Bituminous pavement types; asphalt concrete construction

Types of bituminous pavements

Bituminous pavements are classified by the layer, method of mixing and surface treatment.

Construction of asphalt concrete pavement

Asphalt concrete (bituminous concrete, BC) is a dense-graded hot mix of aggregates, filler and bitumen (VG-30/60-70 penetration grade) laid as a wearing or binder course (IRC:29 / MoRTH Section 509, Nepal DoR specifications).

Materials

• Aggregate: crushed, clean, hard and cubical (LAA below 30 percent, AIV below 24 percent, combined flakiness and elongation index below 30 percent, stripping value below 5 percent, water absorption below 2 percent), graded to BC Grading 1 or 2.
• Filler: stone dust, cement or hydrated lime passing 75 micron.
• Binder: bitumen 60/70 or VG-30; 5 to 6 percent by weight of mix as fixed by Marshall design (stability above 9 kN, flow 2-4 mm, air voids 3-5 percent, VMA 12-14 percent).

Plant and equipment: hot-mix plant (batch or drum mix) with dryer, mixing unit and temperature control, tippers with tarpaulin, bitumen distributor, paver finisher with screed, steel-wheel roller (8-10 t), pneumatic tyre roller (15-20 t), small plate compactor, thermometers.

Construction steps

1. Prepare the base: clean it, repair defects, and apply a tack coat (0.20-0.25 kg/m2 of emulsion) to an old bituminous surface or a prime coat to a granular base.
2. Produce the mix at the plant: aggregate heated to about 160-175 C and bitumen to 150-165 C; the mixed material leaves the plant at not more than 170 C.
3. Transport in covered, insulated tippers; the mix must arrive hot enough to be laid at about 120-165 C, depending on the binder grade.
4. Lay with a paver at uniform speed, using level sensors; hand-lay only small patches.
5. Initial (breakdown) rolling with an 8-10 t steel roller at 110-135 C, intermediate rolling with a pneumatic tyre roller, and final rolling with a tandem roller at 70 C or more to remove roller marks. Roll longitudinally from the edge towards the centre, with overlapping passes.
6. Joints: cut transverse joints vertical and apply tack coat; stagger longitudinal joints.
7. Open to traffic when the surface cools below 60 C.

Quality control: tests on aggregates and bitumen (penetration, softening point, ductility); mix design by the Marshall method; at site: temperature of mix, binder content (extraction), gradation, Marshall stability and flow, density from cores (above 98 percent of laboratory Marshall density), thickness (+-5 mm), and surface regularity with a 3 m straight edge (max 3 mm undulation) and bump integrator.