Permeable Pavement 1, Porous Pavement 0

Nashville, TN

Nashville's local government thought it had found the best solution to reduce runoff for the heavily trafficked parking lot of its Richard H. Fulton Complex back in 2007 with a new pavement system much touted for its low impact benefits-porous asphalt. But just a few years later, it became clear the solution was short-lived. Wear and tear on the surface led the local government to seek alternatives, this time looking to permeable interlocking concrete pavement...

The 52,000 sf (4,800 m2) lot serves as parking for hundreds of daily visitors to the city's Richard H. Fulton Complex, which houses the county court clerk, property taxes and assessment department and metro finance offices, among others. The campus receives a constant flow of traffic to the lot's 108 parking spaces. Beginning in 2005, city officials planned for the installation of porous asphalt to handle stormwater runoff, meeting new local stormwater requirements under the Environmental Protection Agency and the Clean Water Act. Despite the fanfare around the low impact design featuring stormwater infiltration instead of runoff, the asphalt eventually began to deteriorate, posing a liability hazard and a maintenance headache for the city.

Over a period of five to six years, the brooms started sweeping the aggregate off, says Majel Carr, project manager in Design and Construction for Metro General Services, Nashville. "We kept sweeping it like we sweep other parking lots. The surface became hard to walk on." The city needed another cost-effective solution for managing stormwater runoff in the same way, but one that wouldn't wear out like the porous asphalt.

"We started looking at the costs, and came up with concrete pavers, which were being used more in this area," Carr says. "We looked at the cost for asphalt again, concrete and permeable pavers, and decided with a three year warranty on the pavers, we'd go with the pavers." The retrofit project was completed within four weeks at a cost of roughly $200,000 and met the budget goal by using the existing aggregate base reservoir while replacing the original surface with permeable interlocking concrete pavers, aggregate jointing and bedding materials.

Retrofitting for runoff
The 2007 installation featured 1½ in. (38 mm) of hot mix porous asphalt over 3½ in. (90 mm) of asphalt-treated permeable base course. Under the layers of asphalt, the system included 2 in. (50 mm) of ASTM No. 7 aggregate, followed by a 9 in. (230 mm) layer of ASTM No. 3 aggregate over a compacted soil subgrade.

When the surface began to fail, it presented hazardous conditions. Rather than install an entirely new system, the inlay of the permeable concrete pavers could be done after removal of the asphalt. However, the design team had little idea what they would find until the demolition started. The project managers were pleasantly surprised with what they found. The base was in excellent condition. The layer of No. 7 stone required some re-leveling before installing the permeable pavers, but without disturbing the existing base. With only about the top 5 in. (125 mm) of asphalt replaced, project costs averaged about $1,843 per parking space. This was considered a bargain because it saved time and money due to this discovery. In addition, existing bioswales were not damaged and the original storage capacity of the base was maintained
as designed.

Business as usual
The project team kept the parking lot open during renovation, which involved a two-part process: Demolish the existing surface on half of the lot during the first part of the project while keeping the other half open for parking, then open the completed half to drivers and pedestrians while completing work on the remainder. This project is one of several permeable interlocking concrete pavements that enable use of a facility during reconstruction.

Installing the concrete pavers manually wasn't an option due to time and budget constraints, so the team contracted with an installer that mechanically placed the permeable pavers in 12 sf (1.2 m2) layers in the final laying pattern. The project began Aug. 13, 2012, with the area blocked off initially for six weeks, but it ultimately reopened two weeks early thanks to mechanized installation. Once an area of pavers was installed, it could be opened for parking almost immediately because interlocking concrete pavers require no curing time.

Nashville currently has several projects underway, including another city center as well as a school where concrete pavers are the pavement. The solution has been touted for its efficacy, durability and the ability to repair the surface as needed rather than removing and installing entire areas, as was the case with the Richard H. Fulton parking lot. The permeable interlocking concrete pavement surface has been in place for a year with regular surface cleaning to maintain infiltration while subject to daily high-volume traffic.