AC for very thin layers

ACs for thin or very thin layers were developed in France during the 1980s. The aim was to produce an AC for surface courses with better surface characteristics than traditional dense AC, with the capability of being laid at a thickness of less than 4 cm. 

Two types of mixtures were developed (using their French names) BBTM (beton bitu-mineux tres mince) and BBUM (beton bitumineux ultra mince). 

The difference between BBTM and BBUM was that the first mixture could be laid in 20 to 30 mm thickness, whereas the second could be laid thinner (i.e. less than 20 mm thickness). 

With the publication of the relevant European standard in 2006 (CEN EN 13108-2 2008), the BBTM type was adapted and named AC for very thin layers. To avoid confusion, the abbreviation to describe this type of mixture was kept the same, that is, BBTM. 

BBUM mixtures remain proprietary mixtures and thus are not covered by European specifications. 

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Over the last 40 years or more, new mixtures were developed in order to improve some features, such as drainage ability, noise reduction, durability and the ability to be laid in less than a 4 cm thick layer, hence economising on materials. The mixtures developed were porous asphalt (PA) in the United Kingdom, stone mastic asphalt (SMA) in Germany and AC for very thin layers (BBTM) in France. 

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