Bituminous mixes

A bituminous mix is a blend of graded aggregate, filler and bitumen, heated, mixed and laid hot, then compacted by rollers. The aggregate skeleton carries load by stone-on-stone contact, the bitumen binds it and seals it against water, and air voids left after compaction must be neither too many (allowing water and air in, causing ageing and stripping) nor too few (causing bleeding and rutting in hot weather).

Common Indian mixes include bituminous macadam and dense bituminous macadam for base and binder courses, bituminous concrete for surfacing, stone matrix asphalt with a gap-graded stone skeleton held by mastic for heavy-duty surfaces, and open-graded friction courses for drainage and noise. Choice depends on traffic, climate and layer position.

Mix design traditionally uses the Marshall method: specimens with a range of bitumen contents are compacted, tested for stability and flow, and analysed for bulk density, air voids, voids in mineral aggregate and voids filled with bitumen. The design bitumen content meets all criteria, typically giving air voids around 4 percent in the compacted mix. Superpave gyratory compaction better simulates field compaction and is used increasingly. On site, temperature control during mixing, laying and rolling is crucial; field density, thickness, surface regularity and binder content by extraction are checked. Warm mix additives and recycled asphalt are now part of mainstream practice.