Another concise source here:
The Construction & Makeup of Ancient Roman Roads
"...the upper layers of the road are always laid carefully, “of finer material well-rammed down”. He says, “It is apparent from many well-preserved portions that this was often done in several successive layers, for if it is excavated for a section, the material can be seen to break away in flat slabs of gravel, etc., corresponding to these original layers”.
"...a layer of material such as sand is also a common part of this middle layer, serving to lend the road resilience. Adam calls this middle layer the rudus, a layer of “sand, or gravel and sand, sometimes mixed with clay”.
"The topmost surface, or metalling, of the road varies; and most roads were defined by curb stones on each side. The ancient Roman roads are not always paved, especially along difficult stretches, but when available, the roads are paved at least with gravel. Flint and other small, broken stones are also used to pave the road, and Adam says that sometimes slabs of stone are used, though this did not appear before the beginning of the second century B.C. “A text of Livy gives precise information that in 174 B.C. roads had to be paved in towns, but simply surfaced with sand or pebbles on country sections”.
"...“In districts where iron was being worked, the hard slag [the waste metal created in producing iron] provided an almost ideal metalling, which seems in some cases to have increased its effectiveness by rusting together into an uncommonly hard concrete-like mass”... Some other roads were metalled with a sort of rough concrete; a combination of gravel bound with a little mortar..."