Characteristic types of asphalts

Asphalts are characterised by the particle size distribution of the aggregate mixture. Theoretically, there are unlimited types of asphalts, namely, from asphalts consisting only of almost single-sized coarse aggregates to mixtures consisting only of fine aggregates (sand). All types of asphalts used range between these two extreme cases. 

Some countries, such as the United States, England, Germany and France, pioneers in invention and production of asphalts, for many years now, have developed asphalts that are used worldwide. These mixtures were asphalt concrete (AC), macadam, hot rolled asphalt (HRA), mastic asphalt (MA) and gussasphalt. 

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. 

The basic feature of all asphalts is the gradation curve of the aggregate mixture, which may be continuously graded or gap graded. In the first case, all particle sizes exist in the mix at an appreciable proportion. In the second case, certain sizes are in a small proportion, hence creating a kind of a gap. 

Depending on the portion of each particle size, asphalts are designated as porous, open, semi-open or dense-graded mixtures. Thus, their voids vary from high to low content, and as a consequence, some are more permeable than others. 

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