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 book stacks below. In the mid-80s the decorative walking surface was removed, as well as the asphalt membrane below. New bilevel 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 to provide 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 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.In 2006 the university decided it was time to remove the decorative concrete and the membrane and start over. Zera Construction, Niles, Ill., became the general contractor, and Everlast Concrete, Steger, Ill., was contracted to do the decorative concrete work.

Installing a new membrane

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. The product was specified by the engineer but was one with which Zera had experience. Made by American Hydrotec, the rubberized asphalt was heated to 350° F before application. Zera installed a 215 mil-thick membrane that included a fabric reinforcing material. Workers covered the hot material with an additional 60 mil thick protection board and the installation carried a 20-year warranty. Zera also installed new bilevel drain heads at the existing locations. Alex Zera, the company's owner, said that all areas were subjected to a 48-hour water test before construction proceeded.

Placing a drainage mat allowed water that made it past the concrete topping to freely flow to the lower drain heads. Workers also placed 2-inch-thick insulation board and a plastic slip sheet.

Decorative work

Setting bulkheads over membrane is a time-consuming process because you can't penetrate the membrane but 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 sand bags.

Schrementi said that the concrete mix approved for the project was designed for stamping patterns, as well as for good durability. It consisted of 541 pounds of cement, 40 pounds of fly ash, 1280 pounds of sand, 1780 pounds of 3/4-inch stone, 4% to 7.5% air entrainment, a midrange water reducer, and 31 gallons of water (a 0.44 water/cementitious materials ratio). The architect for the project specified both integral color and color hardenereach 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 for this project 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. Schrementi reports that the flat tops didn't leave any markings on the surface of the concrete. 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. The new plaza reflects a quiet warmth and will hopefully last much longer than 20 years.