Considering the technology of thin SMA layers, but not only gradation of the aggregate blend, but the quantity and type of binder as well, must provide a mix that can be placed by mechanical spreading of a layer up to approximately 4 cm (usually less than 3 cm) thick using a standard paver. Furthermore, the component materials and the final mix itself have to produce a layer that has the following qualities (Sybilski and Styk, 1996):
• Is impervious to water and deicers (excluding porous asphalt)
• Has a suitably high coefficient of friction
• Is resistant to permanent deformation
• Is resistant to low-temperature cracking
• Is resistant to fatigue
• Has the potential to reduce traffic noise
It is also worth remembering that, despite their many strengths, thin courses neither reinforce the pavement substantially nor solve the problem of fatigue (net) and reflected (transverse) cracking (Pandyra et al., 1994). Many countries have their own original technological solutions for courses less than approximately 4 cm thick, which usually consist of various gap-graded mixes, including SMA.
Finer SMA (e. g., 0/5, 0/8) mixes are used for thin layers rather than the 0/11 mm and 0/12.5 mm SMAs. These finer SMAs are noted for many good points, such as similar or only slightly worse resistance to permanent deformation compared with 0/11 mm and 0/12.5 mm mixes. The finer SMAs are also characterized by lower water permeability at the same void contents as coarser SMAs (Cooley and Brown, 2003). Additionally, higher contents of binder in the finer mixes lead to an increase of durability and improved mix workability. If properly designed, finer SMAs tend to reduce the appearance of fat spots, so there is a possibility to reduce the stabilizer content.
The following remarks about SMA technology deserve mention: [72]
• Excessively heavy rollers should not be used because they may crush aggregate grains; vibration can only be used occasionally and with great care; on ultra-thin courses the vibrations should be turned off.
• Attention should be paid to temperature drops in the mix during its spreading because thin layers are very susceptible to fast cooling caused by cool crosswinds or a cold sublayer.
• Almost always, a thin SMA layer can be opened to traffic sooner than can a conventional thickness layer; in the case of ultra-thin layers, owing to their rapid cooling, opening to traffic can be done in as little as 30 minutes after the end of compaction (Carswell, 2002).
Thin SMA layers have been used all over the world. Descriptions from Argentina, the United States, the United Kingdom, Sweden, Poland, and other countries are available in literature (Bolzan, 2002; Carswell, 2002; Carswell, 2004; Cooley and Brown, 2003; Richardson, 1997). An interesting review of the performance (e. g., macrotexture, skid resistance connected to aggregates’ PSV, visual condition) of thin layer sections after 15 years can also be found (Nicholls et al., 2008).
Examples of thin SMA layers evaluated after a minimum of 10 years in operation can be the most interesting. One case in point may be the wearing course on the DK3 route in Poland. The 2.5- to 3-cm thin SMA layer of 0/6.3 mm grading was laid in 1993. It was still in very good condition after 13 years in operation (Figure 13.3).
