AskDefine | Define macadam

Dictionary Definition



1 broken stone used in macadamized roadways
2 a paved surface having compressed layers of broken rocks held together with tar [syn: tarmacadam, tarmac]

User Contributed Dictionary



Named after John Loudon McAdam, who invented the process of macadamization. Used for describing road surfaces originally constructed using the McAdam method, but now sometimes used for any road or street.


  • RP:
    • /məˈkædəm/
    • /m@"k

Extensive Definition

Macadam is a type of road construction pioneered by the Scotsman John Loudon McAdam in around 1820. It consisted of creating three layers of stones laid on a crowned subgrade with side ditches for drainage. The first two layers consisted of angular hand-broken aggregate, maximum size 3 inches (75 mm), to 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). Each layer would be compacted with a heavy roller, causing the angular stones to lock together.

Water-bound macadam

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 constructed in this manner were described as "macadamized".

Tar-bound macadam

With the advent of motor vehicles, dust became a serious problem on macadam roads. The vacuum created under fast-moving vehicles sucks dust from the road surface, creating unpleasant dust clouds and a gradual raveling (pulling apart) of the road material. This problem was later rectified by spraying tar on the surface to create "tar-bound macadam" (tarmac). While macadam roads have now been resurfaced in most developed countries, some are preserved along stretches of roads such as the United States' National Road. Due to uses of macadam as a road surface in former times, roads in some parts of the United States (as parts of Pennsylvania) are often referred to as macadam, even though they might be made of asphalt or concrete.
macadam in Danish: Makadam
macadam in German: Makadam
macadam in Esperanto: Makadamo
macadam in Basque: Macadam (sistema)
macadam in Finnish: Murske
macadam in French: Macadam (route)
macadam in Italian: Macadam
macadam in Dutch: Macadam
macadam in Norwegian: Makadamisering
macadam in Portuguese: Macadame
macadam in Swedish: Makadam

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