The balanced mix

Feb. 1, 2018

Will there be more SMA applications in the U.S.?

Fifty years ago, in northern Germany, a new asphalt pavement mixture began being placed.

This mix, dubbed “Kiel mixture” after the city where the mixture was first used, was designed to provide a strong structure to resist deformation and an open surface structure to ensure good friction. Over the years, these mixtures gained wider acceptance under the name stone matrix asphalt (SMA).

In the U.S., a wave of interest in these highly durable mixtures followed the 1990 FHWA/AASHTO/NAPA European asphalt study tour. The first U.S. SMA pavements were constructed about 25 years ago, and by 2002 several state DOTs were leading the way at implementing SMA on interstates and other heavy traffic routes.

SMA is a tough, stable, rut-resistant mixture. The design concept relies on stone-on-stone contact to provide strength and a rich mortar binder to provide durability. These objectives are usually achieved with a gap-graded aggregate coupled with fiber and/or a polymer-modified asphalt cement in a high asphalt content mixture.

The expectation expressed at that time by FHWA, state DOTs, NCAT and NAPA was that SMA would become the preferred premium surface for heavy-duty, high traffic-volume pavements. Larry Michael, who was with the Maryland State Highway Administration at the time, said: “It is, without a doubt, the most tenacious mix I have seen. It is almost impossible to make it rut, and will outlast any other mix.”

Despite this early enthusiasm and successful implementation of SMA in several states, it has not yet become the widespread, national surface mix of choice for high-traffic volume applications in the U.S.

So, what stalled the nationwide use of SMA here? Did we lose interest in SMA, or did the high cost of liquid AC and polymer in 2008/2009 and shrinking highway budgets make SMA too expensive for many agencies? Perhaps the successful introduction of warm-mix technologies to U.S. markets in 2004 shifted the focus away from SMA?

What about those key states that have used SMA pavements for more than a quarter century? What lessons can we learn from their successful adoption of this technology? And, what about our partners in Europe, especially Germany? Does SMA continue to be the material of choice for the autobahns? What innovations are being used with SMA elsewhere in the world?

These are the sorts of questions that we seek to answer at the 1st International Conference on Stone Matrix Asphalt, which convenes Nov. 5-7 in Atlanta, Ga. Agencies and industry from across the U.S. and around the world will share case studies and best practices that highlight the production and construction of SMA pavements that provide high value and performance capabilities.

SMA is not just a premium asphalt pavement; it is an engineered mixture worthy of a fresh look and an intensive discourse, especially given that dedicated freight corridors (i.e., the federal Critical Commerce Corridors initiative) is a fast-approaching reality.

Academia will provide insight into how SMA can continue to evolve through material choices such as reclaimed asphalt pavement, polymer modifiers, recycled tire rubber, fibers and careful aggregate selection. Specifiers and producers will discover enhanced mixture characteristics in terms of life-cycle cost, performance and sustainability.

The conference will give attendees a new understanding of what it means to specify, produce and construct SMA.

Even though the national economy is stronger than when SMA first arrived in the U.S., state DOTs and other public agencies are still charged with delivering the best value and performance possible to the public. Therefore, it makes good sense to expand the use of SMA pavements.

About The Author: Acott is president of the National Asphalt Pavement Association, Lanham, Md.

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