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Ultra Flexibility – The Latest Advancement in Asphalt Interlayers
Asphalt interlayers originated in Europe and were brought to the U.S. by Koch Materials Company in the 1990s. 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 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%, or 25 mm/y after the first year. That means a 50 mm overlay will see all cracks reflect through in 2 to 3 years, and a 100 mm overlay will crack in 4 to 5 years. Clearly, we still need something better to disrupt the rigid/flexible dynamic. And any solution needs to absorb at least 80% 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% strain absorption. Even a 25 mm-thick asphalt-rich interlayer approaching 9% 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. The advanced solution: Supercharge the asphalt mix 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 is locally available to any contractor using a blend of their fine aggregates. Moreover, it is 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. 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 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. |
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With all the various solutions still falling short, the temptation is to say, “It can’t be done.” Oh, but it can!
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