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.
