Stone Matrix Asphalt is Catching on in the U.S.

It’s more expensive than Superpave, but SMA stands up
 to traffic even better, and that’s turning heads.

by Tom Kuennen, Contributing Editor

Use of stone matrix asphalt, the durable, imported asphalt mix design that holds up to heavy traffic, is growing in the United States, even as state and local road agencies have specified ever-increasing tons of Superpave mix designs as well.

Stone matrix asphalt came to the U.S. from Europe in 1991, one year before the debut of Superpave. Today, more than 28 states are using SMA in wearing surface course applications while 48 states are using Superpave.

States like Georgia and Maryland have studied SMA very closely on their systems and are now expanding their use of the pavement — and spreading the word to other states.

Users of this imported design are won over by its longevity: SMA outperforms Superpave, all things being equal, with a service life from 20 to 30% longer than a conventional dense-graded hot-mix asphalt pavement.

The drawback to stone matrix asphalt is cost. Compared to conventional hot-mix asphalt, SMA carries a cost premium of 10 to 30% — and some have estimated a 50% premium. This, at a time when the cost differential between Superpave and conventional mixes has been greatly narrowed.

SMA wearing course here in Maryland is over nine years old.	In SMA, stone-on-stone contact develops internal friction and resistance to shear, easily resisting rutting.	Material transfer vehicle is used to place SMA.

The extra cost for SMA mixes stems from higher liquid asphalt content, stronger crushed aggregate, and, frequently, the need for fibers or other additives. The mix also requires higher mixing temperatures and longer mixing times at the plant, and more intensive quality control at plant and on job site.

But despite the price premium, use of the super-strong mix is spreading to cities as well as states. Aurora, Colorado, for example, is pleased with the durability, spray reduction, and noise attenuation of its SMA mixes and plans to place another 23,000 tons this year.

Usage of this material will only increase in the years to come. New research is affirming the value of gap-graded Superpave mixes that are actually SMA mixes incorporating Superpave technology, and new research is highlighting the potential for adapting SMA to thin asphalt overlays using finer aggregates (see related sidebar).

And while Superpave and SMA burst on the U.S. paving scene together, experts contacted by Better Roads editors dismissed the notion that the two mix designs are sibling rivals vying for favor in the U.S. market. Instead, the two mixes complement each other well, giving agencies the flexibility to deal effectively with a wider range of traffic and truck loads. Just as Superpave provides a cost-effective alternative to conventional pavements for heavily used roads, SMA is giving engineers an alternative to Superpave for the worst case loadings.

Why use SMA?

SMA is a gap-graded asphalt mix (minimizing medium-sized aggregate and fines) which combines strong, coarse aggregate with a high asphalt cement content — as much as 6 to 8% liquid asphalt. The result is a structurally strong mix incorporating a stone-on-stone skeleton. This stone-on-stone contact develops internal friction and resistance to shear, which easily resists rutting.

There are drawbacks to this structure, however. Because the gap-graded mix lacks medium-sized aggregate, conventional low-penetration-grade asphalt used in other mixes can drain out of the coarse aggregate skeleton. To avoid this “drain-down,” SMA mixes use cellulose fibers or other modifiers to hold the binder in place.

The percentage of fines is less than in conventional U.S. mixes — about 15% of the aggregate weight.

The acronym, SMA, is derived from the German term splittmastixasphalt; the Americanized “stone matrix asphalt” means much the same thing. While other central and northern European countries claim to have invented SMA mix — and so do some Americans — the design appears to have been first used in Germany in the 1960s to resist damage from studded tires. Its virtues soon proved to include exceptional resistance to shoving and rutting as well.

“SMA is a premium paving material that is best suited for demanding applications,” observed the Maryland State Highway Administration in its important paper, Construction and Performance of Stone Matrix Asphalt Pavements in Maryland: An Update (2002).

“The performance of SMA pavements in Maryland has been outstanding,” the Maryland SHA researchers say. “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. Other notable benefits of SMA include reduced tire splash and reduced tire noise. Many of the Maryland SMA pavement sections look better today than when first opened to traffic.”

Maryland is important to the growth of SMA in this country because that state is one of its biggest proponents. The Maryland SHA has constructed over 85 projects with the mix since 1992, totaling over 1,300 lane miles of paving. These mixes have been placed as a surface course exclusively on high-volume, high-speed roadways with more than 20,000 average daily traffic with posted speeds of 55 miles per hour.

