Telford began his work by elevating the road's foundation to drain water and built roads on relatively flat grades. He laid his roads in three layers, making his road about 18 inches deep. The bottom layer was composed of large stones about 4 inches wide and 7 inches in depth. The next two layers were stones about 2.5 inches topped with gravel to a thickness of about 9 inches. Telford estimated his roads could support a load of 500 lbs. per inch. Telford built many roads in England and Wales during the 1800s.
Macadam pavements began the use of angular aggregates rather than the rounded stones used previously. As water can cause up to 90 percent of all road damage, MacAdam pioneered a sloped subgrade surface for drainage as opposed to Telford's flat subgrade surface. On top of the subgrade, he put angular aggregates of up to 3 inches in two layers to about 8 inches in depth. The wearing course, or the surface of the road, comprised gravel no larger than 1 inch. The first macadam road was built in Maryland in 1823.
A basic macadam roadbed with a tar-bound surface is a tar macadam pavement. The first tar macadam road was built in 1848 in Nottingham, England. The surface was coal tar, a residue from coal-gas lighting. In 1871, a series of streets and roads were built in Washington, D.C., using a tar-concrete mixture of tar, concrete, sawdust and ashes. These streets, however, failed because the tar was not the right percentage of the overall mix of the ingredients, which included sulfuric acid (as a hardening agent). As a result, tar was discredited, and the asphalt industry began to flourish.
Sheet asphalt pavements began to be built in the mid-1800s. These roads have a bottom layer of pavement rubble or cement concrete, a binder layer of broken stones and asphalt cement and a top layer of asphalt cement and sand. Asphalt is what's left at the bottom of a barrel of petroleum after all the useful stuff has been processed. The first sheet asphalt road in the U.S. was in Newark, New Jersey, in 1870.
Although Portland cement concrete (PCC) was invented in 1824, it wasn't until the late 1800s that it began to be used in road building. In 1891, George Bartholomew built the first PCC pavement in Bellefontaine, Ohio. The PCC pavement is the first rigid pavement mixed on the road site. It was placed in square forms. Bartholomew scored 4-inch squares into the pavement to provide horses better footing. By 1914, more than 2,000 miles of roads were constructed using PCC pavement.
