Macadam is a type of road construction pioneered by the Scotsman John Loudon McAdam in around 1820. The method simplified what had been considered state-of-the-art at that point.

Before McAdam

Before McAdam, French road director Pierre-Marie-Jérôme Trésaguet had recommended a roadway consisting of 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.

McAdam's method was simpler and yet more effective at protecting roadways: he discovered that massive foundations of rock upon rock were unnecessary, and asserted that native soil alone would support the road and traffic upon it, as long as it was covered by a road crust that would protect the soil underneath from water and wear. He used 2-inch broken stones in a layer 6-10 inches deep and depended on the road traffic to pack it into a dense mass, although for quicker compacting, a cast-iron roller could be used.

Water-bound macadam

This basic method of construction is sometimes known as "water-bound macadam". Although this method required a great deal of manual labour, it resulted in a strong and free-draining pavement. Roads constructed in this manner were described as "macadamised".

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 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.

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