Tarmac: What is it?

We've all heard of Tarmac but probably don't know much about it

Tarmac is short for Tarmacadam, a portmanteau for Tar-penetration Macadam, a type of road surface. Tarmac refers to a material patented by Edgar Purnell Hooley in 1901.

A bit of Tarmac trivia is that Baghdad is the first city known to have its streets paved with tar in the 8th century AD.

Over a 1,000 years later, John Loudon McAdam invented Macadamisation, a method of road construction. These roads were adequate for use by horse and carriage or coaches, but they were very dusty and eroded in heavy rain.

With higher speed motor vehicle use they didn't hold up. 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, 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.

The use of tar in road construction was used on a large scale with the arrival of the motor car 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.

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