A permanent installation found in Chicago's Grant Park features Polish artist Magdalena Abakanowicz's 9-foot-tall iron cast sculptures of 106 walking torsos frozen in motion. The project's name Agora means a public marketplace or forum in Greek. To see them is to feel that you are part of a crowd. The concrete for the sculpture makes a plaza that could be between buildings that people use everyday. Its shallow texture was the perfect material to base the sculptures.
Abakanowicz had specific concepts for the color and finish texture of the concrete. “It was to be natural, neutral, and not noticed,” says Ken Schaefges, owner of Schaefges Brothers, Wheeling, Ill., who works as both a general and concrete contractor. He asked Vince Schrementi, owner of Everlast Concrete, Steger, Ill., to join the project because of previous projects they had worked on together.
The 11,700-square-foot plaza has a series of meandering irregular rectangles joined together with a few outlying pads placed separately. It was engineered so Abakanowicz could place the heavy sculptures anywhere on the slab. A 12-inch by 3-foot-tall foundation wall underlies the perimeter.
Schaefges Brothers trenched, placed concrete, backfilled the wall, and placed the stone subgrade. The 8-inch-thick concrete slab was reinforced in each direction with number 4 rebar on 18-inch centers. Concrete was placed in 4-inch lifts on the same day—4 inches of natural gray concrete topped with four inches of colored concrete to create a monolithic slab. Coloring only the top 4 inches saved money for the private foundation. The concrete included liquid dispensed integral color using L.M. Scofield, Los Angeles, Chromix Admixture in dark gray.
Over the two-week installation, there was some shifting of color. “If it was a chilly day, the concrete would bleed differently than on a hot sunny day and the amount of bleed water affects the end color,” says Schrementi. He adds that Abakanowicz wanted the entire area to be monotone in color. “Her concern was that the project was not just for today but for future generations. The color and finish of the concrete were vital to that purpose,” he says. “She would take me by the hand to point out the areas that needed correction.” In the end, using a tinted penetrating sealer rectified the uneven final color.
The fresh concrete was bull floated and given a hand float finish for a flat and consistent work surface. Depressions were filled from a separate wheelbarrow of concrete to keep the revealed surface consistent. The time it took for bleed water to leave the slab depended on the temperature and humidity of Chicago November days. “A critical next step was to determine the right time to spray Topcast surface retarder by WR Grace, Columbia, Md., onto the concrete,” says Schrementi. Timing affects the depth of its penetration into the slab.” When the bleed water was off the surface, a uniform spray of Topcast 15 was applied. Typically, concrete placed in the morning was scrubbed for the reveal in about four hours depending again upon weather and the hardness of the concrete. The shallow reveal began with a broom and water hose scrub and progressed to a pressure wash depending on the concrete's set.
Abakanowicz wanted the plaza without sealer. Instead, she wanted an earthy finish without a gloss or film on the concrete's surface. The penetrating sealer used left no film, sealed the substrate, and offered the opportunity to correct the color shifts. Schrementi chose Con Color by Bomanite, Madera, Calif., which features custom-blending tints to achieve the earth tone.