Rolling on the stones

News January 09, 2003
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After 12 years of experience in this country, pavement engineers are concluding that stone-matrix asphalt (SMA) is a strong sur

After 12 years of experience in this country, pavement engineers are concluding that stone-matrix asphalt (SMA) is a strong surface for high-volume, heavy-duty roadways.

SMA is a stone-on-stone, binder-rich surface mix that provides quality rutting and cracking resistance. The technology was brought to the U.S. as a result of a 1990 European study tour sponsored by the National Asphalt Pavement Association, the Federal Highway Administration and the American Association of State Highway & Transportation Officials.

This material has been used for decades in Germany as a premium surface mix for important motorways such as the Autobahn. The first SMA placed in the U.S. was in Wisconsin in 1991, followed in that same year by projects in Michigan, Georgia and Missouri. By 1997, there were well over 100 projects representing over 3 million tons of mix in at least 28 states.

Why the big interest? It's simple:

* Improved performance;

* Noise reduction; and

* Improved safety.

In a 1997 study by the National Center for Asphalt Technology (NCAT), it was found that 90% of the SMA-surfaced pavements had less than 1/4 in. of rutting and had significantly less cracking than those with conventional dense-graded surfaces. The Georgia DOT experience with SMA indicates a 30 to 40% reduction in rutting and a three to five times greater resistance to fatigue cracking compared to conventional mixtures.

A report on SMA performance in Maryland stated that: "Very little rutting, increase in roughness or decrease of friction has been observed, even for pavements that have been in service for as long as 10 years." Recent results from the NCAT Test Track confirm the rut resistance of SMA mixtures. They have seen less than 1/8 in. rutting after applying 10 million equivalent single axle loads in a two-year period. After more than three decades of use, the German performance shows that a 20- to 30-year service life is typical for SMA surfaces.

When compared to normal dense-graded hot-mix asphalt (HMA) or portland cement concrete surfaces, SMA is quieter according to a number of studies. Research in Germany, the United Kingdom, Italy, Maryland and Michigan have shown a 2 to 5 dB(A) drop in road noise when SMA is compared to other types of pavement surfaces. A 3 dB(A) drop is the equivalent to doubling the distance to the source of the noise.

Although an SMA mixture does not drain water through the surface, its coarse surface texture provides improved frictional characteristics. Researchers in France have reported greater skid resistance on SMA as measured at highway speeds between 40 and 60 mph.

For more on the story, read the January issue of ROADS & BRIDGES.

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