4.9.1 Variable Binder Spray Rates
Sealing with a variable rate spray bar could be considered in the following situations:
- where there is a significantly different surface texture across the lane (e.g. where the difference in texture allowance is ≥ 0.3 L/m² between the wheelpath and non-wheelpath areas), and/or
- where there is a high percentage of equivalent heavy vehicles (EHV(%)) and/or a high annual average daily traffic (AADT)
- This can sometimes result in a uniform spray rate across the road which is too low to avert stripping in untrafficked areas (i.e. around the centreline, between wheelpaths and on shoulders) and yet too high to avert flushing in the wheelpaths.
The underlying principle of variable rate seal design is to:
- design for the wheel paths
- increase the spray rate for the non-wheelpath areas to account for the higher risk of stripping in these areas.
When determining the spray rates to be adopted, the limitations of the variable rate sprayer to be used must be considered.
Due to the reduced binder application rate in low-spray areas, sufficient time under traffic at higher temperatures is required to ensure adhesion of the aggregate before the onset of cold weather. Therefore, it is highly desirable for any variable-rate seal to be subjected to at least one month of hot/warm weather under traffic. Variable‑rate seals are not typically used when the expected daily minimum air temperatures are less than 10 °C within one month after completion of the works.