Typical pavement structures sketch; otta seal

Typical pavement structures

 Flexible        Rigid (CC)      Low volume
 (highway)                       (Otta seal)
 +-----------+   +-----------+   +-----------+
 | BC  40-50 |   | PQC slab  |   | Otta seal |
 +-----------+   | 200-300   |   | 16-32     |
 | DBM 50-170|   +-----------+   +-----------+
 +-----------+   | DLC   150 |   | Base      |
 | WMM   250 |   +-----------+   | 150-200   |
 +-----------+   | GSB   150 |   +-----------+
 | GSB   200 |   +-----------+   | Sub-base  |
 +-----------+   | Subgrade  |   | 100-200   |
 | Subgrade  |   | 500       |   +-----------+
 | 500       |   +-----------+   | Subgrade  |
 +-----------+                   +-----------+

Thicknesses in mm (IRC:37, IRC:58, Nepal DoR low volume guidelines); actual values come from the design for the traffic and subgrade CBR. A single Otta seal is about 16 mm thick and a double one about 32 mm.

Construction methodology of Otta seal

Otta seal is a low-cost bituminous surfacing made of a single or double layer of graded natural gravel or crushed stone (maximum size 16-19 mm) rolled into a relatively thick film of soft binder (cutback such as MC-3000, or 150/200 penetration bitumen) sprayed on the base. It originated in Norway (Norwegian Public Roads Administration, Note 31) and is used on low-volume roads in Nepal under DoR guidelines.

Materials: graded aggregate (continuous grading, maximum size 16 or 19 mm; fines below about 10 percent; flakiness below 30 percent) from a crusher or river source; soft binder (150/200 penetration bitumen cut back with kerosene, or emulsion). A prime coat on the base is generally not required, which is one of the advantages of Otta seal.

Construction

1. Prepare the base (WBM, crushed gravel or stabilised base) to a smooth, tight surface; patch defects and sweep it clean (prime only if the specification asks for it).
2. Spray the binder with a distributor at the specified rate (about 1.4 to 2.0 litres per m2 for the first layer, depending on the grading and the base) at its spraying temperature (about 120-150 C for hot binders).
3. Immediately spread the aggregate (about 13-16 litres per m2, slightly damp) by chip spreader or tipper to cover the binder fully.
4. Roll with a pneumatic tyre roller (at least 12 t) with many passes (about 15 or more); the kneading forces the binder up through the aggregate.
5. Broom loose stones back onto the surface during the first days. For a double Otta seal, apply the second layer (binder and a finer graded aggregate) in the same way after the first layer has bedded in under traffic.
6. Allow traffic at low speed. The seal improves with traffic over the first weeks as the binder works up into the aggregate.