By Chris McDaniel, president of McD Concrete
As with any environmentally minded organization, the Cincinnati Zoo and Botanical Gardens is always looking for progressive ways to lessen its impact on the planet. One of the primary ways it is doing this is by striving to remove itself from the stormwater system at the Cincinnati Metropolitan Sewer District. It has employed many innovative techniques through the years to accomplish this goal such as pervious pavers, rain gardens, green roofs, etc. Now it has entered a new and more dramatic phase of its goals.
The Zoo's Senior Director of Facilities and Planning, Mark Fisher recognized that the biggest impact that the Zoo has is the amount of runoff water coming from its impervious parking areas. In order to combat this he directed his construction team to come up with a design to begin to reduce the amount of runoff. After months of planning and deliberation, Fisher's plan became reality as the Cincinnati Zoo added over 40,000 square feet of pervious concrete paving.
In designing the pervious paving, it became apparent that percolation into the soil was going to be a difficult challenge. With the expansive clays prevalent in Southwest Ohio, the option of simply allowing the excess rainwater to return into the ground did not exist. Therefore, the Zoo chose to treat the parking lots like its own detention system. In so doing, the pervious concrete was designed with 15 to 21 inches of #2 washed, recycled concrete as a base, and then the top 2 inches was choked with washed, recycled 304s. Under the gravel, they placed a woven filter fabric with drain pipes running parallel the length of the parking stalls and perpendicular to the main at approximately every 40 feet. Then, the catch basins were set at elevations to allow the storm water to bleed into the storm sewer only if the entire system was overloaded. Finally, the pervious was designed at a 6-inch thickness in the parking spaces and the drive lanes were installed with 8 inches of conventional concrete.
Critical to the success of this operation was the selection of the appropriate placing technique and the contractor with an appropriate level of knowledge and experience. The Zoo and its construction manager HGC Construction selected McD Concrete of Alexandria, Kentucky, due to many factors, not the least of which was their experience placing pervious concrete in parking lots, horse wash facilities, streetscapes, and many other applications.
The schedule was very intense with many trades having to execute work in a short period of time. To further complicate matters, the Zoo's staff was still in the process of acquiring certain portions of the jobsite that required paving. With an opening date of less than three weeks, McD realized that conventional techniques would not permit installation and proper curing time. For that reason, they used its Somero SXP Laser Screed with pervious head. In so doing, they were able to place 250 to 300 cy of pervious every day for three consecutive days. This afforded the project the adequate curing time in order to allow the necessary opening date. HGC Project Manager, Jerry Muchmore said, "If we had to use conventional techniques, the pervious limitations and duration would not have allowed the Zoo access to their new parking lots in time for the grand opening. That option was clearly not acceptable."
After and during placement, a product known simply as "The Bean" (soybean oil) was sprayed on top of the pervious concrete to aid in the curing process. The paving was then completely covered with 4 mil poly which was left in place for 5 to 7 days. Once removed, the paving was swept, striped, and ready for use.
As with any project, this one required a great deal of planning and coordination. Thanks to the efforts of all involved, it was turned over on time and has removed a great deal of stormwater from the lines in Cincinnati