"Mortar mechanism" vs. "stone contact mechanism" in bituminous materials

'Stone contact mechanism' applies to well graded aggregates coated with bitumen (for example dense bitumen macadam) where traffic loads on bituminous roads are resisted by stone-to-stone contact and by interlocking and frictional forces amid the aggregates. It is vital to adopt aggregates with a high crushing strength.  Bitumen coatings on the surface of aggregates merely serve to cement the aggregates together.

'Mortar mechanism' involves the distribution of loads within the mortar for gap-graded aggregates (for example hot rolled asphalt). Mortar has to possess high stiffness to prevent excessive deformation under severe traffic loads. It is general practice to introduce some filler to stiffen the bitumen.