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What is Macadam?

Macadam was a variety of roads constructed in the 19th century using three layers of stone with gravel packed on top.
Macadam roads are rarely used in modern construction but can still be found in the countryside, in developing nations, and in historically-protected areas.
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  • Written By: Jessica Ellis
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  • Last Modified Date: 12 December 2014
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Macadam is a variety of road covering invented in the 19th century. The inventor, John Loudon McAdam, pioneered his technique as a replacement for dirt roads. Even though this type of road is rarely used in modern times, its influence is still seen in some forms of road construction.

John McAdam was born in Scotland in 1756, and grew up to working in English road construction efforts around the city of Bristol. After several decades of work, he published two treatises on the need for an improved road system using layered rocks. In 1816, as the surveyor on Bristol Turnpike, McAdam tested out his process for creating roads, called macadamizing.

The original system involved a triple layer of stone. The bottom two layers were comprised of hand-broken rocks laid to a depth of 8 in (20.3 cm) over a formation level called a subgrade. The top layer was much smaller rocks, made to be no more than 2 in (5 cm) thick. The entire road was then compacted and crushed together by use of an enormous roller. In addition, macadamized roads had a slightly convex shape, so that water would run off into drains on either side, rather than collecting on the road.

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The paving process became popular throughout the world, particularly in the quickly expanding American Northeast. The first American macadamized road was a remade 10-mile (16-km) section of unpaved road that connected the Maryland towns of Boonsboro and Hagerstown. The Boonsboro Turnpike was completed in 1823, using McAdam’s specific directions. In 1830, work was completed on the 73-mile (117.5-km) National Road, which remains one of the only roads in America to still contain macadam sections.

These roads were initially created for use with carriages and horse-powered travel. With the advent of automobiles, the process underwent a variety of changes to meet new challenges posed by the vehicles. Dust thrown up by automobile wheels became a serious problem for travelers, leading to the invention of tar-bound macadam or tarmac. The new process used a layer of tar on the subgrade and bound the rock layers together during rolling with sand and tar. Many early airports used tarmac pavement around the terminals, leading to the modern usage of the term for the disembarking area around a plane.

Advances in road construction lead to the gradual phase-out of macadam in industrialized nations. Replacements, such as concrete and asphalt, became popular as technology and synthesized materials became available to aid production. In America, the passage of the 1956 Federal Highways act lead to the modernization of most of the country’s major roads, mostly eliminating macadamized constructions. Some developing nations still use the process, and a few remaining areas of such road are protected as historical sites in some American towns.

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discographer
Post 4

@ankara-- I don't think that's macadam. Macadam is made up of smaller stone pieces that are all different sizes and gravel on the surface. And they look very compact on the ground.

Larger stones are used for macadam but those are underneath the smaller stones, so they're not visible.

What you describe doesn't sound like macadam. It's probably just a brick road.

bluedolphin
Post 3

I saw some stone roads when I visited Europe this summer. But I'm not sure if those were a macadam surface or something else. The pieces of stone were perfectly cut into cubes and placed equally apart from one another and bound by cement.

Is that macadam?

bear78
Post 2

Switching to macadam roads must have been really great for people living in that time.

We have some dirt trails around our house and our house is about one hundred years old. The trails become mud in rain and snow and it's so difficult to walk on them.

So I can just imagine how much easier people lives must have become when they could walk and ride on macadam paving instead of muddy, dirt roads.

anon217151
Post 1

This is very nice article. I am a civil engineering student from Kamban Engineering College.

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