4.5.2 Aggregate Size
Nominal aggregate sizes ranging from sand up to 20 mm are used in sprayed seals.
Aggregate sizes up to 14 mm are common in single/single seals; 10 mm and 14 mm aggregate are usually used where there is sufficient traffic volume to warrant the use of large sized aggregates. For lower traffic volume roads and low speed environments, the use of 7 mm aggregate is often sufficient.
Large aggregates such as 16 mm and 20 mm are occasionally used in single/single seals and will feature high binder application rates. They have the following disadvantages:
- high tyre/road noise
- increased risk of damage to vehicles from large, loose aggregate particles on new work
- increased cost.
The primary use of 16 mm and 20 mm aggregates is in combination with a smaller sized aggregate in double/double or single/double seals. It is desirable for aggregate in the second application to be approximately half the average least dimension (ALD) of the aggregate in the first application. Common combinations of aggregate in double/double and single/double seals are:
- 10 mm with a 5 or 7 mm aggregate
- 14 mm with a 5 or 7 mm aggregate
- 16 or 20 mm with a 7 or 10 mm aggregate.
Smaller aggregate sizes such as 5 mm and 7 mm may be used:
- as a second application of a double/double or single/double seal
- as a surfacing treatment for very lightly trafficked pavements, or for pavements where a fine surface texture is required
- as part of a temporary treatment to waterproof and cure the pavement after construction before trafficking, e.g. new construction which will not be opened to traffic for some time
- as part of a scatter coat or racked-in treatment
- for corrective treatments prior to resurfacing.
A guide to the most commonly used sizes is shown in Table 4.3.
Table 4.3: Generally recommended aggregate sizes for sprayed seal treatments
| Treatment | Conditions | Common sizes | Comment |
|---|---|---|---|
| Initial seal | To be resealed before opening to traffic | 5 or 7 mm | A small-sized aggregate will carry construction traffic at lowest cost and avoid presenting a very coarse texture that may require additional binder when applying the final seal. |
| Under traffic | 7 mm for firm pavements and low traffic 10 mm, 10/5, 10/7 or 14/7 in other cases | ||
| Prime and sprayed seal | To be resealed before opening to traffic | 5 or 7 mm | For low-traffic roads, 7 and 10 mm sprayed seals can provide adequate service at lowest initial cost; 14 mm sizes are initially more expensive but can be cost-effective in some light traffic applications where surface enrichment is used to extend the life of the seal (surface enrichment may be undertaken a number of times). |
| Under traffic | 10 or 14 mm for low stress environments 14/7 or 20/7 for heavy traffic and high stresses | ||
| Reseal | Existing seal 7 mm, or asphalt surface | Generally 7 or 10 mm for low traffic, and 14 mm for high traffic (but reduce to 10 mm if noise is an issue) | If the existing surface texture is very high or uneven, it may be very difficult to successfully reseal. In such cases a corrective treatment using 5 or 7 mm aggregate, or microsurfacing may provide a more even surface texture which can subsequently be resealed with a larger aggregate size. |
| Existing seal 10 mm | 7 mm for low traffic or 14 mm for high traffic (see comment) | ||
| Existing seal 14 or 16 mm | 7 mm for low traffic or 10 mm for high traffic (see comment) | ||
| SAM | Existing seal 7 mm or an asphalt surface | Generally 10 mm for low traffic and 14 mm for high traffic (but reduce to 10 mm or use double/double seal if noise is an issue) | If the existing surface texture is very high or uneven, it may be very difficult to successfully reseal. In such cases a corrective treatment using 5 or 7 mm aggregate, or microsurfacing may provide a more even surface texture which can subsequently be resealed with a larger aggregate size. Generally, a SAM seal requires 1.5 L/m2 of PMB to resist reflection cracking. The aggregate size may need to be selected to accommodate this rate or accept that a reduced life may result if a lower rate is used. |
| Existing seal 10 mm | 10 mm for low traffic or 14 mm for high traffic (see comment) | ||
| Existing seal 14 mm | 10 mm for low traffic or 14 mm for high traffic (see comment) | ||
| SAMI | Interlayer beneath asphalt | Generally 10 mm | Generally, a SAMI seal requires 1.8 to 2.2 L/m2 of PMB to resist reflection cracking. Risk of flushing of a 10 mm seal at such application rates is minimal where the seal is only trafficked for a short period before applying asphalt. |
| GRS | To remain as a sprayed seal surface | Generally 14/7 mm | Double application geotextile reinforced sprayed seals using PMB or Class 170 binder are preferred for most GRS applications. If the existing surface on which a single application geotextile sprayed seal is placed is coarse (e.g. surface texture higher than 1.5 mm), a 7 mm sprayed seal may first be used to lower the texture and reduce the risk of premature stripping. |
| To be surfaced with asphalt | Generally 10 mm | ||
| Special seals | Floodways/areas subject to inundation | A heavy robust surfacing is required. Generally this involves a double application seal using combinations of 20 mm or 16 mm with 10 or 7 mm aggregate, or 14 mm with 7 mm aggregate. | |
| Flushed areas | If single application seals or corrective treatments are considered unsuitable, then an effective alternative can be a double application seal using 20 mm with 10 or 7 mm aggregate or an inverted seal. | ||
| Shoulders on highways and freeways | Whilst small size aggregate seals can be used, generally the greatest durability is obtained with double/double seals using a combination of 14 mm with 7 mm (or 20 mm with 10 mm aggregate where a heavier treatment is required). | ||
| Arid country | Consider a double application seal where the role of the top seal coat is to protect the binder in the bottom coat from deterioration/contamination from dust. Consider a single/single seal with an aggregate spread rate 5% less than the design spread rate to provide coverage of binder with aggregate. | ||