tarmacadam

Tarmacadam, commonly shortened to tarmac, is a road surface material consisting of crushed stone bound with a binder. The name combines tar with macadam, the paving technique devised by John Loudon McAdam in the 18th century. McAdam's method used well-graded layers of compacted stone to provide drainage and load distribution, but the surfaces required little binding material. In 1901 the Irish-British engineer Edgar Hooley introduced tar as a binding medium to macadam, producing a tar-bound surface that soon came to be called tarmacadam.

Tarmacadam surfaces offered improved waterproofing and cohesion compared with bare macadam, and they became common for roads, airfields, and driveways in the early to mid-20th century. The binding medium was originally coal tar or pitch; later, cheaper and more resilient bitumen-based binders were used in some variants. Over time, tar-based binders proved prone to softening in heat and emitting odor, and many regions transitioned to asphalt concrete using bitumen rather than tar.

Today the term tarmac is often used colloquially to refer to asphalt or asphalt concrete surfaces, particularly in Britain. Technically, modern tarmacadam is rarely produced as a distinct material, having largely been superseded by bitumen-bound asphalt mixes. Legacy tarmac surfaces remain in use in some roads, airfields, and private driveways, where maintenance involves resurfacing with asphalt.