Seal, prime, tack coat and bituminous concrete

Prime coat, tack coat and seal coat

• Prime coat: low-viscosity cutback bitumen (MC-30 / MC-70) or emulsion sprayed on an unbound granular (WBM/WMM) surface before bituminous layers are laid (rate 0.6-1.0 kg/m2). It binds loose dust, plugs the surface voids, hardens the top, and provides adhesion between the base and the bituminous layer (IRC:16).
• Tack coat: a thin film of bitumen emulsion (RS-1 / SS-1) or cutback (rate 0.20-0.30 kg/m2) sprayed on an existing bituminous or cement concrete surface before overlaying. It bonds the old and new layers, preventing slippage and delamination.
• Seal coat: a thin surface treatment of bitumen (about 0.7-1.0 kg/m2) with 6.7 mm chips or sand, laid on a newly laid open-textured surface (such as premix carpet) or on an old surface (IRC:14 / MoRTH Section 513). It seals the voids, makes the surface waterproof, protects it against weathering and improves skid resistance.

Bituminous concrete pavement

Asphalt concrete (bituminous concrete, BC) is a dense-graded hot mix of aggregates, filler and bitumen (VG-30/60-70 penetration grade) laid as a wearing or binder course (IRC:29 / MoRTH Section 509, Nepal DoR specifications).

Materials

• Aggregate: crushed, clean, hard and cubical (LAA below 30 percent, AIV below 24 percent, combined flakiness and elongation index below 30 percent, stripping value below 5 percent, water absorption below 2 percent), graded to BC Grading 1 or 2.
• Filler: stone dust, cement or hydrated lime passing 75 micron.
• Binder: bitumen 60/70 or VG-30; 5 to 6 percent by weight of mix as fixed by Marshall design (stability above 9 kN, flow 2-4 mm, air voids 3-5 percent, VMA 12-14 percent).

Plant and equipment: hot-mix plant (batch or drum mix) with dryer, mixing unit and temperature control, tippers with tarpaulin, bitumen distributor, paver finisher with screed, steel-wheel roller (8-10 t), pneumatic tyre roller (15-20 t), small plate compactor, thermometers.

Construction steps

1. Prepare the base: clean it, repair defects, and apply a tack coat (0.20-0.25 kg/m2 of emulsion) to an old bituminous surface or a prime coat to a granular base.
2. Produce the mix at the plant: aggregate heated to about 160-175 C and bitumen to 150-165 C; the mixed material leaves the plant at not more than 170 C.
3. Transport in covered, insulated tippers; the mix must arrive hot enough to be laid at about 120-165 C, depending on the binder grade.
4. Lay with a paver at uniform speed, using level sensors; hand-lay only small patches.
5. Initial (breakdown) rolling with an 8-10 t steel roller at 110-135 C, intermediate rolling with a pneumatic tyre roller, and final rolling with a tandem roller at 70 C or more to remove roller marks. Roll longitudinally from the edge towards the centre, with overlapping passes.
6. Joints: cut transverse joints vertical and apply tack coat; stagger longitudinal joints.
7. Open to traffic when the surface cools below 60 C.

Quality control: tests on aggregates and bitumen (penetration, softening point, ductility); mix design by the Marshall method; at site: temperature of mix, binder content (extraction), gradation, Marshall stability and flow, density from cores (above 98 percent of laboratory Marshall density), thickness (+-5 mm), and surface regularity with a 3 m straight edge (max 3 mm undulation) and bump integrator.