5.5.3 Initial Treatments
There is no formal design method for the selection and design for priming which is usually based on experience with the local pavement materials and prevailing weather conditions. A general guide to the selection of the grade of cutback bitumen primer and primer application rates is shown in Table 5.4.
|Pavement types||Grade (AS 2157)||Primer application rate (L/m2)|
|Tightly bonded||AMC 00||0.6 to 1.1|
|Medium porosity||AMC 0||0.8 to 1.1|
|Porous||AMC 1||0.9 to 1.3|
|Very porous (limestone and sandstone)||AMC 1||2 application rates:|
1st @ 0.7 to 0.9
2nd @ 0.5 to 0.7
|Hill gravels, granitic sands||AMC 0||0.8 to 1.1|
|Stabilised||AMC 00(1)||0.5 to 0.8|
|Concrete||AMC 00(1)||0.2 to 0.4|
- Can also consider using proprietary materials, or additional cutter in these cases.
The most suitable grade and application rate of primer will depend on the surface finish of the basecourse. Small‑scale trials can be conducted on pavement materials in situ to test and compare the appropriateness of the prime grade and application rate.
Standard grades of bitumen emulsion are not suitable for priming but proprietary grades have been developed that should be used in accordance with manufacturer’s guidelines.
Where a pavement stabilised with cementitious or chemical binders is to be primed with 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.
Binder application rates are calculated as per the typical Austroads seal design methods and are reported in terms of residual binder.
Rates will generally need to be adjusted in line with allowances for absorption and aggregate embedment.
For higher traffic loadings, design binder application rates may be undesirably low, in which case alternative treatments should be considered to provide a more robust and waterproof initial treatment. Design binder application rates may be checked for appropriateness by comparing to those of performing initial seals in similar conditions and traffic loadings, and by seeking specialist advice.
The design procedure is as follows:
- Apply steps 1 to 6 of the design procedure for single/single (Section 5.5.1) or double/double (Section 5.5.2).
- Apply allowances
- Surface texture, As
Some pavement basecourses present a coarse-textured surface and it may be possible to measure surface texture. The allowances as suggested for texture measured on an existing 5 or 7 mm seal in Table 6.3 can be applied in this case.
If surface texture cannot be measured, the texture allowance is generally in the order of 0.0 to +0.3 L/m2.
Practitioners should proceed with caution when applying texture allowances where large surface texture is to be matched with small aggregate particles, as the aggregate may sit inside the texture of the basecourse.
- Binder absorption, Aba
Allowances for absorption are required for initial seals, except for unusually tight and hard surfaces, or impermeable pavement materials such as stabilised basecourses. Guidance for the absorption allowance is provided in Section 6.2.4.
When pavements are stabilised with chemical binders, refer to the Guide to Pavement Technology Part 4D: Stabilised Materials (Austroads 2006).
- Embedment, Ae
Embedment allowances can be applied as per the instructions in Section 6.2.3.
For ball penetration values exceeding 3 mm, the reduction in binder application rate from the embedment allowance may lead to a situation where heavily trafficked sprayed seals have inadequate binder present to waterproof the pavement.
Where ball embedment exceeds 4 mm, re-preparation of the pavement, including possibilities for improvement in quality of the pavement material, armour coating with a thin layer of good quality material, stabilisation and other treatments should be considered.
- Surface texture, As
- Determine design aggregate spread rate for single/single (Section 6.7.3) or double/double (Section 6.7.4).
- Calculate design binder application rate, Bd (Section 6.3).