These days we take for granted the impact that computers, cell phones, digital imagery, and internet technologies have on our lives. But when we think of art and decorative concrete, we think it's still about creatively working with your hands and using hand tools to apply materials to a canvas—concrete in this case. But every now and then new technology opens the door to creative possibility. That's what happened when the Morton Arboretum in Lisle, Ill., wanted to place large leaf representations on barrier walls and bridge abutments along a neighboring tollway.
The project started when the Illinois Tollway Commission decided to add a lane in each direction to its I-88 Tollway. Where the toll road runs alongside Arboretum property, retaining walls would be needed. So the Tollway Commission decided to invite the Arboretum to plan the aesthetic design work. Peggy Pelkonen, the landscape architect for the Arboretum, says they were enthusiastic about the idea and put together a design team that even included staff from the village of Lisle. The team considered a number of ideas and decided on a leaf motif.
Creating a digital image
Pelkonen says that the decision to use actual leaves from the Arboretum as the basis for creating molds was suggested by her predecessor. He selected a number of leaves from their archive collection, including an ash leaf collected in 1936, a white oak from 1985, an elm from 1985, a maple from 1988, a red oak from 1990, and a ginko collected in 2006.
Each leaf was pressed flat and mounted on a piece of cardboard. Kevin O'Connell, a digital artist working for ThinkBigFactory.com, Minneapolis, says they were selected to do the image work and received the leaves with the instructions that the specimens were to remain attached to the mounting boards and untouched. The Arboretum wanted an exact representation of each leaf so that would mean starting with digitally scanning each one, he recalls. The problem was that some of the leaves weren't in good condition, so work on the digital images would be needed.
The software that Think Big Factory chose to use for the images is called Zbrush. O'Connell says that the recent introduction of a new version of this program made it possible to work with organic shapes, thus making this project possible. “Using the software is kind of like sculpting with clay,” he adds. “The leaves were pressed flat when we received them so the software enabled us to make natural looking three-dimensional shapes. We also were able to restore deteriorated portions of leaves.” When work on the shapes was complete, the images were converted into a computer assisted design (CAD) file so that each leaf could be increased in size to 5 or 6 feet in length. Switching to a CAD format also made it possible to transfer the files to a computer numerically controlled (CNC) router.
Making a positive mold
Previous to CNC router technology, shapes usually were cut from polystyrene foam with hot wire cutting equipment. The results are crude, however, when compared to the detail and accuracy of CNC router work. So this equipment was used to construct the positive forms needed to create the formliners for the project.
CNC routers consist of a table to hold the stock and a router mounted on a moving platform that can travel in x, y, and z directions. The movement of the router, in this case, is guided by a computer with the CAD files. O'Connell says they used a ¼-inch router bit with a ball shaped end to do the work. Blocks of poly foam 3 inches thick and 6 feet square having a density of 2 pounds per cubic foot were chosen as the materials used for the positive form. He reports that it took approximately six hours of cutting time to complete each leaf.
To finish the positive mold and make it ready for molding formliners, the manufacturer of the liners sprayed a light coat of poly urea resin over the form to eliminate the porous surface of the polystyrene.
Forming and placing concrete
The formliners were cast with foam urethanes with a total thickness of 4 inches—3 inches of depth for each leaf with 1 inch of backing for the form-liner. Because the Arboretum wanted the leaf forms to project out from the concrete surface approximately 3 inches, contractor Albin Carlson, Melrose Park, Ill., had to build a plywood false form in front of the wall forms that was flush with the formliner surface to make the leaves project outward.
Self-consolidating concrete (SCC) is increasingly being used to cast walls with architectural and formlined surfaces. But Scott Propper, Albin Carlson's project engineer, says they elected to use a standard approved concrete mix with a superplasticizer, performing careful internal vibration work to ensure that there would be few bugholes. He reports that work to date is progressing well, with the completion of the project scheduled for the fall of 2008.
Pelkonen says they are delighted with the results so far. “The detail in the molds is incredible.” In the specifications for the project, the Arboretum also called for inspections and approvals for the completed images, the positive mold, and of course the completed concrete work. Pelkonen says that you can see all the details of each leaf including their veins in the cast concrete. By making the leaves large and projecting them out from the wall surfaces, motorists will easily see them when they drive by.
When complete, the wall surfaces will be stained an earth brown hue with the leaves left in a natural concrete color. Lights will highlight them at night.