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
- 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.
|Property requiring improvement||Sprayed seal treatments||Microsurfacing||Combined treatments|
|Surface enrichment||Single application sprayed seal|
|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).