Creating dedicated open park space in densely populated urban areas is a consistent challenge for city planners. One innovative approach is to install green-roof systems on existing structures within the city limits. For example, in Chicago, Ill., an entire network of interconnected parks is built on top of underground parking garages. They comprise the largest parking facility in the United States!

Recently, the Chicago branch of Western Specialty Contractors completed a restoration and waterproofing project for the Millennium Lakeside Garage located underneath Maggie Daley Park. This 700,000-square-foot (65,032.1 m²) project was divided into three phases: removal of the existing park, waterproofing and repairs to the concrete parking structure, and re-installation of the park and construction of its new features, including a rock-climbing wall and ice-skating ribbon. Western Specialty Contractors was tasked with repairing and preparing the concrete substrate before installing the American Hydrotech Monolithic Membrane (MM) 6125 waterproofing system.

No Room for Error
The concrete parking structure was originally waterproofed in the 1970s, and the concrete slab was showing signs of wear and tear, as well ground water infiltration. As it is essentially a giant green roof, the parking structure needed to have proper waterproofing to maintain the park’s integrity and sustainability. In addition, any leaks or drainage issues that occurred in the future would require the new park to be removed and the entire structure to be re-waterproofed — an expensive, labor intensive endeavor that Next Realty, the owner of the parking garage, wanted to avoid at all costs.

After the removal of the existing park by James McHugh Construction Company, all drainage issues were addressed and fixed with the installation of new drains. Then, a plan was made for a composite drainage mat system to be installed as a part of the waterproofing system.  It was then time for Western Specialty Contractors to begin work on the concrete roof slab.

“The entire deck was visually inspected for delaminated or possibly delaminated areas of concrete. Those areas were then marked and inspected by Desman Associates, the engineering firm on the job, and they indicated the areas they felt were in need of repair,” explained Collin Tierney of Western Specialty Contractors.  Those areas were chipped out to an average depth of three inches (7.6 cm). All exposed reinforcing bar was visually inspected and repaired as instructed by Desman Associates. “At that point, all reinforcing bar was sand blasted and coated using an epoxy primer. Patches were then installed using ready-mix concrete that was delivered to the site by Prairie Materials,” stated Tierney.

Hot Stuff
With the concrete slab repaired, surface preparation of the entire concrete area began.  Western Specialty Contractors power washed the concrete and then performed all perimeter surface preparation at horizontal to vertical transitions by abrasive blasting to achieve a Concrete Surface Profile (CSP) 3–4 as defined by International Concrete Repair Institute (ICRI) standards. “The remaining horizontal surface preparation was subcontracted to Nelson Industrial Services. The Nelson crew achieved a CSP3–4 surface profile by shot blasting the concrete surface,” said Tierney.

Once the substrate was properly prepped, the Western Specialty Contractors team was ready to apply the American Hydrotech MM6125 waterproofing system. The first step was to apply a primer to the concrete. According to Tierney, this surface conditioner is an asphalt-based, fluid-applied material that improves the bond between the waterproofing membrane and the concrete.  The surface conditioner was sprayed onto the deck using hand-held Hudson sprayers capable of holding solvent-borne material. The rate of application was approximately 300 to 500 square feet per gallon (27.9–46.5 m² per 3.8 L), as stated by Tierney.

The next layer of the system consisted of melting rubberized asphalt bricks into a liquid state by heating the bricks to 400° F (204.4° C) in a kettle. The rubberized asphalt was then poured onto the concrete while still extremely hot. The crew then spread material evenly over the area.

Constant measurements were taken during the entire process to confirm that the liquid-applied material was spread at a consistent rate. “The MM6125 system was applied at an average thickness of 90 mils [2,286.0 microns] per coat for a total average system thickness of 180 mils [4,572.0 microns],” said Tierney. Overall, this project required over 1.3 million pounds of rubberized asphalt!

Multiple crews consisting of 40 workers per day started from opposite corners of the deck and made their way to the middle of the slab.  All crew members working with the hot-applied material wore special gloves to protect themselves from getting burned. In addition, the crew wore hard hats, respirators, eye protection, long-sleeved shirts, long pants, and boots.

While still liquid, the crew rolled reinforcing felt fabric into the asphalt. Then another layer of rubberized asphalt was applied over the top.  The final layers of the system consisted of sheets of heavy-duty rubberized asphalt protection board, specially designed to prevent tree roots from puncturing the waterproofing; composite drainage mat; filter fabric; and approximately 400,000 cubic feet (11,326.7 m³) of gravel overburden.

Challenges Abound
A project of this size and scope is bound to have its share of challenges to overcome.  Security, contractor coordination, and logistics were a massive undertaking, but, with the help of high-definition cameras installed high above the jobsite, the project ran smoothly. These cameras recorded all activity at the jobsite 24/7, and the recordings were used on a daily basis by the management team to coordinate daily activities, work locations, material staging locations, and jobsite safety and security.

Another challenge faced by the Western Specialty Contractors team was the sheer size of the roof deck and the tight timeline of the project. The duration of the job was two years, with work put on hold during the cold and snowy Chicago winter. Knowing that the job would need to be halted for several months due to weather, the team looked for ways to save time and stay on schedule. American Hydrotech helped out in this regard by manufacturing special extra-large rolls of drainage mat made specifically for the job.

Waterproofing Success
Once all aspects of the waterproofing project were complete, tests were conducted using cutting-edge technology to ensure that there were no breaches in the system. A third-party leak detection specialist, International Leak Detection (ILD), used electric field vector mapping (EFVM) to locate even the smallest leak in the membrane. This allowed repairs to be made before there were any issues with the system.

Through creative problem solving, great attention to detail, and the utmost coordination, Tierney and his team completed this complex, challenging, yet extremely successful restoration and waterproofing project and delivered the foundation for an urban oasis for Chicago residents.