Surface dressing and DBSD construction

Surface dressing

Surface dressing is a thin wearing surface made by spraying a film of bituminous binder on the road base or old surface and covering it immediately with a layer of single-size stone chippings, which are then rolled (IRC:17). It may be single (one layer of binder and chips) or double (two layers). It seals the surface, makes it waterproof, gives skid resistance and renews worn-out surfaces, but adds no structural strength.

Construction method

1. Prepare the surface: repair potholes and cracks, sweep and clean; apply a tack or prime coat if needed. The weather should be dry (surface temperature above 20 C, no rain).
2. Spray binder: heat paving bitumen (VG-10/30 or 80/100) to 150-170 C (cutback is heated to a lower temperature, as specified) and spray it uniformly with a pressure distributor at the specified rate (about 1.0-1.5 kg/m2).
3. Spread chips: immediately cover the binder fully with clean, dry single-size chips (13.2 mm, or 11.2/9.5 mm for the second coat) by chip spreader or by hand, in a single layer without overlap.
4. Roll: roll with an 8-10 t smooth-wheel or a pneumatic tyre roller from the edges to the centre until the chips are embedded (about 50-70 percent of their depth).
5. Brush and cure: sweep off loose chips after traffic has bedded them in; restrict speed to 20-30 km/h for the first few days.
6. For double surface dressing, the second coat of binder and smaller chips is applied in the same way after the first.

Construction procedure of DBSD

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.