The Guggenheim hired a renovation team of engineers, architects, materials conservator and contracting firm. The team knew it was under pressure to design the best renovation plan it could, not only because the Guggenheim is one of the most famous concrete buildings in the world but because there wasn't room, or money, for mistakes. "Generally speaking, when you start a project you do a conditions survey, you determine the needs, you select materials to do that, you write a specification, you look for qualified contractors, you ask them to bid, and then you find out how much it's going to cost," explains Norman Weiss, a professor of Concrete Technology at Columbia University and senior scientist with Integrated Conservation Resources (ICR), the materials conservator on the museum project. "Instead someone told us, 'Here's the price and get started.' So that was one of the challenges with this project."
Another challenge to the renovation team was it wanted to capture the as-built existing geometry of the building, both for drawings and a structural analysis model. This prompted a laser survey of the building. "The engineers used the drawing to do their structural analysis, and we used that analysis to make a set of drawings so we could look at cracking and materials lost," Weiss says. In addition, a survey of the building's reinforcement was performed using non-destructive testing, including ground penetrating radar.
Another step in determining the proper repair design was removing the multiple layers of paint and coatings that had accumulated over the years in order to not only see the cracks that had made it through the coatings but also the cracks underneath the paint. It took several months to safely remove the 12 layers of paint that had been applied since 1959.
With the paint layers removed, the renovation team set out to map every visible crack in the museum wall. The architect recorded each crack into one of six degrees, each degree requiring a different repair approach. The cracks varied from small hairline cracks to large cracks that spanned the entire depth of the wall. The larger cracks were found near the embedded Ts and web walls. ICR and Wank Adams Slavin Associates compared the exterior to archival photographs taken after construction and matched some of today's existing cracks to cracks seen in construction-era photos. "In a way, it made us feel good that we weren't presented with a problem that had accelerated tremendously recently but not so good when we thought Wright's experimental construction methods were part of the issue," Weiss says.
A unique problem arose on the sixth floor where the design team encountered reinforcement that was installed differently than in the lower floors. The renovation team used a carbon fiber wrap (FRP) to stabilize the sixth floor. "The main issue is the wall has a different geometry than the other floors. It's a much taller wall - the lower floors have 8-ft. walls and the sixth floor has a 16-ft. wall," says Nancy Hudson, associate with Robert Silman Associates. "When they were installing reinforcement on the sixth floor they changed the way the reinforcing was laid out, and it was not continuous between embedded Ts. The wire mesh in the walls was providing continuity, but in isolated locations that corroded over time.
"Our approach with the carbon fiber was to reestablish the continuity. We could only work on the inside face of the wall so we put on vertical and horizontal strips of carbon fiber. The horizontal strips reestablished the continuity that should have been there with the original reinforcing," Hudson explains.
An additional problem with the rotunda walls due to poor rebar placement was they appeared as though they were slipping away from the web walls, sometimes up to 1/2 in. The engineering team designed an anchor and dowel system to reinforce the connection between the rotunda walls and the web walls.
Although the renovation team was startled by many of the problems it encountered in the building, it found a lot of positive things about the concrete. For one, through core samples it found the strength of the shotcrete to be 7,000 psi in most places. Carbonation tests also revealed some unexpected results. "In any area that was uncracked we found a carbonation depth of 2 or 3 mm, which is almost unheard of in a 50-year-old building. I would have expected 2 or 3 mm in the first few years, and at this point five to 10 times as much," Weiss says. He speculates the low carbonation can be attributed in part to the compaction achieved by shooting the gunite from the inside against the forms on the outside rotunda walls and partly to the original vinyl elastic coating and subsequent coating applications.