Pavement

A general schematic of the process for determination of binder and aggregate application rates for double/double seals is shown in Figure 5.3.

First application

The voids relationship in the first application seal of a double/double seal is affected by the use of the smaller aggregate in the second application, which partially fills and therefore reduces the air voids. The reduction in air voids varies from about 10% at high traffic to 30% at very low traffic volumes. This is compensated for by reducing the basic voids factor compared to a single/single seal. These plots only apply to double/double seals constructed on the same day or both seals applied consecutively with minimal trafficking of the first application seal.

The first application seal is designed using the same procedure as for a single/single seal, but taking into account the fact that the voids in this layer will be reduced by the smaller aggregate in the second application seal.

The design process is as follows:

1. Determine design traffic (Section 5.2).
2. Determine basic voids factor, Vf1 (Figure 6.3 or Figure 6.4).
3. Apply voids factor adjustments (Section 6.1.2)
   1. Aggregate, Va1
   2. Traffic effects, Vt.
4. Calculate design voids factor, VF1 (Section 6.1.3).
5. Determine basic binder application rate, Bb1 (Section 6.2.1).
6. Determine modified binder application rate, Bbm1 (Section 6.3.2).
7. Apply allowances
   1. Surface texture, As (Table 6.3)
   2. Binder absorption, Aba1 (Section 6.2.4)
   3. Embedment, Ae (Section 6.2.3).
8. Calculate design binder application rate, Bd1 (Section 6.4).
9. Determine design aggregate spread rate (Section 6.7.4).

Second application

Where, as recommended, the second application is to be applied immediately after the first with little or no trafficking between applications, the following design procedure is used. Guidance for delaying the second application is provided next.

The design traffic is the same for both layers but for the design of the second application seal there is nil reduction in the air voids.

The design process is as follows:

1. Use the same design traffic as determined for the first application.
2. Determine basic voids factor, Vf2 (Figure 6.3 or Figure 6.4).
3. Apply voids factor adjustments (Section 6.1.2)
   1. Aggregate, Va2
   2. Traffic effects as applied in the first application, Vt.
4. Calculate design voids factor, VF2 (Section 6.1.3).
5. Determine basic binder application rate, Bb2 (Section 6.2.1).
6. Determine modified binder application rate, Bbm2 (Section 6.3.2).
7. Apply allowances, which are generally nil for the second application; the only allowance that could be considered is binder absorption into the aggregate, but this is unlikely for normal aggregates
   1. Binder absorption, Aba2 (Section 6.2.4).
8. Calculate design binder application rate, Bd2 (Section 6.4).
9. Determine design aggregate spread rate (Section 6.7.4).

Second application delayed

For double/double seals, it is recommended that the second application is to be applied immediately after the first with little or no trafficking between applications. If this is not possible, the double/double should be considered as two single/single seal layers, with some modification to the design process as outlined below.

If the second application is delayed, and the seal will be trafficked during this period, the outcomes become more difficult to predict. Designers must modify binder application rate allowances for surface texture, as well as aggregate spread rates, depending on the extent of trafficking of the first application.

If unsure, it is recommended that the second application be delayed for 12 months, or longer if practical, to allow the first application to settle down and provide a reasonable indication of final orientation of the aggregate and surface texture of the mosaic.

The design procedure is as follows:

1. First application – design the first application as a single/single seal (Section 5.5.1).
2. Second application – design the second application as another single/single seal (Section 5.5.1). In this design, measure the surface texture of the first application and determine an appropriate allowance from Table 6.3. The texture allowance values in this table are an indicative guide only. If the time delay is less than 12 months between applications, include only part of the surface texture allowance as shown in Table 5.3.

The longer the delay, the more of the standard texture allowance is applied. After about 12 months, the full allowance is generally applicable. The texture must be measured each time the design is being considered, and the appropriate allowance determined.

Note: It is preferable to delay the second application up to 12 months to let the first application settle down, and this should provide a better indication of any texture allowance that should be applied. If the binder is still lively in the first application, the surface texture allowance should be reduced, possibly to about 75% of the value given in Table 6.3.