By: Dave Newcomb, Contributing Author
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 with conventional mixtures.
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 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 with 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.
Proper treatment
The first step to getting SMA right is to get the right
ingredients. A hard, angular and coarse aggregate with a carefully controlled
gradation is first on the list. Then comes a binder with the right properties
to resist both rutting and thermal cracking. Mineral filler and fibers round
out the list of needed materials for the mixture. It is important to proportion
these components properly in order to obtain the desired characteristics and
performance from SMA.
The aggregate should have a gapped gradation with a
relatively narrow band. For two of the more commonly used SMA sizes, 9.5 mm and
12.5 mm nominal maximum aggregate size (NMAS), the recommended gradations are
shown in the table on p 36.
It is important to note that the percentages in the table
are by volume, not weight. This is because the volume occupied by the aggregate
is crucial in obtaining stone-on-stone contact. The definition of coarse
aggregate changes according to the NMAS; in a 12.5-mm mix, it is the material
coarser than the 4.75-mm sieve, and in the 9.5-mm mix, it is the material
coarser than 2.36 mm.
A parameter called the "voids in coarse aggregate"
(VCA) is used in determining the proper proportion of aggregate to achieve
stone-on-stone contact. First, the amount of dry-rodded coarse aggregate needed
to fill a specified container is determined. This defines the amount of space
available between the aggregate particles for air, fine aggregate, binder,
filler and fiber. In mix design, the voids in coarse aggregate is determined as
a function of the amount of the bulk density, specific gravity of the coarse
aggregate and the weight percentage of coarse aggregate. The objective is to
have a lower VCA in the mixture than in the dry-rodded condition in order to
ensure the stone-on-stone contact.
The hardness and angularity of the coarse aggregate may be
determined by the Los Angeles abrasion test and the number of fractured faces
in the particles. There also is a restriction on the number of flat and
elongated particles in the coarse aggregate. The angularity of the fine
aggregate is defined by the voids in the uncompacted particles.
The binder used in SMA mixtures should be the same used in
Superpave applications in the surface course of high-volume roads. In other
words, a high-reliability PG binder for high and low temperatures with grade
bumping for high traffic volume and slow-moving traffic, if applicable. In most
instances, this will result in the use of a binder with polymer modification.
The selection of mineral fillers and fibers is crucial in
the formation of the matrix in the mixture. Either mineral or cellulose fibers
have been used successfully in SMA. Fibers usually constitute between .3 and
.4% of the mass of the mix. They may come in a dry, loose state either in bags
or in bulk, or they may come in a pelletized form. Fibers, along with
polymer-modified asphalt, can serve to enhance rutting and cracking resistance.
Commercial mineral fillers, in addition to the natural aggregate fines, are
generally needed to fulfill the requirements of material passing the .075-mm
sieve.
The mix design for SMA mixtures can be done either by a
Marshall or Superpave process. When using a Superpave gyratory compactor, a
compaction level of 100 gyrations is typically used. It is recommended that the
mix have a minimum asphalt content of 6% and a design air void content of 4%. A
minimum voids in mineral aggregate (VMA) of 17% is recommended in production. A
minimum value of 70% is recommended for the retained tensile strength. It also
is very important to evaluate the potential for draindown during the mix design
process. The mixture should have a total draindown of less than .3% at the
production temperature.
The production and construction of SMA mixtures require a
great attention to detail. Aggregates need to be carefully stockpiled to
minimize variability and segregation. Additional cold feed bins are normally
required from what is needed for conventional HMA mixes. Bulk or bagged fibers
usually require special handling systems that are adapted according to the type
of plant being used. Mineral fillers also require special storage and handling
to properly meter them into the mixture. The mixing temperature at the plant is
generally the usual temperature or slightly higher. However, SMA mixtures
should not be stored for long periods of time prior to paving in order to
minimize the potential for draindown.
At the paving site, it is imperative that good construction
practices be followed carefully. The paver should operate continuously and
smoothly, and this can best be accomplished by balancing the production rate to
the paver. Care should be taken to minimize segregation and the occurrence of
cold spots in the mat. Rolling should occur immediately behind the paver, and
compaction should be achieved very quickly before the mat has cooled. It is
recommended that two or three rollers be used in compaction, and that the
minimum in-place density be 94% of maximum density. Longitudinal joints should
be carefully constructed so that the paver overlap between mats is no greater
than 1/2 in. and the breakdown roller operates to maximize the density in this
region.
SMA has emerged as the premier asphalt surfacing for
high-volume roadways. Careful attention to the selection of materials, the mix
design and the construction process will lead to smooth, long-lasting asphalt
pavements.
About The Author: Newcomb is vice president, research and technology, at the National Asphalt Pavement Association, Lanham, Md.
