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Tarmac (short for tarmacadam, or tar-penetration macadam) is a type of road surfacing material patented by Edgar Purnell Hooley in 1901. The term is also used, with varying degrees of correctness, for a variety of other materials, including tar-grouted macadam, bituminous surface treatments, and even modern asphalt concrete.
The streets of Baghdad were paved with tar in the 8th century AD. More than 1000 years later, John Loudon McAdam invented a road construction method called macadamisation. These roads were adequate for use by horses and carriages or coaches, but they were very dusty and subject to erosion with heavy rain. Later on, they did not hold up to higher-speed motor vehicle use. Methods to stabilise macadam roads with tar date back to at least 1834 when John Henry Cassell, operating from Cassell's Patent Lava Stone Works in Millwall, patented "Pitch Macadam". This method involved spreading tar on the subgrade, placing a typical macadam layer, and finally sealing the macadam with a mixture of tar and sand. Tar-grouted macadam was in use well before 1900, and involved scarifying the surface of an existing macadam pavement, spreading tar, and re-compacting. Although the use of tar in road construction was known in the 19th century, it was little used and was not introduced on a large scale until the motorcar arrived on the scene in the early 20th century.
Hooley's 1901 patent for Tarmac involved mechanically mixing tar and aggregate prior to lay-down, and then compacting the mixture with a steamroller. The tar was modified by adding small amounts of Portland cement, resin, and pitch.
As petroleum production increased, the by-product asphalt became available in greater quantities and largely supplanted tar due to its reduced temperature sensitivity. The Macadam construction process quickly became obsolete because of its high manual labour requirement; however, the somewhat similar tar and chip method, also known as bituminous surface treatment (BST) or "chip-seal", remains popular.
While the specific tarmac pavement is not common in some countries today, many people use the word to refer to generic paved areas at airports, especially the apron near airport terminals despite the fact that these areas are often made of concrete. The Wick Airport at Wick in Caithness, Scotland, is one of the few airports that still has a real tarmac runway. Similarly in the UK the word "tarmac" is commonly used as an alternative term for asphalt concrete.
Tarmac is a registered trademark although it is frequently used with a lower-case initial letter.
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