--- title: "Prime vs tack coat; surface dressing method" subject: "Transportation Engineering II" chapter: 3 key: "semester_7/transportation_engineering_ii:prime-vs-tack-coat-surface-dressing" source: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/prime-vs-tack-coat-surface-dressing site: BCT Engineering --- # Prime vs tack coat; surface dressing method Transportation Engineering II · Chapter 3: Road Construction Technology - Asked in: 2068 Baisakh, 2+6 marks - Video: [Difference between Prime Coat, Tack Coat and Seal Coat](https://www.youtube.com/watch?v=_h_q1DYdsco) — SDO Aspirants - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/prime-vs-tack-coat-surface-dressing - Whole chapter: https://www.bctengineer.com/md/semester_7/transportation_engineering_ii/chapter-3 ## Question Distinguish between prime coat and tack coat. Explain construction method of surface dressing. ## 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. ## Answer #### 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 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Prime coat is applied on, while tack coat is applied on: A. an old bituminous surface; a granular base B. a granular/untreated base; an existing bituminous or concrete surface C. the subgrade only; the sub-base only D. a concrete slab; a granular base Answer: B. Prime coat goes on granular (WBM/WMM) bases; tack coat goes on existing bituminous or concrete surfaces or between bituminous layers. (Key point: Where each is applied) 2. Which pair of spray rates is correct? A. Prime 0.20-0.30 kg/m2; tack 0.6-1.0 kg/m2 B. Prime 0.6-1.0 kg/m2; tack 0.20-0.30 kg/m2 C. Both 1.0-1.5 kg/m2 D. Prime 5 kg/m2; tack 2 kg/m2 Answer: B. Prime coat penetrates the base so needs 0.6-1.0 kg/m2; tack coat is only a thin film at 0.20-0.30 kg/m2. (Key point: Spray rates) 3. Why can prime coat use a low-viscosity cutback (MC-30/70)? A. It must penetrate 3-6 mm into the base, plugging voids B. It must remain only on the surface C. It must set instantly for traffic D. It needs to be heated to 170 C Answer: A. Prime coat penetrates the granular base to bind loose particles and plug voids, which needs a low-viscosity binder. (Key point: Penetration) 4. In surface dressing, after spraying binder at about 1.0-1.5 kg/m2 the chips should be spread: A. after the binder has fully cooled and hardened B. immediately, in a thin layer covering the binder fully C. only after rolling the binder D. after 24-48 hours of curing Answer: B. Chips must be spread immediately while the binder is hot so that they stick, covering it fully without overlap. (Key point: Surface dressing binder step) 5. Rolling in surface dressing is done: A. from the centre to the edges with a vibratory roller B. from edges to the centre with an 8-10 t smooth-wheel or pneumatic roller until chips are embedded 50-70 percent C. once with a 1 t roller to avoid crushing chips D. not at all, traffic does the work Answer: B. Rolling goes from edges to centre until chips are embedded about 50-70 percent of their depth. (Key point: Rolling) ## Related in Transportation Engineering II - [Seal, prime, tack coat and bituminous concrete](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aseal-prime-tack-coat-bituminous-concrete) — Transportation Engineering II, chapter 3 - [Construction equipment; prime, tack and seal coats](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aequipment-plants-prime-tack-seal-coat) — Transportation Engineering II, chapter 3 - [Surface dressing and DBSD construction](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Asurface-dressing-dbsd-procedure) — Transportation Engineering II, chapter 3 - [Explanatory note: surface dressing](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Asurface-dressing-note) — Transportation Engineering II, chapter 3