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Origins of Tarmacadam – Macadam Roads
In the origins of tarmacadam John Loudon McAdam, the Scottish inventor, is remembered for designing roads using broken stones laid in tight symmetrical patterns covered with further layers of smaller stones which had been crushed, broken and then compacted to create a hard surface. He discovered that the best results were obtained by grading the various layers of stones into similar sized pieces and compressing them into the road surface.
McAdam’s invention became known as a method of macadamising roads or macadam roads and it was considered the greatest advancement in road construction during the late eighteenth century – fine for horses and carriages. Along with the effects of rain and freeze / thaw cycles during winter damaging the surface however, the newly developed motor cars also caused the surface to break up, spitting stones into the air together with accompanying clouds of dust.
There are a number of differing stories of how macadamized roads became tar-mac roads, some of them dating back to the 1830’s (though these involved the addition of tar to existing stone roads). And although tar or pitch was said to be used on highways thousands of years ago, the following version seems to have acquired credibility over the years: –
Origins of tarmacadam – Tar Macadam
In 1901, Edgar Purnell Hooley, a British businessman and Nottingham’s county surveyor, was travelling past a factory (some say ironworks, others claim tar works) when he noticed that a patch of macadamised road was free of dust. Apparently a barrel of tar had spilled onto the road and someone had covered the sticky mess with stone (or perhaps slag from the ironworks).
Hooley developed his own mixture of aggregates, tar and cement, he patented the invention, called it tar-mac and formed a company to manufacture it – Tar Macadam (Purnell Hooley’s Patent) Syndicate Limited. Whilst the Macadam process involved compacting ever smaller pieces of stone and then applying tar, Hooley’s patented invention crucially involved preparing the mix of aggregates and tar together prior to compacting it with a steam roller.
Origins of tarmacadam – The Birth of Tarmac Limited
Hooley had difficulty making his invention work however, and when Sir Alfred Hickman MP for Wolverhampton heard of the product he acquired the patent. It did help that Hickman also owned a large steelworks in the area as he now had an excellent way of profiting from the large quantities of waste slag it generated.
Sir Alfred Hickman MP re-located the business from Nottingham to Wolverhampton and called the new business Tarmac Limited. The rest as they say is history. Unfortunately, John Loudon McAdam, the Scottish inventor who started it all had already died in 1836 so never saw his macadamised roads as tar-mac ones.
Origins of tarmacadam – Asphalt
The development of motor cars may have played a part in the need for tar-mac roads but the increasing production of petroleum to power them took it a stage further. Asphalt is a by-product of petroleum manufacture and it soon took over from tar as it is less affected by changes in temperature. As a result, the method of road construction developed by John Loudon McAdam quickly became obsolete, though a version of it is still used to rejuvenate road surfaces even today.
Commonly referred to as tar and chip, bituminous surface treatment involves spraying liquid asphalt onto a worn out road, followed by a coating of loose aggregate, which is rolled into the surface. Tar and chip is used by local authorities to lengthen the usable life of a road surface but it is no substitute for the much more expensive option of full replacement.