Stone Mastic (or Matrix) Asphalt (“SMA”) is a high quality hot asphalt mix developed in Germany during the 1970's to resist rutting & wear from studded tires. However, due to SMA’s proven performance as a long-lived pavement material it has become widely adopted in Europe, Australia, USA, Canada, China, & many other countries worldwide, as a preferred surface for heavily trafficked roads. Structurally, SMA has a high coarse aggregate content that interlocks to form a stone skeleton that resists permanent deformation due to the stone-to-stone contacts. The voids between the aggregate’s skeleton are filled with mastic of bitumen & filler to which fibers are added to prevent drain down of the binder during transport & placement. The stone-to-stone contact provides a strong skeleton, & the mastic enhances the durability of the SMA. It should be noted that not all surfacing called “SMA” is identical in terms of composition & performance. SMA is a generic term that refers to a broad range of paving mixes with the same general characteristics.
Due to its many functional & structural advantages (eg. longer life, rutting resistance, skid resistant texture, noise reduction, etc.) SMA mixes are considered the best engineering choice for heavily trafficked pavements. However, because of its unique composition & materials, the production, hauling & paving of SMA also pose certain disadvantages:
• Draindown of the asphalt binder due to the higher binder content & VMA ;
• The need to use cellulose fibers in the mix to prevent drain down;
• The need to use excess amounts of asphalt binder because some of it is absorbed by the fibers;
• The need to use special production equipment when putting the fibers into the asphalt mix which adds operational problems such as increased complexity & maintenance issues;
• The need for more mixing time resulting in lower productivity due to the higher binder content & diminished workability of the mix caused by the fibers;
• The need for a higher mixing temperature due to the diminished workability of the binder (usually a modified Bitumen) & fibers in the mastic ;
• All of the above contribute to making SMA substantially more expensive to produce as compared to conventional HMA (dense mixes, Superpave mixes, etc.).
In general, it is common knowledge that "SMA requires more attention to detail when mixing & being produced, transported & placed in the field" . It is often considered "an art" to produce & lay a quality SMA mix due to the fact that "SMA is an unforgiving mix & requires changes & modifications to the mix design to enhance the characteristics of its performance. (1)
The root cause behind all of the above disadvantages related to SMA is the use of cellulose fibers whose sole function is to prevent drain down of the liquid binder during production, transportation application. The limited usefulness of fibers is confirmed in an NCAT report dated February 2009, (2) "Fibers serve no real purpose after the mix is compacted in place". These disadvantages combined with the high cost of fibers has constrained the growth of the SMA market, despite the well documented advantage of its high quality performance.
(1) Allen, G. K., Problems of Stone Mastic Asphalt Use in North Queensland. A Dissertation Submitted to the Faculty of Engineering and Surveying, University of Southern Queensland, November, 2006.
(2) Prowell, D. P., D. E. Watson, G.C. Hurley, and E. R. Brown. Evaluation of Stone Matrix Asphalt (SMA) for
Airfields Pavements. AAPTP 04-04 Final Report, National Center for Asphalt Technology, Auburn
University, February 2009.