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Definition

Since the 1960s, Stone Mastic Asphalt (SMA) pavement surfaces have been used successfully in Germany on heavily trafficked roads as a durable road surfacing to resist wear from studded tyres. Because of its excellent performance characteristics, road authorities in Germany as well as major European Countries quickly adopted SMA as a standard wearing course. During the last few years, SMA has become one of the most popular asphalt pavements.

The deformation resistant capacity of SMA stems from a coarse stone skeleton providing more stone-on-stone contact than with conventional Dense Graded Asphalt (DGA) mixes. Improved binder durability is a result of higher bitumen content, a thicker bitumen film, and lower air voids content. This high bitumen content also improves flexibility. Addition of a small quantity of  cellulose  or mineral  fibre  prevents drainage of bitumen during transport and placement. The essential features, which are the coarse aggregate skeleton and mastic composition, and the consequent surface texture and mixture stability, are largely determined by the selection of aggregate grading and the type and proportion of  filler  and binder. SMA has proved superior on heavily trafficked roads and industrial applications. SMA has distinct advantages as a surfacing, due to its potential for high resistance to fatigue and rutting.

Composition Of SMA

Stone Mastic Asphalt is characterised by its high stone content which forms a gap-graded skeleton-like stone structure. The voids of the structural matrix are filled with high viscosity bituminous mastic. The high stone content of at least 70% ensures stone-on-stone contact after compaction. The required degree of mastic stiffness is achieved through the addition of crushed sand.

SMA mixes have a bitumen content of minimum 6.5%. The bitumen in the gap-graded mix is stabilised during the mixing process, intermediate storage, transportation, surfacing and compaction through the addition of cellulose fibre stabilising additive.

Addition of cellulose fibre does not chemically modify bitumen, but rather enhances physical property of the finished product by allowing the use of higher bitumen contents. It tends to thicken or bulk the bitumen so that it does not run off the aggregate prior to compaction. The content of cellulose fibre is 0.3% by weight of mixture. If the technological requirements of SMA are fully met, good results can be obtained by just using standard bitumen and a cellulose fibre drainage inhibitor.

SMA provides a deformation resistant, durable, surfacing material, suitable for heavily trafficked roads. SMA has found use in Europe, Australia and the United States as a durable asphalt surfacing option for residential streets and highways. SMA has a high coarse aggregate content that interlocks to form a stone skeleton that resist permanent deformation. The stone skeleton is filled with mastic of bitumen and filler to which fibres are added to provide adequate stability of bitumen and to prevent drainage of binder during transport and placement. Typical SMA composition consists of 70-80% coarse aggregate, 8-12% filler, 6.0-7.0% binder, and 0.3 per cent fibre. The deformation resistant capacity of SMA stems from a coarse stone skeleton providing more stone-on-stone contact than with conventional dense graded asphalt (DGA) mixes.

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