3.2.2 Initial Seal
An initial seal is an initial treatment, being an application of a sprayed seal to a prepared basecourse, which has not been primed. These are termed ‘first coat seals’ in New Zealand. It is intended that the seal adhere to the base, whilst providing a wearing course for traffic (Figure 3.3). It allows both immediate trafficking and permits a delay in placing of the final surfacing.
The success of initial seals relies on:
- selection of an appropriate initial sealing treatment
- appropriate binder application and aggregate spread rates
- preparation and condition of base materials
- application procedures (workmanship)
- careful control of traffic speed (e.g. speed reductions) in the first few days after application.
Figure 3.3: Initial seal
When considering the suggested initial seal treatments, it is vital to select a treatment that matches the purpose and requirements of the road, and the subsequent construction and maintenance schedule.
Short-term treatments, typically consisting of a single/single seal with small aggregate size, can be employed to provide a trafficable surface in conditions where priming is not practicable. A secondary treatment, providing an ongoing wearing course will be required after a short period (Section 7.3.2). Particular care must be taken in these situations to provide adequate curing time between treatments, while respecting the limited life expectancy of the initial seal.
Utilising larger aggregate sizes, double/double seals, and lower cutter oil contents, may be used as more robust treatments that provide longer-term wearing courses, particularly for low traffic volumes.
Longer-term treatments that provide an ongoing wearing course are available, utilising larger aggregate sizes, double/double seals, and lower cutter oil contents. However, it must be recognised that a direct application of an initial seal to a basecourse presents a higher risk of failure, and shorter potential life span than what can be expected from a prime and seal approach.
Initial sealing is most effectively applied during warm and dry conditions. Care should be exercised when undertaking initial sealing during cooler and damp conditions. Low temperatures may extend curing time due to a slow cutter evaporation rate and can increase difficulties of achieving successful adhesion of the aggregate to the binder, which typically leads to higher cutter contents being required to counteract poor adhesion.
The choice of binder is mainly influenced by the prevailing weather conditions, as well as the desired life of the treatment, and timing and type of final treatment.
The requirements for AMC cutback bitumen grades are provided in AS 2157. Equivalent field‑blended grades may also be used.
Bitumen emulsions may be considered when a subsequent surfacing is to be applied before adequate curing of a cutback bitumen binder can occur. The requirements for bitumen emulsion grades are provided in AS 1160. Proprietary emulsion binders are also available.
Where a pavement stabilised with cementitious or chemical binder is to be initial sealed using a bitumen emulsion, a check on the compatibility of the emulsion with the stabilised material should be undertaken. Emulsions will always be compatible with bitumen-stabilised pavements.
Binders for very heavy traffic and/or very warm to hot conditions include proprietary grades of polymer modified binder, polymer modified emulsion, cutback bitumen manufactured with Class 320 base bitumen in place of Class 170 and cutback bitumen with low proportions of cutter oil. These binders provide for more rapid curing and reduced risk of bleeding in more demanding performance applications.
Whether a cutback or emulsion binder is used for an initial seal, the pavement surface should be lightly dampened (not wet) immediately prior to binder application to assist in the ‘wetting’ process.