A prime is an initial treatment, being an application of a primer to a prepared granular base (Figure 3.2). It usually consists of a bitumen and cutter oil or specially formulated bitumen emulsion primer and is placed without a cover aggregate. Penetration of the prime into the base varies, but for granular base materials it is typically 5 to 10 mm, depending on the type and grade of prime.
As it has no aggregate, it should not be trafficked until the prime has dried and is no longer tacky. Public traffic should only be permitted once a more substantial surfacing treatment (secondary treatment) has been applied after the prime has cured. Curing times will vary depending on the weather conditions and nature of the base, but generally a minimum 3-day curing period is required for cutback primes, and a minimum 1‑day curing period is required for emulsion primes, depending on the prevailing weather.
A prime is used to:
- penetrate the surface of an unbound granular layer filling all surface pores
- provide a bond onto which a subsequent bituminous surfacing can adhere
- provide a surface which retards the absorption of the bitumen, from a subsequent bituminous surfacing, into the pavement
- assist in waterproofing and protecting the pavement
- assist with the curing of stabilised pavements
- provide a temporary surface for traffic (though it should be noted that the life of the primed surface exposed to traffic is brief).
A prime is always recommended over granular pavements that are to be surfaced with a sprayed seal, or to be surfaced with asphalt where the depth of surfacing is less than 100 mm. A prime is also recommended for bonding of sprayed seals or asphalt to timber or concrete surfaces.