3.4.1 Variable Rate Spraying
Sealing with a variable rate spray bar is the process of spraying different binder application rates across the width of a spray run.
This process facilitates the optimisation of the seal design to address the most common types of defects in sprayed seal surfacings, which are:
- flushing/bleeding in the wheelpaths
- stripping in non-wheelpath areas (i.e. around the centreline, between wheelpaths and on shoulders).
Existing surface texture has an important influence on the amount of binder required to produce an effective sprayed seal. It is difficult to design a suitable single binder application rate when there is variable texture across a pavement, for instance when wheelpaths are flushed, yet the non-trafficked areas between wheelpaths and down the centreline are highly textured.
Variable binder application rates may be achieved by methods including pre-spraying (described in Austroads 2003), and utilising specialised equipment that sprays variable rates in a single pass.
Texture variation across and along the pavement may mean that an effective corrective treatment, or effective reseal, cannot be achieved and an alternative surfacing such as hot mix asphalt or microsurfacing needs to be considered. In some instances, it may be more effective and economic to remove the existing surfacing entirely and re-prepare the pavement for an initial treatment.