August 2011 saw a finished four-acre (1.62 hectare), pervious-paved heavy equipment lot in Chattanooga, Tennessee, for Astec, Inc. ARCADIS U.S. Inc., the lot's designer, partnered with Talley Construction Co. Inc. to deliver a TDOT (Tennessee Department of Transportation) approved lot that would meet Astec's needs. Talley Construction paved the six-layer lot with the help of Roadtec's equipment, whose Shuttle Buggy® and track-style paver were ideal for keeping the No. 57 stone in place during the paving process. The layers of material used to create the pervious pavement included:
- Placing a non-woven filter fabric over the subgrade
- A 14-inch (35.56 cm) layer of No. 3 stone
- A 4-inch (10.16 cm) lift of No. 57 "choke" stone
- Two 4-inch (10.16 cm) lifts of 307-AS warm-mix asphalt (WMA) designed specifically for the project
- A 1.25-inch (3.18 cm) OGFC (open-graded friction course) material surrounded by riprap
LOOKING BACK—A PROJECT OVERVIEW
Scott Thompson, vice president of Talley Construction, explained some of the specifics: "We used the warm mix technology and higher grade asphalt PG70 liquid, which allowed for 5.8 percent liquid asphalt content and avoided having to use fibers in the mix, which saved about $5 a ton." The two layers of 307-AS contained 10 percent RAP (reclaimed asphalt pavement) with the OGFC surface mix containing 15 percent. The latest technology for pervious paving at the time of the project led to money savings in materials, the use of recycled materials in its mixes, and aided in environmental safety by absorbing the "first flush" of stormwater.
Pervious pavement is designed to absorb at least the first inch (2.54 cm) of rainwater—Astec's paved lot can absorb 3-inches (7.62 cm)—in order to reduce the level of pollution from stormwater. The water is absorbed through the asphalt, into the stone, and eventually finds its way into the subgrade where soil chemistry and biology can "treat" stormwater pollutants. Pervious pavement does not require hold basins, retention pods, or piping.
"Money is saved when contractors do not have to build additional stormwater management systems," said Thompson. "This pavement design allows for maximum buildout without the costly extras to hold stormwater."
HOW DOES IT HOLD UP?
According to Malcolm Swanson, president of Astec, Inc., the lot "Still looks good. Actually, it's worn very little; it appears to be very durable." Not only was stormwater runoff absorption a factor in Astec's choosing pervious pavement, but also the ability to stand up to heavy-duty equipment. Astec needed a durable option for its equipment lot. Because the lot was intended for holding Astec's finished silos and conveyors for asphalt plants, Talley Construction used a heavy-mix design for the project. "What we expected it to do was support heavy equipment that we parked on it and be able to move the water away from the equipment and keep it off the surface," said Swanson.
Another reason Astec chose pervious paving for its lot was to provide a demonstration of what pervious pavement would do in a heavy-duty environment. "We were surprised when we first put it in by how well it transferred the water off the surface," Swanson said. "We took a firehose and doused a small area with its full force and the distribution of the water wouldn't run off, it just disappeared into the pavement." While the water absorption properties impressed, how well the pavement would hold up to housing heavy-duty equipment was a significant concern.
"We haven't done any repairs. There are areas where it can stand superficial repairs. We allowed supports for extremely heavy equipment to set for a while on the pavement. Some of the equipment we make is heavy enough that if you put the supports straight onto the pavement without something to spread the load—such as timbers or a steel plate—, it's going to leave some indentations," Swanson said.
Astec has been pleased with the pervious pavement decision. The sturdy lot lives up to the company's expectations. As far as the lot's use as a demonstration of pervious pavement's abilities? "I don't know if anyone has actually made their decision to use pervious pavement based on seeing our equipment lot, but people have seen it and commented on it, and have been impressed by what it does," said Swanson.
The Roadtec Shuttle Buggy® and track-style paver worked well for putting down the pervious pavement mix.
First course of asphalt.
Finish course of asphalt.
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Words from Ben Brock
The Vaccari Group operates one of the largest aggregate mining operations and asphalt production facilities in Italy