Tuesday, December 10, 2013

Stone Mastic Asphalt Details

There are three major types of asphalt surfacing, characterized by a mixture of bitumen and stone aggregate. These are: Dense Graded asphalt (DGA); Stone Mastic Asphalt (SMA) and Open Graded Asphalt (OGA). Asphalt surfacing differs by the proportion of different size aggregate, the amount of bitumen added and the presence of other additives and material. 

Stone Mastic Asphalt
Stone mastic asphalt (SMA), also called stone-matrix asphalt, was developed in Germany in the 1960s. It provides a deformation resistant, durable surfacing material, suitable for heavily trafficked roads. SMA has found use in Europe, Australia, the United States, and Canada 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 resists permanent deformation. The stone skeleton is filled with mastic of bitumen and filler to which fibers 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 fiber.

Difference Between SMA & Conventional Mixes:
 
Stone Mastic Asphalt Composition
Stone Mastic Asphalt Composition
SMA is successfully used by many countries in the world as highly rut resistant bituminous course, both for binder (intermediate) and wearing course. The major difference between conventional mixes and SMA is in its structural skeleton .The SMA has high percent about 70-80 percent of coarse aggregate in the mix. This increases the interlocking of the aggregates and provides better stone to stone contact, which serves as load carrying mechanism in SMA and hence provides better rut resistance and durability. On the other hand, conventional mixes contain about 40-60 percent coarse aggregate. They does have stone to stone contact, but it often means the larger grains essentially float in a matrix composed of smaller particles, filler and asphalt content .The stability of the mix is primarily controlled by the cohesion and internal friction of the matrix which supports the coarse aggregates.

The second difference lies in the binder content, which lies between 5-6 percent for conventional mixes. Below this the mix becomes highly unstable. Above this percent will lead to abrupt drop of stability because the binder fills all the available voids and the extra binder makes the aggregates to float in binder matrix. The SMA uses very high percent of binder > 6.5 percent which is attributed to filling of more amount of voids present in it, due to high coarse aggregate skeleton. The high bitumen content contributes to the longevity of the pavements.

The third difference is the use of stabilizing additives in SMA, which is attributed to the filling up of large number of voids in SMA so as to reduce the drain down due to presence of high bitumen content. On the contrary, there is no stabilizing agent in conventional mixes since the bitumen content is moderate, which only serves the purpose of filling the moderate amount of voids and binding the aggregates

Composition of SMA:

1. Asphalt (Binder)
2. Aggregate
3. Fibers
4. Mineral filler

Advantages:

• High stability against permanent deformation (rutting) and high wear resistance.
• Slow aging and durability to premature cracking of the asphalt
• Longer service-life
• SMA has a higher macro-texture than dense-graded pavements for better friction
• Reduced spray, reduced hydroplaning and reduced noise.
• Good low temperature performance
• Even though SMA has a higher cost than conventional dense mixes, approximately 20 to 25 percent, the advantages of longer life (decreased rutting and increased durability).

Disadvantages:

• Increased cost associated with higher binder and filler contents, and fiber Additive,
• High filler content in SMA may result in reduced productivity. This may Be overcome by suitable plant modifications,
• Possible delays in opening to traffic as SMA mix should be cooled to 40 oC to prevent flushing of the binder surface, and
• Initial skid resistance may be low until the thick binder film is worn off the top of the surface by traffic.


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