Stone mastic asphalt

SMA was developed in Germany and Scandinavian countries in the mid-1960s. The prime reason for the development of this mixture was the demand to produce a mixture that can sustain the destructive action of studded tyres, to have good resistance to permanent deformation and long service life. 

The specificity of the SMA is that the aggregate skeleton consisting of coarse aggregates is bound with a mastic mortar (bitumen, sand, filler and fibres). SMA is a dense gap-graded mixture. 

SMA differs from HRA, since the mortar fills the voids, whereas in HRA, it constitutes the base of the mixture to which coarse, almost single-sized aggregates are added. 

The SMA mixture is characterised by the higher bitumen content used compared to AC. This provides a longer service life but increases the cost. The incorporation of higher bitumen content is achieved by the addition of inorganic or organic fibres. SMA is produced in typical asphalt plants, but small modification for the provision of the fibre dosage system is needed. SMA is specified by European and American standards. 

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Fig. 5.6 Structure of stone mastic asphalt (SMA), a modern asphalt matrix bonded road surfacing material.

FHWA (1991) Stone mastic asphalt - SMA - technology synopsis and work plan, draft. Federal Highway Administration, Washington, DC, February 1991, p 452... 

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. 

Source Reproduced from CEN EN 13108-5, Bituminous mixtures - Material specifications - Part 5, Stone Mastic Asphalt, Brussels CEN, 2008. With permission ( CEN). 

Because of the geometry of the specimens tested, the test is not applicable to open-graded asphalts or stone mastic asphalts (SMAs). It is appropriate for dense-graded mixtures with 19 mm or smaller nominal maximum aggregate size. 

The minimum rolling time depends on the type of material. The minimum rolling time for asphalt concretes and porous asphalts is 20 min, whereas the minimum rolling time for stone mastic asphalt, hot rolled asphalt and mastic asphalt is 30 min. In the case of coated chippings, the minimum rolling time is 10 min. 

For the surface layer, in particular, the asphalts most commonly used nowadays are (a) asphalt concrete for very thin layers (AC-VTL), (b) porous asphalt (PA), (c) open-graded friction course mixture (OGFC), (d) stone mastic asphalt (SMA), (e) single or double surface dressing (S-SD or D-SD) and (f) micro-surfacing (MS). Table 10.17 gives the comparative properties/characteristics for all the abovementioned surfacing materials. 

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