- 2068 Baisakh · 2+6 marks
Prime vs tack coat; surface dressing method
Short answer
Prime coat (cutback, 0.6-1.0 kg/m²) binds and waterproofs granular bases; tack coat (emulsion, 0.20-0.30 kg/m²) bonds between bituminous layers. Surface dressing construction: spray hot binder (1.0-1.5 kg/m²), cover with chips, roll, sweep loose chips, and cure; for double dressing apply a second coat with smaller chips.
Distinguish between prime coat and tack coat. Explain construction method of surface dressing.
Prime coat vs tack coat
| Basis | Prime coat | Tack coat |
|---|---|---|
| Applied on | Granular (WBM/WMM) or untreated base | Existing bituminous or concrete surface, or between bituminous layers |
| Purpose | Binds loose particles, plugs surface voids, gives bond to the layer above, waterproofs the base | Provides bond between old surface and new bituminous layer |
| Binder | Low-viscosity cutback (MC-30/70) or slow-setting emulsion | Emulsion (RS-1/SS-1) or rapid curing cutback |
| Rate | 0.6-1.0 kg/m2 | 0.20-0.30 kg/m2 (thin film) |
| Penetration | Penetrates 3-6 mm into base | Remains on the surface |
| Curing | Cures 24-48 hours before laying | Cures (breaks) in a short time before laying |
Construction method of surface dressing
- 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).
- 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).
- 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.
- 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).
- 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.
- For double surface dressing, the second coat of binder and smaller chips is applied in the same way after the first.
✓ Answer checked by Claude Opus 5.5 (high effort)
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