Porous Warm Mix Asphalt

The environmental aspects of using warm mix on a porous project were also attractive. “We have the benefit of using a mix that is green from both the standpoint of being warm mix and recycled material. Warm mix was chosen not only for the environmental aspects ­— it also allowed us to produce an open-graded material without cellulose fibers.”

Cellulose fibers are often used for stability in open-graded mixes as well as to prevent drain-down. Drain-down occurs when liquid asphalt cement migrates to the bottom of a load of asphalt or in a lift of asphalt during placement.

“Fibers are used to increase the AC content and prevent the AC from draining off the material to improve long-term durability,” says Thompson. “With the lower temperatures of the WMA and the use of PG 70-22, we were able to reduce the drain-down of the asphalt cement from the final product; in turn that increased the film thickness to improve the durability.

“We wanted to be able to do this porous project without the fibers because fibers are expensive, and we wanted the most cost-effective project possible,” he adds.

For this paving project, the WMA was produced at Talley Construction with the Astec Double Barrel Green System. The Double Barrel Green System eliminates the need for additives and special asphalt cement by mixing a small amount of water into the AC to create microscopic bubbles. These small bubbles act to reduce the viscosity of the AC coating on the rock, allowing the mix to be handled and worked at lower temperatures.

The challenges

Thompson says that Talley Construction had no challenges on this storage lot project from the construction standpoint. It was the early stages of the project — convincing officials this was the best way to go on the project was the challenge.

“Most all of our challenges on this project came from simply trying to explain the concept to local officials to issue permits,” says Thompson. “I will say that they now completely understand the product and its benefits.

“Astec and Roadtec, the property owners, and Arcadis, the design firm, took the jump when others would not,” he continues. “Without them, this project would not have happened.”
Porous pavements have been around for several decades, but they are rapidly gaining popularity as property owners and developers look for cost-effective ways to meet new stormwater regulations. “The very concept of porous asphalt is not new,” says Thompson. “I feel like it needs to be reintroduced, simply because today’s materials are of a lot higher quality.”