Pavements are among the most financially important highway-system assets due to their high construction and maintenance costs. Their costs are largely responsible for making the U.S. highway system the most expensive public works project undertaken by any society. Because pavements are so costly to construct and maintain, it is important for highway engineers to have an understanding of basic pavement design principles.
In the United States, there are over 4 million miles of highways. Of these, roughly 45% are lower-volume roads that are not paved (these roads generally have a gravel surface, or are composed of a stabilized material consisting of an aggregate bound together with a cementing agent such as Portland cement, lime fly ash, or asphaltic cement). However, environmental conditions such as moisture can seriously compromise the structural integrity of unpaved highway surfaces. As a result, highways that carry higher volumes of traffic, typically with heavy axle loads, require surfaces with asphalt concrete or Portland cement concrete (PCC) to provide for all-weather operations. Paved highways can cost several million dollars per mile to construct (based on the number of lanes, shoulders, soil conditions, etc.) and many thousands of dollars annually (per mile) to maintain. In the United States, roughly $40 billion dollars are spent annually on the construction and maintenance of pavements. Given the magnitude of this pavement-asset investment, it is easy to understand why the construction, maintenance, and rehabilitation of the pavement infrastructure must be done in a cost-effective manner.