Stone mastic asphalt (SMA) developed in Germany in late sixties. It is characterized by having a high proportion of coarse aggregates that interlock to form a strong aggregate skeleton. Typical SMA composition
consists of 70-80% coarse aggregates, 10% filler and 6% binder.
The concept of SMA design is that severe traffic loads is carried by the stone skeleton, and the mastic (a combination of fine aggregates, filler and binder) is introduced in the mix to fill out the remaining void spaces with an aim in achieving durability. To ensure that the course aggregate stone contact function, it is essential to prevent immediate sizes of aggregate to hold course aggregates apart.
Owing to high binder content, drainage inhibitor is added to prevent binder drainage during transport and placing.
In essence, SMA provides a textured, durable and rut resistant wearing course and hence they are commonly used in heavily trafficked roads with high traffic stress.
This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.
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