Sant Kumar Sehgal, Technical Consultant For Stone Matrix Asphalt Roads, J. Rettenmaier & Sohne, Germany
SMA - ConceptThe SMA-CONCEPT can be briefly summarised as:
- High stability to permanent deformation & high wear resistance by an excellent particle interlock and a high content of crushed premium aggregates.
- Longevity & durability to premature cracking and ravelling by a very high content of bitumen and a void less mastic mortar fills the voids of the stone skeleton and binds it together: high quantity & quality of binder are prerequisite for a long useful service life.
- Stabilizing additives [cellulose fibres] addition assures homogeneity [no binder drainage], during the manufacturing, transportation, laying and compaction.
- High quality & high quantity of bitumen (thick binder films) & coarse aggregates are the essentials for a long service life of SMA.
|Pic 1: Structure of a typical dense graded asphalt mix (BC)||Pic 2: Stone on stone skeleton structure of coarse aggregate in gap graded asphalt mix (SMA)|
Dense Graded Asphalt Mix (BC) vs Gap Graded Asphalt Mix (SMA)BC which is a dense graded asphalt mix, the coarse aggregates do have no contact with each other & are separated by fine aggregates [sand] particles which have to carry the load. SMA is a gap graded asphalt mix, which lacks certain sizes of aggregates, it has abundance of coarse aggregate particles [material retained on 4.75 mm sieve]. SMA has high filler content, [passing on 0.075 mm sieve], usually between 8-12%, compared to BC which has filler content less than 5%. SMA has a higher bitumen content > 6%, hence require the use of cellulose fibre pellets, to inhibit high amount of bitumen from draining out.
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