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Form of pavement invented by John McAdam. McAdam's road cross-section consisted of a compacted subgrade of crushed granite or greenstone designed to support the load, covered by a surface of light stone to absorb wear and tear and shed water to the drainage ditches. In modern macadam construction, crushed stone or gravel is placed on the compacted base course and bound together with asphalt cement or hot tar. A third layer to fill the spaces is then added and rolled. Cement-sand slurry is sometimes used as the binder.
This entry comes from Encyclopædia Britannica Concise. For the full entry on macadam, visit Britannica.com.