Georgia also is a leader in SMA use, specifying it for use on interstates as an intermediate course, but topped with a free-draining, noise-attenuating open-graded friction course (see A New Era for Permeable Pavements, April 2003, pp 28-32). Georgia also uses SMA as a friction course for state routes with more than 50,000 ADT.

Together, these two states have placed almost as much SMA as all the other states combined. More recently, the Alabama DOT now specifies SMA for all its interstate pavements.

SMA outperforms Superpave

That SMA outperforms Superpave is clearly documented in new research presented early in 2003 at the Transportation Research Board meeting in Washington, D.C. The peer-reviewed paper, An Updated Review of SMA and Superpave Projects, advised “SMA mixtures appeared to be more durable than the Superpave mixtures evaluated.”

“We went back after a period of time to see how those projects were still performing,” says the paper’s author, Don Watson, P.E., research engineer at the National Center for Asphalt Technology, Auburn University. “Most of the SMA and Superpave mixes were very rut-resistant, but some of the SMA mixes were seven or eight years old, were performing very well with no distresses, and in some cases still looked as if they had been recently placed,” wrote Watson, while some of the Superpave sections had cracking.

Good construction practices, and good quality materials, are keys to success with SMA, Watson told Better Roads editors. “There have been several changes over the years, with both SMA and Superpave, trying to refine the mix design procedures and make the mixes more durable and easier to construct,” he said.

Moreover, research in recent years has been blurring the demarcation between Superpave and SMA. “There has been work to make SMA fit into the Superpave criteria,” Watson says.

“SMA and Superpave are very rut-resistant, but SMA has the advantage of having a much-higher asphalt content,” he says. “That thicker asphalt film surrounding the aggregate particles is going to give you better long-term durability, as opposed to Superpave, in which you get rut-resistance while sacrificing binder content.” This was borne out in his paper which reported considerably more cracking in some Superpave surfaces.

Wait-and-see attitude

“When rut-resistant Superpave came out, a lot of states took a wait-and-see approach, because SMA mixes are more expensive,” Watson says. “They wanted to see if the more economical Superpave mix would perform just as well. They found that Superpave has great rut resistance, but there are concerns about its long-term durability. As a result, we’re seeing more and more states renew their interest in SMA.”

This was borne out by the interest in the SMA in the USA II workshop, held in March 2002 in Maryland. Over 430 participants from 26 states and seven countries heard speakers describe their experiences with SMA.

“SMA provides a rut-resistant mix with relatively high asphalt content,” says Larry Michael, regional engineer and asphalt team leader, Maryland SHA, and a co-author of the Maryland SHA paper. He’s also a member of the TRB Superpave Committee. “Superpave mixes are rut-resistant, but sometimes we are concerned that they don’t have enough asphalt,” he says. “SMA gives us the opportunity to put more asphalt in a rut-resistant mix.

“In Maryland, we’re concerned about the durability of Superpave, particularly on low-volume roads, because of the leaner asphalt contents,” Michael told Better Roads editors. “On low-volume roads you don’t have enough traffic on the coarse-graded, low-asphalt mix to keep it ‘alive’, and higher asphalt contents are needed for durability in low-traffic areas. It’s an entirely different situation than with high-volume pavements.”

States play catch-up

“Other states are beginning to catch up,” Watson said. “They’re seeing the same thing: With higher asphalt contents of SMA, you will improve its long-term durability.”

One of those states is Colorado, where engineers are using SMA in hard-wearing situations, such as intersections, even when state highways intersect a non-state-owned roadway.

“We were hoping to get a rut-resistant, abrasion-resistant mix,” says Bob LaForce, P.E., Colorado Department of Transportation Region 1 materials engineer. Region 1 extends from the Continental Divide to the Kansas border, excluding the Denver metro area. “We found out later that it’s also one of the quieter mixes. It’s a multipurpose mix that gives us a rut-resistant wearing course that will be quieter than a [conventional] dense-graded mix.”

Colorado has six regions, but most of the SMA has been placed in the Denver area. “We’ve had several in Region 1,” said LaForce. “We started off with experimental projects in 1994, and have had scattered projects since. We’re working into it gradually.”

