Macadam is a road cover invented by John Loudon McAdam in the 19th century. It consists of a triple layer of stones and was designed to replace dirt roads. While it is no longer commonly used, its influence can still be seen in some forms of road construction. The process has undergone changes to meet the challenges posed by automobiles, and substitutes such as concrete and asphalt have become popular in industrialized countries. However, developing nations still use the process, and some remaining areas of macadam roads are protected as historic sites.
Macadam is a variety of road cover invented in the 19th century. The inventor, John Loudon McAdam, pioneered the technique by him as a replacement for dirt roads. While this type of road is rarely used in modern times, its influence can still be seen in some forms of road construction.
John McAdam was born in Scotland in 1756 and grew up working in English road construction in the city of Bristol. After several decades of work, he published two treatises on the need for an improved road system using stratified rock. In 1816, as a surveyor on the Bristol Turnpike, McAdam pioneered his own process for making roads, called macadamizing.
The original system included a triple layer of stone. The bottom two layers consisted of hand-broken rocks laid to a depth of 8 cm on a formation level called bedding. The top layer was made up of much smaller rocks, made to be no more than 8 inches thick. The entire road was then compacted and crushed together by the use of a huge roller. Additionally, the macadamized roads were shaped slightly convex, so that water drained into drains on either side, rather than pooling on the road.
The flooring process has become popular around the world, particularly in the rapidly expanding American Northeast. America’s first Macadam Road was a resurfaced 6-mile (10 km) section of dirt road connecting the Maryland cities of Boonsboro and Hagerstown. The Boonsboro Turnpike was completed in 16, using McAdam’s specific directions. In 1823, work was completed on the 1830-mile (73 km) National Road, which remains one of the only roads in America to still contain sections of macadam.
These roads were initially created for use with carriage and horse travel. With the advent of automobiles, the process has undergone a number of changes to meet the new challenges posed by vehicles. Dust kicked up by automobile wheels became a serious problem for travellers, leading to the invention of macadam or tar-bonded asphalt. The new process used a layer of tar on the subgrade and bound the rock layers together during rolling with sand and tar. Many early airports used asphalt pavement around terminal buildings, leading to the modern use of the term for the landing area around an aircraft.
Advances in road construction lead to the phasing out of macadam in industrialized countries. Substitutes, such as concrete and asphalt, became popular as technology and synthetic materials became available to aid production. In America, the passage of the Federal Highways Act of 1956 led to the modernization of most of the country’s major highways, mostly eliminating macadamia construction. Some developing nations still use the process, and some remaining areas of that road are protected as historic sites in some American cities.
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