Jose wrote:

Tarmac (short for tarmacadam) is actually a trade name for
the substance; it (the word)is formed from "tar" and "macadam". Macadam
(the paving substance made of crushed stone and a binder, usually tar)
is named after its inventer, John L. McAdam, a Scottish engineer.

Macadam is just the gravel, without any binder. John L. McAdam was the
first who built roads using several layers of gravel, without any
binder, each layer being pounded before the next layer was applied. This
method was new. The resulting surface was called macadam.

Tar-macadam most probably was the same thing with a binder, originally
tar, as I would assume.

Stefan