Used for entertaining by the building's owner, Duke Realty, and its tenants, the plaza is home to the family of Clear Channel radio stations, which broadcast from the top of the Towers of Kenwood. The plaza sits atop of a parking garage with seven stories of glass curtain wall above. Needing a renewal and a new deck, the owner chose exposed aggregate and Lithocrete—a patented system for seeding small aggregates into the surface of the concrete. Lithocrete's glass aggregate bands surround fields of lightly exposed river sand mined from the nearby Ohio River.
“After the demolition and waterproofing of the deck was complete, it was of prime concern for us to have an even subgrade for placement of the 6-inch reinforced concrete. We were able to engineer the layout,” says Robert Bennett of Architectural Concrete Systems. They installed a lightweight aggregate up to 8 inches thick on the 16,500-square-foot plaza. The varied aggregate thickness and the pitch of the garage deck provided a drainage layer and ensured an even placement of concrete that helped prevent unwanted cracking.
Business continued at the Towers of Kenwood during construction and made access to the plaza difficult. “The concrete was placed in 5- to 6-yard increments with a 10-man crew,” says Bennett. The 4500-psi concrete mix included fiber reinforcing, water reducer, and Lithocrete's proprietary conditioner to lubricate the fines of the cement. Bennett described the paste as easy to work with around the seeded aggregate. The seeded aggregate was a combination of ¼-inch aqua and cobalt blue glass and was broadcast onto integrally colored concrete, which used 920 smoke by Solomon Colors. The seeded glass aggregate was sprayed with Lithocrete's etching agent and scrubbed with brushes to remove the surface paste within 24 hours of concrete placement. The goal was an even and smooth reveal of the tiny aggregate. “When working with the small glass aggregate, try to use an integral color in the concrete that matches the glass, then the coverage is easier. The smoke color was used here because blue integral color was not available. It required careful, even broadcasting to the point of rejection,” says Bennett.
The clean look of the plaza was further enhanced with a careful joint design. It met a vehicular traffic requirement using a system of “hidden” expansion joints with load-transfer dowels. Concrete was placed against the hidden joints, finished to meet the neighboring pad, and when saw cut, appeared the same as the control joints. Careful layout and execution of saw cuts were used to control cracking, and use of ¼-inch expansion material between vertical structures and horizontal pavements helped to ensure minimal stress on the slab's joints due to lateral movement. The freeze-thaw climate required all the expansion joints to be caulked with a urethane sealant and the saw cut joints to be filled with clear silicone. A penetrating Lithocrete sealer protects against alkali silica reaction (ASR)—the factor that can cause glass to react over time with concrete. Prosoco's Gloss-n-Guard protected and deepened the field color of the exposed aggregate and Diamond Clear sealer by Euclid added gloss to the embedded glass.