6.7.4 Double/Double Seals
As for a single/single seal, the design aggregate spread rate is based on ALD, but the first layer of aggregate is reduced by about 10% to provide a slightly more open mosaic to allow the second application of aggregate to firmly interlock.
Design for little or no trafficking between applications
Aggregate spread rates for the first application of a double/double seal are shown in Table 6.11.
Table 6.11: Double/double seal design aggregate spread rates for the first application seal, with little or no trafficking between applications
| Binder | Aggregate spread rate (m2/m3) |
|---|---|
| C170, C240, C320, multigrade bitumen, PMB | 950/ALD |
| Emulsion, AMC4 & AMC5 cutback binders | 850/ALD |
The spread rate of both layers should be adjusted to achieve an interlocking aggregate mosaic with just enough small stone to fill the voids in the first application. Suggested aggregate spread rates for the second application are shown in Table 6.12 as a guide to design the appropriate aggregate spread rates.
Table 6.12: Double/double seal design aggregate spread rates for second application, little or no trafficking between applications
| Binder | Aggregate size (mm) | Aggregate spread rate (m2/m3) |
|---|---|---|
| All binder types | 10, 7 | 900/ALD |
| 5 (no ALD) | 225 |
Note: For the second aggregate application, the design aggregate spread rate may be up to 20% less (i.e. 1100/ALD) than for a normal single/single seal design depending on the spread rate of the first aggregate application.
When specifying aggregate spread rates, values should be rounded to the nearest 5 m2/m3.
Design for second application delayed
A double/double seal with the second application delayed is to be considered as two separate single/single seals for design purposes.
For both applications, the aggregate spread rate is determined as appropriate for a single/single seal, based on the type of binder as per Table 6.8.