Roads are made from crushed rock, sand and gravelor
aggregateplus either asphalt or cement. This cross-section beneath
an interstate highway shows theres more to a road than meets the eye.
Thats because roads must be both strong, to support heavy loads, and
durable, to withstand weather, seasons, and climate. Even the strongest
blacktop roads last only about 8 to 10 years. Concrete roads
last longer, but are much more expensive to build.
Roads are made up of four layers. The composition of each layer depends
upon the roads specifications, and the materials that are locally
available. The uppermost layers are either asphalt or concrete.