A tarmac is one of several different things, depending on usage and location. Traditionally, the word describes a technique or approach to paving that involves crushed rock or gravel held together with heated tar. The word describes the process and components rather than the end result, but roadways and driveways made of these elements sometimes carry the name in colloquial speech. In many places the word is also closely associated with airports, and in this context the word can describe any runway or paved concourse over which airplanes taxi regardless of how that surface was constructed. Purists insist that this usage is incorrect, but it has nevertheless become pervasive in much of the word.
The art of road paving has changed and advanced over time. The earliest roads were made of loose gravel scattered over dirt; this gave cars and carriages some traction and helped prevent dust and mud, but wasn’t a permanent solution and posed certain risks when it came to vehicle damage and safety. Tarmac is one of the earliest and also most successful ways of binding crushed gravel together in order to form a uniform, level surface. The technique is used almost exclusively for roadways and driveways over which automobiles pass.
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There are only two essential elements in most cases, namely crushed gravel and tar. Warmed tar is usually the easiest to work with. Any sort of crushed gravel will do, but modern manufacturers often mechanically grind rocks of similar origin in order to end up with a powder that is uniform in both size and overall texture. This is combined with the tar, sometimes by hand in a bucket or pail but more often in a machine or truck that can agitate the contents and ensure an even distribution. It is then poured into moulds or over a prepared roadway and allowed to dry.
Most researchers agree that the process for making this type of roadway was discovered quite by accident. Near the end of the 18th century, a man by the name of John MacAdam began paving roads by adding a layer of crushed gravel to an existing dirt surface. He called this process “macadamizing,” a name he derived from his own. However, over time the gravel tended to grind and disintegrate. While it was fine for carriages and horses, newly invented motor cars would turn up huge dust clouds and frequently sent rocks flying from beneath their wheels.
As the story goes, in 1901, British businessman E. Purnell Hooley was passing a tarworks factory when he reportedly noticed a barrel of tar that had spilled over the macadamized roadway. Someone had dumped gravel on the tar to cover it, and in traveling over this section of road, Hooley observed there was far less dust. Based on this discovery, Hooley set out to make his own pavement mixture and launched a company to sell it. The company was Tar Macadam, and after changing hands in 1905, "Tarmac" became a huge success.
Local Differences in Terminology
Americans commonly use the term "blacktop" to refer to paving or asphalt, largely due to its color. In British, Australian, and Indian English, however, the “tarmac” term is still quite commonly employed. In most cases, though, people use it to refer to any paved road or surface. In most parts of the world, tarless asphalt mixtures have largely replaced the traditional process. The word has become so ubiquitous in so many places that it is still very commonly used, though, even if wrongly.
In Airport Settings
One of the most common incorrect usages concerns airport runways. Many people use the term to refer to an airplane runway, most likely due to the fact that Tarmac was used extensively to construct runways during World War II. To this day, any large paved area at an airport is commonly referred to using this term, regardless of how it was made.