Stone Matrix Asphalt

Sant Kumar Sehgal, Technical Consultant For Stone Matrix Asphalt Roads, J. Rettenmaier & Sohne, Germany

Introduction

Sant Kumar Sehgal
Over the years India has seen a tremendous rise not only in the traffic volume, but heavy loading as well. Vehicles carrying heavy loads, have special designed tyres, which are inflated to high pressure, causing high stress on pavement. Under these conditions, conventional dense grades mixes are not rut resistant. STONE MATRIX ASPHALT – SMA, a gap graded mix, is highly rut resistant, tough, stable, skid resistant, with high quantity of coarse aggregates, relies on stone on stone contact, to provide strength & rich in binder, to provide durability. SMA was developed in Germany in the late 1960's & since 1970's has been used extensively in Europe & since 1990's in USA. SMA is used for wearing courses and binder courses (DBM).

SMA - Concept

The 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.
Structure Of A Typical Dense Graded Asphalt Mix
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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