The Design of Asphalt Roads

About this course.

A pavement is designed for a predicted traffic loading and a nominated design life because the damage a road accumulates comes overwhelmingly from repeated heavy-vehicle loads, not from the passage of time or the number of cars. Designing to a fixed "standard thickness" would either waste money on lightly trafficked roads or fail prematurely on heavily trafficked ones. Instead, the designer estimates how many heavy axle loads the pavement must survive over its intended life, and proportions the structure to carry that cumulative loading with an acceptable risk of failure.

The key insight is that road damage scales very steeply with axle load. A widely used engineering approximation, derived from full-scale road testing decades ago, holds that the damage caused by an axle rises with roughly the fourth power of its load. Under such a relationship, a single fully laden heavy-vehicle axle can cause thousands of times more structural damage than a passenger-car axle. The practical consequence is that private cars are almost irrelevant to structural pavement design, while the number, weight and axle configuration of trucks and buses are decisive. This is why traffic surveys for pavement design focus on counting and classifying heavy vehicles rather than total traffic volume.