Pervious-concrete practitioners are monitoring a major test slab in Denver, where a premier freeze-thaw application for the specialty pavement was recently completed in the form of a Safeway grocery story parking lot. Pouring of the pervious concrete began on December 15 by Atlanta-based specialist PCI Systems Inc. with material supplied by local producer Ready Mixed Concrete Co. (The main contractor for the job was Roche Construction of Denver.) The test case slab also is an important factor in demonstrating that pervious concrete may gain market acceptance outside of warmer U.S. regions like the Southeast and California.

Dan Huffman, National Ready Mixed Concrete Association's (NRMCA) national resource director — west (also note “Turner” item, p. 8) and advisor for the placements concluding New Year's Eve, says the project was a milestone for several reasons. The first major use of a PCI vibratory screed on the 3-acre project was preceded by a fresh 2-in. snowfall and ambient temperatures in the 20s at the time of placement. “Typically pervious concrete is used in the warmer parts of the country,” he adds. “The Denver project is the first of its size and profile in a freeze-thaw environment.”

The job is part of an existing store reconstruction (including asphalt parking lot replacement), with many time and other logistical restrictions putting the parking lot into service ahead of American Concrete Institute-recommended timetables. Already, the lot has been subjected to sub-freezing and sub-zero ambient temperatures, as well as snowplows, very early in its service life.

Due to water runoff issues addressed by pervious concrete, the city of Denver's Department of Urban Drainage heavily influenced Safeway's decision to go with the material for its parking lot. The company moved forward with the pervious concrete following two years of discussions and six months of instructional meetings with NRMCA and the Colorado Ready Mixed Concrete Association (CRMCA). “From an environmental perspective, traditional drainage methods don't work as well as pervious concrete,” explains Huffman. “Conventional pavement takes ground water away from the site, carrying pollutants like oil directly into rivers and streams, and the native soil is put at risk. But porous pavement is supported as an EPA Best Management Practice and gets points under the LEED [U.S. Green Building Council's Leadership in Energy and Environmental Design] system.”

The use of pervious concrete replaces the need to create an area for water collection and treatment, thus giving more room for the parking lot or the store. “By not having to spend money on conventional stormwater retention, the builder is actually saving money,” says Huffman. “Another benefit is that snow melts faster on pervious pavement. “Since the melted water drains through, the snow on top melts faster without surface water freezing at night.”

NRMCA's new National Accounts Program, which attempts to influence major corporations — such as Wal-Mart, Safeway and others who build coast to coast — to use concrete in upcoming projects, tends to focus its attentions on company headquarters. Since many of these companies are decentralized, local concrete promotion groups set their sights on the divisional offices. Huffman and members of CRMCA began serious discussions with Safeway's Denver office in the summer of 2004 by meeting with the company's director of construction. The City of Denver is so enthusiastic about the the pervious concrete concept that additional meetings are scheduled for discussion on wider-scale usage of the process, a huge steppingstone toward national acceptance.

Since no project of this magnitude had been attempted, NRMCA produced technical documents for Safeway and the city of Denver that included what amounted to educated guesses as to the results of using pervious concrete. “The documents were based on observations made on smaller projects conducted in similar freeze-thaw situations in areas like Pennsylvania,” says Huffman.

A critical aspect of pervious concrete is the base design, explains Huffman. The concrete needs to be placed over a granular base that will hold water like a reservoir. For the Safeway pavement, 6 in. of concrete was poured onto a 6-in. base.

The mix design for this type of cold-weather job is also unusual, according to Huffman. “There is no sand in this. Instead we used a rounded pea gravel, which defies traditional placement for strength,” he explains. “The gravel results in a 25 percent void structure, which is what makes it pervious. Although we didn't use it for this job, a producer can also put fly ash into the mix or a water-reducing admixture.”

The heated mixture arrived on the job site from Ready Mixed Concrete's plant in the city. Using the facility's central mix batch plant, near-boiling water is introduced to the mixture as the aggregate goes in with the cement. Because of the cold weather application, materials are introduced into the mix slower than usual. The completed mix is loaded into trucks and delivered to the job site at or above 70°F, usually resulting in rather steamy pour. Following concrete placement, a seven-day cure is required, so crews work quickly after the pour to cover up the pavement with insulating blankets to maintain a minimum 55°F temperature.

“There are no state specifications for pervious concrete, no standards as yet,” says Huffman. “We looked at specs from warmer states, but we had to rely on PCI Systems to make this work. They played a huge role.

“The truth is, we don't know yet the realities of this process. We know that this was covered as prescribed, but it's too soon to tell whether seven days was enough time to get the strength we need. Because of space restrictions for the rest of the store reconstruction, we had to open up much of the parking lot after exactly seven days.

“The site will absolutely need to be monitored at least until May, so we can know our exact strength and durability results.”