Macadam
|
|
Macadam is a type of road construction pioneered by John Loudon McAdam in the early 1800s. It consists of three layers of stones laid on a sloped subgrade, with side ditches for drainage. The first two layers consisted of angular aggregate hand-broken, maximum size 3 inches (75 mm) for a total depth of about 8 inches (200 mm). The third layer was about 2 inches (50 mm) thick with a maximum aggregate size of 1 inch (25 mm). The layers would be compacted with a heavy roller. This caused the angular stones to lock to their neighbours. This basic method of construction is sometimes known as water-bound macadam. Although this method required a great deal of manual labor, it resulted in a strong and free-draining pavement.
Roads which were constructed in this manner were described as Macadamized.
With the advent of motor vehicles, dust became a serious problem on macadam roads. The vacuum created under fast moving vehicles sucked the dust out of the surface leading to a gradual raveling of the larger size materials, as well as an unpleasant dust cloud. This problem was later rectified by spraying tar on the surface thus creating tar-bound macadam, or tarmac. This may have led to the common misuse of the word macadam to refer to a road made with asphalt concrete. While macadam roads are extinct in modern America, sections are preserved along the National Road. The misidentified relative - tarmac - can still be sometimes found in New England.