Tarmac

Tarmac

[tahr-mak]

Tarmac (short for tarmacadam, a portmanteau for tar-penetration macadam) is a type of highway surface, pioneered by John Loudon McAdam in around 1820. Strictly speaking, Tarmac refers to a 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, Tarvia, bituminous surface treatments and even modern asphalt concrete.

Origins

The first city to have its streets paved with tar was Baghdad in the 8th century AD. Macadamized roads were adequate for use by horses and carriages or coaches, but they were very dusty, subject to erosion with heavy rain and did not hold up to higher speed motor vehicle use. Methods to stabilise macadam roads with tar date back to at least 1834, when Henry Cassell patented "Pitch Macadam". This method involved spreading tar on the subgrade, then placing a typical macadam layer and then sealing the macadam with a mixture of tar and sand. Tar-grouted macadam was also 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 motor car 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 steam roller. The tar was modified with the addition of small amounts of Portland cement, resin and pitch.

Later developments

As petroleum production increased, the by-product asphalt became available in huge quantities and largely supplanted tar due to its reduced temperature sensitivity. The Macadam construction process also became quickly obsolete due to its high manual labour requirement; however, the somewhat similar tar and chip method, also known as bituminous surface treatment (BST), 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 airport ramp or "apron", near the terminals despite the fact that many of these areas are in fact made of concrete. This term seems to have been popularised when it became part of the news lexicon following live coverage of the Entebbe hijacking in 1976, where "Tarmac" was frequently used by the on-scene BBC reporter in describing the hijack scene. The Wick Airport at Wick in Caithness, Scotland is one of the few airports that still has a real Tarmac runway.

Patents

  • Hooley, E. Purnell, , "Apparatus for the preparation of tar macadam", July 26, 1904.

References

See also

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