Monday, March 3, 2008

What Roads are Made of

In the year 1900 there were only ten miles of paved road in the United States. Today, there are two million miles of paved roads and streets!

Asphalt Road

Unlike early drivers, you don’t have to consider whether or not a road exists to your destination. You just get out the map, plot your course and take off. You can make a quick trip downtown, head out to the seashore or up to the mountains. Hit the interstate and you can visit your uncle in Kalamazoo, Michigan, see Mount Rushmore in South Dakota, or visit Disney World in Orlando, Florida. You never run out of road!

Did you ever stop to wonder what all those miles of road are made of? Wherever you go in the world, and as far back as 4,000 BC, stone is the common ingredient in roads. Simple stone roads were often rough, uneven, and pitted with ruts and holes that filled up with rain and mud in the winter. It wasn’t until the 1700s that the smooth, even roads we know today became possible. We have three Scottish engineers and their improved road building techniques to thank.

Although he was blind, John Metcalfe was able to design and build firm, three-layer roads. First he placed large stones on the bottom layer, then he took the materials excavated from the roadbed such as smaller rocks and earth and used them for the middle layer, and finally he spread a layer of gravel on top.

A second Scottish gentleman by the name of Thomas Telford designed a way to raise the center of the road so that rainwater would drain down the sides. He also devised a method to analyze how thick the road stones had to be to withstand the weight and volume of the horses and carriages that were common in his day.

The last of the three, John McAdam, mixed the necessary road stones with tar. The tar “glued” all the stone together and created a harder and smoother surface for the carriage wheels to roll on. “Tarmacadam roads” became the standard used everywhere until the 1870s. “Tarmacadam” was a mouthful, so eventually people shortened the word to “tarmac.”

A natural rock known as asphalt had been used to construct buildings for many years. In 1824 large blocks of natural asphalt rock were placed on the wide boulevard in Paris known as the Champs-Élysées. This was the first time this type of rock was used for a road.

In the United States during the 1870s, a Belgian immigrant by the name of Edward de Smedt created a man-made asphalt that was of a higher density and quality than the natural stone. And like the tar that McAdam used, asphalt could harden and smoothe the road. Smedt’s new product was soon put to the test on Fifth Avenue in New York City and on Pennsylvania Avenue in Washington, D.C.

Today almost all the roads in the U.S. are surfaced with this man-made asphalt. Asphalt comes from the processing of crude oils. Everything that is valuable in crude oil is first removed and put to good use. Then what remains (hydrogen and carbon compounds with minor amounts of nitrogen, sulfur, and oxygen) is made into asphalt cement for pavement.

Ribbons of firm, well-drained, smoothly paved roads and highways are ready to take you and your family anywhere you want to go this summer, thanks to the construction methods pioneered by three Scottish engineers and the invention of man-made asphalt.


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