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John Loudon McAdam

John Loudon McAdam (1756-1836) was a Scottish engineer and road-builder who invented a new process for building roads with a smooth hard surface that would be more durable and less muddy than plain dirt. This was called 'macadamization'.

McAdam travelled to New York in 1770, returning to Scotland in 1783 after a successful career as a merchant and prize agent during the American Revolution. As an estate-owner and road trustee, he then commenced work on finding ways to improve the notoriously bad roads of Great Britain. His eventual conclusion was that roads needed to be raised above the surrounding ground, and carefully constructed from layered rocks and gravel. He wrote two treatises documenting his research, Remarks on the Present System of Road-Making (1816) Practical Essay on the Scientific Repair and Preservation of Roads (1819). In 1820 Parliament awarded him 2,000 pounds for his efforts and in 1827 he was made Surveyor-General of metropolitan roads.

When a macadam road was built, side ditches were dug, and the road bed was laid with three layers of decreasingly-sized rocks, carefully pulverized "so as not to exceed 6 ounces in weight or to pass a two-inch ring." The finished road was compacted with a cast-iron roller, and the compaction process was completed by passing traffic.

The first macadamized road in North America was completed in 1830.

Although macadamization was replaced by more modern techniques in the early 1900s, the name lives on. Tarmac was originally marketed as tar-macadam, because it was a macadamized road incorporating a binder of tar.

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