15 May 2001
fiber) which forms a mastic portion of the SMA mixture. This mastic stabilizes the coarse aggregate and
reduces the final air voids in the SMA to about 3 to 4 percent. SMA is comparable to hot-mix asphalt in
regards to structural design, mix design, and construction. SMA originated in Europe and has recently
been placed by state and federal agencies on projects throughout the country. The design and
construction of SMA pavements are described in this appendix.
(1) Aggregates. The gradation used for SMA is gap-graded for the coarse aggregate retained
on the 4.75 millimeter (No. 4) sieve. This coarse aggregate will make up from 72 to 80 percent of the
aggregate in the mix. The coarse aggregate should be 100 percent passing the 19 millimeter (3/4-inch)
sieve. The amount of the fine material passing the 75 Fm (No. 200) sieve will be from 8 to 10 percent.
Table 2-12 lists the gradation as recommended by the FHWA. This gradation is based on the
recommendations of a technical working group that reviewed the performance of SMA mixtures in place.
Table 2-13 lists the recommended coarse and fine aggregate properties for SMA.
SMA Gradation Guideline (After NAPA 1994)
19 mm (3/4 inch)
12.7 mm (1/2 inch)
9.5 mm (3/8 inch)
4.75 mm (No. 4)
2.36 mm (No. 8)
600 Fm (No. 30)
300 Fm (No. 50)
75 Fm (No. 200)
(2) Filler. As presented in table D1 for SMA mixtures, the recommended amount of aggregate
filler (dust) passing the 75 Fm (No. 200) sieve is 8 to 10 percent. This amount of filler in the SMA is
higher than that usually found in dense graded hot mix asphalt (HMA). The amount of filler is important
in terms of obtaining the desired mixture air voids and in affecting the optimum asphalt content. The
SMA asphalt content is sensitive to the aggregate fines and filler content. In Europe, SMA mixtures
commonly employ a filler-to-asphalt ratio of approximately 1.5. In contrast, conventional dense graded
hot mix in the United States typically recommend a filler-to-asphalt ratio of less than 1.2. A well-graded
filler with no more than 20 percent of the total filler smaller than 20 microns is required. Commercial
fillers are added by mineral filler feeder systems. Fly ash, limestone dust, and other types of rock dust
have been used successfully as fillers for SMA applications.
(a) General. There is a tendency for the asphalt binder to drain from the aggregate during
storage, transportation, or placement because of the high asphalt content in the mix, the thick asphalt
coating on the coarse aggregate, and the high voids in the aggregate skeleton. In order to reduce this
drainage potential, stabilizers are used to stiffen the mastic or to increase the asphalt binder viscosity.
These stabilizers can be categorized into two groups: either (cellulose fibers or mineral fiber) or