Contractors will soon begin drilling into the steel-and-concrete Gateway Arch in St. Louis to create monitoring ports and obtain core samples as authorities deepen their study of the growing corrosion plaguing the nation’s tallest monument.
A structural steel study, commissioned in September, is now underway to determine the cause and extent of the decay and staining on both the stainless-steel exterior and carbon-steel interior of the iconic Arch, which opened in 1967, eight years after groundbreaking.
The Arch’s exterior never received a protective coating, because planners and builders considered stainless steel impervious to corrosion. Cleaning every 50 years was all that was required for the structure to last 1,000 years, they said.
Time proved otherwise, however, with interior and exterior corrosion and decay becoming visible over the years. The study now underway follows one conducted in 2006 that reportedly noted corroding bolt heads and staircases; a leaky, sometimes-fog-shrouded, rusting interior; and a degenerating exterior.
Some sort of battleship-gray lead-based protective coating was applied to the interior during construction, but except for spot repairs and touch-ups, the reinforced-concrete-and-steel structure has never been recoated or even thoroughly cleaned, according to the National Park Service.
Monitoring and Data Collection
Consultants are now mapping up to two dozen spots in the concrete fill area where ports will be drilled into the legs, said Frank Mares, deputy superintendent of the Jefferson National Expansion Memorial, which includes the Arch. The concrete fill area extends about 300 feet up the Arch legs; above that, the structure is steel only.
The holes—about a dozen in each leg—will range in size up 10 inches wide, Mares said. Some will be used to obtain concrete core samples of a depth yet to be determined. Other ports will be used to install humidity and temperature sensors—and, later on, possibly cameras—to gather data and monitor the corrosion. Those areas will be covered with removable plates,
Other monitors will be installed on the higher steel-only sections to analyze the composition of the area and obtain a better view of the damage, Mares said. The data will be collected for about a year, beginning in January.
Although Park Service officials say the structure is safe, they also acknowledge that they cannot yet determine the extent of the damage.
Part of the current effort will be to determine the source of the moisture infiltration. “Water is in there somehow, and it’s hard to say how,” Mares said.
Among the possibilities:
• Leaking into the ventilation louvers installed on top of the Arch. However, the louvers are inspected annually and officials consider this the least likely possibility.
• Infiltration through welds and seams—a strong possibility, but perhaps not the whole problem, officials say.
• Naturally occurring condensation that results from fluctuations in temperature and humidity extremes. Again, some of this is almost a certainty, but officials want to know how the corrosion became so advanced.
• A problem with the concrete, perhaps caused by the addition of a chemical curing agent during construction. This is a new suspicion, and a strong one, Mares said. “Normally, a curing agent isn’t a problem,” he said, “but when you seal it up like a tomb, that could cause a problem.”
Consultants are now examining archival records to more precisely determine the structure’s coatings and concrete formulations, Mares said.
Also on the consultants’ to-do list: Determine a way to fully access the structure’s exterior, to collect samples and eventually make repairs.
“We presume that it will involve some sort of rappelling,” Mares said. “Cranes could reach it, but there’s really no way to bring a crane in.”
When the Arch was constructed, two 100-ton creeper derricks were used to raise the 12-foot-high, 50-ton sections at the top. Workers then used the derricks to apply the final touches, welding, grinding, polishing and cleaning as they worked their way down the structure.
All of the mapping and research work must be completed within about a month to allow for review for historical compliance, lead abatement and other issues, Mares said. Officials plan to keep the structure open throughout the study period.
Park Service employees will perform the drilling and other work once the plan is approved. After a year of data collection, Mares said, the consultant’s report should be available in January 2012, and the actual restoration and repairs can proceed.
“The aesthetic prettying-up of it,” he added, “will be the least of the problem.”