High RAP, balanced mix design excels in long-term results

In 2020, the City of Louisville tested a new asphalt mix design that offered the best of environmentally friendly qualities: A higher than usual percentage of recycled asphalt pavement (RAP) and longer-term resistance to rutting and cracking.

They paved a downtown street with a new asphalt mix that contains 36% recycled asphalt pavement (RAP) and the BATT Lab devised a new mixing method that combines an EPD-Listed aramid fiber product from Surface Tech with a bio-oil. The new mix design holds great promise for helping other municipalities reduce the amount of RAP pilling up or going to landfills. After two years, Lampton Street shows virtually no cracking or rutting. It's a remarkable achievement given the fact that most asphalt mixes can add only 20% RAP.

In standard mix designs, too much RAP can make the pavement brittle, but the high-RAP mix design utilized by the City achieved a balanced mix that went down smoothly and has resisted wear through two years of traffic, plus hot summers and snowy winters.

The BATT Lab utilized ACE XP Polymer Fiber™ from Surface Tech and a TUFFTREK 4007 bio-oil from Bakelite. Surface Tech now offers the mix design as REARM HR — Rejuvinator + Aramid with High RAP.

Bio-oils are used to soften the RAP in the initial stages of blending. The ACE XP wax-coated fibers get introduced into the mix at a later stage to reinforce it with millions of aramid tendrils that form a strong 3D matrix. The addition of the aramid reinforcer is what allows a higher percentage of RAP in the mix.

Zack McKay, BATT Lab Operations Manager, oversaw the testing.

“We saw a 68% increase in the resistance to cracking in the IDEAL-CT. In the rutting test, the mix was able to reach 20,000 passes with about 4 mm of rutting, an excellent result. The overall performance of the mix was interstate-level,” said McKay.

Matt Haines and the other city engineers have been very forward thinking in their approach to trying mix designs that can improve performance and add sustainability to pavements. In the case of downtown Lampton Street, adding more RAP meant the City could save money on raw aggregates and binders, with the added bonus of creating a more environmentally responsible roadway.