6.2.4 Binder Absorption Allowance (Aba)
It will be necessary to increase the binder application rate to allow for any binder absorption by pavement and/or aggregate, Aba, but it is not possible to give a general allowance.
Binder absorption by pavement
It is strongly recommended that all new pavement surfaces should be primed. However, where an initial seal is applied directly to the prepared granular pavement, and binder from a sprayed seal may drain into voids in the surface of the basecourse. This is most likely to occur in sandy or silty rubble base materials (sandstone, limestone or silty gravels) in a hot dry climate.
The following binder absorption allowances provide a guide for use with various pavements:
allow +0.1 to +0.2 L/m2
allow +0.0 to +0.1 L/m2
allow –0.1 to +0.1 L/m2
for the use of chemical binders, refer to Austroads (2006).
For highly absorptive pavement surfaces, particularly in hot climates, long-term absorption of the binder into the basecourse can occur. The allowance for this will generally be between +0.1 to +0.2 L/m2. Where more than 0.2 L/m2 is required, an alternative treatment should be considered.
Alternative treatments may comprise:
- use of a different class of binder, including PMB
- modification or stabilisation of the basecourse
- use of a 5 or 7 mm initial seal, followed by a larger aggregate seal one or two years later.
In extreme cases, binder absorption into base materials may lead to the need for surface enrichment or a small aggregate reseal being required within one or two years.
Binder absorption into existing spray sealed surfaces will seldom be a problem unless the existing surface is visibly open and porous.
Binder absorption can occur when sealing over aged and brittle asphalt and microsufacing. In these cases, a binder absorption allowance of +0.1–0.2 L/m2 may be appropriate.
Binder absorption by aggregate
Absorptive aggregates may fall into two general categories:
- porous, e.g. sandstone, rhyolite
- vesicular (full of cavities), e.g. scoria, slags.
In general, binder absorption into aggregate is not applicable, but if an allowance is required, it should not usually exceed 0.1 L/m2.