“I routinely get calls from other states, asking for information or a copy of our specs,” Maryland’s Michael said. “I think the fact that it has performed in Maryland — with promotional support from the National Asphalt Pavement Association — is very instrumental in its resurgence elsewhere in the country.”

Where Superpave, SMA come together

Now, Maryland is going where few have gone before, as it integrates Superpave design technology into SMA mixes, to create a hybrid design now called gap-graded Superpave.

“Gap-graded Superpave is in fact SMA designed using Superpave, including gyratory compaction, PG binders, etc., and European SMA technology,” says NAPA president Mike Acott. “It merges the best of both technologies and provides a rut-resistant, durable mix without the need for that elusive performance test.” Proof of performance is gap-graded Superpave’s experience in over 100 existing installations, Acott points out.

“If you look at gap-graded Superpave, it’s very close to a German SMA,” Michael says. “It doesn’t have the middle screens in it, and has a whole lot less such material than a dense-graded mix.” Maryland uses PG-graded liquid asphalts in its gap-graded Superpave mixes.

In 1998, the Maryland SHA used a gap-graded Superpave mix on a massive resurfacing of the Baltimore Beltway, I-695. The installation demonstrated how SMA concepts can be incorporated into the Superpave system of mix design, and it also showed how gap-graded Superpave mixes could be constructed on a large scale for urban expressway applications.

Three base courses using 37.5-mm nominal maximum aggregate size are thought to be the world’s first large-stone Superpave mixes ever placed, reported NAPA in its journal, HMAT.

The 2-inch SMA surface course used performance-graded (PG 76-22) polymer-modified liquid asphalt, and a gyratory compactor, NAPA said. SMA over newly constructed lanes in the highway’s widening used a nominal maximum aggregate size of 12.5 mm, while the overlay above an existing section that was not widened had a nominal maximum aggregate size of 19 mm.

The SMA-style gap-graded Superpave design was used for the surface course, NAPA said, while conventional Superpave mixes were used for all but one leveling course of the base.

Results support gap-graded Superpave

The exploration of how SMA and Superpave is coming together as gap-graded Superpave took place in the 1990s, especially through the work of the National Center for Asphalt Technology.

Although SMA has been used in the United States since 1991, it was not until 1995 that a standard mixture design procedure was developed by NCAT for the TRB under National Cooperative Highway Research Program Project 9-8.

In 1998 — seven years after the mix was first introduced — the formal guidelines for SMA mix design, construction, and QC/QA were published in Designing Stone Matrix Asphalt Mixtures for Rut-Resistant Pavements (NCHRP Report 425, Project 9-8, 1998).

“SMA is a simple idea,” wrote NCAT’s Ray Brown and L.A. Cooley, Jr., in Report 425, as it’s now called.

“Find a hard, durable, quality stone,” they wrote, “fracture it into roughly cubical shape and of a size consistent with the proposed layer thickness, and then glue the stones together with a durable, moisture-resistant mortar of just the right quantity to give stone-on-stone contact among the coarse aggregate particles. For the asphalt technologist, the trick is getting the various parameters right.”

Even before then, guidelines had been developing. In 1998 the Superpave lead states recommended the following modifications when using SMA within the Superpave mix design system:

Binders selected in accordance with AASHTO MP-2 should have the high temperature grade increased by at least one grade. A PG 70-XX should be the minimum high grade specified. While there is no maximum high temperature grade recommended, consideration should be given to a PG 82-XX as the practical upper limit.

A fiber modifier should be used to facilitate placement regardless of the binder grade selected. This recommendation can be eliminated based on acceptable drain-down test results.

Voids in the Total Mixture at an N-design gyration level of 100 should be 4.0%. There are no recommended N-initial or N-maximum values.

Use 150-mm diameter Superpave specimens in mixture design development.

Also, in March 1999, NCHRP Project 9-9, Refinement of the Superpave Gyratory Compaction Procedure, observed that gap-graded mixtures can be designed without problem in the Superpave gyratory compactor.

And in January 1999, those conclusions on the role of aggregates where SMA and Superpave come together were further supported by a TRB peer-reviewed paper, Evaluation of Aggregate Size Characteristics in Stone Matrix Asphalt and Superpave Mixtures, by Lynn, Cooley and Brown.

