Shortly after the Core Library at Northwestern University in Evanston, Ill., was built in the 1950s, the 40,000-square-foot plaza began leaking into the bookstacks below. In the mid-1980s the decorative walking surface and the asphalt membrane below were removed. New bi-level drains were added and the contractor installed fill concrete on the structural slab so that water would run to the drains.
A polyvinyl chloride (PVC) membrane was then installed, with the seams heat-welded together. A drainage mat and insulation were placed on top of the membrane. Finally, a topping of colored stamped concrete was installed for the walking surface.
The decorative concrete stood the test of time very well. The problem was with the membrane, which was guaranteed for 20 years and, indeed, lasted that long. But the primary failure of the membrane was from electrical improvements made throughout the years. There were many conduit penetrations, most of which leaked.
The university recently decided it was time to remove the decorative concrete and the membrane and start over. Everlast Concrete of Steger, Ill., was contracted to do the decorative concrete work.
Decorative concrete is only as good as its foundations. Workers began by shotblasting the deck to provide a bondable surface for the new waterproofing membrane. Made by American Hydrotec, the rubberized asphalt was heated to 350º F before being applied.
A 215 mil-thick membrane was installed. This included a fabric reinforcing material. Workers covered the hot material with an additional 60 mil-thick protection board; the installation carried a 20-year warranty. Crews also installed new bi-level drain heads at the existing locations. All areas underwent a 48-hour water test before construction proceeded.
Placing a drainage mat allowed water that made it past the concrete top-ping to freely flow to the lower drain heads. Workers also placed 2-inch-thick insulation board and a plastic slip sheet.
Setting bulkheads over membrane is a time-consuming process because you can't penetrate the membrane, yet it must be held secure and vertical. Vince Schrementi, the owner of Everlast Concrete, said the forming was more difficult because the thickness of the concrete varied from 3 to 5 inches to provide proper pitch to the drains in all locations. They held their bulkheads in place with sandbags.
The architect specified both integral color and color hardener–each of the same color and supplied by the L.M. Scofield Co., which had also supplied the color for the original work 20 years earlier.
The specially designed stamps included a textured border with a 6x12-inch smooth brick pattern infill area. An unusual aspect of the stamp was the infill units, which had a closed top to the stamp without any texture. The flat tops didn't leave any markings on the surface. And each stamp was sized to stamp a complete infill area.
To finish the project, the work was sealed with a solvent acrylic sealer, control joints were filled with polyurethane caulk, and ADA ramps and railings were added.
-Joe Nasvik is senior editor ofConcrete Construction and Concrete Surfaces' sister publication.