4.4.2 Performance Requirements
There are several parameters that require careful assessment in order to be able to select the most suitable seal treatment for a particular situation. These include:
- traffic volume, composition, speed, mass and turning/acceleration/deceleration movements
- existing surface condition (cracking type and severity, texture depth variation)
- performance requirements
- skid resistance
- surface texture
- noise
- water spray characteristics
- conspicuity of pavement markings
- appearance (aesthetics)
- availability of equipment, materials and expertise.
Austroads (2009a) provides advice on the effects of all of these parameters as well as a methodology for assessing sprayed seal, asphalt and concrete surfacings for a particular set of needs. Table 4.2 (from Austroads 2009a) provides an indication of the likely effects of resurfacing existing surfaces using different seal types. It gives a generic description of the effect of surfacing treatments.
The properties referred to in Table 4.2 are for newly placed surfacings after a settling-in period. For example:
- Skid resistance of new surfacings will increase after application when the precoating material has worn or been washed off the aggregate.
- New seals may be tender until they are bedded down, some of the cutter oils evaporated or, for emulsions, until the emulsion has cured.
- Asphalt surfaces may be tender until cooled sufficiently or tightened by the action of traffic.
Table 4.2: Effectiveness of sprayed seal, microsurfacing and combined resurfacing treatments on existing surfacing characteristics
| Property requiring improvement | Sprayed seal treatments | Microsurfacing | Combined treatments | ||||
|---|---|---|---|---|---|---|---|
| Surface enrichment | Single application sprayed seal (single/single) | Multiple application sprayed seal | Geotextile reinforced sprayed seal | Correction or regulation course plus SAM | Correction or regulation course plus SAMI with asphalt surface | ||
| Bitumen ageing/ oxidation | Delays further oxidation | ||||||
| Roughness | No effect | Suitable for an uneven surface but has little impact on loss of longitudinal shape | Good | Very good | |||
| Waterproofing properties | Minimal | Good | Very good | Excellent | Minor improvement | Excellent | |
| Skid resistance | Possible short-term reduction | Excellent | Good at low speeds but may reduce at high speeds due to fine texture | Excellent | As for asphalt | ||
| Structural strength | No effect | Minimal but depends on thickness of asphalt layers | |||||
| Robustness (relating to sharp turning traffic) | No effect | Poor, but improved with modified binders | Some improvement over single coat seals due to interlocking of aggregate | Moderate | More robust if double application used | As for asphalt | |
| Water spray reduction | No effect | May achieve some improvement depending on aggregate size | Minimal effect | Good | As for asphalt | ||
| Permeability of surface | Some reduction | Low | Moderate to high | Low | |||
| Flexibility | Very good | Very good | Poor | Good | As for asphalt | ||
| Shape correction ability | No effect | Suitable for correcting shallow wheelpath ruts with single or multiple layers | Good | Very good | |||
| Surface reflection cracking | Little effect | Good(2) | Excellent | Poor | Excellent | ||
| Likely life of treatment(1) | 2 to 5 years | 5 to 15 years | 8 to 15 years | 8 to 15 years | 5 to 10 years | 5 to 10 years | 5 to 12 years |
- Depends on the condition of the existing surface and the structural condition of the pavement.
- The performance of a single/single seal will depend upon the width of the cracks and their extent. Where wide cracks are in the surface then the performance of a single/single seal may be between good and poor.
Source: Austroads (2009a).