Low Noise SMA

Earlier, common practice has designated porous mixtures as the most effective way of reducing traffic noise. One — or two-layer pavements have been used, while the latter ones generally are a more effective option. However, that is an expensive solu­tion. On the other hand, it has been found (von Bochove and Hamzah, 2008) that gap-graded mixes composed in accordance with the SMA concept—with an air void content of 9-16% (v/v)—provide a worthwhile alternative to porous asphalt in urban areas. They are marked by a higher resistance to the loads occurring in urban traf­fic conditions, a longer service life, and good noise reduction properties (up to -5 dB[A]). At the same time, the authors have added that such a mixture cannot be a conventional SMA, but it has to distinguish itself by a significant gap grading and a strong skeleton of coarse particles.

The concept of a “silent” SMA (SMA LA), which is being developed in Germany, is an example of such a solution. Some test sections on roads in Bavaria (Germany) made of SMA LA mixtures 0/5 and 0/8 have been described (Gartner et al., 2006). The following are the expected values of SMA LA:

• Content of voids above 10% (v/v)

• Gap-graded aggregate mix

• Grading 0/5 mm for layers 15-25 mm thick

• Grading 0/8 mm for layers 20-30 mm thick

The SMA LA course is not gritted since noise reduction has been given high priority. Very positive results of skid resistance with the SKM (SKM — Seitenkraft- Messverfahren — Griffigkeit) (results greater than 0.58) method have been achieved, and noise tests with close-proximity (CPX) method have yielded reductions both at 80 km/h and 120 km/h. Finally the SMA 0/8 LA mixture has turned out to be more effective in noise reduction than the SMA 0/5 LA. Figures 13.5 and 13.6 depict the grading curves of SMA 0/5 LA and SMA 0/8 LA.

Also, in Denmark the road administration, together with industry and consul­tants, has created a system of classifying the noise reduction effects of various types of asphalt surfacings (Andersen and Thau, 2008). Assessment of the surfacing is carried out according to the CPX method at two speeds, 50 km/h (reference noise level 94.0 dB[A]) and 80 km/h (reference noise level 102 dB[A]). It is worth noting that in Denmark two types of SMA—6 + SRS and SMA 8 SRS—are used for noise — reducing asphalt surfaces as follows:

•

SMA 6 + SRS—maximum aggregate size of 8 mm, air voids between 4 and 10% (v/v), ratio of binder volume to aggregate volume of at least 0.18, minimum thickness of 45 kg/m2

• SMA 6 + SRS—maximum aggregate size of 11 mm, air voids between 3 and 10% (v/v), ratio of binder volume to aggregate volume of at least 0.17, minimum thickness of 50 kg/m2

• SMA 8 SRS—maximum aggregate size of 8 mm, air voids between 4 and 12% (v/v), ratio of binder volume to aggregate volume of at least 0.18, mini­mum thickness of 55 kg/m2

These requirements are part of the first generation specifications used in tenders (contracts) in Denmark.