Roads are made from crushed rock, sand and gravel—or aggregate—plus either asphalt or cement. This cross-section beneath an interstate highway shows there’s more to a road than meets the eye. That’s 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 road’s specifications, and the materials that are locally available. The uppermost layers are either asphalt or concrete.

			Bituminous Surface The uppermost part of a blacktop road is called the bituminous (or wearing) surface. This layer is about 6.5 cm (2.5 inches) thick and made of a mixture of asphalt (6%) and aggregate (94%). The aggregate particles are about 2 cm (0.75 inches) in diameter. The bituminous surface can be removed and replaced as it "wears" out.
			Bituminous Binder The intermediate layer of a road is called the bituminous binder. It is about 7.5 cm (3 inches) thick. Like the bituminous surface, it is a mixture of asphalt (6%) and aggregate (94%), and is designed to be the main load-carrying layer. The aggregate particles are about 4 cm (1.5 inches) in diameter.
			Aggregate A road’s base layer consists entirely of aggregate. It is about 30 cm (12 inches) thick and made of various sizes of aggregate. The largest particles are about 5 cm (2 inches) in diameter. The base layer dissipates the weight of cars and trucks into the underlying layer, the subgrade.
			Subgrade The very bottom of a road is its subgrade. This is the local soil that has been compacted with a roller. The subgrade provides the stable base upon which pavement is constructed.