Bituminous premix carpet construction

Bituminous premix carpet (PMC) is a 20-25 mm thick open-graded wearing course in which pre-coated chippings are laid and compacted over a tack-coated base (IRC:14, MoRTH Section 508; used on rural and low-volume roads in Nepal under DoR/DoLIDAR specifications).

Materials

• Aggregate: 13.2 mm and 11.2 mm nominal size crushed stone/gravel; AIV below 30 percent, LAA below 35 percent, flakiness below 25 percent, water absorption below 1 percent, stripping value below 5 percent.
• Binder: bitumen 80/100 or 60/70 grade, or cutback / emulsion; quantities of binder and chippings per 10 m2 are taken from the IRC:14 table for the 20 mm carpet (per 10 m2: 0.18 m3 of 13.2 mm and 0.09 m3 of 11.2 mm chippings, with 9.5 kg and 5.1 kg of bitumen respectively, 14.6 kg in all).
• Tack coat: 0.25-0.30 kg/m2 of bitumen emulsion.

Equipment: bitumen boiler with sprayer, mixer (drum mixer or mixing platform), 8-10 t smooth wheel roller, tipper, brooms, hand tools, thermometers.

Construction procedure

1. Prepare the base: clean, repair, apply tack coat (0.25-0.30 kg/m2) and allow to break.
2. Heat aggregates to 130-165 C and bitumen to 120-165 C, mix in a mixer so that each chipping is uniformly coated (a mixing platform for small jobs).
3. Spread the premix at 100-120 C immediately by paver or by hand rake, to uniform camber and thickness.
4. Roll with an 8-10 t roller from edges towards the centre, starting at about 100 C, until the chippings are firmly keyed and the surface is true.
5. Apply the seal coat (a bitumen film with 6.7 mm chips or sand) after the carpet has set; this closes the open texture and makes it waterproof.
6. Open to traffic after the surface cools.

Quality control: aggregate and bitumen tests; temperature checks of mix at laying; rate of spread of materials by measuring the area covered; thickness (+- 5 mm) and cross-section; surface evenness with a 3 m straight edge (max 8 mm); bitumen content by extraction on random samples.