Ultra Flexibility — the latest advancement in asphalt interlayers
Asphalt interlayers originated in Europe and were brought to the US by Koch Materials Company in the 1990’s. The first project was placed around 1995. The idea then, as now, was to pave an asphalt interlayer over concrete or old asphalt surfaces to help isolate cracking, waterproof, improve ride, and extend the surface overlay life.
I have spent most of my decades-long career improving interlayer technology for applications in the U.S., applying performance-based design strategies to create a hot-mix interlayer solution that provides exceptional flexibility (nearly 100 times more flexibility) than traditional asphalt.
As further research and field investigations have found, flexibility is a crucial property for an asphalt interlayer because it allows the pavement to absorb stress and movement.
Rigid vs. Flexible — the underlying cause of reflective cracking problems
Reflective cracking occurs when cracks or joints in an underlying pavement layer propagate upward into a new asphalt overlay. The rocklike hardness of underlying concrete or old asphalt creates a rigid layer that shears and moves under the forces of traffic and earth movements. It’s little wonder then, that flexible asphalt pavements placed on top of concrete or old asphalt don’t stand a chance of holding tight against these extreme forces. The asphalt overlay will crack directly over the joints in the underlying concrete slabs or cracks in asphalt pavement, providing a path for water to infiltrate the underlying base.
For decades the industry has tried a variety of solutions, from grids, to paving fabrics, to a plethora of asphalt surface layer designs. All have worked to some degree in combatting the problem of shear movement but “some degree” is still not enough, especially today with the increased emphasis on sustainability and long-life pavements.
Asphalt overlays generally reflective crack at a rate of 25 percent, or one inch per year after the first year. That means a 2-inch overlay will see all cracks reflect through in two to three years, and a 4-inch overlay will crack in four to five years. Clearly, we still need something better to disrupt the rigid/flexible dynamic. And any solution needs to absorb at least 80 percent of the underlying movement to ensure survival of the surface overlay.
Polymerized chip seals add resistance to cracking but can’t achieve enough thickness to reach 80 percent strain absorption. Even a one-inch-thick asphalt-rich interlayer approaching 9 percent asphalt content (AC) would still not effectively combat the underlying rigid/flexible dynamic due to rutting potential, and a higher asphalt content increases the cost of the overlay as well.
With all the various solutions still falling short, the temptation is to say, “It can’t be done.”
Oh, but it can!
The advanced solution: Supercharge the asphalt mix
Today it’s possible to provide unparalleled crack resistance in hot mixes that can be modified to meet specifications required for your pavement’s particular performance needs and customer expectations. It’s all due to the successful addition of a fine aggregate, high-asphalt binder, and a razor-edge design that incorporates high-tech additives (polymer and aramid fiber). This design leads to an interlayer that meets or exceeds a 750 CTindex at 7 percent air voids. Also, it’s not unusual for this CTindex to approach or exceed 1,500.
By achieving this high index, the supercharged interlayer design provides a leap forward over traditional mix designs, allowing it to absorb shear movements and delay reflective cracking.
The real beauty of this interlayer design is that it’s locally available to any contractor using a blend of their fine aggregates. Moreover, it’s plant-produced, paver-laid, and provides a quick return to traffic.
Most recently BATT has designed several successful interlayer projects, starting in 2019 with a the Taylor County Airport runway that is still in good shape today
(See https://sway.cloud.microsoft/XSnKPLftoELSbX08?ref=Link&loc=play). In 2024 alone, BATT has assisted in specifying, designing, and constructing four interlayers to combat severe reflective cracking in Canada and the U.S. These include interlayers in Alberta, Ontario, and New Brunswick, and on a section of I-76 on the Pennsylvania Turnpike.
For best results, think holistically on the pavement solution
In addition to a supercharged, flexible interlayer, a complementary, more flexible overlay will provide added crack resistance to absorb the remaining 20% movement not controlled by the interlayer. This option provides the famous one-two punch to win the round against cracking, as most top layers handle only about 10 percent of the shifting slabs.
Keep in mind that a well-designed flexible interlayer helps distribute loads more evenly, preventing reflective cracking from underlying layers. This adaptability to shearing enhances the pavement’s durability and improves its performance under varying traffic and environmental conditions.
For the long run, my best advice is: Be purposeful in your asphalt mix design. Align the solution to target the pavement distress and avoid replacing trouble spots with the same mix formula as the original pavement. By addressing the root cause—whether it's cracking, rutting, or moisture damage—solutions should be tailored for maximum durability and efficiency.
This strategic approach not only extends pavement life but also minimizes future maintenance, reducing overall costs and disruptions. Investing in the right fix for the right problem leads to safer, stronger, and more sustainable roads.
For more information on interlayer and other paving solutions, contact BATT. You may also access this technical paper for more in-depth reporting on our interlayer asphalt solutions and results:
