Characteristics of Base, Binder and Regulating Courses
A binder course layer is normally thinner than a base layer and higher binder content with finer aggregate grading can provide better compacted density and lower air voids. The smaller nominal size also helps to minimise segregation during placement of the layer.
ased on an interlocking coarse aggregate skeleton with a mastic of fine aggregate, filler and bitumen
For heavily trafficked roads the coarse aggregate is normally crushed rock
Coarse aggregates should conform to the relevant Clauses in IS EN 13043
The PSV of aggregates used in bases and binder courses is rarely a consideration
and therefore a wide range of rock types, which occur in Ireland can be used. Limestone is often the preferred material as it is relatively easy to quarry and has good affinity with binder
SMA binder courses, permit crushed rock sands or a blend of crushed rock and natural sand with a maximum of 50% natural sand
The choice of binder for SMA is limited to 40/60 or 70/100 paving grade bitumen, but polymer modified binder is permitted with a grade of 65/105-60
Minor constituents usually contribute less than 0.5%, sometimes much less, to the total mixture. The most common are fibres and adhesion agents. Fibres may be natural or man-made, and are normally used to enable a thicker binder film (higher binder content) to be achieved without risking binder draining from the aggregate. They are most frequently used in SMA binder courses
SMA binder course is typically used in maintenance situations where a new inlaid surface course is thinner than the surface course that has been planed out
Stone Mastic Asphalt (SMA) consists of discrete and almost single sized aggregate particles (typically around 70% by mass) forming a skeletal structure bonded together by mastic. This mixture is of often used as a surface layer in cases where high stability is needed
The requirements for the mix design and designation for SMA mixtures are set out in NRA HD 300
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