Not mutually exclusive

The bottom line is that SMA and Superpave actually complement each other, given the right application. And in some respects, Superpave mix design methods are making SMA even more durable.

“They’re not mutually exclusive,” Maryland’s Michael said. “We’re using the Superpave mix design system, the gyratory compactor, and other Superpave design tools to design SMA.”

And more refinements to SMA may be coming in the near future. “A number of us just were back in Germany, looking at SMA and seeing what kind of changes they’ve made recently,” Michael says. “There, the SMA mix of choice is the 9.5 mm, and I’m sure we’ll be looking at some finer SMA mixes as well.” 

Stone Matrix Asphalt suitable for thin lifts, new research shows

Stone matrix asphalt is associated with big, beefy road projects, but it may also be suitable for thin lift hot-mix asphalt overlays if finer aggregate is used, according to new research released in April from the National Center for Asphalt Technology at Auburn University.

“Fine SMAs could be successfully designed to have stone-on-stone contact,” report L.A. Cooley, manager, Southeastern Superpave Center at NCAT, and Ray Brown, director, NCAT, in their paper, Potential of Using Stone Matrix Asphalt for Thin Overlays. “Rut susceptibility testing with the asphalt pavement analyzer confirmed that the designed fine SMA were rut-resistant,” they write.

“To date almost all of the SMA mixes have had either a 12.5- or 19.0-mm nominal maximum aggregate size,” Cooley and Brown write. “These two nominal maximum aggregate sizes have been predominant because they conform to information obtained from European experiences with SMA.”

However, they say, a “fine” SMA mix could be beneficial because it could be placed in thinner lifts, it could be used as part of a preventive maintenance or pavement preservation program, and it ought to be more workable.

Cooley and Brown defined a “fine” SMA as having either a 4.75- or 9.5-mm nominal maximum aggregate size. Data accumulated from this study showed that these fine SMAs could have stone-on-stone contact and be rut-resistant.

“Permeability testing indicated that these fine SMA mixes were less permeable than conventional SMA mixes, at similar air void levels, and thus should be more durable,” they say. “Based upon all information from this study, it was concluded that fine SMAs are a viable option for thin overlays.”

The complete study is downloadable in .pdf format from NCAT at www.eng.auburn.edu/center/ ncat/reports/rep03-01.pdf. 

For More Information

Much information on stone matrix asphalt is available for road agency personnel and contractors. Here’s a selection:

A technical paper, Construction and Performance of Stone Matrix Asphalt Pavements in Maryland: An Update (2002) is available in .doc format from the Maryland State Highway Administration’s Office of Materials and Technology; contact Gloria Burke at gburke@sha.state.md.us, or call 800-477-7453.

The proceedings of the SMA in the USA II workshop, March 2002, on CD-ROM, are available from the Federal Highway Administration. Contact John Bukowski, P.E., Office of Pavement Technology, at john.bukowski@fhwa. dot.gov, or call 202-366-1287

Larry Michael’s graphic presentation on Maryland’s experience with SMA is downloadable as a .pdf file from the Web site of the 30th Rocky Mountain Asphalt Conference and Equipment Show 2003, off the Web site of Colorado State University, College of Applied Human Sciences, at www.cahs.colostate.edu/mtcm/RMACES%20Presentations.htm.

An essential, 43-page publication, Designing and Construction SMA Mixtures: State-of-the-Practice (QIP 122), is for sale by the National Asphalt Pavement Association. Contact NAPA at 888-468-6499, or via napa@hotmix.org.

The 2003 National Center for Asphalt Technology paper, Potential of Using Stone Matrix Asphalt for Thin Overlays, is downloadable as a .pdf from NCAT at www.eng.auburn.edu/center/ ncat/reports/rep03-01.pdf. Visit NCAT’s home page at www.eng.auburn.edu/center/ncat/ for much more information on SMA, Superpave, HMA, and their applications.

The paper, Summary of Georgia’s Experience with Stone Matrix Asphalt Mixes, is available from the Georgia DOT Office of Materials and Research, as a .pdf file, at www.dot.state .ga.us/ dot/construction/materials-research/b-admin/ research/onlinereports%5Cr-SMA2002.pdf.

Reprinted from Better Roads Magazine
September 2003

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