining an ESAL factor for each classification of vehicle using the pavement. An ESAL factor is a ratio relating the damage caused by a passing vehicle of specified weight relative to the damage caused by an 18-kip axle load, or

where

Studies have shown that...