Goal One: Sustainability
When Nashville was seeking a parking solution for the complex—which houses its property taxes and assessment department, county clerk and metro finance services—it looked for a pavement system with low impact benefits to meet new local stormwater requirements under the EPA and the Clean Water Act. Like many municipalities across the country that are seeking to meet stormwater regulations and take environmental stewardship to the next level, the primary goal was sustainability.
When porous asphalt was chosen, it was a sound decision at the time based on industry buzz around the low impact design that promised stormwater infiltration instead of runoff. The system was intended to reduce peak flows as the water would pass through the wearing course and then travel to bioretention islands and bio-retention swales before being introduced into the traditional storm water system. An under drain configuration of pipes was put in place to make sure the water was directed to the various bioretention islands and swales. However, a few years later the porous asphalt on top of the system began to deteriorate. Aggregate loosened with cleaning and weather, and the surface became a liability risk and challenge for maintenance efforts.
Take Two: A Long-Term Solution
In 2012, the City of Nashville elected to replace the failing porous asphalt with permeable interlocking concrete pavers, basing the decision on the performance benefits of PICP— strength, durability, superior functionality and lower lifetime maintenance—and the effective results PICP was gaining in other projects.
Nearby shopping center, Gateway Village—a 92,000 square-foot mixed-use parking lot in Murfreesboro—had recently installed a PICP system and was able to show exceptional results and success in the two years following the install. Working with a grant from the City of Murfreesboro, the Concrete Industry Management program at Middle Tennessee State University studied water quality and quantity using an ISCO sampler situated at the site for two years.
In that period of time there was 41 inches of rain (2.3 million gallons of water) and they found that there was no water discharge at the outlet located at the back of the site. All of the water from the rooftops and parking surface was infiltrated back into the soil and replenished into the groundwater aquifer system. With this case study in hand and several other area commercial projects seeing success in permeable pavers, the City of Nashville had a clear path for a long-term solution.
No Time Lost
Permeable interlocking concrete pavers also offered a short-term benefit. During construction, half the parking area needed to remain open to the public to conduct business with the various government offices that share the lot.
Contractor Cumberland Valley Construction and installer Aqua Paving Construction began by demolishing and removing the existing four to five inches of porous asphalt. The area was regraded to bring the existing #57 open-graded aggregate to a compacted elevation of -5”. The project team placed and screeded #8 washed chips as a setting bed and support of the 80mm (3”) thick concrete permeable pavers.
They mechanically placed the Belgard Aqua Bric Type 4 permeable pavers in 12 square foot layers in the final laying pattern. The pavers were saw-cut at the edges, as required, and the installation crew used a 5,000-pound plate compactor to force setting bed and pavers into position.
Once that was done, they were able to sweep and compact washed 1/4-inch chips into joints (the open spaces between the pavers). The job was estimated to take six weeks, but the mechanical installation helped reduce the schedule by two weeks. Additionally, the areas completed were immediately available the next day for business parking, since no curing time is required for PICP.
In addition to keeping the parking lot open during renovation, the project team also was able to save time during the installation phase thanks to the condition of the base. The layer of No. 57 stone needed very little re-leveling, and the existing bioswales and original storage capacity were in good condition and did not need replacing or reconstruction.
Putting the Pavement to the Test
After the installation, the City of Nashville enlisted the help of the local fire department to conduct a demonstration of the new paver parking lot’s infiltration capabilities. Thousands of gallons of water were released in an isolated area to show the infiltration rates. The test proved the stormwater benefits; permeable pavers efficiently reintroduce rain water back into the ground, reducing the amount of stormwater runoff entering the Cumberland River and carrying with it contaminants and pollutants.
Additionally, the engine from the Nashville Fire Department illustrated the structural capability of the paved parking surface, whose concrete pavers offer both durability and aesthetics. Permeable interlocking concrete pavers provided the City of Nashville with a post-structural Best Manufacturing Practice that reflects its commitment to sustainability and safety.