Asphalt concrete construction and quality control

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, flakiness index below 30 percent combined, stripping value below 5 percent, water absorption below 2 percent), graded to the 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, repair defects, and apply a tack coat (0.20-0.25 kg/m2 of emulsion) to the old 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 by paver with proper level sensors and uniform speed; hand-lay only at small patches.
5. Initial (breakdown) rolling by 8-10 t steel roller at 110-135 C, intermediate rolling with pneumatic tyre roller, final rolling with tandem roller at 70 C or more to remove marks. Roll longitudinally from edge towards centre, overlapping.
6. Joints: cut transverse joints vertical and apply tack coat; stagger longitudinal joints.
7. Traffic opens when the surface cools below 60 C.

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