28. Components Sub-grade (roadbed) course: natural material that serves as the foundation of the pavement structure Sub-base course: above the sub-grade, superior to sub-grade course Base course: above the sub base, granular materials such as crushed stone, crushed or uncrushed slag, gravel, and sand Surface course: upper course of the road pavement, should withstand tire pressures, resistant to abrasive forces of traffic, provide skid-resistant driving surface, prevent penetration of surface water 3 inches to > 6 inches