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STONE MASTIC ASPHALT
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ABSTRACT
Since the 1960s, Stone Mastic Asphalt (SMA) pavement surfaces have been used successfully in Germany on heavily trafficked roads as a durable road surfacing to resist wear from studded tyres. Because of its excellent performance characteristics, road authorities in Germany as well as major European Countries quickly adopted SMA as a standard wearing course. During the last few years, SMA has become one of the most popular asphalt pavements.
The deformation resistant capacity of SMA stems from a coarse stone skeleton providing more stone-on-stone contact than with conventional Dense Graded Asphalt (DGA) mixes. Improved binder durability is a result of higher bitumen content, a thicker bitumen film, and lower air voids content. This high bitumen content also improves flexibility. Addition of a small quantity of cellulose or mineral fibre prevents drainage of bitumen during transport and placement. The essential features, which are the coarse aggregate skeleton and mastic composition, and the consequent surface texture and mixture stability, are largely determined by the selection of aggregate grading and the type and proportion of filler and binder. SMA has proved superior on heavily trafficked roads and industrial applications.
SMA has distinct advantages as a surfacing, due to its potential for high resistance to fatigue and rutting (wear and tear due to traffic loading).
Limitations of SMA are the increased material cost, increased mixing time, possible delays in openings (the road) etc.
This report gives an overview of the history of SMA, its performance characteristics, composition, advantages, disadvantages as well as its applications.
. INTRODUCTION
Stone Mastic asphalt (SMA), otherwise known as Stone Matrix Asphalt / Split Mastic Asphalt, was developed in Germany in the mid of 1960's and it has spread throughout Europe and across the world in 1980's and 1990's respectively. The excellent performances include resistant to mechanical and temperature deformation, cracking, and particularly rutting, resistant to weathering actions such as aging and low temperature cracking. Durability is excellent even under slow moving heavy traffic. The textured surface increases skid resistance and provides environmental and driving comfort by reduced noise level, and improved visibility in rainy days.
SMA provides a deformation resistant, durable, surfacing material, suitable for heavily trafficked roads. SMA has found use in Europe, Australia and the United States as a durable asphalt surfacing option for residential streets and highways. SMA has a high coarse aggregate content that interlocks to form a stone skeleton that resist permanent deformation. The stone skeleton is filled with mastic of bitumen and filler to which fibres are added to provide adequate stability of bitumen and to prevent drainage of binder during transport and placement. Typical SMA composition consists of 70−80% coarse aggregate, 8−12% filler, 6.0−7.0% binder, and 0.3 per cent fibre. The deformation resistant capacity of SMA stems from a coarse stone skeleton providing more stone-on-stone contact than with conventional dense graded asphalt (DGA) mixes. Improved binder durability is a result of higher bitumen content, a thicker bitumen film and, lower air voids content. This high bitumen content also improves of flexibility. Addition of a small quantity of cellulose or mineral fibre prevents drainage of bitumen during transport and placement. The essential features, which are the coarse aggregate skeleton and mastic composition, and the consequent surface texture and mixture stability, are largely determined by the selection of aggregate grading and the type and proportion of filler and binder.
SMA is characterized by a stone-on-stone structure. SMA uses a high proportion of larger stones or aggregate that contacts each other. This skeleton of larger stones resists heavy loads by transmitting them to the pavement below. If the under laying pavement is sufficiently strong then the SMA will resist the heavier loads effectively. (A surfacing cannot compensate for a weak pavement).
The bituminous mastic is intended to hold the aggregate in place and to inhibit the ingress of moisture into the pavement and to provide durability. The mastic consists of bitumen and fine aggregate particles; it may also include a polymer modified bitumen and filler material to increase the mastic’s strength. Fibres may also be added to stabilize the bitumen and to prevent the binder segregating from the aggregate during transport and placement.
It is important that the aggregate material consist of only the larger stones (in the structure) and fines to provide effective mastic. The intermediate size aggregates are not included, as these keep the larger aggregate apart and reduce the strength of the SMA.
HISTORY OF SMA
Stone Mastic Asphalt (SMA), an asphalt paving mixture, was originated in Germany in the 1970s to provide maximum resistance to rutting caused by the studded tyres on European roads. Strabag, a large German construction company, led to the development of SMA. After the use of studded tyres was no longer allowed, it was found that SMA provided durable pavements which exhibited such high resistance to rutting by heavy truck traffic and proved to be extremely effective in combating wear. In recognition of its excellent performance a national standard was set in Germany in 1984. Since then SMA has spread throughout Europe, North America and Asia Pacific. Several individual Countries in Europe now have a national standard for Stone Mastic Asphalt (SMA), and CEN, the European standards body, is in the process of developing a European product standard. In the United States, Australia, New Zealand and in Asia, the use of SMA is increasing in popularity amongst road authorities and the asphalt industry.
PERFORMANCE CHARACTERISTICS OF SMA
The development of modern pavement technology is needed to accelerate significant improvement of pavement quality of highways, airport runways and urban roads.
SMA meets the following demands upon an asphalt pavement:
• Good stability at high temperatures
• Good flexibility at low temperatures
• High wearing resistance
• High adhesive capacity between the stone granules and the bitumen
• A mix with no tendency to separate
• Good skid resistance
• Reduced water spray
• Lower traffic noise
Good stability at high temperatures
SMA mix has a self-supporting stone skeleton of crushed high quality coarse aggregate, which provides an increase in internal friction and shear resistance and hence its extremely high stability.
Good flexibility at low temperatures
SMA mix has a binder rich mastic mortar which has superior properties over dense graded asphalt in resisting thermal cracking.
High wearing resistance
SMA mix has low air voids, which make the mix practically impermeable, and provide satisfactory ageing resistance, moisture susceptibility and durability.