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) for a total depth of about 8 inches. The third layer was about 2 inches thick with a maximum aggregate size of 1 inch. 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.

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.