Double surface treatment materials and procedure

Double bituminous surface dressing (DBSD) is a surface treatment of two successive applications of bitumen and stone chips on a prepared WBM or granular base, forming a 20-25 mm waterproof wearing surface (IRC:17 and IRC:23; Nepal DoR "Standard Specifications for Road and Bridge Works"). It is used for low-cost roads.

Materials

• Binder: bitumen 80/100 or 60/70, cutback or emulsion; typical rates are about 1.2 kg/m2 for the first coat (with 13.2 mm chips) and about 1.0 kg/m2 for the second coat (with 11.2 mm chips), as fixed by the specification.
• Aggregate: crushed stone or gravel, AIV below 30 percent, flakiness below 25 percent, water absorption below 1 percent, stripping value below 5 percent; first coat 13.2 mm (about 0.15 m3 per 10 m2); second coat 11.2 mm or 6.7 mm (about 0.09 m3 per 10 m2).
• Prime coat over a WBM base if required: 0.6-1.0 kg/m2 of cutback or emulsion.

Construction procedure

1. Prepare the base: sweep and clean it, fill all depressions, patch potholes and ensure the correct camber. Apply a prime (or tack) coat and let it cure.
2. Spray the first coat of binder with a pressure distributor at the specified rate and spraying temperature (about 150-170 C for paving bitumen; lower for cutback, and emulsion is sprayed cold or warm), as a uniform film, one lane width at a time.
3. Immediately spread the first-coat chippings (13.2 mm) by chip spreader or manually, covering 100 percent of the binder without piling.
4. Roll with an 8-10 t roller, moving from the edge to the centre; brush off surplus chips.
5. Apply the second coat of binder and the smaller chips (11.2 mm) in the same way, then roll.
6. Do the final rolling with a pneumatic roller and sweep off loose chips; open to traffic after 24 hours at low speed (20 km/h) for a few days.
7. Quality control: bitumen temperature and rate of spread (tray test), chip spread rate, grading, cleanness and surface regularity.