macadam

n. broken stones used for paving roads; road or other surface paved with layers of broken stones bound with tar or asphalt

Macadam is a type of road construction pioneered by the Scotsman John Loudon McAdam in around 1820. It consisted of creating three layers of stones laid on a crowned subgrade with side ditches for drainage. The first two layers consisted of angular hand-broken aggregate, maximum size 3 inches (75 mm), to a total depth of about 8 inches (200 mm). The third layer was about 2 inches (50 mm) thick with a maximum aggregate size of 1 inch (25 mm). Each layer would be compacted with a heavy roller, causing the angular stones to lock together with their neighbours.
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John Loudon McAdam (September 21 1756 - November 26 1836) was a Scottish engineer and road-builder. He invented a new process, "macadamisation", for building roads with a smooth hard surface that would be more durable and less muddy than soil-based tracks. Modern road construction still reflects McAdam's influence. Of subsequent improvements, the most significant was the introduction of tar (originally coal tar) to bind the road surface's stones together – "tarmac" (for Tar Macadam) – followed later by the use of hot-laid tarred  aggregate or tar-sprayed chippings to create better road metalling. More recently, oil-based asphalt laid on reinforced concrete has become a major road surface, but its use of granite or limestone chippings still recalls McAdam's innovation.
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Noun
1. broken stone used in macadamized roadways
(hypernym) paving material
(derivation) macadamize, macadamise, tarmac
2. a paved surface having compressed layers of broken rocks held together with tar
(synonym) tarmacadam, tarmac
(hypernym) paved surface
(derivation) macadamize, macadamise, tarmac

n. macadam, tarmac, broken stones used for paving roads; road or other surface paved with layers of broken stones bound with tar or asphalt

n. macadam, broken stones used for paving roads